Lightweight refractory high entropy alloys with excellent specific strength and enhanced malleability by in-situ heterogeneous structure

A novel category of lightweight Tix(x=0.5–2)AlV0.5CrMo refractory high-entropy alloys (RHEAs) have been designed by tailoring the in-situ forming of heterogeneous structures during spark plasma sintering (SPS). The effect of Ti on the microstructure evolution and mechanical properties was thoroughly investigated. All the as-milled Tix(x=0.5–2)AlV0.5CrMo powders were found to present a single body-centered cubic (BCC) phase structure. At high temperature above 1200 °C, however, the disordered BCC2 and hexagonal Al2O3 phases formed from the metastable BCC solid solution. The grain sizes of both the BCC and the newly-formed BCC2 and Al2O3 phases were in the range of 0.13–0.41 μm. By referring to the empirical parameters and non-equilibrium solidification theory, the phase transformation in the SPS sintering process was discussed in detail. Mechanical tests showed that the Tix(x=0.5–2)AlV0.5CrMo RHEAs possessed ultra-high specific strength, due to the solid solution strengthening, grain size refinement strengthening, precipitation strengthening and hetero-deformation induced (HDI) hardening. In particular, the Ti1.5AlV0.5CrMo RHEA presented an excellent specific compressive yield strength as high as 385.3 MPa.cm3/g combined with an enhanced plastic strain of 10.6%. The current research turned out to be a new strategy for preparing RHEAs with heterogeneous structures to achieve the strength- malleability trade-off.