Dislocation: dynamics, interactions and plasticity

1. Slip systems in bcc/fcc/hcp metals • Dislocation dynamics: cross-slip, climb • Interaction of dislocations • Intersection of dislocations Dislocation: dynamics, interactions and plasticity

2. Edge/screw/mixed dislocations? • Screw: Burgers vector parallel to the dislocation line. • Edge: Burgers vector normal to the dislocation line.

3. b=n1xn2= (111)x( ) = n=(111) n=( ) n=(111) b u Dislocation dynamics Edge Screw Slip Direction || to b || to b  between line and b  || Line movement rel. to b ||  How can disloc. leave slip plane climb cross-slip Climb: diffusion controlled. Important mechanism in creep.

4. Slip systems in crystals {321} {110} {211} • BCC • FCC • HCP <111> Fe, Mo, W, Na Fe, Mo, W,  brass Fe, K {111} <110> <11-20>{0001} <11-20>(10-10) <11-20>(10-11)

5. b Superdislocation and partial dislocations Motion of partials b Separation of partials Superdislocations in ordered material are connected by APB Partial Dislocations b = b1 + b2 If energy is favorable, Gb2 > Gb12 + Gb22 then partial dislocation form. ( Ga2/2 > Ga2/3)

6. b Sessile dislocation in fcc Lormerlock Lormer-Cottrell lock motion motion n n=(001) motion motion Unless lock (sessile dislocation) is removed, dislocation on same plane cannot move past.

7. Sessile dislocation in bcc [001] is not a close-packed direction -> brittle fracture

8. Edge dislocation stress field y=x y=–x

9. Edge dislocations interaction edges dislocations with identical b attractive repulsive X=Y Stable at X=0 for identical b; Stable at X=Y for opposite b.

11. Edge dislocations interaction (general case) For an edge dislocations

12. 2 1 r b1 Screw dislocations interaction Example: two attracting screws u(1)= (001) =u(2) b(1)= (001)b = –b(2) radial force b1

13. before b1e b2e after b2e b1e Edge-Edge Interactions: creates edge jogs **Dislocations each acquire a jog equal to the component of the other dislocation’s Burger’s vector that is normal to its own slip plane. This dislocation got a jog in direction of b1e. Dislocation 1 got a “jog” in direction of b2e of the other dislocation; thus, it got longer. Extra atoms in half-plane increases length.

14. Dislocation intersection Edge jog on the edge Edge kink on the screw Edge jogs on screws Interaction of two edges with parallel b Two screw kinks (screw)