•include和containTypes of Materials 
材料的类型
        Materials may be grouped in several ways. Scientists often classify materials by their state: solid, liquid, or gas. They also separate them into organic (once living) and inorganic (never living) materials.
        材料可以按多种方法分类。科学家常根据状态将材料分为：固体、液体或气体。他们也把材料分为有机材料(曾经有生命的)和无机材料(从未有生命的)。
      For industrial purposes, materials are divided into engineering materials or nonengineering materials. Engineering materials are those used in manufacture and become parts of products.
      就工业效用而言，材料被分为工程材料和非工程材料。那些用于加工制造并成为产品组成部分的就是工程材料。
Nonengineering materials are the chemicals, fuels, lubricants, and other materials used in t
he manufacturing process, which do not become part of the product.
非工程材料则是化学品、燃料、润滑剂以及其它用于加工制造过程但不成为产品组成部分的材料。
        Engineering materials may be further subdivided into: ①Metal ②Ceramics ③Composite ④Polymers, etc.
        工程材料还能进一步细分为：①金属材料②陶瓷材料③复合材料 ④聚合材料，等等。
•Metals and Metal Alloys
金属和金属合金
      Metals are elements that generally have good electrical and thermal conductivity. Many metals have high strength, high stiffness, and have good ductility.
      金属就是通常具有良好导电性和导热性的元素。许多金属具有高强度、高硬度以及良好的延展性。
Some metals, such as iron, cobalt and nickel, are magnetic. At low temperatures, some metals and intermetallic compounds become superconductors.
某些金属能被磁化，例如铁、钴和镍。在极低的温度下，某些金属和金属化合物能转变成超导体。
      What is the difference between an alloy and a pure metal? Pure metals are elements which come from a particular area of the periodic table. Examples of pure metals include copper in electrical wires and aluminum in cooking foil and beverage cans.
      合金与纯金属的区别是什么？纯金属是在元素周期表中占据特定位置的元素。例如电线中的铜和制造烹饪箔及饮料罐的铝。
Alloys contain more than one metallic element. Their properties can be changed by changing the elements present in the alloy. Examples of metal alloys include stainless steel which is an alloy of iron, nickel, and chromium; and gold jewelry which usually contains an alloy of gold and nickel.
合金包含不止一种金属元素。合金的性质能通过改变其中存在的元素而改变。金属合金的例子有：不锈钢是一种铁、镍、铬的合金，以及金饰品通常含有金镍合金。
        Why are metals and alloys used? Many metals and alloys have high densities and are used in applications which require a high mass-to-volume ratio.
      为什么要使用金属和合金？许多金属和合金具有高密度，因此被用在需要较高质量体积比的场合。
Some metal alloys, such as those based on aluminum, have low densities and are used in aerospace applications for fuel economy. Many alloys also have high fracture toughness, which means they can withstand impact and are durable.
某些金属合金，例如铝基合金，其密度低，可用于航空航天以节约燃料。许多合金还具有高断裂韧性，这意味着它们能经得起冲击并且是耐用的。
        What are some important properties of metals?
        Density is defined as a material’s mass divided by its volume. Most metals have relatively high densities, especially compared to polymers.
      金属有哪些重要特性？
      密度定义为材料的质量与其体积之比。大多数金属密度相对较高，尤其是和聚合物相比较而言。
Materials with high densities often contain atoms with high atomic numbers, such as gold or lead. However, some metals such as aluminum or magnesium have low densities, and are used in applications that require other metallic properties but also require low weight.
高密度材料通常由较大原子序数原子构成，例如金和铅。然而，诸如铝和镁之类的一些金属则具有低密度，并被用于既需要金属特性又要求重量轻的场合。
        Fracture toughness can be described as a material’s ability to avoid fracture, especially when a flaw is introduced. Metals can generally contain nicks and dents without weakening very much, and are impact resistant. A football player counts on this w
hen he trusts that his facemask won’t shatter.
      断裂韧性可以描述为材料防止断裂特别是出现缺陷时不断裂的能力。金属一般能在有缺口和凹痕的情况下不显著削弱，并且能抵抗冲击。橄榄球运动员据此相信他的面罩不会裂成碎片。
        Plastic deformation is the ability of bend or deform before breaking. As engineers, we usually design materials so that they don’t deform under normal conditions. You don’t want your car to lean to the east after a strong west wind.
      塑性变形就是在断裂前弯曲或变形的能力。作为工程师，设计时通常要使材料在正常条件下不变形。没有人愿意一阵强烈的西风过后自己的汽车向东倾斜。
However, sometimes we can take advantage of plastic deformation. The crumple zones in a car absorb energy by undergoing plastic deformation before they break.
然而，有时我们也能利用塑性变形。汽车上压皱的区域在它们断裂前通过经历塑性变形来吸收能量。
      The atomic bonding of metals also affects their properties. In metals, the outer valence electrons are shared among all atoms, and are free to travel everywhere. Since electrons conduct heat and electricity, metals make good cooking pans and electrical wires.
      金属的原子连结对它们的特性也有影响。在金属内部，原子的外层阶电子由所有原子共享并能到处自由移动。由于电子能导热和导电，所以用金属可以制造好的烹饪锅和电线。
It is impossible to see through metals, since these valence electrons absorb any photons of light which reach the metal. No photons pass through.
因为这些阶电子吸收到达金属的光子，所以透过金属不可能看得见。没有光子能通过金属。

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