《2022年机械设计方案制造及其自动化专业英语翻译》由会员分享，可在线阅读，更多相关《2022年机械设计方案制造及其自动化专业英语翻译（27页珍藏版）》请在金锄头文库上搜索。
1、精品学习资源Unit 1 Metals金属欢迎下载精品学习资源The use of metals has always been a key factor in the development ofthe socialsystems of man. Ofthe roughly100basic elements of whichall matter is composed, about half are classified as metals. Thedistinction between a metal and a nonmetal is not always clear-cut. The
2、most basic definitioncenters around the type of bonding existing between the atoms of the element, and around the characteristics of certain of the electrons associated with these atoms. Ina more practicalway, however,a metal can be defined as an elementwhich has a particular package of properties.M
3、etals are crystalline when in the solid state and, withfewexceptions e.g. mercury,are solid at ambient temperatures. They are good conductorsofheatandelectricityandare opaquetolight.Theyusuallyhavea comparativelyhighdensity. Manymetals are ductile-thatis,theirshape canbechanged permanentlybytheappli
4、cationofaforce without breaking. The forces required to cause this deformation and those required to break or fractureametalare comparatively high, although, the fracture forces is not nearly as high as would be expected from simple consideration of the forces required to tear apart the atoms of the
5、 metal.Oneofthemoresignificantofthese characteristics from our point of view is that of crystallinity. A crystalline solid is one in whichthe constituent atoms are located in a regular three-dimensional array as if they were located atthecornersofthesquaresofathree- dimensionalchessboard. The spacin
6、g ofthe atoms in the array is of the same order as the size of the atoms, the actual spacing being a characteristicoftheparticularmetal.The directions of the axes of the array define the orientation of the crystal in space. The metals commonlyused inengineeringpracticeare composed of a large number
7、of such crystals, calledgrains. Inthe most general case, the crystals ofthe variousgrainsare randomly oriented in space. The grains are everywhere in intimatecontact withone another and joined togetheron an atomicscale. Theregionat which they join is known as a grain boundary.An absolutely pure meta
8、l i.e. one composed of onlyonetypeofatomhasneverbeen produced. Engineers would not be particularly interested in such a metal even if it were to be在人类社会的进展中，金属的应用起着关键性的作用；构成物质的大约100 种基本元素中，大约有一半为金属；金属和非金属之间的区分不是特殊明显；最基本的定义集中在元素原子间存在的连接形式和与这些原子相关联的电子的某些特性；然而，在实际应用中，可以将具有某些特性集合金属定义为某种元素；除了少数例外金属在常温下是固
9、态的；它们是热和电的良导体，不透光；它们往往具有较高的密度；很多金属具有延展性，也就是说，在不被破坏的情形下它们的外形在外力的作用下可以发生变化；引起永久变形所需的力和最终使金属断裂所需的力相当大，尽管发生断裂所需的力远没有像所预期的撕开金属原子所需的力那么大；从我们的观点来看，在全部的特性中结晶性是最重要的；结晶体是这样一种结构，组成它的原子定位在规章的三维排列中，似乎位于三维棋盘的方格的角上；原子间距随着原子大小呈规律性变化，原子间距是金属的一种特性；三维排列的轴线打算了晶体在空间中的方向；在工程实践中应用的金属由大量的晶体组成，这些晶体称之为晶粒；在大多数情形下，晶粒在空间中是自由排列的
10、；在原子范畴内，晶粒之间相互接触紧密结合；晶粒之间连接区域被称为晶界；欢迎下载精品学习资源produced, because it would be soft and weak. Themetalsusedcommerciallyinevitably contain small amounts of one or more foreign elements, either metallic or nonmetallic. These foreign elements may be detrimental, they may be beneficial, or they may have no
11、influence at all on a particular property. If disadvantageous, the foreignelements tendtobe knownas impurities. Ifadvantageous, they tend to be known as alloying elements. Alloying elements are commonly added deliberately in substantial amounts in engineering materials. The result is known as an all
12、oy.The distinction between the descriptors“metal”and “alloy ”is not clear-cut. The term “metal” may be used to encompass both a commercially pure metal and its alloys. Perhaps it can be said that the more deliberately an alloyingadditionhas been made and the larger the amount of the addition, the mo
13、re likely itis that the product will specifically be called an alloy.In any event, the chemical composition of a metal or an alloy must be knownand controlledwithincertain limitsifconsistent performanceistobe achieved in service. Thus chemical composition has to be taken into account when developing
14、 anunderstandingofthefactorswhich determine the properties ofmetals and their alloys.Of the 50 or so metallic elements, only a few are produced and used in large quantities in engineering practice. The most important by far is iron, on whichare based the ubiquitous steels and cast irons basicallyall
15、oys ofiron and carbon. They account for about 98% by weightofallmetalsproduced.Nextin importanceforstructuraluses thatis,for structures that are expected to carry loads are aluminum,copper,nickel,andtitanium. Aluminumaccounts for about 0.8% by weight of all metals produced, and copper about 0.7%, leavingonly0.5%forallothermetals. As might be expected, the remainders are all used inratherspecial applications.Forexample, nickel alloys are usedprincipally
