Plasticity and strength of an equiatomic and a non-equiatomic HfNbTaTiZr high entropy alloy during uniaxial loading : a molecular dynamics simulation study

Understanding plasticity and strength of high entropy alloys of HfNbTaTiZr is extremely significant in building nuclear reactors, gas turbines, aerospace devices etc. Here we study an equiatomic (Hf0.20-Nb0.20-Ta0.20-Ti0.2-Zr0.20) and a non-equiatomic (Hf0.35-Nb0.20-Ta0.15-Ti0.15-Zr0.15) mixture of two alloys under uniaxial tensile loading from molecular dynamics simulations. Modified Embedded atom potential is used to model both these bcc alloys and all simulations are performed at 300 K with three different tensile strain rates-0.0002, 0.0005 and 0.001 ps-1. Radial distribution functions, bond-orientational parameters and OVITO are used to analyse the MD trajectories. At 0.001 ps-1 strain, both these alloys deform similarly, but differences are observed at 0.0005 and 0.0002 ps-1 strains. At these rates, both alloys deform elastically till 3%, thereafter they deform plastically till 15%-20% strain. Yield strengths are comparable in the elastic limit but in the plastic limit non-equiatomic alloy have higher strength. In equiatomic alloy, bcc phase transforms to fcc whereas in non-equiatomic alloy bcc phase transforms to both fcc and hcp. Formation of hcp atoms (50%) decrease the plasticity of the non-equiatomic alloy but increases its strength. We also observe that in both these alloys and at all strain rates, bcc atoms transform to fcc/hcp atoms through an intermediate amorphous like state. Local coordination and orientation of all atoms change similarly in equiatomic mixture. But in non-equiatomic mixture local orientation in Hf, Ti and Zr changes differently compared to Nb and Ta.