By conducting In-situ XRD measurement during tensile deformation while oscillating the tensile tester, it was possible to measure the change in dislocation density of a pure aluminum alloy having coarse grains with the grain size of 20 μm. In the coarse-grained material, the dislocation density during tensile deformation changed through four regions, as in the case of the fine-grained material. Since the dislocation multiplication start stress was very low at 22MPa, the elastic deformation region was very short. Thereafter, the dislocations multiplied rapidly, but when the stress and dislocation density reached 33 MPa and 1.57×1014m-2, respectively, the dislocation multiplication rate was greatly reduced. This is considered to be due to the low dislocation density required to progress the deformation by plastic deformation in coarse-grained aluminum.
Time-resolved X-ray tomography was performed to observe three-dimensional equiaxed-grain structure in Al-15mass%Cu alloy refined by inoculant TiB2 particles. The time resolution of as short as 0.5 s was achieved by using a pink X-ray beam at a beamline BL28B2 of SPring-8. A filtering method using a phase field model was used for improving quality of reconstructed images, and consequently motion of equiaxed grains were observed in-situ. The time and spatial resolutions allowed to analyze the three-dimensional microstructural evolution. Nucleation events of α-Al preferably occurred in the melt of the bottom part, where the temperature was lower. The dendritic grains were formed during floating after the nucleation because of the buoyancy force. The floating equiaxed grains were stacked due to the floating, and the solid fraction of the stacked regions ranged from 0.11 to 0.19. The observations suggested that the coherency point of dendritic grains was less than 0.2. The observations also showed the grain motion influenced the formation of equiaxed grain structure.
This paper proposes a technique for evaluating the nucleation sequence and grain refinement phenomena during solidification of a metallic alloy. Time-resolved and in-situ transmission imaging using a monochromatized X-ray in a synchrotron radiation facility, SPring-8, showed wave-like nucleation events, in which nucleation sequentially occurred along the temperature gradient, in an Al-4mass%Si alloy with addition of TiB2 (Al-3mass%Ti-1mass%B alloy). The position and time of nucleation events were measured to analyze the nucleation wave. The wave motion, which was calculated from the measured data, was used to evaluate the difference of nucleation temperature for each dendritic grain from the wave front. The standard deviation of the temperature difference in the Al-4mass%Si alloy with TiB2 particles was evaluated to be as small as 0.25 K. The addition of TiB2 reduced the standard deviation to more than one tenth. This study shows that the dispersion or the standard deviation is a representative figure for evaluating the potency of nuclei.
This study investigated cluster formation in the early stages of natural aging in Al-1.04 mass%Si-0.55 mass%Mg alloys by soft X-ray XAFS measurements and first-principles calculation. XAFS measurements at the Mg-K and Si-K edges were carried out at the BL27SU beamline at SPring-8. It was found that the absorption edge energies changed as aging proceeded. Density functional theory (DFT) calculations were used to determine the valence electron densities near Si and Mg atoms and to simulate the Si-K and Mg-K edge spectra for some cluster models. On the basis of the results, it was demonstrated that Si and Mg atoms formed clusters in four stages (I-IV) during natural aging. In stage I, Si-vacancy pairs, Mg-vacancy pairs, and a combination of both were formed. In stage II, vacancies were released from the clusters formed in stage I. In stage III, Mg-vacancy pairs were included in the clusters. In stage IV, the clusters coarsened through the release of vacancies. These results indicate that soft X-ray XAFS, which is capable of identifying individual elements, has the ability to provide information on such clusters.
The deformation behavior and microstructure evolution of Ti-6Al-4V alloy under room temperature creep was investigated using mechanical test and scanning electron microscope observation with electron back-scatter diffraction method. The alloys were creep deformed and ruptured under initial stresses of 874MPa, 889MPa and 904MPa at room temperature. The rapid stress change test revealed that creep deformation was controlled by the viscous slip motion of dislocations. The stress exponent was estimated as 59. The strain rate of acceleration creep region calculated by the Nortonʼs law with the high stress exponent was inconsistent with that measured by the experimental creep test. Using the slip trace analysis, it was found that single dislocation slip in basal and prism were mainly activated in the early stage of creep, and multiple slips were often observed as the deformation progresses. Especially, the multiple slip including 1st pyramidal slip believed to be effective for suppressing strain rate acceleration in creep. In addition, the work hardening behavior during creep showed a strain rate dependence, indicating that the lower the strain rate is, the more work hardening occurs.
Energy absorbing materials with isotropic mechanical properties are necessary for landing gears of spacecrafts. Porous Al-10Si-0.3Mg alloys with an acorn shape are designed for JAXA small lander for investigating moon. The ordered cellular structure with the porosity of 94% is manufactured through a powder bed fusion process. Compression tests are carried out on the inclined plane. Acorn shaped porous aluminum alloys showed good energy absorption behavior on the inclined plane up to 20°. In addition, compression tests of the acorn shaped porous aluminum alloys are carried out on the plane with a hemispherical boulder. Experimental results can be explained using a theoretical calculation of the contact area between the acorn shaped specimen and the inclined plane or between the specimen and the boulder. Energy absorption diagram revealed that the absorbed energy became high at the compression on the small size of the boulder. It was found that the acorn shaped porous aluminum alloy had a potential as an energy absorbing material for the small lander on the moon.