Yoji Miyajima

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.

Microstructural change due to isochronal annealing in severely plastic-deformed commercial purity aluminium

Microstructural changes, such as the density of grain boundary (GB) and dislocation density, due to isochronal annealing in severely plastic-deformed commercial purity aluminium up to 523 K was evaluated using electrical resistivity measurements and scanning transmission electron microscopy. Eventually, the GB density decreases from about 7.2  ×  106 to about 2  ×  106 m−1, whilst the dislocation density decreases from an initial value of around 1.3  ×  1014 m−2 down to around 4  ×  1013 m−2.

Dislocation Density of FCC Metals Processed by ARB

Accumulative roll bonding (ARB) was applied to three FCC metals, such as Al, Cu and Ni. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) were made to evaluate dislocation density ? of the ARB processed FCC metals. The values of ? of ARB processed Cu and Ni increased rapidly after the first ARB cycle, and then, tend to saturate after the following cycles. The evaluated values of ? of both ARB 8-cycled Cu and Ni using DSC were around 2?1015 m?2. Whereas, those using XRD were around 5?1014m?2 (for Cu) and 3?1014m?2 (for Ni). Although the ? values depend on the measurement methods, the trends that DSC values are about an order of magnitude higher than XRD values seem to be common. In the case of Al, dislocation density evaluated using XRD increased to about 1?1013m?2 at the first ARB cycle, and then gradually decreased to about 1?1012m?2 with increasing number of ARB cycles.

Effects of Rolling Reduction and Strength of Composed Layers on Bond Strength of Pure Copper and Aluminium Alloy Clad Sheets Fabricated by Cold Roll Bonding

Three types of clad sheets, Cu/Al, Cu/AA5052, and Cu/AA5083, were produced by cold roll bonding with the rolling reduction of 50% and 75%. Tensile shear tests which give tensile shear strength were performed in order to assess the bond strength. Scanning electron microscopy was performed on the fractured interface produced by the tensile shear tests, which suggests that the fracture occurs within the Al alloy layer. The tensile shear strengths considering the area fraction of deposit of Al alloy on Cu side were compared with the shear stress converting from the ultimate tensile strengths. As a result, the tensile shear strength of the clad sheets is attributed to the shear strength of Al alloy layer close to the well bonded interface. A simple model was proposed that explains the effects of the rolling reduction and area fraction of deposit of Al alloy.

Contactless electrical conductivity measurement of metallic submicron-grain material: Application to the study of aluminum with severe plastic deformation

We measured the electrical conductivity σ of aluminum specimen consisting of submicron-grains by observing the AC magnetic susceptibility resulting from the eddy current. By using a commercial platform for magnetic measurement, contactless measurement of the relative electrical conductivity σn of a nonmagnetic metal is possible over a wide temperature (T) range. By referring to σ at room temperature, obtained by the four-terminal method, σn(T) was transformed into σ(T). This approach is useful for cylinder specimens, in which the estimation of the radius and/or volume is difficult. An experiment in which aluminum underwent accumulative roll bonding, which is a severe plastic deformation process, validated this method of evaluating σ as a function of the fraction of high-angle grain boundaries.

Texture Evolution in ARB Processed Commercial Purity Aluminium

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.

Recrystallization Texture of Heavily Cold Rolled Polycrystalline Nickel Sheets with and without Strong Starting Cube Texture

The evolution of texture is studied in high purity (~99.7%) nickel sheets with widely different starting cube texture ({001}) intensities following heavy cold rolling and annealing. For this purpose two nickel sheets with strong and weak starting recrystallization cube texture (SSCT and WSCT, respectively) prepared by Accumulative Roll Bonding and conventional rolling, respectively, followed by annealing are used as the starting materials for subsequent processing. These sheets are cold rolled to 90% reduction in thickness and annealed at different temperatures. Profuse cube oriented bands could be identified in the SSCT nickel sheet after 90% cold rolling as opposed to rather insignificant presence of cube regions in the WSCT nickel sheet. However, the WSCT nickel sheets consistently show stronger cube texture after annealing treatments as compared to the SSCT material. The failure to observe recrystallization cube texture in SSCT is attributed to the inhibited nucleation of cube grains owing to the unfavorable misorientation environment surrounding cube regions in the deformed matrix.

In-situ X-ray diffraction during tensile deformation of ultrafine-grained copper using synchrotron radiation

Abstract At the synchrotron facility, Super Photon Ring – 8 GeV, in-situ X-ray diffraction during tensile deformation was conducted on ultrafine-grained Cu with a grain size of about 300 nm fabricated by equal-channel angular pressing. The diffraction profile was observed with the time resolution of about 1 s using multiple MYTHEN detectors, and the diffraction angle and the full-width at half-maximum of some Bragg peaks were determined using the pseudo-Voigt function. From the analysis of Bragg peaks, it was found out that there are microscopically three regions; elastic, plastic and transition regions. The 0.2% proof stress obtained from the stress–strain curve locates within the microscopic transition region. Microstrain was evaluated using the Williamson–Hall method and the dislocation density was also obtained from the microstrain. The dislocation density starts increasing before 0.2% proof stress, which is associated with dislocation bow-out and emission from grain boundaries. The Taylor relationship seems to be still satisfied after 0.2% proof stress.

Dislocation Density Changes in Ultrafine‐Grain Aluminum during Tensile Deformation

It has been reported that the tensile strength of UFG aluminum is several times higher than that of coarse-grain aluminum, and that UFG aluminum exhibits unique mechanical phenomena under tensile deformation. This implies that dislocation behavior in UFG aluminum differs from that in coarse-grain aluminum. In this research, changes in dislocation density during tensile deformation were examined by in-situ X-ray diffraction, and the effect of grain size on dislocation multiplication behavior was investigated. In samples with smaller grains, the increase in dislocation density during tensile deformation was larger and the decrease in dislocation density after unloading via fracture was also larger.

Stability of Cube oriented grains during cold-rolling of highly Cube-oriented polycrystalline nickel

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.