Local Bonding States of Titanium and Germanium-doped Tetragonal Zirconia Polycrystal and Their Correlation to High Temperature Ductility

Abstract

The chemical bonding states of GeO 2 and/or TiO 2 -doped tetragonal zirconia polycrystal (TZP) are calculated by a first principle molecular orbital method using model clusters. It is clarified that Ge 4+ and Ti 4+ ions, which are substituted into a lattice of TZP, have a high covalent bond with oxygen ions rather than Zr-O bond. Covalency of TZP is more increased by solution of germanium ions than that of titanium ones. In superplastic deformation of TZP, an addition of GeO 2 or TiO 2 enhances tensile ductility of TZP. Germanium ion is more effective to improve ductility than titanium. The increment of covalency is in a good agreement with the improvement of elongation to failure in doped TZP. Dopant cations segregate at grain boundaries and form no secondary phase. Assuming that a dopant effect on chemical bonding states in grain boundaries is similar to that in grain interior, segregation of germanium or titanium ion increases covalent bonding strength nearby grain boundaries. Such increasing of covalency is likely to enhance cohesion of grain boundaries. The enhancement of grain boundary cohesion suppresses intergranular failure during tensile deformation at elevated temperatures. This must be the reason why an addition of GeO 2 and TiO 2 is effective to improve the high temperature ductility of TZP. Our calculation suggests that the covalency nearby grain boundaries have a critical role in the tensile ductility of TZP.

Flat Cavities formed in TZP caused by Superplastic Deformations at High Strain-Rates and Their Effect on Elongation

“…(2) The addition of dopants, which can suppress the generation of flat cavities even when the stress applied to the specimen is high, improves the elongation as well. The former idea coincides with the results given by Kim et al,24) while the latter coincides with that shown by Kuwabara et al, 23) though qualitatively. In conclusion, it is most probable that whether the generation of flat cavities is easy to occur or not is a crucial phenomenon to determine the elongation to fracture of superplastic ceramics.…”

“…Recently, it has been reported [20][21][22] that an addition of dopants which segregate and alter the chemical bonding state at and nearby grain boundaries sensitively changes the elongations to fracture of TZP. More recently, Kuwabara et al 23) have shown that the elongation to fracture of TZP has a close relation with covalency at and nearby grain boundaries; an addition of dopants, which enhances the covalency, improves the elongation considerably. They 23) have explained that the enhancement of the grain boundary covalency makes grain boundary cohesion stronger and suppresses intergranular failure accordingly.…”

“…More recently, Kuwabara et al 23) have shown that the elongation to fracture of TZP has a close relation with covalency at and nearby grain boundaries; an addition of dopants, which enhances the covalency, improves the elongation considerably. They 23) have explained that the enhancement of the grain boundary covalency makes grain boundary cohesion stronger and suppresses intergranular failure accordingly. Combining the results of Kuwabara et al 23) with our present ones, we can conclude that the addition of dopants, which enhances covalency and accordingly cohesion of grain boundaries, can suppress the formation of the flat cavities.…”

“…They 23) have explained that the enhancement of the grain boundary covalency makes grain boundary cohesion stronger and suppresses intergranular failure accordingly. Combining the results of Kuwabara et al 23) with our present ones, we can conclude that the addition of dopants, which enhances covalency and accordingly cohesion of grain boundaries, can suppress the formation of the flat cavities.…”

Grain Boundary Energy and Tensile Ductility in Superplastic Cation-doped TZP

“…[9][10][11] Recent studies have pointed out that covalent bonding states nearby grain boundaries correlate with the tensile ductility of TZP and Al 2 O 3 . [12][13][14] Further analyses to the grain boundaries are indispensable to fully understand the tensile ductility of TZP.…”