TaC and HfC are recognized as potential ultra-high temperature ceramics for application in refractories and coating materials. However, the test of mechanical property under high temperature is difficult. In this paper, the mechanical properties of TaC and HfC at high temperature are systematically investigated by density functional theory. The results revealed that their elastic constants, elastic moduli and elastic anisotropy all display a monotonic decreasing trend with increasing temperature, and the anisotropy of TaC is stronger than that of HfC at a given temperature. TaC and HfC both have eight possible dislocations. As the applied shear stress increases, the activation energies and critical resolved shear stress (CRSS) of all dislocations are decreased. As the temperature increases, the CRSS is decreased. Moreover, the yield strength and hardness of TaC and HfC both decreased with the increase of temperature. The hardness decrease from 0 to 400 K is mainly due to the proportion change of the two major dislocations (1/2 0 degrees perfect dislocations and (1/6 30 degrees partial dislocations). Then the hardness sharply decreases at 400 K, which is attributed to the activation of (1/6 90 degrees partial dislocations. The hardness decrease from 500 K is attributed to the CRSS change of (1/6 90 degrees partial dislocations and (1/6 30 degrees partial dislocations. In addition, the influence of strain rate and dislocation density are revealed, which shows that their hardness are both increased with increasing strain rate, while decreased with increasing dislocation density.
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