Understanding crystallization and amorphization in liquid Ti cooled at different rates: A molecular dynamics...

Crystallization and amorphization are important processes and different cooling rates cause these transitions. Obtaining pure metals from their molten state is a challenge unless these two are well understood. Here we study both these transitions in liquid Ti using molecular dynamics simulations wherein Ti is modeled with embedded atom potential. At 1 bar, Ti crystal is melted and then cooled from 2200 to 300 K with cooling rates of 0.1, 1, and 10 K/ps. When cooled at 0.1 and 1 K/ps, molten Ti crystallizes to bcc phase between 1100 and 1000 K, and when cooled at 10 K/ps, it amorphizes between these temperatures. From radial distribution functions and Voronoi tessellation, we observe that liquid to bcc transition takes place through short range distorted hcp/bcc-like structures already present in it. Relaxation dynamics is studied using velocity-autocorrelation functions (VACFs), intermediate scattering function, and dynamic structure factor. For all cooling rates, relaxations in VACFs increase with cooling. However, correlations in them are stronger when the system is cooled at 10 K/ps. Relaxation times decrease and increase between 1100 and 1000 K for crystallization and amorphization, respectively, thereafter they increase again with further cooling. The dynamic structure factor shows stronger damping in thermal diffusive motion when systems are cooled at 0.1 and 1 K/ps and vibration peaks shift to higher frequencies when crystallization take place. Our findings support Binder's [K. Binder, Proc. Natl. Acad. Sci. U. S. A. 111, 9374 (2014)] argument that if we cool the system faster than the minimum time needed for the liquid to relax, it will amorphize. This also prevents the growth of pre-ordered domains in it to establish long-range order.