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 章材料的結(jié)構(gòu)和變形 2.1 說明 2.2 晶體的結(jié)合能 2.3 晶體材料的結(jié)構(gòu) 2.4 彈性形變和理論強(qiáng)度 2.5 非彈性形變 2.6 總結(jié) 目標(biāo) 回顧固體材料間晶體結(jié)構(gòu)的結(jié)合能在同一基本能級(jí)，并與這些差異力作用在不同類型的材料。 理解彈性形變的物理基礎(chǔ)，由于其化學(xué)成鍵所以使用這種方法估計(jì)固體的理論強(qiáng)度。理解由塑性和蠕變 引起的非彈性變形的基本機(jī)制。 學(xué)習(xí)為什么實(shí)際材料打破化學(xué)鍵強(qiáng)度的優(yōu)勢(shì)遠(yuǎn)低于理論。 2.1 說明 各種各樣的材料應(yīng)用在抵抗機(jī)械負(fù)荷是必要的，可以大致分為金屬化合物、聚合物、陶瓷和玻璃和復(fù)合材料這些統(tǒng)稱為工程材料。一些典型類別材料見表2.1。 不同材料的化學(xué)鍵和微觀結(jié)構(gòu)影響力學(xué)性能，引起材料間的相對(duì)優(yōu)勢(shì)和相對(duì)劣勢(shì)。情況總結(jié)為圖 2.1,為例。在陶瓷和玻璃間強(qiáng)烈的化學(xué)鍵提高了機(jī)械強(qiáng)度和剛度（高 E），溫度和耐腐蝕性能，但導(dǎo)致材料變脆。相反，許多聚合物相對(duì)鏈分子之間弱鍵合，在這種情況下，材料的蠕變變形導(dǎo)致低的強(qiáng)度、剛度和 敏感度。 圖 2.1 9 圖 2.2 從對(duì)工程大小規(guī)模開始，大約一米，這是十個(gè)數(shù)量級(jí)的尺寸的跨度，下至大約 10-10 米的原子。這種情況下，對(duì)此感興趣各種的中等尺度見圖 2.1。在任何給定的尺度，理解該行為可以為通過查看較小規(guī)模時(shí)會(huì)發(fā)生什么。一臺(tái)機(jī)器，車輛或結(jié)構(gòu)的行為可由其組成部分的作用來解釋。并且這些行為可以依次通過使用小的（ 10-1 到 10-2 米）試樣，以及物料進(jìn)行說明。同樣，材料的宏觀性質(zhì)是由晶粒的晶體缺陷，聚合物鏈和存在于 10-3 到 10-9 米的其他微觀結(jié)構(gòu)特性進(jìn)行說明。因此，在從 1 米的范圍的大小降低 到 10-10 米有助于預(yù)測(cè)和了解機(jī)器、車輛、結(jié)構(gòu)的性能 本章將回顧一些理解材料力學(xué)所需基底。我們將開始在圖 2.2。 增加一個(gè)向下的力。有 的主題包括化學(xué)鍵，晶體結(jié)構(gòu)，晶體中的缺陷和物理原因的彈性，塑性，蠕變變形。下一章將運(yùn)用到這些概念于討論每個(gè)工程材料課程的更多細(xì)節(jié)。 10 圖 2.3 2.2 晶體間結(jié)合能 在固體的分子和原子中有幾種類型化學(xué)鍵，離子鍵、共價(jià)鍵、金屬鍵三類統(tǒng)稱為主要縛束。主鍵強(qiáng)大，穩(wěn)定而且不容易隨逐漸增加的溫度而融化。他們負(fù)責(zé)金屬和陶瓷的結(jié)合，他們提供高彈性模量（ E）材料。范德華力和氫鍵是比較弱的， 被稱為次級(jí)共價(jià)鍵。這些都是非常重要的在測(cè)定液體間作用力與聚合物中碳鏈分子之間的鍵。 2.2.1 主要化學(xué)鍵 三種類型的主鍵示于圖 2.3.離子鍵涉及轉(zhuǎn)移的一個(gè)或多個(gè)不同類型的原子間的選擇。注意到，電子圍繞原子的外殼是穩(wěn)定的，如果它包含 8 個(gè)電子（除了穩(wěn)定的數(shù)量是兩個(gè)或兩個(gè)氫或氦的單殼），因此，一個(gè)金屬鈉原子，只有一個(gè)電子在其外殼，氯原子可以得到這個(gè)電子，與其 7 個(gè)電子組成穩(wěn)定的外殼。反應(yīng)后，鈉原子有一個(gè)空殼， 8 個(gè)氯原子有一個(gè)穩(wěn)定的外殼。原子成為帶電離子，如鈉離子和氯離子，由于其相反的的靜電荷，相互吸引而形成化學(xué)鍵 。這樣帶電的離子，在此情況下，相等數(shù)目的集合，形成一個(gè)電中性的固體布置成常規(guī)的結(jié)晶排列，如圖 2.4.轉(zhuǎn)移的電子數(shù)目可能不是一個(gè)。