Strategic enhancement of CoCrFeMnNi high-entropy alloy mechanical properties through a high-strength nano-scale nitride layer without geometrical or tolerance constraints

Abstract

Plasma nitriding is a class of surface treatment method that improves wear, corrosion, and fatigue properties along with the benefits of excellent geometry freedom and minimal dimensional distortion. Yet, previous plasma nitriding studies related to tensile properties have mostly compromised strength or ductility mainly due to grain growth or the brittle nature of bulky micrometer-scale nitride layer. We propose a strategy to simultaneously improve mutually exclusive strength and elongation through a high-strength nano-scale nitride layer fabricated via plasma nitriding, overcoming the typical trade-off relationship; for example, ultimate tensile strength and uniform elongation were improved by ∼74.6 MPa and ∼7.9 %, respectively. Using extraordinarily controlled processing parameters (e.g., low-pressure, short-time, warm-temperature), we successfully produced CoCrFeMnNi HEA with a nano-scale nitride layer of ∼291.9 nm near the surface without any change in grain size. The enhanced mechanical properties of the plasma nitrided CoCrFeMnNi HEA are attributed to the combined effects of pre-existing dislocation density, high-strength nano-scale nitride layer, and compressive residual stress. This work introduces an innovative approach to nano-scale hard regions, providing a novel framework for post-processing strategies ranging from fundamental research to various industrial applications.