2014考研英語：閱讀經(jīng)典試題及詳解(十七)_跨考網(wǎng)

　　2014考研英語：閱讀經(jīng)典試題及詳解(十七)

　　The intensive work of materials scientists and solid-state physicists has given rise to a class of solids known as amorphous metallic alloys or glassy metals. There is a growing interest among theoretical and applied researchers alike in the structural properties of these materials.

　　When a molten metal or metallic alloy is cooled to a solid, a crystalline structure is formed that depends on the particular alloy composition. In contrast, molten nonmetallic glass-forming materials when cooled do not assume a crystalline structure, but instead retain a structure somewhat like that of the liquid — an amorphous structure. At room temperature the natural long-term tendency for both types of materials is to assume the crystalline structure. The difference between the two is in the kinetics or rate of formation of the crystalline structure which is controlled by factors such as the nature of the chemical bonding and the ease with which atoms move relative to each other. Thus, in metals, the kinetics favors rapid formation of a crystallines structure whereas in nonmetallic glasses the rate of formation is so slow that almost any cooling rate is sufficient to result in an amorphous structure. For glassy metals to be formed, the molten metal must be cooled extremely rapidly so that crystallization is suppressed.

　　The structure of glassy metals is thought to be similar to that of liquid metals. One of the first attempts to model the structure of a liquid was that by the late J. D. Bernal of the University of London, who packed hard spheres into a rubber vessel in such a way as to obtain the maximum possible density. The resulting dense, random-packed structure was the basis for many attempts to model the structure of glassy metals.

　　Calculations of the density of alloys based on Bernal-type models of the alloys metal component agree fairly well with the experimentally determined values from measurements on alloys consisting of a noble metal together with a metalloid such as alloys of palladium and silicon or alloys consisting of iron phosphors, and carbon, although small discrepancies remained. One difference between real alloys and the hard spheres area in Bernal models is that the components of an alloy have different size, so that models based on two sizes of spheres are more appropriate for a binary alloy for example. The smaller metalloid atoms of the alloys might fit into holes in the dense random-packed structure of the larger metal atoms.

　　One of the most promising properties of glassy metals is their high strength combined with high malleability. In usual materials, one finds an inverse relation between the two properties, whereas for many practical applications simultaneous presence of both properties is desirable. One residual obstacle to practical applications that is likely to be overcome is the fact that glassy metals will crystallize at relatively low temperatures when heated slightly.