Papers by Author: Yasuhiro Aruga

Abstract: The effects of Sn addition on clustering and age-hardening behavior in an Al-0.6Mg-1.0Si (mass %) alloy were investigated. Addition of Sn delayed the age-hardening in single aging at 170 ̊C. On the other hand, Sn promoted the age-hardening response in 3-step aging process which comprises a pre-aging (PA) at 90 ̊C for 18ks followed by natural aging (NA) for 604.8ks and artificial aging (AA) at 170 ̊C. The characteristics of clusters formed during PA and NA were evaluated by differential scanning calorimetry (DSC) analysis and atom probe tomography (APT). The DSC results show that the endothermic peak at around 160 ̊C to 200 ̊C was observed in the Sn-free alloy. On the other hand, in the Sn-added alloy, endothermic peak was not observed. It is suggested that Sn addition suppresses the formation of the clusters formed during NA. The APT results show that the Sn addition decreases the number density of clusters, especially smaller clusters. No Sn precipitates were found in Mg-Si precipitates formed during AA at 170 ̊C for 3.6ks. It is speculated that suppression of smaller cluster formation by addition of Sn promotes the age-hardening response

Abstract: In this study we aim at combining the results from transmission electron microscopy (TEM) and atom probe tomography (APT) to study the early stages of phase decomposition in the age hardening alloy AA 6016. Samples are subjected to different periods of natural ageing or artificial pre-ageing at elevated temperature in order to produce different types of clusters and early stages of precipitation before age hardening commences. APT is utilized to detect clusters and identify their compositions, whereas TEM is applied to analyse and quantify number density and sizes of the particles during artificial ageing at 185°C. It is shown that the two techniques, TEM and APT, are complementary and a combined approach yields more detailed insight into the early stages of phase decomposition in age hardening 6xxx series alloys than possible by the sole use of either technique individually.

Abstract: The effects of pre-aging and natural aging on the bake hardening behavior of Al-0.62Mg-0.93Si (mass%) alloy with multi-step aging process were investigated by means of Vickers hardness test, tensile test, differential scanning calorimetry analysis (DSC) and transmission electron microscopy (TEM). The characteristics of nanoclusters (nano scale solute atom clusters) formed during pre-aging and natural aging were also investigated using the three dimensional atom probe (3DAP) analysis. The results revealed the occurrence of natural age hardening and that the bake hardening response was decreased after the extended natural aging even though the pre-aging was conducted before natural aging. Since the 3DAP results exhibited the Si-rich clusters were newly formed during extended natural aging, it was assumed that the Si-rich clusters caused the natural age hardening and the reduced bake hardening response corresponding to Cluster(1). The decrease of the bake hardening response was markedly higher in the later stage of bake hardening than in the early stage. The size of the β’’ precipitates were reduced with increasing the natural aging time. Exothermic peaks of Peak 2 and Peak 2’ were observed in the DSC curves for the alloys pre-aged at 363K. Peak 2’ became larger with the natural aging time. This is well understood by the following model. The transition from Cluster(2) to the β’’ phase occurs preferentially at the early stage of the bake hardening. Then the growth of the β’’ phase is inhibited by the presence of Cluster(1) at the later stage of bake hardening. The combined formation of Cluster(1) and Cluster(2) by the multi-step aging essentially affects the bake hardening response and the β’’ precipitates in the Al-Mg-Si alloys.

Abstract: The relationship between the cluster morphology formed during natural or artificial aging and the paint-bake hardening response in an Al-0.62Mg-0.93Si (mass%) alloy have been investigated using atom probe tomography (APT). Increasing the subsequent aging time at 170 °C causes a gradual increase in hardness in the artificially aged materials, while the retardation period of the hardness increase appears in the naturally aged materials at the early stage of aging. The statistically-proved records in the APT analysis have shown that the artificially aged materials have some large clusters. It is revealed that the hardening at the early stage of the subsequent aging at 170 °C is not promoted in the long-time naturally aged material although the number density of small clusters increases approximately 1.3 times by prolonged natural aging.Hence, we believe that the small clusters are hard to transform continuously into the β'' phase during aging at 170 °C. As for the naturally aged materials, the long-time aging leads to a significant drop in hardness at the early stage of aging at 170 °C. It is speculated that the Mg-Si mixed clusters formed after long-time natural aging can be reversed during the subsequent heat treatment.

Abstract: The serrated flow phenomena in Al-Mg alloys with and without Zn were investigated after aging on several conditions, focusing on the role of precipitates. Al-6mass%Mg-0~3mass%Zn alloys were solution treated at 753~803K, quenched, and then aged at room temperature. Further artificial aging at 323~573K for 86.4ks was performed for some of them after natural aging for 2.6Ms. The serrated flow behavior was evaluated by tensile test. Microstructure was characterized by differential scanning calorimetry, transmission electron microscopy, atom probe tomography, and positron annihilation lifetime spectroscopy. The increase in the amount of Zn addition and the natural aging time lead to a delayed onset of serrated flow. The artificial aging at higher temperatures after natural aging, on the other hand, decreases the onset strain. A large number of small coherent Zn-Mg clusters are formed during natural aging in the Al-Mg-Zn alloys, which are transformed to the larger incoherent meta-stable precipitates during subsequent artificial aging. These results suggest that the mechanism of interfering with serrated flow is related to the vacancy trapping effect, which is enhanced by the coherent clusters.

Abstract: The strain-amplitude dependence of internal friction in Cu-0.41Ni-0.11P (mass%) alloys has been evaluated to reveal the relation between the amplitude-dependent internal friction and the stress relaxation performance. Annealing at 250°C after cold rolling causes a suppression of the strain-amplitude dependence with increasing annealing time in the range between 10 s and 10⁴ s. Analysis of the amplitude-dependent internal friction reveals the plastic strain of the order of 10^-9 as a function of effective stress on dislocation motion. It is found that the microflow stress at a constant level of plastic strain increases with increasing annealing time. This result is in line with the improvement in the stress relaxation performance but disagrees with a decrease in the tensile strength and yield stress after annealing. We believe that the increase in the microflow stress after annealing is caused by inhibition of dislocation motion due to Ni-P clusters, which were revealed by three-dimensional atom probe (3DAP) experiments.

Abstract: Three-dimensional zinc mapping based on X-ray K-edge scanning has been performed. By microtomographies with energies above and below the K-absorption edges of the elements, the concentration distribution of the elements is evaluated during in-situ experiments, respectively. It is found that the Zn concentration distribution during the heat treatment was changed inside the cell wall of the aluminum foams and it has been homogenized. Also several precipitated phase transformation can be three-dimensionally visualized by the CT-method tuning X-ray energies.

Abstract: The effect of the amount of dispersoids on softening behavior and recrystallized microstructure of Cu-Fe-P alloy was examined by the extracted residue analysis method. The degrees of contribution of larger particles (larger than 1μm in an average diameter) and smaller ones (less than 0.1μm) to the softening behavior were considered in the quantitative aspect, respectively. It was found that the change of the order of 10-1mass% in the amounts of both particles has a great effect on softening behavior. The difference in the amount of fine particles changes recrystallized grain size distributions at similar hardness. In the specimen with a small amount of fine particles, coarse grains and wide distribution of grain size were observed after annealing. As a result, it was revealed that fine and homogeneous recrystallized microstructure was obtained due to just 0.35mass% of fine partcles, even if the amount of large particles increased.

Abstract: X-ray CT method is a kind of nondestructive inspection, but has strong limitation in sample size due to a small field of view (FOV). The higher the resolution, the smaller FOV is, mainly due to the element number of available detectors commercially. Therefore, sample machining is more or less necessary so that the sample size is fit within the small FOV in the case of the high-resolution observation. Local tomography technique enables a high resolution reconstruction of small region of interests within a sample without the sample machining. In this study, we have evaluated the size effects of aluminum foam samples in terms of the 3D image quality by the local tomography techniques.

Abstract: Synchrotron X-ray microtomography has been utilized for the 3D characterisation of microstructure in the cell materials of aluminium foams. Tomographs, consisting of about 109 isotropic voxels with a maximum of 1.0µm edge, were collected at the SPring-8 in Japan. A combination of high-resolution phase contrast imaging technique and several state-of–the-art application techniques has enabled the quantitative image analyses of micro-pore, intermetallic particles and grain boundary as well as the assessment of their effects on compressive deformation and fracture behaviours in two kinds of aluminium foams.