Role of chemical short-range order in atomic dynamics decoupling

Using molecular dynamics simulation, α-relaxation times τ_α 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 τ_Fe/τ_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 α-relaxation and diffusion). The relaxation decoupling is clear in Al₆₀Fe₄₀ melt, less clear in Al₇₀Fe₃₀ melt and not shown in Al₈₀Fe₂₀ and Al₉₀Fe₁₀ 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.