工程材料基本概念之一

一、晶体结构和结晶过程的概念

Structures of the metals and concepts of crystallization

1. 晶体、非晶体的概念、联系和区别concepts 、relations and difference between crystal and amorphous body（noncrystal ）

概念：

∙ Composition/concentration:

∙ allotropic transformations 同素异晶转变：there are no compositional alterations ∙ incongruent transformations：Eutectic and eutectoid reactions, as well as the melting of an alloy

∙ Crystal and amorphous state：

∙ Pure metal纯金属

∙ Metallic Solid solution (Sosoloid)固溶体

∙ Metallic Compound：cementite 金属化合物（ Fe3C）

∙ Mechanical mixture：pearlite 机械混合物：珠光体

∙ Binary eutectic systems (Eutectic reaction)

∙ BCC 体心立方晶格 Body-centered cubic

∙ FCC 面心立方晶格

∙ HCP 密排六方晶格Hexagonal close-packed lattice (HCP)

∙ Crystallographic planes 滑移面

∙ ferrite [‘ferait] 铁素体

∙ cementite [si'mentait, si'men.tait] Fe3C

∙ eutectoid [ju:‘tektɔid]共析混合物

∙ Proeutectoid 先共析体

∙ hypoeutectoid [haipəuju(:)‘tektɔid] 亚共析

∙ hypereutectoid [haipərju‘tektɔid]过共析

∙ pearlite [‘pə:lait, ’pə.lait] 珠光体、珍珠岩

∙ austenite [‘ɔ:stə.n ɑit] 奥氏体

∙ peritectic[peri‘tektik, .perə’tektik]包晶的，包晶体

∙ congruent transformations 一致性转变

∙ allotropic transformation同素异晶转变

∙ isomerism 同分异构

Transformation 晶体与非晶体的转化

∙ 晶化（crystallizing ）/脱玻化（devitrification ）: （1）can be happened

spontaneously because noncrystal with high internal energy，unstable 晶化过程可以自发进行，因为非晶态内能高、不稳定，而晶态内能低、稳定。（2）非晶化（noncrystallizing ）：damage of regular arrangement of inner atoms 因内部质点的规则排列遭到破坏而转化为非晶态。Examples ：非晶化一般需要外能。如天然石英是晶体, 熔融过的石英却是非晶体；把晶体硫加热熔化（温度超过300℃）再倒进冷水中, 会变成柔软的非晶硫, 再过一段时间又会转化为晶体硫．

∙ Most solids are crystals因为晶体比非晶体稳定，所以晶体的分布十分广泛，自然界的固体物质中，绝大多数是晶体。

Difference

∙ anisotropic property ；isotropic （各项同性）

∙ fixed melting temperature

∙ X-ray diffraction

∙ low internal energy and stable

2. 晶格、晶胞和晶格常数的含义crystal lattice/a reduced sphere、unit cell and lattice parameters（length of sides and angles of planes）

∙ coordination number（配位数）：For metals, each atom has the same number of nearest-neighbor or touching atoms, which is the coordination number

∙ atomic packing factor (APF)（致密度）：is the sum of the sphere volumes of all atoms within a unit cell (assuming the atomic hard-sphere model) divided by the unit cell volume

3. 三种常见晶格的介绍及其相关计算

∙ The Face-Centered Cubic Crystal Structure、The Body-Centered Cubic Crystal Structure and the hexagonal close-packed crystal structure

Figure： For the face-centered cubic crystal structure, (a) a hard-sphere unit cell

representation, (b) a reduced sphere unit cell, and (c) an aggregate of many atoms. Figure ： For the body-centered cubic crystal structure, (a) a hard-sphere unit cell representation, (b) a reduced sphere unit cell, and (c) an aggregate of many atoms.

Figure ： For the hexagonal close-packed crystal structure（HCP ）, (a) a reduced-sphere unit cell (a and c represent the short and long edge lengths, respectively), and (b) an aggregate of many atoms.

Thinking ：why c=1.66a？

see the following picture for hint

密排六方晶胞是一个六方柱体。柱体的上、下底面六个角及中心各有一个原子, 柱

体中心还有3个原子。属于这类晶胞类型的金属有:铍(Be)、镁(Mg)、锌(Zn)、镉(Cd)等。

晶胞常数：底面边长 a 和高 c，

原子半径:r=1/2a

原子数:n=12×1/6+2×1/2+3 =6

致密度： K= nV原子/V晶体= 0.74

原子配位数：12

高的计算过程：

取密排六方中间层某原子，为保证密排，其投影位于底面三个原子三角形的中心，该投影距离底面某原子的距离为d1=a/（3^(1/2)）, 而由于该中间层原子距离底面原子距离为d2=a,故c=2*（d2*d2-d1*d1）^(1/2)=1.633

二、晶体的结晶过程Processes of crystallization

1. Concept of crystallization、conditions and steps of crystallization: crystallization, crystal structures

2. 塑性变形plastic deformation、再结晶recrystallization 、冷热加工cold work的基本概念：

3. 实际晶体中晶体缺陷介绍imperfection in real crystal、塑性变形和再结晶的实质nature of plastic deformation and recrystallization

概念：

∙ Imperfection 晶体缺陷：点、线、面（体）Point Defects点缺陷( cation [美'kætaɪən]and anion vacancy)、阴离子空隙anion[美'ænaɪən]interstitial；

Dislocations —linear defects线缺陷（edge dislocation刃型位错 and screw dislocation 螺型位错）；Interfacial defects面缺陷(grain boundary晶界/亚晶界、twin planes孪晶) ；bulk or volume defects体缺陷：pores, cracks, foreign inclusions, and other phases

∙ Imperfection 晶体缺陷：点、线、面（体）Point Defects点缺陷( cation [美'kætaɪən]and anion vacancy)、阴离子空隙anion[美'ænaɪən]interstitial；

Dislocations —linear defects线缺陷；Interfacial defects面缺陷(grain boundary晶界/亚晶界、twin planes孪晶) ；bulk or volume defects体缺陷：pores, cracks, foreign inclusions, and other phases

∙ 单晶体塑性变形的实质 Nature of plastic deformation：dislocation motion

4. 三种典型晶体结构及金属晶体中晶面和晶向的表示方法。

Slip system ：Slip occurs on specific crystallographic planes and within these planes only in certain directions. A slip system represents a slip plane–slip direction combination. ∙

∙

Operable slip systems depend on the crystal structure of the material. The slip plane is that plane that has the densest atomic packing, and the slip direction is the direction within this plane that is most closely packed with atoms. The slip system for the FCC crystal structure is{111} , so the number of slip direction is 4x3=12; for BCC several are possible: {110} ,{211} and {321} , so it is 6x2=12; for HCP: it is1x3=3.

三、Composition and structures of alloys合金的组成与结构

1. Solid Solutions 固溶体 single phase

2. Metals compounds金属化合物single phase

3. Mechanical mixtures 机械混合物single phase?