Low-temperature deformation of body-centred cubic metals I. Yield and How stress measurements

Abstract
Measurements of the tensile yield stress and of the temperature and strain-rate sensitivity of the flow stress are reported for single crystals of niobium and for polycrystalline speci­mens of niobium, vanadium and tantalum over the temperature range 4.2 to 373°K. The temperature dependences of yield and flow stresses are nearly identical, and the results show that the high yield stresses at low temperatures are attributable mainly to a frictional force opposing the motion of free dislocations. The yield stress is very dependent on the purity of the metal, and the temperature and strain rate sensitivities vary slightly with purity, especially at higher temperatures. At very low temperatures, the stress needed to cause macroscopic deformation at a strain rate of 10-4s-1isca. 1% of the shear modulus in all specimens examined. The relation of the results to the interpretation of the para­meters in the Hall-Petch equation for the variation of yield stress with grain size is briefly discussed.