外文翻譯--材料結構與變形

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 材料結構與變形 2.1 簡介 2.2 固體內部鍵 2.3 晶體材料的結構 2.4 彈性變形和理論強度 2.5 非彈性變形 2.6 小結 學習目標 回顧基本固體材料化學鍵和晶體結構，并聯(lián)系比較各種材料力學性能的差別。 理解彈性變形的物理基礎，利用這評估由于化學鍵產生的固體理論強度 。理解由于塑性和蠕變引起非彈性變形的基本機制。 學習材料的實際強度要遠遠低于理論強度時化學鍵發(fā)生破壞的原因。 2.1 簡介 金屬合金，高分子材料，陶瓷，玻璃及復合材料這些工程材料經(jīng)常在需承受機械載荷的情況下使用每種材料的一些典型情況在表格 2.1 給出。 這些材料的化學鍵與微觀結構的差異影響著它們的力學性能，導致了這些種類材料的相對優(yōu)勢和劣勢。這種情形被概括在圖形 2.1 中。比如在陶瓷和玻璃中的強大化學鍵賦予它們高的力學強度和剛度（高彈性模量），還有溫度和抗腐蝕能力，但是會導致發(fā)生脆性行為。相反，一些高分子材料在鏈 狀分子間被相對較弱的鍵連接，在這種情況下材料強度剛度低且易發(fā)生蠕變變形。 9 圖 2.1 圖 2.2 在工程上從基本的尺寸規(guī)模開始，粗略一米，在大小上有一個 10 數(shù)量級的跨度，低至原子的規(guī)模，大約在 10-10m。這種情況和各種中間尺寸規(guī)模在圖 2.2中列出。通過觀察發(fā)生在更小規(guī)模上的情況來尋求對性能的了解。一個機器，車輛或者結構可以通過其組成部分的性能來體現(xiàn)，而這些組成部分的性能反過來可以通過小的試樣和材料的使用來體現(xiàn)。 圖 2.3 相似地，材料的宏觀性能通過晶粒，晶體中的缺陷，高分子鏈和存在于尺寸范圍為 10-3m 到 10-9m 的微觀結構特征來解釋。因此，整個從 1m 到 10-10m 的大小范圍的性能知識有助于理解和預測機器，車輛和結構的性能。這個主題包括化學鍵，晶體結構，晶體中的缺陷，彈性塑性以及蠕變變形的物理原因。下一章將運用這些概念詳細地討論每一個種類的工程材料。 2.2 固體內部鍵 有幾種類型的化學鍵使得原子和分子聚集在固體中。三種類型的化學鍵 -離子鍵，共價鍵，金屬鍵 -被統(tǒng)稱為基本鍵。它們是形成金屬和陶瓷中的鍵的原因。它們在材料中提 供了高彈性模量。相對較弱的范德華鍵和氫鍵被稱為副鍵。對于決定流體屬性非常重要，正如聚合物中碳鏈分子間的鍵。 2.2.1 基礎化學鍵 三種類型的基本鍵在圖 2.3 中已列出。離子鍵在不同類型的原子之間轉移一個或多個電子。需要指出的是如果原子外層包含 8 個電子，那外層電子包含原子是穩(wěn)定的（除了穩(wěn)定數(shù)目為兩個或是氫或氦的單殼）。因此，外層只有一個電子的金屬鈉原子可以貢獻一個電子給外層有 7 個電子的氯原子。反應后，鈉原子外 10 層無電子，氯原子外層有穩(wěn)定的 8 電子。原子變成帶電離子。比如 Na+和 Cl-，由于它們相反的靜電荷氯原子吸引 了一個電子形成化學鍵。一組這樣的帶電離子，每種都有相同數(shù)量，形成一個電中性的固體排列成規(guī)則的結晶陣列。如圖 2.4 所示。 圖 2.4 被轉移的電子的數(shù)量可能不止一個。比如說，在鹽 MgCl2和在氧化物 MgO 中 ,從一個 Mg2+離子轉移 2 個電子。在倒數(shù)第二層的電子也可能被轉移，比如，鐵有 2 個外層電子，可能來自 Fe2+抑或是 Fe3+離子。許多常見的鹽類，氧化物，和其他固體中都有鍵，大多或者部分都是離子鍵。這些材料往往是硬又脆。共價鍵包含電子的共用，發(fā)生在外層電子為半滿或多于半滿的情況。共用電子可