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Materials Science and Technology

 Materials Science and Technology, the study of materials, nonmetallic as well as metallic, and how they can be adapted and fabricated to meet the needs of modern technology. Using the laboratory techniques and research tools of physics, chemistry, and metallurgy, scientists are finding new ways of using plastics, ceramics, and other nonmetals in applications formerly reserved for metals.



Recent Developments
The rapid development of semiconductors  for the electronics industry, beginning in the early 1960s, gave materials science its first major impetus. Having discovered that nonmetallic materials such as silicon could be made to conduct electricity in ways that metals could not, scientists and engineers devised ways of fashioning thousands of tiny integrated circuits (see Integrated Circuit) on a small chip of silicon. This then made it possible to miniaturize the components of electronic devices such as computers.
In the late 1980s, materials science research was given renewed emphasis with the discovery of ceramics that display superconductivity at higher temperatures than metals do. If the temperature at which these new materials become superconductive can be raised high enough, new applications, including levitating trains and superfast computers, are possible.
Although the latest developments in materials science have tended to focus on electrical properties, mechanical properties are also of major, continuing importance. For the aircraft industry, for instance, scientists have been developing, and engineers testing, nonmetallic composite materials that are lighter, stronger, and easier to fabricate than the aluminum and other metals currently used to form the outer skin of aircraft.


Mechanical Properties of Materials
Engineers must know how solid materials respond to external forces, such as tension, compression, torsion, bending, and shear. Solid materials respond to these forces by elastic deformation (that is, the material returns to its original size and form when the external force is lifted), permanent deformation, or fracture. Time-dependent effects of external forces are creep and fatigue, which are defined below.
Tension is a pulling force that acts in one direction; an example is the force in a cable holding a weight. Under tension, a material usually stretches, returning to its original length if the force does not exceed the material's elastic limit . Under larger tensions, the material does not return completely to its original condition, and under even greater forces the material ruptures.
Compression is the decrease in volume that results from the application of pressure. When a material is subjected to a bending, shearing, or torsional (twisting) force, both tensile and compressive forces are simultaneously at work. When a rod is bent, for example, one side of it is stretched and subjected to a tensional force, and the other side is compressed.
Creep is a slowly progressing, permanent deformation that results from a steady force acting on a material. Materials subjected to high temperatures are especially susceptible to this deformation. The gradual loosening of bolts, the sagging of long-span cables, and the deformation of components of machines and engines are all noticeable examples of creep. In many cases the slow deformation stops because the force causing the creep is eliminated by the deformation itself. Creep extended over a long time eventually leads to the rupture of the material.
Fatigue can be defined as progressive fracture. It occurs when a mechanical part is subjected to a repeated or cyclic stress, such as vibration. Even when the maximum stress never exceeds the elastic limit, failure of the material can occur even after a short time. With some metals, such as titanium alloys, fatigue can be avoided by keeping the cyclic force below a certain level. No deformation is apparent during fatigue, but small localized cracks develop and propagate through the material until the remaining cross-sectional area cannot support the maximum stress of the cyclic force. Knowledge of tensile stress, elastic limits, and the resistance of materials to creep and fatigue are of basic importance in engineering.


"Materials Science and Technology," Microsoft® Encarta® 97 Encyclopedia. © 1993-1996 Microsoft Corporation. All rights reserved.

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