Enhancing the oxidation resistance of MoTiSiB-based composites by the design of multi-principal element BCC solid solution phase

In this study, two ultrahigh-temperature composites incorporating multi-principal element BCC solid solution phases were developed by alloying the MoTiSiB quaternary system with Cr and Al. Both alloys, along with a Cr/Al-free reference one, exhibited the same phase equilibrium (BCC+T 2 +D8 8 ) after high-temperature annealing and shared a similar microstructure with BCC solid solution constituting over 50 vol.%. Oxidation tests revealed that the Cr/Al-free quaternary alloy suffered from catastrophic oxidation (pest phenomenon) at 800°C, though its oxidation resistance improved when the temperature was elevated to 1100°C. In contrast, the introduction of Cr and Al, which promoted the formation of multi-principal element BCC solid solution phases, significantly enhanced the oxidation resistance of the alloys at both temperatures. Notably, the Cr-Al co-added alloy showed the smallest mass change and the slowest oxidation rate attributed to the formation of protective scales such as dense Cr 2 O 3 , 2Al 3 O 4 ·SiO 2 or Al 2 O 3 layers. These results demonstrate that the design of multi-principal element BCC solid solution phases can effectively improve the oxidation resistance of MoTiSiB-based composites without reducing the volume fraction of BCC, thereby avoiding potential degradation in fracture toughness. This approach represents a promising strategy for balancing the trade-off between mechanical properties and oxidation resistance in such ultrahigh-temperature materials.