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type:Full length article, We directly captured, classified, and evaluated 3D particle debonding and fracture behavior in a H-charged 7075 Al alloy throughout the entire tensile deformation using synchrotron X-ray tomography and microstructural feature tracking techniques. The effects of particle size, shape, spatial clustering and stress state on strain-dependent particle damage were identified and isolated from each other. Moreover, state-of-the-art imaging and tracking techniques enabled the establishment of spatially and time-resolved hydrogen distributions during deformation. Based on realistic hydrogen partitioning among various nanoscopic trap sites, the contributions of particles to hydrogen trapping and the hydrogen effect at individual damaged particles were assessed quantitatively. Fracturing of coarse and irregular Al7Cu2Fe particles was found to be the predominant particle damage mode due to the spatial clustering and brittleness of these particles, but a hydrogen effect was not observed. The debonding of Mg2Si particles seemed to be the result of competition between hydrogen and clustering-induced stress localization, but detrimental effects of hydrogen on ductile fracture induced by accelerating interfacial debonding were found to be limited. The quantitative evaluation of particle damage in the present model material clarified a viable strategy for mitigating hydrogen embrittlement, which involves introducing and modifying intermetallic particles with strong hydrogen trapping capacities.

type:Mechanics of Materials, Crystallographic assessment of the hydrogen embrittlement behavior of Al–Zn–Mg alloy was performed by means of a technique combining fracture trajectory analysis and synchrotron X-ray diffraction contrast tomography. The 3D microstructure reconstructed using diffraction contrast tomography contained 119 grains. Fracture surfaces revealing intergranular fracture, ductile fracture, and quasi-cleavage fracture were observed in the alloy. While the intergranular crack initiated at a grain boundary with high grain boundary energy and a high angle between the grain boundary plane and loading direction, the crack propagation itself was not observed to be sensitive to these two parameters. The quasi-cleavage fracture surfaces were not characterized by any specific crystal orientation because of variation in the free surface segregation energy of hydrogen uniforms without depending on surface orientation.

To develop high strength Al-Zn-Mg alloys, suppression of hydrogen embrittlement is indispensable. The hydrogen embrittlement behavior of different prepared Al-10.1-1.2Mg alloys with various hydrogen trap sites was observed using in situ synchrotron X-ray tomography in this study. Furthermore, we quantified the hydrogen partitioning based on hydrogen occupancies and hydrogen trap site densities in the prepared alloys. The combined analysis of hydrogen embrittlement and hydrogen partitioning showed that initial trapped hydrogen content in grain boundaries, vacancies, and dislocations before deformation was not crucial for inducing both intergranular fracture and quasi-cleavage fracture. However, hydrogen accumulates at grain boundaries and precipitate interfaces during deformation, inducing intergranular and quasi-cleavage fracture, respectively. Due to hydrogen accumulation, intergranular and quasi-cleavage fracture initiate when the hydrogen content at the grain boundary enriches 10^3～10^4 times the initial content and hydrogen content at the precipitate interface enriches 3.9×10^2 times the initial content, respectively. The change in hydrogen trap sites by processing and heat treatments did not suppress the intergranular and quasi-cleavage fracture. We conclude that generating new hydrogen trap sites (e.g., the interior of the intermetallic particle) in which hydrogen trap site density and binding energy are higher than the precipitate interface (> 33.87 kJ/mol) is beneficial to suppress hydrogen embrittlement.

In this study, high-energy x-ray nanotomography (nano-computed tomography, nano-CT) based on full-field x-ray microscopy was developed. Fine two-dimensional and three-dimensional (3D) structures with linewidths of 75 nm-100 nm were successfully resolved in the x-ray energy range of 15 keV-37.7 keV. The effective field of view was similar to 60 mu m, and the typical measurement time for one tomographic scan was 30 min-60 min. The optical system was established at the 250-m-long beamline 20XU of SPring-8 to realize greater than 100x magnification images. An apodization Fresnel zone plate (A-FZP), specifically developed for high-energy x-ray imaging, was used as the objective lens. The design of the A-FZP for high-energy imaging is discussed, and its diffraction efficiency distribution is evaluated. The spatial resolutions of this system at energies of 15 keV, 20 keV, 30 keV, and 37.7 keV were examined using a test object, and the measured values are shown to be in good agreement with theoretical values. High-energy x-ray nano-CT in combination with x-ray micro-CT is applied for 3D multiscale imaging. The entire bodies of bulky samples, similar to 1 mm in diameter, were measured with the micro-CT, and the nano-CT was used for nondestructive observation of regions of interest. Examples of multiscale CT measurements involving carbon steel, mouse bones, and a meteorite are discussed.