Self-ion-irradiation-induced grain formation in nickel

• Radiation damage in a cross-sectionally wedge-shaped single-crystal nickel Transmission Electron Microscopy (TEM) foil causes the build-up of stress in the implanted layer; • Point defects evolve into dislocation with a preferred orientation that leads to a radial bending of the foil; • MD simulations indicate that radiation damage in Ni with a constrained underlying layer is likely to give rise to the formation of interstitial-type dislocation loops with a preferential orientation that favors the radial bending of the foil as observed experimentally; • The loops combine to form lines and the lateral stress results in the “pile-up” of these dislocations to form low-angle grain boundaries, converting single-crystal material into long thin grains ≈ 100 – 200 nm in width but > 1 µm in length. • The width of the grain boundaries is determined by the interaction of glisssile dislocations with the stress induced by the radiation-damaged layer and that from a neighbouring boundary. A series of experiments has been conducted in which thin foils containing large polycrystals of Ni (single crystals from the perspective of transmission electron microscopy) have been irradiated with 300 keV Ni ions at temperatures from 25° to 475°C. The aim was to examine the fundamental aspects of the build-up of extended defects in a “simple” system with no implantation of foreign species and without the likelihood of segregation, precipitation or formation of new phases. Experiments were carried out using the MIAMI-2 facility in which the development of radiation damage is observed (and recorded) whilst ion-irradiating in-situ in a transmission electron microscope. Surprisingly, all irradiations of the electrochemically-thinned foils of Ni resulted in the accumulation of dislocations to form low-angle grain boundaries such that single crystal material was converted into a series of grains, each typically less than 200 nm in width but generally more than 1 µ m in length with the long axis approximately parallel to the edge of the foil. The early stages of this process have been modelled using Molecular Dynamics simulations and an interpretation of this process of radiation-induced grain-boundary formation is discussed in terms of the coupled effects of irradiation, temperature and stress induced by the radiation damage. The stress arises due to swelling in the thin irradiated region of the jet-polished specimens (with a wedge-shaped radial cross section) which is constrained by deeper-lying unirradiated material. The position in which a grain boundary forms is determined by the interaction of glisssile dislocations with the stress induced by the radiation-damaged layer and that from a neighbouring boundary.