Investigations of Phase Transformations in AISI 5160 Steel and Ceria Partially Stabilized Zirconia via Electron Backscatter Diffraction Based Techniques

Abstract

In this dissertation, electron backscatter diffraction (EBSD) is applied to study the phase transformation in AISI 5160 steel and 10 mol% Ce doped ZrO2. For AISI 5160 steel, quantitative characterization of the volume fractions and spatial arrangements of the major constituents of microstructure is needed for continued progress toward the next generation of advanced steels. EBSD data can be used to simultaneously characterize orientation relationships as well as determine the volume fraction and size of grains of retained austerite and ferrite due to the difference in diffraction patterns caused by the different crystal structures of the two phases; however, distinguishing among martensite, ferrite, pearlite, and bainite using EBSD remains a challenge. A detailed analysis of the capabilities of EBSD-based methods for separation of the individual microstructural constituents is performed on samples of a commercial steel - AISI 5160. It is demonstrated that kernel average misorientation (KAM) data can be used to distinguish and quantify the constituents.

Zirconia is one of many materials that experiences a displacive phase transformation. In zirconia, the tetragonal to monoclinic martensitic transformation can be induced either thermally or by applied stress. Since transformation from tetragonal to monoclinic phase is accompanied by a volume increase, it may be engineered to utilize the martensitic transformation as a toughening mechanism. In the present work, EBSD was used to observe the morphology and orientation relationships simultaneously. According to the EBSD study, all six orientation relationships were present. The theoretical predictions based on the phenomenological theory of martensitic transformations were also calculated for 10 mol% ceria doped zirconia and compared with experimental findings. The calculation of the strains showed that the correspondence B has the lowest lattice invariant strain among the three correspondences, and for the shape strain, the three correspondences are nearly equivalent.