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.
2022考研初復(fù)試已經(jīng)接近尾聲,考研學(xué)子全面進(jìn)入2023屆備考,跨考為23考研的考生準(zhǔn)備了10大課包全程準(zhǔn)備、全年復(fù)習(xí)備考計(jì)劃、目標(biāo)院校專業(yè)輔導(dǎo)、全真復(fù)試模擬練習(xí)和全程針對性指導(dǎo);2023考研的小伙伴針也已經(jīng)開始擇校和復(fù)習(xí)了,跨考考研暢學(xué)5.0版本全新升級,無論你在校在家都可以更自如的完成你的考研復(fù)習(xí),暑假集訓(xùn)營帶來了院校專業(yè)初步選擇,明確方向;考研備考全年規(guī)劃,核心知識點(diǎn)入門;個性化制定備考方案,助你贏在起跑線,早出發(fā)一點(diǎn)離成功就更近一點(diǎn)!
考研院校專業(yè)選擇和考研復(fù)習(xí)計(jì)劃 | |||
2023備考學(xué)習(xí) | 2023線上線下隨時學(xué)習(xí) | 34所自劃線院??佳袕?fù)試分?jǐn)?shù)線匯總 | |
2022考研復(fù)試最全信息整理 | 全國各招生院??佳袕?fù)試分?jǐn)?shù)線匯總 | ||
2023全日制封閉訓(xùn)練 | 全國各招生院??佳姓{(diào)劑信息匯總 | ||
2023考研先知 | 考研考試科目有哪些? | 如何正確看待考研分?jǐn)?shù)線? | |
不同院校相同專業(yè)如何選擇更適合自己的 | 從就業(yè)說考研如何擇專業(yè)? | ||
手把手教你如何選專業(yè)? | 高校研究生教育各學(xué)科門類排行榜 |
相關(guān)推薦
跨考考研課程
班型 | 定向班型 | 開班時間 | 高定班 | 標(biāo)準(zhǔn)班 | 課程介紹 | 咨詢 |
秋季集訓(xùn) | 沖刺班 | 9.10-12.20 | 168000 | 24800起 | 小班面授+專業(yè)課1對1+專業(yè)課定向輔導(dǎo)+協(xié)議加強(qiáng)課程(高定班)+專屬規(guī)劃答疑(高定班)+精細(xì)化答疑+復(fù)試資源(高定班)+復(fù)試課包(高定班)+復(fù)試指導(dǎo)(高定班)+復(fù)試班主任1v1服務(wù)(高定班)+復(fù)試面授密訓(xùn)(高定班)+復(fù)試1v1(高定班) | |
2023集訓(xùn)暢學(xué) | 非定向(政英班/數(shù)政英班) | 每月20日 | 22800起(協(xié)議班) | 13800起 | 先行階在線課程+基礎(chǔ)階在線課程+強(qiáng)化階在線課程+真題階在線課程+沖刺階在線課程+專業(yè)課針對性一對一課程+班主任全程督學(xué)服務(wù)+全程規(guī)劃體系+全程測試體系+全程精細(xì)化答疑+擇校擇專業(yè)能力定位體系+全年關(guān)鍵環(huán)節(jié)指導(dǎo)體系+初試加強(qiáng)課+初試專屬服務(wù)+復(fù)試全科標(biāo)準(zhǔn)班服務(wù) |