In-Situ Electron Channeling Contrast Imaging of Cyclic Deformation Mechanisms in CrCoNi Medium-Entropy Alloy

Abstract

This study investigates the microstructural evolution and cyclic stress response of a CrCoNi medium-entropy alloy as a function of strain amplitude (0.7% and 0.9%), cycle number (N, up to 200 cycles), and Taylor factor (M = 2.3–3.6) using in-situ electron channeling contrast imaging (ECCI). In an initial stage (N < 20 cycles), dislocations multiply uniformly across all grains for both strain amplitudes. Subsequently (N > 50 cycles), grains with low Taylor factors (M = 2.3–2.8) retain planar slip arrays, those with medium Taylor factors (M= 2.8–3.2) develop transitional dislocation structures, while grains with high Taylor factors (M = 3.2–3.6) form fully heterogeneous structures through multi-slip activation. This microstructural evolution directly correlates with the mechanical response: low-M grains exhibit continuous hardening as a result of planar slip, medium-M grains display slightly lower hardening, while high-M grains undergo cyclic softening. The softening in high-M grains stems from the formation of low-density dislocation channels, increasing the dislocation mean free path and reducing back stress. A constitutive model incorporating the Taylor factor distribution and these evolving dislocation configurations quantitatively predicts the cyclic stress response, confirming that softening originates from heterogeneous structures in grains with high Taylor factor. This work thus identifies the Taylor factor-dependent formation of heterogeneous structures, observed via ECCI, as the critical mechanism driving cyclic softening. These findings offer a crystallographic perspective for designing fatigue-resistant multi-principal element alloys.