THE DEFORMATION OF NIOBIUM SINGLE CRYSTALS

Various orientations of single crystals of niobium, purified by ultra-high vacuum annealing, have been tested in compression at 295 °K and in tension at temperatures between 77 °K and 295 °K. The shear stress – shear strain curves show three-stage hardening in a manner similar to f.c.c. crystals. Analysis of the orientation dependence of the operative slip system suggests an asymmetry in the critical resolved shear stress for slip on {112} planes which increases with decreasing temperature. Explanations for this in terms of the various ways in which a [Formula: see text] type screw dislocation can dissociate have been proposed. It is found that dissociation on two {112} planes or composite dissociation on {110} and {112} planes leads to a satisfactory qualitative explanation of the experimental results.The dislocation distribution occurring as a function of strain has been studied for crystals of a single glide orientation deformed in tension at 295 °K. [Formula: see text] sections from crystals deformed into stage I contain clusters of primary edge dipoles. The density of secondary dislocations is low (~10%). Sections from crystals deformed into stage II were chosen such that the three-dimensional nature of the dislocation arrays could be investigated. The distribution shows strong similarities to those observed in copper crystals (Steeds 1966), i.e. edge multipole walls, tilt and twist boundaries. The density of secondary dislocations is high, being of the same order as the primary density.