Using 3D EBSD, additional information is obtained about the effect of grain volume and of martensite distribution on strain accommodation. We quantify orientation gradients, pattern quality and GND density variations at ferrite–ferrite and ferrite–martensite interfaces. The latter assumption is analyzed using high-resolution electron backscatter diffraction (EBSD).

These effects are attributed to the higher second phase fraction and the inhomogeneous transformation strain accommodation in ferrite.

The steel with higher martensite fraction has a lower elastic limit, a higher yield strength and a higher tensile strength. We study orientation gradients and geometrically necessary dislocations (GNDs) in two ultrafine grained dual-phase steels with different martensite particle size and volume fraction (24 vol.% and 38 vol.%). The procedures to post-process the ASTAR™/PED datasets in order to obtain the important (and currently largely hidden) details of nanotwins as well as quantifications of dislocation density distributions are described in this study. This orientation microscopy technique provides the raw data required for the determination of these parameters. The remarkable spatial resolution of this technique (~ 2 nm) enables highly detailed characterization of nanotwins, grain boundaries and the configuration of dislocations. In this paper, such details are provided using a transmission electron microscope-based orientation microscopy technique called ASTAR™/precession electron diffraction. Therefore, it is vital to determine the orientation, width and distance of nanotwins, the angle and axis of grain boundary misorientations as well as the type and the distributions of dislocations in an automatic and statistically meaningful fashion in a relatively large area. The presence and interaction of nanotwins, geometrically necessary dislocations, and grain boundaries play a key role in the mechanical properties of nanostructured crystalline materials.

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