Hideharu Nakashima

Characterization of High Temperature Creep Properties in Recrystallized 12Cr-ODS Ferritic Steel Claddings

The high temperature strengthening mechanism of previously manufactured 12Cr-ODS ferritic steel claddings was clarified. In the recrystallized 12Cr-2W-0.3Ti-0.24Y2O3-ODS ferritic steel cladding, αY2TiO5 type complex oxide formation was responsible for the drastic reduction of oxide particle size and the resulting shortened distance between particles, which led to superior internal creep rupture strength at 973 K because of the high resistance to gliding dislocation. Internal creep deformation was considered to be controlled by the grain boundary sliding associated with grain morphology: the near Σ11, Σ and Σ19 coincidence boundaries with a (110) common axis.

Low-temperature fabrication of polycrystalline Si thin film using Al-induced crystallization without native Al oxide at amorphous Si/Al interface

Low-temperature fabrication of polycrystalline silicon (poly-Si) thin film has been performed by Al-induced crystallization (AIC), and the structural properties have been investigated. In our experiments, to prevent native oxidation of Al film, an amorphous silicon (a-Si)/Al bilayer was formed on the SiO2/Si substrate by electron beam evaporation without breaking the vacuum. The a-Si/Al/SiO2/Si structure was then heated at a low temperature of 400°C to induce AIC. It was confirmed that layer exchange of the a-Si/Al bilayer is induced even though there is no native oxidation of Al film, which was demonstrated by scanning transmission electron microscopy and energy dispersive X-ray analysis. The mechanism for layer exchange of the a-Si/Al bilayer has been discussed. Furthermore, it was verified by scanning electron microscopy and spectroscopic ellipsometry that the a-Si/Al thickness ratio of roughly 1:1 is suitable to achieve a flat surface morphology of poly-Si. In addition, it was found, by X-ray diffraction and orientation imaging microscopy, that the Si(111)-oriented grain becomes dominant with decreasing thickness of the a-Si/Al bilayer.

Influence of top surface passivation on bottom-channel hole mobility of ultrathin SiGe- and Ge-on-insulator

Bottom-channel hole mobility was examined by a pseudo-metal-oxide-semiconductor field-effect transistors method for ultrathin SiGe-on-insulator (SGOI) and Ge-on-insulator (GOI), which were fabricated using Ge condensation by dry oxidation. By comparing samples with and without a top SiO2 layer, we investigated the influence of top surface passivation on bottom-channel hole mobility. Mobility degradation was found in an ultrathin SGOI/GOI layer without top SiO2 and became more serious with a decrease in the thickness of the SGOI/GOI layer, which strongly suggested that top surface passivation is necessary to evaluate accurate channel mobility. A 13-nm-thick GOI with passivation showed a high mobility value of 440 cm2/V s.

Change in electrical resistivity of commercial purity aluminium severely plastic deformed

Commercial purity aluminium sheets were severely plastic deformed by accumulative roll bonding (ARB). Changes in electrical resistivity at 77 K and microstructure during the ARB process were traced up to 12 cycles, which corresponded to an equivalent strain of 10. The resistivity at 77 K increased with increasing number of ARB cycles, then saturated after about the sixth ARB cycle with a maximum increment of resistivity from starting material of about 1.1 nΩ m. Since lattice defects affect the resistivity of metals, the internal dislocation density and the density of grain boundaries were evaluated from scanning transmission electron microscopy images using Hams method and grain boundary maps obtained from electron back-scattering diffraction, respectively. The relationship between the change in resistivity and the lattice defects is discussed.

Atomic periodicity of 〈001〉 symmetric tilt boundary in molybdenum

Abstract The grain boundary microstructure and its periodicity of [001] symmetric tilt boundary in molybdenum have been examined by applying the high-resolution transmission electron microscope (HRTEM) observation and the molecular dynamics (MD) method. As a result, it was found that the grain boundary structure consisted of the combination of structure units, which is based on the stable boundaries ((00) ∑1 and (10) ∑1 single crystals, and (1—0) ∑5 coincidence boundary), and the periodicity can be completely described in atomic-scale by the concept of structure unit model.

Annealing Behavior and Recrystallized Texture in ARB Processed Copper

The recrystallization behavior and texture development in copper accumulative roll-bonding (ARB) processed by various cycles (2, 4 and 6 cycle) were studied by differential scanning calorimetry (DSC) analysis and SEM/EBSP method. The exothermic peaks caused by recrystallization appeared at 210 ~ 253 in each sample. The peak positions shifted to lower temperature as the number of ARB cycles increased. This result indicated that the evolution of finer microstructure with increasing number of the ARB cycles enhanced the occurrence of recrystallization at lower temperature. The stored energy calculated from the DSC curve of the ARB processed copper increased with the increasing strains. During an annealing, the preferential growth of cube-oriented grains ({100}<001>) occurred in each sample. The recystallization behavior of ARB processed copper having low stacking fault energies was distinguished from that of so-called “recovery type” materials, i.e. aluminum and low carbon steels, which shows rather continuous changes in microstructure during annealing. The accumulated strains provided the driving force for the preferential growth, which was the same mechanism as the preferential growth in normally rolled copper. The sharpest cube texture developed in ARB processed copper by 4 cycles. The difference of cube texture development between 2 cycles and 4 cycles was caused by the distribution of cube-oriented regions which corresponded to the nucleation sites of recrystallized grains before annealing. More nanocystalline layers in the vicinity of bonded interfaces were distributed in ARB processed copper by 6 cycles than 4cycles. The nanocystalline structure could grow faster than the cube-oriented grains and led to the inhibition of sharp cube texture in the ARB processed copper by 6 cycles.

Grain boundary structures in silicon carbide: verification of the extended boundary concept

A grain boundary layer of ca. 0.5 nm in thickness is present in B+C added SiC and SiC without any sintering aids. Since these materials do not show a significant strength-decrease at high temperatures, Ikuhara et al. presumed that the layer is not a second phase of sintering aids or impurities but a reconstructed structure formed to reduce the high energy of the grain boundary, and they called such a boundary an extended boundary. The concept of the extended boundary, however, has not yet been generally accepted for lack of convincing evidence. In the present work, the elements analysis of the boundary layer was made and some additional collateral verifications were conducted in order to inspect the extended boundary concept.

Creep strengthening by lath boundaries in 9Cr ferritic heat-resistant steel

Abstract The interactions between dislocations and lath boundaries in Grade 91 steel were observed by an in situ transmission electron microscopy tensile test at 973 K. Dislocations glided slowly and bowed out in a martensite lath interior. The ends of the dislocation were connected to the lath boundaries. In a tempered specimen, the pinning stress caused by the lath boundary was estimated to be >70 MPa with a lath width of 0.4 μm. In crept specimens, lath coarsening reduced the pinning effect.

Photoluminescence evaluation of defects generated during SiGe-on-insulator virtual substrate fabrication: Temperature ramping process

Crystal qualities of Si∕SiGe∕Si-on-insulator structures with different SiGe thicknesses were evaluated by photoluminescence (PL). The wafers were annealed at different temperatures with a ramping rate of 5°C∕min. Free exciton PL peaks were clearly observed for the as-grown wafers and decreased with an increase in the annealing temperature. For the annealed wafers, defect-related PL signals were observed at around 0.82, 0.88, 0.95, and 1.0eV, which varied according to the annealing temperature and the SiGe thickness. They were also correlated to dislocation-related defects by transmission electron microscopy.

Effect of lower bainite/martensite/retained austenite triplex microstructure on the mechanical properties of a low-carbon steel with quenching and partitioning process

We present a study concerning Fe–0.176C–1.31Si–1.58Mn–0.26Al–0.3Cr (wt%) steel subjected to a quenching and partitioning (Q&P) process. The results of scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and tensile tests demonstrate that the microstructures primarily consist of lath martensite, retained austenite, lower bainite (LB), and a small amount of tempered martensite; moreover, few twin austenite grains were observed. In the microstructure, three types of retained austenite with different sizes and morphologies were observed: blocky retained austenite (~300 nm in width), film-like retained austenite (80–120 nm in width), and ultra- fine film-like retained austenite (30–40 nm in width). Because of the effect of the retained austenite/martensite/LB triplex microstructure, the specimens prepared using different quenching temperatures exhibit high ultimate tensile strength and yield strength. Furthermore, the strength effect of LB can partially counteract the decreasing strength effect of martensite. The formation of LB substantially reduces the amount of retained austenite. Analyses of the retained austenite and the amount of blocky retained austenite indicated that the carbon content is critical to the total elongation of Q&P steel.