Simulated electron diffraction patterns of ω-Fe in Fe-C martensite

A metastable hexagonal omega-Fe phase with its particle size of about 1-2 nm coexists at the {112}< 111 >-type twinning boundaries in Fe-C martensite. The omega-Fe lattice parameters are dependent on their corresponding body centered cubic (BCC) phase (a(omega) = root 2 a(bcc), c(omega) = root 3/2a(bcc)). The above dependence and the unique distribution of the omega-Fe particles at the BCC {112}< 111 >-type twinning boundaries inevitably cause the overlapping of electron diffraction spots of fine omega-Fe with those of BCC and its twin during transmission electron microscope (TEM) observations. Thus, in order to understand the nature of the ultrafine omega-Fe phase in carbon steels, we have simulated its electron diffraction patterns by means of the commercial CrystalMaker software. In most of the zone axes, which show the electron diffraction pattern with three sets of diffraction spots [(1) the BCC matrix crystal, (2) the corresponding twin crystal, and (3) the possible twinning double diffraction spots] together, the simulated results have shown that the diffraction spots from the omega-Fe phase are all overlapped with those three sets of spots. However, in some specific zone axes, for example, in BCC [112] directions, the diffraction spots from the omega-Fe phase can be uniquely recognized since the BCC {112}< 111 >-type twinning boundary plane is perpendicular to the observation direction, and thus, no twinning double diffraction spots should be observed theoretically. Published under license by AIP Publishing.