Effect of small aluminum additions on microstructure and mechanical properties of molybdenum di-silicide

Abstract

The effect of Al alloying on the microstructure and properties of MoSi 2 was investigated. MoSi 2–2.8 and 5.5 at% Al (1.5 and 3 wt %, respectively) alloys showed a two phase microstructure comprising of α-Al 2O 3 and MoSi 2–Al alloy, while the MoSi 2–9 at% (5 wt %) Al alloy possessed a 3 phase microstructure: MoSi 2–Al alloy, Mo(Si,Al) 2 and α-Al 2O 3. Al additions to MoSi 2 reduced the SiO 2 phase, which is detrimental for high temperature strength of MoSi 2, forming Al 2O 3. The surplus Al entered into solid solution with the MoSi 2 and altered the lattice parameters. The Al in solid solution was 0.5 at% in MoSi 2–2.8 at% Al alloy, 2.5 at% in MoSi 2–5.5 at% Al alloy and 3.1 at% in the MoSi 2–9 at% Al alloy. The fracture toughness underwent only a moderate improvement in MoSi 2–2.8 at% and 5.5 at% Al alloys, but exhibited a significant 49% rise in the MoSi 2–9 at% Al alloy. Replacement of SiO 2 by Al 2O 3 in MoSi 2–Al alloys led to a significant improvement in the high temperature yield strength between 1100°C and 1250°C.