Transmission electron microscopy/electron energy loss spectroscopy measurements and ab initio calculation of local magnetic moments at nickel grain boundaries.

We have determined local magnetic moments at nickel grain boundaries using a transmission electron microscopy/electron energy loss spectroscopy method assuming that the magnetic moment of Ni atoms is a linear function of the L3/L2 (white-line ratio) in the energy loss spectrum. The average magnetic moment measured in the grain interior was 0.55 µB, which agrees well with the calculated magnetic moment of pure nickel (0.62 µB). The local magnetic moments at the grain boundaries increased up to approximately 1.0 µB as the mis-orientation angle increased, and showed a maximum around 50°. The respective enhancement of local magnetic moments at the Σ5 (0.63 µB) and random (0.90 µB) grain boundaries in pure nickel was approximately 14 and 64% of the grain interior. In contrast, the average local magnetic moment at the (111) Σ3 grain boundary was found to be 0.55 µB and almost the same as that of the grain interior. These results are in good agreement with available ab initio calculations.

In situ 3D crystallographic characterization of deformation-induced martensitic transformation in a metastable Fe-Cr-Ni austenitic alloy by X-ray microtomography.

Excellent strength-ductility balance in metastable Fe-Cr-Ni austenitic alloys stems from phase transformation from austenite (fcc structure) to α' martensite (bcc structure) during deformation, namely deformation-induced α' martensitic transformation (DIMT). Here, DIMT in a metastable Fe-17Cr-7Ni austenitic alloy was detected in situ and characterized in three dimensions (3D) by employing synchrotron radiation X-ray microtomography. This technique utilizes refraction contrast, which is attributable to the presence of phase boundaries between the parent austenite and the newly formed α' martensite phase. By combining microtomography and position-sensitive X-ray diffraction, we succeeded in crystallographically identifying multiple α' martensite phases continuously transformed in four groups from a single parent austenitic phase.

Switching nanoprecipitates to resist hydrogen embrittlement in high-strength aluminum alloys.

Hydrogen drastically embrittles high-strength aluminum alloys, which impedes efforts to develop ultrastrong components in the aerospace and transportation industries. Understanding and utilizing the interaction of hydrogen with core strengthening elements in aluminum alloys, particularly nanoprecipitates, are critical to break this bottleneck. Herein, we show that hydrogen embrittlement of aluminum alloys can be largely suppressed by switching nanoprecipitates from the η phase to the T phase without changing the overall chemical composition. The T phase strongly traps hydrogen and resists hydrogen-assisted crack growth, with a more than 60% reduction in the areal fractions of cracks. The T phase-induced reduction in the concentration of hydrogen at defects and interfaces, which facilitates crack growth, primarily contributes to the suppressed hydrogen embrittlement. Transforming precipitates into strong hydrogen traps is proven to be a potential mitigation strategy for hydrogen embrittlement in aluminum alloys.

High-energy x-ray nanotomography introducing an apodization Fresnel zone plate objective lens.

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 ∼60 µm, 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 100× 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, ∼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.