First-principles molecular dynamics studying the solidification of Ti-6Al-4V alloy

Abstract

Solidification process plays a significant role in macroscopic properties of metallic materials. However, the understanding of this process at the atomic level is still far from complete. In this work, for the first time, the first-principles molecular dynamics simulations are carried out to systematically explore the structural evolution and transport properties of Ti-6Al-4V alloy, one of the commonly used alloys for selective laser melting technology, upon cooling from 2400 K to 1300 K. Firstly, the melting range of this alloy is predicted by simulated density to be about 1850–1950 K, which is in good agreement with the experimental value of 1875–1925 K. Afterwards, the local atomic structure variation with temperature is statistically analyzed by structure factors, partial radial distribution functions, coordination number, bond angle distributions, and Voronoi tessellation. It reveals that Ti-6Al-4V alloy experiences a phase transition from 2400 K to 1300 K. The system changes from a liquid dominated by the icosahedral-like cluster to a solid with body-centered cubic structure, where the 〈0, 5, 2, 6〉 and 〈0, 4, 4, 6〉 polyhedra play an important role. Eventually, the diffusion coefficients of Ti, Al, and V are estimated to investigate the kinetic property of liquid Ti-6Al-4V alloy. From 2400 K to 1950 K, the diffusion coefficients of atoms all obey Arrhenius function, and V atom has the largest diffusion coefficient, followed by the Ti atom, which is very close to the self-diffusion coefficient of pure liquid Ti, and finally the Al atom. This abnormal mass dependence of diffusion coefficient is well explained by activation energy. In conclusion, the present work provides insights into the understanding of structural dynamics and the kinetic properties of such alloy melts, which could guide selective laser melting technology to obtain desired microstructure.