"Formation of Rutile Type Oxides in Oxidation of Refractory Complex Concentrated Alloys" 

Akhil Bejjipurapu, MSE PhD Candidate 

Advisor: Professor Mike Titus

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ABSTRACT

Refractory Complex Concentrated Alloys (RCCAs), a subset of high entropy alloys, are considered as potential candidates for the replacement of Ni-based superalloys and conventional refractory alloys because of their enhanced mechanical properties at high temperatures. However, RCCAs do not exhibit satisfactory high-temperature oxidation because of multiple oxidation mechanisms, including internal oxidation, formation of volatile oxides, external scale formation, and pesting. The protective oxide scales such as Cr2O3, Al2O3, or SiO2 do not form rapidly on the external surface of the RCCAs. Instead, most of these RCCAs form mixed complex oxides, which are either porous or have different volume expansion coefficients than the metal substrates, which leads to spallation. Contrary to this, the formation of mixed rutile type ABO4 complex oxides on RCCAs has shown promising results in increasing the oxidation resistance. Rutile-type oxides, specifically CrTaO4, have provided adequate protection against oxidation by slowing down the diffusion of cations and inhibiting the formation of simple oxides that have detrimental effects. It was reported in the literature that internal oxidation and nitridation zones are prevalent in all alloy systems with these types of oxides. Albeit this negative mechanism the RCCAs that contain these types of oxides have shown lower mass gain values even for long hours of oxidation. With these promising properties, this study reviews all the available properties of these type of oxides, their thermodynamics, and the oxidation mechanisms of the few RCCA systems that reported the formation of the CrTaO4 oxide.