Role of chemical short-range order in atomic dynamics decoupling

CH Li and YW Luan and XJ Han and JG Li, MOLECULAR SIMULATION, 43, 1056-1060 (2017).

DOI: 10.1080/08927022.2017.1342121

Using molecular dynamics simulation, a-relaxation times tau(alpha) and self-diffusion coefficients D for Al90Fe10, Al80Fe20, Al70Fe30, Al60Fe40 and Al80Ni20 (as a contrast system) melts have been systematically computed over a wide temperature range (1000-2000 K). The computed results reveal that tau(Fe)/tau(Al) (or D-Al/D-Fe) for the Al90Fe10 and Al80Fe20 melts exhibit an accelerating increase with cooling at temperatures lower than 1400 K, implying a clear decoupling of dynamics of Al and Fe (here referred to as component decoupling). This component decoupling diminishes in Al70Fe30, melt and disappears in Al60Fe40 melt. We simultaneously checked the relaxation decoupling (i.e. the decoupling between a-relaxation and diffusion). The relaxation decoupling is clear in Al60Fe40 melt, less clear in Al70Fe30, melt and not shown in Al80Fe20 and Al 90 Fe 10 melt. It exhibits a tendency counter to that of component decoupling with changing composition, arguing that relaxation decoupling does not necessarily lead to component decoupling. This finding is contradicted against the conventional view that component decoupling is believed as a result of relaxation decoupling. We further attributed such a contradiction to the difference in the degree of chemical short-range order (CSRO) in melts. The existence of CSRO can increase the cooperativity in dynamics of different components. So it is better to consider component decoupling as a combined effect of relaxation decoupling and CSRO. This work would be helpful in improving our understanding of the relationship between the two kinds of decoupling.

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