Twinning and the low temperature mechanical properties of polycrystalline niobium and molybdenum

Abstract

The mechanical properties of high-purity niobium and molybdenum specimens have been investigated over a range of grain sizes from 2O°C to − 269°C. Interpretation of the load-elongation curves, in compression and tension, along with detailed metallographic investigation has led to the following conclusions: 1. (i) twinning can occur more readily in coarse-grain than fine-grain material, 2. (ii) twins can occur over the entire range of temperature 20°C to −269°C in molybdenum but only in the range −150° to −269°C in niobium at g3 = 4.2 × 10 −4 sec −1, 3. (iii) enhanced stresses ahead of a crack can cause twinning but there is no definite evidence to show that twins cause cracking. The results have been discussed by interpreting the effect of grain size in terms of dislocation density, the effect of temperature and strain rate in terms of dislocation mobility and the advent of twinning in terms of a model independent hypothesis based on simple considerations of the dissipation of energy by various deformation mechanisms.