Abstract
In this study, interrupted oxidation behavior of synthetic NiAlCr–Ca (Ca = 0.3, 1.4, 2 at.%) and NiAlCr–Sr (Sr = 0.4 at.%) alloys in the air at 1027 °C for 192 h was investigated. Parabolic rate constants (kp) showed that the Sr-containing alloy exhibited the best oxidation resistance among the alloys investigated in this study. The oxide scale formed on the Sr-containing alloy was composed of α-Al2O3 phase with Sr-rich nodules. Increasing the Ca concentration in the alloys was found to reduce the oxidation resistance due to the formation of non-protective Ca-rich complex oxides and consumption of α-Al2O3 scale by the reaction between Al2O3 and CaO. The Ca-rich complex oxides were initially formed on the Ca-rich interdendritic region and grew with time. Very little scale spallation was observed for the Sr-containing alloy, while it was notable for 0.3 at.% Ca-containing alloy. Spallation was attributed to the coefficient of thermal expansion (CTE) mismatch arisen from the formation of CaAl4O7 phase, a compound with a very low CTE.
Highlights
One of the reasons for material failure and high maintenance costs in the industry is the high-temperature oxidation of metallic materials under service conditions
Since the solubility of Ca [29] and Sr [30] is very low in Ni, they segregated in the interdendritic region, which solidified the last
The EM–energy-dispersive spectrometer (EDS) examinations showed that the amount of Ca/Sr in β, γ, γ′ and α-Cr phases was lower than 0.5 at.%
Summary
One of the reasons for material failure and high maintenance costs in the industry is the high-temperature oxidation of metallic materials under service conditions. The effects of RE addition include the facilitation of the scale nucleation, alteration of the scale growth mechanism by inhibiting the cation diffusion, allowing oxygen diffusion, and forming oxide pegs [17, 18], which are usually believed to increase scale adhesion. Another beneficial effect of RE addition is the inhibition of sulfur’s adverse effects on the alloy properties. The influence of Ca as an impurity on the oxidation behavior of high-temperature alloys is not clear
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