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 im-
purities 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 mi-
croscopy 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. ... mehrAdditionally,
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 un-
derstanding 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.