Orientation gradients and geometrically necessary dislocations were  studied in two ultrafine-grained dual-phase steels with differing martensite particle sizes and volume fractions (24 and 38vol%). The steel with higher martensite fraction has a lower elastic limit, a higher yield strength and a higher tensile strength. These effects were attributed to the higher second phase fraction and the inhomogeneous transformation strain accommodation in ferrite. The latter assumption was analyzed using high-resolution electron backscatter diffraction. Orientation gradients, pattern quality and geometrically necessary dislocation density variations at ferrite–ferrite and ferrite–martensite interfaces were quantified. Using three-dimensional electron back-scattering diffraction, additional information was obtained about the effect of grain volume and of martensite distribution on strain accommodation. Two methods were demonstrated to calculate the geometrically necessary dislocation density from the electron back-scattering diffraction data based on the kernel average misorientation measure and on the dislocation density tensor, respectively. The overall geometrically necessary dislocation density was shown to increase with increasing total martensite fraction, decreasing grain volume, and increasing martensite fraction in the vicinity of ferrite.

Orientation Gradients and Geometrically Necessary Dislocations in Ultrafine Grained Dual-Phase Steels Studied by 2D and 3D EBSD. M.Calcagnotto, D.Ponge, E.Demir, D.Raabe: Materials Science and Engineering A, 2010, 527[10-11], 2738-46