New perspectives on collision cascade damage in self-ion irradiated tungsten from HR-EBSD and ECCI

Understanding defect production and evolution under irradiation is a long-standing multi-scale problem. Conventionally, experimental examination of irradiation-induced dislocation loops (IIDLs) has mainly relied on transmission electron microscopy (TEM), which offers high spatial resolution but only limited strain sensitivity (strains less than 0.1% are challenging to evaluate). TEM also requires very thin samples, making multi-scale characterisation and quantitative strain measurements difficult. Here, we explore the potential of using advanced techniques in the scanning electron microscope (SEM) to probe irradiation damage at the surface of bulk materials. Electron channelling contrast imaging (ECCI) is used to image nano-scale irradiation-induced defects in 20 MeV self-ion irradiated tungsten, the main candidate material for fusion reactor armour. The results show an evolution of the damage microstructure from uniformly and randomly distributed nano-scale defects at 0.01 dpa (displacement per atom) to raft structures extending over hundreds of nanometres at 1 dpa. Cross-correlation based high-resolution EBSD (HR-EBSD) is used to probe the lattice strain fields associated with IIDLs. While there is little strain fluctuation at 0.01 dpa, significant heterogeneity in the lattice strains is observed at 0.1 dpa, increasing with dose until saturation at 0.32 dpa. The characteristic length scale of strain fluctuations is ~500 nm. Together, ECCI and HR-EBSD reveal a transition from a structure where defects are disordered to a structure with long-range order driven by elastic interactions between pre-existing defects and new cascade damage.