Chlorination treatment to improve the oxidation resistance of Nb-Mo-Si-B alloys

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. MoO₃ evaporates from the surface scale at about 650 °C, leaving a porous borosilicate glassy scale. Nb₂O₅ 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 Nb₂O₅ from the scale of the quaternary alloy as volatile NbCl₅. A Nb-Mo-Si-B alloy was studied with a nominal composition of 63(Nb,Mo)-30Si-7B (at. pct) with Nb/Mo = 1:1. The alloy consisted of a three-phase microstructure of (Nb,Mo)₅Si₃B _x (T1)-(Nb,Mo)₅(Si,B)₃ (T2)-(Nb,Mo)₅Si₃B _x (D8₈). The oxidation behavior of these alloys in air was studied both before and after chlorination. Results showed that Nb₂O₅ 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. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.