Effect of Off-Stoichiometry on Elastic Modulus of TiC Phase in Mo-TiC Ternary System

MoSiBTiC alloy is a promising material for advanced aerospace applications and next generation high pressure turbine blades in jet engines and gas turbines. It mainly consists of Mo solid solution, TiC and Mo₅SiB₂ phases and has creep strength much stronger than Ni-base superalloys and better than SiC/SiC ceramic matrix composites. Furthermore, the fracture toughness of the alloy is much better (>15 MPa(m)^1/2) than Mo-Si-B ternary alloys (<10 MPa(m)^1/2) even if the volume fraction of Mo solid solution is less than 50 %. The improvement of fracture toughness would be caused not only by the continuity of Mo solid solution in solidification microstructure but also by TiC phase affecting as a fracture-resistant phase. In order to understand the microstructure evolution during solidification and the effect of TiC phase on the fracture toughness of the MoSiBTiC alloy, Mo-Ti-C ternary model alloys are dealt with in this study. Then, (1) liquidus surface projection and (2) isothermal section and the elastic moduli of TiC phase in equilibrium with Mo solid solution were focused on. The obtained liquidus surface projection suggests that the ternary transition peritectic reaction (L+ Mo₂C->Mo+TiC) takes place in Mo-rich region. At 1800 °C, TiC phase in equilibrium with Mo phase contains at least 20.2 at% Mo and the Mo/TiC/Mo₂C three phase region should exist around Mo-15Ti-10C.