Verification of Short-Range Order and Its Impact on the Properties of the CrCoNi Medium Entropy Alloy

TL;DR

This study provides the first experimental evidence of chemical short-range order in the CrCoNi medium entropy alloy, linking it to enhanced mechanical properties and suggesting new ways to tailor these properties through processing.

Contribution

First direct observation of short-range order in CrCoNi MEA, demonstrating its impact on mechanical properties and potential for property tuning.

Findings

Increased SRO correlates with higher stacking fault energy.

Higher SRO levels lead to increased hardness.

Chemical ordering can be controlled via thermomechanical processing.

Abstract

Traditional metallic alloys are mixtures of elements where the atoms of minority species tend to distribute randomly if they are below their solubility limit, or lead to the formation of secondary phases if they are above it. Recently, the concept of medium/high entropy alloys (MEA/HEA) has expanded this view, as these materials are single-phase solid solutions of generally equiatomic mixtures of metallic elements that have been shown to display enhanced mechanical properties. However, the question has remained as to how random these solid solutions actually are, with the influence of chemical short-range order (SRO) suggested in computational simulations but not seen experimentally. Here we report the first direct observation of SRO in the CrCoNi MEA using high resolution and energy-filtered transmission electron microscopy. Increasing amounts of SRO give rise to both higher stacking fault energy and hardness. These discoveries suggest that the degree of chemical ordering at the nanometer scale can be tailored through thermomechanical processing, providing a new avenue for tuning the mechanical properties of MEA/HEAs.