Short-range ordering suppresses mechanical annealing in CoCrNi alloy nanopillars

Comprehensive atomic simulations have been conducted to compare the effects of pre-existing dislocation densities on the intermittent plastic behaviors of CoCrNi medium-entropy alloy (MEA) single-crystalline nanopillars with that of pure metal nanopillars. In contrast to pure metal nanopillars that demonstrate prolonged nearly elastic loading and reloading segments, the MEA nanopillars show short loading and reloading segments and high dislocation densities throughout the entire deformation process, suggesting that mechanical annealing is substantially suppressed in MEA nanopillars. The closely spaced junctions between the short-range-order domains and adjacent Ni clusters exert exceptionally strong local Peierls friction forces that not only slow down dislocation slip, but also increase the probability for dislocation entanglement. As a result, high densities of dislocations can be accumulated during the plastic deformation of the MEA nanopillars, leading to suppression of mechanical annealing and transition from exhaustion hardening to strain hardening. This work provides new insights to the plastic deformation of MEA nanopillars that are distinctive from pure metal nanopillars.