elastoplastic phenomena, such as plastic deformation and failure, are multi-scale, deformation-pathdependent, and mechanical-field-sensitive problems associated with metals. Accordingly, visualization of the microstructural deformation path under a specific mechanical field is challenging for the elucidation of elastoplastic phenomena mechanisms. To overcome this problem, a dislocation-resolved in-situ technique for deformation under mechanically controllable conditions is required. Thus, we attempted to apply electron channeling contrast imaging (ECCI) under tensile loading, which enabled the detection of lattice defect motions and the evolution of elastic strain fields in bulk specimens. Here, we presented the suitability of ECCI as an in-situ technique with dislocation-detectable spatial resolution. In particular, the following ECCI-visualized plasticity-related phenomena were observed: (1) pre-deformation-induced residual stress and its disappearance via subsequent reloading, (2) heterogeneous dislocation motion during plastic relaxation, and (3) planar surface relief formation via loading to a higher stress. Metals have been extensively used in structural applications due to their superior ductility and toughness. These characteristics are attributed to plasticity-driven strain evolution and associated stress redistribution. The plasticity is originated from dislocation glides, which have a variable behavior sensitive to local shear stress, depending on external load and stress concentration. Therefore, in order to understand ductility and toughness, the plasticity mechanisms under certain mechanical conditions need to be shown. In this context, observations using a dislocation-resolved in-situ technique and done under mechanically controllable conditions are the most suitable approach. From a spatial resolution perspective, transmission electron microscopy (TEM) is an effective technique for resolving dislocations 1-4. However, regarding in-situ deformation experiments, the special boundary conditions in a thin foil-type specimen may be problematic, giving unwanted signals such as effects of image force 5-9. Consequently, in addition to the TEM approach, there is a high demand for a dislocation-resolved in-situ technique suitable for bulk specimens. Electron channeling contrast imaging (ECCI) enables visualization of dislocations 10-13 , stacking faults 6,11,12,14 , twins 11,12,15,16 , and elastic strain fields using a field-emission scanning electron microscope. Thus, ECCI can be used to characterize bulk specimens, allowing for the use of the conventional geometry of the mechanical test specimens 13,17-19. Therefore, dislocation-resolved ECCI was chosen as a mechanically specified in-situ technique for the analysis of deformation. Moreover, since lattice defects can be detected under optimal specimen surface orientation, nanometer-scale deformation heterogeneity can be visualized in an observational area greater than a 10 µm square.

To understand the mechanism of FCC–HCP martensitic transformation, we applied electron channeling contrast imaging under cooling to −51°C and subsequent heating to 150°C. The stacking faults were randomly extended and aggregated during cooling. The stacking fault aggregates were indexed as HCP. Furthermore, the shrink of stacking faults due to reverse motion of Shockley partials was observed during heating, but some SFs remained even after heating to the finishing temperature for reverse transformation (A f : 104°C). This fact implies that the chemical driving force of the FCC/HCP phases does not contribute to the motion of a single SF but works for group motion of stacking faults.

Microstructural changes during the martensitic transformation from face centred cubic (FCC) to body centred cubic (BCC) in an Fe–31Ni alloy were observed by scanning electron microscopy (SEM) with a newly developed Peltier stage available at temperatures to −75 °C. Electron channelling contrast imaging (ECCI) was utilized for the in situ observation during cooling. Electron backscatter diffraction (EBSD) analysis at ambient temperature (20 °C) after the transformation was performed for the crystallographic characterization. A uniform dislocation slip in the FCC matrix associated with the transformation was detected at −57 °C. Gradual growth of a BCC martensite was recognized upon cooling from −57 °C to −63 °C.

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Toward establishing in-situ electron channeling contrast imaging (ECCI), we present three insights in ECC images, which are related to {111} surface traces in face-centered cubic alloys. For this purpose, ECCI was carried out under tensile loading in an austenitic steel. First, dislocation slip and stacking fault formation during deformation can be distinguished in ECCIs. Second, crystallographic orientation can be determined by three {111} traces in ECCIs. Third, shrinkage of stacking faults on {111} planes under unloading was observed by in-situ ECCI even in a smooth specimen, which implies a significant difference between in-situ and ex-situ observations.