On the grain boundary network characteristics in a dual phase steel

The relative areas of interfaces in dual-phase steel containing an equal fraction of ferrite and martensite have been measured and classified according to five crystallographic interface parameters. When the martensite–martensite (M–M), ferrite–ferrite (F–F), and ferrite–martensite (F–M) interfaces were analysed separately, it was apparent that the distribution in each category was determined by the dominant phase transformation mechanism (diffusional vs displacive) upon the formation of a given phase (ferrite vs martensite). The misorientation angle distribution of the M–M interfaces showed a bimodal distribution, with one mode in the range of 5°–22° and the second mode in the range of 45° to 60°, with a significant peak at ~ 60°. The F–F interfaces were spread across all misorientation angles, revealing two broad peaks at ~ 13° and ~ 60°. The F–M interfaces displayed a mixed character inherited from both martensite and ferrite interfaces. The grain boundary plane distribution was also anisotropic. For example, the relative area of M–M interfaces terminated on {110} planes was greater than two multiples of a random distribution (MRD). The 60°/[111] misorientation revealed symmetrical tilt {112} boundary planes for the F–F interfaces with a relative area > 40 MRD, which correspond well with the low energy configuration, whereas the most common M–M interfaces were symmetrical tilt {110} boundaries with a relative area > 20 MRD, that result from the crystallographic constraint associated with the displacive transformation. For the case of F–M interfaces, the 60°/[111] misorientation exhibited multiple peaks spread along the zone of tilt boundaries, inherited from both diffusional ferrite and displacive martensite phase transformations.