ANOMALOUS TEMPERATURE-DEPENDENCE OF THE YIELD STRENGTH IN IVA-VIII INTERMETALLIC COMPOUNDS WITH B2 STRUCTURE

The mechanical properties of the binary compound CoHf, and the pseudo-binary compounds CoTi-CoZr and CoTi-NiTi with B2 structure were measured as functions of testing temperature, composition and strain rate. In all of investigated compounds, the anomalous (positive) temperature dependence of the yield strength was observed at intermediate temperatures. The yield strength of CoHf compounds was dependent on composition through the contained second phase particles. The pseudo-binary compounds CoTi-CoZr were strengthened by a mutual solid solution of Ti and Zr, due to the atomic size difference between two atoms. The pseudo-binary compounds CoTi-NiTi were, by a mutual solid solution of Co and Ni, strengthened at low temperatures but softened at intermediate and high temperatures. A positive strain rate dependence of the yield strength was clearly found at high temperatures in CoHf compounds. Very high strain-hardening rates were observed at intermediate temperatures, usually accompanied by serrations on the stress-strain curves. Based on these results, it was proposed that the intermetallic compounds with B2 structure consisting of IVa and VIII transition metals universally exhibit the anomalous temperature dependence of the strength and therefore involve the similar deformation mechanism. Also, a possible deformation mechanism was considered in each temperature region.