Discovery of Ni activated sintering of MoNbTaW predicted by a computed grain boundary diagram

Activated sintering of refractory metals represents a classical phenomenon in powder metallurgy. This study discovers that Ni addition can enhance sintering of MoNbTaW both above and below the bulk solidus compo-sition at 1800 degrees C, thereby demonstrating the first example of activated sintering of a high-entropy alloy. To probe the underlying mechanism, experiments reveal complete grain boundary (GB) wetting above the bulk solidus composition, which logically infers the stabilization of a liquid-like interfacial phase (GB complexion) in a prewetting region below the bulk solidus composition. Furthermore, bulk CALPHAD (calculation of phase dia-gram) methods have been extended to model GBs in Ni-doped MoNbTaW to compute a GB "phase" diagram to forecast high-temperature GB disordering in the prewetting region and rationalize the observed Ni-activated sintering of MoNbTaW. Only weak GB segregation of Ni is found in furnace-cooled specimens, which suggests GB "drying" during cooling, consistent with the observed nanoscale precipitates along GBs.