The strength–ductility trade-off in metal alloys is a well-known phenomenon: increasing strength tends to decrease ductility and toughness and vice versa. This arises from the intrinsic nature of the interactions of dislocations with the surrounding microstructural elements. Novel approaches to overcome this challenge are focused on changing the nature of these interactions. To this end, high-entropy alloys (HEAs) based on multiple components in equiatomic proportions, like the well-known Fe20Mn20Ni20Co20Cr20 HEA, provide new opportunities. Until recently, HEA referred to alloys that had at least five major elements; the high mixing entropy, or configurational entropy, of such systems stabilized the single solid-solution phase, which confers strengthen by impeding dislocation motion.

By reconsidering the typical aim of producing HEAs with single-phase solid-solution microstructures, and introducing a second, metastable phase, researchers at the Massachusetts Institute of Technology (MIT) and the Max Planck Institut für Eisenforschung in Düsseldorf, Germany, have eliminated the high strength–high ductility trade-off.

“We show that when aiming for novel high-entropy alloys with improved properties, one should focus more carefully on the microstructures and the mechanisms rather than just trying to get a single phase solid solution,” says Cem Tasan, professor of metallurgy in the Department of Materials Science and Engineering at MIT. “Even if increased configurational entropy could stabilize a single phase solid solution, going for metastability rather than stability turns out to be a better option for the alloy system that we designed to introduce new mechanisms, and in-turn exhibits better property combinations.”

As reported in Nature, Tasan and his colleagues introduced metastability by going from five atomic components to four, investigating Fe80-xMnxCo10Cr10 alloys with x = 45, 40, 35, and 30 at%. Eliminating the relatively expensive, austenite-stabilizing alloying element Ni decreases the stability of the matrix phase in the alloy, and enables the formation of a martensitic second phase through a thermally induced fcc–hcp partial phase transition during cooling. An interesting aspect is that both of these phases are compositionally equivalent solid solutions, and so benefit from the solid solution strengthening effect of both on dislocation movement. Interfaces between the two phases further hinder dislocation glide, contributing to strength. On the other hand, deformation-induced martensitic transformation of the remaining fcc phase contributes to the extended ductility of the system by delaying plastic instability.

X-ray diffraction and electron backscatter diffraction analysis showed a single solid solution fcc phase for the x = 45 and 40 at% alloys. Minor amounts of fcc–hcp transformation were seen at x = 35 at%. Further reduction of the Mn content to 30 at% successfully produced the transition-induced, plasticity-assisted, dual-phase high-entropy alloy (TRIP-DP-HEA) with a composition of Fe50Mn30Co10Cr10 and an hcp content of 28%. The two phases consist of an fcc γ matrix with hcp ε phase laminate layers. The hcp laminates act as nucleation sites for strain-induced martensite, which forms through the overlapping of stacking faults.

Tensile strength testing of the grain-refined Fe50Mn30Co10Cr10 alloy showed major improvements in ultimate tensile strength and in elongation to fracture compared to the five-component, equiatomic Fe20Mn20Ni20Co20Cr20 HEA that has demonstrated the best properties to date.

“This proof-of-principle study breaks new ground by showing that it is possible to synergistically utilize a symphony of strengthening and toughening mechanisms in a dual-phase high-entropy alloy to achieve an excellent combination of strength and ductility,” says Easo P. George, Professor of Materials Design at Ruhr University Bochum in Germany, who was not involved in the research.

For Tasan and his colleagues, George’s words “proof of principle” are key to their plans for future research. Having established that TRIP-DP-HEAs have microstructural features that promote both strength and ductility, they plan to investigate a wide range of atomic components and alloy compositions that they believe will further improve these previously conflicting physical properties while reducing the cost of these materials.

Read the abstract in Nature.