Papers by Author: Tokimasa Kawabata

Abstract: CrAlN/BN nanocomposite coatings were deposited on mirror-polished silicon wafer and high-speed steel (HSS) substrates using reactive cosputtering, i.e., pulsed dc and rf sputtering of CrAl and h-BN targets, respectively. Further, the oxidation resistance of the obtained coatings was investigated. The CrAlN/BN coating exhibited superior oxidation resistance properties when compared with those of the CrAlN coatings; after annealing the sample at 800 °C in air for 1 h, the plastic hardness value of the CrAlN coatings decreased to 50% of the as-deposited hardness value; in contrast, the CrAlN/BN coatings exhibited self-hardening phenomena from 700 to 800 °C in the range of 5 to 30%. In particular, the CrAlN/18 vol% BN coatings showed an increase of approximately 30% in hardness values, and a maximum hardness value of approximately 50 GPa was reached after annealing the sample at 800 °C in air. The plastic hardness value hardly changed when the sample was annealed up to 800 ^°C in nitrogen and argon; this result was contrary to the result obtained for the sample that was annealed in air. The radiofrequency glow discharge optical emission spectroscopy (rf-GD-OES) analysis of the CrAlN/18 vol% BN coating annealed in air revealed that the coating has an oxide layer deposited on the surface to a depth of ~200 nm. Conventional transmission electron microscopy (TEM) observations of the same coating indicate that the columnar structure was disrupted by a thin layer (30–40 nm) of the coating annealed in air. The indentation hardness value of the annealed coating was measured using Ar ion sputtering before and after etching of the annealed surface. Subsequently, when the oxide layer was etched to a depth of 200 nm from the surface, the hardness value decreased from approximately 48 GPa to 43 GPa; this result was similar to the results obtained for the as-deposited coating.

Abstract: The current study was undertaken to investigate the synthesis of CrAlN/BN composite coatings having super high hardness by a reactive co-sputtering using CrAl alloy and BN targets and gaseous mixture of Ar+N2, in order to eliminate the possible formation of boride bonding. CrAlN or BN phase was deposited by pulsed d.c.- and r.f.- sputtering, respectively. Plastic hardness, Hpl, and Young’s modulus, E*, of the coatings increased with BN phase ratio, reaching a maximum value of ~46 GPa and 390 GPa at ~8 vol. % of BN phase; and then decreased to ~20GPa and ~300GPa at ~18 vol.%, respectively. Only B1 structured Cr(Al)N phase was found in XRD and SAED analysis. XPS and TEM/HRTEM results revealed that the CrAlN/8vol%BN coating consists mostly of CrAlN and BN phase, which exists as an amorphous like phase among the CrAlN grains. The CrAlN/8vol%BN coating has a kind of nanocomposite structure and the super high hardness over 40 GPa is probably due to this structure.

Abstract: It is known that the phase-decomposition process of 60/40 Cu-Zn alloy is so-called the bainitic transformation, and decomposition of α-phase from the β’-phase is as follow: β’ → α9R → αfcc. In this work,decomposition of α-phase from the β’ single phase of Cu-40.26at.%Zn alloy has been investigated by high-resolution transmission electron microscopy (HRTEM) to understand the phase transformation of this alloy. Especially, striations in the α-phase has been focused on the special feature for the change of the structure and hardening of this alloy during annealing. The result of a comparison between this alloy and the Si added alloy is also reported.

Abstract: The age hardening precipitates of Mg-4.7mass%Zn alloy aged at 423K,473K were studied by using high-resolution transmission electron microscope (HRTEM). Contrasts of mono layers were confirmed to exist on the (0001) and (1100) matrix planes. It was considered that the contrast of mono layer was plate-like shape, and identified as pre-precipitates from as-quenched stage to early stage of aging at 473K for 32h . In the peak aged specimen of aged at 473K, the β1’ phase was observed. The β1’ phase has a rod-like shape and parallel to c-axis of Mg matrix. It can be observed orientation relationship between Mg matrix and β1’phase has not only same parts to previous reports but also different parts in one β1’ phase .

Abstract: Changes in the mechanical properties on AZ91 Mg alloy cast into sand mold caused by heat-treatment and its microstructure were investigated by the tensile test and observation using optical and scanning electron microscopes, and chemical composition analysis. Tensile test results show that the specimens aged at 441K have larger elongation than those of aged at 489K, although they had same proof stress. The fracture surface observation reveal the cleavage fracture of aged specimen caused by the nodular structure as well as the formation of micro void around the coarse spheroidal Al-Mn-(Fe) phase.

Abstract: High resolution transmission electron microscopy（HRTEM）observation was performed to clarify morphology of precipitates at early stage of aging in Mg-Gd-Zr alloy. In the specimen aged at 423K, monolayer was observed on {1100}Mg planes. With increasing aging time, the monolayer and β” phase grew along the <1120>Mg directions. Arrangement of bright dots corresponding to {020}β’ except for the monolayer and β” phase was observed before peak aging stage. β’ phases formed at the same aging stage. Thus, these precipitates co-existed before peak aging stage. The monolayer, β” phase and β’ phase still co-existed in the peak aged specimen. The β’ phase was dominated in the peak hardness. It is revealed that the monolayer lying on {1100}Mg act as precursor of β” phase and β’ phase.

Abstract: There have been many reports about the bainitic phase decomposition of annealed Cu-Zn alloy. During annealing at 523 K, the hardness of this alloy increased at the early stage of annealing, although the β-phase couldn't be observed in the matrix of β'-phase (CsCl). The crystal structure of β' phase, thus, was investigated by a high-resolution transmission electron microscopy (HRTEM) to find its structural change. As the result, the striations were observed in the HRTEM image of matrix at the early stage of annealing. The selected area diffraction pattern was also obtained from this area, and it showed the extra streaks which couldn't be explained by the normal CsCl structure of the β’-phase.

Abstract: The high resolution transmission electron microscope (HRTEM) observation was carried out to investigate the microstructure of precipitates in Mg-Gd-Y-Zr alloy. In the early stage of aging at 423K, monoatomic layers were observed along the (1 1 00)Mg plane. After 64h aging at 423K, the contrast of β” phase based on ordered D019 hexagonal super lattice structure was appeared. The monoatomic layers, two layers spaced by (1 1 00) Mg planes and β’ phase based on bco-structure were observed in the specimen aged for 128h. In the peak- aged specimen, the β’ phase and monoatomic layers were observed.

Abstract: The α-phase in 60/40 brass added Si was investigated by transmission electron microscope (TEM) in order to understand the relationship between the ability of hardening during annealing and phase decomposition. The ability of hardening ( Δ HV), which is the value of difference between the maximum hardness during annealing and the value of as-quenched sample, was obtained measurement from the alloy. As a result, the Si-bearing alloy is higher in ΔHV than a base alloy without Si. The α-phase in the Si-bearing alloy showed fringes in its TEM image like as stacking faults, and extra reflected spots in its selected area diffraction pattern (SADP), which is corresponding to the 9R structure. Streaks were also observed together with spots in its SADP. Extra reflected spots and streaks in SADP became weak or disappeared when the annealing time increased. After 600 ks annealing, SADP obtained from the α-phase could be indexed as FCC lattice. Change in chemical composition of α-phase was also measured by the energy dispersive X-ray spectroscopy (EDS).