清华大学：《材料科学基础》课程教学资源（PPT课件讲稿）Chapter 6.1 Introduction

Chapter y Diffusion in Solids Lesson twenty-sIX

§6.1 ntroduction Atom movement in solids mechanical movement thermal vibration (short-range migration) Thermal motion diffusion(long-range migration)

mechanical movement thermal vibration (short-range migration) Thermal motion diffusion (long-range migration)

surface diffusion evaporation solidification volume diffusion

evaporation & solidification surface diffusion volume diffusion

e. Classification of diffus ion in solids 1. According to Steady state diffusion and Non-steady state diffusion

Steady state diffusion and Non-steady state diffusion ( )  C

aC Gp. =0. self-diffusion. both in pure metal and in homogeneous alloy aC dr+0. inter-diffusion, down-hill up-hil

( )  0   x C ( )  0   x C

3. According to diffusion path o Volume diffusion (with grain, or lattice) (2 Grain boundary diffusion ③ Surface diffusion short-cut diffusion a diffusion along dislocation 4. According to structure Diffusion in single phase Diffusion in multiphase

① Volume diffusion (with grain, or lattice) ② Grain boundary diffusion ③ Surface diffusion short-cut diffusion ④ diffusion along dislocation Diffusion in single phase Diffusion in multiphase

lll Significance heat treatment, welding, sintering, corrosion IV. Content phenomenological theory I microscopic theory

heat treatment , welding , sintering , corrosion phenomenological theory microscopic theory

§6.2 Fick s Laws 1.Fick’ s first aw 界面 C1,均为固定值 For binary; single-phase system△m∝ aC A△t OX lim An__DaC A diffusion flow dt m D diffusion flux Adt

For binary, single-phase system A t x C m      diffusion flux d d lim diffusion flow d d 0 — — x C D A t m J A x C D t m t m t               界面 x C2 C1 C2 > C1，均为固定值

B 0●●●00 OOOOC 8888°88888 Solution B 80 ● OOOOOOOOO u 860 40 ●o●● OOOOO ●oo●oOO ●o●o●o● OOOOOO 20 Solution 0 ●o●0o●o 30-20-10010203040 ●●O O●o●●O r. um ●0●●ooo●0o● Original cu-Ni interface ●o●●●o●o●o●

/ flux of atoms across plane with area, A dc D x For 3 dimensional diffusion: J=-Dgradc=-DVC D=Do expl RT

J  DgradC  DC         RT Q D D0 exp