Medium Entropy Alloys

Severe lattice distortion is a core effect in the design of multiprincipal element
alloys with the aim to enhance yield strength, a key indicator in structural
engineering. Yet, the yield strength values of medium- and high-entropy alloys
investigated so far do not substantially exceed those of conventional alloys
owing to the insufficient utilization of lattice distortion. Here it is shown that
a simple VCoNi equiatomic medium-entropy alloy exhibits a near 1 GPa yield
strength and good ductility, outperforming conventional solid-solution alloys.
It is demonstrated that a wide fluctuation of the atomic bond distances in
such alloys, i.e., severe lattice distortion, improves both yield stress and its
sensitivity to grain size. In addition, the dislocation-mediated plasticity
effectively enhances the strength–ductility relationship by generating
nanosized dislocation substructures due to massive pinning. The results
demonstrate that severe lattice distortion is a key property for identifying
extra-strong materials for structural engineering applications.

A high density of grain boundaries can potentially increase structural materials' strength, but at the expense of losing the materials' strain hardening ability at high flow stress levels. However, endowing materials with grain size gradients and a high density of internal interfaces can simultaneously increase the strength and strain hardening ability. This applies particularly for through-thickness gradients of nanoscale interface structures. Here we apply a machining method that produces metals with nanoscale interface gradients. Conventional bulk plastic deformation such as rolling, a process applied annually to about 2 billion tons of material, aims to reduce the metal thickness. We have modified this process by introducing severe strain path changes, realized by leading the sheet through a U-turn while preserving its shape, an approach known as ‘hard turning’. We applied this process at both room temperature and 77 K to a NiCrCo medium entropy alloy. Micropillar compression was conducted to evaluate the mechanical response. After hard turning at room temperature, the surface microstructure obtained a ~50% increase in yield stress (0.9 GPa) over the original state with homogeneous grain size (0.4 GPa), but the initial strain hardening rate did not show significant improvement. However, after hard turning at 77 K, the gradient nanolaminate structure tripled in yield stress and more than doubled its initial strain hardening rate. The improvements were achieved by introducing a specific microstructure that consists of gradient nanolaminates in the form of nanospaced twins and martensite in the face center cubic (fcc) phase. This microstructure was formed only at cryogenic temperature. It was found after turning at room temperature that only nanospaced twins were present in the fcc phase inside nanolaminates that had formed at the surface. The origin of the enhanced strain hardening mechanism was studied. Joint density functional theory (DFT) and axial next nearest neighbor Ising (ANNNI) models were used to explain the temperaturedependent phase formation of the NiCrCo nanolaminate at the surface of the hard-turned material.