Early melting of tantalum carbide under anisotropic stresses: An ab initio molecular dynamics study

Tantalum carbide (TaC) is an ultrahigh-temperature ceramic binary compound with the highest melting temperature $(3770{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C})$ of its analogues. Combined with high chemical stability, good thermal conductivity, high hardness, etc., it is identified as an excellent refractory material with wide applications in extremely high temperature environments as thermal protection systems for designing sharp leading edges of hypersonic flying vehicles, thrust nozzles of rockets, tank armors, cutting and drilling tools, etc. In this paper, we present striking results from ab initio molecular dynamics simulations, unveiling that anisotropic stresses under uniaxial compression along the [001] direction can reduce the melting temperature of TaC by as much as $700{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, to within the typical working temperature $(3000{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C})$ in thrust nozzles of rockets. The mechanical strengths of TaC, associated with the peak stresses in full-range stress-strain curves which set the maximum stresses sustainable by TaC under given deformations, are found to be reduced by half as the temperature approaches $3000{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$, displaying a severe deterioration of its plastic mechanical properties at high temperatures in contrast to its elastic properties, represented by the temperature-dependent bulk, Young's, and shear moduli of TaC, which are reduced by about 20% in the same temperature range. These discoveries provide fresh perspectives for understanding the extreme physics of TaC at high temperatures.