High entropy alloys: A focused review of mechanical properties and deformation mechanisms

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.

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

High-entropy alloys (HEAs) are an intriguing new class of metallic materials due to their unique mechanical behavior. Achieving a detailed understanding of structure-property relationships in these materials has been challenged by the compositional disorder that underlies their unique mechanical behavior. Accordingly, in this work, we employ first-principles calculations to investigate the nature of local chemical order and establish its relationship to the intrinsic and extrinsic stacking fault energy (SFE) in CrCoNi medium-entropy solid-solution alloys, whose combination of strength, ductility, and toughness properties approaches the best on record. We find that the average intrinsic and extrinsic SFE are both highly tunable, with values ranging from -43 to 30 mJ⋅m-2 and from -28 to 66 mJ⋅m-2, respectively, as the degree of local chemical order increases. The state of local ordering also strongly correlates with the energy difference between the face-centered cubic (fcc) and hexagonal close-packed (hcp) phases, which affects the occurrence of transformation-induced plasticity. This theoretical study demonstrates that chemical short-range order is thermodynamically favored in HEAs and can be tuned to affect the mechanical behavior of these alloys. It thus addresses the pressing need to establish robust processing-structure-property relationships to guide the science-based design of new HEAs with targeted mechanical behavior.

https://www.pnas.org/content/pnas/115/36/8919.full.pdf

Tunable stacking fault energies by tailoring local chemical order in CrCoNi medium-entropy alloys.

Mechanical behavior of high-entropy alloys

High-entropy alloys (HEAs) were presumed to have a configurational entropy as high as that of an ideally mixed solid solution (SS) of multiple elements in near-equal proportions. However, enthalpic interactions inevitably render such chemically disordered SSs rare and metastable, except at very high temperatures. Here we highlight a structural feature that sets these concentrated SSs apart from traditional solvent-solute ones: the HEAs possess a wide variety of (local) chemical ordering (LCO). Our atomistic simulations employing an empirical interatomic potential for NiCoCr reveal that the LCO of the multi-principal-element SS changes conspicuously with alloy processing conditions, producing a wide range of generalized planar fault energy in terms of both its sample-average and spatial variation. We further demonstrate that the LCO heightens the ruggedness of the energy landscape and raises activation barriers governing dislocation activities. This not only influences the selection of dislocation pathways in slip, faulting, twinning, and martensitic transformation, but also increases the lattice friction to dislocation motion via a new mechanism of nanoscale segment detrapping that elevates the mechanical strength. All these open a vast playground not accessible to ground-state SSs or intermetallics, offering rich opportunities to tune properties.

/pdf/strengthening-in-multi-principal-element-alloys-with-local-2r1n7o1jel.pdf

Strengthening in multi-principal element alloys with local-chemical-order roughened dislocation pathways

CrCoNi alloy exhibits a remarkable combination of strength and plastic deformation, even superior to the CrMnFeCoNi high-entropy alloy. We connect the magnetic and mechanical properties of CrCoNi, via a magnetically tunable phase transformation. While both alloys crystallize as single-phase face-centered-cubic (fcc) solid solutions, we find a distinctly lower-energy phase in CrCoNi alloy with a hexagonal close-packed (hcp) structure. Comparing the magnetic configurations of CrCoNi with those of other equiatomic ternary derivatives of CrMnFeCoNi confirms that magnetically frustrated Mn eliminates the fcc-hcp energy difference. This highlights the unique combination of chemistry and magnetic properties in CrCoNi, leading to a fcc-hcp phase transformation that occurs only in this alloy, and is triggered by dislocation slip and interaction with internal boundaries. This phase transformation sets CrCoNi apart from the parent quinary, and its other equiatomic ternary derivatives, and provides a new way for increasing strength without compromising plastic deformation.

/pdf/magnetically-driven-phase-transformation-strengthening-in-4wep13rq1c.pdf

Magnetically-driven phase transformation strengthening in high entropy alloys

https://hal-amu.archives-ouvertes.fr/hal-02082220/document

Carlyn R. LaRosa

Papers

Magnetically-driven phase transformation strengthening in high entropy alloys

Solid solution strengthening theories of high-entropy alloys

Deactivating deformation twinning in medium-entropy CrCoNi with small additions of aluminum and titanium

A “local” stacking fault energy model for concentrated alloys