清华大学：《材料科学基础》课程教学资源（PPT课件讲稿）Chapter 6.7 Reaction Di ffusion Diffusion n Mul tiple Phase Zone

36.7 Reaction Diffusion Diffusion in Multiple Phase Zone I Course of reaction di ffus ion diffusion phase change

§6.7 Reaction Diffusion (Diffusion in Multiple Phase Zone) ◆ Ⅰ. Course of reaction diffusion diffusion phase change

IB C C +d (a) (b) (c) y相变为a相

1. There have no two-phases area in diffusion area, every layer is single-phases 2. Phase' s interface moves according to parabola law 3. Phase's width

1. There have no two-phases area in diffusion area, every layer is single-phases. 2. Phase’s interface moves according to parabola law. 3. Phase’s width

Ⅱ. Basic features: 1. Diffusion is accompanied by reaction or phase transformation. In general, diffusion is rate-determining step while reaction take place instantaneously 2. For diffusion in binary systems there appear only a series of single phase, one adjacent to another, while mixed phase zone doesnt appear 3. Concentrating changes abruptly at phase boundary 4. The phase boundaries and the width of each phase change parabolic ally with time

Ⅱ.Basic features: 1. Diffusion is accompanied by reaction or phase transformation. In general, diffusion is rate-determining step while reaction take place instantaneously. 2. For diffusion in binary systems there appear only a series of single phase, one adjacent to another, while mixed phase zone doesn’t appear. 3. Concentrating changes abruptly at phase boundary. 4. The phase boundaries and the width of each phase change parabolic ally with time

ll Reaction Di ffus i on dynamics 1. Speed of interface aC C、 (DO)Ba(D.aB dt =(CBa-CaB)dx OX aC dx (D) aB-(d)Ba ax dt Ba B xX let dc OX d

◆ Ⅲ.Reaction Diffusion dynamics ( ) ( ) d = ( − )d 1         − −   − t C C x x C D x C D                 d 1 d let ( ) ( ) d d C x t C t x C C x C D x C D t x =   = −   −    = 1. Speed of interface

dc dc (D DaB(d) dt ap dt K(C) dt B B x=√·K'(C) 2 Phase width W B" rk dwo d MBa dx dt d dt W.=Bvt B

W x x B t j = j, j+1 − j−1, j = j      W = x − x t A t x t x t W      = − = d d d d d d W B t  =  ( ) 1 ( ) 1 ) d d ) ( d d ( d d x t K C t K C C C t t C D t C D t x  =    = − −  =         2. Phase width

3. Rule of phase appearing ,F==c (a) (b)

3. Rule of phase appearing

◆Ⅳ. Discuss ion 1. Phases that can appear 2. Growth law: in general W2=K(C)t n special cases: x"=K(C)t n=1 (1)is tenable in volume(lattice) diffusion factors infiuencing growth a. Diffusion path(lattice grain boundaries pore surface) b. Relative rate of diffusion and reaction

◆ Ⅳ.Discussion 1. Phases that can appear 2. Growth law: in general in special cases: ～ (1) is tenable in volume (lattice) diffusion ( ) (1) 2 W = K C t x = K(C)t n =1 4 n factors influencing growth: a. Diffusion path (lattice grain boundaries pore surface). b. Relative rate of diffusion and reaction

Examples and Discussions 1.F-N 900 800 (y-Fe) (b) 700 680±5℃ 2.8t650℃ 密 Cy a-Fe) 4.55 喱600 590℃ 500 150 5.7% 6.1% 400 Fe 1 6 8C0910 %

Examples and Discussions 1. F-N

2.F -C a y+FeaC 相区界面 T C la+Fe3C C /%

2. F-C