Abstract
Molecular dynamic simulations have been carried out to investigate the crystallization mechanism during rapid cooling and isothermal relaxation for Zirconium (Zr) under a pressure of 100 GPa. The properties of the simulated system are analysed in terms of the system energy, the pair distribution function (PDF), and the largest standard cluster analysis. It is found that the so called topologically close-packed (TCP) local structures dominate in number in liquid, supercooled liquid and the final solid. Both solidification and relaxation crystallize into an A15 phase rather than the simple bcc or hcp crystals, the metastable and stable crystal of Zr under the ambient condition. Interestingly on PDF curves for liquids and super-liquids, left to the first major peak (at around r = 3.0 Å) there is a mini-peak (at around r = 1.87 Å) that decreases with the decrease of temperature and vanishes when crystallization ends. Further analysis reveals that the mini-peak originates from dynamical compact atomic pairs that are however almost nothing to do with TCP local structures. In addition, the multi characteristic lengths of the A15 unit (composed of Z12 and Z14 local structures with ratio of 1:3) result in the distinct split of the first and second peaks on the PDF curves.
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