Local order and phase selection in undercooled transition metal based systems: ab initio molecular dynamics study

An overview of a recent series of ab initio molecular dynamics ( MD) simulations for pure liquid transition metals as well as for transition metals (TM) based liquid alloys is presented. The aim is to investigate the local structure of these systems and their evolution upon undercooling, and our results are analysed through a three-dimensional image of the short-ranger order (SRO) by means of the common-neighbour analysis. Recent diffraction experiments indicate that the structure of both pure metals and alloys in undercooled states is dominated by an icosahedral SRO. Such a SRO is predicted to influence the energy of the interface between the liquid and a solid nucleus, depending on the structure of the solid phase. This, in turn, decisively impacts the nucleation behaviour of solid phases from the undercooled melts. We find that the five-fold symmetry is already present in the liquid state of all the studied systems. However, our findings show that the five-fold symmetry in the liquid state as well as its evolution upon undercooling depends on the system under consideration. For Ni, Zr, and Ta, local configurations are more complex than that given by the simple icosahedron. For Al1-xMnx alloys, local configurations are the result of a strong competition between chemical and topological effects; more particularly, our results indicate the predominance of the fivefold symmetry around x = 0.14 in agreement with the experimental quasicrystal forming range.