足立 大樹

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.

Type-B serrations were observed during room-temperature tensile deformation of Al-2.17mass%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.

22Mn-0.6C (wt. %) high-Mn austenitic steel having fully recrystallized microstructure was obtained through 4 cycles of repeated cold-rolling and annealing process. Mechanical properties were evaluated by tensile test at an initial strain rate of 8.3 × 10-4 s-1 at room temperature and the local strain and strain-rate distribution of the specimen were analyzed using digital image correlation (DIC) method. The results revealed that a strain localization behavior characterized by the formation, propagation, and annihilation of strain-localized bands, called Portevin-Le Chatelier (PLC) bands, determines the global mechanical properties including serration behavior on the stress-strain curve. In addition, in-situ synchrotron XRD measurement during tensile test clarified that the dislocation density was significantly accumulated as the PLC band passed through the X-ray beam position, which led to the local strain hardening in the propagating PLC band. Such a local strain hardening was repeated during the whole tensile deformation, and the global strain hardening progressed in the 22Mn-0.6C steel.

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.

Grain refinement is one of the methods applied to strengthen metallic materials, and various peculiar mechanical properties have been reported to be expressed when the grain size is reduced to less than submicron dimensions. This is considered to be due to a change in the behavior of dislocations that are associated with plastic deformation. In situ synchrotron radiation measurements of microstructural changes during deformation in face-centered cubic (fcc) metals with grain sizes of 20 μm to 5 nm were performed to systematically investigate the effects of grain size on dislocation behavior during plastic deformation. In pure aluminum with grain sizes of 20 to 3 μm, the dislocation density during plastic deformation was approximately 1014 m−2, regardless of the grain size. However, when the grain size was less than 3 μm, the dislocation density increased monotonically in proportion to the grain size to the power of -1. Furthermore, in a nickel alloy with a grain size of less than 10 nm, this relationship was no longer satisfied, and the results suggested that deformation progresses due to partial dislocations. In materials with a grain size of less than 1 μm, the dislocation density after unloading became much smaller than that during loading.