Abstract: The excellent dislocation storage ability of bulk multi-principal element alloys (MPEAs) has been widely reported. To date, however, the underlying mechanisms of dislocation escape behavior in small-size face-centered cubic (FCC) MPEAs have rarely been studied. Here, we quantitatively control the initial dislocation densities (∼10¹⁵ m^-2 and ∼10¹⁶ m^-2) by large-scale molecular dynamics (MD) simulations and perform uniaxial compression simulations to compare the dislocation starvation behavior of CrCoNi with pure Cu single crystal pillars (SCPs). The analysis reveals that the CrCoNi SCPs with low initial dislocation density (∼10¹⁵ m^-2) can continuously accommodate mobile dislocations, and the critical dimension for dislocation starvation is about 30 nm. In particular, the CrCoNi SCPs with chemical short-range ordering (SRO) exhibit better dislocation storage and multiplication abilities than the random solid solution (RSS) samples even when the initial dislocation density is low. However, the presence of a large number of pre-existing dislocation locks governs the strong dislocation multiplication ability of the small-size RSS CrCoNi SCPs, in obvious contrast to the deformation of all pure Cu SCPs which is completely dominated by intermittent mobile dislocation starvation. Most importantly, we reveal the fundamental physics for the good dislocation storage of CrCoNi SCPs at small sizes from the perspective of chemical heterogeneity. The new phenomena reported in this work provide a unique atomic-scale perspective for understanding the microscopic physical origin of the mechanical behavior of MPEAs and the discovery of extremely slow dislocation escape behavior in small-scaled pillars, providing a reliable basis for the development of the dislocation starvation model.

Key words: Molecular dynamics, Medium entropy alloy, Single crystal pillar, Dislocation starvation, Chemical short-range order, Size effect