Just another WordPress site - Ruhr-Universität Bochum

Abstract

Ni-Al alloys are a basic component of high-temperature superalloys, the ordered L₁₂ phase in Ni₃Al is of particular interest. Nucleation is a crucial step in solidification, but atomistic insight still remains elusive due to the extended time scale and complexity of the nucleation process in binary alloys. We investigate the nucleation in Ni₃Al on the atomic scale. We employ transition path sampling and generate an ensemble of molecular dynamics trajectories to extract thermodynamic and kinetic properties, such as nucleation rates and free energies. A strong competition between fcc, hcp and bcc in the growing nucleus predetermines the final dominant polymorphs in the crystallization. In particular, we find that the composition of the growing nucleus remains constant while there is a decreased short-range chemical order in the nuclei. The size of the growing nucleus alone is not a complete reaction coordinate for the nucleation process, in contrast to what we observed in elemental metals. A multi-dimensional description is needed to fully characterize the nucleation process in binary alloys.