Papers by Author: Shinji Muraishi

Abstract: The recrystallization behavior of the cold-rolled AA3003 aluminum alloy with the reduction rate of 20%, 50% and 90% during annealing at the temperature ranging from 300°C to 400°C was investigated. As increasing reduction rate, the cold rolled specimens exhibit deformation bands with elongated grain microstructure consisting of straight grain boundary parallel to rolling direction. Therefore, large density of nucleation sites for recrystallization would be expected with increase of strain energy. The grain size of the cold-rolled specimens decreased with increase of reduction rate, c.f., as the rolling reduction increased to 90%, grain size along the direction normal to the sheet decreased to about 8μm in thick. When the sample annealed at 350°C for 5s, the first recrystallized grains were observed in the vicinity of the grain boundary. The relaxation and recrystallization kinetics under different annealing conditions were characterized in terms of the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model. The apparent activation energies of recrystallization for the cold-rolled specimens with reduction rate of 20%, 50% and 90% were determined as 332 kJ/mol, 239 kJ/mol and 115 kJ/mol, respectively. XRD analysis by using modified Williamson–Hall plots revealed that the tendency of the change in dislocation density is varied depending on reduction rate. These results indicate that the apparent activation energy for recrystallization and the crystallites size decrease with increase of the reduction rate, which leads to a decrease in the size of the recrystallized grains.

Abstract: Effect of precipitation strengthening on metal is generally attributed to the dislocation interaction with the precipitate which acts as the barrier to the dislocation motion on the slip plane. In order to achieve better understanding of critical events of dislocation motion and evolution of dislocation microstructure, we have developed numerical simulation method of dislocation-dislocation and dislocation-particle interactions by means of discrete dislocation dynamics at mesoscopic scale. In this work, Green’s function method is utilized for the computation of the stress fields of dislocation and misfitting particle, and the interaction forces acting on the dislocation. We also proposed the efficient algorithm of the connectivity vector for the dislocation line elements, linked-list data structure, to deal with the flexible interaction of dislocation line elements. The geometrical effect of dislocation slip planes on the dislocation bypassing behaviors is tested by changing the relative height of dislocation slip plane against the center plane of spherical particle, where cross slip event is also taken into account for the dislocation motion. Simulation results show a wide variety of topological changes of dislocation during motion on the slip planes around the particle, which results from the stress field of the particle varied with the relative height between the dislocation slip plane and center plane of particle. The full analysis of the mechanisms of dislocation line bypassing misfitting particle has been explained in this study.

Abstract: Misfit precipitates greatly contribute to precipitation hardening in wrought aluminum alloys, where attractive and repulsive interactions are expected by stress-strain field of fine misfit precipitates. There are two types of dislocation cutting manner of {001} GP-zone and θ’ phase in Al-Cu alloys; one is dislocation burgers vector intersects (001) variant by 0 deg. (Type A), the other is dislocation Burgers vector intersects (001) variant by 60 deg. (Type B). In order to simulate the interaction of dislocation and fine misfit precipitates, internal stress fields by dislocation and precipitate are computed by Micromechanics based Green’s function method. The elastic field inside and outside a precipitate is deduced from Eshelby’s inclusion theory, where misfit strain of a (001) precipitate is assumed by unidirectional eigenstrain across the disk shaped inclusion. Dislocation motion under three different kinds of dislocation Burgers vector is tested by computing interaction force acted on the discretized dislocation line elements. The interaction force caused by (001) misfit precipitate is varied with types of dislocation cutting manner, magnitude of the interaction force associated with dislocation glide is increased by Type B variant (60 deg.), whereas that is minutely zero for Type A variant (parallel).

Abstract: Mechanism of mechanoluminescence of the composite of Mn-doped ZnS particles and polyester resin is developed based on the principle of Fermi-level shift, associated by piezoelectric effect. As the electron-hole recombination is responsible for light emission, it is possible to determine the number of emitted photons from the difference in number of conducting electrons between the stressed state and the thermal equilibrium condition. For the given emission color, characteristics of photomultiplier tube and deformation conditions, the emitted photons can be approximated. The model is expressed as A[exp (Bσ²) - 1] where σ is the applied stress, a pre-exponential A illustrates the efficiency of luminescent centers and the influences of dielectric and piezoelectric constants of ZnS host and temperature are displayed as the exponent factor B.

Abstract: Magnetic pulse welding (MPW) which is one of the impact welding methods is suitable for a wide variety of combinations of similar and dissimilar metals. The flyer plate is accelerated by electromagnetic force and collided to the parent plate. A characteristic wavy interface is formed. The impact velocity and impact angle of the flyer plate during impact are important parameters which affect the interface morphology. In the case of dissimilar metals (e.g. Al/Cu, Al/Fe), the intermediate layer (such as intermetallic compound (IMC)) is formed by wavy interface formation and local temperature increase. The intermediate layer often decreases the bonding strength. Wavy interface formation mechanism and temperature increase at the joint interface should be investigated in order to obtain the dissimilar metal joint with high bonding strength. In this study, the impact velocity and impact angle of the flyer plate were obtained by using ANSYS Emag-Mechanical. Based on the obtained impact velocity and impact angle of the flyer plate in the MPW, the wavy interface formation and temperature change were reproduced by using ANSYS Autodyn for solving non-liner dynamics problems. Al sheets and Cu sheets were joined by the MPW. The joint interface was observed by OM and SEM and compared to the simulation result.

Abstract: For decades, twin-roll casting has been applied for manufacturing aluminum alloy sheets. This conventional process contributes to make thin aluminum sheets from the molten metal directly. Recently, vertical-type high-speed twin-roll casting (HSTRC) has been developed with much higher casting speed rather than the horizontal-type. Some modifications such as feeding nozzle and water-cooling system of copper rolls contribute to increase cooling rates. This characteristic leads to many metallurgical advantages like grain refinement, super-saturation of alloying elements and fine distribution of secondary particles. The objective of this study is to investigate the constituent particles in HSTRC aluminum alloy. The commercial Al-Mn alloy strip was successfully fabricated by HSTRC. Clearly different microstructure was found in thickness direction. Many constituent particles observed along the grain boundaries/cell boundaries as well as inside of Al matrix on the surface area, while eutectic structure around globular grain boundaries was observed in mid-central area. The morphology as well as chemical compositions of the constituent particles were investigated.

Abstract: The size dependent hardening in Al-4wt%Cu thin film on Si substrate has been investigated by the numerical calculation of indentation stress field and the observation of plastic zone microstructure of indented film. Distribution of internal stress predicted by triangular dislocation loop (TDL) model shows no size dependency with the different number of dislocations when the constant line density is assumed at the contact surface. TEM cross-sectional observation reveals that the plastic deformation is dominantly induced inside the film, and the growth of plastic zone is restricted at the interface of hard Si substrate. The size dependent hardening in soft film and hard substrate system is discussed from the change in dislocation density with respect to the plastic zone microstructure.

Abstract: We have used ferromagnet/antiferromagnet/ferromagnet sandwich structure to probe the antiferromagnetic layer thickness dependence of exchange bias in sputter-deposited Co/CoO/Co trilayer. The exchange coupling occurring at the upper ferromagnetic/antiferromagnetic interface is always found to be stronger than the one at the lower antiferromagnetic/ferromagnetic interface. The grain growth with increasing antiferromagnetic layer thickness can lead to a gradient of grain size distribution through the whole antiferromagnetic layer. Consequently, the relatively large grains at the upper interface would results in a rougher interface which we treat as structural defects and can significantly enhance exchange bias through domain state model. The slightly decrease of exchange coupling with increasing antiferromgnetic layer thickness indicates that the exchange bias is only governed by the grains that are thermally stable but whose anisotropy energy is low enough to be set.

Abstract: Preferred orientation of AlN films has been improved to c-axis using a highly (111) textured Pt layer. The highly textured (111) Pt layer is obtained by inserting an AlN layer between the Pt layer and substrate. Thus, Pt/AlN/substrate could be termed a substrate for preferred orientation controlled AlN films. X-ray diffraction (XRD) profiles reveal that the degree of preferred orientation of such highly (111) textured Pt layer surpasses the one originated from the crystal structure of Pt. The (2θ, ψ) intensify maps of diffracted X-ray collected as a function of the diffraction angle (2θ) and the tilting angle (ψ) exhibit that the films are perfectly (111) preferred orientated, however, they do not show in-plane texture. The (2θ, ψ) maps also demonstrate that a residual stress in films is subject to compressive.

Abstract: Preferred orientation of AlN film has been changed from random to c-axis textured by inserting the ultra-thin Pt/AlN underlayer. The ultra-thin Pt underlayer also exhibits the same effect on the preferred orientation control of AlN film. According to intensity distributions of diffracted X-ray collected as the function of 2θ and ψ on these films, it is clearly shown that c-axis of AlN is altered from titled to the surface normal. AlN film and AlN films with underlayers demonstrate tensile stress, and it can be reduced by inserting underlayers. The effects of underlayers on the development of the preferred orientation and the reduction of stresses are similar to that of decreasing sputtering gas pressure.