Arshad Muhammad, Bano Saira, Amer Mohamed, Janik Vit, Hayat Qamar, Bai Mingwen
Centre for Manufacturing and Materials-Materials Science, Coventry University, Coventry CV1 5FB, UK.
Department of Chemical Engineering, University of Engineering and Technology, Peshawar 25120, Pakistan.
Materials (Basel). 2024 Jul 19;17(14):3579. doi: 10.3390/ma17143579.
The high-temperature oxidation behaviour and phase stability of equi-atomic high entropy AlCrCoFeNi alloy (HEA) were studied using in situ high-temperature X-ray diffraction (HTXRD) combined with ThermoCalc thermodynamic calculation. HTXRD analyses reveal the formation of B2, BCC, Sigma and FCC, phases at different temperatures, with significant phase transitions observed at intermediate temperatures from 600 °C-100 °C. ThermoCalc predicted phase diagram closely matched with in situ HTXRD findings highlighting minor differences in phase transformation temperature. ThermoCalc predictions of oxides provide insights into the formation of stable oxide phases, predominantly spinel-type oxides, at high (O), while a lower volume of halite was predicted, and minor increase observed with increasing temperature. The oxidation behaviour was strongly dependent on the environment, with the vacuum condition favouring the formation of a thin, AlO protective layer, while in atmospheric conditions a thick, double-layered oxide scale of AlO and CrO formed. The formation of oxide scale was determined by selective oxidation of Al and Cr, as further confirmed by EDX analysis. The formation of thick oxide in air environment resulted in a thick layer of Al-depleted FFC phase. This comprehensive study explains the high-temperature phase stability and time-temperature-dependent oxidation mechanisms of AlCrCoFeNi HEA. The interplay between surface phase transformation beneath oxide scale and oxides is also detailed herein, contributing to further development and optimisation of HEA for high temperature applications.
采用原位高温X射线衍射（HTXRD）结合ThermoCalc热力学计算，研究了等原子高熵AlCrCoFeNi合金（HEA）的高温氧化行为和相稳定性。HTXRD分析揭示了在不同温度下B2、体心立方（BCC）、西格玛（Sigma）和面心立方（FCC）相的形成，在600℃至100℃的中间温度下观察到显著的相变。ThermoCalc预测的相图与原位HTXRD结果密切匹配，突出了相变温度的微小差异。ThermoCalc对氧化物的预测为在高氧环境下主要形成尖晶石型氧化物等稳定氧化物相提供了见解，同时预测了较低含量的岩盐型氧化物，且随温度升高略有增加。氧化行为强烈依赖于环境，真空条件有利于形成薄的AlO保护膜，而在大气条件下形成了由AlO和CrO组成的厚双层氧化膜。通过能谱分析（EDX）进一步证实，氧化膜的形成是由Al和Cr的选择性氧化决定的。在空气环境中形成的厚氧化膜导致了一层厚的贫Al FFC相。这项综合研究解释了AlCrCoFeNi HEA的高温相稳定性以及随时间和温度变化的氧化机制。本文还详细阐述了氧化膜下表面相变与氧化物之间的相互作用，有助于进一步开发和优化用于高温应用的HEA。
