Design of lightweight high temperature structural materials based on Ti–Mo–Ta–Cr–Al refractory compositionally complex alloys, Part II: high temperature oxidation behavior

Three Ti-rich Ti–Mo–Ta–Cr–Al Refractory Compositionally Complex Alloys (RCCAs) with low
density of ~7.5 g/cm³, identified in Part I for their stable single-phase disordered A2 structure, with
room-temperature compressive plasticity and exceptional strength retention up to 1000 °C, are
investigated here for their high-temperature oxidation behavior. Isothermal oxidation experiments
from 800 to 1100 °C revealed the formation of multilayered oxide scales comprising four sublayers
with rutile- and corundum-structured oxides. Mo initially precipitates as metallic particles within
the inner oxide layer before undergoing oxidation and volatilization. Volatilization of MoO3
becomes noticeable at temperatures above 900 °C, occurring more locally with lower Mo content.
Two primary oxide scale degradation mechanisms were identified: localized scale disruption from
Mo oxide volatilization and oxide scale spalling due to excessive TiN precipitation. The findings
highlight the need for careful compositional balancing of Mo and Ti in these Ti-rich RCCAs to
enhance oxidation resistance by simultaneously suppressing Mo-oxide volatilization and internal
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nitridation. Overall, they exhibit improved oxidation performance compared to conventional TiAl-
based alloys and display oxidation kinetics comparable to equimolar TaMoCrTiAl at elevated
temperatures.