Competition between TCP and crystalline clusters during phase transition of rapidly super-cooled aluminum.

• High cooling rate 10 K/ps results in vitrification while two lower ones lead to crystal. • The topologically close-packed (TCP) atoms can better describe the structure of liquid and super-cooled liquid. • Crystallization cannot happen until TCP atoms drops off in number at the end of saturation stage. • The competition between TCP and crystalline atoms persists long enough will result in vitrification. Undercooling is usually a necessary condition for the starting of crystallization of molten metal under cooling. What structure hampered crystallization is however still unclear yet. The rapid cooling of pure aluminum has been conducted by a large-scale molecular dynamics simulation at different cooling rates. The evolution of local structures and phase transition mechanism are investigated in terms of the atom energy and the largest standard clusters. It is found that with high local density and low average potential energy (APE), the topologically close-packed (TCP) clusters in liquids increase exponentially with decreasing temperature, and have a number saturation stage whose survival time is related to cooling rate. When the crystalline clusters' APE is lower than that of the system, crystal nucleation does not begin until the TCP clusters' APE become higher than the system. Thus the competition between TCP and crystalline clusters results in the saturation of TCP clusters. These findings will provide new understandings on solidification and nucleation theory. [ABSTRACT FROM AUTHOR]