Papers by Author: Tatsuo Sato

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: Two types of nanoclusters are formed during low temperature aging and play important roles in age-hardening of Al-Mg-Si alloys. The formation behavior of these nanoclusters depends on the alloy composition and heat-treatment process. In this work, the various alloys with different Mg and Si concentration were used in order to clarify the influence of alloy composition on the nanocluster formation using differential scanning calorimetry (DSC), hardness and electrical resistivity measurements. Based on the DSC results, two overlapped exothermic peaks were clearly detected, showing the formation of Cluster (1) and Cluster (2) in all examined alloys with different alloy composition. These two overlapped peaks are separated by the Gaussian function method to analyze the volume fraction of nanoclusters quantitatively. It is found that the Si and Mg concentration of Al-Mg-Si alloys has a marked effect on the nanocluster formation. The formation of Cluster (1) is more related with the Si concentration, whereas Cluster (2) is correlated with both of the Mg and Si concentration. Furthermore, the important point is that the formation behavior of nanoclusters strongly depends on the Mg/Si ratio of the alloys. The formation of nanoclusters is most enhanced when the Mg/Si ratio is approximately 1.0.

Abstract: Microstructures and mechanical properties of the D-SSF (Deformation Semisolid Forming) processed Al-Zn-Mg alloys with high Fe content up to 2 mass% were investigated. A high ductility alloy even containing 1 mass% Fe was successfully produced by applying the D-SSF process. Especially, the D-SSF processed alloy with 1 mass% Fe was superior to the conventionally processed alloy with Fe-free in the properties of the tensile strength and elongation. These results clearly indicate that the harmful influence by the addition of 1 mass% Fe is effectively modified into not harmful one by applying the D-SSF process. Furthermore, the former liquid phase regions (FLP regions) which are finally solidified during the D-SSF process are completely extinguished by the solution treatment and resultantly the mechanical properties are greatly improved. The extinction process of the FLP regions during the solution treatment was examined using an EBSD method in terms of the crystal orientations of the α-Al grains.

Abstract: The characteristic age-hardening response of Al-3.0Mg-1.0Cu (mass%) alloys with and without Ag addition has been investigated by the hardness measurement, differential scanning calorimetry (DSC) and electrical resistivity measurement. The alloy compositions locating in the (α+S+T) phase field of the Al-Mg-Cu phase diagram are known to be effective to harden in two stages separated by a distinct and often prolonged hardness plateau. The first stage of hardening occurs very rapidly (e.g. within 60 s at 443 K) and contributes to increase hardness as much as 50 % of the total age-hardening. In the Ag-added alloy, the hardness change during the first stage of hardening is larger and the plateau stage is shortened as a result of the fast arrival at the second stage of hardness. Small amount of Ag changes the age hardening response of the Base alloy dramatically. In the low temperature aging, the incubation stage at which no clear hardness and electrical resistivity increase appears for a long period before the first stage of hardening. After the pre-aging at this incubation period, a characteristic two-step aging response is observed. The peak hardness dramatically changes in the Al-3.0Mg-1.0Cu alloy, while no clear change in the Ag-added alloy.

Abstract: Ultrathin (2 emulsion (SCE). Incomplete coverage of the Cu plate, the working electrode, by electroplated Ni and non-uniform Ni films with defects were obtained when conventional electroplating at 1 A/dm² with 30 sec of deposition time was used. When electroplating with SCE (ESCE) was applied, complete coverage, defect-free and uniform UTNFs were obtained. SEM and AFM showed surface morphology of the UTNFs was covered by spherical-shaped particles with ~10 nm in diameter, which was expected to be individual Ni grains because the size was consistent with grain size of Ni films reported when ESCE was applied. High H₂ solubility in CO₂, periodic-plating-characteristic after applying ESCE, and improved transport efficiency of the reactive species are believed to be the main reasons to cause effects of grain refinement and suppression in formation of the defects. Thickness of the UTNFs was 11.97±1.82 nm when the deposition time was 15 sec, and the thickness increased to 38.45±1.71 nm when the deposition time was increased to 45 sec.

Abstract: The effects of pre-aging before natural aging on the bendability of an asymmetric-rolled Al-Mg-Si alloy were investigated. In the specimen without pre-aging, hardness increased with natural aging time due to the formation of nanoscale cluster (nanocluster; Cluster (1)). The suppression of Cluster (1) during natural aging is clearly seen in the pre-aged specimens though the formation of Cluster (2). It was found that tensile properties were not so affected by the types of clusters. Bending test clearly showed that the pre-aging improves the bendability of this alloy. This effectiveness of pre-aging means Cluster (2) well contributes the deformability of this alloy compared with Cluster (1). Such an improvement of bendability is considered to be derived from the structure of nanoclusters and its interactions with the factor of plastically deformation of alloys.

Abstract: It has been known that Cu- or Ag-addition Al-1.0mass%Mg₂Si (balanced) alloys shows higher hardness and elongation than Cu-free or Ag-free balance alloy. In this study, the alloys with Cu or Ag addition and the alloys with Si / Mg in excess have been investigated by hardness and tensile tests and HRTEM observation. Cu addition is effective for higher hardness, and Ag-addition is useful for improvement of elongation for peak-aged samples. Precipitates in peak aged these alloys have been confirmed by HRTEM. Cu-addition alloy almost includes Q’-phase, and Ag-addition alloy includes b’-phase. The precipitation sequence of Ag- or Cu addition Al-Mg-Si alloy was investigated using HRTEM, SAED, and EDS. The precipitates obtained in the two alloys were classified into several kinds by HRTEM images and SAED patterns. The relative frequencies of precipitates were also investigated and compared with that in the alloy.

Abstract: In the present work, b’ phase in alloys Al -1.0 mass% Mg₂Si -0.5 mass% Ag (Ag-addition) and Al -1.0 mass% Mg₂Si (base) was investigated by high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) to understand the effect of Ag addition. The b’ phase is rod-shape with the longest direction parallel to <001>_Al. HRTEM images and SAED patterns obtained along the direction were similar for the b’ phase in both alloys. The unit cell of b’-phase in Ag-addition alloy is hexagonal with the same c-axis dimension as the Ag-free b’, but shorter a-axis. Ag was found in the composition of the rod-shaped precipitates in Ag-addition alloy by energy dispersive X-ray spectroscopy (EDS). In addition, the distribution of Ag was investigated by energy filtered mapping and high annular angular dark field scanning transmission electron microscopy (HAADF-STEM). The Ag-containing atomic column was observed in every b’ unit cell, and the unit cell symmetry is slightly changed as compared with the Ag-free b’. The Ag-containing b’ rods have complicated domain structures. The interfaces of these particles are enriched with Ag atoms that occupy the lattice positions in the Al matrix. The occupancy of the Ag-containing atomic columns seem to vary both inside particles, as well as at the interfaces.

Abstract: A planetary ball milling (PBM) technique was employed to fabricate mechanically alloyed (MA processed) Al-Nb2O5 composite powder. Nano or sub-micron sized Nb2O5 particles were homogeneously embedded in the Al particles after milling for various periods. None of cracks, by-products and pores were observed in the areas between embedded Nb2O5 particulates and Al matrix powder after milling. The sequence of the in-situ reaction was confirmed by DSC, XRD measurements, optical microscopy and EPMA. The specific temperature of the in-situ reaction was between 650 and 700°C. Al-based metal matrix composites (MMC) reinforced with the sub-sieve sized θ-Al2O3 particulates and Al3Nb intermetallic compound were successfully fabricated by the in-situ reaction process. The substituted Nb by the in-situ reaction was fully reacted with Al to form the Al3Nb intermetallic compound during sintering. A number of sub-sieve sized θ-Al2O3 particulates and Al3Nb intermetallic compound formed by the in-situ reaction between Al and Nb2O5 were homogeneously distributed in the Al matrix during sintering. Nano sized θ-Al2O3 particulates are preferentially distributed near the Al3Nb intermetallic compound and no by-products are formed in the interfaces with the Al matrix.