Ab initio molecular dynamics study of early-stage aluminum incorporation and short-range ordering at a Ni(111)/Ti(0001) interface

Pack cementation is an established route for producing protective aluminide layers on nickel-based superalloys, yet the first adsorption and incorporation events are difficult to resolve experimentally. Here, ab initio molecular dynamics (AIMD) and climbing-image nudged elastic band calculations were used to examine Al motion at a simplified Ni(111)/Ti(0001) interface at 593 K. Five independent 0.5 ps AIMD trajectories were considered. Al adatoms relaxed rapidly from atop toward three-fold hollow environments and subsequently sampled lateral hollow-bridge-hollow paths. A short-time two-dimensional mobility estimate obtained from the ensemble mean-square displacement was (1.3 ± 0.2) × 10 −4 cm 2 s −1 ; because the fitting interval was only 0.15 ps, this value is not interpreted as an equilibrium macroscopic diffusivity. In the trajectories, thermally assisted fluctuations enabled occupation of near-surface octahedral environments, whereas no Al atom crossed into the Ti region. The local Al coordination increased from approximately three to six Ni neighbors, producing Al-centered motifs compatible with the nearest-neighbor geometry of L1₂-Ni₃Al. These motifs indicate short-range chemical ordering, not phase nucleation. The results therefore describe ultrafast local relaxation pathways within the limits of a thin-slab, sub-picosecond model and do not represent long-range coating growth.