Influences of Al content and secondary phase of Mo5(Si,Al)3 on the oxidation resistance of Al-rich Mo(Si,Al)2-base composites

The microstructures and oxidation behavior of Mo(Si,Al)2-base composites were investigated. A high Al content up to 38 mol% and the effect of the secondary Mo5(Si,Al)3 phase were specifically examined. Microstructure observations indicated that the Al solubility limit at 1680 K extends to 43 and 10 mol% for Mo(Si,Al)2 and Mo5(Si,Al)3, respectively. The composites containing about 10 vol.% Mo5(Si,Al)3 exhibits excellent oxidation resistance in the temperature range between 780 and 1780 K, so that the Mo5(Si,Al)3 up to about 10 vol.% is acceptable and has no detrimental influence on the oxidation resistance. At temperatures from 1480 to 1780 K, a continuous alumina layer develops on the Mo5(Si,Al)3 as well as on the Mo(Si,Al)2. The alumina growth rate is nondistinctive in the Al range investigated, and it is controlled by inward oxygen diffusion through the alumina at 1580 K and higher.