Dislocation characterization by the direct-fitting/modified williamson-hall (DF/mWH) method in ultra-low carbon martensitic steel

The Williamson-Hall (WH) plots are the basic approach for the dislocation characterization. However, the elastic anisotropy affects full width at half maximum in diffraction peaks and this makes the dislocation characterization difficult. In order to correct the effect of elastic anisotropy, Ungár developed a unique methodology using the contrast factor, so called the modified Williamson-Hall (mWH) method. On the other hand, authors developed a new methodology termed as "direct-fitting (DF) method" in which the elastic anisotropy is corrected directly applying the correction parameter; ωhkl. By the DF method, reliable values are obtained for the parameter α which contains an information of crystallite size. In this paper, the α-value obtained by the DF method was applied to the mWH method and the dislocation characterization was performed in an ultra-low carbon martensitic steel (Fe-18%Ni alloy) with cold rolling up to 20% thickness reduction. It was found that high dislocation density ρ of 2.1×1015/m2 is obtained in as-quenched specimen and the cold rolling does not give significant effect on dislocation density ρ. However, the parameter φ obtained by the mWH method changes markedly by charging small amount of cold rolling. As a result, the parameter A, that depends on the values of ρ and φ, changes markedly by charging small amount of cold rolling: A=0.77 in as-quenched specimen but A=0.60 in specimens with cold rolling. This result indicates that the dislocation arrangement has been changed from homogeneous to inhomogeneous distribution by cold rolling.