Diffusion and viscous flow in bulk glass forming alloys

We review radiotracer diffusion and isotope measurements in bulk glass forming alloys from the glassy state to the equilibrium melt and compare diffusion and viscous flow. In the glassy as well as in the deeply supercooled state below the critical temperature Tc, where the mode coupling theory predicts a freezing-in of liquid-like motion, very small isotope effects indicate a highly collective hopping mechanism. Not only in the glassy state but also in the supercooled state below Tc the temperature dependence of diffusion is Arrhenius-like with an effective activation enthalpy. A clear decoupling takes place between the diffusivities of the individual components of the alloys and between time scales related to diffusive transport and viscous flow. While the component decoupling is small for the smaller components a vast decoupling of more than 4 orders of magnitude is observed in Pd–Cu–Ni–P alloys between the diffusivity of the large majority component Pd and of the smaller components at the glass transition temperature Tg. The diffusivities of all components merge close to the critical temperature Tc of mode coupling theory. Above Tc, the onset of liquid-like motion is directly evidenced by a gradual drop of the effective activation energy. This strongly supports the mode coupling scenario. The isotope effect measurements show atomic transport up to the equilibrium melt to be far away from the regime of uncorrelated binary collisions. For Pd, in contrast to the behavior of single component molecular glass formers, the Stokes–Einstein equation even holds in the entire temperature range below Tc over at least 14 orders of magnitude. Apparently, the majority component Pd forms a slow subsystem in which the other elements move fast. Rearrangement of the Pd atoms thus determines the viscous flow behavior. The decoupling of atomic mobility seems to arise from a complex interplay between chemical short order and atomic size effects that gets more pronounced on approaching the glass transition temperature. The ability of the bulk glass forming alloys to form a slow subsystem in the liquid state appears to be a key to the understanding of their excellent glass forming properties.

Research highlights▶ We measured radiotracer diffusivities of all components in a Pd43Cu27Ni10P20 melt. ▶ We see a vast decoupling between the diffusivity of Pd and of the smaller components at Tg. ▶ We see no decoupling between Pd diffusion and viscous flow. ▶ The Stokes-Einstein equations holds for Pd in the hole supercooled range. ▶ Pd forms a slow subsystem.