Investigation of element volatilization and impurity removal behavior in electron beam melting of VNbTaTi refractory high-entropy alloys

Refractory high-entropy alloys show great promise for extreme environments due to their excellent mechanical properties. However, their sensitivity to interstitial impurities under high-temperature conditions leads to embrittlement, limiting their practical applications. This study utilizes electron beam melting to purify VNbTaTi alloy. Thermodynamic calculations indicate that the volatilization sequence of the alloy components is Ti > V > Nb > Ta. The study focuses on investigating the influence of refining power and time on element and impurity content. The volatilization behavior of each element exhibits a nonlinear relationship with process parameters. As the melting time increases, the O content shows a trend of first decreasing then increasing, while the N content slightly increases. Increasing power causes O content to first drop then stabilize. The C content exhibits a pattern of first increasing and then decreasing with the increase of power or time. Among these, O and N are primarily removed through interface evaporation, while C is removed through carbon-oxygen reactions and high-density impurity deposition. Strong convection and the Marangoni effect in the melt significantly influence impurity movement. The reversal of melt flow promotes the aggregation of inclusions to high-temperature regions, and in-situ impurity removal can be achieved by local overheating, evaporation, dissolution, and decomposition reactions. By optimizing the process parameters, the O content in the alloy is reduced to 84 ppmw, successfully producing a high-purity VNbTaTi alloy with C, N, and O all below 100 ppmw. This work balances low impurity levels with high composition accuracy, offering important theoretical guidance for producing high-purity refractory high-entropy alloys.