材料科学基础第04章1

1、材料科学基础Fundamental of MaterialsProf: Tian Min Bo Tel: 62795426 ，62772851 E-mail: Department of Material Science and EngineeringTsinghua University. Beijing 100084Chapter Plastic Deformation of CrystalLesson twelve4.1 Introduction1. Deformation process of polycrystalline materials Deformation process

2、of industrial material consists of three stages: Elastic, which is reversible Plastic or permant Fracture2. Deformation mode of crystals 1) Slip 2) Twinning 3) Grain boundary sliding followed by g.b. migration 4) Diffusion3. Basic feature of slip and twinning Both slip and twinning are shear process

3、es under shear stresses, i.e. it is a translational movement of one portion of the crystal with respect to the other on a specific plane and along a specific direction. sliptwinning The main difference between slip and twinning is that the former does not cause any change in orientation in the sense

4、 that any crystallographic direction remain the same with respect to the laboratory coordinate systems. While the latter results in change in orientation. After twinning the two parts of the crystal, twinned and untwinned, are symmetrical to some plane or orientation. Or say, one is the twin of the

5、other.4.2 Slip Systems. Definition： Slip Systems is the combination of a crystallographic plane and a direction lying on it. On the plane and along the direction slip takes place. A slip plane plus a corresponding slip direction is called a slip system. Slip systems of typical crystals FCC: S.S. (Sl

6、ip Systems) is 111 at R.T. (temperature). There are 12 S.S. in all. HCP: S.S. BCC: CsCl: 110for metals with large c/a for other HCP metalsfor metals with large c/afor other HCP metals. Schmids law (The condition for slip to start)The shear stress acting on slip plane along slip direction is .Slip oc

7、curs when the resolved shear stress acting on slip plane along slip direction reaches a Critical value, called the Critical stress, Critical Resolved Shear Stress (C.R.S.S) and is characteristic of the crystal in question.When , slip starts as when slip starts. depends on material and temperature.Fo

8、r FCC: For BCC: and other structure: Condition for slip to start:Orientation factorSoft orientationHard orientation0.1 0.2 0.4 0.3 0.5 0.4 0.3 0.2 0.1 2. Discussion is a material constant Which one will first slip in crystallographiclly equivalent slip systems? It is an approximate law. For single crystal is not a material co