Texture evolution of friction stir-processed dual-phase steel

Abstract

Evolution of crystallographic texture and local strains in different zones of a dual-phase steel subjected to severe plastic deformation by friction stir process was examined by electron backscatter diffraction technique. Misorientations, which were expressed by the values of grain orientation spread and kernel average misorientation (KAM), increased from the thermomechanical affected zone (TMAZ) to pin-affected stir zone (PE-SZ). KAM maps showed that the highest local strain appeared in the PE-SZ. Misorientation distributions were found to be consistent with microhardness distributions in the processed region. The crystallographic texture of base metal, heat-affected zone, and TMAZ was similar. Recrystallization and deformation texture components were observed at stir zones, and recrystallization texture components decreased drastically at PE-SZ. Rotated cube, goss and rotated goss texture components were detected at PE-SZ. Rotated cube texture component indicates partial recrystallization after plastic deformation. Shear texture components were determined using shear direction (SD) as reference. Different shear texture components (111), (112), and (101) || SD were found in the processed region revealing the differences in material flow and plastic deformation at each zone. Some other texture components were found to be resulting from transformation of austenite into ferrite/martensite, by taking orientation relationship between those parent and product phases. Taylor factor values of all zones were calculated in order to understand the influence of texture on mechanical properties. The results indicate that crystallographic texture has a minor influence on hardness compared to grain refinement and dislocation density.