Papers by Author: Hideharu Nakashima

Abstract: Grain boundary structures in the commercial purity aluminum (1100Al) highly deformed by the accumulative roll bonding (ARB) process was observed by using conventional transmission electron microscopy (CTEM) and high resolution transmission electron microscopy (HRTEM). In the low angle grain boundary with a tilt angle (2θ) of 2.1o consisted of the periodic dislocations array, the interval of those dislocations could be explained by the dislocation model for grain boundary. However, the dense dislocation region locally existed at the vicinity of the low angle boundary. On the other hand, we also observed the high angle grain boundary of which the common axis and 2θ was <110> and 125.9o, respectively. In this grain boundary, we could describe the boundary configuration in terms of the combination of the kite-shaped structure unit characterized by Σ11 coincidence boundary with the 2θ of 129.52o around <110> and the additional dislocations to compensate the difference of the actual and geometrically coincided one.

Abstract: Grain boundary mobility in preferential growth of cube grains ({100}<001>) was evaluated by in-situ electron back scattering diffraction pattern (EBSP) analysis in order to clarify the fundamental mechanism of primary recrystallization in pure aluminum foils of 99.9% purity thermo-mechanically processed in the industrial production route for aluminum foils for electrolytic capacitors. We have carried out the continuous EBSP measurements during recrystallization of the aluminum foils heated to various temperatures in the chamber of scanning electron microscopy (SEM). We have succeeded in dynamic observation of the preferential growth of cube grains by the in-situ EBSP analysis. The in-situ EBSP analysis could reveal the migration rate of grain boundaries surrounding the cube grains. It was clarified that the proportional relation between migration rate and annealing time was satisfied. The stored energy providing the driving force for the grain boundary migration during primary recrystallization could be estimated from the misorientation within the deformed grains. The mobility of the grain boundary could be evaluated using the measured grain boundary migration rate and stored energy. Then the activation energy could be estimated by the in-situ EBSP analysis at various temperatures ranging from 270°C to 310°C. The obtained activation energy was 124 kJ/mol, which approximately corresponded to that for the diffusion of impurity such as iron or silicon in aluminum. This suggested that the rate-determining process of the grain boundary migration of cube grains was impurity diffusion in the pure aluminum foils.

Abstract: Al-Mg-Si alloy is the suitable material for the automotive body application. However, it is found that a rope-like profile (ridging) develops when the Al-Mg-Si alloy sheet is stretched along the transverse direction. In this study, in order to clarify the formation process of ridging developed in Al-Mg-Si alloy, the relation between ridging and texture components of the sheet was investigated by 3D profile microscope and SEM/EBSD method. It was found that the ridging developed remarkably in the hot-rolled (at higher temperature) and annealed sheets. In ridging sample, there was the band of cube oriented grains (cube band). It was also found that the region of cube band corresponded to the ridges and valleys which caused a rope-like profile in the sheets. It could be thought that the difference of plastic deformability between cube-oriented grains and other oriented grains led to the development of ridging. This assumption is supported by the lower Taylor factor of cube oriented grains than other oriented grains. From these results, it was concluded that the development of ridging was strongly affected by the distribution of cube oriented grains.

Abstract: Creep tests and microstructural observations in several high Cr ferritic steels bearing V and Nb were carried out in order to investigate effects of addition of V and Nb on
, which is a coefficient of
method. Creep strength was improved with increasing the V content and was slightly lowered by increasing the Nb content.
in all the steels bearing V or Nb was lager than that in fundamental steel. The dependence of
on the amount of additions was different between V and Nb. In steels bearing V,
became larger with increasing V content. In steels bearing Nb,
became smaller. These results revealed that
corresponds to the creep strength, indicating that the value of
can be estimated using the strengthening effects of the additive element.

Abstract: Grain boundary structures in the Accumulative roll-bonding (ARB) processed copper (ARB-Cu) have been studied. The grain boundary structures were observed by high-resolution transmission electron microscopy (HRTEM). In order to clarify the difference between the grain boundaries in ARB-Cu and equilibrium boundaries, calculated atomic structure of symmetric tilt grain boundaries with <110> common axis (<110> symmetric tilt grain boundary; <110> STGB) in Cu were used. The near 14° boundary in the ARB-Cu could be described by the dislocation model, but the dense dislocation region existed near the grain boundary. The high angle boundaries in ARB-Cu could be described by the structural units which were obtained by molecular dynamics (MD) simulation. Furthermore, in the 2 cycles and 6 cycles ARB-Cu (2cARB-Cu and 6cARB-Cu), the deformation twin boundaries could be observed and described by the structural unit. Therefore, it was concluded that the grain boundary structure in the ARB-Cu was not much different from the normal equilibrium grain boundary and explained by conventional dislocation and structural unit models.

Abstract: 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.

Abstract: Migrations of <110> tilt S 11 and S 27 boundaries in 99.99% purity aluminum have been investigated by Sun and Bauer technique as a function of temperature. In the S 11 tilt boundary, the activation energy for grain boundary migration is about 1/2 of the energy for Al-atom bulk-diffusion, indicating that the boundary motion may be governed by the grain boundary diffusion. While in the S 27 tilt boundary, the activation energy for grain boundary migration is about 125kJ/mol, which agrees with the energy for Al-atom bulk-diffusion. Study of boundary structure observation by high resolution electron microscopy revealed that the grain boundary structure of S 27 tilt boundary was consisted of edge dislocation array in which a space between dislocations was very short. It is considered that climb motion of the dislocations controlled to the motion of tilt boundaries consisted of edge dislocation array. From these results, it is concluded that the boundary motion of S 27 tilt boundary may be governed by climb motion of their dislocations controlled by Al-atom bulk-diffusion.