材料成型金属学ch01-1位错理论

1、1.位错理论基础 1.1 晶体缺陷概论n晶体中的缺陷缺陷： 原子排列偏离完整性的区域n点缺陷在三个方向上尺寸都很小 n线缺陷在二个方向上尺寸很小 n面缺陷在一个方向上尺寸很小 点缺陷 point defectsn空位 vacancyn间隙原子 interstitial atomn杂质原子 impurity atomSchematic illustration of types of defects in a single-crystal lattice: self-interstitial, vacancy, interstitial, and substitutional.Extra ato

2、mMissing atomExtra atomForeign atomPoint Defects in a Single Crystal LatticePoint Defects 点缺陷Substitutional atom 替换原子Self- interstitial atomInterstitial atom间隙原子Vacancy空位 Equilibrium concentration varies with temperature! How?空位平衡浓度Boltzmanns constant (1.38 x 10-23 J/atom K) (8.62 x 10-5 eV/atom K)

3、NDN=exp-QDkT No. of defectsNo. of potential defect sites.Formation energyTemperatureEach lattice site is a potential vacancy siteSelf Diffusion Via Vacancy Mechanism点缺陷对晶体性能的影响n间隙原子体积膨胀12个原子体积n 空位体积膨胀0.5个原子体积n屈服强度 n对扩散、高温形变和热处理等过程均有重要影响 面缺陷 plane defectn小角度晶界 low angle grain boundaryn大角度晶界 high angl

4、e grain boundaryn相界面 interphase boundary小角度晶界特点*10 由位错构成 *位错密度 位向差晶格畸变晶界能大角度晶界 10 小角度晶界 类型（1）对称倾侧晶界相邻晶粒各转/2同号刃位错垂直排列（2）不对称倾侧晶界相互垂直的两组刃位错垂直排列 Low Angle Grain Boundary 小角晶界grain boundariesSchematicAdapted from Fig. 4.7, Callister 6e. GRAIN BOUNDARIES 晶界晶界特点 1） 晶界畸变晶界能向低能量状态转化晶粒长大、晶界变直晶界面积减小； 2） 阻碍位错运动

5、 b 细晶强化； 3） 位错、空位等缺陷多晶界扩散速度高； 4） 晶界能量高、结构复杂容易满足固态相变的条件固态相变首先发生地； 5） 化学稳定性差晶界容易受腐蚀； 6） 微量元素、杂质富集Variation in Grain Boundary Energy With Misorientation for Copper. 晶界能角度00.10.20.30.40.50.60102030405060DegreesEnergy (J/m2)相界面n两个不同相之间的界面 共格界面 半共格界面 非共格界面弹性能 与 界面能？晶界处新晶粒晶界与亚晶界新晶粒与位错Laminar structure 片层状组

6、织High resolution TEM 高分辨电子显微镜高分辨电子显微镜成百上千个原子成百上千个原子单个原子排列单个原子排列ELECTRON MICROSCOPYnJuan Villegas, Ph.D. thesis (UConn MSE)advancing twin001110Nickel alloy线缺陷 line defectn刃型位错 Edge dislocationn螺型位错 Screw dislocation n混合位错 Mixed dislocation1.2 位错概念的引入n1934 解释实测的晶体临界切应力值与理论计算值相差千倍以上的问题理想晶体的临界切应力实际晶体的逐步

7、滑移2Gm= Effect of dislocations in the lattice structure under stressIn the series of diagrams, the movement of the dislocation allows deformation to occur under a lower stress than in a perfect lattice1.3 位错的原子模型及柏氏矢量Edge Dislocation 刃型位错刃型位错From A. G. Guy, Elements of Physical Metallurgy, Addison-We

8、sleyPublishing Co., Inc., Reading, Mass., 1959.Contains an additional plane of atoms which extends indefinitely along the crystal 多余原子面Edge Dislocation 刃型位错刃型位错Screw Dislocation 螺型位错 From J.F. Shackelford, Introduction to Material Science for Engineers,MacMillan Publishing Co, New York, NY, 1985Spir

9、al Stacking of crystal planes. The direction of motion is perpendicular to the shearing stress. Screw Dislocation 螺型位错 Mixed Dislocation 混合位错 From J.F. Shackelford, Introduction to Material Science for Engineers,MacMillan Publishing Co, New York, NY, 1985Has both edge andscrew character.Screw Disloc

10、ationEdge DislocationMixed Dislocation 混合位错 n柏氏矢量的确定 （1）包含位错线做一封闭回路柏氏回（2） 将同样的回路置于完整晶体中不能闭合 （3） 补一矢量（终点指向起点）使回路闭合柏氏矢量BURGERS VECTOR 柏氏矢量nEdge dislocation: Burgers vector perpendicular to dislocation linenScrew dislocation: Burgers vector parallel to dislocation linenMixed dislocation: Orientation of

11、 Burgers vector with respect to dislocation changes with positionEdge dislocationScrew dislocationDislocation line: into pageDislocation lineBurgers vectorBurgers vector特性（1） 满足右螺旋规则时，柏氏矢量与柏氏回路路径无关 唯一性 （2） 用柏氏回路求得的柏氏矢量为回路中包围的所有位错柏氏矢量的总和（矢量和） 可加性 （3） 同一位错，柏氏矢量处处相同 同一性位错密度n单位体积中位错的总长度：3/,cmcmVL=3/1 ,c

12、mAnALnL=将位错线看作于垂直某一平面的直位错线位错的观察Etching to Reveal Dislocation MotionMovement of dislocations in LiF.“A” are sessile dislocations & “B” are glissile dislocations.When a stress is applied the glissile dislocation slides, hence, when it is etched again it leaves a new trace where as the old trace gets wider, but not deeper.Sessile dislocations do not move and the etch pits get deeper and wider on repeated