清华大学：《材料科学基础》课程教学资源（PPT课件讲稿）Chapter 4.9 Comparison Between Slip and Twinning

84.9 Comparison Between Slip and Twinning I. S imi l ar ity Both slip and twinning are shear process under shear stress T ◆‖|. Differences 1. Orientation 2. Atom movement 3.y,△ a)Structure: Twinning is easy 4. Conditions: b) Temperature de d c) Strain rate (or

§4.9 Comparison Between Slip and Twinning ◆ Ⅰ.Similarity: Both slip and twinning are shear process under shear stress . ◆ Ⅱ.Differences: 1. Orientation 2. Atom movement 3. 4. Conditions:   , l a) Structure: Twinning is easy b) Temperature: c) Strain rate ) d d (or d d t t  

5. Surface morphology Etching treatment with an etchant removes slip steps leaving no trace, but reveals twins by delineating the interface between the twin and the matrix 6. Conditions ① Structure: twinning is easy to take place in crystals of low symmetry ② Temperature dy ③ Strain rate dt 7. Twinning plays an essential role in BCC and HCP lattices

5. Surface morphology: Etching treatment with an etchant removes slip steps leaving no trace, but reveals twins by delineating the interface between the twin and the matrix. 6. Conditions: ① Structure: twinning is easy to take place in crystals of low symmetry. ② Temperature ③ Strain rate dt d 7. Twinning plays an essential role in BCC and HCP lattices

34.10 Basic Features of The Plastic Deformation of Polycrystals Basic distinction between single crystal and polycrystals (1 A great deal of grains with different orientation ② Grain boundary G B is a thin layer of material between neighboring grain where atoms arranged in transitional position

§4.10 Basic Features of The Plastic Deformation of Polycrystals Basic distinction between single crystal and polycrystals ① A great deal of grains with different orientation ② Grain boundary G.B. is a thin layer of material between neighboring grain where atoms arranged in transitional position

grain gr rain boundary single crystal polycrystal

single crystal polycrystal grain grain boundary

I Grain boundary and its role i deformation (1 Impede slip(as obstacles)at R.T. and low temperature 2 Coordinate the deformation in different grains 3 Enhance(assist) the plastic deformation at high temperature through grain boundary sliding There exists an Equicohesive Temperature at which grain Ob(g b)=ob(grain T≈0.5T

◆ Ⅰ.Grain boundary and its role in deformation ① Impede slip (as obstacles) at R.T. and low temperature. ② Coordinate the deformation in different grains. ③ Enhance (assist) the plastic deformation at high temperature through grain boundary sliding.  b graing.b. Tet T There exists an Equicohesive Temperature at which  b(g.b.) = b(grain) Tet  0.5Tm

I. Deformation features of pol l o Plastic deformation can take place in various ways slip, twinning, g.b. sliding, migration, diffusion ② Multislip+ rotation

◆ Ⅱ.Deformation features of polycrystal ◆ Plastic deformation can take place in various ways. slip, twinning, g.b. sliding, migration, diffusion ② Multislip + rotation

lll. Internal stress (residual stress) 1 Definition 2 Formation macroscopIc 3. Classification mIcroscopIc Bauschinger effect Previous extension will increase the strength in subsequent extension while decrease the strength in subsequent compression

◆ Ⅲ.Internal stress (residual stress) 1. Definition: 2. Formation: 3. Classification: Baushinger effect: Previous extension will increase the strength in subsequent extension while decrease the strength in subsequent compression. macroscopic microscopic

6 2 ②2⑥软位向,μ大,已发生塑性变形 ①③硬位向,μ小,未发生塑性变形

1 2 3 5 4 6 ②⑥ 软位向，μ大，已发生塑性变形 ①③ 硬位向，μ小，未发生塑性变形

◆Ⅳ. Strain harden ing( or work hardening The increase of strength of materials with strain is called strain hardening. It is shown by o-scurve 2. Hall-Petch formula: 0=0 +kd 2 Where o and k are material constant d: The size of grains(diameter) do hardening rate de

◆ Ⅳ.Strain hardening (or work￾hardening) 1. The increase of strength of materials with strain is called strain hardening. It is shown by curve. 2. Hall-Petch formula: Where and k are material constant. d: The size of grains. (diameter)  − 2 1 − = + k d  ys  o   hardening rate d d  

◆V. Morpho logy 1. Band structure 2. Orange peal structure due to the diferent in account of deformation between grain center and grain boundary 3. 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

◆ Ⅴ.Morphology 1. Band structure. 2. Orange peal structure due to the different in account of deformation between grain center and grain boundary. 3. 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