Growth of deformation twins in zinc crystals

Abstract

The incorporation of slip dislocations at the coherent twin boundary has been analysed for the (112) [111] type twins in hexagonal close-packed zinc crystal by applying matrix algebra. The effects of the various dislocation incorporation processes on the growth and untwinning of the existing twin are discussed from the geometric and the energetic points of view. The results indicate that for the (112) [111] twin a basal screw dislocation in the matrix with [110] Burgers vector may cross glide onto the basal plane in the twin without causing the twin either to grow or to shrink, whereas a pair of [100] mixed dislocations of the basal slip system or a [111] mixed dislocation of the second-order pyramidal slip system can be incorporated only into the corresponding {100} first-order prism plane in the twin leaving a double twin dislocation at the coherent twin boundary. Other second-order pyramidal slip dislocations would have to be incorporated into high index planes of the twin to become sessile dislocations with high index Burgers vectors. A mechanism is proposed for the growth and untwinning based on the movement of the twin dislocations left at the coherent twin boundary under the shear stress acting on them.