The origin and control of interstitial impurities in refractory complex concentrated alloys

Interstitial impurities, primarily O and N, inadvertently introduced during the processing of refractory complex concentrated alloys (RCCAs) significantly limit the mechanical properties of the alloys at room temperature. Plasma arc melting (PAM) has facilitated the quest for RCCAs, showcasing remarkable combinations of strength and ductility. In this work, the composition and chemistry of residual gases in the PAM chamber environment during arc melting and the interstitial impurities in elemental feedstocks were analyzed to quantify the origin of O and N during synthesis. Moreover, the thermodynamic mechanisms governing the origin of interstitial impurities in arc-melted MoNbTaW RCCAs were discussed. With an understanding of the mechanisms governing the content of interstitial impurities in RCCAs during PAM, arc-melted MoNbTaW RCCAs with fewer than 100 ppm O were synthesized. The arc-melted MoNbTaW RCCAs were then characterized using electron microscopy and atom probe tomography to establish the compositional architecture of grain boundaries and thereby elucidate the thermodynamic drive for segregation and oxide formation in MoNbTaW. Additionally, density functional theory calculations of a MoNbTaW grain boundary were employed to analyze the effect of local chemical fluctuations on the O segregation and oxide formation at the grain boundary. This detailed understanding of the thermodynamic mechanisms governing the origin of interstitial impurities and their grain boundary segregation in MoNbTaW RCCAs provides greater context surrounding O interactions in RCCAs, which is key for the successful development of refractory alloys with unique combinations of strength and ductility. • The role of interstitial impurities in RCCAs. • Plasma arc melting synthesis of RCCAs to control content of intersitial impurities. • APT measurements and DFT analysis of GB segregation and oxide formation in RCCAs.