外文翻译--材料的结构和变形

1 Chapter 2 Structure and Deformation in Materials 2.1 INTRODUCTION 2.2 BONDING IN SOLIDS 2.3 STRUCTURE IN CRYSTALLINE MATERIALS 2.4 ELASTIC DEFORMATION AND THEORETICAL STRENGTH 2.5 INELASTIC DEFORMATION 2.6 SUMARRY OBJECTIVES Review chemical bonding crystal structure in solid materials at a basic level, and relate these to differences in mechanical behavior among various classes of materials. Understand the physical basis of elastic deformation, and employ this estimate the theoretical strength of solids due to their chemical bonding. Understand the basic mechanisms of inelastic deformation due to plasticity and creep. Learn why actual strengths of materials fall far below the theoretical strength to break chemical bonds. 2.1 INTRODUTION A wide variety of materials are used in applications where resistance to mechanical loading is necessary. These are collectively called engineering materials and can be broadly classified as metals alloys, polymers, ceramics and glasses, and composites. Some typical members of each class are given in Table 2.1. Differences among the classes of materials as to chemical bonding and microstructure affect mechanical behavior, giving rise to relative advantages and disadvantages among the classes. The situation is summarized by Fig .2.1.For example .the strong chemical bonding in ceramics and glasses imparts mechanical strength and stiffness (high E), and also temperature and corrosion resistance, but cause brittle behavior. In contrast, many polymers are relatively weakly bonded between the chain molecules, in which case the material has low strength and stiffness and is susceptible creep deformation. 2 Starting from the size sale of primary interest in engineering ,rough one meter ,there is a span of 10 orders of magnitude in size ,down to the sale of the atom ,which is around 10-10m .This situation and various intermediate size scales of interest are indicated in Fig.2.2.At any given size scale ,an understanding of the behavior can be sought by looking at what happens at a smaller scale ；The behavior of a machine ,vehicle ,or structure is explained by the behavior of its component parts ,and the behavior of these can in turn be explained by the use of small (10-1to 10-2m) test specimens ,and the materials .Similarly ,the macroscopic behavior of the material is explained by the behavior of crystal grains ,defects in crystals, polymer chains ,and other microstructure features that exist in size range of 10-3to 10-9m .Thus ,knowledge of behavior over the entire range of size from 1m down to 10-10m contributes to understanding and predicting the performance of machines ,vehicles ,and structures . 3 This chapter review some of the fundamentals needed to understand mechanical behavior of materials. We will start at the lower end of the size scale in Fig.2.2 and progress upward .The individual topics include chemical bonding ,crystal structures ,defects in crystals ,and the physical causes of elastic ,plastic ,and creep deformation .The next chapter will then apply these concepts in discussing each of the classes of engineering materials in more details . 2.2 BONDING IN SOLIDS These are several types of chemical bonds that hold atoms and molecules together in solids .Three types of bonds -ionic ,covalent ,and metallic -are collectively termed primary bonds ,Primary bonds are strong and stiff and do not easily melt with increasing temperature .They are responsible for the bonding of metals and ceramics ,and they provide the relaxing high elastic modules (E)in these materials .Van der Waals and hydrogen bonds ,which are relatively weak ,are called secondary bonds .These are important in determining the behavior of liquids and as bonds between the carbon-chain molecules in polymers . 2.2.1 Primary Chemical Bonds The three types of primary bonds are illustrated in Fig .2.3.Ionic bonding involves the transfer of one or more elections between atoms of different types .Notes that the outer shell of electrons surrounding an atom is stable if it contains eight electrons (except that the stable number is two or the single shell of hydrogen or helium ),Hence ,an atom of the metal sodium ,with only one electron in its outer shell ,can donate an electron to an atom of chlorine ,which has an outer shell with seven electrons .After the reaction ,the sodium atom has an empty outer shell and the chlorine 4 atom has a stable outer shell of eight elections .The atoms become charged ions ,such as Ma +and Cl -,which attract one another and form a chemical bond due to their opposite electrostatic charges .A collection of such charged ions ,equal numbers of each in this case ,forms an electrically neutral solid arrangement into a regular crystalline array ,as shown in Fig .2.4. The number of electrons transferred may differ from one .For example, in the salt MgCl2 and in that in the oxide MgO, two electrons are transferred from an Mg2+ ion. Electrons in the next-to-last shell may also be transferred .For example ,iron has two outer shell electrons ,but may from either Fe2+or Fe3+ions .Many common salts ,oxides ,and other solids have bonds that are mostly or partially ionic .These materials tend to be hard and brittle. Covalent bonding involves the sharing of electrons and occurs where the outer shell are half full or more than half full .The shared electrons can be thought of as allowing both atoms involved to have stable outer shells of eight (or two )electrons .For example ,two hydrogen atoms each share an electron with an oxygen atom to make water ,H2O,or two chlorine atoms share one electron to form the diatomic molecules Cl 2.The tight covalent bonds make such simple molecules relatively independent of one another ,so that collections of them tend to form liquids or gases at ambient temperatures . Metallic bonding is responsible for the usually solid form of metals and alloys .For metals ,the outer shell of electrons is in most cases less than half full each atom donates its outer electrons to a cloud of electrons .These electrons are shared in common by all of the metal atoms ,which have 5 become positively charged ions as a result of giving up electrons .The metal ions are thus held together by their mutual attraction to the electron cloud . 2.2.2 Discussion of Primary Bonds Covalent bonds have the property -not shared by the other primary bonds of being strongly directional .This arises from covalent bonds being depended on the sharing electrons with specific neighboring atoms, whereas ionic and metallic solids are held together by electrostatic attraction involving all neighboring ions . A continues arrangement of covalent bonds can form a three -dimensional to make a sold .An example is carbon in the form of diamond ,in which each carbon atoms shares an electron with four adjacent ones ,These atoms are arranged at equal angles to one anther in three -dimensional space ,as illustrated in Fig 2.5.As a result of the strong directional bonds ,the crystal is very hard and stiff .Another important continuous arrangement of covalent bonds is the carbon chain .For example ,in the gas ethylene ,C2H4,each molecule is formed by covalent bonds as shown in Fig 2.6.However ,if the double bond between the carbon atoms is replaced by a single bond to each of two adjacent carbon atoms ,then a long chain ,molecule can form .The result is the polymer called polyethylene . Many solids ,such as SiO2 and other ceramics have chemical bonds that have a mixed ionic -covalent character .The examples given previously of NaCl for ionic bonding and diamond for covalent bonding do represent cases of nearly pure bonding of these types ,but mixed bonding is more common . Metals of more than one type may be melted together to form an alloy .Metallic bonding is the dominant type in such cases .However, intermetallic, compounds may from with alloys ,often as hard particles .These compounds have a define chemical formula ,such as TiAl3 or Mg2Ni,and 6 their bonding is generally a combination of the metallic and ionic or covalent types . 2.2.3 Secondary Bonds Secondary bonds occur due to the presence of an electrostatic dipole ,which can be induced by a primary bond .For example ,in water ,the side of a hydrogen atom away from the covalent bond to the oxygen atom has a positive charge ,due to the sole electron being predominantly on the side toward the oxygen atom .Conservation of charge over the entire molecule then requires a negative charge molecules ,as illustrated in Fig. 2.7. Such bonds, termed permanent dipole bonds ,occur between various molecules .They are relatively weak ,but are nevertheless sometimes sufficient to bind materials into solids ,water ice being an example. Where the secondary bond involves hydrogen as in the case of water, it is stronger than other dipole bonds and is called a hydrogen bond . Vander Waals bonds arise from the fluctuating positions of electrons relative to an atoms nucleus .The uneven distribution of electric charge that thus occurs causes a weak attraction between atoms or molecules ,This type of bond can also be called a fluctuating dipole -distinguished from a permanent dipole bond because the dipole is not fixed in direction as it is in a water molecule. Bonds of this type allow the inert gases to form solids at low temperature. 7 In polymers, covalent bonds form the chain molecules and attach hydrogen and other atoms to the carbon backbone .Hydrogen bonds and other secondary bonds occur between the chain molecules and tend to prevent them from sliding past one another .This is illustrated in Fig.2.8for polyvinyl chlorine .The relative weakness of the secondary bonds accounts for the low melting temperatures ,and the low strengths and stiffness of these materials . 8 第 2 章材料的结构和变形 2.1 说明 2.2 晶体的结合能 2.3 晶体材料的结构 2.4 弹性形变和理论强度 2.5 非弹性形变 2.6 总结 目标 回顾固体材料间晶体结构的结合能在同一基本能级，并与这些差异力作用在不同类型的材料。 理解弹性形变的物理基础，由于其化学成键所以使用这种方法估计固体的理论强度。理解由塑性和蠕变 引起的非弹性变形的基本机制。 学习为什么实际材料打破化学键强度的优势远低于理论。 2.1 说明 各种各样的材料应用在抵抗机械负荷是必要的，可以大致分为金属化合物、聚合物、陶瓷和玻璃和复合材料这些统称为工程材料。一些典型类别材料见表2.1。 不同材料的化学键和微观结构影响力学性能，引起材料间的相对优势和相对劣势。情况总结为图 2.1,为例。在陶瓷和玻璃间强烈的化学键提高了机械强度和刚度（高 E），温度和耐腐蚀性能，但导致材料变脆。相反，许多聚合物相对链分子之间弱键合，在这种情况下，材料的蠕变变形导致低的强度、刚度和 敏感度。 图 2.1 9 图 2.2 从对工程大小规模开始，大约一米，这是十个数量级的尺寸的跨度，下至大约 10-10 米的原子。这种情况下，对此感兴趣各种的中等尺度见图 2.1。在任何给定的尺度，理解该行为可以为通过查看较小规模时会发生什么。一台机器，车辆或结构的行为可由其组成部分的作用来解释。并且这些行为可以依次通过使用小的（ 10-1 到 10-2 米）试样，以及物料进行说明。同样，材料的宏观性质是由晶粒的晶体缺陷，聚合物链和存在于 10-3 到 10-9 米的其他微观结构特性进行说明。因此，在从 1 米的范围的大小降低 到 10-10 米有助于预测和了解机器、车辆、结构的性能 本章将回顾一些理解材料力学所需基底。我们将开始在图 2.2。 增加一个向下的力。有 的主题包括化学键，晶体结构，晶体中的缺陷和物理原因的弹性，塑性，蠕变变形。下一章将运用到这些概念于讨论每个工程材料课程的更多细节。 10 图 2.3 2.2 晶体间结合能 在固体的分子和原子中有几种类型化学键，离子键、共价键、金属键三类统称为主要缚束。主键强大，稳定而且不容易随逐渐增加的温度而融化。他们负责金属和陶瓷的结合，他们提供高弹性模量（ E）材料。范德华力和氢键是比较弱的， 被称为次级共价键。这些都是非常重要的在测定液体间作用力与聚合物中碳链分子之间的键。 2.2.1 主要化学键 三种类型的主键示于图 2.3.离子键涉及转移的一个或多个不同类型的原子间的选择。注意到，电子围绕原子的外壳是稳定的，如果它包含 8 个电子（除了稳定的数量是两个或两个氢或氦的单壳），因此，一个金属钠原子，只有一个电子在其外壳，氯原子可以得到这个电子，与其 7 个电子组成稳定的外壳。反应后，钠原子有一个空壳， 8 个氯原子有一个稳定的外壳。原子成为带电离子，如钠离子和氯离子，由于其相反的的静电荷，相互吸引而形成化学键 。这样带电的离子，在此情况下，相等数目的集合，形成一个电中性的固体布置成常规的结晶排列，如图 2.4.转移的电子数目可能不是一个。