Molecular dynamics study of sintering in CoCrNi medium-entropy alloy powders: Effects of particle size and pressure modality.

CoCrNi medium-entropy alloy powders were studied by molecular dynamics to clarify how particle-size distribution and pressure mode affect sintering at the atomic scale. Polycrystalline multi-particle models with random crystallographic orientations were constructed for monodisperse powders with particle radii of 2, 4, and 6 nm, together with a mixed-size assembly spanning 2-6 nm. Free-pressure sintering, hot isostatic pressing, and oscillatory pressure sintering were then compared using the same heating and cooling schedule but route-specific isothermal durations (200 ps for FPS and HIP, 400 ps for OPS). Particle size was found to affect both the available solid-state sintering window and the subsequent pore-evolution pathway. Mixed-size packing improved densification because coarse particles formed the main structural framework, while fine particles filled the interstitial space. External pressure did not simply increase atomic displacement; instead, it directed mass transport and plastic accommodation toward interparticle contacts and pore-closure regions. Among the protocols examined, the OPS protocol produced the lowest residual porosity, and in the mixed-size system it also gave the highest dislocation density. After sintering, HIP showed a slightly higher average yield and flow strength, whereas OPS combined closely comparable strength with lower porosity and a more homogeneous microstructure. These results show that particle-size heterogeneity and the applied pressure-time protocol should be considered together in the design of pressure-assisted consolidation routes for CoCrNi powders.