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.

Crystal orientation governs the plasticity of intermetallic alloys, yet the atomic-scale mechanisms linking defect dynamics to mechanical properties remain elusive. Here, we unveil unprecedented deformation pathways in single-crystal γ-TiAl through large-scale molecular dynamics simulations under uniaxial tension across four crystallographic orientations: [100], [112], [110], and [111]. Strikingly, a metastable body-centered cubic (BCC) phase emerges transiently during [100]-oriented stretching, acting as a critical bridge between elastic and plastic regimes—a phenomenon unreported in γ-TiAl. For [110] and [111] orientations, we identify a hierarchical defect evolution cascade (intrinsic stacking faults→extrinsic stacking faults→twin boundary (ISF→ESF→TB)) driven by intersecting stacking faults and Shockley partial dislocation interactions, which govern twin boundary nucleation and growth. In contrast, [112]-oriented deformation adheres to conventional dislocation-mediated plasticity. These findings reveal how crystallographic anisotropy dictates defect dynamics, offering atomic-scale insights into deformation twinning and transient phase transitions. This work bridges atomistic processes to macroscopic properties, advancing the design of next-generation lightweight high-temperature materials.

This study is to examine the effectiveness of the two ways forming the incommensurate contacts on reducing friction. Two dimensional (2D) penta–graphene (PG) and B₂N₄ possessing the same lattice structure and excellent lubricating property were chosen to construct the homogenous and heterogenous interfaces. We studied the frictional properties at the homogenous interfaces of PG/PG and B₂N₄/B₂N₄, and the heterogenous interface of B₂N₄/PG. Our calculations show that the friction coefficients at the homogenous interface of B₂N₄/B₂N₄ are smaller than those at the heterogenous interface of B₂N₄/PG. The different compositional elements at the commensurate contacting surfaces hardly reduce the interlayer friction. It indicates the ineffectiveness of heterogenous interface on reducing friction when it is commensurate contacting. Additionally, the incommensurate contact formed via biaxial stretching one layer can significantly reduce the interlayer friction, which is the superposition of uniaxial stretching. In addition, the interlayer friction was elucidated from the point views of potential energy, charge density difference, and registry index (RI). This study highlights the importance and effectiveness of twisting in reducing the interlayer friction compared to stretching.

Surface moisture or humidity impacting the lubrication property is a ubiquitous phenomenon in tribological systems, which is demonstrated by a combination of molecular dynamics (MD) simulation and experiment for the organic friction modifier (OFM)-containing lubricant. The stearic acid and poly-α-olefin 4cSt (PAO4) were chosen as the OFM and base oil molecules, respectively. The physical adsorption indicates that on the moist surface water molecules are preferentially adsorbed on friction surface, and even make OFM adsorption film thoroughly leave surface and mix with base oil. In shear process, the adsorption of water film and desorption OFM film are further enhanced, particularly under higher shear rate. The simulated friction coefficient (that is proportional to shear rate) increases firstly and then decreases with thickening water film, in good agreement with experiments, while the slip length shows a contrary change. The wear increases with humidity due to tribochemistry revealing the continuous formation and removal of Si–O–Si network. The tribological discrepancy of OFM-containing lubricant in dry and humid conditions is attributed to the slip plane’s transformation from the interface between OFM adsorption film and lubricant bulk to the interface between adsorbed water films. This work provides a new thought to understand the boundary lubrication and failure of lubricant in humid environments, likely water is not always harmful in oil lubrication systems.