DEVELOPMENT OF THE DISLOCATION ARRANGEMENT IN HIGH-PURITY NB-34AT-PERCENT-TA ALLOY SINGLE-CRYSTALS DEFORMED IN TENSION

The deformation behaviour and the dislocation arrangement of high-purity Nb-34 at% Ta alloy single crystals after tensile deformation at 330 K into different stages are studied by slip-line observations, Laue X-ray back-reflection, X-ray topography, and transmission electron microscopy (TEM). In stage I of the work-hardening curve, the dislocation arrangement consists predominantly of primary dislocations, which are clustered into bundles in the form of braids. Fine slip traces on the surface of the specimens show that the slip proceeds homogeneously in the whole volume of the crystal. Stage II, which is a characteristic property of f.c.c. metals, is missing. Instead, the experimental findings suggest that a gradual transition occurs from stage I into stage III. The transition region is characterized by a pronounced formation of dislocation sheets approximately parallel to the primary slip plane. The dislocation sheets are crossed grids of primary and secondary dislocations and occur in pairs. The secondary slip necessary for the formation of the dislocation sheets is found to occur in the direction favoured by the applied shear stress. 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, whereby the plane that carries the maximum resolved shear stress (MRSS) is the active macroscopic slip plane.