Dislocation mechanisms and 3D twin architectures generate exceptional strength-ductility-toughness combination in CrCoNi medium-entropy alloy

TLDR

High strength and ductility are difficult to achieve together in metals, but twinning‑induced plasticity offers an exception, and ab initio modeling predicts that CrCoNi has negative stacking‑fault energy and a strong tendency to twin. The study aims to overcome the strength‑ductility trade‑off by investigating deformation mechanisms in CrCoNi, a medium‑entropy alloy with record strength, ductility, and toughness, using in‑situ aberration‑corrected scanning transmission electron microscopy. The authors employ in‑situ aberration‑corrected scanning transmission electron microscopy to observe deformation mechanisms in CrCoNi. Deformation generates a 3D hierarchical twin network from three twinning systems that both blocks dislocations at twin boundaries and provides pathways for dislocation glide and cross‑slip, and this stable twin architecture remains intact, continuously supplying strength, ductility, and toughness.

Abstract

Abstract Combinations of high strength and ductility are hard to attain in metals. Exceptions include materials exhibiting twinning-induced plasticity. To understand how the strength-ductility trade-off can be defeated, we apply in situ , and aberration-corrected scanning, transmission electron microscopy to examine deformation mechanisms in the medium-entropy alloy CrCoNi that exhibits one of the highest combinations of strength, ductility and toughness on record. Ab initio modelling suggests that it has negative stacking-fault energy at 0K and high propensity for twinning. With deformation we find that a three-dimensional (3D) hierarchical twin network forms from the activation of three twinning systems. This serves a dual function: conventional twin-boundary (TB) strengthening from blockage of dislocations impinging on TBs, coupled with the 3D twin network which offers pathways for dislocation glide along, and cross-slip between, intersecting TB-matrix interfaces. The stable twin architecture is not disrupted by interfacial dislocation glide, serving as a continuous source of strength, ductility and toughness.