AbstractsEngineering

High purity vanadium, both in the polycrystalline and monocrystalline form, was strained in tension in order to study the controlling deformation mechanism at temperatures between 77 and 293°K. The temperature and strain rate dependence of the flow stress for two grades of material, 99.99% and 99.93%, was evaluated using temperature change and strain rate change procedures. An abrupt change of yield and flow stresses, strain rate sensitivity, activation enthalpy and volume at approximately 200°K indicates that a change of rate controlling mechanism occurs at this temperature. It was found that at temperatures below 200°K, the activation enthalpy is less than 1 ev. and the activation volume is less than 30 b³. A high strain hardening rate in the early stages of straining and an orientation dependence of flow stress were also noticed. Among all possible controlling activation processes, the pseudo- Peierls mechanism, i.e, the slightly dissociated screw dislocation model, agrees best with the observations. In the temperature range from 200 to 293°K, the yield and flow stresses of vanadium were remarkably affected by the level of impurity content. On the basis of this observation, the controlling process in this temperature range is most probably the interaction between dislocations and impurities. Mechanical twins were formed on single crystals at 77 and 120°K; twins occur on {211} planes. Slip lines could not in general be identified, but were noted in one test. The slip plane determined on a single crystal with <110> axis deformed at 160°K was {211} . Single crystals generally showed a chisel edge type of ductile fracture.