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

Abstract

The microstructure, phase composition and mechanical properties of the AlMo0.5NbTa0.5TiZr and Al0.4Hf0.6NbTaTiZr high-entropy alloys are reported. The AlMo0.5NbTa0.5TiZr alloy consists of two body-centered cubic (bcc) phases with very close lattice parameters, a1=326.8pm and a2=332.4pm. 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=7.40gcm−3 and Vickers hardness Hv=5.8GPa. Its yield strength was 2000MPa at 298K and 745MPa at 1273K. The Al0.4Hf0.6NbTaTiZr had a single-phase bcc structure, with the lattice parameter a=336.7pm. This alloy had a density ρ=9.05gcm−3, Vickers microhardness Hv=4.9GPa, and its yield strength at 298K and 1273K was 1841MPa and 298MPa, respectively. The properties of these Al-containing alloys were compared with the properties of the parent CrMo0.5NbTa0.5TiZr 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.