Deformation of high purity tantalum single crystals at 4.2 K

Abstract

High purity Ta single crystals of various orientations were deformed in tension and compression at 4.2 K. The expected (011) slip was the dominant mode of deformation in tension, except for orientations near [011], where twinning and some anomalous ( 1 01) slip were observed. In tension, stress-strain curves were always serrated owing to the occurrence of adiabatic slip. In compression, the dominant mode of deformation was twinning; slip was only observed to a small extent for orientations near [001]. The occurrence of adiabatic slip in tension obscured any possible effects of normal stress on the critical resolved shear stress for slip. However, a pronounced increase of twinning stress with increasing normal stress was observed in compression. This can be explained in terms of the effect of normal stress on the nucleation of twins by the transformation of the screw dislocation core into a three-layer twin configuration. The addition of up to 4% Re to Ta eliminated twinning at 77K, and promoted the occurrence of slip at 4.2 K for orientations displaying only twinning in unalloyed specimens. This effect is most probably related to effects of solute atoms on the extended screw dislocation core.