Atomic diffusivities in amorphous and liquid Cu-Zr: Kirkendall effects and dependence on packing density

A novel method for measurement of atomic interdiffusivity is applied to amorphous Cu-Zr close to its glass-transition temperature Tg. Sputter-deposited multilayers are examined in cross-section by transmission electron microscopy and energy-dispersive X-ray spectroscopy. Mapping the evolution of composition profiles gives the interdiffusivity, which is orders of magnitude higher than if coupled to the viscosity expected near Tg. Kirkendall drift of interlayer interfaces in both amorphous and supercooled liquid states (i.e. below and above Tg), and associated voiding in the liquid, show that the diffusivity of copper greatly exceeds that of zirconium. Amorphous Cu-Zr is known to show maxima in atomic packing density at sharply defined compositions. The comparison of the two compositions in the present work provides the first direct evidence that denser packing is associated with lower atomic interdiffusivity. The lower interdiffusivity is governed by a lower diffusivity of copper, and reflects a lessened degree of decoupling of the copper (fast) and zirconium (slow) diffusivities in an efficiently packed glass. The new insights help to understand issues ranging from glass-forming ability to the controlled generation of nanovoided structures.