Tailoring hardness and deformation slip mechanisms in Hf-Ta-C

Transition metal carbides (TMCs) compose a class of ultra-high temperature ceramics characterized by their exceptionally high melting temperatures and hardness. The most refractory of these carbides are B1 phase HfC and TaC with melting temperatures in excess of 3800 ˚C. In low temperature indentation studies, HfC has been shown to slip on the {110} planes whereas TaC slips on {111}. This difference has been contributed to an intrinsic stacking fault present in TaC and absent in HfC. In the present work, we have expanded those studies to investigate how alloying the B1 crystal structure with mixtures of Ta and Hf concentrations can alter the slip behavior as well as the hardness. The experimental slip systems were quantified by dynamical electron diffraction. The results of which were then compared to a series of density functional theory (DFT) calculations of the stability of the intrinsic stacking fault. Furthermore, the elastic constants of the mixed transition metal carbides were computed and the theoretical hardness was compared to experimental values. We noted the highest hardness was predicted for the Hf3TaC4 composition. Finally, we will discuss the asymmetry of transition metal mixing of TaC and HfC powders to produce a single phase B1 structure. The Hf-rich compositions were found to be more difficult to yield this single B1 phase because of the associated higher metal vacancy formation energy. Please click Additional Files below to see the full abstract