Observation of Retained gamma Grains in TRIP Steels Using SEM-FIB/EBSD Method and Examination of Stability Evaluation Method

Morphologies of retained gamma grains in TRIP steels are a very important factor for understanding the relationship between the microstructure and mechanical properties of TRIP steels. We have investigated a serial sectioning technique using a scanning electron microscope (SEM) equipped with an electron backscatter diffraction (EBSD) and a focused ion beam (FIB) to reconstruct 3D microstructure of TRIP steels. In this paper, FIB fabrication condition dependences of reconstructed 3D structures are presented in TRIP steels and gamma-grains stabilities are discussed in terms of ion beam effects. Retained gamma-grains are not recognized by EBSD map from a FIB fabricated surface using normal ion accelerating voltage of 30 kV, because gamma grains are easily transformed into martensite due to ion irradiation. It was found that the transformation was suppressed by using a low acceleration voltage of 7 kV or less, and the three-dimensional morphology could be observed using low acceleration voltage serial sectioning technique. It becomes possible to evaluate morphology of each phase and we proposed evaluation method of gamma stabilities by ion irradiation transformation.