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

First-principles based molecular-dynamics simulations have been performed for binary <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:msub><a:mi>Cu</a:mi><a:mi>x</a:mi></a:msub><a:msub><a:mi>Ti</a:mi><a:mrow><a:mn>1</a:mn><a:mo>−</a:mo><a:mi>x</a:mi></a:mrow></a:msub></a:mrow></a:math> (<b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mi>x</b:mi></b:math> = 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 fivefold 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 the interatomic distances of Ti atoms, which increase with increasing Ti content. The <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mrow><c:msub><c:mi>Cu</c:mi><c:mn>50</c:mn></c:msub><c:msub><c:mi>Ti</c:mi><c:mn>50</c:mn></c:msub></c:mrow></c:math> 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 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.