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
The author is primarily concerned with discussing
A.crystalline solids and their behavior at different temperatures
B.molten materials and the kinetics of the formation of their crystalline structure
C.glassy metals and their structural characteristics
D.metallic alloys and problems in determining their density
E.amorphous materials and their practical utilization
The author implies that the rate at which the molten materials discussed in the passage are cooled is a determinant of the
A.chemical composition of the resulting solids
B.strength of the chemical bonds that are formed
C.kinetics of the materials' crystalline structure
D.structure the materials assume
E.stability of the materials' crystalline structure
The author's speculation about the appropriateness of models using spheres of two sizes for binary alloys would be strongly supported if models using spheres of two sizes yielded
A.values for density identical to values yielded by one-sphere models using the smaller spheres only
B.values for density agreeing nearly perfectly with experimentally determined values
C.values for density agreeing nearly perfectly with values yielded by models using spheres of three sizes
D.significantly different values for density depending on the size ratio between the two kinds of spheres used
E.the same values for density as the values for appropriately chosen models that use only medium-sized spheres
The author's attitude toward the prospects for the economic utilization of glassy metals is one of
Which of the following best describes the relationship between the structure of liquid metals and the structure of glassy metals, as it is presented in the passage?
A.The latter is an illustrative example of the former.
B.The latter is a large-scale version of the former.
C.The former is a structural elaboration of the latter.
D.The former provides an instructive contrast to the latter.
E.The former is a fair approximation of the latter.
It can be inferred from the passage that, theoretically, molten nonmetallic glasses assume a crystalline structure rather than an amorphous structure only if they are cooled
A.very evenly, regardless of the rate
B.rapidly, followed by gentle heating
D.to room temperature
E.to extremely low temperatures
正确答案:D C D B C E C
5天搞定800组GRE近/反义词 活动日期 : 2018.10.19-2018.10.23
价格 : ￥19.9