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.
Highlights
Combinations of high strength and ductility are hard to attain in metals
Our ab initio calculations based on the density-functional theory (DFT) suggest that this alloy has a negative stacking-fault energies (SFEs) at 0 K, similar to what has been reported for some other systems[37,38,39,40,41] and high propensity for twinning when compared with pure fcc metals[42]
On the basis of the in situ straining of the CrCoNi, we have found that a three-dimensional hierarchical twin network is established within individual grains in this medium-entropy alloy (MEA), associated with its very low stacking-fault energy calculated at 0 K; this network presents substantial barriers for dislocation motion, and contributes to its high strength and significant strain hardening
Summary
Combinations of high strength and ductility are hard to attain in metals. Exceptions include materials exhibiting twinning-induced plasticity. This is apparent in twinning-induced plasticity (TWIP) steels, which are known to form multiple types of twins that result in high strength with substantial uniform ductility[6,10,11,12] Another notable example includes certain equiatomic multielement alloys, termed high-entropy alloys[30] (HEAs), based on the CrMnFeCoNi composition that have a single-phase facecentered cubic structure (fcc) structure[31,32] with relatively low-stacking-fault energies (SFEs), comparable with 304 stainless steels[33] and relatively large separations between the Shockley partials[34]. We seek to demonstrate that a 3D twin network is formed within the grains of this fcc CrCoNi MEA and that the interactions of dislocations with these twins can lead to a simultaneous increase of strength and ductility, that is, to identify the origin of its excellent mechanical properties
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