Multistage work hardening assisted by multi-type twinning in ultrafine-grained heterostructural eutectic high-entropy alloys

High strength of materials usually comes with low ductility due to the lost or short-lived strain hardening. Here, we uncover a sequentially-activated multistage strain hardening (SMSH) that allows for sustained and effective strain-hardening capability in strong ultrafine-grained eutectic high-entropy alloy (EHEA). Consequently, exceptional ductility is realized in an ultrafine-grained EHEA, accompanied with high ultimate strength. We demonstrate that the SMSH is derived from a coordinated three-level design on structural heterogeneity, grain-size control, and intragranular composition modification, which enables the sequential activation of stress-dependent multiple hardening mechanisms. Furthermore, despite the well-known low twinning propensity due to ultrafine grains and medium-to-high stacking fault energies of prototype EHEAs, our coordinated design sequentially activates three types of deformation twinning to assist this SMSH. This work sheds light on the SMSH effect assisted by multi-type twinning previously unexpected in ultrafine-grained EHEAs, and thereby represents a promising route for improving ductility of high-strength materials.