Papers by Keyword: TEM Observation

Abstract: We investigated the effect of a melt-back process on the quality of the grown crystal in 4H-SiC solution growth. In our experiments, the crystal was grown by top-seeded solution growth (TSSG) method with and without melt-back, following which the quality of the obtained crystals was compared. When solution growth was carried out without melt-back, solvent inclusions and a different polytype were observed. When molten KOH etching was conducted, the dislocation density in the crystals at the early stage of growth became much higher than that in a seed crystal. Solvent inclusions, a different polytype, and an increase in dislocations were suppressed when solution growth was performed with melt-back. It was confirmed that melt-back is necessary to prevent the deterioration of crystal quality at the early stage of solution growth.

Abstract: Traditionally, Mg-Gd alloys have been strengthened by dispersed precipitates. Several reports are available about Sc addition to Mg alloys for improving a creep resistance. In this research, aging behavior of Mg-Gd, Mg-Sc and Mg-Gd-Sc alloys including the same amount of solute elements were investigated to understand the effect of Sc on microstructures and mechanical properties during aging. Hardness measurement revealed that Sc addition delayed to form precipitate. Close inspection of TEM micrographs, β” phase formed at an initial stage of aging and β’ phase was observed at a peak-aged stage in Mg-Gd and Mg-Gd-Sc alloys. In Mg-Sc alloy, there is no evidence of precipitate formation during aging at 473K.

Abstract: Erbium oxide (Er₂O₃) was shown to be a high potential candidate for tritium penetration barrier and electrical insulator coating for advanced breeding blanket systems such as liquid Li, Li-Pb or molten-salt blankets. Recently, Hishinuma et. al. succeeded to form Er₂O₃ coating layer on large interior surface area of metal pipe using Metal Organic Chemical Vapor Deposition (MOCVD) process. In this work, Er₂O₃ coating layer of hydrogen penetration control on stainless steel 316 (SUS 316) substrate have been investigated to the effect of hydrogen penetration and using SEM, TEM and SAED patterns. Er₂O₃ coating layer of before hydrogen penetration was granular structures from SEM observation. Er₂O₃ coating layer was columnar grains from TEM observation. TEM image and SAED patterns obtained from Er oxide film can be understood that the growth direction of Er₂O₃ film was [110].

Abstract: Two-phase single-crystal intermetallic alloys composed of Ni₃Al (L1₂) and Ni₃V (D0₂₂) with some orientations were compressed at various temperatures, and their deformation microstructures were observed by transmission electron microscopy (TEM). The deformation at room temperature was governed by the glide motion of dislocations in the primary Ni₃Al precipitates and the activation of the microtwins in the Ni₃V variant structures in the channel regions. The interfaces between the primary Ni₃Al precipitates and the Ni₃V variant structures are suggested to work as the barriers to the dislocation motion. While, at temperature above the peak temperature (873 K), the deformation microstructures of the two-phase intermetallic alloy exhibited the ribbon-like deformation microstructures penetrating the constituent phases i.e. through the interfaces between primary Ni₃Al precipitates and the Ni₃V variant structures in the channel regions. It was also suggested that the superior strength in the two-phase intermetallic alloys is due to the high flow strength of the Ni₃V phases and to the interfacial hardening receiving when the dislocations activated in the primary Ni₃Al precipitates propagate to the channel regions.

Abstract: We investigated the crystallization and resulting morphology of a stoichiometric barium-silicate glass 1BaO-2SiO₂ (glassy sanbornite) by using transmission electron microscope and Raman spectroscopy. A crystalline aggregate structure consisting of a Ba₅Si₈O₂₁ phase (trunk) and a β-BaSi₂O₅ phase (branch) was observed in the heat-treated glassy sanbornite. The Ba-richer phase of Ba₅Si₈O₂₁ precipitated prior to the formation of the stoichiometric β-BaSi₂O₅ phase, suggesting the presence of the region in which the Ba ions are enriched or cohesively distributed. A considerable structural similarity between the glassy sanbornite and the Ba₅Si₈O₂₁ phase was also suggested in this study.

Abstract: The mechanical properties and the oxidation resistance of -TiAl at elevated temperatures have to be improved to be used in the severe environmental conditions. It has become clear that the addition of more than 4at.%Ta in TiAl demonstrates a superior oxidation to the other TiAl-X compounds, according to the weight gain results of cyclic oxidation experiments at 1173 and 1273K. Oxidation behaviors are strongly influenced by the Ta concentration in TiAl. XRD, SEM-EDS, and TEM-EDS observations have been carried out to determine the microstructures and the surface compositions of multi-layered oxide scales. It was revealed that a protective intermediate phase simultaneously formed between the substrate γ-TiAl and the oxide scale layer. The Ti53Al32Ta15 ternary compound exists as an equilibrium phase at 1373K, according to the published Ti-Al-Ta ternary phase diagram. This ternary compound can work as a barrier to some extent. It contributes to decelerating the diffusion of Ti and Al atoms and to decreasing the oxidation rate. The formation mechanism of the intermediate phase has been discussed in conjunction with diffusion in TiAl.

Abstract: The structural stability of hollow Cu2O and NiO nanoparticles, which were obtained via oxidation of Cu and Ni nanoparticles in air, was studied by transmission electron microscopy (TEM). Hollow Cu2O and NiO were observed to have shrunk at 473 and 623 K in annealing under 5.0×10-5 Pa, respectively, where the reduction reactions from oxides to metals started. As a result of shrinking associated with reduction, hollow oxides turned into solid metal nanoparticles after annealing at higher temperatures for a long time. In addition, hollow oxides shrunk and collapsed through high-temperature oxidation. It was found that shrinking of hollow oxides during oxidation occurs at temperature where the diffusion coefficients of slower diffusing species reach around 10-22 m2s-1. It seems that the hollow oxide nanoparticles tend to shrink and collapse at high temperatures because the hollow structures are energetically unstable.

Abstract: The formation of hollow metal oxide nanoparticles through oxidation process has been studied by transmission electron microscopy for Cu, Zn, Al, Pb and Ni in order to clarify the detailed formation mechanism of hollow oxide nanoparticles and their governing factors. For Cu, Zn, Al and Ni nanoparticles, hollow oxide nanoparticles are obtained as a result of vacancy aggregation in the oxidation processes. These results arise from faster outward diffusion of metal ions through the oxide layer in the oxidation processes. On the other hand, Pb nanoparticles turn to solid PbO, because the diffusivity difference DPb < DO in PbO leads to no formation of vacancy clusters.

Abstract: The microstructure of the friction-bonded interface of Al alloys to low C steel has been investigated by TEM observations to reveal the controlling factor of the formation and growth of the IMC (Intermetallic Compound) layer, which caused the premature fracture at the interface even when its thickness was less than 1 μm, as reported in a previous paper. The thickness of the IMC layer observed at the interfaces of Al-Mg alloy/steel and pure-aluminum/steel joints increased almost in proportion to the friction time, but did not obey the parabolic law a characteristic kinetics of the diffusion-controlled process. Analyses of SAD patterns from the IMC layer indicate that it consisted of Fe2Al5, Fe4Al13, (Fe, Mn)Al6 and FeAl2, depending on the alloying elements. These IMCs were granular and distributed almost randomly within the IMC layer, suggesting that mechanical mixing of the steel with the Al alloy occurred at the interface. In the low C steel adjacent to the IMC layer, a zone of much finer grains than those of the base metal was observed. Its width increased with friction time and pressure, and with the growth of the IMC layer, as well. These results suggest that the superficial region of the steel underwent a heavy plastic deformation during the friction process and it had a close relation with the growth of the IMC layer.

Abstract: The oxidation behavior of Ti-48Al-2Cr-2Nb, Ti-48Al-2Cr-2W and Ti-48Al-2Cr-2Fe was studied in a simulated combustion gas, 10O2-7CO2-6H2O-bal.N2 (vol%), at 1173 K and TEM observation was performed for understanding the initial stage of oxidation behavior. Ti-48Al-2Cr-2Nb and Ti-48Al-2Cr-2W show excellent oxidation resistance in the test gas by forming thin and protective Al2O3-rich scales, while Ti-48Al-2Cr-2Fe shows poor oxidation resistance. The superior oxidation resistance of W-containing alloy is explained in terms of the formation of a bcc phase with low Al content in the alloy which was confirmed by TEM observation and also possible enhanced Al diffusion from the substrate to the scale in this phase. The oxidation resistance of the former two alloys in the test gas is better than in laboratory air, due to the lower O2 content in the test gas. The presence of H2O and CO2 in the test gas enhances the oxidation of Ti-50Al, while it has almost no influence on the oxidation behavior of these two alloys, indicating that these gases are influential to a TiO2-rich scale but not to Al2O3-rich scales.