Atomic transport in dense multicomponent metallic liquids

Pd₄₃Ni₁₀Cu₂₇P₂₀ has been investigated in its equilibrium liquid state with incoherent, inelastic neutron scattering. As compared to simple liquids, liquid Pd₄₃Ni₁₀Cu₂₇P₂₀ is characterized by a dense packing with a packing fraction above 0.5. The intermediate scattering function exhibits a fast relaxation process that precedes structural relaxation. Structural relaxation obeys a time-temperature superposition that extends over a temperature range of 540 K. The mode-coupling theory of the liquid to glass transition [mode-coupling theory (MCT)] gives a consistent description of the dynamics that governs the mass transport in liquid Pd-Ni-Cu-P alloys. MCT scaling laws extrapolate to a critical temperature Tc at about 20% below the liquidus temperature. Diffusivities derived from the mean relaxation times compare well with Co diffusivities from recent tracer diffusion measurements and diffusivities calculated from viscosity via the Stokes-Einstein relation. In contrast to simple metallic liquids, the atomic transport in dense, liquid Pd₄₃Ni₁₀Cu₂₇P₂₀ is characterized by a drastic slowing down of dynamics on cooling and a q^-2 dependence of the mean relaxation times at intermediate q as a result of a highly collective transport mechanism. At temperatures as high as 2Tc diffusion in liquid Pd₄₃Ni₁₀Cu₂₇P₂₀ is as fast as that in simple liquids at the melting point. However, the difference in the underlying atomic transport mechanism indicates that the diffusion mechanism in liquids is not controlled by the value of the diffusivity but rather by that of the packing fraction.