手机APP下载

您现在的位置: 首页 > GRE > GRE阅读 > GRE阅读每日一题 > 正文

GRE阅读理解(Barron模考)每日一题 第114期

来源:可可英语 编辑:mike   可可英语APP下载 |  可可官方微信:ikekenet

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
A.disinterest
B.impatience
C.optimism
D.apprehension
E.skepticism

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
C.extremely slowly
D.to room temperature
E.to extremely low temperatures

重点单词   查看全部解释    
formation [fɔ:'meiʃən]

想一想再看

n. 构造,编队,形成,队形,[地]地层

联想记忆
assume [ə'sju:m]

想一想再看

vt. 假定,设想,承担; (想当然的)认为

联想记忆
presence ['prezns]

想一想再看

n. 出席,到场,存在
n. 仪态,风度

 
tendency ['tendənsi]

想一想再看

n. 趋势,倾向

联想记忆
gentle ['dʒentl]

想一想再看

adj. 温和的,轻柔的,文雅的,温顺的,出身名门的

 
component [kəm'pəunənt]

想一想再看

n. 元件,组件,成份
adj. 组成的,构成

联想记忆
speculation [.spekju'leiʃən]

想一想再看

n. 沉思,推测,投机

联想记忆
determinant [di'tə:minənt]

想一想再看

n. 行列式(决定因素) adj. 决定性的

 
vessel ['vesl]

想一想再看

n. 容器,器皿,船,舰
n. 脉管,血

 
liquid ['likwid]

想一想再看

adj. 液体的,液态的
n. 液体

 

发布评论我来说2句

    最新文章

    可可英语官方微信(微信号:ikekenet)

    每天向大家推送短小精悍的英语学习资料.

    添加方式1.扫描上方可可官方微信二维码。
    添加方式2.搜索微信号ikekenet添加即可。