Application of orientation imaging microscopy to study phase transformations in steels

In this investigation, the methodology, advantages, and limitations of usin g Orientation Imaging Microscopy (OIM) to study microstructures in multipha se steels are examined. In particular, proeutectoid ferrite precipitation h as been investigated in a hypoeutectoid steel (Fe-0.12%C-3%Ni), while the p roeutectoid cementite transformation has been studied in a hypereutectoid s teel (Fe-1.3%C-13%Mn). By utilizing misorientation maps and inverse pole fi gure maps to determine the orientation differences between individual cryst als, the internal morphology of the solid-state precipitates has been deter mined. What appear by other types of imaging to be monolithic single crysta ls are shown to be aggregates of finer crystals with misorientations betwee n them. The relative quality of the Electron Backscatter Diffraction (EBSD) patterns has been correlated to specific phases and to interphase and grai n boundaries using pattern quality maps (also referred to as image quality maps). These techniques have been adapted for quantitative identification o f different microconstituents, including volume fraction measurements. By m aking comparisons among optical micrographs, conventional SEM micrographs, pattern quality maps, inverse pole figure maps, and misorientation maps, th e advantages and limitations of different types of OIM mapping for studying different aspects of phase transformations in steels are elucidated. (C) 2 001 Elsevier Science Inc. All rights reserved.