Effect of Chemical Bonding State on High-temperature Plastic Flow Behavior in Fine-grained, Polycrystalline Cation-doped Al<SUB>2</SUB>O<SUB>3</SUB>

Abstract

High-temperature plastic deformation characteristics such as flow stress and elongation to failure in polycrystalline Al 2 O 3 with an average grain size of about 1 µm is remarkably changed by the doping of 0.1 mol% YO 1.5 , SiO 2 , TiO 2 or ZrO 2 at 1400 • C under an initial strain rate of 1.2 × 10 −4 s −1 . The difference in the flow stress and the tensile ductility is considered to be originated from change in the grain boundary chemistry in Al 2 O 3 due to segregation of the dopant cation in the vicinity of the grain boundary. A change in the chemical bonding state in the cations-doped Al 2 O 3 is examined by first-principle molecular orbital calculations using DV-Xα method based on [Al 5 O 21 ] 27− cluster model. Chemical shift observed in electron energy-loss spectra (EELS) at the grain boundary due to the dopant segregation can be roughly reproduced by the molecular orbital calculations. A correlation is found between the flow stress and product of net charges of aluminum and oxygen ions. Moreover, the tensile elongation to failure seems to correlate with bond overlap population between Al and O. The change in the chemical bonding strength at the grain boundary must dominantly affect to the grain boundary diffusion and bond strength at the grain boundary, and thus seems to be an important factor to determine the plastic flow behavior in polycrystalline Al 2 O 3 .

Densification Behavior of Ti-Doped Polycrystalline Alumina in a Nitrogen-Hydrogen Atmosphere

“…Our previous studies revealed that a small amount of doped cations tends to segregate in the vicinity of the grain boundaries in polycrystalline Al 2 O 3 sintered in air. [13][14][15][16][17] The present result of the TiO 2 -doped Al 2 O 3 sintered in the reducing atmosphere is consistent with the previous reports for that sintered in air. Figure 4 shows an EELS spectrum for the TiO 2 -doped Al 2 O 3 s sintered in the N 2 þ H 2 atmosphere at 1573 K. The Ti-L 2;3 and O-K edges can be seen in the spectrum, though the intensity of the Ti-L 2;3 edge is fairly weak compared to the O-K edge.…”

“…6,15,16) The previous results indicate that the high-temperature matter transport in Al 2 O 3 increases with the decreasing absolute value of the effective ionic charge. The effective ionic valence in the Al cation and O anion must correspond to Coulomb's attractive force between the ions.…”

Change in equilibrium position of misfit dislocations at the GaN/sapphire interface by Si-ion implantation into sapphire. II. Electron energy loss spectroscopic study

“…They 14 examined the effect of doping of 0.1 mol% SiO 2 on the chemical bonding state in α-Al 2 O 3 by a first-principles molecular orbital calculation using DV-Xα based on [Al 5 O 21 ] 27− model cluster. According to their calculation, 14 by the substitution of Si for Al,the product of Al and O net charges, which is a measure of the ionic bonding strength in ionic crystals, 15,16 such as α-Al 2 O 3 , becomes lower than that of the undoped α-Al 2 O 3 . This is thus expected to lead to a lower elastic modulus.…”

“…1(d)] show a sign of the substitution of Si for Al in sapphire.The softening effect of Si isattributed to the reduction of the ionic bond strength by the substitution of Si for Al. 14 …”

Criterion for High Temperature Failure and Grain Boundary Chemistry in Superplastic TZP

“…13) On the other hand, a first principle molecular orbital calculation revealed that the tensile elongation to failure in ceramics correlates well with bond overlap population between anions and cations, which corresponds to covalent bond strength between atoms. 14,15) The change in the chemical bonding strength at grain boundaries must dominantly influence grain boundary diffusion and bond strength at the grain boundaries, and thus seems to be an important factor to determine the plastic flow in superplastic ceramics. However, the correlation between the interatomic reaction and the critical assessment for high temperature dutility has not been clarified yet.…”