Fig. 1. Pristine microstructures of NC CrCoNi, CG CrCoNi and NC Ni. (a) XRD patterns of the NC/CG CrCoNi and NC Ni samples, revealing only a single FCC phase in each sample. Insets are (a1) an EBSD band contrast image and (a2) a bright-field TEM image of CG CrCoNi. (b, c) EDS elemental maps of the NC CrCoNi and CG CrCoNi, respectively, showing uniform elemental distributions. (d, h) Bright-field plan-view TEM images, (e, i) cross-section TEM images, (f, j) grain-size distributions and (g, k) HRTEM images of NC CrCoNi and NC Ni, respectively.

Fig. 2. Typical indentation P-h curves for the three materials: (a) NC CrCoNi, (b) CG CrCoNi and (c) NC Ni. Multiple pop-ins appear in the P-h curves of NC and CG CrCoNi, as marked by red arrows, while no obvious pop-in presents for NC Ni. (d) Pop-in width versus pop-in load under the loading strain rate of 0.004/s for NC CrCoNi and CG CrCoNi.

Fig. 3. Representative P-h curves of NC CrCoNi and CG CrCoNi with Hertzian fitting analysis as presented by solid lines. The Hertzian fitting analysis is presented by solid red lines.

Fig. 4. Maximum shear stress and activation volume of the first pop-in. Cumulative probability of the pop-in events versus maximum shear stress of (a) NC CrCoNi and (b) CG CrCoNi at different loading strain rates. Relation between $\text{ln}\left[ -\ln \left( 1-F \right) \right]$ and ${{P}^{1/3}}$ for (c) NC CrCoNi and (d) CG CrCoNi at three loading strain rates.

Fig. 5. Incipient plasticity of NC CrCoNi and NC Ni in MD simulations. The simulated P-h curve of (a) NC Ni and (d) NC CrCoNi. (b, c) The snapshots of microstructure evolution during the indentation in NC Ni, which correspond to the points c and d in the P-h curves in (a). Defect structures immediately (e) before and (f) after the first pop- in NC CrCoNi, which correspond to the points e and f in the P-h curves in (d). Two pre-existing SF (marked by red arrows) were erased, leaving a SF debris (marked by a blue arrow). (g) Typical atomic configurations upon the erasing of pre-existing stacking faults by Shockley partials upon nano-indentation in CrCoNi. (g1) Partials nucleate from the junction of the stacking faults and grain boundaries. (g2) Partials move toward the opposite direction of pre-existing stacking faults, which will be erased after the partial slipping. (g3) Atomic configuration after a full erasing of the pre-existing stacking faults. Green atoms have a perfect FCC local structure while dislocations and GBs are colored by red and white, respectively.

Fig. 6. Composition profile of the grain interior and GB regions. EDS line scan data for various elements were acquired along the yellow line.

Fig. 7. Influence of GB chemistry on the nano-scale pinning-depinning events during dislocations motion. (a) Comparison of the displacement of dislocations with time under different GB chemistries. The corresponding evolution of dislocation line morphology is shown in (b) NC Ni and (c) NC CrCoNi.

Fig. 8. Schematics illustration showing the influence of GB on the dislocation expansion. Schematics showing the revolution of a dislocation loop beneath the indentation tip in (a) CG CrCoNi and (b) NC CrCoNi.

Fig. 9. Typical deformed microstructure around an indent in NC CrCoNi. (a) Bright-field TEM image showing nano-grains in the vicinity of the indent. The inset shows the indent with triangle geometry. (b) HRTEM image showing tangled dislocations, SFs and nano-twins within the nano-grains. Dislocations are marked by red “⊥”, and SFs are marked by green arrows.