Plastic deformation and dislocation arrangement of Nb[sbnd]34 at.% Ta alloy single crystals

Abstract

Nb₆₆ Ta₃₄ alloy single crystals having orientations in the central part of the stereographic triangle and an interstitial impurity content of less than 10p.p.m. (C + N + O) have been deformed in tension at 77, 195, 295, 330 or 470 K. The dislocation behaviour and arrangement have been studied by slip-line observations, Laue X-ray back-reflection, X-ray topography and transmission electron microscopy (TEM). The slip-line investigations indicate that at each deformation temperature the plane that carries the maximum resolved shear stress (MRSS) is the active macroscopic slip plane. At low deformation temperatures (i.e. 77 and 195 K) the work-hardening curves are parabolic and the dislocation arrangement consists of primary and secondary screw dislocations and of tangles of non-screw dislocations. At intermediate temperatures (i.e. 295, 330 and 470 K) the work-hardening curves resemble that of f.c.c. crystals with an extended stage I. However, the experimental findings suggest that a gradual transition occurs from stage I into stage III and that the characteristic features of stage II are missing. In stage I, primary edge dislocations are predominantly observed. In stage III the dislocation arrangement consists mainly of dislocation sheets almost parallel to the primary slip plane. These dislocation sheets are crossed grids of primary and secondary dislocations. In spite of the different dislocation arrangements, the slip processes in stage I and stage III seem to be essentially the same. In both cases the macroscopic slip proceeds by glide of primary dislocations over large slip distances.