Yutaka Shinoda

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.

Magnesium dititanate (MgTi2O5) with pseudobrookite structure: a review

Abstract Magnesium dititanate (MgTi2O5, MT2) has been synthesized since the early 1930s. It has the pseudobrookite structure (general formula Me3O5), corresponding to the Mg-enriched artificial endmember of the Fe2TiO5 (pseudobrookite)–FeTi2O5 (ferropseudobrookite)–Mg0.5Fe0.5Ti2O5 (armalcolite) solid solution. Since MgTi2O5 has relativity high thermal stability among pseudobrookite-type phases, it is expected to be a well-balanced low-thermal-expansion material. Here we review both the historical and recent studies on MgTi2O5, particularly on its crystal structure, cation order–disorder, physical properties and synthesis methods.

Multifunctional porous titanium oxide coating with apatite forming ability and photocatalytic activity on a titanium substrate formed by plasma electrolytic oxidation

Plasma electrolytic oxidation (PEO) was used to make a multifunctional porous titanium oxide (TiO2) coating on a titanium substrate. The key finding of this study is that a highly crystalline TiO2 coating can be made by performing the PEO in an ammonium acetate (CH3COONH4) solution; the PEO coating was formed by alternating between rapid heating by spark discharges and quenching in the solution. The high crystallinity of the TiO2 led to the surface having multiple functions, including apatite forming ability and photocatalytic activity. Hydroxyapatite formed on the PEO coating when it was soaked in simulated body fluid. The good apatite forming ability can be attributed to the high density of hydroxyl groups on the anatase and rutile phases in the coating. The degradation of methylene blue under ultraviolet radiation indicated that the coating had high photocatalytic activity.

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.

Tensile Ductility of Liquid-Phase Sintered β-Silicon Carbide at Elevated Temperature

Nano-grain sized β-SiC was prepared with various additives by hot-pressing in Ar. The relative bulk densities were more than 95 %. As-sintered bodies were composed of equiaxed grains. Amorphous phase existed at grain boundaries in each as-sintered material. Tension tests were performed at the initial strain rates from 3 X 10 -4 s -1 to 3 X 10 -5 S * at 1973 - 2048 K. Tensile elongation more than 60 % was obtained. In deformed specimens, grain growth was observed. However, anisotropy of grain growth was not observed. The grain-boundary phase was vaporized during tension tests. The β phase of SiC was stable even after tension test. Therefore, critical deformation mechanism was thought to be grain-boundary sliding. Deformation behavior was influenced by the grain-boundary phase.

Large increase in fracture resistance of stishovite with crack extension less than one micrometer

The development of strong, tough, and damage-tolerant ceramics requires nano/microstructure design to utilize toughening mechanisms operating at different length scales. The toughening mechanisms so far known are effective in micro-scale, then, they require the crack extension of more than a few micrometers to increase the fracture resistance. Here, we developed a micro-mechanical test method using micro-cantilever beam specimens to determine the very early part of resistance-curve of nanocrystalline SiO2 stishovite, which exhibited fracture-induced amorphization. We revealed that this novel toughening mechanism was effective even at length scale of nanometer due to narrow transformation zone width of a few tens of nanometers and large dilatational strain (from 60 to 95%) associated with the transition of crystal to amorphous state. This testing method will be a powerful tool to search for toughening mechanisms that may operate at nanoscale for attaining both reliability and strength of structural materials.

TEM analysis of nanocrystalline SiC ceramics sintered by SPS using Al2O3–TiO2 additive

Abstract Nanocrystalline SiC ceramics was fabricated by spark plasma sintering (SPS) using Al2O3 and TiO2 additives with the composition of 5, 10 and 20 mass% (Al2O3–TiO2). XRD analysis revealed formation of TiC phase. And also mullite phase with preferred orientation was detected in SiC with 20 mass% addition. From TEM observation and EDS analysis, TiC, aluminosilicate glass and mullite crystal phases were identified. Segregation of titanium atoms in addition to aluminum and oxygen atoms was observed at intergranular glassy phase with the thickness of 1–2 nm. The electrical conductivity was the same level as the Al2O3 single addition to SiC ceramics.

Statistics of grain disappearance in three-dimensional normal grain growth

Abstract Three-dimensional grain growth was analysed using the Surface Evolver program that simulates grain-boundary motion by curvature. When the grain-boundary network reached a statistically self-similar steady state structure in the normal grain growth period, the mean life span until the disappearance of grains could be predicted from the distribution function of the normalized grain size and the distribution function of the number of faces in normal grain growth period.

Effect of the Elastic Deformation of a Point-Sharp Indenter on Nanoindentation Behavior

The effect of the elastic deformation of a point-sharp indenter on the relationship between the indentation load P and penetration depth h (P-h curve) is examined through the numerical analysis of conical indentations simulated with the finite element method. The elastic deformation appears as a decrease in the inclined face angle β, which is determined as a function of the elastic modulus of the indenter, the parabolic coefficient of the P-h loading curve and relative residual depth, regardless of h. This indicates that nominal indentations made using an elastic indenter are physically equivalent to indentations made using a rigid indenter with the decreased β. The P-h curves for a rigid indenter with the decreased β can be estimated from the nominal P-h curves obtained with an elastic indenter by using a procedure proposed in this study. The elastic modulus, yield stress, and indentation hardness can be correctly derived from the estimated P-h curves.

Deformation Behavior of SiO2 Doped Nanocrystalline Monoclinic Zirconia at Low Temperatures

The deformation behavior of SiO2 doped nanocrystalline monoclinic zirconia (MZP) was studied at 1323-1223 K in compression tests. The strain rate of SiO2 doped nanocrystalline MZP was slower than that of high-purity MZP by one order of magnitude. SiO2 doped nanocrystalline MZP exhibited a stress exponent n ≈ 2. The apparent activation energy for the deformation of SiO2 doped nanocrystalline MZP was characterized by a higher value than that observed for high-purity MZP. 1wt% SiO2 doped nanocrystalline MZP was deformed at constant flow stress, while the flow stress of high-purity MZP increased significantly with the strain (strain hardening). While no grain growth was observed after the compressive deformation of 1wt % SiO2 doped nanocrystalline MZP, remarkable grain growth was observed after the deformation of high-purity MZP. The addition of SiO2 into nanocrystalline MZP is effective in limiting grain growth at low temperatures