Abnormal hardening and amorphization in an FCC high entropy alloy under extreme uniaxial tension

Abstract

Ever more extreme environments in advancing industrial applications motivate efforts to design new generation of alloys with excellent mechanical properties. High entropy alloys (HEAs) are considered as promising candidates for bearing extreme loadings. However, the reported results of correspondence between extremely mechanical behaviors and micromechanisms are still in infancy. Here we showed mechanical responses over wide strain rate and temperature ranges and microscopic observations recovered from dynamic uniaxial tension of face-centered cubic (FCC) HEA to reveal plasticity mechanisms transition under various loading conditions. In particular, with an increasing strain during the extreme dynamic tension a sequence of mechanisms, including twinning, detwinning-induced localization, nanoscale body-centered cubic (BCC) phase transformation and symbiotic amorphization, were progressively activated for continued plasticity. At strains over 35%, twin boundaries hindrance to localized band propagation and more dislocations emitting from BCC phases even collectively caused a second sharp rise in hardening. Additionally, the plasticity kept increasing during crystal to amorphization transition, which should be promoted by severe lattice distortion in super-dense dislocations areas. By combining large scale molecular dynamic simulations, the causes of phase transition to BCC lattice structure or even to amorphous bands were explained in depth. The thorough uncovering of these brand-new mechanisms can help understand the plasticity and amorphization of HEAs under the extreme loadings.