Impact of local structure on melt dynamics in Cu-Ti alloys: Insights from ab-initio molecular dynamics simulations

TL;DR

This study uses ab-initio molecular dynamics to explore how local structural order influences melt dynamics in Cu-Ti alloys, revealing complex short-range order and its impact on glass-forming ability.

Contribution

It provides new insights into the relationship between local structure and melt dynamics in Cu-Ti alloys, emphasizing the role of complex short-range order over Cu-Ti pairing.

Findings

High five-fold symmetry (FFS) around Cu atoms in undercooled melts.

Cu50Ti50 composition shows the highest FFS and slowest melt dynamics.

Local structural complexity, not Cu-Ti pairs, influences glass-forming ability.

Abstract

First-principle based molecular-dynamics simulations have been performed for binary Cu$_x$Ti$_{1-x}$ (x = 0.31, 0.50, and 0.76) alloys to investigate the relationship between local structure and dynamical properties in the liquid and undercooled melt. The undercooled melts show a pronounced short-range order, majorly a five-fold symmetry (FFS) around the Cu atoms, which competes with bcc ordering. This complex SRO is also reflected in the partial coordination numbers, where mainly a Z12 coordination is present around Cu, which corresponds to an icosahedral ordering. Higher coordination numbers were obtained for Ti compatible with Frank-Kasper polyhedra. The increasing Frank-Kasper polyhedra coordination scenario around Ti impacts on the interatomic distances of Ti atoms, which increase with increasing Ti content. The Cu$_{50}$Ti$_{50}$ composition exhibits the highest FFS ordering and amount of Frank-Kasper polyhedra, which explains the slowest melt dynamics, found experimentally and in simulations for this composition. Thus, our results suggest that the high undercooling degree and glass-forming ability of binary CuTi alloys, originates from the high complexity of the local structure rather than due to the preferred formation of Cu-Ti pairs, as Cu-Ti interactions were found to be weak.