Mass Dependence of Diffusion in a Supercooled Metallic Melt

Abstract

The isotope effect $E\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\left({D}_{\ensuremath{\alpha}}/{D}_{\ensuremath{\beta}}\ensuremath{-}1\right)/(\sqrt{{m}_{\ensuremath{\beta}}/{m}_{\ensuremath{\alpha}}}\ensuremath{-}1)$ 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, $E\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0.09\ifmmode\pm\else\textpm\fi{}0.03$, 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.