Deformation twinning in high-hydrogen-solubility refractory alloy crystals

Abstract

Deformation twinning was studied in single crystals of three high-hydrogen-solubility refractory alloys; Ta-10 at.% Nb, Nb-10 at.% Mo, and Nb-10 at.% V. The hydrogen concentration was maintained close to 150 p.p.m. by weight. Four different crystal orientations were tested in uniaxial tension and compression; in each case, at four different temperatures: 77K, 142K, 195K and 300K. Trace analyses of the deformation markings were carried out on pairs of perpendicular surfaces to determine the crystallographic nature of slip, twinning, cleavage and hydride precipitates. The sense of the deformation twin shear strains conforms with their occurrence under tensile or compressive stresses for the different crystal orientations. The activation energy for the nucleation of twins is determined by means of dislocation modelling of the twin lamella. It is shown that a stress concentration, either from planar dislocation arrays formed during microslip or from a cleavage crack tip, enables the activation energy to be overcome. Different twin morphologies in two temperature ranges, at 77K and from 142K to 195K, are described. Particular electron microscopy observations of the dislocation substructures associated with twinned regions in the Nb-10 at.% V alloy are presented.