Papers by Author: Nobuhiro Tsuji

Abstract: In the present work, effects of loading scheme and strain reversal on structure evolution are studied by using high pressure torsion (HPT) and twist extrusion (TE) techniques. High purity aluminum (99.99%) was processed at room temperature up to a total average equivalent strain of ~4.8 by TE and HPT with two deformation modes: monotonic and reversal deformation with a step of 12˚ rotation. It was revealed that microstructural change with straining observed in pure Al was a common consequence of the SPD processing and was not affected significantly by the loading scheme. At the same time, it was found that strain reversal retarded grain refinement in comparison with monotonic deformation.

Abstract: Aging behavior and mechanical properties of ultra fine grained Cu-Cr-Zr alloy sheet produced by accumulative roll bonding (ARB) process were investigated. A Cu-0.85Cr-0.07Zr (in mass%) alloy was solution treated and then cold-rolled at ambient temperature in the sheet of 1 mm thick. The sheets were heavily deformed by ARB process at ambient temperature up to 5 cycles. The grain size was reduced down to 210 nm and the fraction of high angle grain boundaries (HAGB’s) in the specimen after ARB process was 63%. The proof stress ( σ 0.2) and elongation were 540 MPa and 10%, respectively. Due to the aging treatment, a little grain growth took place (240 nm) and the fraction of HAGB’s was increased to 67%. The proof stress and elongation of the aged one increased to 605 MPa and 15%, respectively. It was noteworthy that the electrical conductivity remarkably increased from 35% to 79%IACS by the aging treatment. It was concluded that the aging treatment after ARB process enhanced not only the mechanical properties but also the electrical conductivity in the Cu-Cr-Zr alloys.

Abstract: An Al-0.2wt%Zr alloy was severely deformed up to a strain of 8.0 by accumulative roll bonding (ARB) process, started from the solution-treated state. The microstructural evolution during ARB and its aging behavior were investigated. With increasing the number of ARB cycles, Vickers hardness of the specimens increased and reached to a constant value. The microstructural evolution during the ARB could be understood in terms of grain subdivision. The ultrafine grained (UFG) materials whose mean grain size was 0.4 -m were obtained by 10-cycle ARB process. In aging of the ARB processed specimens at high temperatures above 673K, the UFG microstructures quickly coarsened. On the other hand, it was suggested that the precipitation behaviors of the ARB specimen at 623K were quite unique and completely different from those of the conventionally solution-treated material with coarse grain size.

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: Effect of solid solution elements on microstructure evolution and mechanical properties was investigated using a high purity Al (purity 99.99%) and Al-0.5 at.% X ( X = Si, Ag, Mg ) alloys deformed by accumulative roll bonding (ARB) process up to 7 cycles (equivalent strain of 5.6) at ambient temperature. The ARB-processed high purity Al showed the equiaxed microstructure having mean grain size of 750 nm. On the other hand, the microstructure of the ARB-processed Al-0.5at.%X alloys showed lamellar boundary structures elongated along RD. The mean lamellar boundary spacing significantly differed depending on the alloying elements, which suggested that solute atoms had a significant effect on microstructure evolution. The difference in the grain size was regarded to be caused by the difference in recovery processes in the alloys. The tensile strength of the alloys increased with increasing the number of ARB cycles. In the Al-Si and Al-Ag alloys, the post-uniform elongation increased with increasing the number of the ARB cycles. On the other hand, the elongation of the Al-Mg hardly changed during the ARB process.

Abstract: All the SPD techniques introduce reversal straining principally, but effects of the reversal deformation on structure evolution were not studied directly yet. In the present work, an attempt was made to manage structure in pure (99.99%) Al by strain reversal through high pressure torsion (HPT). Total accumulated deformation up to equivalent strain ~8 was used. General trend of the grain refinement is similar for both deformation modes; and it is typical with all other SPD processed FCC metals. At the same time, the difference in microstructure evolution at the vicinity of the specimen axis and with increasing distance in the radial direction introduces microstructural heterogeneities which are specific features of the reversal straining. In the monotonic deformation process the A ({111}<011>) fiber is gradually substituted by the C component ({ 0 0 1}< 1 1 0>) with increasing strain before it is found to weaken. In the reverse straining process the A fiber is found to dominate the deformation texture in the low strain region. In the reverse straining process at high strain level, a {001}<100> component appear.

Abstract: Recrystallization behavior of SPD processed high purity iron was studied. The 99.95% iron sheet was deformed by the accumulative roll-bonding (ARB) process up to 8 cycles (equivalent strain of 6.4) at ambient temperature. Subsequently, the ARB-processed specimens were annealed for 1.8ks at various temperatures from 300°C to 500°C. The microstructures of these specimens were characterized by TEM and SEM/EBSP. The microstructure of the as-ARB-processed specimens showed the lamellar boundary structure elongated along RD, which was the typical microstructure of the ARB-processed materials. The mean interval of the lamellar boundaries was about 100 nm. After annealing at 400°C, the ARB specimen showed a partially recrystallized microstructure composed of equiaxed grains with grain size larger than 10 5m and the recovered lamellar boundary structure. After annealing above 500°C, the microstructures were filled with equiaxed recrystallized grains. These results suggest that conventional discontinuous recrystallization characterized by nucleation and growth occurs during annealing at annealing temperature above 400 °C. In previous work reported about the annealing behavior of the low carbon IF steel ARB processed, the continuous recrystallization occurred during annealing at annealing temperature above 600 °C. The recrystallization temperature of the pure iron was much lower than the IF steel and the recrystallization process were significantly different. This difference was suggested to be caused by inhomogeneous microstructure in the pure iron ARB-processed.

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: Pure Al (99%) and pure Fe (99.5%) sheets were mutually stacked and severely deformed up to equivalent strain of 16 by the accumulative roll bonding (ARB) process in an attempt to achieve bulk mechanical alloying. The deformation was carried out at RT. The Al/Fe sheets ARB processed by 1 cycle showed a number of shear bands penetrating the stacked layers. The Fe layers, which were harder than the Al layers, were subdivided by the shear bands into diamond-shaped regions. Dissolution of Fe into Al was observed and a supersaturated solid solution was formed in the specimen ARB processed by 10 cycles. It was also found that local amorphization occurred at interface regions via formation of Al5Fe2 intermetallic compound.

Abstract: Microstructures and mechanical properties of Fe-15wt.%Cr-10wt.%Ni alloy ARB processed by various cycles were studied. The starting material showed lath martensite structure. However, the austenitic phase became stable by the high straining (ARB) above Af temperature. The volume fraction of austenitic phase greatly increased to around 90 % by 2 ARB cycles and nearly saturated at about 95 % after higher ARB cycles. The grain refinement progressed during the ARB, which leaded to the formation of ultrafine lamellar UFG austenitic microstructure with mean lamellar spacing of about 300 nm. The UFG Fe-Cr-Ni alloy performed both high strength and large elongation. Especially, the yield strength of the alloy ARB processed by 5 cycles reached to 900 MPa, and the total elongation was 40 %. The good ductility of the present specimens was attributed to the occurrence of transformation-induced plasticity (TRIP).