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

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

Dislocation dynamics EdgeScrew Slip Direction|| to b|| to b  between line and b  || Line movement rel. to b||  How can disloc. leave slip planeclimb cross-slip n=( ) n=(111) b u b=n 1 xn 2 = (111)x ( ) = Climb: diffusion controlled. Important mechanism in creep.

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

b Superdislocation and partial dislocations Superdislocations in ordered material are connected by APB b Motio n of partial s Separation of partials Partial Dislocations b = b 1 + b 2 If energy is favorable, Gb 2 > Gb Gb 2 2 then partial dislocation form. ( Ga 2 /2 > Ga 2 /3)

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

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

Edge dislocation stress field y=x y=–x

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

Edge dislocations interaction (general case) For an edge dislocations

Screw dislocations interaction Example: two attracting screws u (1) = (001) =u (2) b (1) = (001)b = –b (2) b1b1 radial force b1b1 r 1 2

Edge-Edge Interactions: creates edge jogs Dislocation 1 got a “jog” in direction of b 2e of the other dislocation; thus, it got longer. Extra atoms in half-plane increases length. This dislocation got a jog in direction of b 1e. after b 1e b 2e before b 1e b 2e **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.

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