Molecular dynamics simulation on structural evolution during crystallization of rapidly super-cooled Cu50Ni50 alloy

A molecular dynamics simulation of rapid cooling for fcc Cu50Ni50 alloy was conducted. The structural evolution was analysed by using the pair distribution function, LSCA (the Largest Standard Cluster Analysis) and tracing method. The results indicated that the crystallization process undergoes three stages: (1) individual nucleation, where the nuclei size is small but their number increases rapidly, with crystalline atoms stacking in defected layers; (2) rapid growth, where individual growing and multi-nuclei merging occur simultaneously to form large cluster with different stacking directions, resulting in the rapid increase of nuclei size but the decrease of nuclei number; and (3) slow growth where the merging process becomes slower with the crystals evolving to the fcc-dominated structures. Results also demonstrated that besides some hcp crystallites, hcp atoms mainly locate at the interface of fcc regions; and bcc structures act as an intermediate metastable state which are dominated in the initial nuclei and are often formed on the surface of crystallites and eventually transformed into the fcc and hcp structures during crystallization, which validates the step rule of Ostwald. These findings can improve the understandings of crystallization of liquid alloy or metal systems under rapid cooling.