Site specific He ion irradiation damage studies in nanolayered thin films by cross-coupling Helium Ion Microscopy with TEM and APT V. Shutthanandan 1 , A. Devaraj 1 , R. S. Vemuri 1 , C. M. Wang 1 , T. Varga 1 , CH. Henager Jr. # , S. Thevuthasan 1 1 EMSL, #Pacific Northwest National Laboratory, Richland, WA 99354, USA Over recent years materials with a high density of nanoscale interfaces received increasing attention due to their improved radiation tolerance in comparison to their bulk forms [1-3]. The efficient trapping and recombination of radiation induced point defects such as vacancies and interstitials at such interfaces are proposed to be the fundamental reason for their increased radiation tolerance. Several different ODS steels, nanostructured ceramic materials and nanolayered thin films have been recently investigated to understand the fundamental mechanisms associated with radiation damage. In many of these investigations, high energy He ion irradiations were carried out in a large area over the entire specimen followed up with characterization of radiation damage. The spot size of ion irradiation beams from conventional sources was in the order of 100s of microns or larger preventing a site specific irradiation damage investigation of individual microstructural features. In such cases often the overall irradiation damage evolution in the material would be a cumulative response of the entire material microstructure (grain boundaries, interphase-interfaces, second phase precipitates and other preexisting defects) to the ion beam irradiation. A nanoscale site specific He ion irradiation method, if made possible, can aid in decoupling and individually analyzing the He ion irradiation response of different microstructural features in a mutually exclusive manner. He ion microscopy (HIM) developed in recent years offer such a capability for obtaining coherent He ion beams that can be precisely controlled and directed to areas as small as few square nanometers. In EMSL, a DOE national user facility in PNNL, efforts are underway to look at irradiation response of nanoscale microstructural features in nanolayered metallic thin films by cross coupling site specific He ion irradiations with TEM and Atom probe. Sample preparation methods using Focused ion beam (FIB) are ideally suited to make successful measurements. A schematic of the site specific irradiation set up using Helium ion microscope is shown in fig (1) where the red squares are representative of site specific irradiation of a part of a grain boundary or a second phase precipitate. Proof of principle experiments are being conducted in nanoscale PVD synthesized Ti/Al nanolayer thin films using He ion irradiation doses ranging from 1E14 to 1E17 ions/cm 2 . TEM sample preparation using FIB based liftout method helps in direct comparison of irradiated and unirradiated regions within the same TEM sample as shown in Fig 2(a). Fig 2 (b) shows the bright field TEM image obtained from such a sample of Ti/Al 5nm Multilayer thin film, where half of the area is irradiated and the other half is unirradiated. FIB lift out based APT sample preparation and subsequent atom probe analysis from the irradiated and unirradiated area aided in estimating the compositional intermixing that occurred as a result of irradiation. Detailed characterization carried out in such a manner help to develop an in-depth understanding of the interface damage, crystal lattice damage, amorphization and He bubble formations during ion irradiation of nanoscale multilayer thin films and metallic materials. These insights were used to investigate whether the internal interfaces in metallic materials can be manipulated at the nanoscale to enhance dynamic