Fracture toughness of Mo–Si–B-quartz particulate composites

Abstract

Multiphase Mo–Si–B alloys are potential candidates for high temperature applications owing to their favorable high temperature strength, creep and acceptable oxidation resistance. The inherent brittle nature of these alloys coupled with their low fracture toughness (<8 MPa m) below 1000 °C, limits their application in industry. The addition of α-quartz as particulate reinforcement in various multiphase alloys containing MoSS, T2 (Mo5SiB2) and Mo2B resulted in increased fracture toughness values, from ∼6 to 12 MPa m to ∼17–23 MPa m, in the temperature range 200 °C–800 °C. Interrupted fracture toughness testing revealed that markedly higher loads were required to propagate the crack tip as compared to those required for the comparable alloy without quartz. The improvement in fracture toughness behavior is also seen when quartz is added to two-phase alloys containing MoSS and the T2 phase. Tensile tests above 900 °C show that the addition of quartz results in softening and decreased ductility as compared to the base alloy. It is proposed that compressive elastic stresses induced in the composite due to differential thermal expansion of quartz and the Mo–Si–B matrix contribute to the observed increase in fracture toughness of the composites.