Effect of aluminum on the microstructure and properties of two refractory high-entropy alloys

Abstract The microstructure, phase composition and mechanical properties of the AlMo 0.5 NbTa 0.5 TiZr and Al 0.4 Hf 0.6 NbTaTiZr high-entropy alloys are reported. The AlMo 0.5 NbTa 0.5 TiZr alloy consists of two body-centered cubic (bcc) phases with very close lattice parameters, a 1  = 326.8 pm and a 2  = 332.4 pm. One phase was enriched with Mo, Nb and Ta and another phase was enriched with Al and Zr. The phases formed nano-lamellae modulated structure inside equiaxed grains. The alloy had a density of ρ  = 7.40 g cm −3 and Vickers hardness H v  = 5.8 GPa. Its yield strength was 2000 MPa at 298 K and 745 MPa at 1273 K. The Al 0.4 Hf 0.6 NbTaTiZr had a single-phase bcc structure, with the lattice parameter a  = 336.7 pm. This alloy had a density ρ  = 9.05 g cm −3 , Vickers microhardness H v  = 4.9 GPa, and its yield strength at 298 K and 1273 K was 1841 MPa and 298 MPa, respectively. The properties of these Al-containing alloys were compared with the properties of the parent CrMo 0.5 NbTa 0.5 TiZr and HfNbTaTiZr alloys and the beneficial effects from the Al additions on the microstructure and properties were outlined. A thermodynamic calculation of the solidification and equilibrium phase diagrams was conducted for these alloys and the calculated results were compared with the experimental data.