Refractory alloying additions on the thermal stability and mechanical properties of high-entropy alloys

In this study, alloying effects of Mo and W refractory elements on the microstructural evolution of high-entropy alloys (HEAs) were systematically studied. High-density L1₂-type precipitates formed during the isothermal treatment at 800 °C. Alloying additions of Mo and W displayed different partitioning behaviors between the matrix and precipitate phases, with Mo partitioning to the matrix phase (K_Mo = 0.45) and W partitioning to the precipitates (K_w = 1.52) in the 1.5 at.% Mo and 1.5 at.% W alloyed HEA, respectively. A reversal in the partition of W back to the matrix (K_w = 0.95) was identified for the combined Mo and W alloying. It was demonstrated that W not only destabilized the Heusler phase at grain boundaries but also increased the volume fraction of the precipitates. In addition, lattice misfit was significantly reduced after alloying with these refractory additions. The coarsening kinetics was also quantitatively described according to the modified-Lifshitz-Slyozov-Wagner model. The coarsening rate constant for the HEAs was significantly reduced as comparison with that for Ni- and Co-based superalloys, implying an improved thermal stability of HEAs. Moreover, a reduced interfacial energy together with inherently small diffusivity of the refractory elements attributed to the improved thermal stability. Our findings show the remarkable thermal stability for HEAs and the potential for HEAs to be developed as new high-temperature structural materials.