Microstructure and oxidation behavior of Nb-Mo-Si-B alloys

The microstructure and oxidation behavior of sintered Nb-Mo-Si-B alloys were investigated for the phase assemblies T, (Mo5Si3Bx)MoSi2-MoB, T-1-T-2 (Mo5SiB2)-Mo3Si, and Mo-T-2-Mo3Si in the Mo-Si-B system. In the Nb-Si-B system, T-2 (Nb-5(Si,B)(3)) and D8(8) (Nb5Si3Bx) were investigated, while in the quaternary Nb-Mo-Si-B system, T-1-T-2-D8(8) was investigated. Alloys were oxidized at 1000 degrees C in flowing air. For Mo-Si-B compositions, the alloys showed excellent oxidation stability and initial mass loss of the alloy varied according to its Mo content. Minor quantities of MoO2 were observed in the Mo-Si-B scales. The oxidation rates of Nb-Si-B and Nb-Mo-Si-B alloys were much larger than that of Mo-Si-B alloys. Their overall mass gains were significantly dependent on the initial heating atmosphere. In the Nb-Si-B system, T2 and D88 alloys were more resistant to oxidation when heated to the test temperature in high purity argon. The quaternary Nb-Mo-Si-B alloy containing less D88 phase showed lower mass gains than that containing more D88 phase. Scales of the order of 1050 mu m thick were observed on Mo-Si-B alloys while much thicker scales, of 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 scales and reduced the oxidation rate of Nb-Mo-Si-B alloys. Published by Elsevier Ltd.