No abstract

Multiphase composite alloys based on the Mo-Si-B system are candidate materials for ultra-high temperature applications. In non load-bearing uses such as thermal barrier coatings or heat exchangers in fossil fuel burners, these materials may be ideally suited. The present work investigated the effect of water vapor on the oxidation behavior of Mo-Si-B phase assemblages. Three alloys were studied: Alloy 1 = Mo 5 Si 3

Recent studies have shown that the quaternary Nb-Mo-Si-B system is not oxidation resistant. The difference in oxidation resistance between Mo-Si-B and Nb-Mo-Si-B may be interpreted in terms of the volatility of the metal oxide that forms. MoO 3 evaporates from the surface scale at about 650 °C, leaving a porous borosilicate glassy scale. Nb 2 O 5 persists as a rapidly growing condensed phase that overwhelms the ability of the borosilicate glass to form a protective layer. In the present work, a novel chlorination process was employed to selectively remove Nb 2 O 5 from the scale of the quaternary alloy as volatile NbCl 5 . A Nb-Mo-Si-B alloy was studied with a nominal composition of 63(Nb,Mo)-30Si-7B (at. pct) with Nb/Mo ϭ 1:1. The alloy consisted of a three-phase microstructure of (Nb,Mo) 5 Si 3 B x (T1)-(Nb,Mo) 5 (Si,B) 3 (T2)-(Nb,Mo) 5 Si 3 B x (D8 8 ). The oxidation behavior of these alloys in air was studied both before and after chlorination. Results showed that Nb 2 O 5 can be selectively removed from the scale to leave a borosilicate-rich scale, which then forms a dense scale after heat treatment at 1100°C in argon. The oxidation rate of the chlorinated alloy was about one-third that of the unchlorinated alloy under identical conditions. Alloy oxidation during heating to the test temperature was studied, and a plausible mechanism for the formation of porosity in the oxide scale has been offered.