《材料科学基础--skja_12》由会员分享，可在线阅读，更多相关《材料科学基础--skja_12（19页珍藏版）》请在金锄头文库上搜索。
1、材料科学基础Fundamental of Materials,Prof: Tian Min Bo Tel: 62795426 ，62772851 E-mail: Department of Material Science and Engineering Tsinghua University. Beijing 100084,Chapter ,Plastic Deformation of Crystal,Lesson twelve,4.1 Introduction,1. Deformation process of polycrystalline materials Deformation p
2、rocess of industrial material consists of three stages: Elastic, which is reversible Plastic or permant Fracture,2. Deformation mode of crystals 1) Slip 2) Twinning 3) Grain boundary sliding followed by g.b. migration 4) Diffusion,3. Basic feature of slip and twinning Both slip and twinning are shea
3、r processes 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.,slip,twinning,The main difference between slip and twinning is that the former does not cause any change in orientation in
4、 the sense 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 tw
5、in of the 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
6、 FCC: S.S. (Slip Systems) is 111 at R.T. (temperature). There are 12 S.S. in all. HCP: S.S. BCC: CsCl: 110,for metals with large c/a for other HCP metals,for metals with large c/a for other HCP metals,. Schmids law (The condition for slip to start) The shear stress acting on slip plane along slip di
7、rection is .,Slip occurs 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 materi
8、al and temperature. For FCC: For BCC: and other structure:,Condition for slip to start:,Orientation factor,Soft orientation Hard orientation,0.1,0.2,0.4,0.3,0.5,0.4,0.3,0.2,0.1,2. 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 constant.,The determination of active slip system(s),angle, Application of Schmids law,Examples and Discussions,Exercise,Thank you !,12,
