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We report simultaneous radiotracer diffusion experiments of Co-57 and of Zr-95 in binary $\mathrm{Z}{\mathrm{r}}_{64}\mathrm{N}{\mathrm{i}}_{36}, \mathrm{Z}{\mathrm{r}}_{36}\mathrm{N}{\mathrm{i}}_{64}$, and ternary $\mathrm{Z}{\mathrm{r}}_{60}\mathrm{N}{\mathrm{i}}_{25}\mathrm{A}{\mathrm{l}}_{15}$ alloys above but near the liquidus temperature (${T}_{l}$). In contrast to the multicomponent $\mathrm{Z}{\mathrm{r}}_{46.75}\mathrm{T}{\mathrm{i}}_{8.25}\mathrm{C}{\mathrm{u}}_{7.5}\mathrm{N}{\mathrm{i}}_{10}\mathrm{B}{\mathrm{e}}_{27.5}$ (Vit4), where a significant component decoupling at the ${T}_{l}$ is observed, the ratio between Zr and Co self-diffusion coefficients is smaller than two in the alloys with fewer components. The difference in the degree of decoupling compared to Vit4 can be explained in terms of different ${T}_{l}$'s. Moreover, owing to the high accuracy of the simultaneous tracer diffusion technique, we are able to resolve a small but notable composition dependence of the ratio ${D}_{\mathrm{Co}}/{D}_{\mathrm{Zr}}$, which decreases with increasing Zr content for all glass forming alloys reported here. In contrast to a hard sphere (HS)-like mixture, where decoupling is controlled only by atomic sizes, this indicates a coupling of the Co/Ni and Zr diffusion, due to the strong chemical affinity between the diffusing components. Our results are in very good agreement with recent simulations in $\mathrm{Z}{\mathrm{r}}_{64}\mathrm{N}{\mathrm{i}}_{36}$ based on mode coupling theory (MCT).

We report (95)Zr and (57)Co radiotracer diffusivities and viscosity data in the equilibrium liquid state of a bulk metallic glass forming Zr(46.75)Ti(8.25)Cu(7.5)Ni(10)Be(27.5) melt (Vitreloy 4) far above the liquidus temperature T(l) that are not affected by convection, as evidenced via quasielastic neutron scattering. Zr diffusion is strongly decoupled from diffusion of the smaller components by more than a factor of 4 at T(l), although it obeys the Stokes-Einstein equation. The results suggest that, in the present Zr-based metallic glass forming systems, diffusion and viscous flow start to develop solidlike, i.e., energy-landscape-controlled, features already in the stable liquid state more than 300 K above the mode coupling temperature T(c).