外文翻译--材料结构与变形

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 2 between the chain molecules, in which case the material has low strength and stiffness and is susceptible creep deformation. 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 . This chapter review some of the fundamentals needed to understand mechanical behavior of 3 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 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. 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 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 . 5 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 their bonding is generally a combination of the metallic and ionic or covalent types . 2.2.3 Secondary Bonds 6 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 材料结构与变形 2.1 简介 2.2 固体内部键 2.3 晶体材料的结构 2.4 弹性变形和理论强度 2.5 非弹性变形 2.6 小结 学习目标 回顾基本固体材料化学键和晶体结构，并联系比较各种材料力学性能的差别。 理解弹性变形的物理基础，利用这评估由于化学键产生的固体理论强度 。理解由于塑性和蠕变引起非弹性变形的基本机制。 学习材料的实际强度要远远低于理论强度时化学键发生破坏的原因。 2.1 简介 金属合金，高分子材料，陶瓷，玻璃及复合材料这些工程材料经常在需承受机械载荷的情况下使用每种材料的一些典型情况在表格 2.1 给出。 这些材料的化学键与微观结构的差异影响着它们的力学性能，导致了这些种类材料的相对优势和劣势。这种情形被概括在图形 2.1 中。比如在陶瓷和玻璃中的强大化学键赋予它们高的力学强度和刚度（高弹性模量），还有温度和抗腐蚀能力，但是会导致发生脆性行为。相反，一些高分子材料在链 状分子间被相对较弱的键连接，在这种情况下材料强度刚度低且易发生蠕变变形。 9 图 2.1 图 2.2 在工程上从基本的尺寸规模开始，粗略一米，在大小上有一个 10 数量级的跨度，低至原子的规模，大约在 10-10m。这种情况和各种中间尺寸规模在图 2.2中列出。通过观察发生在更小规模上的情况来寻求对性能的了解。一个机器，车辆或者结构可以通过其组成部分的性能来体现，而这些组成部分的性能反过来可以通过小的试样和材料的使用来体现。 图 2.3 相似地，材料的宏观性能通过晶粒，晶体中的缺陷，高分子链和存在于尺寸范围为 10-3m 到 10-9m 的微观结构特征来解释。因此，整个从 1m 到 10-10m 的大小范围的性能知识有助于理解和预测机器，车辆和结构的性能。这个主题包括化学键，晶体结构，晶体中的缺陷，弹性塑性以及蠕变变形的物理原因。下一章将运用这些概念详细地讨论每一个种类的工程材料。 2.2 固体内部键 有几种类型的化学键使得原子和分子聚集在固体中。三种类型的化学键 -离子键，共价键，金属键 -被统称为基本键。它们是形成金属和陶瓷中的键的原因。它们在材料中提 供了高弹性模量。相对较弱的范德华键和氢键被称为副键。对于决定流体属性非常重要，正如聚合物中碳链分子间的键。 2.2.1 基础化学键 三种类型的基本键在图 2.3 中已列出。离子键在不同类型的原子之间转移一个或多个电子。需要指出的是如果原子外层包含 8 个电子，那外层电子包含原子是稳定的（除了稳定数目为两个或是氢或氦的单壳）。因此，外层只有一个电子的金属钠原子可以贡献一个电子给外层有 7 个电子的氯原子。反应后，钠原子外 10 层无电子，氯原子外层有稳定的 8 电子。原子变成带电离子。比如 Na+和 Cl-，由于它们相反的静电荷氯原子吸引 了一个电子形成化学键。一组这样的带电离子，每种都有相同数量，形成一个电中性的固体排列成规则的结晶阵列。如图 2.4 所示。 图 2.4 被转移的电子的数量可能不止一个。比如说，在盐 MgCl2和在氧化物 MgO 中 ,从一个 Mg2+离子转移 2 个电子。在倒数第二层的电子也可能被转移，比如，铁有 2 个外层电子，可能来自 Fe2+抑或是 Fe3+离子。许多常见的盐类，氧化物，和其他固体中都有键，大多或者部分都是离子键。这些材料往往是硬又脆。共价键包含电子的共用，发生在外层电子为半满或多于半满的情况。共用电子可