【土木建筑】材料科学基础_16.pptVIP

材料科学基础Fundamental of Materials §4.9 Comparison Between Slip and Twinning Ⅰ.Similarity: Both slip and twinning are shear process under shear stress . Ⅱ.Differences: Orientation Atom movement Conditions: §4.10 Basic Features of The Plastic Deformation of Polycrystals Ⅰ.Grain boundary and its role in deformation Impede slip (as obstacles) at R.T. and low temperature. Coordinate the deformation in different grains. Enhance (assist) the plastic deformation at high temperature through grain boundary sliding. Ⅱ.Deformation features of polycrystal Plastic deformation can take place in various ways. slip, twinning, g.b. sliding, migration, diffusion Multislip + rotation. Ⅲ.Internal stress (residual stress) Definition: Formation: Classification: Baushinger effect: Previous extension will increase the strength in subsequent extension while decrease the strength in subsequent compression. Ⅳ.Strain hardening (or work- hardening) The increase of strength of materials with strain is called strain hardening. It is shown by curve. Hall-Petch formula: Where and k are material constant. d: The size of grains. (diameter) Ⅴ.Morphology Band structure. Orange peal structure due to the different in account of deformation between grain center and grain boundary. Fibre structure (mechanical texture). In plastic deformation, both the grains and the impurities or second phases may be elongated along the principal direction of flow, forming fibre structure. Ⅵ.Preferred orientation (crystallographic texture) Definition: If certain crystallographic directions (or planes) of grains in polycrystalline material aligned to a common reference directions (or planes) the material is said to have preferred orientation. Description By indicating the ideal position u v w or {h k l} By pole figure Classification: Deformation texture