Mass Dependence of Diffusion in a Supercooled Metallic Melt

Abstract

The isotope effect $E=\left(D_{\alpha }/D_{\beta }-1\mathrm{}\right)/(\sqrt{m_{\beta }/m_{\alpha }}-1\mathrm{})$ of cobalt diffusion in the deeply supercooled melt of the metallic alloy ${\mathrm{Zr}}_{46.7}{\mathrm{Ti}}_{8.3}{\mathrm{Cu}}_{7.5}{\mathrm{Ni}}_{10}{\mathrm{Be}}_{27.5}$ has been measured employing the radiotracers ${}^{57}\mathrm{Co}$ and ${}^{60}\mathrm{Co}$. The isotope effect is very small, $E=0.09\mathrm{}\pm 0.03$, and exhibits no significant temperature dependence in a range up to 120 K above the calorimetric glass transition temperature $T_g$, encompassing almost 3 orders of magnitude in the diffusivity. This result suggests that long-range diffusion in the deeply supercooled melt is not mediated by viscous flow but rather proceeds by collective hopping processes involving about ten atoms.