Effect of elemental combination on microstructure and mechanical properties of quaternary refractory medium entropy alloys

Abstract

The present work reports the effect of elemental combination on microstructure and mechanical properties of quaternary refractory medium entropy alloys (RMEAs) having equi-atomic compositions. As-cast RMEAs ((1) HfNbTaTi, (2) HfNbTaZr, (3) HfNbTiZr, (4) HfTaTiZr, and (5) NbTaTiZr) were fabricated by vacuum arc-melting of pure elements under Ar atmosphere, homogenization was then performed at 1150°C for 24 hours with Ar atmosphere. Firstly, microstructures of both as-cast and homogenized RMEAs were observed by SEM-BSE. Three kinds of microstructures consisting of annealed grains (AG), granular morphology (GM) and dendritic morphology (DM) were found to be distributing along solidification direction in the as-cast RMEAs. Inter-dendritic segregation in the as-cast RMEAs was characterized by SEM-EDX. At the same time, grain boundary precipitates were found in the as-cast (2) HfNbTaZr and (4) HfTaTiZr alloys. After homogenization at 1150°C, a fraction of AG greatly increased while that of DM largely decreased. It was also found that the degree of segregation was largely reduced after homogenization. In addition, grain boundary precipitates having equiaxed morphology and HCP structure were observed in the homogenized (2) HfNbTaZr alloy. Subsequently, tensile tests of both as-cast and homogenized RMEAs were performed at room temperature (RT) to characterize mechanical properties of the RMEAs. After homogenization, ductility of (1) HfNbTaTi, (3) HfNbTiZr, (4) HfTaTiZr, and (5) NbTaTiZr alloys was highly improved while (2) HfNbTaZr alloy still showed early brittle fracture. Better ductility of the homogenized (1) HfNbTaTi, (3) HfNbTiZr, (4) HfTaTiZr, and (5) NbTaTiZr alloys could be attributed to the elimination of inter-dendritic segregation as well as grain boundary precipitates through homogenization.