Ion-beam mixing of Ni/Pd layers: II. Thermally assisted regime (>500K)

Atomic mixing in Ni/Pd bilayer films due to 120 keV Ar⁺ irradiation in the thermally assisted regime (523−673 K) has been measured, in situ, using Rutherford backscattering with 2.0 MeV ⁴He⁺ ions. The mean diameter of grains in these polycrystallinc films increased from 10 to 60 nm, following Ar⁺ bombardment at 573 K. Initial mixing was rapid due to grain boundary diffusion and incorporation of the metal solute into the solvent metal matrix by grain growth; this mixing stage was essentially complete within 10 min for annealed films or after an Ar⁺ dose of 4 × 10¹⁵ cm⁻² in irradiated films (10 min irradiation). No further measurable mixing occurred in the annealed, unirradiated films. For the irradiated samples the initial rapid mixing (6−35 atoms/ion) was followed by a slower mixing stage of 0.7–1.8 atoms/ion for irradiation doses of up to 2.5 × 10¹⁶ Ar⁺ cm⁻². The Ar⁺ bombardment gave rise to much smaller mixing levels when the Pd films were deposited on large-grain or single-crystal Ni. A diffusion analysis demonstrates that the effective diffusivity, Deff, for ion-irradiation-enhanced mixing in the thermally assisted regime satisfied the relation D_l < D_eff < D_B, where the ratio of the grain boundary to lattice diffusivity was D_B/D_l > 10⁶.