The temperature dependence of microhardness of the transition-metal carbides

Microhardness as a function of temperature has been measured for single crystals of TiC_0.96, ZrC_0.94, and NbC_0.87 from room temperature through their ductile-brittle transitions and for single crystals of V₆C₅ through the order-disorder transition. The present data confirm the result observed for polycrystalline specimens that for T<0.5 T_m, the decrease in hardness with increasing temperature for the carbides is substantially more rapid than for metals and semiconductors. Above the ductile-brittle transitions, the ratios of hardness to yield stress are factors of 1.5 to 3 larger than calculated from theory. We suggest that the larger values result from the higher strainrates and larger strains involved in the hardness tests. Arrhenius plots of the hardness data for T>0.25 T_m are linear and exhibit changes in slope near 0.33 and 0.44 T_m. The former temperature is near the ductile-brittle transition temperature for the disordered carbides, and the latter is at the temperature for which a similar change in Arrhenius slope occurs in published yield-stress data. In addition, a change in slope is found for VC at 0.48 T_m (1275 °C) which is the order-disorder transition temperature for V₆C₅→VC_0.84. These changes in slope are suggestive of corresponding changes in the thermally-activated processes governing plastic flow.