4D-STEM Strain Mapping of Nanometre-Scaled Defect Clusters

Energetic neutron irradiation changes the mechanical and physical properties of materials due to the accumulation and evolution of the primary defects into stable nanometre sized interstitial and vacancy clusters. The statistics of sizes of these defect clusters has direct implications for microstructural evolution and the degradation of properties of a material under irradiation. The current state-of-the-art for modelling the accumulation and evolution of irradiation-induced defects, i.e. rate theory, cluster dynamics and object kinetic Monte Carlo, all use highly simplified models of defect geometry and interactions. Molecular dynamics (MD) captures the full complexity of atomic behaviour, at the expense of handling only limited length- and time-scales. Despite the importance of the question, no experimental technique yet exists to provide observations at atomic level to validate the results provided by MD simulations, especially for defect clusters that are less than 2 nm in size. It is important to note that, according to MD estimation, they may amount to 97% of the overall defect population and will have significant contribution to the evolution of matrix defects and diffusion of alloying elements that resulted in the hardening effects on the irradiated material. Hence, it is crucial to be able to characterise these small defect clusters, i.e. &lt; 2nm, for mechanistic understanding of the degradation behaviour of materials under irradiation, and for physics understanding for simulation and modelling purposes.<br/>AtomCRaD, a research project that was funded by the EUROfusion consortium and Research Council UK, focuses on developing modern advanced Scanning Transmission Electron Microscopy (STEM) characterisation techniques dedicated for the radiation damage, especially those small defect clusters. The characterisation techniques development used new generation S/TEM and coupled with latest STEM image simulation capability, such as uSTEM and Prismatic.<br/>AtomCRaD demonstrated the importance of convergence angle for defect clusters detection and its influence on the strain contrast of the defect clusters. With a suitable convergence angle, STEM can detect defect clusters with diameter down to ~1 nm with confidence. Using STEM image simulation, it indicated that the limitation of pushing the detection limit further, i.e. &lt; 1nm, is on the TEM specimen surface quality and thickness.<br/>Furthermore, AtomCRaD demonstrated that strain maps can be generated from 4DSTEM dataset of nanometer-scaled defect cluster, i.e. &lt;3nm. collected using pixelated detector. The results indicated that high spatial resolution strain variation, i.e. ~1 nm, can be detected and measured. The unique strain profile induced by various defect clusters with difference burger vectors can be compared with simulated MD models. Early results show that the strain profile measured from experimental data has a good match with simulated data. The discrepancy and future application of such method will be further discussed.