Evaluation of Dislocation Density of Coarse and Fine Grains in Bimodal Harmonic Structured Steel Observed by Diffraction Contrast Tomography using Ultrabright Synchrotron Radiation

Abstract

Austenitic stainless steels with a bimodal harmonic structure, in which the fine grain structure (Shell) exists around the coarse grain structure (Core), are prepared by powder metallurgy to improve both strength and ductility. Herein, X‐ray diffraction contrast tomography, a 3D grain mapping technique for polycrystalline materials using ultrabright synchrotron radiation X‐rays, is used to reconstruct the grain shape and location and to evaluate the average excess dislocation density of the Core and Shell structures. This technique allows one to evaluate the excess dislocation density not only on the surface, but also inside the sample where damage occurs in tensile tests. The results show that the excess dislocation density of the Shell structure is higher than that of the Core structure. The excess dislocation density of homogeneous austenitic stainless steels with grain sizes similar to the Core structure of the harmonic structured stainless steel is higher than that at comparable stresses, indicating that the deformation of the bimodal harmonic structured alloy is localized in the fine grain structure. This is consistent with the results obtained from electron backscatter diffraction analysis, in which the surface grains are evaluated.