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 T1 (Mo5Si3Bx)–MoSi2–MoB, T1–T2 (Mo5SiB2)–Mo3Si, and Mo–T2–Mo3Si in the Mo–Si–B system. In the Nb–Si–B system, T2 (Nb5(Si,B)3) and D88 (Nb5Si3Bx) were investigated, while in the quaternary Nb–Mo–Si–B system, T1–T2–D88 was investigated. Alloys were oxidized at 1000 °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 10–50 μm thick were observed on Mo–Si–B alloys while much thicker scales, of the order of 200–600 μm, 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.