Papers by Author: Nobuhiro Tsuji

Abstract: A study has been carried out on the evolution of microstructure, grain boundary character and mechanical properties in a Twinning Induced Plasticity steel heavily cold rolled and subsequently annealed.The cold rolled mcrostructures showed fine lamellar boundaries with many shear bands.With progress of annealing, numerous numbers of recrystallized grains were generated.The fully recrystallized steel showed equi-axed nanocrystalline grains with a mean grain size of 400 nm that enhanced the yield strength significantly while retaining tensile ductility.

Abstract: The evolution of cube texture has been investigated in pure Ni (~99.97%) following ultra high straining by Accumulative Roll Bonding (ARB) and annealing. Fully annealed sheets of pure Ni with average recrystallized grain size of ~28 (henceforth termed as fine grained starting material) and 650 µm (henceforth termed as coarse grained starting material) were used as the starting material for the ARB processing. ~50% reduction was imparted in each cycle of ARB and the process was carried up to 8 cycles resulting in an accumulated equivalent strain (ε_eq) of 6.4. The deformed materials were subsequently annealed at temperatures ranging between 300 to 800°C for one hour. In addition few samples were annealed at 300°C for different time intervals to observe the evolution of texture components. In all cases textures were carefully characterized using Electron Back Scatter Diffraction (EBSD) technique.

Abstract: We have studied the crystal defects in severely deformed pure iron, by combining mechanical spectroscopy with measurements of Vickers hardness and electrical resistivity. Iron of 99.99% purity and a material in which the C content is further reduced to below 1 at. ppm were deformed to strains up to 5.6 by accumulated roll-bonding. Mechanical loss and dynamic shear modulus of as-deformed samples were measured over the temperature range from –195°C to 800°C. Effects due to dislocations and grain boundaries have been identified. Thermal stabilities of the defects have been examined through variations in the mechanical loss during heating-run and cooling-run measurements. It is found that the small difference in the carbon content influences the densities of defects and microstructure, as well as the thermal stability. They are consistent with the recovery behaviour observed through hardness and resistivity in isochronal annealing experiments.

Abstract: We studied the crystallographic features, especially the orientation relationship with respect to austenite, of martensite in a steel transformed from coarse-grained equiaxed austenite (35 μm), ultrafine-grained equiaxed (2.5 μm) or lamellar (300nm) austenite fabricated by sever plastic deformation. With decreasing the grain size of equiaxed austenite, the orientation relationship changed from Kurdjumov - Sachs relationship to Greninger - Troiano relationship. We inferred that this change of orientation relationship could be attributed to the small size of martensite plate transformed from the ultrafine-grained equiaxed austenite. The martensite transformed from the ultrafine-grained lamellar austenite did not have a definite orientation relationship with austenite. We considered that a high density of dislocations or a high density of low angle boundaries within the ultrafine lamellar austenite grain resulted in the large deviation of orientation relationship.

Abstract: The effect of ultrahigh straining on the evolution of cube texture in high purity nickel (99.97%) processed by Accumulative Roll Bonding (ARB) is investigated in the present study. Fully recrystallized nickel sheet having average grain size of 28 μm is deformed by ARB to the strain levels of εeq=3.2 and εeq=6.4 corresponding to 4 and 8 cycles of ARB processing, respectively, and subsequently annealed at different temperatures. The 8 cycle ARB processed material consistently shows much higher cube volume fraction as compared to the 4 cycle processed material after different annealing treatments. Almost 100% cube volume could be obtained in the 8 cycle processed material after annealing at 800°C. The development of extremely sharp cube texture in the 8 cycle processed material could be attributed to the oriented nucleation of cube grains at early stages of recrystallization and subsequent growth of these grains. The results indicate that ultrahigh straining could be useful for applications requiring near perfect cube texture such as substrates for coated superconductors applications.

Abstract: Texture of the ARB processed commercial purity aluminium was investigated in order to explore the stable component in rolling texture more than 95 % of rolling reduction in FCC metal sheets having high stacking fault energy. Weak location dependence along normal direction was observed in the sheet up to ARB 8 cycle, whereas the texture could be regarded to be uniform after ARB 8 cycle. The main orientation was Copper component, whereas the minor orientations were Brass and S orientations in the ARB processed pure aluminium sheets rolled up to around 99.9 % reduction.

Abstract: A pure Ni sheet was heavily deformed up to an equivalent strain of 6.4 at room temperature and then annealed to obtain highly Cube textured material, which is a polycrystal subdivided by many low-angle grain boundaries. The highly Cube-oriented sheets were cold-rolled to various reductions up to 90%. It was found that large fraction of Cube oriented grains remained stable in cold-rolling although the orientation is theoretically unstable. The stability of Cube orientation was considered to be associated with the constraint by grain boundaries in the materials.

Abstract: In order to study about dynamic transformation phenomenon, Fe-6Ni-0.1C alloy was hot-deformed in uniaxial compression using thermo-mechanical simulator at various temperatures ranging from 600 to 1000 °C at various strain rates from 0.001 to 1 s^-1 after austenitization. As the value of Zener-Hollomon (Z) parameter increased, softening of the stress from the empirically expected value, which was extrapolated from stresses deformed at low Z value, was observed through systematical analysis of peak stresses. It suggested that this softening phenomenon was attributed to the dynamic transformation, since ferrite is softer than austenite at elevated temperature. The microstructural observation also supported that ferritic transformation occurred during compressive deformation. Even above Ae₃ temperature the softening of the peak stress of austenite was still observed, which implied that dynamic ferritic transformation might occur above Ae₃ temperature.

Abstract: An attempt has been made to fabricate highly cube textured ({001}<100>) polycrystalline nickel substrate tapes for coated superconductor applications using severe plastic deformation route. For this purpose high purity Ni (~99.97%) plates with widely different average grain size (28 µm or fine grained starting material and 650 µm or coarse grained starting material, respectively) are severely deformed by Accumulative Roll Bonding (ARB) up to an equivalent strain level of (εeq = 6.4) and annealed at various temperatures between 300° to 800°C for one hour. Upon annealing at different temperatures, the fine grained starting material developed a much stronger cube texture than the coarse grained starting material. By suitable annealing treatment near 100% cube textured tape could be obtained in the SPD processed fine grained nickel tapes having a predominantly low angle boundary network. The observed difference in the strength of cube texture in the two materials could be explained based on the oriented nucleation mechanism of formation of cube texture. The present results indicate that SPD processing route such as ARB is very effective in fabricating high purity Ni or Ni alloys intended for coated superconductor substrate applications.

Abstract: An ultra-low carbon IF steel was heavily deformed up to an equivalent strain of 36 at various high temperatures of ferrite single-phase region and various strain rates. Effects of temperature and strain rate on the microstructures evolved in torsion deformation were clarified. On the other hand, it was found that homogeneous ultrafine grained structures were not obtained by the present torsion deformation though very high strain was applied. The coarser grain sizes than those obtained by conventional severe plastic deformation (like ARB) were due to the deformation at higher temperature and lower strain rate, but lower fraction of high-angle grain boundaries in the torsion specimen was suggested to be attributed to the characteristics of monotonic torsion (or simple shear) deformation including the way of strain evaluation.