Microstructural Evolution and Vacancy Defect Formation in Mn–Mo–Ni RPV Steel Under Low Cycle Fatigue: Insights From EBSD and PALS

Low cycle fatigue (LCF) is a critical degradation mechanism in reactor pressure vessel (RPV) steels, potentially compromising the structural integrity of nuclear power plants. This study investigates the microstructural evolution and defect formation in a Mn–Mo–Ni low‐alloy RPV steel subjected to LCF at a total strain amplitude of ± 0.7% at room temperature. Electron backscatter diffraction (EBSD) and positron annihilation lifetime spectroscopy (PALS) were employed to assess changes in dislocation substructure, grain orientation, and defect configurations after LCF loading. EBSD analysis revealed enhanced intragranular misorientations indicating significant dislocation activity and localized strain accumulation. PALS measurements identified an increase in positron lifetime components associated with microvoids and dislocations, suggesting the formation and growth of subsurface defects during fatigue cycling. The complementary use of EBSD and PALS provided a comprehensive understanding of the damage mechanisms operative during LCF.