机械设计制造及其自动化专业英语翻译.docVIP

Unit 1 Metals 金属 The use of metals has always been a key factor in the development of the social systems of man. Of the roughly 100 basic elements of which all matter is composed, about half are classified as metals. The distinction between a metal and a nonmetal is not always clear-cut. The most basic definition centers around the type of bonding existi ng betwee n the atoms of the element, and around the characteristics of certain of the electrons associated with these atoms. In a more practical way, however, a metal can be defi ned as an eleme nt which has a particular package of properties. Metals are crystalline when in the solid state and, with few exceptions (e.g. mercury), are solid at ambient temperatures. They are good con ductors of heat and electricity and are opaque to light. They usually have a comparatively high density. Many metals are ductile-that is, their shape can be changed permanently by the application of a force without break ing. The forces required to cause this deformati on and those required to break or fracture a metal are comparatively high, 在人类社会的发展中，金属的应用 起着关键性的作用。构成物质的大约 100种 基本兀素中，大约有一半为金属。金属和非 金属之间的区别不是特别明显。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态 的。它们是热和电的良导体，不透光。它们 往往具有较咼的密度。 许多金属具有延展性， 也就是说，在不被破坏的情况下它们的形状 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，在所有的特性中结 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则的三维排列中，仿 佛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。三维排列的轴线决定了晶体在 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 大多数情况下，晶粒在空间中是自由排列的。 在原子范围内，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。 although, the fracture forces is not nearly as high as would be expected from simple con siderati on of the forces required to tear apart the atoms of the metal. One of the more significant of these characteristics from our point of view is that of crystalli nity A crystalline solid is one in which the constituent atoms are located in a regular three-dimen sio nal array as if they were located