Examination of the Oxidation and Metal-Oxide Layer Interface of a Cr-Nb-Ta-V-W High Entropy Alloy at Elevated Temperatures

The authors report on the evaluation of the oxide scale and the interface microstructure of a Cr-Nb-Ta-V-W refractory high entropy alloy (HEA) at elevated temperatures. The Cr-Nb-Ta-V-W HEA is oxidized at 700 and 800 degrees C in lab air and the substrate/oxide interface is investigated. Combined in situ X-ray diffraction (XRD) coupled with ex situ scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) analyses characterize the oxide scale and confirm the phases present in the substrate which have been previously identified in this alloy. The microstructure near the interface is studied for an indication of selective oxidation of this alloy. Cracking and porosity are found along the interface layer which grows directionally outward. Two main oxides are identified: a W-based oxide with a needle-like structure and a Cr oxide containing Ta that has a granular structure, primarily found in clusters. The oxide layers are porous, and no dense protective oxide is identified. It is found that when the temperature is increased to 800 degrees C, the oxide layer exhibits an increase in thickness. In situ XRD indicates that V is the first element to oxidize.