Revisiting the phase transformations of wüstite scale formed on low carbon steel

This work used x-ray diffraction, electron backscatter diffraction and transmission electron microscopy to explore phase transformations of wüstite formed on the surface of a low carbon steel. Wüstite scale soaked at 450°C first transformed into magnetite through heterogeneous nucleation and growth at the magnetite-wüstite interface, followed by the formation of topotactically transformed magnetite with the same orientation as that of the parent wüstite at both the magnetite-wüstite and wüstite-substrate interfaces. Finally, the residual wüstite was exhausted by eutectoid decomposition. The topotactic magnetite grains possessed rugged semicoherent boundaries and occasionally contained untransformed wüstite islands in their interior. In addition, the surrounding parent wüstite exhibited an ordered defect structure which resulted in weak <110> type diffraction spots in electron diffraction patterns. This study explored the potential origins of topotactic transformation.