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标题: 求助国内题92.10 amorphous metallic alloys中最后一题 [打印本页]

作者: 我Me是个好人 时间: 2017-6-2 17:16:31 标题: 求助国内题92.10 amorphous metallic alloys中最后一题

本帖最后由我Me是个好人于 2017-6-3 02:17 编辑

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 crystalline 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 consisting 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 models 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 applications that is likely to be overcome is the fact that glassy metals will crystallize at relatively low temperatures when heated slightly.
27. 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

很明显是要定位到这句话:
Thus, in metals, the kinetics favors rapid formation of a crystalline 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.

答案给的是C, 但C是怎么看出来的啊?

作者: onelittlemajia 时间: 2017-6-2 23:45:12

这道题我是C,D不知取舍
“At room temperature, the natural long-term tendency for both types of materials is to assume the crystalline structure”
这句话是说“室温下，两种物质的长期趋势都是形成晶体”，那就是说不管有金属还是没金属的玻璃体在室温下慢慢的最后都能形成晶体。

“in nonmetallic glasses the rate of formation is so slow that almost any cooling rate is sufficient to result in an amorphous structure”
含金属的玻璃体很快冷却就能形成形成晶体，不含金属的玻璃体很慢才会形成晶体，所以几乎任何冷却速度都不形成晶体，言下之意要想冷却成晶体，冷却速度要非常非常慢。

但是D “to room temperature”怎么排除？
还是因为题目问的“they are cooled”特指“冷却”这个动作

作者: 我Me是个好人 时间: 2017-6-3 02:15:02

本帖最后由我Me是个好人于 2017-6-3 02:26 编辑

onelittlemajia 发表于 2017-6-2 23:45
这道题我是C,D不知取舍
“At room temperature, the natural long-term tendency for both types of mater ...

C还是觉得不太对啊, 我的理由是,

At room temperature, the natural long-term tendency for both types of materials is to assume the crystalline structure.
这句话说在室温条件下两种物质都呈crystalline structure right?
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.
这句话接着上一句, 说(在室温条件下)structure相同, 但其不同就在于这个structure的formation rate不同, 而这个formation rate又是被等等这些factors所控制的.
Thus, in metals, the kinetics favors rapid formation of a crystalline 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.
因此, 金属的crystalline structure的formation rate很快, 而在非金属中却很慢, 以至于慢到无论各种cooling rate都会产生amorphous structure.

所以我的看法是, 这里的formation rate和cooling rate是两种不同的rate, 而非同一种rate.
所以, whereas in nonmetallic glasses the rate of formation is so slow that almost any cooling rate is sufficient to result in an amorphous structure, 也就是说无论cooling rate如何, 最后也都只能形成amorphous structure, 于是cooling rate就和crystalline structure没有任何关系了.
所以C我觉得这里就不能选了....能选的只有D. 理由是, 我们既然要对molten materials(I assert they must be extremely hot)进行冷却, 还要让它形成crystalline structure, 根据原文就只有把它冷却到room temperature才可以.

The problem is, nevertheless, that any sort of answer I have found yet does point exclusively to C...

作者: onelittlemajia 时间: 2017-6-3 03:35:45

我Me是个好人发表于 2017-6-3 02:15
C还是觉得不太对啊, 我的理由是,

At room temperature, the natural long-term tendency for both t ...

非金属玻璃体也会结晶，只是要很长时间。

头两句“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. ”
金属融化物冷却结晶，非金属融化物冷却不结晶，而是保持amorphous structure

但是后面就说了实际上这两种东西在室温下，自然的长期趋势都是结晶
“At room temperature, the natural long-term tendency for both types of materials is to assume the crystalline structure.”

后面说导致结晶的原因。结论就是一个冷却很快结晶，一个冷却几乎不结晶。
Thus, in metals, the kinetics favors rapid formation of a crystalline 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.
结晶的速度很慢，几乎任何冷却速度都可以保持amorphous structure。
并不是说非金属不结晶，只是非金属可以很长时间保持变形结构。

我现在想排除D可能是因为，非金属结晶速度很慢，冷却到室温它还没结晶，只是在室温下很长时间后最终结晶。
这篇文章介绍的知识可能是这个意思，液态合金做成需要的形状最后定型。液态金属遇冷很快就定型了，液态非金属失温后还可以保持液态很长时间，这样可以不是在滚烫的液体状态比如常温下做成需要的形状，最终定型。

作者: 我Me是个好人 时间: 2017-6-3 10:34:50

onelittlemajia 发表于 2017-6-3 03:35
非金属玻璃体也会结晶，只是要很长时间。

头两句“When a molten metal or metallic alloy is cooled ...

太感谢了, 这么一说就懂了....好惭愧啊, 之前一直在抠局部内容的字眼, 忘了把整篇文章联系起来(*/ω＼*)

我对D的理解是, 题目是infer题, 但D却给了一个explicitly presented in the passage的答案, 所以它尽管对, 但却是irrelevant的, 不符合题目要求. 而C就恰恰相反.

Anyway, thank you soooooooo much!٩( 'ω' )و

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