Oxidation mechanism in a refractory multiple-principal-element alloy at high temperature

Refractory multiple-principal-element alloys (RMPEAs) are promising structural materials to enable increased power efficiency in high-temperature oxidation environments, but the oxidation behavior and microstructures of the oxides, especially at the beginning of the oxidation, have received limited attention. The oxidation mechanism in an equimolar W-Mo-Ta-Nb-V was investigated at 1300 degrees C and compared with the equimolar W-Mo-Ta-Nb alloy without V. The oxide scale on WMoTaNbV after 1 min exposure is shown to be composed of a degradation layer at the interface of the alloy/oxide, an initial oxide transition layer, followed by the main phase aggregate oxide layer, and an outermost oxide layer. At the early stage of oxidation, the absorption of oxygen by the RMPEA substrate forms a solid solution. Vanadium accelerates the initial degradation process as it forms VO. The initial oxidation-induced degradation of RMPEA follows a sequence governed by the free energy change accompanied by local element segregation. Liquid V-Mo oxides aggregate in the intermediate oxide layer. The inward growth of the oxide scale is controlled by the local composition changes, the orientation of the substrate, the crystal structure, and physical properties such as melting points of the oxides.

Automated summary

Refractory multiple-principal-element alloys (RMPEAs) have potential as structural materials in high-temperature oxidation environments, but their oxidation behavior and microstructures, especially at the beginning of oxidation, have not been well studied. In this research, the oxidation mechanism of an equimolar W-Mo-Ta-Nb-V alloy was investigated and compared with an equimolar W-Mo-Ta-Nb alloy without V at 1300 degrees C. The oxide scale on the RMPEA alloy was found to consist of a degradation layer, an initial oxide transition layer, a main phase aggregate oxide layer, and an outermost oxide layer. The presence of vanadium accelerated the initial degradation process by forming VO.