Reactive synthesis and ultrafast sintering of refractory compositionally complex alloy-carbide composites

Abstract

Refractory compositionally complex alloy (RCCA) NbMoTaW with a well-dispersed compositionally complex carbide (CCC) reinforcing secondary phase is made through high-energy ball milling (HEBM) and ultrafast high-temperature sintering (UHS). Stearic acid is used as a process control agent (PCA) for HEBM and a carbon source to form a controllable fraction (∼2–20 vol%) of CCC in-situ via reactive synthesis. Ultrahigh heating rates of ∼100 °C/s enable sintering of dense (∼92–95 % relative densities) bulk compacts of RCCA-CCC composites at ∼1850 °C in ∼70 s. With the addition of 0.1–0.75 wt% PCA, ultrafast sintering enables the formation of a favorable P$\overline{3}$m1 subcarbide CCC phase, (Nb_0.37Mo_0.11Ta_0.39W_0.13)₂C. A thermodynamic model is developed to explain the observed Nb and Ta enrichments in (Nb_0.37Mo_0.11Ta_0.39W_0.13)₂C. An additional rock-salt monocarbide CCC phase forms with more carbon source addition (1 wt% PCA). The well-dispersed and fine-grained CCC precipitates result in grain size refinement and a corresponding increase in hardness.