Effects of impurities on stability of TiC, TaC and ZrC particles in tungsten

Titanium carbide (TiC), tantalum carbide (TaC) and zirconium carbide (ZrC) in form of particles are widely added in W to improve its mechanical and anti-irradiation properties. However, TiC, TaC and ZrC particles are decomposed to Ti-C-O, Ta-C-O and Zr-C-O in W, respectively. To understand the micro-mechanisms, we carry out systematical simulations and find that the presence of impurities such as O, N, and P has great influence on the bond length and bond strength of Ti-C, Ta-C and Zr-C in W. The bond lengths of Ti-C, Ta-C and Zr-C are generally increased when O meets Ti-C, Ta-C and Zr-C. The bond strength of O to Ti-C, Ta-C and Zr-C is much larger than that of C to Ti-O, Ta-O and Zr-O. In contrast, N has little effect on the Ti-C bond, but has great influence on the Ta-C bond. P significantly increases Ta-C bond length, while its influence on Zr-C bond is negligible. The kinetics calculations elucidate that the diffusion barriers of C away from TiO are significantly increased by about 1 eV. It is very difficult for C to escape from TiO and ZrO, and it is also extremely difficult for O to escape from TiC, TaC and ZrC. This may be the reason why Ti-C-O, Ta-C-O, and Zr-C-O particles are found in W when TiC TaC and ZrC compounds are dispersed in W.