The influence of pre-strain on deformation twinning in niobium single crystals

Abstract

Single crystals of niobium were grown by the electron beam floating zone technique and purified by annealing in ultra high vacua. Crystals orientated for either single or double slip were predeformed at room temperature or 158°K in order to investigate the effects of various dislocation distributions on the twinning behaviour. It was found that pre-strain always raises the twinning stress and that more twins form in prestrained specimens. Pre-strain at 158°K in both double and single slip orientations and at room temperature in double slip orientations, is effective in suppressing twinning at temperatures down to 77°K, but not at 55°K and below. The twinning stress continues to increase slightly with decreasing temperature down to 20°K and there is evidence that twinning is always accompanied by some slip. In very pure pre-strained crystals, twinning is not restored by annealing. It is concluded that a homogeneous distribution of dislocations is the main requirement for effective twin suppression and that dislocations which intersect the twinning plane are more effective than those which are parallel to it. A possible nucleation mechanism is the dissociation of screw dislocations, formed by micro-slip, at or below the macroscopic yield stress at which these dislocations move.