GRE Reading Comprehension
The intensive work of materials scientists and solidstate 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 crystal-line 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 agreed 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 con- sisting of iron, phosphorus, and carbon, although small discrepancies remained. One difference between real alloys and the hard spheres used in Bernal models is that the components of an alloy have different sizes, so that mode, based on two sizes of spheres are more appropriate for a binary alloy, for example. The smaller metalloid atoms of the alloy 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 crystalline 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 appli-cations that is likely to be overcome is the fact that glassy metals will crystallize at relatively low temper- atures when heated slightly.
According to the passage, which of the following determines the crystalline structure of a metallic alloy?
- A At what rate the molten alloy is cooled
- B How rapid the rate of formation of the crystalline phase is
- C How the different-sized atoms fit into a dense, random-packed structure
- D What the alloy consists of and in what ratios
- E At what temperature the molten alloy becomes solid
Correct Answer: D