Vikas Behrani
Published: 2004
Total Pages: 164
Get eBook
Microstructure and oxidation of M-Si-B alloys, where M=Nb, Mo and (Nb, Mo) with phase assemblies T1(Mo5Si3B[subscript x])-MoSi2-MoB, T1T2(Mo5SiB2)- Mo3Si, Mo- T2- Mo3Si in Mo-Si-B system, T2 (Nb5(Si, B)3), D88(Nb5Si3B[subscript x]) in Nb-Si-B system, and T1-T2-D88 in (Nb, Mo)-Si-B system were investigated. In Mo-Si-B compositions, alloys showed excellent oxidation stability and initial mass loss of alloy varied according to Mo content. Nb-Si-B and Nb-Mo-Si-B alloys displayed large parabolic rate constants (in the range of 0.5-120 mgs2/cm4.hr) indicating that these systems are not as oxidatively stable as Mo-Si-B alloys. Oxidation kinetics was significantly dependent on initial heating atmosphere. In the Nb-Si-B system T2 and D88 alloys were more resistant to oxidation when heated to test temperature in pure argon. Quaternary Nb-Mo-Si-B alloy containing less D88 phase was more oxidation resistant than that containing more D88 phase. Scales on the order of 20-80 [mu]m were observed on Mo-Si-B alloys and relatively thicker scales (on the order of 200-600 [mu]m) were observed on Nb-Si-B and Nb-Mo-Si-B alloys. Initial heating in argon resulted in denser scale and reduced the parabolic rate constants of ternary alloys by [difference]17-22% and quaternary compositions by [difference]30-40%. 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, leaving an oxidation resistant borosilicate glassy scale. Nb2O5 persists as a rapidly growing condensed phase that overwhelms the ability of the borosilicate glass to form a protective layer. A novel chlorination processing was employed to selectively remove Nb2O5 from the scale of the quaternary alloy as volatile NbCl5 from the scale of alloy comprised of a three phase microstructure of (Nb, Mo)5Si3B[subscript x](T1)-(Nb, Mo)5(Si, B)3(T2)-(Nb, Mo)5Si3B[subscript x](D88). Results show that Nb2O5 can be selectively removed from the scale to leave a borosilicate rich scale. The chlorinated scale forms a dense scale after heat treatment at 1000°C in argon. Oxidation rate of the chlorinated alloys was about one-third that of the unchlorinated alloy under identical conditions. Alloy oxidation during heating to test temperature has been studied and a plausible mechanism for formation of porosity in oxide scale has been offered.