Mechanical behavior and microstructure-property correlation of a metastable interstitial high entropy alloy with hierarchical gradient structures

Metastable high entropy alloys (MHEAs) are attractive materials due to their exceptional mechanical properties, particularly in addressing the challenge of the strength-ductility trade-off. However, one of the major hurdles of MHEAs is that their yield stresses are still relatively low compared to the engineering requirements, even with carbon-doped treatment and grain refinement. To further improve the strength of MHEAs, surface mechanical attrition treatment (SMAT) was applied to an interstitial MHEA (iHEA) with Fe49.5Mn30Co10Cr10C0.5 (at.%) compositions. Mechanical tests demonstrated that the yield stress of the iHEA had been improved by 32.6% after only 15 min of treatment without sacrificing ductility, and a 60-min treatment made the ultimate tensile strength approach 1 GPa. In-depth quantitative analysis indicated that hierarchical gradient microstructures were generated by SMAT, including grains, dislocations, twins, and dual phases. The theoretical analysis confirmed the synergetic extra-strengthening caused by the gradient distributions of various microstructures. Furthermore, the microstructural heterogeneity resulted in the hetero-deformation induced (HDI) hardening, contributing to strain hardening and maintaining sufficient ductility. The microstructure-property correlation revealed by this study provides guidance for the mechanical property design of MHEAs via gradient structure.