String-like collective motion mediates the martensitic α-β transition in titanium

Recent computational studies have examined the structural relaxation time τα of cooled liquids, the atomic diffusivity D within grain boundaries of crystalline materials, and the interfacial regions of bulk, thin film, and nanoparticles of crystalline materials under equilibrium conditions. They have revealed the general occurrence of string-like collective atomic motion and its importance for understanding the typically non-Arrhenius dynamics of all these materials. In the present work, we extend our study of this type of collective motion in crystalline metal materials to consider the α-β displacive structural transition of titanium from its lower temperature hexagonal close-packed to body-centered cubic structure as the temperature (T) is elevated. In particular, we employ molecular dynamics simulation and focus on the role of string-like collective motion in mediating this displacive transition and the dynamics of the phase transformation process. Above the α-β transition temperature, as the temperature increases, we observe that the scale of collective motion progressively decreases, and the activation free energy of the moving interface of the new phase is well-described by the average string length Ls, similarly to τα of glass-forming liquids. As a special feature of this transition, we find that the strings are localized in channels having a temperature-dependent width within the crystal. We thus find another physical example of a class of materials in which collective motion plays a crucial role in material dynamics.