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.nɑit] 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