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

Abstract

Mo–9Si–18B alloy with a T 2/Mo ss two-phase structure was pack-cemented in a Si-base pack mixture [Si:NaF:Al 2O 3=5:1:44 (wt.%)] and its oxidation behavior was examined. The deposited layer of as-cemented alloy consists of MoSi 2 with the C11 b structure. Upon heating to temperatures above 1500 °C, the deposited layer is transformed into B-doped Mo 5Si 3 with the D8 m 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 mg 2 cm −4 h −1, respectively. Those at both temperatures are almost equal to those of MoSi 2. The weight loss after isothermal oxidation at 1500 °C for 24 h and 26 × 1h cyclic oxidation was about −0.4 mg/cm 2. No significant increase in weight loss was observed during 26 × 1 h cycles. In long-term cyclic testing (50 h × 2 cyclic oxidation), the Mo 5Si 3 layer is oxidized and a cristobarite layer formed at the expense of Mo 5Si 3, 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.