Fumihiro Wakai

Three-dimensional microstructural evolution in ideal grain growth—general statistics

Abstract The idealized grain growth in three dimensions (3D) was studied by the Surface Evolver method that models the process of boundary motion by curvature to minimize the boundary energy. Even starting from arbitrary packing of uniform grains, the boundary network reached steady structure after the incubation and transient period. The parabolic law in grain growth was observed only in a region where the steady structure was maintained. The more general von Neumann–Mullins law on kinetics of grain growth held in both transient period and normal grain growth period. The grain size distribution function and the distribution of the number of faces in steady structure were analyzed in 3D, and compared with the microstructure in cross-section. The perimeter law and Aboav–Weaire law in 3D on topological nature of boundary network structure held not only in the steady structure but also in transient structures.

Effects of solute ion and grain size on superplasticity of ZrO2 polycrystals

The deformation of ZrO2 polycrystals containing 2 to 8 mol% Y2O3 or 12 mol% CeO2 were investigated by uniaxial tension and tensile creep tests at elevated temperatures. It was found that there were two deformation mechanisms. The stress exponent was close to 2 for the finegrained materials (less than 1 μm), but the exponent decreased with increasing grain size. This behaviour was analysed using a model based on grain-boundary sliding with diffusion accommodation, in which the diffusion creep controlled by interface-reaction and that controlled by diffusion of cations were incorporated. The diffusion coefficient of cations was greatly affected by the concentration of the solute ions. It was observed that there was a negative correlation between interface-reaction rate and Y2O3 concentration.

Equilibrium configuration of particles in sintering under constraint

Abstract The equilibrium configuration of identical particles evolved during sintering was determined by minimizing the sum of surface energy and grain boundary energy for the case where the interparticle distance was constrained. The energy was calculated for the equilibrium configuration as functions of the distance between particles. The energy–displacement curve exhibited hysteresis loop at bonding and separation of two particles. The sintering force, a force necessary to just stop the sintering contraction, was calculated from the slope of energy–displacement curve.

Superplasticity of ceramics

Superplastic elongation of fine-grained ceramics can be observed when some mechanical and microstructural requirements are satisfied at appropriate test conditions, temperature and strain rate. The phenomenon can be utilized in superplastic forming, solid-state bonding, sinter-forging, hot press, and hot-isostatic pressing. Recent advances in ceramic processing brought about superplastic ZrO2 composites, covalent polycrystals, bioceramics, and nanocrystalline materials. Numerous works on superplasticity of ZrO2 ceramics revealed some characteristics, microstructural features, and possible mechanisms. Attempts to achieve high-strain rate superplasticity were performed by modifying the nature of the grain boundary.

Sintering through surface motion by the difference in mean curvature

Abstract The diffusion of atoms changes the morphology of particles in sintering. When the attachment kinetics of adatoms is slower than the diffusion of atoms, the sintering is controlled by the local processes taking place at the interface. This interface-controlled sintering, for example, the sintering by evaporation–condensation, is described as the sintering through the surface motion by the difference between mean curvature and the average of mean curvature. The sintering of two identical spherical particles has been studied by the Surface Evolver program until they reach the final equilibrium shape. Although the evaporation–condensation is a “non-densifying mechanism”, the shrinkage occurred due to the motion of mass centers of particles in a long time. The shrinkage rate was approximately proportional to the sintering force.

Topological transformation of grains in three-dimensional normal grain growth

The topological transformation of grains in three-dimensional normal grain growth was analyzed by Brakkes Surface Evolver method that simulated the boundary motion by curvature. The statistics on elemental processes, which change the number of faces f of a grain, were determined from the simulation. The distribution function of the number of faces P ( f ) in a steady structure could be predicted from the difference in the current of grains arriving at and leaving from state f. For the disappearance of one grain, face-creation switching occurred 3.7 times and face-elimination switching occurred 13.2 times on the average.

Topological transformation of grains in superplasticity-like deformation

Abstract The maintenance of the equiaxed shape of grains after a large deformation is a common feature of superplasticity in polycrystalline solids. This feature is similar to that seen in the deformation of soap froth. The topological transformation of the grains was analyzed by using the Surface Evolver which simulated the three-dimensional simple shearing flow of soap froth. In this model the equiaxed shape of the grains could be maintained in a disordered system of N grains when both face-creation switching and face-elimination switching occurred αγN times in shear strain γ . The simulation gave α -values from 4.9 to 4.5 in the monodispersed structure and α =4.7 in the polydispersed structure. The strain, which is associated with grain switching, is represented by γ ″=Δ F / αN , where Δ F is the mean cumulative number of grain switchings. The intragranular strain, which is related to the aspect ratio of the grains, is represented by γ ′= γ − γ ″. The back stress by boundary tension was proportional to γ′ . It can act to restore the equiaxed shape, and is one of the origins of anelasticity.

High temperature deformation of silicon nitride ceramics with different microstructures

Abstract The deformation of some silicon nitride ceramics which consist of elongated β-Si 3 N 4 grains in a fine-grained matrix containing an amorphous grain boundary phase was studied by tensile testing at elevated temperature. Superplastic elongations larger than 250% could be observed, and characteristic microstructural development during deformation was studied.

Ceramics superplasticity: Deformation mechanisms and microstructures

The superplasticity shall be generally achieved by grain refinement of various ceramics (ionic polycrystals and covalent polycrystals). This nature can be utilized for novel deformation processing in ceramic industry, for example, superplastic forming and superplastic forging. The superplasticity is a macroscopic phenomena that is very sensitive to slight difference in atomistic structure, and nature and chemical bonding of grain boundary. The mechanism of superplasticity is grain boundary sliding accommodated by matter transport through grain boundary. The models developed for superplasticity are classified by the structure of grain boundary. The experimental results on superplasticity of ZrO{sub 2} and Si{sub 3}N{sub 4} were reviewed and compared to the predictions from the theories.

Fabrication of zirconia-alumina functionally gradient material by superplastic diffusion bonding

Functionally gradient ZrO2-Al2O3 material was fabricated by superplastic diffusion bonding in the present study. Conditions included a bonding temperature of 1550 °C, a time of 30 min, and strains of 17, 33 and 50%, Complete bonding was obtained under all of these bonding conditions. The apparent bending strength of functionally gradient material fabricated by superplastic diffusion bonding was 1860 MPa at the strain of 50%.