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

Abstract

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.

Effects of Si, W and W–Mo on isothermal oxidation behaviors of Nb/Nb5Si3 in situ composites at high temperature

“…Therefore, the surface diffusion of oxygen atomic in the scale is improved significantly. The result of this investigation is consistent with that of the published literatures [21][22][23]. Furthermore, comparing to Nb-20Si-10W alloy, Mo atoms were substituted for most of W atoms solid solution in Nb, and the W phase with solid solution of little Nb was formed.…”

“…But, the MoO 3 phases can rapidly evaporate at exceeding 800 • C [21], and leaves a porous scale. Therefore, the surface diffusion of oxygen atomic in the scale is improved significantly.…”

Effects of Zirconium and Yttrium Oxide on Mechanical and Oxidation Properties of Mo–3Si–1B–1Zr–1Y2O3 (wt.%) Alloy

“…It also can be seen from the fracture section that the grains are very tightly bonded, and there are almost no holes at the triple node of the grains, indicating that the denseness is high. The actual measured density has reached 9.8 g/cm 3 , which is in line with the expected result.…”

“…The effect of the Si/B ratio on the oxidation behavior of Mo-Si-B alloy has been discussed in Supatarawanich's report [2]. The results show that when the Si/B ratio is increased to 1, the eutectic alloy has the best oxidation resistance at 1300 • C because Mo 3 Si provides a silicon source for the protective layer of silicified glass flakes at 1300 • C. Behrani et al studied the oxidation behavior of Nb-Mo-Si-B alloy in flowing air at 1000 • C [3]. Experimentally, due to the similar structure of niobium and molybdenum, they can form complete mutual solubility, which greatly improves the fracture toughness at room temperature and the toughness at high temperature.…”

Rare Earth Elements Enhanced the Oxidation Resistance of Mo-Si-Based Alloys for High Temperature Application: A Review

“…Previous studies have pointed out that adding Nb to the Mo-Si-based materials can play a satisfactory effect in improving mechanical properties due to damaging the stability of the Mo 3 Si phase [137][138][139]. However, the presence of Nb will lead to catastrophic oxidation of the material [140][141][142]. Inspired by the above study that adding Y can enhance the antioxidant properties of Zr-doped Mo-Si-B alloys, we further discussed the role of adding Y on the oxidation behavior of Nb-doped Mo-Si alloys.…”