Structural relaxation in a binary metallic melt: Molecular dynamics computer simulation of undercooled Al₈₀Ni₂₀

Kurzfassung

Molecular dynamics computer simulations are performed to study structure and structural relaxation in the glassforming metallic alloy Al₈₀Ni₂₀. The interactions between the particles are modeled by an effective potential of the embedded atom type. Our model of Al₈₀Ni₂₀ exhibits chemical short-range order (CSRO) that is reflected in a broad prepeak around a wave number of 1.8 Å⁻¹ in the partial static structure factor for the Ni-Ni correlations. The CSRO is due to the preference of Ni atoms to have Al rather than Ni atoms as nearest neighbors. By analyzing incoherent and coherent intermediate scattering functions as well as self-diffusion constants and shear viscosity, we discuss how the chemical ordering is reflected in the dynamics of the deeply undercooled melt. The q dependence of the alpha relaxation time as well as the Debye-Waller factor for the Al-Al correlations show oscillations at the location of the prepeak in the partial static structure factor for the Ni-Ni correlations. The latter feature of the Debye-Waller factor is well reproduced by a calculation in the framework of the mode coupling theory (MCT) of the glass transition, using the partial static structure factors from the simulation as input. We also check the validity of the Stokes-Einstein-Sutherland formula that relates the self-diffusion coefficients with the shear viscosity. We show that it breaks down already far above the mode coupling critical temperature Tc. The failure of the Stokes-Einstein-Sutherland relation is not related to the specific chemical ordering in Al₈₀Ni₂₀.