Atomic transport in amorphous ${\mathrm{Pd}}_{40}$${\mathrm{Ni}}_{40}$${\mathrm{P}}_{20}$ near the glass-transition temperature: Au diffusivity and viscosity

Measurements of the diffusivity of ion-implanted Au in amorphous ${\mathrm{Pd}}_{40}$ ${\mathrm{Ni}}_{40}$ ${\mathrm{P}}_{20}$ have been performed at temperatures just below the glass-transition temperature. Concentration profiles before and after annealing were determined with Rutherford-backscattering spectrometry. It is found that the diffusivity in as-quenched material decreases to an equilibrium value presumably due to structural-relaxation processes. The equilibrium values of the diffusivity show a very strong temperature dependence and can be described by ${\mathit{D}}_{\mathrm{eq}}$(T)=1.7×${10}^{17}$ exp[-(413 kJ/mol)/RT]${\mathrm{m}}^2$/s. The results have been quantitatively compared to the viscosity of the same material revealing the breakdown of the Stokes-Einstein relation for this metallic glass. This implies the existence of two types of atomic-transport mechanisms in amorphous ${\mathrm{Pd}}_{40}$ ${\mathrm{Ni}}_{40}$ ${\mathrm{P}}_{20}$. Moreover, from cycling between different equilibrium values of the diffusivity, we found that once amorphous ${\mathrm{Pd}}_{40}$ ${\mathrm{Ni}}_{40}$ ${\mathrm{P}}_{20}$ has reached a fully relaxed state, the relation between the viscosity and diffusivity is lost.