Atomic structure evolutions and mechanisms of the crystallization pathway of liquid Al during rapid cooling.

The solidification of pure aluminum has been studied by a large-scale molecular dynamic simulation. The potential energy, position D , height H , and width W of the first peak and valley of PDF curves, and the local structures were investigated. It was found that the FCC-crystallization ability of pure Al is so strong that still local crystal regions exist in the amorphized solid. As the temperature decreases, besides the counter-intuitive increase in D p ( D of the first peak), H p increases monotonically; W p , D v , and H v decrease monotonically; only W v first decreases and then increases. They all change critically when phase transition happens. After the nucleation, orientation-disordered HCP-regions, as the grain boundaries or defects of FCC crystals, rapidly transform into FCC structures, and then the surviving HCP-regions regularize into few parallel layers or orientation-disordered HCP-regions. If parallel layers result in dislocation pinning, structural evolution terminates; otherwise, it continues. These findings will have a positive impact on the development of the solidification and nucleation theory.