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.

Misorientation of Grains in Fatigue of Harmonic Structured Steel Observed by Diffraction Contrast Tomography Using Ultrabright Synchrotron Radiation

“…Ludwig et al proposed diffraction contrast tomography (DCT) to reconstruct the shape and position of each grain in a polycrystalline material using ultrabright synchrotron radiation X-rays [4]. Using DCT, Nakai et al developed a method to evaluate the excess dislocation density ρ from the orientation spread of diffraction spots [5]. DCT can reveal the excess dislocation density of each grain inside the sample as well as on the sample surface [5][6][7].…”

“…Using DCT, Nakai et al developed a method to evaluate the excess dislocation density ρ from the orientation spread of diffraction spots [5]. DCT can reveal the excess dislocation density of each grain inside the sample as well as on the sample surface [5][6][7]. It is a nondestructive observation technique that enables a continuous observation of the local plastic deformation of grains.…”

“…DCT imaging was performed using the BL46XU beamline of the synchrotron radiation facility of Super Photon ring 8 GeV (SPring-8), which uses an undulator beam and provides an ultrabright X-ray beam with high coherence, enabling microtomography with high spatial resolution in the μm range. Details of the DCT measurement conditions are given elsewhere [5][6][7]. Evaluation of Excess Dislocation Density.…”

“…Evaluation of Excess Dislocation Density. The total misorientation β can be calculated by measuring the rotation angle spread that satisfies the diffraction condition Δωdiff, the diffraction angle θ, and the Debye ring angle ψ for each diffraction spot of a grain that satisfies the Bragg condition [5].…”