Atomic-scale insights into two-stage nucleation and microstructure evolution in cobalt solidification

The homogeneous nucleation and microstructure evolution of cobalt (Co) during solidification are investigated via molecular dynamics simulations, with a focus on the effects of the cooling rate (1.0×10¹¹–1.0×10¹³ K/s) and degree of undercooling (300–1400 K). The results reveal a two-stage crystallization mechanism: (i) formation of undercooled dense liquids with short-range order (SRO), particularly icosahedral (ICO) clusters, followed by (ii) transformation into long-range FCC/HCP crystalline phases. The final microstructure exhibited two dominant types—lamellar (stacked FCC/HCP phases) and nanocrystalline (highly twinned)—with the former stabilizing at low cooling rates and the latter stabilizing at high quenching rates. The critical nucleus sizes (0.93–5.0 nm) align with classical nucleation theory, whereas the maximum nucleus number peaks at intermediate undercooling (~1000 K), reflecting a trade-off between the thermodynamic driving force and kinetic barriers. Notably, ICO-rich regions serve as nucleation precursors, with their rapid depletion coinciding with crystalline phase formation, as evidenced by bond-orientational Q₆ and common neighbor subcluster analyses. The cooling rate critically governs the ICO lifetime and transformation pathway: low rates enable complete ICO→FCC/HCP conversion into lamellar structures, whereas high rates kinetically trap ICO clusters, leading to nanocrystalline or amorphous composites. The glass transition temperature (T_g ≈ 580 K) and fractal bond reorganization below the T_g further elucidate the amorphous-to-crystalline transition. This work provides atomic-scale insights into the stepwise nucleation pathway in Co, emphasizing the roles of SRO and the cooling rate in microstructure control, with implications for designing advanced Co-based alloys.