足立 大樹

We propose a novel method for the visualization of the shape and stress–strain evolution in the local deformation of metallic materials. We combined in situ X-ray diffraction with simultaneous transmittance measurements while scanning the specimen, to visualize the temporal evolution of the spatial distributions of thickness, stress, and inhomogeneous strain in the sample. Through a comparison of stress and strain distributions within the local deformation regions, it was demonstrated that work hardening is the dominant factor contributing to the increase in stress in the local deformation area of pure copper. This method is expected to contribute to the elucidation of local deformation in manufacturing processes.

Type-B serrations were observed during room-temperature tensile deformation of Al-2.17 mass%Mg alloy with an average grain size of 12 µm. Digital image correlation was used to visualize Portevin-Le Chatelier (PLC) bands, and microstructural changes in these bands were observed by in-situ X-ray diffraction measurements using synchrotron radiation at SPring-8. The results indicated that the overall density of dislocations, including both mobile and pinned dislocations inside the PLC bands, increased substantially as the bands formed. This suggests that serration occurs due to an increase in the mobile dislocation density resulting from the formation of new dislocations from sources inside the PLC bands.

The microstructural factors contributing to the high strength of additive-manufactured Al–Si alloys using laser-beam powder bed fusion (PBF-LB) were identified by in-situ synchrotron X-ray diffraction in tensile deformation and transmission electron microscopy. PBF-LB and heat treatment were employed to manufacture Al–12%Si binary alloy specimens with different microstructures. At an early stage of deformation prior to macroscopic yielding, stress was dominantly partitioned into the α-Al matrix, rather than the Si phase in all specimens. Highly concentrated Si solute (∼3%) in the α-Al matrix promoted the dynamic precipitation of nanoscale Si phase during loading, thereby increasing the yield strength. After macroscopic yielding, the partitioned stress in the Si phase monotonically increased in the strain-hardening regime with an increase in the dislocation density in the α-Al matrix. At a later stage of strain hardening, the flow curves of the partitioned stress in the Si phase yielded stress relaxation owing to plastic deformation. Therefore, Si-phase particles localized along the cell walls in the cellular-solidified microstructure play a significant role in dislocation obstacles for strain hardening. Compared with the results of the heat-treated specimens with different microstructural factors, the dominant strengthening factors of PBF-LB manufactured Al–Si alloys were discussed.

This study investigated the cluster formation process in the early stages of 353 K aging in Al1.04 mass%Si0.55 mass%Mg alloys by means of soft X-ray absorption fine structure (XAFS) measurements and first-principles calculations. XAFS at the Si-K and Mg-K edges was carried out at the BL27SU beamline at SPring-8. To observe the structural changes in detail, an XAFS apparatus able to hold the sample at 353 K in a vacuum chamber and cool it rapidly to suppress the progress of clustering was developed. Density functional theory (DFT) calculations were used to simulate the Si-K and Mg-K edge spectra for various cluster models. Based on the results, the cluster formation process in the early stages of aging at 353 K was qualitatively clarified. Initially, MgVa (Va: vacancy) pairs and SiVa pairs were formed, then 2-MgVa clusters formed by bonding between MgVa pairs along (100); subsequently, L10 clusters were formed by Mg atoms ordered along (100), and then SiVa-py clusters with Va adjacent to the first-nearest-neighbor atom of Si atoms and Si-py without adjacent Va were formed, in which MgVa pairs and SiVa pairs were individually united, respectively. Monolayer and multilayer structures then developed as aging proceeded, involving Mg and Si atoms ordered along (100), in which Mg and Si atoms were bonded.