Metals

Metals account for about two thirds of all the elements and about 24% of the mass of the planet. Metals have useful properties including strength, ductility, high melting points, thermal and electrical conductivity, and toughness. From the periodic table, it can be seen that a large number of the elements are classified as being a metal. A few of the common metals and their typical uses are presented below.

Common Metallic Materials

• Iron/Steel - Steel alloys are used for strength critical applications
• Aluminum - Aluminum and its alloys are used because they are easy to form, readily available, inexpensive, and recyclable.
• Copper - Copper and copper alloys have a number of properties that make them useful, including high electrical and thermal conductivity, high ductility, and good corrosion resistance.
• Titanium - Titanium alloys are used for strength in higher temperature (~1000° F) application, when component weight is a concern, or when good corrosion resistance is required
• Nickel - Nickel alloys are used for still higher temperatures (~1500-2000° F) applications or when good corrosion resistance is required.
• Refractory materials are used for the highest temperature (> 2000° F) applications.

The key feature that distinguishes metals from non-metals is their bonding. Metallic materials have free electrons that are free to move easily from one atom to the next. The existence of these free electrons has a number of profound consequences for the properties of metallic materials. For example, metallic materials tend to be good electrical conductors because the free electrons can move around within the metal so freely. More on the structure of metals will be discussed later

. Ceramics

A ceramic has traditionally been defined as “an inorganic, nonmetallic solid that is prepared from powdered materials, is fabricated into products through the application of heat, and displays such characteristic properties as hardness, strength, low electrical conductivity, and brittleness." The word ceramic comes the from Greek word "keramikos", which means "pottery." They are typically crystalline in nature and are compounds formed between metallic and nonmetallic elements such as aluminum and oxygen (alumina-Al₂O₃), calcium and oxygen (calcia - CaO), and silicon and nitrogen (silicon nitride-Si₃N₄).

Depending on their method of formation, ceramics can be dense or lightweight. Typically, they will demonstrate excellent strength and hardness properties; however, they are often brittle in nature. Ceramics can also be formed to serve as electrically conductive materials or insulators. Some ceramics, like superconductors, also display magnetic properties. They are also more resistant to high temperatures and harsh environments than metals and polymers. Due to ceramic materials wide range of properties, they are used for a multitude of applications. 

The broad categories or segments that make up the ceramic industry can be classified as:

• Structural clay products (brick, sewer pipe, roofing and wall tile, flue linings, etc.)
• Whitewares (dinnerware, floor and wall tile, electrical porcelain, etc.)
• Refractories (brick and monolithic products used in metal, glass, cements, ceramics, energy conversion, petroleum, and chemicals industries)
• Glasses (flat glass (windows), container glass (bottles), pressed and blown glass (dinnerware), glass fibers (home insulation), and advanced/specialty glass (optical fibers))
• Abrasives (natural (garnet, diamond, etc.) and synthetic (silicon carbide, diamond, fused alumina, etc.) abrasives are used for grinding, cutting, polishing, lapping, or pressure blasting of materials)
• Cements (for roads, bridges, buildings, dams, and etc.)
• Advanced ceramics
  • Structural (wear parts, bioceramics, cutting tools, and engine components)
  • Electrical (capacitors, insulators, substrates, integrated circuit packages, piezoelectrics, magnets and superconductors)
  • Coatings (engine components, cutting tools, and industrial wear parts)
  • Chemical and environmental (filters, membranes, catalysts, and catalyst supports)

The atoms in ceramic materials are held together by a chemical bond which will be discussed a bit later. Briefly though, the two most common chemical bonds for ceramic materials are covalent and ionic. Covalent and ionic bonds are much stronger than in metallic bonds and, generally speaking, this is why ceramics are brittle and metals are ductile.

Polymers

A polymeric solid can be thought of as a material that contains many chemically bonded parts or units which themselves are bonded together to form a solid. The word polymer literally means "many parts." Two industrially important polymeric materials are plastics and elastomers. Plastics are a large and varied group of synthetic materials which are processed by forming or molding into shape. Just as there are many types of metals such as aluminum and copper, there are many types of plastics, such as polyethylene and nylon. Elastomers or rubbers can be elastically deformed a large amount when a force is applied to them and can return to their original shape (or almost) when the force is released.

Polymers have many properties that make them attractive to use in certain conditions. Many polymers:

• are less dense than metals or ceramics,
• resist atmospheric and other forms of corrosion,
• offer good compatibility with human tissue, or
• exhibit excellent resistance to the conduction of electrical current.
  

The polymer plastics can be divided into two classes, thermoplastics and thermosetting plastics, depending on how they are structurally and chemically bonded. Thermoplastic polymers comprise the four most important commodity materials – polyethylene, polypropylene, polystyrene and polyvinyl chloride. There are also a number of specialized engineering polymers. The term ‘thermoplastic’ indicates that these materials melt on heating and may be processed by a variety of molding and extrusion techniques. Alternately, ‘thermosetting’ polymers can not be melted or remelted. Thermosetting polymers include alkyds, amino and phenolic resins, epoxies, polyurethanes, and unsaturated polyesters.

Rubber is a natural occurring polymer. However, most polymers are created by engineering the combination of hydrogen and carbon atoms and the arrangement of the chains they form. The polymer molecule is a long chain of covalent-bonded atoms and secondary bonds then hold groups of polymer chains together to form the polymeric material. Polymers are primarily produced from petroleum or natural gas raw products but the use of organic substances is growing. The super-material known as Kevlar is a man-made polymer. Kevlar is used in bullet-proof vests, strong/lightweight frames, and underwater cables that are 20 times stronger than steel.

Composites

A composite is commonly defined as a combination of two or more distinct materials, each of which retains its own distinctive properties, to create a new material with properties that cannot be achieved by any of the components acting alone. Using this definition, it can be determined that a wide range of engineering materials fall into this category. For example, concrete is a composite because it is a mixture of Portland cement and aggregate. Fiberglass sheet is a composite since it is made of glass fibers imbedded in a polymer.

Composite materials are said to have two phases. The reinforcing phase is the fibers, sheets, or particles that are embedded in the matrix phase. The reinforcing material and the matrix material can be metal, ceramic, or polymer. Typically, reinforcing materials are strong with low densities while the matrix is usually a ductile, or tough, material.

Some of the common classifications of composites are:

• Reinforced plastics
• Metal-matrix composites
• Ceramic-matrix composites
• Sandwich structures
• Concrete

Composite materials can take many forms but they can be separated into three categories based on the strengthening mechanism. These categories are dispersion strengthened, particle reinforced and fiber reinforced. Dispersion strengthened composites have a fine distribution of secondary particles in the matrix of the material. These particles impede the mechanisms that allow a material to deform. (These mechanisms include dislocation movement and slip, which will be discussed later). Many metal-matrix composites would fall into the dispersion strengthened composite category. Particle reinforced composites have a large volume fraction of particle dispersed in the matrix and the load is shared by the particles and the matrix. Most commercial ceramics and many filled polymers are particle-reinforced composites. In fiber-reinforced composites, the fiber is the primary load-bearing component. Fiberglass and carbon fiber composites are examples of fiber-reinforced composites.

If the composite is designed and fabricated correctly, it combines the strength of the reinforcement with the toughness of the matrix to achieve a combination of desirable properties not available in any single conventional material. Some composites also offer the advantage of being tailorable so that properties, such as strength and stiffness, can easily be changed by changing amount or orientation of the reinforcement material. The downside is that such composites are often more expensive than conventional materials.

Structure of Materials

It should be clear that all matter is made of atoms. From the periodic table, it can be seen that there are only about 100 different kinds of atoms in the entire Universe. These same 100 atoms form thousands of different substances ranging from the air we breathe to the metal used to support tall buildings. Metals behave differently than ceramics, and ceramics behave differently than polymers. The properties of matter depend on which atoms are used and how they are bonded together.

The structure of materials can be classified by the general magnitude of various features being considered. The three most common major classification of structural, listed generally in increasing size, are:

Atomic structure, which includes features that cannot be seen, such as the types of bonding between the atoms, and the way the atoms are arranged.

Microstructure, which includes features that can be seen using a microscope, but seldom with the naked eye.

Macrostructure, which includes features that can be seen with the naked eye)

The atomic structure primarily affects the chemical, physical, thermal, electrical, magnetic, and optical properties. The microstructure and macrostructure can also affect these properties but they generally have a larger effect on mechanical properties and on the rate of chemical reaction. The properties of a material offer clues as to the structure of the material. The strength of metals suggests that these atoms are held together by strong bonds. However, these bonds must also allow atoms to move since metals are also usually formable. To understand the structure of a material, the type of atoms present, and how the atoms are arranged and bonded must be known. Let’s first look at atomic bonding.

Atomic Bonding

(Metallic, Ionic, Covalent, and van der Waals Bonds)

From elementary chemistry it is known that the atomic structure of any element is made up of a positively charged nucleus surrounded by electrons revolving around it. An element’s atomic number indicates the number of positively charged protons in the nucleus. The atomic weight of an atom indicates how many protons and neutrons in the nucleus. To determine the number of neutrons in an atom, the atomic number is simply subtracted from the atomic weight.

Atoms like to have a balanced electrical charge. Therefore, they usually have negatively charged electrons surrounding the nucleus in numbers equal to the number of protons. It is also known that electrons are present with different energies and it is convenient to consider these electrons surrounding the nucleus in energy “shells.” For example, magnesium, with an atomic number of 12, has two electrons in the inner shell, eight in the second shell and two in the outer shell.

All chemical bonds involve electrons. Atoms will stay close together if they have a shared interest in one or more electrons. Atoms are at their most stable when they have no partially-filled electron shells. If an atom has only a few electrons in a shell, it will tend to lose them to empty the shell. These elements are metals. When metal atoms bond, a metallic bond occurs. When an atom has a nearly full electron shell, it will try to find electrons from another atom so that it can fill its outer shell. These elements are usually described as nonmetals. The bond between two nonmetal atoms is usually a covalent bond. Where metal and nonmetal atom come together an ionic bond occurs. There are also other, less common, types of bond but the details are beyond the scope of this material. On the next few pages, the Metallic, Covalent and Ionic bonds will be covered in more detail.

Ionic Bonds

Ionic bonding occurs between charged particles. These may be atoms or groups of atoms, but this discuss will be conducted in terms of single atoms. Ionic bonding occurs between metal atoms and nonmetal atoms. Metals usually have 1, 2, or 3 electrons in their outermost shell. Nonmetals have 5, 6, or 7 electrons in their outer shell. Atoms with outer shells that are only partially filled are unstable. To become stable, the metal atom wants to get rid of one or more electrons in its outer shell. Losing electrons will either result in an empty outer shell or get it closer to having an empty outer shell. It would like to have an empty outer shell because the next lower energy shell is a stable shell with eight electrons.

Since electrons have a negative charge, the atom that gains electrons becomes a negatively charged ions (aka anion) because it now has more electrons than protons. Alternately, an atom that loses electrons becomes a positively charged ion (aka cations). The particles in an ionic compound are held together because there are oppositely charged particles that are attracted to one another.

The images above schematically show the process that takes place during the formation of an ionic bond between sodium and chlorine atoms. Note that sodium has one valence electron that it would like to give up so that it would become stable with a full outer shell of eight. Also note that chlorine has seven valence electrons and it would like to gain an electron in order to have a full shell of eight. The transfer of the electron causes the previously neutral sodium atom to become a positively charged ion (cation), and the previously neutral chlorine atom to become a negatively charged ion (anion). The attraction for the cation and the anion is called the ionic bond.

Generally, solid materials with ionic bonds:

• are hard because particles cannot easily slide past one another.
• are good insulators because there are no free electrons or ions (unless dissolved or melted).
• are transparent because their electrons are not moving from atom to atom and less likely to interact with light photons.
• are brittle and tend to cleave rather than deform because bonds are strong.
• have high melting point because ionic bonds are relatively strong.

Covalent Bonding

Where a compound only contains nonmetal atoms, a covalent bond is formed by atoms sharing two or more electrons. Nonmetals have 4 or more electrons in their outer shells (except boron). With this many electrons in the outer shell, it would require more energy to remove the electrons than would be gained by making new bonds. Therefore, both the atoms involved share a pair of electrons. Each atom gives one of its outer electrons to the electron pair, which then spends some time with each atom. Consequently, both atoms are held near each other since both atoms have a share in the electrons.

More than one electron pair can be formed with half of the electrons coming from one atom and the rest from the other atom. An important feature of this bond is that the electrons are tightly held and equally shared by the participating atoms. The atoms can be of the same element or different elements. In each molecule, the bonds between the atoms are strong but the bonds between molecules are usually weak. This makes many solid materials with covalent bonds brittle. Many ceramic materials have covalent bonds.

Compounds with covalent bonds may be solid, liquid or gas at room temperature depending on the number of atoms in the compound. The more atoms in each molecule, the higher a compound’s melting and boiling temperature will be. Since most covalent compounds contain only a few atoms and the forces between molecules are weak, most covalent compounds have low melting and boiling points. However, some, like carbon compounds, can be very large. An example is the diamond in which carbon atoms each share four electrons to form giant lattices.

Some Common Features of Materials with Covalent Bonds:

• Hard
• Good insulators
• Transparent
• Brittle or cleave rather than deform

Metallic Bonding

A common characteristic of metallic elements is they contain only one to three electrons in the outer shell. When an element has only one, two or three valence electrons (i.e. electrons in the outer shell), the bond between these electrons and the nucleus is relatively weak. So, for example, when aluminum atoms are grouped together in a block of metal, the outer electrons leave individual atoms to become part of common “electron cloud.” In this arrangement, the valence electrons have considerable mobility and are able to conduct heat and electricity easily. Also, the delocalized nature of the bonds, make it possible for the atoms to slide past each other when the metal is deformed instead of fracturing like glass or other brittle material.

Since the aluminum atoms lose two electrons, they end up having a positive charge and are designated Al³⁺ ions (cations). These ions repel each other but are held together in the block because the negative electrons are attracted to the positively charged ions. A result of the sharing of electrons is the cations arrange themselves in a regular pattern. This regular pattern of atoms is the crystalline structure of metals. In the crystal lattice, atoms are packed closely together to maximize the strength of the bonds. An actual piece of metal consists of many tiny crystals called grains that touch at grain boundaries.

Some Common Features of Materials with Metallic Bonds:

• Good electrical and thermal conductors due to their free valence electrons
• Opaque
• Relatively ductile

Van der Waals Bond

The van der Waal bonds occur to some extent in all materials but are particularly important in plastics and polymers. These materials are made up of a long string molecules consisting of carbon atoms covalently bonded with other atoms, such as hydrogen, nitrogen, oxygen, fluorine. The covalent bonds within the molecules are very strong and rupture only under extreme conditions. The bonds between the molecules that allow sliding and rupture to occur are called van der Waal forces.

When ionic and covalent bonds are present, there is some imbalance in the electrical charge of the molecule. Take water as an example. Research has determined the hydrogen atoms are bonded to the oxygen atoms at an angle of 104.5°. This angle produces a positive polarity at the hydrogen-rich end of the molecule and a negative polarity at the other end. A result of this charge imbalance is that water molecules are attracted to each other. This is the force that holds the molecules together in a drop of water.

This same concept can be carried on to plastics, except that as molecules become larger, the van der Waal forces between molecules also increases. For example, in polyethylene the molecules are composed of hydrogen and carbon atoms in the same ratio as ethylene gas. But there are more of each type of atom in the polyethylene molecules and as the number of atoms in a molecule increases, the matter passes from a gas to a liquid and finally to a solid.

Polymers are often classified as being either a thermoplastic or a thermosetting material. Thermoplastic materials can be easily remelted for forming or recycling and thermosetting material cannot be easily remelted. In thermoplastic materials consist of long chainlike molecules. Heat can be used to break the van der Waal forces between the molecules and change the form of the material from a solid to a liquid. By contrast, thermosetting materials have a three-dimensional network of covalent bonds. These bonds cannot be easily broken by heating and, therefore, can not be remelted and formed as easily as thermoplastics.

Solid State Structure

In the previous pages, some of the mechanisms that bond together the multitude of individual atoms or molecules of a solid material were discussed. These forces may be primary chemical bonds, as in metals and ionic solids, or they may be secondary van der Waals’ forces of solids, such as in ice, paraffin wax and most polymers. In solids, the way the atoms or molecules arrange themselves contributes to the appearance and the properties of the materials.

Atoms can be gathered together as an aggregate through a number of different processes, including condensation, pressurization, chemical reaction, electrodeposition, and melting. The process usually determines, at least initially, whether the collection of atoms will take to form of a gas, liquid or solid. The state usually changes as its temperature or pressure is changed. Melting is the process most often used to form an aggregate of atoms. When the temperature of a melt is lowered to a certain point, the liquid will form either a crystalline solid or and amorphous solid.

Amorphous Solids

A solid substance with its atoms held apart at equilibrium spacing, but with no long-range periodicity in atom location in its structure is an amorphous solid. Examples of amorphous solids are glass and some types of plastic. They are sometimes described as supercooled liquids because their molecules are arranged in a random manner some what as in the liquid state. For example, glass is commonly made from silicon dioxide or quartz sand, which has a crystalline structure. When the sand is melted and the liquid is cooled rapidly enough to avoid crystallization, an amorphous solid called a glass is formed. Amorphous solids do not show a sharp phase change from solid to liquid at a definite melting point, but rather soften gradually when they are heated. The physical properties of amorphous solids are identical in all directions along any axis so they are said to have isotropic properties, which will be discussed in more detail later

.

Crystalline Solids

More than 90% of naturally occurring and artificially prepared solids are crystalline. Minerals, sand, clay, limestone, metals, carbon (diamond and graphite), salts ( NaCl, KCl etc.), all have crystalline structures. A crystal is a regular, repeating arrangement of atoms or molecules. The majority of solids, including all metals, adopt a crystalline arrangement because the amount of stabilization achieved by anchoring interactions between neighboring particles is at its greatest when the particles adopt regular (rather than random) arrangements. In the crystalline arrangement, the particles pack efficiently together to minimize the total intermolecular energy.

The regular repeating pattern that the atoms arrange in is called the crystalline lattice. The scanning tunneling microscope (STM) makes it possible to image the electron cloud associated individual atoms at the surface of a material. Below is an STM image of a platinum surface showing the regular alignment of atoms.

Courtesy: IBM Research, Almaden Research Center.

Crystal Structure

Crystal structures may be conveniently specified by describing the arrangement within the solid of a small representative group of atoms or molecules, called the ‘unit cell.’ By multiplying identical unit cells in three directions, the location of all the particles in the crystal is determined. In nature, 14 different types of crystal structures or lattices are found. The simplest crystalline unit cell to picture is the cubic, where the atoms are lined up in a square, 3D grid. The unit cell is simply a box with an atom at each corner. Simple cubic crystals are relatively rare, mostly because they tend to easily distort. However, many crystals form body-centered-cubic (bcc) or face-centered-cubic (fcc) structures, which are cubic with either an extra atom centered in the cube or centered in each face of the cube. Most metals form bcc, fcc or Hexagonal Close Packed (hpc) structures; however, the structure can change depending on temperature. These three structures will be discussed in more detail on the following page.

Crystalline structure is important because it contributes to the properties of a material. For example, it is easier for planes of atoms to slide by each other if those planes are closely packed. Therefore, lattice structures with closely packed planes allow more plastic deformation than those that are not closely packed. Additionally, cubic lattice structures allow slippage to occur more easily than non-cubic lattices. This is because their symmetry provides closely packed planes in several directions. A face-centered cubic crystal structure will exhibit more ductility (deform more readily under load before breaking) than a body-centered cubic structure. The bcc lattice, although cubic, is not closely packed and forms strong metals. Alpha-iron and tungsten have the bcc form. The fcc lattice is both cubic and closely packed and forms more ductile materials. Gamma-iron, silver, gold, and lead have fcc structures. Finally, HCP lattices are closely packed, but not cubic. HCP metals like cobalt and zinc are not as ductile as the fcc metals.

Primary Metallic Crystalline Structures

(BCC, FCC, HCP)

As pointed out on the previous page, there are 14 different types of crystal unit cell structures or lattices are found in nature. However most metals and many other solids have unit cell structures described as body center cubic (bcc), face centered cubic (fcc) or Hexagonal Close Packed (hcp). Since these structures are most common, they will be discussed in more detail.

Body-Centered Cubic (BCC) Structure

The body-centered cubic unit cell has atoms at each of the eight corners of a cube (like the cubic unit cell) plus one atom in the center of the cube (left image below). Each of the corner atoms is the corner of another cube so the corner atoms are shared among eight unit cells. It is said to have a coordination number of 8. The bcc unit cell consists of a net total of two atoms; one in the center and eight eighths from corners atoms as shown in the middle image below (middle image below). The image below highlights a unit cell in a larger section of the lattice.

The bcc arrangement does not allow the atoms to pack together as closely as the fcc or hcp arrangements. The bcc structure is often the high temperature form of metals that are close-packed at lower temperatures. The volume of atoms in a cell per the total volume of a cell is called thepacking factor. The bcc unit cell has a packing factor of 0.68.

Some of the materials that have a bcc structure include lithium, sodium, potassium, chromium, barium, vanadium, alpha-iron and tungsten. Metals which have a bcc structure are usually harder and less malleable than close-packed metals such as gold. When the metal is deformed, the planes of atoms must slip over each other, and this is more difficult in the bcc structure. It should be noted that there are other important mechanisms for hardening materials, such as introducing impurities or defects which make slipping more difficult. These hardening mechanisms will be discussed latter.

Face Centered Cubic (FCC) Structure 

The face centered cubic structure has atoms located at each of the corners and the centers of all the cubic faces (left image below). Each of the corner atoms is the corner of another cube so the corner atoms are shared among eight unit cells. Additionally, each of its six face centered atoms is shared with an adjacent atom. Since 12 of its atoms are shared, it is said to have a coordination number of 12. The fcc unit cell consists of a net total of four atoms; eight eighths from corners atoms and six halves of the face atoms as shown in the middle image above. The image below highlights a unit cell in a larger section of the lattice.

In the fcc structure (and the hcp structure) the atoms can pack closer together than they can in the bcc structure. The atoms from one layer nest themselves in the empty space between the atoms of the adjacent layer. To picture packing arrangement, imagine a box filled with a layer of balls that are aligned in columns and rows. When a few additional balls are tossed in the box, they will not balance directly on top of the balls in the first layer but instead will come to rest in the pocket created between four balls of the bottom layer. As more balls are added they will pack together to fill up all the pockets. The packing factor (the volume of atoms in a cell per the total volume of a cell) is 0.74 for fcc crystals. Some of the metals that have the fcc structure include aluminum, copper, gold, iridium, lead, nickel, platinum and silver.

Hexagonal Close Packed (HPC) Structure

Another common close packed structure is the hexagonal close pack. The hexagonal structure of alternating layers is shifted so its atoms are aligned to the gaps of the preceding layer. The atoms from one layer nest themselves in the empty space between the atoms of the adjacent layer just like in the fcc structure. However, instead of being a cubic structure, the pattern is hexagonal. (See image below.) The difference between the HPC and FCC structure is discussed later in this section.

The hcp structure has three layers of atoms. In each the top and bottom layer, there are six atoms that arrange themselves in the shape of a hexagon and a seventh atom that sits in the middle of the hexagon. The middle layer has three atoms nestle in the triangular "grooves" of the top and bottom plane. Note that there are six of these "grooves" surrounding each atom in the hexagonal plane, but only three of them can be filled by atoms.

As shown in the middle image above, there are six atoms in the hcp unit cell. Each of the 12 atoms in the corners of the top and bottom layers contribute 1/6 atom to the unit cell, the two atoms in the center of the hexagon of both the top and bottom layers each contribute ½ atom and each of the three atom in the middle layer contribute 1 atom. The image on the right above attempts to show several hcp unit cells in a larger lattice.

The coordination number of the atoms in this structure is 12. There are six nearest neighbors in the same close packed layer, three in the layer above and three in the layer below. The packing factor is 0.74, which is the same as the fcc unit cell. The hcp structure is very common for elemental metals and some examples include beryllium, cadmium, magnesium, titanium, zinc and zirconium.

Similarities and Difference Between the

FCC and HCP Structure

The face centered cubic and hexagonal close packed structures both have a packing factor of 0.74, consist of closely packed planes of atoms, and have a coordination number of 12. The difference between the fcc and hcp is the stacking sequence. The hcp layers cycle among the two equivalent shifted positions whereas the fcc layers cycle between three positions. As can be seen in the image, the hcp structure contains only two types of planes with an alternating ABAB arrangement. Notice how the atoms of the third plane are in exactly the same position as the atoms in the first plane. However, the fcc structure contains three types of planes with a ABCABC arrangement. Notice how the atoms in rows A and C are no longer aligned. Remember that cubic lattice structures allow slippage to occur more easily than non-cubic lattices, so hcp metals are not as ductile as the fcc metals.

The table below shows the stable room temperature crystal structures for several elemental metals.

Metal	Crystal Structure	Atomic Radius (nm)
Aluminum	FCC	0.1431
Cadmium	HCP	0.1490
Chromium	BCC	0.1249
Cobalt	HCP	0.1253
Copper	FCC	0.1278
Gold	FCC	0.1442
Iron (Alpha)	BCC	0.1241
Lead	FCC	0.1750
Magnesium	HCP	0.1599
Molybdenum	BCC	0.1363
Nickel	FCC	0.1246
Platinum	FCC	0.1387
Silver	FCC	0.1445
Tantalum	BCC	0.1430
Titanium (Alpha)	HCP	0.1445
Tungsten	BCC	0.1371
Zinc	HCP	0.1332

A nanometer (nm) equals 10^-9 meter or 10 Angstrom units.

Solidification

The crystallization of a large amount of material from a single point of nucleation results in a single crystal. In engineering materials, single crystals are produced only under carefully controlled conditions. The expense of producing single crystal materials is only justified for special applications, such as turbine engine blades, solar cells, and piezoelectric materials. Normally when a material begins to solidify, multiple crystals begin to grow in the liquid and a polycrystalline (more than one crystal) solid forms.

The moment a crystal begins to grow is know as nucleation and the point where it occurs is the nucleation point. At the solidification temperature, atoms of a liquid, such as melted metal, begin to bond together at the nucleation points and start to form crystals. The final sizes of the individual crystals depend on the number of nucleation points. The crystals increase in size by the progressive addition of atoms and grow until they impinge upon adjacent growing crystal.

a) Nucleation of crystals, b) crystal growth, c) irregular grains form as crystals grow together, d) grain boundaries as seen in a microscope.

In engineering materials, a crystal is usually referred to as a grain. A grain is merely a crystal without smooth faces because its growth was impeded by contact with another grain or a boundary surface. The interface formed between grains is called a grain boundary. The atoms between the grains (at the grain boundaries) have no crystalline structure and are said to be disordered.

Grains are sometimes large enough to be visible under an ordinary light microscope or even to the unaided eye. The spangles that are seen on newly galvanized metals are grains. Rapid cooling generally results in more nucleation points and smaller grains (a fine grain structure). Slow cooling generally results in larger grains which will have lower strength, hardness and ductility.

Dendrites

In metals, the crystals that form in the liquid during freezing generally follow a pattern consisting of a main branch with many appendages. A crystal with this morphology slightly resembles a pine tree and is called a dendrite, which means branching. The formation of dendrites occurs because crystals grow in defined planes due to the crystal lattice they create. The figure to the right shows how a cubic crystal can grow in a melt in three dimensions, which correspond to the six faces of the cube. For clarity of illustration, the adding of unit cells with continued solidification from the six faces is shown simply as lines. Secondary dendrite arms branch off the primary arm, and tertiary arms off the secondary arms and etcetera.

During freezing of a polycrystalline material, many dendritic crystals form and grow until they eventually become large enough to impinge upon each other. Eventually, the interdendriticspaces between the dendrite arms crystallize to yield a more regular crystal. The original dendritic pattern may not be apparent when examining the microstructure of a material. However, dendrites can often be seen in solidification voids that sometimes occur in castings or welds, as shown to the right..

Shrinkage

Most materials contract or shrink during solidification and cooling. Shrinkage is the result of:

• Contraction of the liquid as it cools prior to its solidification
• Contraction during phase change from a liquid to solid
• Contraction of the solid as it continues to cool to ambient temperature.

Shrinkage can sometimes cause cracking to occur in component as it solidifies. Since the coolest area of a volume of liquid is where it contacts a mold or die, solidification usually begins first at this surface. As the crystals grow inward, the material continues to shrink. If the solid surface is too rigid and will not deform to accommodate the internal shrinkage, the stresses can become high enough to exceed the tensile strength of the material and cause a crack to form. Shrinkage cavitation sometimes occurs because as a material solidifies inward, shrinkage occurred to such an extent that there is not enough atoms present to fill the available space and a void is left.

Crystal Defects

A perfect crystal, with every atom of the same type in the correct position, does not exist. All crystals have some defects. Defects contribute to the mechanical properties of metals. In fact, using the term “defect” is sort of a misnomer since these features are commonly intentionally used to manipulate the mechanical properties of a material. Adding alloying elements to a metal is one way of introducing a crystal defect. Nevertheless, the term “defect” will be used, just keep in mind that crystalline defects are not always bad. There are basic classes of crystal defects:

• point defects, which are places where an atom is missing or irregularly placed in the lattice structure. Point defects include lattice vacancies, self-interstitial atoms, substitution impurity atoms, and interstitial impurity atoms
• linear defects, which are groups of atoms in irregular positions. Linear defects are commonly called dislocations.
• planar defects, which are interfaces between homogeneous regions of the material. Planar defects include grain boundaries, stacking faults and external surfaces.

It is important to note at this point that plastic deformation in a material occurs due to the movement of dislocations (linear defects). Millions of dislocations result for plastic forming operations such as rolling and extruding. It is also important to note that any defect in the regular lattice structure disrupts the motion of dislocation, which makes slip or plastic deformation more difficult. These defects not only include the point and planer defects mentioned above, and also other dislocations. Dislocation movement produces additional dislocations, and when dislocations run into each other it often impedes movement of the dislocations. This drives up the force needed to move the dislocation or, in other words, strengthens the material. Each of the crystal defects will be discussed in more detail in the following pages.

Point Defects

Point defects are where an atom is missing or isin an irregular place in the lattice structure. Point defects include self interstitial atoms, interstitial impurity atoms, substitutional atoms and vacancies. A self interstitial atom is an extra atom that has crowded its way into an interstitial void in the crystal structure. Self interstitial atoms occur only in low concentrations in metals because they distort and highly stress the tightly packed lattice structure.

A substitutional impurity atom is an atom of a different type than the bulk atoms, which has replaced one of the bulk atoms in the lattice. Substitutional impurity atoms are usually close in size (within approximately 15%) to the bulk atom. An example of substitutional impurity atoms is the zinc atoms in brass. In brass, zinc atoms with a radius of 0.133 nm have replaced some of the copper atoms, which have a radius of 0.128 nm.

Interstitial impurity atoms are much smaller than the atoms in the bulk matrix. Interstitial impurity atoms fit into the open space between the bulk atoms of the lattice structure. An example of interstitial impurity atoms is the carbon atoms that are added to iron to make steel. Carbon atoms, with a radius of 0.071 nm, fit nicely in the open spaces between the larger (0.124 nm) iron atoms.

Vacancies are empty spaces where an atom should be, but is missing. They are common, especially at high temperatures when atoms are frequently and randomly change their positions leaving behind empty lattice sites. In most cases diffusion (mass transport by atomic motion) can only occur because of vacancies.

Linear Defects - Dislocations

Dislocations are another type of defect in crystals. Dislocations are areas were the atoms are out of position in the crystal structure. Dislocations are generated and move when a stress is applied. The motion of dislocations allows slip – plastic deformation to occur.

Before the discovery of the dislocation by Taylor, Orowan and Polyani in 1934, no one could figure out how the plastic deformation properties of a metal could be greatly changed by solely by forming (without changing the chemical composition). This became even bigger mystery when in the early 1900’s scientists estimated that metals undergo plastic deformation at forces much smaller than the theoretical strength of the forces that are holding the metal atoms together. Many metallurgists remained skeptical of the dislocation theory until the development of the transmission electron microscope in the late 1950’s. The TEM allowed experimental evidence to be collected that showed that the strength and ductility of metals are controlled by dislocations.

There are two basic types of dislocations, the edge dislocation and the screw dislocation. Actually, edge and screw dislocations are just extreme forms of the possible dislocation structures that can occur. Most dislocations are probably a hybrid of the edge and screw forms but this discussion will be limited to these two types.

Edge Dislocations

The edge defect can be easily visualized as an extra half-plane of atoms in a lattice. The dislocation is called a line defect because the locus of defective points produced in the lattice by the dislocation lie along a line. This line runs along the top of the extra half-plane. The inter-atomic bonds are significantly distorted only in the immediate vicinity of the dislocation line.

Understanding the movement of a dislocation is key to understanding why dislocations allow deformation to occur at much lower stress than in a perfect crystal. Dislocation motion is analogous to movement of a caterpillar. The caterpillar would have to exert a large force to move its entire body at once. Instead it moves the rear portion of its body forward a small amount and creates a hump. The hump then moves forward and eventual moves all of the body forward by a small amount.

As shown in the set of images above, the dislocation moves similarly moves a small amount at a time. The dislocation in the top half of the crystal is slipping one plane at a time as it moves to the right from its position in image (a) to its position in image (b) and finally image (c). In the process of slipping one plane at a time the dislocation propagates across the crystal. The movement of the dislocation across the plane eventually causes the top half of the crystal to move with respect to the bottom half. However, only a small fraction of the bonds are broken at any given time. Movement in this manner requires a much smaller force than breaking all the bonds across the middle plane simultaneously.

Screw Dislocations

There is a second basic type of dislocation, called screw dislocation. The screw dislocation is slightly more difficult to visualize. The motion of a screw dislocation is also a result of shear stress, but the defect line movement is perpendicular to direction of the stress and the atom displacement, rather than parallel. To visualize a screw dislocation, imagine a block of metal with a shear stress applied across one end so that the metal begins to rip. This is shown in the upper right image. The lower right image shows the plane of atoms just above the rip. The atoms represented by the blue circles have not yet moved from their original position. The atoms represented by the red circles have moved to their new position in the lattice and have reestablished metallic bonds. The atoms represented by the green circles are in the process of moving. It can be seen that only a portion of the bonds are broke at any given time. As was the case with the edge dislocation, movement in this manner requires a much smaller force than breaking all the bonds across the middle plane simultaneously.

If the shear force is increased, the atoms will continue to slip to the right. A row of the green atoms will find there way back into a proper spot in the lattice (and become red) and a row of the blue atoms will slip out of position (and become green). In this way, the screw dislocation will move upward in the image, which is perpendicular to direction of the stress. Recall that the edge dislocation moves parallel to the direction of stress. As shown in the image below, the net plastic deformation of both edge and screw dislocations is the same, however.

The dislocations move along the densest planes of atoms in a material, because the stress needed to move the dislocation increases with the spacing between the planes. FCC and BCC metals have many dense planes, so dislocations move relatively easy and these materials have high ductility. Metals are strengthened by making it more difficult for dislocations to move. This may involve the introduction of obstacles, such as interstitial atoms or grain boundaries, to “pin” the dislocations. Also, as a material plastically deforms, more dislocations are produced and they will get into each others way and impede movement. This is why strain or work hardening occurs.

In ionically bonded materials, the ion must move past an area with a repulsive charge in order to get to the next location of the same charge. Therefore, slip is difficult and the materials are brittle. Likewise, the low density packing of covalent materials makes them generally more brittle than metals.

Planar Defects

Stacking Faults and Twin Boundaries

A disruption of the long-range stacking sequence can produce two other common types of crystal defects: 1) a stacking fault and 2) a twin region. A change in the stacking sequence over a few atomic spacings produces a stacking fault whereas a change over many atomic spacings produces a twin region.

A stacking fault is a one or two layer interruption in the stacking sequence of atom planes. Stacking faults occur in a number of crystal structures, but it is easiest to see how they occur in close packed structures. For example, it is know from a previous discussion that face centered cubic (fcc) structures differ from hexagonal close packed (hcp) structures only in their stacking order. For hcp and fcc structures, the first two layers arrange themselves identically, and are said to have an AB arrangement. If the third layer is placed so that its atoms are directly above those of the first (A) layer, the stacking will be ABA. This is the hcp structure, and it continues ABABABAB. However it is possible for the third layer atoms to arrange themselves so that they are in line with the first layer to produce an ABC arrangement which is that of the fcc structure. So, if the hcp structure is going along as ABABAB and suddenly switches to ABABABCABAB, there is a stacking fault present.

Alternately, in the fcc arrangement the pattern is ABCABCABC. A stacking fault in an fcc structure would appear as one of the C planes missing. In other words the pattern would become ABCABCAB_ABCABC.

If a stacking fault does not corrects itself immediately but continues over some number of atomic spacings, it will produce a second stacking fault that is the twin of the first one. For example if the stacking pattern is ABABABAB but switches to ABCABCABC for a period of time before switching back to ABABABAB, a pair of twin stacking faults is produced. The red region in the stacking sequence that goes ABCABCACBACBABCABC is the twin plane and the twin boundaries are the A planes on each end of the highlighted region.

Grain Boundaries in Polycrystals

Another type of planer defect is the grain boundary. Up to this point, the discussion has focused on defects of single crystals. However, solids generally consist of a number of crystallites or grains. Grains can range in size from nanometers to millimeters across and their orientations are usually rotated with respect to neighboring grains. Where one grain stops and another begins is know as a grain boundary. Grain boundaries limit the lengths and motions of dislocations. Therefore, having smaller grains (more grain boundary surface area) strengthens a material. The size of the grains can be controlled by the cooling rate when the material cast or heat treated. Generally, rapid cooling produces smaller grains whereas slow cooling result in larger grains. For more information, refer to the discussion on solidification.

Bulk Defects

Bulk defects occur on a much bigger scale than the rest of the crystal defects discussed in this section. However, for the sake of completeness and since they do affect the movement of dislocations, a few of the more common bulk defects will be mentioned. Voids are regions where there are a large number of atoms missing from the lattice. The image to the right is a void in a piece of metal The image was acquired using a Scanning Electron Microscope (SEM). Voids can occur for a number of reasons. When voids occur due to air bubbles becoming trapped when a material solidifies, it is commonly called porosity. When a void occurs due to the shrinkage of a material as it solidifies, it is called cavitation.

Another type of bulk defect occurs when impurity atoms cluster together to form small regions of a different phase. The term ‘phase’ refers to that region of space occupied by a physically homogeneous material. These regions are often called precipitates. Phases and precipitates will be discussed in more detail latter.

Elastic/Plastic Deformation

When a sufficient load is applied to a metal or other structural material, it will cause the material to change shape. This change in shape is called deformation. A temporary shape change that is self-reversing after the force is removed, so that the object returns to its original shape, is called elastic deformation. In other words, elastic deformation is a change in shape of a material at low stress that is recoverable after the stress is removed. This type of deformation involves stretching of the bonds, but the atoms do not slip past each other.

When the stress is sufficient to permanently deform the metal, it is called plastic deformation. As discussed in the section on crystal defects, plastic deformation involves the breaking of a limited number of atomic bonds by the movement of dislocations. Recall that the force needed to break the bonds of all the atoms in a crystal plane all at once is very great. However, the movement of dislocations allows atoms in crystal planes to slip past one another at a much lower stress levels. Since the energy required to move is lowest along the densest planes of atoms, dislocations have a preferred direction of travel within a grain of the material. This results in slip that occurs along parallel planes within the grain. These parallel slip planes group together to form slip bands, which can be seen with an optical microscope. A slip band appears as a single line under the microscope, but it is in fact made up of closely spaced parallel slip planes as shown in the image.

Fatigue Crack Initiation

While on the subject of dislocations, it is appropriate to briefly discuss fatigue. Fatigue is one of the primary reasons for the failure of structural components. The life of a fatigue crack has two parts, initiation and propagation. Dislocations play a major role in the fatigue crack initiation phase. It has been observed in laboratory testing that after a large number of loading cycles dislocations pile up and form structures called persistent slip bands (PSB). An example of a PSB is shown in the micrograph image to the right.

PSBs are areas that rise above (extrusion) or fall below (intrusion) the surface of the component due to movement of material along slip planes. This leaves tiny steps in the surface that serve as stress risers where fatigue cracks can initiate. A crack at the edge of a PSB is shown in the image below taken with a scanning electron microscope (SEM).

Diffusion

Diffusion is the migration of atoms from a region of high concentration to a region of low concentration. In a homogeneous material, atoms are routinely moving around but the movement is random (i.e. there is always an equal number of atoms moving in all directions). In an inhomogeneous material, all the atoms are moving near randomly, but there is a migration of atoms to areas where their concentrations are lower. In other words, there is a net diffusion.

Atom diffusion can occur by the motion of host or substitutional atoms to vacancies (vacancy diffusion), or interstitial impurities atoms to different interstitial positions (interstitial diffusion). In order to move, an atom must overcome the bond energy due to nearby atoms. This is more easily achieved at high temperatures when the atoms are vibrating strongly. Carburizing, which will be discussed later, is an example of diffusion is used.

Property Modification

Many structural metals undergo some special treatment to modify their properties so that they will perform better for their intended use. This treatment can include mechanical working, such as rolling or forging, alloying and/or thermal treatments. Consider aluminum as an example. Commercially pure aluminum (1100) has a tensile strength of around 13,000 psi, which limits its usefulness in structural applications. However, by cold-working aluminum, its strength can be approximately doubled. Also, strength increases are obtained by adding alloying metals such as manganese, silicon, copper, magnesium and zinc. Further, many aluminum alloys are strengthened by heat treatment. Some heat-treatable aluminum alloys obtain tensile strengths that can exceed 100,000 psi.

Strengthening/Hardening Mechanisms

As discussed in the previous section, the ability of a crystalline material to plastically deform largely depends on the ability for dislocation to move within a material. Therefore, impeding the movement of dislocations will result in the strengthening of the material. There are a number of ways to impede dislocation movement, which include:

• controlling the grain size (reducing continuity of atomic planes)
• strain hardening (creating and tangling dislocations)
• alloying (introducing point defects and more grains to pin dislocation)

Control of Grain Size

The size of the grains within a material also has an effect on the strength of the material. The boundary between grains acts as a barrier to dislocation movement and the resulting slip because adjacent grains have different orientations. Since the atom alignment is different and slip planes are discontinuous between grains. The smaller the grains, the shorter the distance atoms can move along a particular slip plane. Therefore, smaller grains improve the strength of a material. The size and number of grains within a material is controlled by the rate of solidification from the liquid phase.

Strain Hardening

Strain hardening (also called work-hardening or cold-working) is the process of making a metal harder and stronger through plastic deformation. When a metal is plastically deformed, dislocations move and additional dislocations are generated. The more dislocations within a material, the more they will interact and become pinned or tangled. This will result in a decrease in the mobility of the dislocations and a strengthening of the material. This type of strengthening is commonly called cold-working. It is called cold-working because the plastic deformation must occurs at a temperature low enough that atoms cannot rearrange themselves. When a metal is worked at higher temperatures (hot-working) the dislocations can rearrange and little strengthening is achieved.

Strain hardening can be easily demonstrated with piece of wire or a paper clip. Bend a straight section back and forth several times. Notice that it is more difficult to bend the metal at the same place. In the strain hardened area dislocations have formed and become tangled, increasing the strength of the material. Continued bending will eventually cause the wire to break at the bend due to fatigue cracking. (After a large number of bending cycles, dislocations form structures called Persistent Slip Bands (PSB). PSBs are basically tiny areas where the dislocations have piled up and moved the material surface out leave steps in the surface that act as stress risers or crack initiation points.)

It should be understood, however, that increasing the strength by cold-working will also result in a reduction in ductility. The graph to the right shows the yield strength and the percent elongation as a function of percent cold-work for a few example materials. Notice that for each material, a small amount of cold-working results in a significant reduction in ductility.

Effects of Elevated Temperature on Strain Hardened Materials

When strain hardened materials are exposed to elevated temperatures, the strengthening that resulted from the plastic deformation can be lost. This can be a bad thing if the strengthening is needed to support a load. However, strengthening due to strain hardening is not always desirable, especially if the material is being heavily formed since ductility will be lowered.

Heat treatment can be used to remove the effects of strain hardening. Three things can occur during heat treatment:

1. Recovery
2. Recrystallization
3. Grain growth

Recovery

When a stain hardened material is held at an elevated temperature an increase in atomic diffusion occurs that relieves some of the internal strain energy. Remember that atoms are not fixed in position but can move around when they have enough energy to break their bonds. Diffusion increases rapidly with rising temperature and this allows atoms in severely strained regions to move to unstrained positions. In other words, atoms are freer to move around and recover a normal position in the lattice structure. This is known as the recovery phase and it results in an adjustment of strain on a microscopic scale. Internal residual stresses are lowered due to a reduction in the dislocation density and a movement of dislocation to lower-energy positions. The tangles of dislocations condense into sharp two-dimensional boundaries and the dislocation density within these areas decrease. These areas are called subgrains. There is no appreciable reduction in the strength and hardness of the material but corrosion resistance often improves.

Recrystallization

At a higher temperature, new, strain-free grains nucleate and grow inside the old distorted grains and at the grain boundaries. These new grains grow to replace the deformed grains produced by the strain hardening. With recrystallization, the mechanical properties return to their original weaker and more ductile states. Recrystallization depends on the temperature, the amount of time at this temperature and also the amount of strain hardening that the material experienced. The more strain hardening, the lower the temperature will be at which recrystallization occurs. Also, a minimum amount (typically 2-20%) of cold work is necessary for any amount of recrystallization to occur. The size the new grains is also partially dependant on the amount of strain hardening. The greater the stain hardening, the more nuclei for the new grains, and the resulting grain size will be smaller (at least initially).

Grain Growth

If a specimen is left at the high temperature beyond the time needed for complete recrystallization, the grains begin to grow in size. This occurs because diffusion occurs across the grain boundaries and larger grains have less grain boundary surface area per unit of volume. Therefore, the larger grains lose fewer atoms and grow at the expense of the smaller grains. Larger grains will reduce the strength and toughness of the material.

Alloying

Only a few elements are widely used commercially in their pure form. Generally, other elements are present to produce greater strength, to improve corrosion resistance, or simply as impurities left over from the refining process. The addition of other elements into a metal is called alloying and the resulting metal is called an alloy. Even if the added elements are nonmetals, alloys may still have metallic properties.

Copper alloys were produced very early in our history. Bronze, an alloy of copper and tin, was the first alloy known. It was easy to produce by simply adding tin to molten copper. Tools and weapons made of this alloy were stronger than pure copper ones. The typical alloying elements in some common metals are presented in the table below.

Alloy	Composition
Brass	Copper, Zinc
Bronze	Copper, Zinc, Tin
Pewter	Tin, Copper, Bismuth, Antimony
Cast Iron	Iron, Carbon, Manganese, Silicon
Steel	Iron, Carbon (plus small amounts of other elements)
Stainless Steel	Iron, Chromium, Nickel

The properties of alloys can be manipulated by varying composition. For example steel formed from iron and carbon can vary substantially in hardness depending on the amount of carbon added and the way in which it was processed.

When a second element is added, two basically different structural changes are possible:

1. Solid solution strengthening occurs when the atoms of the new element form a solid solution with the original element, but there is still only one phase. Recall that the term ‘phase’ refers to that region of space occupied by a physically homogeneous material.
2. The atoms of the new elements form a new second phase. The entire microstructure may change to this new phase or two phases may be present.

Solid Solution Strengthening

Solid solution strengthening involves the addition of other metallic elements that will dissolve in the parent lattice and cause distortions because of the difference in atom size between the parent metal and the solute metal. Recall from the section on crystal point defects that it is possible to have substitutional impurity atoms, and interstitial impurity atoms. A substitutional impurity atom is an atom of a different type than the bulk atoms, which has replaced one of the bulk atoms in the lattice. Substitutional impurity atoms are usually close in size (within approximately 15%) to the bulk atom. Interstitial impurity atoms are much smaller than the atoms in the bulk matrix. Interstitial impurity atoms fit into the open space between the bulk atoms of the lattice structure.

Since the impurity atoms are smaller or larger than the surrounding atoms they introduce tensile or compressive lattice strains. They disrupt the regular arrangement of ions and make it more difficult for the layers to slide over each other. This makes the alloy stronger and less ductile than the pure metal. For example, an alloy of 30% nickel raises the cast tensile strength of copper from 25,000 PSI to 55,000 PSI.

Multiphase Metals

Still another method of strengthening the metal is adding elements that have no or partial solubility in the parent metal. This will result in the appearance of a second phase distributed throughout the crystal or between crystals. These secondary phases can raise or reduce the strength of an alloy. For example, the addition of tin, zinc, or aluminum to copper will result in an alloy with increased strength, but alloying with lead or bismuth with result in a lower strength alloy. The properties of a polyphase (two of more phase) material depend on the nature, amount, size, shape, distribution, and orientation of the phases. Greek letters are commonly used to distinguish the different solid phases in a given alloy.

Phases can be seen on a microscopic scale with an optical microscope after the surface has been properly polished and etched. Below is a micrograph take at 125x of lead-tin alloy composed of two phases. The light colored regions are a tin-rich phase and the dark colored regions are a lead-rich phase.

Alloying (continued)

Phase Diagrams

As previously stated, the phase diagram is simply a map showing the structure of phases present as the temperature and overall composition of the alloy are varied. It is a very useful tool for understanding and controlling the structures of polyphase materials. A binary phase diagram shows the phases formed in differing mixtures of two elements over a range of temperatures. When an alloy exhibits more than two phases, a different type of phase diagram must be used, such as a ternary diagram for three phase alloys. This discussion will focus on the binary phase diagram.

On the binary phase diagram, compositions run from 100% Element A on the left, through all possible mixtures, to 100% Element B on the right. The composition of an alloy is given in the form A - x%B. For example, Cu - 20%Al is 80% copper and 20% aluminum. Weight percentages are often used to specify the proportions of the alloying elements, but atomic percent are sometimes used. Weight percentages will be used throughout this text.

Alloys generally do not have a single melting point, but instead melt (or alternately solidify) over a range of temperatures. At each end of the phase diagram only one of the elements is present (100% A or 100% B) so a specific melting point does exists. Additionally, there is sometimes a mixture of the constituent elements which produces melting at a single temperature like a pure element. This is called the eutectic point.

At compositions other than at the pure A, pure B and the eutectic points, when the alloy is cooled from a high temperature it will begin to solidify at a certain temperature but will remain in a mushy (liquid plus solid) condition over a range of temperatures. If experiments are conducted over a range of compositions to determine the temperature at which the alloys start to solidify, this data can be potted on the phase diagram to produce a curve. This “start of solidification curve” will join the three single solidification points and is called the liquidus line.

Up to a few percent of composition, it is possible for one element to remain dissolve in another while both are in the solid state. This is called solid solubility and the solubility limit normally changes with temperature. The extent of the solid solubility region can be plotted onto the phase diagram. In this example, the alpha phase is the region of solid solution where some of B atoms have dissolved in a matrix of A atoms. The beta phase is the region where a small percentage of A atoms have dissolved in a matrix of B atoms. It is important to note that some elements have zero solid solubility in other elements. An example is aluminum/silicon alloys, where aluminum has zero solid solubility in silicon.

If an alloy's composition does not place it within the alpha or beta solid solution regions, the alloy will become fully solid at the eutectic temperature. The eutectic line on the phase diagram indicates where this transformation will occur over the range of compositions. At alloy compositions and temperatures between the liquidus temperature and the eutectic temperature, a mushy mix of either alpha or beta phase will exist as solid masses within a liquid mixture of A and B. These are the alpha plus liquid and the beta plus liquid areas on the phase diagram. The region below the eutectic line, and outside the solid solution region, will be a solid mixture of alpha and beta.

Alloying (continued)

Tie and Lever Rules

Simply by looking at a phase diagram it is possible to tell what phase or phases an alloy will have at a given temperature. But, it is also possible to get quantitative information from the diagram. Consider the alloy at the temperature shown on the phase diagram. It is easy to see that at this temperature, it is a mixture of alpha and liquid phases. Using a tie line it is also

possible to determine the composition of the phases at this temperature. A tie line is an isothermal (constant temperature) line drawn through the alloy's position on the phase diagram when it is in a two phase field. The points where the ends of the tie line intersect the two adjacent solubility curves indicate the compositions of the two phases that exist in equilibrium at this temperature. In this example, the tie line shows that the alpha phase is 5.2%B and the liquid phase is 34.5%B at this temperature. It is important to keep in mind that the tie rule addresses the determination of the compositions of the constituent phases within the sample and it does not address the overall chemical composition of the sample, which remains unchanged.

It is also possible to determine how much of each phase exists at the given temperature using the lever rule. It is important to know the amounts of each phase present because the properties of the alloy depend on the amount of each phase present. The lever rule uses the tie line and the basic scientific principle of the conservation of mass to determine the ratio of the two phases present. The tie-line gives the chemical compositions of each of the two phases, and the combined amounts of these two compositions must add up to the alloy's overall composition (C_o), which is known. In other words, C_o must be composed of the appropriate amount of α at composition C_α and of liquid at C_liq. So basically, the proportions of the phases present are given by the relative lengths of the two sections of the tie line.

The fraction of alpha phase present is the given by the ratio of the C_o to C_liq portion of the tie line and the total length of the tie line (C_liq to C_α). Mathematically the relationships can be written as f_α << (C_liq – C_o)/(C_liq - C_α). The fraction of liquid phase present is given by the ratio of the C_o to C_α portion of the tie line and the total length of the tie line (C_liq to C_α). Mathematically this relationships can be written as f_liq << (C_o - C_α)/(C_liq - C_α). Of course, the two values must total to equal one.

Note that the right side of the tie line gives the proportion of the phase on the left (α phase in this example) and left side of the tie line gives the proportion of the phase to the right (liquid phase in this example). It is easy to keep this relationship straight by simply considering what the ratio would be near one of the tie line intersect points. For example, if C_o were near the liquidus line the ratio of the liquid section of the line to the total length of the line will be nearly one.

Alloying (continued)

Composition, Microstructure, and the Phase Diagram 

Let’s finish this discussion on phase diagrams by briefly looking at three different compositions of elements A and B, and how their microstructures will differ because of their positions on the phase diagram. First a eutectic alloy, which is an alloy with composition right at the eutectic point, will be considered. Then compositions on both sides of the eutectic point will be discussed. An alloy with a composition that lies to the left of the eutectic point on the phase diagram is called a hypoeutectic alloy, and an alloy with a composition that lies to the right of the eutectic point is called hypereutectic alloy. At this point, only the condition of slow cooling, which will allow the alloy to solidify into it equilibrium condition, will be considered. The microstructure can be controlled by manipulating the speed of cooling the alloy, but this will be covered in the section on heat treatments.

Eutectic Alloys

First, consider the eutectic alloy of elements A and B as it is cooled from a temperature at location 1 to location 4 on the phase diagram. At location 1, the alloy is at a high enough temperature to make the mixture fully liquid. The circles below show a representation of the alloy's microstructure at each of the locations numbered on the phase diagram.

At location 1, there is nothing of interest as the alloy is completely liquid. As the alloy is slow cooled, it remains liquid until it reaches the eutectic temperature (location 2) where it starts to solidify at any favorable nucleation sites. From the microstructure image 2, it can be see that as the alloy solidifies it forms into alternate layers of alpha and beta phase. This layered microstructure is known as lamellar microstructure and the layers are often only of the order of 1 micron across. The reason that a eutectic alloy forms in this way has to do with the diffusion times required to form the solid.

The grains grow by adding alpha to alpha and beta to beta until they encounter another grain (location 3). Further nucleation sites will also continue to form within the liquid parts of the mixture. This solidification happens very rapidly as any given volume of liquid in the melt reaches the eutectic temperature. Remember that a eutectic composition solidifies at a single temperature like a pure element and not over a temperature range.

As the now sold alloy cools to location 4, the composition of the layers of alpha and beta continue to change as it cools. Atoms of A and B will diffuse between the two phases to produce the equilibrium compositions of alpha and beta phase at a given temperature. By drawing tie lines at various temperatures the eutectic point on the phase diagram, it can be seen that the solubility of A in the beta phase and B in the alpha phase decreases as the temperature decreases. Since this phase composition change is due to diffusion, which is a relatively a slow process), it is important that eutectic alloys be allowed to cool slowly to produce the correct microstructure.

Hypoeutectic Alloys

Next, consider an alloy of A and B that has an overall composition that places it to the left of the eutectic point. When an alloy falls to the left of the eutectic point it is called a hypoeutectic alloy. At location 1, the alloy is at a temperature that is high enough to put it in a fully liquid phase.

When the alloy is cooled, it remains in the liquid state until it reaches the temperature where it crosses the liquidus line (location 2). At this temperature, the alpha phase starts to solidify at any favorable nucleation sites. The alpha solidifies as dendrites which grow to become grains of alpha. The first solid phase to form is called the primary phase so, in this case, primary alpha is formed.

As the alloy continues to cool (location 3) the existing nucleation sites will grow as dendrites and further nucleation sites will form within the liquid part of the mixture. The melt will have that mushy consistency of chunks in liquid while it is in the “alpha + liquid” region of the phase diagram. Since the alpha phase is mostly element A (with a small amount of B atoms in solid solution), the remaining liquid becomes slightly richer in B as the liquid cools, which is indicated by the liquidus line. The composition of the solid alpha phase also becomes slightly richer in B atoms as the solid solution line shows.

This primary alpha phase growth and the accompanying phase composition shifts continue until enough A atoms have been removed so that the remaining liquid is of eutectic composition. This composition is achieved at the point where the temperature crosses the eutectic line (location 4). At this point the primary alpha phase stops forming. The remaining liquid starts to solidify into the lamellar (alternating layers of alpha and beta phases) structure of a eutectic composition. The eutectic structure will grow; adding alpha to the layers of alpha and beta to the layers of beta in the eutectic regions, and new solidification sites will continue to form. Remember that solidification occurs rapidly and without the need for a further decrease in temperature once the liquid reaches the eutectic line. At this point, the entire alloy has solidified into a mixture comprised of grains of alpha and grains of eutectic mixture (alpha and beta). The microstructure from this point at the eutectic line down to ambient temperature will look something like that shown in micro 5.

Diffusion occurs as the alloy cools since the amount of element B in the alpha phase changes with temperature. This occurs exactly like it did for the eutectic alloy. Diffusion must also occur in the grains of pure alpha, as the composition of alpha phase also changes with temperature.

Hypereutectic

Finally, consider an alloy of A and B that has an overall composition that places it to the right of the eutectic point. When an alloy falls to the right of the eutectic point it is called a hypereutectic alloy. This alloy will solidify like the hypoeutectic alloy did except it will pass through the “beta + liquid” region of the phase diagram rather than the “alpha + liquid” region. This will result in a microstructure comprised of grains of beta and grains of eutectic mixture (alpha and beta) rather than grains of alpha and grains of eutectic mixture (alpha and beta) as the hypoeutectic alloy had.

At location 1, the alloy is at a temperature that is high enough to put it in a fully liquid phase. When the alloy is cooled, it remains in the liquid state until it reaches the temperature where it crosses the liquidus line (location 2). At this temperature, the beta phase starts to solidify at any favorable nucleation sites. The beta solidifies as dendrites which grow to become grains of beta. The first solid phase to form is called the primary phase so, in this case, primary beta is formed.

As the alloy continues to cool (location 3) the existing nucleation sites will grow as dendrites and further nucleation sites will form within the liquid part of the mixture. Since the beta phase is mostly element B (with a small amount of A atoms in solid solution), the remaining liquid becomes richer in A as the liquid cools, which is indicated by the liquidus line. The composition of the solid beta phase also becomes slightly richer in A atoms as the solid solution line shows.

This primary beta phase growth and the accompanying phase composition shifts continue until enough B atoms have been removed so that the remaining liquid is of eutectic composition. This composition is achieved at the point where the temperature crosses the eutectic line (location 4). At this point the primary beta phase stops forming. The remaining liquid starts to solidify into the lamellar (alternating layers of alpha and beta phases) structure of a eutectic composition. The eutectic structure will grow; adding alpha to the layers of alpha and beta to the layers of beta in the eutectic regions, and new solidification sites will continue to form. At this point, the entire alloy quickly solidifies into a mixture of beta grains and eutectic mixture (alpha and beta) grains. The microstructure from this point at the eutectic line down to ambient temperature will look something like that shown in micro 5.

Diffusion occurs as the alloy cools since the amount of element B in the alpha phase changes with temperature. This occurs exactly like it did for the eutectic alloy. Diffusion must also occur in the grains of pure alpha, as the composition of alpha phase also changes with temperature

Thermal Treatments (Heat-Treating)

In the previous pages on the subjects of alloying and the binary phase diagram, the microstructures of alloys that were allowed to solidify by slow cooling were considered. It should also be known, however, that it is possible to modify the microstructure of an alloy by subjecting it to various thermal treatments. Heat-treating is a term used to describe all of the controlled heating and cooling operations performed on a material in the solid state for the purpose of altering its microstructure and/or properties. The focus of this discussion will be on metals but is should be noted that heat-treatment is also used on ceramics and composites to modify their properties.

The major objectives of the different kinds of thermal treatments are:

1. Soften the material for improved workability.
2. Increase the strength or hardness of the material.
3. Increase the toughness or resistance to fracture of the material.
4. Stabilize mechanical or physical properties against changes that might occur during exposure to service environments.
5. Insure part dimensional stability.
6. Relieve undesirable residual stresses induced during part fabrication.

Different metals respond to treatment at different temperatures. Each metal has a specific chemical composition, so changes in physical and structural properties take place at different, critical temperatures. Even small percentages of elements in the metal composition, such as carbon, will greatly determine the temperature, time, method and rate of cooling that needs to be used in the heat treating process. Depending on the thermal treatment used, the atomic structure and/or microstructure of a material may change due to movement of dislocations, an increase or decrease in solubility of atoms, an increase in grain size, the formation of new grains of the same or different phase, a change in the crystal structure, and others mechanisms.

Since there are so many ways in which metals are heat treated, it is not practical to discuss them all. But, as an example, let’s look at how heat treatment is used to strengthen a copper aluminum alloy.

Precipitation Hardening

In designing alloys for strength, an approach often taken is to develop an alloy with a structure that consists of particles (which impede dislocation movement) dispersed in a ductile matrix. Such a dispersion can be obtained by choosing an alloy that is a single phase at elevated temperature but on cooling will precipitate another phase in the matrix. A thermal process is then developed to produce the desired distribution of precipitate in the matrix. When the alloy is strengthened by this thermal treatment, it is called precipitation strengthening or hardening.

Precipitation hardening consists of three main steps: solution treatment, quenching, and aging. Solution treatment involves heating the alloy to a temperature that allows the alloying atoms (called the solute) to dissolve into the solution. This results in a homogeneous solid solution of one phase. Quenching rapidly cools the solution and freezes the atoms in solution. In more technical terms, the quenching cools the material so fast that the atoms of the alloying elements do not have time to diffuse out of the solution. In the as-quenched condition, the solute is supersaturated meaning that the lattice is overly stressed by the alloying atoms. Aging is the process where the solute particles diffuse out of solution and into clusters that distort and strengthen the material.

The precipitation hardening process for a copper-aluminum alloy is shown graphically in the image below. On the right is phase diagram, which is a very useful tool for understanding and controlling polyphase structures. The phase diagram is simply a map showing the structure of phases present as the temperature and overall composition of the alloy are varied. The images on the right in the image show the resulting microstructure at each step in the process.

Common Heat Treating Processes

A few of the more common terms used in heat treating are introduced below. It should be noted that not all of the term are applicable to all alloys.

Age Hardening is a relatively low-temperature heat treatment process that strengthens a material by causing the precipitation of components or phases of alloy from a super-saturated solid solution condition.

Annealing is a softening process in which metals are heated and then allowed to cool slowly. The purpose of annealing is to soften the material for improve machinability, formability, and sometimes to control magnetic properties.

Normallizing is much like annealing, but the cooling process is much faster. This results in increased strength but less ductility in the metal. Its purpose is to refine grain structure, produce more uniform mechanical properties, and sometimes to relieve internal and surface stresses.

Precipitation Heat Treatment is the three step process of solution treating, quenching, and age hardening to increase the strength or hardness of an alloy.

Solution Heat Treatment involves heating the material to a temperature that puts all the elements in solid solution and then cooling very rapidly to freeze the atoms in place.

Stress Relieving is a low temperature heat treat process that is used to reduce the level of residual stresses in a material.

Tempering involves gently heating a hardened metal and allowing it to cool slowly will produce a metal that is still hard but also less brittle. This process is known as tempering.

Quenching is the rapid cooling of a hot material. The medium used to quench the material can vary from forced air, oil, water and others. Many steels are hardened by heating and quenching. Quenching results in a metal that is very hard but also brittle.

More information on heat treatment can be found in the material (ie aluminum, steel, titanium, etc.) sections

Ceramic Structures

As discussed in the introduction, ceramics and related materials cover a wide range of objects. Ceramics are a little more complex than metallic structures, which is why metals were covered first. A ceramic has traditionally been defined as “an inorganic, nonmetallic solid that is prepared from powdered materials and is fabricated into products through the application of heat. Most ceramics are made up of two or more elements. This is called a compound. For example, alumina (Al2O3) is a compound made up of aluminum atoms and oxygen atoms.

The two most common chemical bonds for ceramic materials are covalent and ionic. The bonding of atoms together is much stronger in covalent and ionic bonding than in metallic. This is why ceramics generally have the following properties: high hardness, high compressive strength, and chemical inertness. This strong bonding also accounts for the less attractive properties of ceramics, such as low ductility and low tensile strength. The absence of free electrons is responsible for making most ceramics poor conductors of electricity and heat.

However, it should be noted that the crystal structures of ceramics are many and varied and this results in a very wide range of properties. For example, while ceramics are perceived as electrical and thermal insulators, ceramic oxide (initially based on Y-Ba-Cu-O) is the basis for high temperature superconductivity. Diamond and silicon carbide have a higher thermal conductivity than aluminum or copper. Control of the microstructure can overcome inherent stiffness to allow the production of ceramic springs, and ceramic composites which have been produced with a fracture toughness about half that of steel. Also, the atomic structures are often of low symmetry that gives some ceramics interesting electromechanical properties like piezoelectricity, which is used in sensors and transducers.

The structure of most ceramics varies from relatively simple to very complex. The microstructure can be entirely glassy (glasses only); entirely crystalline; or a combination of crystalline and glassy. In the latter case, the glassy phase usually surrounds small crystals, bonding them together. The main compositional classes of engineering ceramics are the oxides, nitrides and carbides.