Materials engineering
From Encyclopædia
The practical objective of the materials engineer is the production of materials whose properties meet the requirements for a specific use. The properties of any material are the result of its microstructure, that is, the way in which the groups of
atoms comprising the material have arranged themselves. In turn, the microstructure may be determined--in part, at least--by the processing history of the material. This complex relationship between structure, processing, and properties provides the basis for materials engineering and the closely related
science of
metallurgy.CLASSIFICATION OF MATERIALSEngineering materials are classified into several broad groups:
metals, ceramics, polymers,
semiconductors, intermetallics, and composites. The materials within each group have different and unique properties, resulting from differences in their microstructure and atomic bonding.MetalsIn general, materials classified as
metals are very stiff, have high strength, good electrical and thermal conductivity, and good ductility (or formability) and shock resistance. Therefore,
metals are useful in structural load-bearing applications that must be resistant to cracking or
fracture. Pure
metals may be used to take advantage of specific properties; for example, pure
Copper wire is used to transmit electrical power because of its excellent electrical conductivity.The unique properties that a
metal possesses are a result of the metallic bonding mechanism (see
metal) that produces, among other valuable characteristics, great ductility, allowing it to be molded or shaped into innumerable forms.Alloys are combinations or mixtures of
metals, formulated to provide particular properties or to optimize a combination of properties. For example, aluminum is often alloyed with
silicon and
Copper before it is cast. The
silicon produces a molten
metal that will flow more readily into the small features of a mold; the
Copper adds strength to the aluminum matrix.CeramicsMost CERAMIC materials are compounds of
metal atoms bonded to a nonmetal, usually
oxygen. The ceramic bonding mechanism produces materials that have high strengths; because the bonds will break under
stress, however, ceramic materials--which include brick, glass, china, and porcelain, refractories, and abrasives--tend to be brittle, with low electrical and thermal conductivity. New processing techniques have produced tough, energy absorbing ceramics with great high-temperature resistance. They are used for jet
engine components and such other high-impact functions as in nonsparking hammerheads.PolymersRubber,
plastics, and many types of adhesives are polymers, giant organic molecular structures produced by a process called POLYMERIZATION. Polymers have low electrical and thermal conductivities and low strength, and are not suitable for use at high temperatures; but they are lightweight, corrosion resistant, and relatively inexpensive, and they can be readily formed into a variety of shapes.Most polymers are electrical insulators, although special conductive polymers have been developed. Some are transparent and can replace glass in certain applications. Some are relatively frictionless, others have the ability to convert
light into
electricity, still others provide the basis for nonstick cookware.
plastics whose polymer chains are all oriented, or aligned, in the same direction can be made into superstrong materials, such as the reinforcing fiber
Kevlar; or, in the form of a LIQUID
crystal, provide the basis for re-recordable compact discs.Polymers are classified as thermoplastic or thermosetting materials. (See
plastics for a description of their differing structures.) Thermoplastic polymers have good ductility and formability, and are plastic at slightly elevated temperatures. They can be heated and formed, cooled, and then reheated and reformed, all without changing their basic structure or properties. They are also easily recyclable. Thermosetting polymers--made up of molecular chains that are tightly linked to produce a strong, three-dimensional network of
molecules--are stronger than thermoplastics but more brittle. They cannot be re-formed after they are produced and are not easily recycled.SemiconductorsUsed to make solid-state electronic components,
semiconductors (see
semiconductor) are substances whose ability to conduct
electricity falls somewhere between electrical conductors, like
Copper, and nonconductors, such as glass or ivory.
semiconductor materials include
silicon, germanium, and a
number of compounds (gallium phosphide and gallium arsenide, for example). The conductivity of a device made from
semiconductors can be controlled, and they are therefore used in
transistors, diodes, and integrated circuits.
semiconductor materials tend to be very brittle, however, and they provide considerable challenge in the production and design of components.IntermetallicsAn intermetallic compound is made up of two or more elements that together produce a new substance having its own composition,
crystal structure, and properties. Intermetallic materials are often found in alloys as very
fine particles randomly distributed throughout a
metal matrix. The large
number of particles serves to strengthen the alloy by making it more resistant to deformation. Intermetallic compounds--unlike most
metals and alloys (see
ALLOY)--remain strong at high temperatures but at normal temperatures are almost always very brittle.Although these compounds are used most often to strengthen a softer, more ductile matrix, there is now significant
interest in using these materials by themselves. Components that are fabricated entirely from intermetallic materials have high melting points, good resistance to oxidation at elevated temperatures, and resistance to deformation under extended loads at elevated temperatures ("creep resistance.") A variety of new intermetallic materials are in development and should prove particularly useful as components in the severe environments of aircraft jet
engines or in the giant turbines of power generators. Most of them are based on aluminum-titanium or aluminum-
nickel alloy systems.CompositesComposites are produced when two materials are combined into a new material with properties that cannot be attained in the originals. They offer unusual combinations of stiffness, strength, weight, high-temperature performance, corrosion resistance, hardness, or conductivity. They can be meldings of
metal-
metal,
metal-ceramic,
metal-polymer, ceramic-polymer, ceramic-ceramic, or polymer-polymer. Concrete, plywood, and fiberglass are typical elementary examples of composites.Composites are categorized according to the shapes of the materials from which they are made. Concrete is a particulate composite, consisting of a mixture of
gravel, sand, and cement. Fiberglass, made o? a polymer matrix in which glass fibers are embedded, is a fiber-reinforced composite. Plywood, which consists of alternate layers of wood and wood veneer held together by glues applied under
pressure, is an example of a laminar composite.Composite materials, and the components made from them, often have anisotropic properties--that is, their properties will differ in different directions. The particles in a particulate composite will usually have a random orientation and its properties will be isotropic, the same in all directions. Many new fiber-reinforced composites, however, are engineered in such a way that the fibers are oriented in a particular direction; and the composite will be as strong, for example, as the strength of those fibers when the material is stressed in the same direction as the fibers.Advanced aircraft and aerospace vehicles rely heavily on composites--for example, the
carbon-fiber reinforced polymers used in aircraft bodies. A high-performance wing, made from a
carbon-reinforced epoxy, is very lightweight, stiff, and strong, with an exceptional strength-to-weight ratio.TESTING THE MECHANICAL PROPERTIES OF MATERIALSThe mechanical properties that most concern a materials engineer are tensile strength, brittleness, and fatigue. Each of these properties can be tested.Tensile strength is the response of a material to an applied load.
stress describes the amount of load applied to a material per unit area; strain describes the amount of deformation per unit length. A load, or applied
stress, will result in the material undergoing some amount of deformation, or strain. With low applied stresses, the response of the material, or strain, is usually elastic: it will resume its original size when the
stress is removed. When the applied
stress is sufficiently large, the material will plastically deform. The amount of
stress necessary to begin plastic deformation of a material is called the yield
stress. The ultimate strength is the maximum amount of
stress a material can withstand prior to
fracture. These are all common mechanical properties of materials that the engineer must test, to properly match materials to uses.An impact test is used to evaluate the brittleness of a material when it is subjected to a sudden intense blow. In one test, a heavy pendulum is released from a known height and allowed to fall until it strikes and breaks the test specimen; then it continues to fall to a final, lower position. The difference between the initial and final heights of the pendulum is a measure of the impact energy absorbed by the material during
fracture. The quality of toughness is a material's ability to withstand an impact blow.
