In situ weak-beam scanning transmission electron microscopy observation of geometrically necessary dislocations formed by Mn precipitates in A533B alloy steel

In this study, the Charpy impact test was performed for mechanically introducing cracks in A533B steel. Then, in situ weak-beam scanning transmission electron microscopy (WB-STEM) annealing tests were performed from room temperature to 600 degrees C. A wide area surface polishing method that did not require chemical polishing or resin-filling process for bulk specimens were developed for microsampling a 200 nm thin film. The film was sampled from the strain site at the crack tip (EBSD-KAM value: 2.7 degrees) via Focus ion beam-scanning electron microscopy (FIB-SEM), i.e., the inhomogeneous plastic deformation zone of dislocation density above 2.5 x 1016 /m2 formed by Mn precipitates. In isochronous annealing process imaging, the dynamic behavior of dislocations was successfully visualized using movie files with a spatial resolution of 0.4 nm/pixel and a temporal resolution of 1s/frame via WB-STEM. Results revealed thermal relaxation of local strain as high density dislocations deformed into new subgrain boundaries via the geometrically necessary dislocation network at control temperatures from 500 degrees C to 550 degrees C.