Papers by Author: Eiichi Sato

Abstract: Accommodation processes are crucial for grain boundary sliding in superplasticity though few have been reported on their positive experimental evidences. The present study achieved two-dimensional grain boundary sliding in ODS ferritic steel which had elongated and aligned grain structure and got direct observations of accommodation processes without the surface effect of floating grains: 1) In Region II, diffusional accommodation was confirmed through observing the change in marking-line spacing, which indicates volume inflow and outflow at grain boundaries. 2) Between Regions II and III, dislocation accommodation inside of the mantle region, as proposed by Gifkins, was confirmed through observing curves of marking lines near grain boundaries. 3) In Region III, dislocation accommodation inside of the core region, as proposed by Ball and Hutchison, was confirmed through observing slip bands and sub-boundaries passing through a grain. It is, then, derived that superplasticity relies not on a single mechanism but on several diffusional and dislocation accommodations contributing depending on strain rate condition.

Abstract: A new class of superplastic titanium alloy, Ti–4.5Al–2.5Cr–1.2Fe–0.1C–0.3Cu–0.3Ni, was deformed at 1073 K with strain rates of 1×10⁻⁴–1×10⁻¹ s⁻¹, and microstructures in the condition between superplastic regions II and III (= 1×10⁻² s⁻¹) were observed using scanning electron microscope and electron back-scattered diffraction. Continuous dynamic recrystallization was observed, resulting in grain refinement both in α and β phases. The grain size decreased significantly in α phase at the early stage of the deformation and in β phase at the later stage. In the recrystallized microstructure, the major sub-boundaries formed perpendicularly to slip directions <11−20> in α phase and parallel to slip planes {110} in β phase, which might be caused by the difference in the symmetry of the crystal structures.

Abstract: High-temperature tensile deformation was performed using an oxide-dispersionstrengthened (ODS) ferritic steel,, which has grain structure largely elongated and aligned in one direction, in the perpendicular direction. In the superplastic region II, two-dimensional grain boundary sliding (GBS) was achieved, in which the material did not shrink in the grain-axis direction and grain-boundary steps appeared only in the surface perpendicular to the grain axis. In this condition, a classical grain switching event was observed. Using kernel average misorientation maps drawn with SEM/EBSD, dominant deformation mechanisms and accommodation processes for GBS were examined in the different regions. Cooperative grain boundary sliding, in which only some of grain boundaries slide, was also observed.

Abstract: Two-dimensional grain movements were microscopically observed in high-temperature shear deformation of an oxide-dispersion-strengthened ferritic steel with an elongated and aligned grain structure that was sheared in a direction perpendicular to the grain long axis. The microstructure was analyzed using electron back-scattered diffraction and electron channeling contrast imaging techniques before and after the shear deformation. Clear grain switching events, which are assumed to occur via grain-boundary sliding (GBS), were observed and the switching mechanism was characteristic of the core–mantle superplasticity model proposed by Gifkins; dislocation densities got much higher in narrow areas near the grain boundaries (mantles) than the grain interiors (cores). The mantle regions typically appeared in protruding portions of grains that was likely resistant to GBS, and low-angle boundaries were found to emerge at the core–mantle boundaries via slipping of dislocations within the mantle regions.

Abstract: The grain size dependence of creep behavior from coarse grain to ultrafine grain regions was examined using fully-annealed specimens fabricated from a single process route. For coarse-grained sample, in tensile deformation, stress-strain curves show slow work hardening, and the proof stress shows typical Hall-Petch behavior. On the other hand, creep behavior is observed under the stress above the proof stress, and the creep rate has no grain size dependence. For ultrafine-grained sample, in the tensile deformation, stress-strain curves show yielding behavior, and the yield stress shows Hall-Petch behavior also. On the other hand, creep behavior was observed below the proof stress, but the creep rate decreases with a decrease in grain size.

Abstract: It is understood that grains move by grain boundary sliding, and change their relationship to each other during superplastic deformation. Ideal two-dimensional observation of grain movements from the specimen surface is difficult even in the shear deformation because grains move three-dimensionally according to the stress distribution against the specimen surface. In this study, ODS steel with elongated grains aligned along one direction was deformed perpendicular to the aligned axis to achieve ideal two-dimensional grain movements. Surface height profiles with a laser microscope showed small amount of three-dimensional grain movements, while two-dimensional grain movements and rotations were appeared by observations before and after the deformation with SEM-EBSD.

Abstract: HCP metals show new dislocation creep at temperatures below 0.3 Tm with stresses below σ0.2, while FCC metals show it above σ0.2. In the former, grain boundaries absorb the dislocations through slip-induced grain-boundary sliding, while in the latter dislocations are accommodated by cross slip at cell walls. The difference comes from the difference in the crystal symmetry. In UFG-Al at low temperatures, it is anticipated that grains without cell structure lead creep deformation similar to CG HCP metals rather than CG Al. UFG Al specimens were fabricated by ARB method. They showed remarkable creep behavior at less than σ0.2 similary to CG HCP metals. It posseses stress exponent of about three, grain-size exponent of almost zero, and very low apparent activation energy of 20 kJ/mol, and also grain boundary sliding behavior is obserbed by AFM.

Abstract: Hexagonal close-packed metals and alloys show significant creep behavior with extremely low activation energies at and below ambient temperature even below their 0.2% proof stresses. It is caused by straightly-aligned dislocation arrays in a single slip system without any dislocation cuttings. These dislocation arrays should, then, pile up at grain boundary (GB) because of violation of von Mises' condition in H.C.P. structure. The piled-up dislocations have to be accommodated by GB sliding. Electron back scatter diffraction (EBSD) analyses and atomic force microscope (AFM) observations were performed to reveal the mechanism of GB sliding below ambient temperature in H.C.P. metals as an accommodation mechanism of ambient temperature creep. EBSD analyses revealed that crystal lattice rotated near GB, which indicates the pile up of lattice dislocations at GB. AFM observation showed a step caused by GB sliding. GB sliding below ambient temperature in H.C.P. metals are considered to compensate the incompatibility between neighboring grains by dislocation slip, which is called slip induced GB sliding.

Abstract: Institute of Space and Astronautical Science (ISAS/JAXA) in collaboration with Mitsubishi Heavy Industries (MHI) has developed fuel and gas tanks for reaction control system and orbital control system of satellites; A tank is fabricated through welding of two thin, hemi-spherical or conical parts, which are fabricated by superplastic blow forming. Mass-productivity is not an important factor but the forming precision and flexibiliry in the process are important for this application. ISAS and MHI, therefore, developed a new blow-forming technique, which has high flexibility in terms of tank size because it requires a furnace but not a hot-press machine. Some typical propulsion tanks fabricated through this process are presented.