Atomic-scale structural signature of dynamic heterogeneities in metallic liquids

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. INTRODUCTIONThe nature of local structure in a liquid and its connection with its dynamic properties play a key role in understanding transformation pathways. On cooling, a liquid usually undergoes a first-order transition to form a crystal. However, it is also possible to form a metastable undercooled state by using cooling rates fast enough to avoid crystallization. On further cooling, 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 (D s ), viscosity (η) and structural relaxation time (τ), [1][2][3][4][5] (ii) the occurrence of dynamic heterogeneities (DHs) 6-9 (iii) the breakdown of the Stokes−Einstein (SE) relation.6, 9-15 So far, many studies have been devoted for understanding the physics underneath although it remains a major challenge to characterize order in disorder and to establish structure-dynamics relations.For instance, the increase in the rate of viscosity with falling temperature down in the deep undercooling regime has been used by Angell 1 to classify liquids by introducing the concept of fragility. Fragility has been related to thermodynamic properties, including the excess entropy and specific heat, [16][17][18] suggesting that it should have a structural signature. Only recently, an empirical correlation between fragility and liquid structure has been proposed.

Excess-entropy scaling in supercooled binary mixtures

“…The rather large difference in RDFs between the two compositions might also be anticipated from the relevance of the locally favored structures (bicapped square antiprisms) for the dynamics in these mixtures 94 . Furthermore, this anticipation is supported by a connection between decoupling of component dynamics, dynamical heterogeneity, and development of different local medium-range-like ordering in the supercooled regime for certain alloys where the local ordering is directly detectable in the RDFs 95 .…”

“…The quasiuniversality found in supercooled binary mixtures thus appears to be more subtle than the quasiuniversality observed in single-component atomic liquids at high temperatures. Future work should focus on clarifying the nature behind this observation in the supercooled regime which could be related to local orderings in the supercooled liquid 95 .…”

“…99 ). This behavior could be related to the development of different local orderings in the liquid as seen for a Cr-based alloy 95 . Fundamental questions are therefore: For which component does quasiuniversality hold-and why?…”

The intimate relationship between structural relaxation and the energy landscape of monatomic liquid metals

“…For liquid metals and alloys changes in the dynamics within the equilibrium liquid state have been reported. Violations of the Stokes-Einstein relation in experiment 37 and MD-simulations have been reported 10 and linked to the onset of correlated particle movements 12 – 14 , 38 . The diffusion coefficients for a wide range of glass forming liquids was investigated, including 11 alloys, and an Arrhenius crossover was reported at a temperature deep in the liquid state of alloys at about 39 , a temperature range compatible with our estimate for liquid aluminium.…”

“…It was suggested that this slow process is responsible for the freezing of the liquid and was later confirmed experimentally 8 , 9 . Further MD-simulations on binary alloys found changes in the dynamics far above the glass transition temperature and sometimes even far above the liquidus, which were related to dynamic heterogeneity and the onset of collective particle movements 10 – 14 .…”