外文翻譯--材料結(jié)構(gòu)與變形

1、<p>　　Chapter 2 Structure and Deformation in Materials</p><p>　　2.1 INTRODUCTION</p><p>　　2.2 BONDING IN SOLIDS</p><p>　　2.3 STRUCTURE IN CRYSTALLINE MATERIALS </p><

2、p>　　2.4 ELASTIC DEFORMATION AND THEORETICAL STRENGTH </p><p>　　2.5 INELASTIC DEFORMATION</p><p>　　2.6 SUMARRY</p><p>　　OBJECTIVES</p><p>　　Review chemical bonding cr

3、ystal structure in solid materials at a basic level, and relate these to differences in mechanical behavior among various classes of materials.</p><p>　　Understand the physical basis of elastic deformation,

4、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.</p><p>　　Learn why actual stre

5、ngths of materials fall far below the theoretical strength to break chemical bonds.</p><p>　　2.1 INTRODUTION</p><p>　　A wide variety of materials are used in applications where resistance to mec

6、hanical 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 giv

7、en in Table 2.1.</p><p>　　Differences among the classes of materials as to chemical bonding and microstructure affect mechanical behavior, giving rise to relative advantages and disadvantages among the class

8、es. 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 br

9、ittle behavior. In contrast, many polymers are relatively weakly bonded between the chain molecules</p><p>　　Starting from the size sale of primary interest in engineering ,rough one meter ,there is a span o

10、f 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

11、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</p><p>　　and structures

12、.</p><p>　　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

13、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 clas

14、ses of engineering materials in more details .</p><p>　　2.2 BONDING IN SOLIDS</p><p>　　These are several types of chemical bonds that hold atoms and molecules together in solids .Three types of

15、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 ceram

16、ics ,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</p><p>　　2.2.1 Primary Chemical B

17、onds </p><p>　　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 electr

18、ons 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 s

19、hell ,can donate an electron to an atom of chlorine ,which has an outer shell with seve</p><p>　　The number of electrons transferred may differ from one .For example, in the salt MgCl2 and in that in the ox

20、ide 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 salt

21、s ,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 </p><p>　　Metallic bonding is respon

22、sible 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 electr

23、ons 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 .&l

24、t;/p><p>　　2.2.2 Discussion of Primary Bonds </p><p>　　Covalent bonds have the property -not shared by the other primary bonds of being strongly directional .This arises from covalent bonds being d

25、epended on the sharing electrons with specific neighboring atoms, whereas ionic and metallic solids are held together by electrostatic attraction involving all neighboring ions .</p><p>　　A continues arrange

26、ment 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 a

27、ngles 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 th

28、e carbon chain .For example ,in the gas e</p><p>　　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 ioni

29、c bonding and diamond for covalent bonding do represent cases of nearly pure bonding of these types ,but mixed bonding is more common .</p><p>　　Metals of more than one type may be melted together to form an

30、 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 the

31、ir bonding is generally a combination of the metallic and ionic or covalent types .</p><p>　　2.2.3 Secondary Bonds </p><p>　　Secondary bonds occur due to the presence of an electrostatic dipole

32、,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 t

33、oward the oxygen atom .Conservation of charge over the entire molecule then requires a negative charge molecules ,as illustrated in Fig. 2.7.</p><p>　　Such bonds, termed permanent dipole bonds ,occur between

34、 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

35、 stronger than other dipole bonds and is called a hydrogen bond .</p><p>　　Vander Waals bonds arise from the fluctuating positions of electrons relative to an atom's nucleus .The uneven distribution of e

36、lectric 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 i

37、n direction as it is in a water molecule. Bonds of this type allow the inert gases to form solids at low temperature.</p><p>　　In polymers, covalent bonds form the chain molecules and attach hydrogen and oth

38、er 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 .Th

39、e relative weakness of the secondary bonds accounts for the low melting temperatures ,and the low strengths and stiffness of these materials .</p><p>　　第2章 材料結(jié)構(gòu)與變形</p><p><b>　　2.1 簡介</b

40、></p><p><b>　　2.2 固體內(nèi)部鍵</b></p><p>　　2.3 晶體材料的結(jié)構(gòu)</p><p>　　2.4 彈性變形和理論強(qiáng)度</p><p><b>　　2.5 非彈性變形</b></p><p><b>　　2.6 小結(jié)</b

41、></p><p><b>　　學(xué)習(xí)目標(biāo)</b></p><p>　　回顧基本固體材料化學(xué)鍵和晶體結(jié)構(gòu)，并聯(lián)系比較各種材料力學(xué)性能的差別。</p><p>　　理解彈性變形的物理基礎(chǔ)，利用這評估由于化學(xué)鍵產(chǎn)生的固體理論強(qiáng)度。理解由于塑性和蠕變引起非彈性變形的基本機(jī)制。</p><p>　　學(xué)習(xí)材料的實(shí)際強(qiáng)度要遠(yuǎn)遠(yuǎn)低

42、于理論強(qiáng)度時(shí)化學(xué)鍵發(fā)生破壞的原因。</p><p><b>　　2.1簡介</b></p><p>　　金屬合金，高分子材料，陶瓷，玻璃及復(fù)合材料這些工程材料經(jīng)常在需承受機(jī)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。</p><p>　　這些材料的化學(xué)鍵與微觀結(jié)構(gòu)的差異影響著它們的力學(xué)性能，導(dǎo)致了這些種類材料的相對優(yōu)勢和劣勢。這種

43、情形被概括在圖形2.1中。比如在陶瓷和玻璃中的強(qiáng)大化學(xué)鍵賦予它們高的力學(xué)強(qiáng)度和剛度（高彈性模量），還有溫度和抗腐蝕能力，但是會(huì)導(dǎo)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(qiáng)度剛度低且易發(fā)生蠕變變形。</p><p>　　圖2.1 圖2.2</p><p>　　在工程上從基本的尺寸規(guī)模開始，粗

44、略一米，在大小上有一個(gè)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(gè)機(jī)器，車輛或者結(jié)構(gòu)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)。</p><p><b>　　圖2.3</b></p><p>　　相似地，材料的宏

45、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(jié)構(gòu)特征來解釋。因此，整個(gè)從1m到10-10m的大小范圍的性能知識有助于理解和預(yù)測機(jī)器，車輛和結(jié)構(gòu)的性能。這個(gè)主題包括化學(xué)鍵，晶體結(jié)構(gòu)，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(yùn)用這些概念詳細(xì)地討論每一個(gè)種類的工程材料。</p><p><b>　　2.2固體內(nèi)部鍵</b></p>

46、<p>　　有幾種類型的化學(xué)鍵使得原子和分子聚集在固體中。三種類型的化學(xué)鍵-離子鍵，共價(jià)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。</p><p>　　2.2.1基礎(chǔ)化學(xué)鍵</p><p>　　三種類型的基本鍵在圖2.3中已列出。離子鍵在不

47、同類型的原子之間轉(zhuǎn)移一個(gè)或多個(gè)電子。需要指出的是如果原子外層包含8個(gè)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(gè)或是氫或氦的單殼）。因此，外層只有一個(gè)電子的金屬鈉原子可以貢獻(xiàn)一個(gè)電子給外層有7個(gè)電子的氯原子。反應(yīng)后，鈉原子外層無電子，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(gè)電子形成化學(xué)鍵。一組這樣的帶電離子，每種都有相同數(shù)量，形成一個(gè)電中性的固體排列成規(guī)則的結(jié)晶陣列。如

48、圖2.4所示。</p><p><b>　　圖2.4</b></p><p>　　被轉(zhuǎn)移的電子的數(shù)量可能不止一個(gè)。比如說，在鹽MgCl2和在氧化物MgO中,從一個(gè)Mg2+離子轉(zhuǎn)移2個(gè)電子。在倒數(shù)第二層的電子也可能被轉(zhuǎn)移，比如，鐵有2個(gè)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。

49、共價(jià)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。共用電子可以被認(rèn)為是允許涉及到的2個(gè)原子具有穩(wěn)定的8（或2）電子的外層。比如說，兩個(gè)氫原子每個(gè)都和氧原子共用一個(gè)電子形成水，H2O，或者，兩個(gè)氯原子共用一個(gè)電子形成雙原子分子Cl2。緊的共價(jià)鍵形成的這種簡單的分子彼此相對獨(dú)立，因此，它們的集合在外界溫度下更傾向于形成液體或是氣體。</p><p>　　金屬鍵一般是金屬和合金為固體形式的的原因。對金屬來說，

50、外層電子在大多數(shù)情況下處于半滿狀態(tài)，每個(gè)原子都需貢獻(xiàn)外層原子形成“云”電子，這些電子被所有由于放棄電子已經(jīng)變成帶正電荷離子的金屬原子共同使用。因此，金屬離子通過他們對電子云的相互吸引而集合在一起。</p><p><b>　　圖2.5</b></p><p>　　2.2.2基礎(chǔ)鍵的討論</p><p>　　共價(jià)鍵有這樣的特性—不會(huì)被其他強(qiáng)烈定向

51、的基礎(chǔ)鍵共用。這源于共價(jià)鍵依賴特定相鄰原子的共用電子，然而離子和金屬固體通過所有相鄰離子的靜電吸引而被集合在一起。</p><p>　　共價(jià)鍵的連續(xù)排列可以形成一個(gè)三維空間來形成固體。碳以鉆石的形式存在就是一個(gè)例子。鉆石中每個(gè)碳原子和相鄰4個(gè)原子都共用一個(gè)電子，在三維空間中，這些原子都被布置在彼此之間相同的角度上，正如圖2.5所列出。由于強(qiáng)烈的定向鍵，晶體十分堅(jiān)硬。共價(jià)鍵另一個(gè)重要的連續(xù)分布是碳鏈。比如，在氣體乙

52、烯中，C2H4，每個(gè)分子都是由共價(jià)鍵形成的，如圖2.6所示。但是，如果碳原子之間的雙鍵被單鍵所取代并連接到其他兩個(gè)相鄰的碳原子上，這樣就形成長鏈分子。結(jié)果形成了被稱為聚乙烯的聚合物。</p><p>　　很多固體，比如SiO2和其他陶瓷都有化學(xué)鍵，這化學(xué)鍵有混合離子共價(jià)鍵的特性。之前給出的例子NaCl的離子鍵和鉆石的共價(jià)鍵都代表這些類型的純鍵的情況。但是混合鍵更普遍。</p><p>　　

53、兩種或兩種以上不同類型的金屬熔化形成合金。這種情況下金屬鍵是主導(dǎo)類型，但是，金屬間的化合物可能來自合金，通常為硬顆粒。這些化合物有明確化學(xué)式，比如TiAl3和Mg2Ni,它們的鍵通常是金屬鍵，離子鍵和共價(jià)鍵類型的結(jié)合。</p><p><b>　　2.2.3副鍵</b></p><p>　　副鍵的產(chǎn)生是由于一個(gè)靜電偶極子的出現(xiàn)，而靜電偶極子是由于基礎(chǔ)鍵產(chǎn)生。舉個(gè)例子，

54、在水分子里，由于唯一的電子顯著地偏向氧原子一側(cè)，氫原子一側(cè)遠(yuǎn)離氧原子的共價(jià)鍵，從而帶正電荷。整個(gè)分子的電荷守恒需要一個(gè)負(fù)電荷分子，如圖2.7所示。</p><p><b>　　圖2.7</b></p><p>　　這種鍵被稱為固定偶極鍵，發(fā)生在各種分子之間。這種鍵相對較弱，但有時(shí)足夠使材料凝固成固體。水結(jié)冰就是個(gè)例子。副鍵所包含的水中的氫，它比其他的偶極鍵都要強(qiáng)，被稱

55、為氫鍵。</p><p>　　范德華鍵的產(chǎn)生是電子相對原子核的位置移動(dòng)。電荷的不均勻分布導(dǎo)致了在分子或原子之間的弱吸引力。這種類型的鍵也被稱為波動(dòng)偶極子，不同于固定偶極鍵，因?yàn)榕紭O子在水分子中，在方向上不固定。這種類型的鍵允許惰性氣體在低溫下形成固體。</p><p><b>　　圖2.8</b></p><p>　　在聚合物中，共價(jià)鍵形成分子