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

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.

Figure 1. Stacking faults and their ab initio calculated energies for the CrCoNi alloy.

(a–d) show the atomic configurations of the original fcc structure, intrinsic stacking fault, two-layer extrinsic fault and three-layer twin, respectively. (e) Top view image showing the close-packed (111) plane. (f) Energy barriers along the displacement pathway direction in e that result in the two types of stacking fault and a twin boundary. The smallest displacement along the pathway is given by the magnitude of the Burgers vector of a Shockley partial dislocation, b_s. Here, γ_isf and γ_esf represent the stacking-fault energies of intrinsic (b) and extrinsic (c) stacking faults, respectively. γ_us is the unstable stacking-fault energy, and γ_ut denotes the energy barrier for the formation of the initial twin boundary. After γ_ut is overcome, the subsequent energy barriers (for example, ) for creating multi-layer twins are smaller than γ_ut. The energy barriers along the displacement pathway were obtained with the nudged-elastic band method in our ab initio calculations.

Figure 2. TEM of twin structures in the CrCoNi alloy.

(a) Bright-field TEM image showing the hierarchical twinning architecture in a grain of the CrCoNi alloy. A grain boundary is marked by the yellow line near the top-left corner, and multiple twinning systems are labelled. Scale bar, 1 μm. (b) Low-magnification bright-field TEM image showing dislocation arrays on the twin boundary. Scale bar, 500 nm. (c) SAED pattern along <110> beam direction from the region on the CTB circled in blue in a showing extra spots which belong to the twin. (d) HAADF STEM image showing the structure of a CTB and an ITB which contains a 9R structure. This image was taken from an intersection of CTB and ITB of twin 1 in a. Scale bar, 500 pm.

Figure 3. In situ compression of CrCoNi micro-pillars with and without a twin boundary.

(a) Low-magnification dark field TEM image showing the structure of the pillar containing a twin; the inset in a shows the SAED pattern of the pillar. The spots from the matrix and twin can be distinguished. The twin boundary is marked by a white dashed line. The zone axis is [110]. Scale bar, 200 nm. (b) Low-magnification dark field TEM image showing the pillar with no twin (g=[220]). Scale bar, 200 nm. (c) Engineering stress-displacement curves of compression tests on the two pillars shown in a and b. The pillar containing a twin boundary is 67% stronger than the pillar without the boundary.

Figure 4. In situ imaging shows the dislocation and twin sequences during deformation.

(a) TEM image sequence showing the formation of the twinning architecture and the dynamic process of dislocations gliding from one twin boundary to another. Scale bar, 200 nm. (b) TEM images showing the glide of paired partial dislocations on a CTB. The inset on the right shows the change of stacking at the twin boundary from fcc to hcp due to the glide of a partial dislocation. Scale bar, 200 nm.