Precipitation Studies in the System WC-ZrC

The solubility of WC in ZrC varied from 25 mol% at 1800°C to 65 mol% at 2550°C. There was no detectable solubility of ZrC in WC in the same temperature range. Tungsten carbide precipitates from supersaturated WC-ZrC solutions as rods. The crystallographical relation between the precipitated phase and the matrix established by electron diffraction techniques is: [100]_wc|[110]_ZrC and [010]_wc|[011]_zrc. The interfacial planes were (001) for WC and (111) for ZrC. The hardness of the solid solution of 55 mol% WC and 45 mol% ZrC increased ∼ 10% after it was annealed at 1900°C.     

Solid Solubility and Precipitation in a Single-Crystal Alumina–Zirconia System

Solid solubility was examined in Zr-doped sapphire and Al-doped yttria-stabilized zirconia (YSZ) single crystals from 1200° to 1600°C. Specimens were fabricated via ion implantation of single crystals, followed by annealing in air. Secondary ion mass spectroscopy (SIMS) was used to quantify solute redistribution during annealing. Comparison of SIMS results with analytical electron microscopy (AEM) revealed an alumina solubility of 0.2–0.3 wt% in zirconia, and a zirconia solubility of 0.004–0.027 wt% in alumina. Direct imaging of zirconia precipitates revealed that tetragonal zirconia precipitates from supersaturated sapphire with the following orientation relationship: (100)_tetragonal‖ (0001)_sapphire and [01¯1]_tetragonal‖ [12¯10]_sapphire.     

Crystallography of the Tetragonal to Monoclinic Transformation in MgO-Partially-Stabilized Zirconia

The structure of monoclinic ZrO₂ particles dispersed in MgO-partially-stabilized zirconia has been examined systematically using transmission electron microscopy and electron micro-diffraction. In both particles transformed athermally and those transformed under stress, the product of the martensitic tetragonal to monoclinic transformation comprises parallel variants of the monoclinic structure. The monoclinic domains extend either parallel or normal to the original precipitate habit plane and, to within the accuracy of the technique, adjacent pairs are twin related. For particles with domains parallel to the particle habit plane the boundary between variants is (001)_m and the orientation relationship between tetragonal and monoclinic lattices is such that (001)_m‖ (001)_t and [100]_m‖ [100]_t. In particles with transverse domains, the domain boundaries are parallel to (100)_m, and the orientation relationship is given by (100)_m‖ (100)_t and [001]_m‖ [001]_t. In each case the lattice correspondence implied between parent and product lattices is such that the c _m axis is parallel to the C _m axis. The microcracking associated with transformed particles appears closely related to the substructure adopted by the particles and the origin of this microcracking is briefly discussed.     

Shape Evolution of a Coherent Tetragonal Precipitate in Partially Stabilized Cubic ZrO2: A Computer Simulation

The kinetics of shape evolution of a tetragonal precipitate coherently embedded in a cubic matrix are examined. Specifically, the morphology of tetragonal ZrO₂ particles in partially stabilized cubic ZrO₂ is discussed. A computer simulation, carried out without any a priori constraint on possible kinetic paths and particle morphologies, shows that a lenslike shape appears during growth of a tetragonal particle. Upon further coarsening , the shape relaxes into a rhombus bounded by facets. Depending on the balance between interfacial and strain energies controlled by the particle size, the facets can be smoothly curved or straight. The predicted particle morphologies are in good agreement with the experimental observations. The kinetic model proposed is quite general for simulating microstructural developments during decomposition involving a crystal lattice symmetry change where elastic strain accommodation plays an important role.     

Precipitation in Partially Stabilized Zirconia

Transmission electron microscopy was used to study the sub-structure of partially stabilized ZrO₂ (PSZ) samples, i.e. 2-phase "alloys" containing both cubic and monoclinic modifications of zirconia, after various heat treatments. Monoclinic ZrO₂ exists as (1) isolated grains within the polycrystalline aggregate (a grain-boundary phase) and (2) small plate-like particles within cubic grains. These intragranular precipitates are believed to contribute to the useful properties of PSZ via a form of precipitation hardening. These precipitates initially form as tetragonal ZrO₂, with a habit plane parallel to the {100} matrix planes. The orientation relations between the tetragonal precipitates and the cubic matrix are
and      

Structure-Optical Property Correlation of Epitaxial Potassium Niobate Thin Films Deposited on Magnesium Oxide (100) Substrates Using a Strontium Titanate Transition Layer

Epitaxial thin films of orthorhombic KNbO₃ (100)/(010) were deposited on MgO (100) single-crystal substrates by pulsed laser ablation using an ε120 Å SrTiO₃ (110) transition layer in between. From X-ray diffraction, the orientation relationships were determined as (010) [100] or (100) [010] KNb0₃∥ (110) [110] SrTiO₃∥ (100) [011] MgO and (010) [001] or (100) [001] KNbO₃∥ (110) [001] SrTiO₃∥ (100) [011] MgO. The measured film refractive indices at 632.8 nm were 2.213 ± 0.003, 2.278 ± 0.003, and 2.285 ± 0.003, respectively, along MgO [100], [011], and [011] directions. A model was developed to correlate the measured effective refractive indices of the film to area fractions of domain variants in the film. Using the model, the area fraction of domains with their polarization axis normal to the substrate was estimated to be 60.0 ± 2.7%.     

Metastable Crystal Structure of Strontium- and Magnesium-Substituted LaGaO3

The metastable crystal structure of strontium- and magnesium-substituted LaGaO₃ (LSGM) was studied at room and intermediate temperatures using powder X-ray diffractometry and Rietveld refinement analysis. With increased strontium and magnesium content, phase transitions were found to occur from orthorhombic (space group Pbnm ) to rhombohedral (space group R [Threemacr] c ) at the composition La_0.825Sr_0.175Ga_0.825Mg_0.175O_2.825 and, eventually, to cubic (space group Pm [Threemacr] m ) at the composition La_0.8Sr_0.2Ga_0.8Mg_0.2O_2.8. At 500°C in air and at constant strontium and magnesium content, a phase transformation from orthorhombic (space group Pbnm ) to cubic (space group Pm [Threemacr] m ) was observed. For the orthorhombic modification, thermal expansion coefficients were determined to be α _a ,ortho = 10.81 × 10⁻⁶ K⁻¹, α _b ,ortho = 9.77 × 10⁻⁶ K⁻¹, and α _c ,ortho = 9.83 × 10⁻⁶ K⁻¹ (25°–400°C), and for the cubic modification to be α_cubic= 13.67 × 10⁻⁶ K⁻¹ (500°–1000°C).     

Optical and X-Ray Single Crystal Studies of the Monoclinic ⇌ Tetragonal Transition in ZrO2

The monoclinic ⇌ tetragonal phase transition in ZrO₂ single crystals was studied at temperature by transmission optical microscopy and X-ray diffraction techniques. A series of timelapse photographs illustrated the relations between the events that occur during the transition. The events themselves were recognized by direct observation using a high-temperature microscope stage and by scrutiny of several high-temperature Laue photographs. During heating the monoclinic phase transforms to the tetragonal by the motion of an interface parallel to the (100) _m plane; simultaneous twinning also occurs behind the advancing interface. The tetragonal phase is usually twinned on the (1 2) _bct or ( 12) _bct plane, and the extent of twinning is influenced by the heating rate. Cooling transforms the untwinned tetragonal form into a twinned monoclinic form with the orientation of the monoclinic twins parallel to the trace of the (001) _m plane when observations are made in the (100) _m plane. Transformation of a twinned tetragonal crystal results in twins on the {110} _m and {001} _m planes. Orientation relations in the ZrO₂ transformation are: (100) _m ‖(110) _bct , [010] _m ‖[001] _bct , and by the virtue of twinning, (100) _m ‖(110) _bct , [001] _m ‖[001] _bct . During cooling the same topotaxial relations are maintained.     

High-Pressure Phase Transitions in Zirconia and Yttria-Doped Zirconia

Raman spectroscopy has been utilized to characterize the phase transformations and transition pressures in pure and doped zirconia containing 3, 4, and 5 wt% Y₂O₃. The pressure-induced transformations were investigated to over 6 GPa (at room temperature) using a diamond anvil pressure cell. Pure zirconia single-crystal samples transformed to a "new" tetragonal phase (different from the one obtained at high temperatures at atmospheric pressure) at about 4 GPa. The pressure transformation, like the temperature transition, was reversible and exhibited an approximately 0.45-GPa hysteresis at room temperature. The 3 and 4 wt% Y₂O₃ crystals underwent a monoclinic ( P 2_1/b) to tetragonal ( P 4_{2 nmc}) phase transition similar to that observed at high temperatures. This phase change was found to be irreversible on releasing the pressure. The 5 wt% Y₂O₃ at atmospheric pressure consists of a tetragonal modification in a disordered cubic matrix; a gradual, but reversible, disordering transformation of the tetragonal precipitate takes place with pressure.     

Application of Transmission Electron Microscopy to Engineering Practice in Ceramics: Introduction, Fundamentals, and Application to Precipitation Phenomena

This paper, the first of eight in this issue of the Journal devoted to transmission electron microscopy (TEM) of ceramics, provides a brief introduction to the physics of electron optics as it relates to the study of microstructural defects in crystalline solids. The particular application of TEM discussed in this paper, the study of subsolidus precipitation in star sapphire (Ti-doped AI₂O₃) and in magnesia-partially stabilized zirconia (Mg-PSZ), illustrates the usefulness of this technique in providing detailed crystallographic and microstructural information on precipitation reactions. In star sapphire, TEM was used to identify unambiguously the needle-like precipitate phase responsible for the asterism in gem-quality crystals as rutile, the stable tetragonal form of TO₂. The observed orientation relation between precipitate and matrix allows good lattice matching between the two phases. The work on PSZ described in the present paper consists of "deciphering" the precipitation history of a commercial, sintered, polycrystalline ceramic showing, in particular, that precipitation occurred in three distinguishable stages. Although tetragonal ZrO₂ is the precipitate phase in all three stages, a polymorphic transformation to monoclinic symmetry occurs in two of the three types of precipitates; the tetragonal symmetry is retained metastably in the third. Furthermore, the internal structure of the precipitates and the precipitate/cubic zirconia host microstructural relations are different in each case; these differences can profoundly affect mechanical properties.     

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.     

Relationship between Fracture. Toughness and Phase Assemblage in Mg-PSZ

The phase content, size, and distribution have a marked effect on the thermomechanical properties of a 9 mol% MgO-ZrO₂ (Mg-PSZ) alloy. The work undertaken in this study is the first investigation to simultaneously measure and characterize the quantitative bulk phase contents and correlate these parameters to the fracture toughness and R -curve behavior of a variously aged Mg-PSZ alloy. Quantitative bulk phase characterization entailed a systematic evaluation using neutron diffraction. In addition, microscopy techniques were used to determine the phase distribution and assemblage. The results indicate that the maximum toughness is attained, for a constant tetragonal precipitate size, when the anion-ordered δ-phase (Mg₂Zr₅O₁₂) replaces a major portion of the cubic matrix phase after and 1100°C aging treatment. The shape of the R -curve and the stress required to transform the tetragonal precipitates are also significantly influenced by the δ-pnase content.     

Three-Dimensional Dynamic Calculation of the Equilibrium Shape of a Coherent Tetragonal Precipitate in Mg-Partially Stabilized Cubic ZrO2

The equilibrium shape of a tetragonal precipitate coherently embedded in a cubic matrix is examined in three dimensions by a computer simulation. Independent experimental data of Mg-partially stabilized ZrO₂ are used as the input parameters. The equilibrium shape is obtained diffusional relaxation of an initially nonequilibrium spherical shape. The relaxation is described through a generalized field kinetic model which takes into account the transformation-induced elastic strain arising from a cubic → tetragonal crystal lattice rearrangement. Without any a priori constraint on the geometry of the particle, the equilibrium shape is shown to be a rotation disk formed by two cones with the common base. It is far from the ellipsoidal shape assumed in the conventional analytical calculations based on Eshelby's model.     

Polymorphic Behavior of Thin Evaporated Films of Zirconium and Hafnium Oxides

Polymorphism in thin evaporated films of zirconium and hafnium oxides was investigated from 100° to 1500°C by electron diffraction and transmission electron microscopy. The films have metastable cubic structures at room temperature and at moderate temperatures. Zirconium oxide, depending on temperature, exists in cubic, tetragonal, and monoclinic forms, whereas hafnium oxide transforms directly from the cubic to the monoclinic structure. The transformation temperatures depend on the oxygen partial pressure. Air annealing of thin films of ZrO₂ and HfO₂ lowered the temperature of transformation of the tetragonal and the cubic structure into the monoclinic structure by about 150° and 100°C, respectively. The cubic/tetragonal transformation of ZrO₂ is monotropic, whereas the tetragonal monoclinic transformation occurs by the typical nucleation and growth mechanism. Determination of grain size in ZrO₂ at the tetragonal/monoclinic transformation temperature showed that the transformation occurs when a constant grain size of about 800 Å is reached. The oxygen partial pressure, grain size, and temperatures at which the metastable phases exist were correlated. The rate of grain growth is enhanced by increase in oxygen partial pressure. The accelerated transformation in air is attributed to rapid attainment of the critical size; grain boundary energy is an important controlling factor in transformation.     

Cubic-to-Tetragonal (t') Transformation in Zirconia-Containing Systems

The coexistence of the cubic fluorite and tetragonal phases in rapidly quenched samples was studied in the ZrO₂-MO_1.5 systems for M = Sc, In, Y, and rare earths (R). Spontaneous transformation from metastable cubic phase was triggered at room temperature by a mechanical force. Isolated tetragonal platelets in the cubic matrix were bounded by [101] habit planes and contained anti-phase boundaries. The tetragonality decreased with stabilizer content and vanished at around 18 mol% for M = Y and R, 23 mol% for M = Sc, and 25 mol% for M = In, all at room temperature. With increasing temperature, the tetragonality initially increased because of anisotropic thermal expansion, then decreased rapidly, after reaching a maximum, as the temperature for the tetragonal-to-cubic transformation was approached. Being a first-order martensitic transformation, the cubic-to-tetragonal transformation is accompanied by a discontinuous change of tetragonality and a hysteresis loop as the temperature or composition passes through the equilibrium value.     

Revised Phase Diagram of the System ZrO2-CeO2 Below 1400°C

The phase diagram of the system ZrO₂-CeO₂ was rein-vestigated using hydrothermal techniques. Cubic, tetragonal, and monoclinic solid solutions are present in this system. The tetragonal solid solution decomposes to monoclinic and cubic solid solutions by a eutectoid reaction at 1050°50°C. The solubility limits of the tetragonal and cubic solid solutions are about 18 and 70 mol% CeO₂, respectively, at 1400°C, and about 16 and 80 mol% CeO₂, respectively, at 1200°C. Solubility limits of the monoclinic and cubic solid solutions are about 1.5 and 88 mol% CeO₂ at 1000°C, and 1.5 and 98 mol% CeO₂ at 800°C, respectively. The compound Ce₂Zr₃O₁₀ is not found in this system.     

Phase Transformation and Densification of an Attrition-Milled Amorphous Yttria-Partially-Stabilized Zirconia–20 mol% Alumina Powder

Phase transformations during consolidation treatments of an attrition-milled amorphous yttria-partially-stabilized zirconia (Y-PSZ: ZrO₂–3 mol% Y₂O₃)–20 mol% Al₂O₃ powder and the resulting microstructures have been investigated. A metastable cubic phase ( c -ZrO₂ solid solution) together with an α-Al₂O₃ phase is formed in the amorphous matrix by consolidation at temperatures below 1204 K. The metastable cubic phase transforms to a stable tetragonal phase ( t -ZrO₂ solid solution) with an increase in the consolidation temperature. Fully dense bulk samples consisting of extremely fine tetragonal grains together with a small amount of α-Al₂O₃ particles could be obtained by consolidation at temperatures above 1432 K. Important features concerned with the densification behavior are as follows: (1) Marked increase in the relative density occurs after cubic crystallization and subsequent cubic-to-tetragonal transformation. (2) All of the consolidated bulk samples show extremely fine grain structure with grain sizes of several tens of nanometers, irrespective of the consolidation temperature. (3) The regularity of the lattice fringe contrast in each tetragonal grain seems to be kept in the vicinity of grain boundaries. These results suggest that densification of the attrition-milled amorphous powder proceeds via superplastic flow and/or diffusional creep, rather than viscous flow of the initial amorphous phase before crystallization.     

Cubic-to-Tetragonal Displacive Transformation in Gd2O3–Bi2O3 Ceramics

Gd₂O₃-doped Bi₂O₃ polycrystalline ceramics containing between 2 and 7 mol% Gd₂O₃ were fabricated by pressureless sintering powder compacts. The as-sintered samples were tetragonal at room temperature. Hightemperature X-ray diffraction (XRD) traces showed that the samples were cubic at elevated temperatures and transformed into the tetragonal polymorph during cooling. On the basis of conductivity measurements as a function of temperature and differential scanning calorimetry (DSC), the cubic → tetragonal as well as tetragonal → cubic → teansition temperatures were determined as a function of Gd₂O₃ concentration. The cubic → tetragonal transformation appears to be a displacive transformation. It was observed that additions of ZrO₂ as a dopant, which is known to suppress cation interdiffusion in rare-earth oxide–Bi₂O₃ systems, did not suppress the transition, consistent with it being a displacive transition. Annealing of samples at temperatures 660°C for several hundred hours led to decomposition into a mixture of monoclinic and rhombohedral phases. This shows that the tetragonal polymorph is a metastable phase.     

Raman Scattering Study of Cubic–Tetragonal Phase Transition in Zr1−xCexO2 Solid Solution

A structural phase transition between the cubic (space group, Fm 3 m) and tetragonal (space group, P 4₂ /nmc) phases in a zirconia–ceria solid solution (Zr_1−xCe_xO₂) has been observed by Raman spectroscopy. The cubic–tetragonal ( c–t" ) phase boundary in compositionally homogeneous samples exists at a composition X₀ (0.8 < X₀ < 0.9) at room temperature, where t " is defined as a tetragonal phase whose axial ratio c/a equals unity. The axial ratio c/a decreases with an increase of ceria concentration and becomes 1 at a composition X'₀ (0.65 < X'₀ < 0.7) at room temperature. The sample with a composition between X₀ and X'₀ is t " ZrO₂. By Raman scattering measurements at high temperatures, the tetragonal ( t" ) → cubic and cubic → tetragonal phase transitions occur above 400°C in Zr_0.2 Ce_0.8O₂ solid solution.