Abstract
With sufficiently high cooling rates, liquids will cross their equilibrium melting temperatures and can be maintained in a metastable undercooled state before solidifying. Studies of undercooled liquids reveal several intriguing dynamic phenomena and because explicit connections between liquid structure and liquids dynamics are difficult to identify, it remains a major challenge to capture the underlying structural link to these phenomena. Ab initio molecular dynamics (AIMD) simulations are yet especially powerful in providing atomic-scale details otherwise not accessible in experiments. Through the AIMD-based study of Cr additions in Al-based liquids, we evidence for the first time a close relationship between the decoupling of component diffusion and the emergence of dynamic heterogeneities in the undercooling regime. In addition, we demonstrate that the origin of both phenomena is related to a structural heterogeneity caused by a strong interplay between chemical short-range order (CSRO) and local fivefold topology (ISRO) at the short-range scale in the liquid phase that develops into an icosahedral-based medium-range order (IMRO) upon undercooling. Finally, our findings reveal that this structural signature is also captured in the temperature dependence of partial pair-distribution functions which opens up the route to more elaborated experimental studies.
Highlights
The nature of local structure in a liquid and its connection with its dynamic properties play a key role in understanding transformation pathways
We have is done through ab initio molecular dynamic (AIMD) simulations plotted the self-diffusion coefficient of pure Al liquid
As already discussed above, it is clear that FFS observed in local ordering is not sufficient to reveal a structural mechanism for the dynamic slowdown in the deep undercooling regime
Summary
The nature of local structure in a liquid and its connection with its dynamic properties play a key role in understanding transformation pathways. A liquid usually undergoes a first-order transition to form a crystal. A liquid becomes progressively more viscous and the dynamics changes significantly, exhibiting several intriguing dynamic phenomena such as (i) the Arrhenius-to-non-Arrhenius transition of transport quantities, i.e., diffusivity (Ds), viscosity (η) and structural relaxation time (τ),[1,2,3,4,5] (ii) the occurrence of dynamic heterogeneities (DHs)[6,7,8,9] (iii) the breakdown of the Stokes−Einstein (SE) relation.[6, 9,10,11,12,13,14,15] So far, many studies have been devoted for understanding the physics underneath it remains a major challenge to characterize order in disorder and to establish structure-dynamics relations. An empirical correlation between fragility and liquid structure has been proposed.[5]
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