The role of interstitial constituents in refractory complex concentrated alloys

To meet the growing demand for transportation and energy consumption around the world, more efficient turbine engines and power generators with higher operating temperatures are needed, which require alloys with retained strength levels at elevated temperatures. In the past decade, refractory complex concentrated alloys (RCCAs) have gained prominence through numerous reports of superior strengths at higher homologous temperatures compared to those of conventional refractory and super alloys. However, these RCCAs, comprised of transition metals from subgroups IV, V, and VI, tend to be brittle at room temperature, hindering their broad applicability. Recent findings reveal that interstitial constituents may significantly contribute to, and convolute observations of, the ductility and strength of RCCAs at room temperature. This review of the literature examines and discusses the field’s current understanding of the role of interstitial constituents, specifically oxygen and nitrogen, in the microstructure and mechanical behavior of RCCAs. Moreover, we provide context derived from the binary interactions of interstitial constituents with refractory metals and their contribution to the development and processing of conventional refractory alloys as a framework to gain insight into interstitial constituent mechanisms in RCCAs. In some cases, the mechanisms of interstitial constituents in RCCAs are similar to those of unalloyed subgroup VI transition metals and their dilute alloys, segregating to and embrittling grain boundaries. In other cases, interstitial constituents can be accommodated in solid solution, strengthening the RCCA, similar to interstitial constituents soluble in unalloyed subgroup IV and V metals and their dilute alloys. With the understanding of interstitial constituent element interactions with RCCA constituents, more holistic approaches to the design of RCCAs are suggested to engineer the mechanisms of intended and unintended interstitial constituents through alloy design and processing. For the development of strong and ductile RCCAs, the interactions between interstitial constituents and RCCA constituents and their resulting mechanisms must be understood and controlled.