Competing yielding mechanisms in Cantor alloy nanopillars

Multi-principal element alloys are promising for load-bearing applications, yet the mechanisms underlying nanoscale compressive yield onset across microstructural and loading variables remain incompletely resolved. Classical molecular dynamics simulations of single-crystal nanopillars centered on the equiatomic Cantor alloy (CoCrFeMnNi) are reported here, with pillar diameter, chemical short-range order (SRO), crystallographic orientation, strain rate, and composition varied independently. Rather than following a simple smaller-is-stronger trend, the size dependence reflects the coupled influence of surface compliance and defect escape. At smaller diameters, surface compliance reduces the apparent elastic modulus, whereas at larger diameters, limited defect escape shifts yielding toward propagation-controlled plasticity instead of a starvation regime. SRO increases local lattice distortion and modifies the post-relaxation defect state, shifting yield onset from nucleation to propagation and lowering the initial peak stress. Crystallographic orientation sets the preferred deformation mode, with twinning or slip selected according to the relative Schmid factors of the leading and trailing Shockley partials. Elevated strain rates increase the initial peak stress and promote defect accumulation because dislocation-mediated plasticity cannot fully accommodate the imposed deformation. For CoCrFeMnNi, CoCrFeNi, and CoCrNi, the composition effect is reflected primarily in the nucleation barrier, with peak-stress differences consistent with differences in unstable stacking-fault energy, indicating that the barrier for leading-partial nucleation depends on the specific elemental combination more than on element count alone. These results identify the governing event at yield as a unifying basis for interpreting nanoscale compression in face-centered cubic (FCC) MPEA nanopillars across size, SRO, orientation, strain rate, and composition.