DEFORMATION PROPERTIES OF NIOBIUM SINGLE CRYSTALS

Abstract

Vacuum-purified niobium crystals of various orientations have been tested in tension and compression, mainly at room temperature and 158 °K. The observed slip planes were almost always of the {101} or {211} type, and the asymmetry of the stress for slip on {211} has been confirmed at both temperatures. The critical resolved shear stress was approximately constant at room temperature, but a strong orientation dependence of both the macroyield and the microyield stress was found at 158 °K. Three-stage work-hardening curves were found for both the {101} and {211} single-slip orientations at room temperature; there was also a pronounced stage 0 and large overshoot for {101} slip, but not for {211} slip.Dislocation distributions in stage I consisted mainly of long slip-plane clusters containing edge dislocations, and in stage II complex, densely packed obstacles built up into a cell structure. The secondary dislocation density was very high in stage II. After deformation at 158 °K, the structure contained a fairly uniform distribution of screw dislocations and small loops.Current ideas on the effects of impurities, the origin of the friction stress, the significance of asymmetry, and the orientation dependence are reviewed.