Abstract
We perform ab initio molecular dynamics simulations in stable and undercooled Al1-xCux liquids, with copper composition x = 0.3 and 0.4, to examine the relationship between structure and dynamics as a function of temperature. The temperature evolution of the diffusivity, relaxation times and viscosity display a change from an Arrhenius behavior at high temperature to a non-Arrhenius one at low temperature, which occurs at a crossover temperature TX yet in the vicinity of the liquidus temperature. We find that TX corresponds also to an onset of dynamic heterogeneities (DHs) that develop in the undercooled states. The structure factors and pair-correlation functions display characteristic features compatible with the occurrence of the icosahedral short-range order (ISRO) as well as the development of a medium range order (MRO) upon cooling. A common neighbor analysis further identifies that ISRO undergoes a strong increase below TX. Finally, we demonstrate that the medium-range order is formed by interconnected fivefold bipyramids and is correlated with the emergence of DHs.
Highlights
Understanding evolutions of transport properties in undercooled liquids and their interplay with their structural features represents an important issue for solidification processes of metallic alloys, such as crystallization and formation of quasi-crystalline or amorphous phases.[1,2,3,4] Among the cast alloys, Al-Cu alloys represent an important basic system regarding fundamental aspects as well as for applications
By coupling the spatial distribution of icosahedral short-range order (ISRO) with isoconfigurational ensemble simulations, we directly reveal that the emergence of dynamic heterogeneities (DHs) is related to the development of a medium range ordering formed by interconnected fivefold bipyramids, this finding being more significant for the composition richest in copper
Dynamics We start our analysis by considering the diffusion properties through the mean-square displacements (MSD) determined from Eq (1), which are displayed in Figs. 1(a)-(b) and Figs. 1(c)-(d) for
Summary
Understanding evolutions of transport properties in undercooled liquids and their interplay with their structural features represents an important issue for solidification processes of metallic alloys, such as crystallization and formation of quasi-crystalline or amorphous phases.[1,2,3,4] Among the cast alloys, Al-Cu alloys represent an important basic system regarding fundamental aspects as well as for applications. From an experimental point of view, relatively little amount of studies have been reported in the literature regarding the structural properties of Al-Cu melts, based on X-ray diffraction, and are limited to the total structure factor.[7,8,9,10,11] One of the main feature observed is a pre-peak whose amplitude grows with increasing copper composition, in the Al-rich range It has been argued this feature might be a signature of the chemical short-range order[8] in binary Al-transition metal (TM) alloys with preferred Al-TM bonds and minimization of TM-TM number of bonds. Other interpretations have been put forward such as occurrence of chemically ordered Al-Cu micro-groupings,[9] or aggregates with fivefold symmetry extending to the medium range order (MRO).[10]
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