Transformation-Toughened ZrO2: Correlations between Grain Size Control and Composition in the System ZrO2-Y2O3

The average grain size of ZrO₂(+Y, o,) materials sintered at 1400°C was observed to depend significantly on the Y₂O₃ content. The average grain size decreased by a factor of 4 to 5 for Y₂O₃ contents between 0.8 and 1.4 mol% and increased at Y₂O₃ contents of 6.6 mol%. Grain growth control by a second phase is the concept used to interpret these data; compositions with a small grain size lie within the two-phase tetragonal + cubic phase field, and the size of the tetragonal grains is believed to be controlled by the cubic grains. This interpretation suggests that the Y₂O₃-rich boundary of the two-phase field lies between 0.8 and 1.4 mol% Y₂O₃. Transformation toughened materials fabricated in this binary system must have a composition that lies within the two-phase field to obtain the small grain size required, in part, to retain the tetragonal toughening agent.     

Mechanical properties of toughened ZrO2-Y2O3 Ceramics

Tetragonal zirconia polycrystals were produced from high-purity powders containing 1.5 to 5.0 mol% Y₂O₃ by cold isostatic pressing and sintering, hot-pressing, and hot isostatic pressing. The mechanical properties and microstructures of these resulting materials were examined, with emphasis on the relation between strength and fracture toughness.     

Phase Equilibration in ZrO2-Y2O3 Alloys by Liquid-Film Migration

The tetragonal ( t ) and cubic ( c ) ZrO₂ solid solutions in two-phase ZrO₂-8 wt% Y₂O₃ ceramics have low and high solute content, respectively. Annealing samples sintered at 1600°C between 700° and 1400°C requires a change in the volume fraction of the coexisting phases, as well as their equilibrium Y₂O₃ content. The enrichment in Y₂O₃ content of the c -ZrO₂ grains is accomplished by liquid-film migration involving the ubiquitous silicate grain-boundary phase, while the volume fraction of t -ZrO₂ increases by the nucleation and growth of cap-shaped t -ZrO₂ lenses. The interfaces between the c -ZrO₂ matrix and the growing t -ZrO₂ lenses are semicoherent.     

Phase Equilibria and Ordering in the System ZrO2-Y2O3

The phase diagram for the system ZrO₂-Y₂O₃ was redetermined. The extent of the fluorite-type ZrO₂-Y_zO₃ solid solution field was determined with a high-temperature X-ray furnace, precise lattice parameter measurements, and a hydrothermal technique. Long range ordering occurred at 40 mol% Y₂O₃ and the corresponding ordered phase was Zr₃Y₄OL₁₂. The compound has rhombohedra1 symmetry (space group R 3), is isostructural with UY₆O_l2 and decomposes above 1250±50°C. The results indicate that the eutectoid may occur at a temperature <400°C at a composition between 20 and 30 mol% Y₂O₃ Determination of the liquidus line indicated a eutectic at 83± 1 mol% Y₂O₃ and a peritectic at 76 ± 1 mol% Y₂O₃.     

Room-Temperature Strength and Fracture of ZrO2-Y2O3 Single Crystals

Strength and fracture toughness results are presented for ZrO₂ single crystals stabilized with Y₂O₃. The crystals (2 cm in diameter by 5 cm long) were prepared by skull melting. The partially stabilized compositions with 4 to 6 wt% Y₂O₃ showed a dramatic improvement in mechanical properties over the fully stabilized samples containing 20 wt% Y₂O₃, i.e. a strength exceeding 1000 MPa and a fracture toughness of 8 Mpa,.m ^1/2 were achieved compared to 200 MPa and 2 Mpa.m^1/2, respectively, for fully stabilized ZrO₂ single crystals.     

Subsolidus Phase Equilibria and Ordering in the System ZrO2-Y2O3

The subsolidus phase relations in the entire system ZrO₂-Y₂O₃ were established using DTA, expansion measurements, and room- and high-temperature X-ray diffraction. Three eutectoid reactions were found in the system: ( a ) tetragonal zirconia solid solution→monoclinic zirconia solid solution+cubic zirconia solid solution at 4.5 mol% Y₂O₃ and ∼490°C, ( b ) cubic zirconia solid solutiow→δ-phase Y₄Zr₃O₁₂+hexagonalphase Y₆ZrO₁₁ at 45 mol% Y₂O₃ and ∼1325°±25°C, and ( c ) yttria C -type solid solution→wcubic zirconia solid solution+ hexagonal phase Y₆ZrO₁₁ at ∼72 mol% Y₂O₃ and 1650°±50°C. Two ordered phases were also found in the system, one at 40 mol% Y₂O₃ with ideal formula Y₄Zr₃O₁₂, and another, a new hexagonal phase, at 75 mol% Y₂O₃ with formula Y₆ZrO₁₁. They decompose at 1375° and >1750°C into cubic zirconia solid solution and yttria C -type solid solution, respectively. The extent of the cubic zirconia and yttria C -type solid solution fields was also redetermined. By incorporating the known tetragonal-cubic zirconia transition temperature and the liquidus temperatures in the system, a new tentative phase diagram is given for the system ZrO₂-Y₂O₃.     

Phase Equilibria in the Pseudoternary System ZrO2−Y2O3−Cr2O3 at 1600°C in Air

The pseudoternary system ZrO₂-Y₂O₃-Cr₂O₃ was studied at 1600°C in air by the quenching method. Only one intermediate compound, YCrO₃, was observed on the Y₂O₃−Cr₂O₃ join. ZrO₂ and Y₂O₃ formed solid solutions with solubility limits of 47 and 38 mol%, respectively. The apex of the compatibility triangle for the cubic ZrO₂, Cr₂O₃, and YCrO₃ three-phase region was located at =17 mol% Y₂O₃ (83 mol% ZrO₂). Below 17 mol% Y₂O₃, ZrO₂ solid solution coexisted with Cr₂O₃. Cr₂O₃ appears to be slightly soluble in ZrO₂(ss).     

Possibility of Spinodal Decomposition in ZrO2-Y2O3 Alloys: A Theoretical Investigation

The thermodynamics and kinetics of cubic → tetragonal phase transformations in ZrO₂-Y₂O₃ alloys were investigated by using thermodynamic stability analysis and kinetic computer simulations to explore the possibility of a spinodal mechanism during decomposition. Based on a simple free energy model, it is shown that, depending on the alloy composition, a cubic phase aged within the t + c two-phase field may result in three different sequences of phase transformations: (1) direct nucleation and growth of the equilibrium t-phase from the c-phase matrix; (2) formation of a metastable t'-phase followed by nucleation and growth of the equilibrium c- and t-phases; and (3) formation of a transient t'-phase followed by its spinodal decomposition into two tetragonal phases with one of the tetragonal phases eventually transforming to the equilibrium c-phase. The temporal microstructure evolutions for different compositions were studied by using computer simulations based on the time-dependent Ginzburg-Landau (TDGL) model which incorporates the long-range elastic interactions.     

Crystallographic Analysis of the Cubic-to-Tetragonal Phase Transformation in the ZrO2-Y2O3 System

The crystallography of diffusion-induced and diffusionless cubic-to-tetragonal phase transformations in the ZrO₂-Y₂O₃ system is examined on the basis of the phenomenological crystallographic theory by adopting the lsqb;011rsqb; (0 1 1) twinning system as the lattice invariant deformation system. Numerical calculation indicates that the principal axes of the cubic phase are not exactly parallel to those of the tetragonal phase and that the habit plane orientation is sensitive to the lattice parameters of the cubic and tetragonal phases. The calculated results are compared with the observed crystallography of the tetragonal precipitates formed by diffusion and of the metastable tetragonal phase formed in a diffusionless manner. In many aspects, the present results were in good agreement with experimental observations. In particular, the crystallography and morphology of the so-called "herringbone" structure are very well explained by the present analysis.     

Solubility Limit of MgO in Al2O3 at 1600°C

The solubility of Mg in alumina was measured using wavelength-dispersive spectroscopy mounted on a scanning electron microscope. Careful calibration of the microscope's working conditions was performed in order to optimize the detection limit and accuracy. Measurements were conducted on water-quenched and furnace-cooled samples, without any thermal or chemical etching to avoid alteration of the bulk concentration. The results indicate the solubility limit of Mg in alumina to be 132±11 ppm at 1600°C.     

The Systems ZnO-Fe2O3-SnO2 and MgO-Fe2O3-SnO2 at 1060°C

Zn₂SnO₄, an inverse spinel, and ZnFe₂O₄, a normal spinel, form a complete series of solid solutions in the system ZnO-Fe₂O₃3nO₂. The variation of cell dimensions with composition varies from 0.8439 nm (ZnS₂nO₄) to 0.8660 nm (ZnS₂nO₄) and exhibits a positive deviation from a linear relationship. Mg₂SnO₄ and MgFe₂O₄, both predominantly inverse in nature, have only an incomplete series.     

On the Microstructures Resulting from the Diffusionless Cubic→Tetragonal Transformation in ZrO2-Y2O3 Alloys

A diffusionless cubic (c)→metastable tetragonal (t') phase transformation occurs in certain alloys in the ZrO₂-Y₂O₃ system on quenching from elevated temperatures. Microstructural features due to this phase transformation, principally anti-phase domain boundaries (APB's) and mechanical accommodation twins, have been characterized using transmission electron microscopy. Certain differences between our interpretation and those of other workers are discussed.     

The System HfO2-Y2O3-Er2O3

A mathematical model of the liquidus surface based on a reduced polynomial method was proposed for the system HfO₂-Y₂O₃-Er₂O₃. The results of calculations according to this model agree fairly well with the experimental data. Phase equilibria in the system HfO₂-Y₂O₃-Er₂O₃ were studied on melted (as-cast) and annealed samples using X-ray diffraction (at room and high temperatures) and micro-structural and petrographic analyses. The crystallization paths in the system HfO₂-Y₂O₃-Er₂O₃ were established. The system HfO₂-Y₂O₃-Er₂O₃ is characterized by the formation of extended solid solutions based on the fluorite-type (F) form of HfO₂ and cubic (C) and hexagonal (H) forms of Y₂O₃ and Er₂O₃. The boundary curves of these solid solutions have the minima at 2370°C (15. 5 mol% HfO₂, 49. 5 mol% Y₂O₃) and 2360°C (10. 5 mol% HfO₂, 45. 5 mol% Y₂O₃). No compounds were found to exist in the system investigated.     

Phase Equilibria in the System CaO-MgO-B2O3 at 900°C

Phase equilibria in the system CaO-MgO-B₂O₃ were investigated at 900°C using X-ray powder diffraction techniques. With the exception of MgO-B₂O₃, the binary phases reported previously were confirmed, but no ternary phases were found. Solid solution effects were investigated for the binary phases by comparison of patterns, whereas for CaO and MgO, accurate lattice parameters were compared. No solid solutions were detected. As a result, the isothermal equilibrium diagram at 900°C reduces to three phase triangles. X-ray powder diffraction data for the calcium berates are included.     

Phase Relationships in the ZrO2-Sc2O3 and ZrO2-In2O3 Systems

Zirconia-rich subsolidus phase relationships in the ZrO₂–Sc₂O₃ and ZrO₂–In₂O₃ systems were investigated. Phase inconsistencies in the ZrO₂–Sc₂O₃ system resulted from a diffusionless cubic-to-tetragonal ( t' ) phase transformation not being recognized in the past. Through three different measuring techniques, along with microstructural observations, the solubility limits of the tetragonal and cubic phases were determined.     

Determination of 1600° and 1700°C. Liquidus Lines in CaO-2Al2O3 and AI2O3 Stability Fields of the System CaO—Al2O3—SiO2

The 1600° and 1700°C. liquidus lines in the CaO·2Al₂O₃ and A1₂O₃ stability fields of the system CaO-Al₂O₃-SiO₂ are determined from the chemical analyses of saturated slags at these temperatures.