Oxidation behavior of Mo–9Si–18B alloy pack-cemented in a Si-base pack mixture

Mo–9Si–18B alloy with a T2/Moss two-phase structure was pack-cemented in a Si-base pack mixture [Si:NaF:Al2O3=5:1:44 (wt.%)] and its oxidation behavior was examined. The deposited layer of as-cemented alloy consists of MoSi2 with the C11b structure. Upon heating to temperatures above 1500 °C, the deposited layer is transformed into B-doped Mo5Si3 with the D8m structure through a reaction between the deposited layer and the matrix containing B. Steady-state oxidation is observed at 1300, 1400 and 1500 °C. The steady-state oxidation rates at 1300 and 1500 °C are 1.6 × 10−3 and 2.6 × 10−2 mg2 cm−4 h−1, respectively. Those at both temperatures are almost equal to those of MoSi2. The weight loss after isothermal oxidation at 1500 °C for 24 h and 26 × 1h cyclic oxidation was about −0.4 mg/cm2. No significant increase in weight loss was observed during 26 × 1 h cycles. In long-term cyclic testing (50 h × 2 cyclic oxidation), the Mo5Si3 layer is oxidized and a cristobarite layer formed at the expense of Mo5Si3, and thereby a large weight loss occurs in comparison to short-term testing. However, a multi-layer coating spontaneously formed beneath the cristobarite layer, and thus no significant oxidation of the matrix alloy occurred. Oxidation protective coatings spontaneously formed when as-cemented Mo–9Si–18B alloy is exposed to temperatures higher than 1500 °C have great potential in suppressing high-temperature oxidation of Mo–9Si–18B alloy.