Optimizing Medium- and High-Entropy Alloy Strength

To support the safety and longevity of buildings and infrastructure — and the safety of their inhabitants — structural engineering materials, including concrete, steel, and an assortment of alloys, must be both fracture- and deformation-resistant. Yet conventional methods of strengthening most metallic materials (rendering them resistant to deformation) often make them more vulnerable to brittle fracture, a trade-off that can lead to catastrophic structural failure.

In the article "Partially recrystallized microstructures expand the strength-toughness envelope of CrCoNi medium-entropy alloy," published in the journal Communications Materials, Exponent metallurgical engineer Connor Slone and co-authors explore how controlled recrystallization techniques applied to CrCoNi alloys — a type of medium-entropy alloy composed of chromium, cobalt, and nickel — can optimize this balance.

The authors identify thermomechanical processing techniques capable of producing partially recrystallized, heterogeneous microstructures in equimolar CrCoNi alloys, documenting analysis of these microstructures using advanced equipment, including high-performance scanning electron microscopes (SEM) and specialized, electron backscatter diffraction (EBSD) imaging and analysis.

The paper demonstrates the ability to increase the yield strength of CrCoNi alloys by 175% — and more at cryogenic temperatures — without negatively impacting the exceptional fracture toughness of the material. The authors conclude that these results likely stem "from the composite nature of the microstructure with non-recrystallized grains providing strength and recrystallized grains enabling plasticity that dissipates stresses during crack propagation." They also state that it should be possible to tune the degree of recrystallization and thus control the strength and fracture toughness in similar metals, paving the way for broader use of this technique in structural applications.

From the publication: "This work demonstrates a simple, scalable thermo-mechanical processing route that expands the yield strength-toughness envelope."