Atomic dynamics in Zr-based glass forming alloys near the liquidus temperature

Abstract

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).