Kinking in niobium

Abstract

Single crystals of niobium with a longitudinal axis near to [111] were strained in tension at 78 K and at the yield point rapid localised plastic deformation occurred accompanied by a large drop in load. Surface examination indicated that kinking had occurred and the crystallographic axes had rotated by ∼25° relative to the matrix crystal. The plane of the 'sharp' kink/matrix interface bisected the angle between the slip traces in the kink and those in the matrix. The kink had the form of a macroscopic shear band. In some cases, but not in all, there was an associated coarse slip band. Macroscopic crystallographic traces of the coarse slip band indicated that the slip system appeared to be (211)[-111] but, in fact, the band comprised intense aggregates of fine slip on {101} planes. A dislocation dynamics model of kinking is presented that accounts for the observed upper yield stress, yield drop, and the observed rotations of the kinked crystal and its interface with the matrix. Finally the effect of adiabatic heating of the kink and its effect on the stability of deformation is considered. © 2002 IoM Communications Ltd.