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Abstract

The initial stages of nucleation and growth are a crucial step during solidification. Atomistic insight into nucleation, in particular for binary alloys, still remains elusive due to the extended timescales and the complexity of the process. In this study, we focus on the binary alloy Ni₃Al which is a key component in high-temperature superalloys. We employ transition path sampling to generate an ensemble of molecular dynamics trajectories, from which we extract thermodynamic and kinetic properties of the nucleation process. For Ni₃Al, we obtain two different nucleation pathways that resemble the competition between two distinct bulk crystal structures, face-centred cubic and body-centred cubic. The size of the critical nucleus commonly used as reaction coordinate strongly depends on the structural composition and is therefore not sufficient to describe the nucleation in this binary system. Both the size and crystal structure of the growing nucleus need to be explicitly taken into consideration as reaction coordinates. Furthermore, we observe that the competition between diffusion and interface growth velocities has a decisive impact on the chemical order. In Ni₃Al, the growth velocity surpasses the diffusion even for small undercooling of 1% which leads to disorder trapping with partial chemical order.