Microstructure evolution of metallic nanocrystalline thin-films under ion-beam irradiation

The microstructural evolution of nanocrystalline metallic thin-films under ion irradiation, especially grain growth and second-phase precipitation, was studied with detailed in situ experiments, and a theoretical model was developed to explain the results of grain-growth. Free-standing Zr, Pt, Cu and Au, Cu-Fe, and Zr-Fe nanocrystalline thin films prepared by sputter deposition were irradiated in-situ at the Intermediate Voltage Electron Microscope (IVEM) at Argonne National Laboratory with Ar and Kr ions to fluences in excess of 1016 ion/cm2 at temperatures ranging from 20 to 773 K.; The microstructural evolution of the thin-films was followed in situ by systematically recording bright field images and diffraction patterns at successive ion-irradiation doses. Grain growth was observed as a result of irradiation in all samples at all irradiation temperatures. The results suggest the existence of three regimes with increasing irradiating temperature: a low temperature regime (below about 0.15 to 0.22 Tm) where grain-growth does not depend on the irradiation temperature, a thermally assisted regime where both the grain-growth rate and the final grain size increase with increasing irradiation temperature, and a thermal regime where thermal effects dominate ion beam effects. Similarly to thermal grain growth, the ion-irradiation induced grain growth curves could be best fitted with curves of the type: Dn-Dn0=KF with n∼3 in the low temperature regime. The effect of solute addition on grain-growth was investigated using Zr(Fe) and Cu(Fe) supersaturated solid-solutions. In the case of Zr-Fe, Zr2Fe precipitates formed during irradiation (with the dose-to-precipitation of Zr2Fe decreasing with increasing irradiation temperature), whereas Cu-Fe remained as a solid-solution. The grain-growth rate and final size decreased in both alloys with respect to the pure metallic films as a result of second-phase particle pinning (Zener drag) (Zr-Fe), and solute drag (Cu-Fe). The grain-growth rate was found to depend on material and on irradiation conditions. In particular, neither grain-growth nor precipitation occurred under 1 MeV electron irradiation. Combined with rate-theory calculations, this shows that long range diffusion does not play a role in the process.; A model of grain-growth under ion irradiation in the temperature-independent regime was developed, based on direct impact of irradiation-induced thermal spikes on grain-boundaries. Grain-boundary migration occurs by atomic jumps within the thermal spike biased by the grain-boundary curvature driving force. The model incorporates cascade structure features such as subcascade formation, and the probability of subcascades occurring at grain-boundaries. This results in a power law expression relating the average grain-size with the ion dose, where the exponent is 3 in agreement with the experimental data. In the thermally assisted regime, the increased grain-growth is explained within the framework of the same model, by the increased thermal spike size with higher substrate temperature. At the highest temperatures, other effects such as subcascade overlap and point defect migration to sinks may also enhance the process.