Novel scanning electron microscope (SEM) imaging approaches to study microstructural evolution of alloys at elevated temperatures
Keywords: phase transformation
high-temperature
Scanning Electron Microscope (SEM)
carbon steel
heat treatment
pearlite
EBSD
ferrite
in-situ SEM

This thesis presents an investigation into high-temperature imaging of metals and alloys using a novel heat stage for in situ scanning electron microscopy (SEM). The results demonstrate the advantages and challenges of SEM imaging at elevated temperatures using secondary electron (SE) and electron backscatter diffraction (EBSD) detectors. The data collected using the heat stage showed beam, vacuum and detector stability at high-temperatures, without the need for shielding or detector modification, owing to the optimally designed heat stage and specimen geometry.
The novel heat stage was used to conduct investigations, during the heat treatment of carbon steel, into the impact of surface effects on the surface and bulk microstructural evolution. Results highlighted a significant discrepancy between the surface and bulk grain growth of the specimens during heat treatment at temperatures of 850 °C and above. It was concluded that the combination of thermal etching and oxidation led to the retardation of grain growth on the surface of carbon steel, but that the bulk of the specimen’s grain growth was unaffected. The mechanism that caused the grain retardation was found to be not dissimilar to Zener pinning, where in this case oxide particles pinned the newly exposed etched grain boundaries. However, it was found that for heat treatments of carbon steel conducted at 800 °C for less than 1 hour, the surface microstructural evolution was representative of the bulk, as there was a lack of oxidation present at this temperature. Usefully, though, the presence of oxidation at higher temperatures did allow for the tracking of the morphological evolution that occurred during the thermal oxidation of carbon steel.
Despite some of the challenges, the presence of thermal etching did enable in situ tracking of the microstructural evolution of carbon steel during the heat treatment. High-temperature SE imaging, combined with thermal etching, was used to study the grain and phase transformations during 1-hour heat treatments of carbon steel at 800 °C. The temperature and timescales were chosen to ensure the surface observations were representative of the bulk microstructure. The high-temperature SE data, supported by optical and high-temperature EBSD data, suggested that grain growth in the austenitic region could be described by linear, ‘abnormal’ grain growth. The real-time SE and EBSD data also enabled studies on the phase transformation into the austenitic region from starting structures of ferrite / pearlite and martensite. The results of these studies compared favourably to current phase-field models.
High-temperature SE and EBSD imaging of nickel and steel allowed for the investigation of one of the most notable, but rarely documented, limitations of high-temperature SEM. Namely, the impact of material properties on the specimen beam interaction on EBSD imaging. Data showed high-temperature EBSD imaging was affected by both the evolution of surface roughness during heating and the material’s magnetic properties. In particular, at temperatures close to the Curie temperature of nickel there was a loss of EBSD image quality, but quality was regained when temperatures were further increased. This was attributed to the influence of local magnetization on electron trajectories, likely resulting in a decrease in interaction volume and subsequent loss of EBSD image quality.
Overall, this thesis documents the development, testing, and use of a novel heat stage in the study of the high-temperature microstructural evolution of nickel and carbon steel. In particular, this project provides a unique scientific contribution to the understanding of microstructural changes during the heat treatment of carbon steel, as well as the development and application of a new high-temperature SEM imaging technique.