Simultaneously enhancing strength and plasticity in AlMoNbTaTiZr refractory high-entropy alloys via powder metallurgy

Abstract

This study presented a powder metallurgy (PM) strategy combining mechanical alloying (MA) and spark plasma sintering (SPS) for fabricating the high-performance AlMoNbTaTiZr refractory high-entropy alloys (RHEAs), which are composed of BCC phases, ordered B2 phases and grain boundary Al₄Zr₅ intermetallics. By optimizing ball milling processes, SPS sintering temperatures, and Al/Zr content, a relatively high content of B2 phase with a high degree of structural ordering and desired discontinuous Al₄Zr₅ intermetallics at grain boundaries was achieved. And it was noteworthy that the average grain size of AlMo_0.5NbTa_0.5TiZr RHEA was only 14.8 μm, an order of magnitude smaller than that of as-cast AlMo_0.5NbTa_0.5TiZr RHEA. These endowed the PM AlMo_0.5NbTa_0.5TiZr RHEAs with a low density (7.4 g/cm³) and significantly improved mechanical properties, especially at 1000°C, with the yield strength of 853 MPa and compressive strength of 929 MPa. Moreover, with a decrease in the content of Al and Zr elements, the yield strength, fracture strength and fracture strain at room temperature for the present PM Al_0.5Mo_0.5NbTa_0.5TiZr_0.5 RHEA were up to 2408 MPa, 2783 MPa and 20.8%, which were approximately 400 MPa and 108% higher than that of as-cast AlMo_0.5NbTa_0.5TiZr RHEA, respectively. More importantly, the specific strengths of present PM AlMo_0.5NbTa_0.5TiZr and Al_0.5Mo_0.5NbTa_0.5TiZr_0.5 RHEAs were far higher than that of publicly reported Ni-based superalloys, particularly above 1000°C, having a potential for partial substitution of conventional Ni-based superalloys to meet the dual demands for aerospace’s weight reduction and performance improvement. Finally, the microstructure evolution characteristics and strengthening-toughening mechanisms of PM AlMo_0.5NbTa_0.5TiZr RHEAs were discussed.