Observation of microstructure in thick steel with high voltage TEM

HVEM make possible to conduct experiments with relatively thicker specimen than that of used in a conventional medium voltage TEM. Bulk specimen is of big advantage for studying internal microstructures because true strained state of materials is to be easily relaxed in thin specimen and then various micro structural factors that control mechanical properties are subjected to change. In this study, HVEM, 1250kV acceleration voltage, was applied to study microstructure in high strength steels using specimens over several 100nm thicknesses. In addition to improving transparency of incident electron, reducing chromatic and stigmatic aberration using an energy filtering technique was useful to conduct observation of such thick and magnetic materials. Density of dislocation and fine carbides from direct observation were consistent to the values estimated from XRD and quantitative chemical analysis. In-situ loading experiment using a tensile specimen holder was also successfully performed to track mutual interaction between nano-sized carbide and dislocations. This in-situ observation proved that dispersed fine carbides in studied steel was to be strong obstacle against to mobile dislocations.