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

Mo-9Si-18B alloy With a T-2/Mo-ss 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 Cl1(b) structure. Upon heating to temperatures above 1500 degreesC, the deposited layer is transformed into B-doped Mo5Si3 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 degreesC. The steady-state oxidation rates at 1300 and 1500 degreesC are 1.6 x 10(-3) and 2.6 x 10(-2) mg(2) cm(-4) h(-1), respectively. Those at both temperatures are almost equal to those of MoSi2. The weight loss after isothermal oxidation at 1500 degreesC for 24 h and 26 x 1h cyclic oxidation was about -0.4 mg/cm(2). No significant increase in weight loss was observed during 26 x 1 h cycles. In long-term cyclic testing (50 h x 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 indegrees suppressing high-temperature oxidation of Mo-9Si-18B alloy. (C) 2003 Elsevier Ltd. All rights reserved.