Papers by Author: Nobuhiro Tsuji

Abstract: The sheet of pure Al (99.99%) single crystal having (1 12)[110] orientation was deformed up to equivalent strain of 6.4 by the accumulative roll-bonding (ARB) process. The microstructures and orientation of the single crystal ARB-processed by various cycles were characterized by the EBSP measurement. After 1cycle-ARB process, the crystal was macroscopically subdivided into two matrices (macroscopic grain subdivision). These matrices exhibits two different variants of brass orientation, which are (1 01)[121] and (011)[211]. In addition to the macroscopic grain subdivision, microscopic grain subdivision also occurred within the matrix to form an ultrafine grained structure in the single crystal specimen after high strains.

Abstract: It has been reported that accumulative-roll-bonded (ARB) aluminum having ultrafine grains shows an intriguing mechanical character that it is hardened by annealing and contrarily softened by deformation. In order to understand the mechanism of such a peculiar phenomenon, we have investigated the dislocation damping with ultrasonic spectroscopy and total amount of dislocations with the X-ray diffraction measurement. From the present measurements, we have found that the dislocations introduced by the ARB process are tangled but can be released with a large stress. Such dislocations accumulated in ultrafine grains play a significant role in the plastic deformation.

Abstract: The effect of large strain deformation on microstructural refinement has been intensely investigated by many deformation processes. Two popular processes are equal channel angular extrusion (ECAE) and accumulative roll-bonding (ARB). This work presents the evolution of microstructure in Al-0.13%Mg as a function of von Mises strain for both ECAE and ARB in terms of high and low angle boundary spacing and relative fraction, as well as the stability of the deformed structures after a strain of 10, thereby allowing a direct comparison of the two processes. It is found that microstructural evolution as a function of strain is similar, and that ARB structures after a strain of 10 are more stable than ECAE on annealing.

Abstract: Ultra-low carbon steel (ferritic steel), commercial purity aluminum and high purity copper were heavily deformed by the accumulative roll bonding (ARB) process, and the microstructural evolution during the ARB was analyzed. Significant grain refinement by grain subdivision mechanism was confirmed in all three kinds of materials. On the other hand, microstructure refinement slowed down with increasing strain and the grain size stayed in nearly a constant value in the ultrahigh strain region. The mechanism of the grain size saturation was discussed.

Abstract: Accumulative Roll Bonding (ARB) is a technique of grain refinement by severe plastic deformation, which involves multiple repetitions of surface treatment, stacking, rolling, and cutting. The rolling with 50% reduction in thickness bonds the sheets. After several cycles, ultrafine-grained (UFG) materials are produced. Since ARB enables the production of large amounts of UFG materials, its adoption into industrial practice is favoured. ARB has been successfully used for preparation of UFG sheets from different ingot cast aluminium alloys. Twin-roll casting (TRC) is a cost and energy effective method for manufacturing aluminium sheets. Fine particles and small grain size are intrinsic for TRC sheets making them good starting materials for ARB. The paper presents the results of a research aimed at investigating the feasibility of ARB processing of three TRC alloys, AA8006, AA8011 and AA5754, at ambient temperature. The microstructure and properties of the ARB were investigated by means of light and transmission electron microscopy and hardness measurements. AA8006 specimens were ARB processed without any problems. Sound sheets of AA8011 alloy were also obtained even after 8 cycles of ARB. The AA5754 alloy suffered from severe edge and notch cracking since the first cycle. The work hardening of AA8006 alloy saturated after the 3rd cycle, whereas the hardness of AA5754 alloy increased steadily up to the 5th cycle. Monotonous increase in strength up to 280 MPa was observed in the ARB processed AA8011 alloy.

Abstract: The reaction between the zinc plate (ZP) and the IF steel with near surface ultra fine grains (NSUFG) structure with grain size of about 89 nm was studied in temperature range of 473K to 623K in order to elucidate the temperature dependence of the reactions and its mechanism, by comparison with the reactions of ZP to coarse grains (CG) sheet, superficial cold rolled CG sheet (CG+R) and superficial cold rolled NSUFG sheet (NSUFG+R). It was found that this NSUFG structure considerably affected reactions between IF steel and ZP. There was almost no effect of superficial cold rolling on their reactions, but the NSUFG structure dramatically enhanced the reactions. The incubation times for appearance of the reaction layer and its layer width of ZP /(NSUFG or NSUFG+R) reactions are shorter and thicker than those of the ZP/(CG or CG+R) ones. The activation energy for reaction was 107kJ/mol, which indicates that the volume diffusion in zinc side and the grain boundary diffusion in the iron side play an important role in the reaction. The layer growth up to the layer thickness less than about 10µm was controlled mainly by the interface reaction and it over about 10µm mainly by the diffusion mass transfer.

Abstract: Internal stress field in a severely deformed aluminium with ultrafine grained microstructure has been studied by convergent-beam electron diffraction (CBED) technique in transmission electron microscopy (TEM). A commercial purity aluminium (99.1%Al) sheet was highly strained by the accumulative roll-bonding (ARB) process to evolve an ultrafine grained structure. Higher-order Laue zone (HOLZ) lines in the incidence disk of the ] 12 1 [ zone axis have been observed at various positions within an identical ultrafine grain. The key finding is that the HOLZ line pattern taken from the vicinity (~50nm) of the grain boundary (lamellar boundary) looses ) 1 1 0 ( mirror symmetry, whereas the pattern from the grain centre has the symmetry. The former and the latter represent the existence of a large non-hydrostatic stress field and a small internal stress field, respectively. The magnitude of the internal stress becomes larger with approaching to a grain boundary.

Abstract: Variant selection rule of plate martensite in a coarse-grained Fe-28.5at.%Ni alloy was investigated. Crystallographic analysis of martensite plates was carried out by electron back scattering diffraction (EBSD) analysis in a scanning electron microscope with a field emission type gun (FE-SEM). Certain variant selections were recognized for martensite plates in the Fe-28.5at.%Ni alloy. Equivalent strain, which was calculated from the shape strain of each variant, was used to evaluate the accommodation of the shear strain induced by martensite transformation. Variant selection was ruled not only by the shear strain accommodation but also by the ngle between habit planes of adjacent martensite plates.

Abstract: High purity aluminum (99.99% purity) was severely deformed by accumulative roll-bonding (ARB) to a thickness reduction of 98.4%. Quantitative microstructural characterization of the deformed sample was carried out by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). EBSD scans at various locations from the sample surface to the mid-thickness revealed a fairly uniform and equiaxed structure, although a small fraction of an elongated structure parallel to rolling direction (RD) was also observed. Misorientation angle distributions for grain boundaries of which misorientation angle was larger than 2° were evaluated by EBSD, showing that more than 70% of the boundaries were high-angle ones (>15°). More detailed structural features, such as low-angle boundaries (<2°) and dislocations between boundaries were characterized by TEM. The TEM results indicated that about 17% of the boundaries have misorientations <2° and that the fraction of high-angle boundaries is about 52%. An estimated yield strength based on the structural parameters determined by TEM was in good agreement with the measured value.