Morphology and microstructure of M₂C carbide formed at different cooling rates in AISI M2 high speed steel

In as-cast structure of AISI M2 high speed steel, M₂C carbide prevails, the morphology of which has crucial influence on distribution and dimension of carbides in final products. In this study, the morphology and microstructure of M₂C formed at different cooling rates have been investigated by scanning electron microscope, X-ray diffraction, transmission electron microscope, and electron back-scatter diffraction. The results show that the morphology of M₂C carbide changes from the plate-like type to the fibrous one with increasing cooling rates. Surprisingly, the microstructure between plate-like and fibrous M₂C is significantly different. Twining and stacking faults are observed in the plate-like M₂C, which is supported by great misorientations between adjacent carbides. However, no planar faults are identified in fibrous M₂C and the carbides in one colony have almost identical orientation. It is expected that plate-like M₂C grows as a faceted phase, while fibrous M₂C formed at high cooling rates is likely a non-faceted phase. The difference of liquid/solid interface structure is supposed to result in the morphology change of M₂C.