Oxidation behavior and condition-specific (Nb, X)Si2 depletion-time estimation of silicide-coated Nb–16Si–22Ti–5Cr–3Al (at.%) alloy at 1500 °C

In this study, the oxidation behavior and (Nb, X)Si 2 depletion behavior under isothermal oxidation at 1500 °C in ambient air were investigated for a silicide-coated Nb–16Si–22Ti–5Cr–3Al (at.%) alloy using separate specimens for each oxidation duration. Pack cementation produced an (Nb, X)Si 2 layer (X = Ti, Cr, Al) with an average thickness of approximately 104 μm on the alloy surface. During oxidation, the coating evolved into a SiO 2 /TiO 2 -rich surface scale, an intermediate (Nb, X)Si 2 + (Nb, X) 5 Si 3 region, and an inner (Nb, X) 5 Si 3 layer. The uncoated alloy suffered severe degradation, showing a mass change of −350.5 mg/cm 2 after 20 h. By contrast, the silicide-coated alloy maintained a low mass change of 2.7 mg/cm 2 after 20 h, confirming effective surface protection within the stable regime. A phase-fraction-based analysis of cross-sectional BSE images showed that the retained (Nb, X)Si 2 area fraction decreased linearly with oxidation time, giving an estimated (Nb, X)Si 2 depletion time of ∼22.4 h under the present oxidation condition. After 23 h, the coated alloy showed rapid mass loss (−238.8 mg/cm 2 ) and substrate oxidation, which occurred near the estimated depletion time of the protective (Nb, X)Si 2 phase. These findings demonstrate that the silicide coating can provide short-term oxidation protection for the Nb–16Si–22Ti–5Cr–3Al alloy under the specific 1500 °C condition investigated here, and the phase-fraction analysis provides a condition-specific (Nb, X)Si 2 depletion-time estimate under the present oxidation condition.