Back stress exerted by obstacle on leading dislocation. (It is supposed to be a short-range-force.) nrbdx =tpbdx dx virtual displacement

By cottrell atmosphere model, we can explain the sharp yielding phenomenon

5.16 Dislocation in FCC crystals Perfect dislocation b =na, +nb. +nC

Introduction 1. Definition: Fis a fictitious force which is supposed to be responsible for the motion of dislocation. Fll Criterion: The work done on crystal by stress must be equal to that done on dislocation by force F. 2. Principle of derivation of F: Macroscopic work=microscopic work (or deformation work)

5. 8 The Stress Around a Dislocation 1. Screw dislocation: Displacement:

Discrepancies between theory and practice in Deformation. 1. Morphology of the surface in deformation. 2. The great difference between theoretical and actual strength of single crystals (. >>)

Edge dislocation 1. Slip (glide) Dislocation motion vs. atom movement. Direction:l⊥讠 Plane:lb(l× Dislocation motion vs. crystal movement. → crystal movement→(dislocation motion)

1. Classification y Ideal or perfect crystal real or defect crystal According to dimension o point defect: vacancies, interstitial atoms s line defect dislocation planar defect: stacking fault o volume defect mosaic structure

4.9 Comparison Between Slip and Twinning Similar ity Both slip and twinning are shear process under shear stress T I. Differences 1. Orientation 2. Atom movement 3.y,△ a) Structure: Twinning is easy 4. Conditions b) Temperature da d c) Strain rate (c

4. Rule for elongation or reduction of R D All R.D. in the region where K1 and K2 makes an obtuse(acute) angle will increase (reduce) its length after twinning