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

The effect of Al alloying on the microstructure and properties of MoSi2 was investigated. MoSi2-2.8 and 5.5 at% Al (1.5 and 3 wt %, respectively) allo ys showed a two phase microstructure comprising of alpha-Al2O3 and MoSi2-Al alloy, while the MoSi2-9 at% (5 wt %) Al alloy possessed a 3 phase microst ructure: MoSi2-Al alloy, Mo(Si,Al)(2) and alpha-Al2O3. Al additions to MoSi 2 reduced the SiO2 phase, which is detrimental for high temperature strengt h of MoSi2, forming Al2O3. The surplus Al entered into solid solution with the MoSi2 and altered the lattice parameters. The Al in solid solution was 0.5at% in MoSi2-2.8 at% Al alloy, 2.5 at% in MoSi2-5.5 at% Al alloy and 3.1 at% in the MoSi2-9 at% Al alloy. The fracture toughness underwent only a m oderate improvement in MoSi2-2.8 at% and 5.5 at% Al alloys, but exhibited a significant 49% rise in the MoSi2-9 at% Al alloy. Replacement of SiO2 by A l2O3 in MoSi2-Al alloys led to a significant improvement in the high temper ature yield strength between 1100 degrees C and 1250 degrees C. (C) 1999 El sevier Science Ltd. All rights reserved.