Phase Equilibria, Microstructure, and High-Temperature Strength of TiC-Added Mo-Si-B Alloys

TiC was added to Mo-Si-B alloys using a conventional Ar arc-melting technique, and the phase equilibria, microstructure evolution, and high-temperature strength at 1673 K (1400 A degrees C) were investigated. The primary phase changed to Mo solid solution (Mo-ss), Mo5SiB2 (T-2), or TiC depending on the composition. Following the primary phase solidification, a Mo-ss + TiC, Mo-ss + T-2, or Mo-ss + T-2 + TiC + Mo2C eutectic reaction took place as the secondary solidification step. In some alloys, Mo-ss + T-2 + TiC and Mo-ss + T-2 + Mo2C eutectic reactions were present as higher-order solidification steps. After annealing at 2073 K (1800 A degrees C) for 24 hours, Mo-ss, T-2, TiC, and Mo2C coexisted stably with microstructural coarsening. The coarsening rate was much faster in an alloy with no TiC dispersion, suggesting that TiC has a strong pinning effect on the grain boundary and interface migration. Compression tests conducted at 1673 K (1400 A degrees C) revealed strength properties of almost all the alloys that were better than those of the Mo-Hf-C alloy (MHC). Alloy densities were 9 g/cm(3) or less, which is lighter than pure Mo and MHC (a parts per thousand yen10 g/cm(3)) and competitive with Ni-base superalloys. TiC-added Mo-Si-B alloys are promising candidates for ultrahigh-temperature materials beyond Ni-base superalloys.