Session:

Refractory high entropy alloys (RHEAs) hold the promise of superior mechanical properties at high temperatures but are plagued by a lack of oxidation resistance. Efforts to improve the oxidation resistance of RHEAs by adding classical passivation elements, such as Cr and Al, have been unsuccessful, largely due to the inability to form a continuous protective oxide scale, or the formation of deleterious secondary phases. Here, we present a detailed analysis of the oxidation behavior in a HfNbTaTiZr-based RHEAs with Al additions. Our study presents a case for stable and uniform oxide growth through tuning of the base alloy chemistry, where we examine the limits of passivating element additions to equiatomic RHEAs. Using a combination of multiscale microscopy and thermodynamic calculations, we determined that despite the increase in Al content, metastable phases and discontinuous oxides were dominant over the composition range of Al_xHfNbTaTiZr (x=0,0.25,0.75). As expected, the base RHEA (x=0) showed severe pesting after a one-hour exposure in ambient/atmospheric/etc air at 900° C, where HfO₂, ZrO₂, TiO₂ and TiNb₂O₇ were formed. The addition of Al improved the oxidation resistance of the base alloy and prevented pesting via promoting the formation of Al₂O₃. STEM chemical analysis revealed non-uniform enrichment of Zr and Hf near the metal/oxide interface that could block the diffusion of passivating elements, and thus, act against the formation of a uniform passivation scale. The addition of Al also induced the formation of the ZrAl₂ intermetallic phase that led to reduced density of Zr-rich oxides. We reach the conclusion that even if RHEAs contain enough Al to develop Al₂O₃ scale (i.e., 13 at. %), the passivation layer formed is still non-uniform with a high density of metastable oxides and intermetallic phases. This highlights the need for a passivating approach other than Al additions. Overall, this work provides insight into base alloy chemistry and its interplay with passivating elements as a key determining factor in passive film growth during high temperature oxidation.

.

Co-Author(s): Elaf Anber (Johns Hopkins University), David Beaudry (Johns Hopkins University), Sebastian Lech (Johns Hopkins University), Jean-Phillippe Couzinie (University Paris Est Creteil), James Rondinelli (Northwestern University), Christopher Wolverton (Northwestern University), Elizabeth Opila (University of Virginia)

Mitra Taheri

Johns Hopkins University

Appearances