Microstructure, room-temperature mechanical properties, and oxidation resistance of Ti-added Mo-Si-B alloys fabricated via hot pressing sintering

Microstructure, room-temperature mechanical properties, and oxidation resistance of Mo-12Si-8B-xTi (x = 10, 20, 30, 40, at.%) alloys fabricated via hot pressing sintering are investigated. 10Ti and 20Ti alloys comprise α-Mo + Mo₃Si + Mo₅SiB₂, whereas 30Ti and 40Ti alloys comprise α-Mo + Mo₅SiB₂ + Ti₅Si₃. CALPHAD thermodynamic modeling and EDS indicate a significant increase in Ti solubility in α-Mo and Mo₅SiB₂ phases with higher Ti content, reaching levels of 30–50%. EBSD analysis reveals evident grain coarsening, the average grain size improves from 1.43 to 2.97 μm. HRTEM characterization demonstrates α-Mo/Mo₅SiB₂ and α-Mo/Ti₅Si₃ interfaces are both incoherent. 10Ti alloy exhibits the highest flexural strength of 454 MPa and fracture toughness of 9.9 MPa‧m^1/2, while minor differences in fracture toughness are observed among 20–40Ti alloys. The combination of adequate α-Mo matrix, appropriate grain size, and brittle intermetallic phases collectively contribute to determining the strength and toughness. Simultaneous thermal analysis and cyclic oxidation show 40Ti alloy offers the best oxidation resistance at 800–1200 °C, because of the protective TiO₂‧SiO₂ layer generated through selective oxidation of Ti₅Si₃ and Ti/Si. Using Pilling-Bedworth ratio as the sole criterion to assess the oxidation resistance is inadequate, as temperature and phase constitution must be considered.