DEFORMATION TWINNING IN HIGH-HYDROGEN-SOLUBILITY REFRACTORY ALLOY CRYSTALS

Deformation twinning was studied in single crystals of three high-hydr ogen-solubility refractory alloys; Ta-10 at.% Nb, Nb-10 at.% Mo, and N b-10 at.% V. The hydrogen concentration was maintained close to 150 p. p.m. by weight. Four different crystal orientations were tested in uni axial tension and compression; in each case, at four different tempera tures: 77 K, 142 K, 195 K and 300 K. Trace analyses of the deformation markings were carried out on pairs of perpendicular surfaces to deter mine the crystallographic nature of slip, twinning, cleavage and hydri de precipitates. The sense of the deformation twin shear strains confo rms with their occurrence under tensile or compressive stresses for th e different crystal orientations. The activation energy for the nuclea tion of twins is determined by means of dislocation modelling of the t win lamella. It is shown that a stress concentration, either from plan ar dislocation arrays formed during microslip or from a cleavage crack tip, enables the activation energy to be overcome. Different twin mor phologies in two temperature ranges, at 77 K and from 142 K to 195 K, are described. Particular electron microscopy observations of the disl ocation substructures associated with twinned regions in the Nb-10 at. % V alloy are presented.