A new guide for improving mechanical properties of non-equiatomic FeCoCrMnNi medium-and high-entropy alloys with ultrasonic nanocrystal surface modification process

Ultrasonic nanocrystal surface modification (UNSM) treatment on non-equiatomic medium-and highentropy alloy (HEA) of Fex(CoCrMnNi)100-xis firstly introduced and its impact on microstructure and mechanical properties are revealed.By UNSM,severe plastic deformation-induced dislocation and deformation twins (DTs) arise at the topmost surface.Especially,Fe60(CoCrMnNi)40(Fe60),which is classified as a medium-entropy alloy (MEA),exhibits ε-martensitic transformation.In the room temperature tensile test,a high strength of ~600 MPa and ductility of ~65%elongation (strain to failure) is accomplished in Fe60.Initially formed DTs and ε-martensitic transformation by UNSM treatment plays a key role in retardation of necking point via both twinning-induced plasticity and transformation-induced plasticity.However,Fe20(CoCrMnNi)80(Fe20) comparatively shows low strength of ~550 MPa and ~40% elongation,owing to the low accommodation of DTs than Fe60.Our research will provide new guidelines for enhancing the mechanical properties of MEA and HEA.