Zirconia-Related Phases in the Zirconia/Titanium Diffusion Couple
After Annealing at 11001–15501C
Kun-Lin Lin and Chien-Cheng Lin*
,wDepartment of Materials Science and Engineering, National Chaio Tung University, Hsinchu 30050, Taiwan A diffusion couple of 3 mol% Y2O3–ZrO2 and titanium was
isothermally annealed in argon at temperatures between 11001 and 15501C. The phases and microstructure in the ceramic side were investigated using scanning electron microscopy and trans-mission electron microscopy, both attached to an energy-disper-sive spectrometer. After annealing at 11001C/6 h, zirconia grains did not grow conspicuously and evolved only traces of oxygen, resulting in t-ZrO2xbut not a-Zr. At temperatures above 13001C, a significant amount of oxygen evolved from zirconia, reducing the O/Zr ratio, such thata-Zr was excluded from t-ZrO2x during cooling, yielding a higher O/Zr ratio ( 2). When held at 15501C/6 h, zirconia grains grew rapidly. The a-Zr was segregated on grain boundaries during cooling by the exsolution of zirconium from ZrO2x, while twinned t0-ZrO2xor lenticular t-ZrO2x, which was embedded in or-dered c-ZrO2x, was found. The ordered c-ZrO2xwas
identi-fied by the 1
5 {113} superlattice reflections of its electron diffraction patterns.
I. Introduction
E
XTENSIVEresearch1–3has been performed on the reactions between titanium and zirconia (ZrO2), mostly usingmicro-hardness measurements, metallographic, and X-ray diffraction analyses. Economos and Kingery1indicated that titanium pene-trated along the grain boundaries of ZrO2, without the
forma-tion of a new phase, other than the black oxygen-deficient zir-conia (ZrO2x). Weber et al.2discovered limited dissolution of
titanium in zirconia, as well as the blackening of zirconia, when the melting titanium reacted with the ZrO2crucible. Ruh3found
that up to 4 at.% of titanium was retained at room temperature as a substitutional solid solution in zirconia, while up to ap-proximately 10 mol% zirconia would be dissolved in titanium, with zirconium and oxygen entering into the substitutional and interstitial positions of titanium lattice, respectively.
The darkening of yittria-stabilized zirconia was observed at high temperatures under reducing conditions.4,5Moya et al.6,7 attributed the darkening of partially stabilized zirconia to the formation of Zr31ions. Using the electron spin resonance meth-od, they claimed that the difference in color could be caused by the exsolution of impurities, mainly iron, from the bulk to the surface of zirconia polycrystals under reducing conditions. In contrast, other studies found8,9black-colored zirconia following the formation of nonstoichiometric ZrO2x, where x indicates
the deviation from stoichiometry. Rice10stated that the zirconia was blackened by loss of intrinsic oxygen. Ingo11indicated that the change of valence states of zirconium was responsible for dark-ening of 8 wt% yttria–zirconia plasma-sprayed thermal barrier
coatings, while the segregation and exsolution of impurities (Fe, Al, Si, and Na) could be ruled out as causes of darkening. The phase transformations of zirconia have been subjected to comprehensive investigations in the past several decades.12–20In
a systematic study on 12 wt% Y2O3ZrO2, Chaim et al.21
showed that a tweed-like microstructure existed inside c-ZrO2,
resulting from the strain field of small coherent t-ZrO2
precipi-tates after sintering at 16001C/2 h. While a twinned t0-ZrO 2
phase with antiphase domain boundaries was observed after a subsequent annealing at 15501C/1 h and a rapid cooling, twinned t-ZrO2precipitates (or colonies) among the c-ZrO2
ma-trix were found after long-term annealing at 14001C. However, precipitate-free zones at the perimeter of c-ZrO2grains with fine
t-ZrO2 precipitates were observed after annealing at 12501C.
The twinned t0-ZrO2phases, which resulted from the
untrans-formable t-ZrO2, were characterized by a high Y2O3content up
to 10 wt% as mentioned in several other previous studies.17The twins were able to relieve the strains arising from the small tetragonality of the product phase and the small molar volume change accompanying transformation.
Previous studies1–3 mentioned above did not deal with the
fine features and new phases (except a-Zr) formed in the reac-tion zone of zirconia and titanium because of the instrumental limitation. Using analytical transmission electron microscopy (TEM), Lin and Lin22characterized the microstructure abutting the interface of Ti melt and ZrO2. They reported that the liquid–
solid reaction at 17501C/7 min caused zirconia to transform into oxygen-deficient zirconia (ZrO2x) with an O/Zr ratio as low as
1.53. While a significant amount of oxygen accumulated at grain boundaries of titanium, the remainder was dissolved in titanium as a-Ti(O). An ordered titanium suboxide (Ti3O) was then
formed from a solid solution of a-Ti(O) during cooling. More-over, the twinned a-Zr(O) was excluded from ZrO2x, leading to
the formation of fine crystalline ZrO2xwith a high O/Zr ratio
( 1.9) during cooling. The lamellae of Ti2ZrO and a-Ti(Zr, O)
were also found by TEM/energy-dispersive spectrometer (EDS) analysis.
In the heat-treatment experiments of the ZrO2/Ti diffusion
couple conducted recently by Lin and Lin,23the lamellar and the
spherical Ti2ZrO as well as the orthorhombic b0-Ti were found
to exist in the titanium side after annealing at 15501C/30 min. On heating, the dissolution of a large amount of zirconium and oxygen into titanium gave rise to the metastably supersaturated disordered a-Ti(Zr, O) solid solution where Ti2ZrO
subsequent-ly precipitated, while the b-Ti coexisting with a-Ti at high tem-peratures was transformed to the orthorhombic b0-Ti during cooling. However, the microstructural variation of zirconia in the reaction-affected zone has not been studied thoroughly to date. The purpose of the present study is to investigate the phases and microstructure of zirconia in the Ti/ZrO2diffusion
couple annealed at 11001C to 15501C for 6 h using scanning electron microscopy (SEM) and TEM, both attached to an EDS.
II. Experimental Procedures
Bulk ZrO2specimens used in this study were prepared from the
powder of 3 mol% Y2O3partially stabilized zirconia (494 wt%
Journal
DOI: 10.1111/j.1551-2916.2005.00526.x r2005 The American Ceramic Society
2928 D. Green—contributing editor
The authors would like to thank the National Science Council of Taiwan for financially supporting this research under Contract No. NSC 91-2216-E-009-021.
*Member, American Ceramic Society.
w
Author to whom correspondence should be addressed. e-mail: chienlin@faculty. nctu.edu.tw
ZrO21HfO2, 5.4 wt% Y2O3,o0.001 wt% Fe2O3,o0.01 wt%
SiO2,o0.005 wt% Na2O,o0.005 wt% TiO2,o0.02 wt% Cl,
o0.005 wt% SO42; Toyo Soda Mfg. Co., Tokyo, Japan) by hot
pressing (Model HP50-MTG-7010, Thermal Techno. Inc., San-ta Rosa, CA). The specimens were heated to and held at 3001C for 3 min under 5 MPa at a heating rate of 301C/min, while heating to and being held at 14501C for 30 min under 30 MPa at a heating rate of 251C/min. During cooling, pressure was released at 11001C, and then the furnace was cooled down to room temperature.
The as hot-pressed ZrO2and commercially available pure
ti-tanium (with a nominal composition of 99.31 wt% Ti, 0.25 wt% O, 0.01 wt% H, 0.03 wt% N, 0.10 wt% C, 0.30 wt% Fe, des-ignated as Cp–Ti hereafter) were cut and machined to the
di-mensions of 14 mm 14 mm 5 mm. Their surfaces were
ground and polished to 0.5 mm using a diamond paste, and were then ultrasonically cleaned in acetone. The Cp–Ti was in-serted in between two ZrO2 specimens to form a sandwiched
sample, and then placed in a graphite furnace; it was pressed in a preparatory process at 5 MPa, before the furnace was evacuated to 2 104Torr and then filled with argon to a pressure of one atmosphere. This cycle of evacuation and purging was repeated at least three times. The temperature was increased to 10001C at a rate of 301C/min, and then to 11001, 13001, and 15501C at 251C/min, respectively, where the specimen was then held for 6 h. During cooling, the temperature was lowered to room temper-ature at a rate of 251C/min and then the pressure was released. The cross-sectional TEM specimens perpendicular to the in-terface of zirconia and titanium were cut into pieces of
approx-imately 3 mm 2 mm 0.5 mm. They were ground down to
B80 mm thickness with a diamond matted disk, polished with diamond pastes of 6, 3, and 1 mm in sequence, dimpled to 50 mm thickness, and finally milled by precision ion milling (Model 691, Gatan Inc., Pleasanton, CA). The microstructure of the zirconia side in the reaction-affected zone was characterized using an SEM (Model JSM-6330F, JEOL Ltd., Tokyo, Japan) and an analytical TEM (Model JEM 2000Fx, JEOL Ltd.) equipped with an EDS (Mode ISIS300, Oxford Instrument Inc., London, U.K.). The quantitative composition analyses were performed based on the principle of the Cliff–Lorimer24 standardless
method.
III. Results and Discussion
(1) Annealing at 11001C
Figure 1(a) displays the TEM micrograph of the t-ZrO2x or
tetragonal-deficient oxygen zirconia in the ceramic side of the
ZrO2/Ti diffusion couple after reaction at 11001C/6 h. An
av-erage t-ZrO2xparticle size of around 0.5 mm is obtained so that no grain growth was noticeable. Figure 1(b) shows the diffrac-tion ring pattern of the t-ZrO2x. The first, second, third, and fourth rings correspond to the t-ZrO2x{111}, {200}, {112}, and
{220} planes, respectively. No phases other than the oxygen-deficient zirconia were found in the ceramic side.
(2) Annealing at 13001C
Figure 2 displays an SEM micrograph (backscattered electron image (BEI)) of the zirconia side of the ZrO2/Ti diffusion couple
after reaction at 13001C/6 h. The micrographs reveal the coex-istence of a-Zr, labeled as ‘‘A’’ (bright), and t-ZrO2x, labeled as
‘‘B’’ (gray). The mean particle size of both a-Zr and t-ZrO2x
was approximately 1 mm. The a-Zr appeared brighter than the ZrO2x in the BEI because of the atomic number effect. Also
shown in Fig. 2, the dark areas represent voids, and a coarser intergranular a-Zr is shown by the arrow.
Figure 3 (a) displays the TEM micrograph of a-Zr and t-ZrO2x, corresponding to Fig. 2 on the zirconia side of the
ZrO2/Ti diffusion couple after reaction at 13001C/6 h. The
se-lected area diffraction pattern (SADP) of a-Zr, shown in Fig. 3(b), was identified as that of a hexagonal structure with a 3.18 A˚ and c 5.01 A˚. Figure 3(c) shows the EDS spec-trum of a-Zr, which reveals a composition of 81.03 at.% Zr, 15.29 at.% O, and 3.68 at.% Y. Figure 3(d) displays the SADP of t-ZrO2xon the zone axis of [110] with parameters a 5.05 A˚
and c 5.14 A˚. Figure 3(e) presents the EDS spectrum of t-ZrO2x, consisting of 33.29 at.% Zr, 61.45 at.% O, and 5.26
at.% Y corresponding with ZrO1.846. It is consistent with the
fact that a two-phase region of a-Zr (O) and t-ZrO2xwith a
wide range of oxygen content (30.8 at.% O–63.5 at.% O) exists in the phase diagram of Zr–O at 13001C. It also indicates that the solubility of a-Zr in tetragonal ZrO2xdeclined as the
tem-perature decreased.25Hence, the a-Zr (O) tends to get
segrega-ted from the supersaturasegrega-ted solid solution of ZrO2x during cooling, and excluding Zr can increase the O/Zr ratio of the oxygen-deficient zirconia.
(3) Annealing at 15501C
Figure 4(a) shows a BEI at the zirconia side of the ZrO2/Ti
dif-fusion couple after reaction at 15501C/6 h. It reveals the coarsen-ing of intergranular a-Zr (marked as ‘‘A’’) and t-ZrO2x
(marked as ‘‘B’’) in the c-ZrO2xmatrix (marked as ‘‘C’’). The
a-Zr was approximately 10 mm in size. Parts of the a-Zr easily peeled off to form voids (black) at the grain boundaries of zir-conia by mechanical grinding and polishing. The grain size of the
Fig. 1. (a) Transmission electron microscopy micrograph (bright field image) of zirconia far away from the ZrO2/Ti interface after reaction at
zirconia matrix was about 50 mm on average, which was much larger than that of zirconia after reaction at either 11001 or 13001C.
The high chemical affinity of titanium to oxygen, as well as the high solid solubility of oxygen in titanium (about 14.5%), is the driving force of the oxidation–reduction reaction, causing the formation of oxygen-deficient zircona (ZrO2x) and a phase
in titanium (oxygen-stabilized a phase). The oxidation–reduc-tion reacoxidation–reduc-tion can be demonstrated by the following reacoxidation–reduc-tion:
OxO in ZrO 2 !V O in ZrO2þ 2e 0 in ZrO2 þ Osolid solution in Ti (1)
The oxygen vacancies, resulting from the reduction reaction of ZrO2by Ti on annealing, as indicated by Eq. (1), were believed
to dominate in the present study, even though the equilibrium concentration of oxygen vacancies increases with temperature. It is well known that vacancy is the predominant diffusion mech-anism in zirconia, and the oxygen vacancies increase with the deviation from stochiometry. Therefore, the atoms diffuse more rapidly with decreasing O/Zr ratio because of the reduction re-action of oxygen-deficient zirconia. The grain growth of ZrO2x
during annealing at high temperatures is thus enhanced in that the grain growth is governed by the diffusion phenomenon.
The rate of oxidation–reduction reaction between zirconia and titanium increased with temperature, leading to a significant increase in oxygen vacancies or diffusivity, and an enhanced grain growth of ZrO2xas well. In contrast, the grain size of as
hot-pressed zirconia, which was subjected to the same heat treatment as the zirconia/titanium diffusion couple, was about 2–3 mm on average as shown in Fig. 4(b). This fact indicates that the reaction between zirconia and titanium played a very im-portant role in grain growth. During cooling, a-Zr was segre-gated on grain boundaries by the exsolution of zirconium from ZrO2x. The segregation of a-Zr in the grain boundaries can
suppress further grain growth of ZrO2xbecause of the
drag-ging effect. It is evidenced that no intragranular a-Zr was ob-served in the present study.
Figure 5(a) presents the TEM micrograph at the zirconia side of the ZrO2/Ti diffusion couple after the reaction at 15501C/6 h.
The twinned t0-ZrO
2xwas embedded in the t0-ZrO2xmatrix.
Reflections of the type odd, odd, even, not allowed for c-ZrO2,
were applied extensively to distinguish the c-ZrO2 from the
t-ZrO2phase. A /111S zone axis orientation was used as all
possible t-ZrO2variants were identified using {112}-type
reflec-tions.12–20Figures 5(b) and (c) show the microdiffraction pat-terns of the t0-ZrO2x matrix and the twin t0-ZrO2x,
respectively, with the electron beam along the zone axis [111]. The fact that reflections {112} were absent in Figs. 5(b) and (c) indicated that both the matrix and the twin were tetragonal rather than cubic ZrO2x. These results were basically in agree-ment with the observations conducted by Heuer et al.16,17,21,
Fig. 2. Scanning electron microscopy micrograph (backscattered elec-tron image) of the zirconia side in the ZrO2/Ti diffusion couple after
reaction at 13001C/6 h, indicating the coexistence of a-Zr (marked as ‘‘A’’) and t-ZrO2x, (marked as ‘‘B’’). Also shown is the coarsening of
a-Zr (arrowed).
Fig. 3. (a) Transmission electron microscopy micrograph (bright field image) of a-Zr and t-ZrO2xin the ZrO2/Ti diffusion couple after reaction at
13001C/6 h; (b) selected area diffraction pattern (SADP) of the a-Zr along the [101] zone axis; (c) an energy-dispersive spectrum of the a-Zr shown in (b); (d) SADP of the t-ZrO2x, along the [110] zone axis; (e) an energy-dispersive spectrum of the t-ZrO2xshown in (d).
who described the displacive c-t0 transformation in ZrO 2
alloys.
The ZrO2had a high Y2O3content up to more than 10 wt%
in previous studies;16,21 however, the Y2O3 concentration in
ZrO2was as low as 5.36 wt% (3 mol%) in this study.
Accord-ing to the ZrO2–Y2O3phase diagram, the ZrO2with 3 mol%
Y2O3at 15501C was predominantly tetragonal rather than cubic
ZrO2,26indicating that the c-t0displacive transformation could
not have taken place in 3 mol% Y2O3–ZrO2. However, a
sig-nificant increase in oxygen vacancies, as a consequence of the reaction between zirconia and titanium, triggered the stabiliza-tion effect of zirconia. Therefore, it was inferred that the
zir-conia would be in the cubic phase solid solution region during annealing at 15501C, and the t0phase was formed as the diffu-sion couple was cooled in a furnace to room temperature fol-lowing annealing.
At a position further away from the interface, it was found that lenticular t-ZrO2xwith three {100}-type variants
precipi-tated in the c-ZrO2xmatrix on the zirconia side after annealing
at 15501C/6 h, as shown in Fig. 6(a). It was inferred that the specimen was cooled down from the two-phase (c1t) region in the ZrO2–Y2O3 phase diagram. On the other hand, the cubic
oxygen-deficient zirconia would exist at 15501C, as the eutectoid reaction, as expressed in the following equation, took place at about 15251C:27
ZrO2xðcubicÞ2ZrO2xðtetragonalÞ þ ZrðaÞ (2)
Figure 6(b) clearly shows, at a higher magnification, the contrast of the stress field because of the ordering of oxygen in the c-ZrO2x matrix. The EDS spectrum of c-ZrO2x, as shown in
Fig. 6(c), revealed that lenticular t-ZrO2x consisted of 35.5
at.% Zr, 58.48 at.% O, and 6.02 at.% Y, corresponding to an oxygen-deficient zirconia of 7.25 mol% Y2O3–ZrO1.41. The
low-er limit of the solidus of the cubic ZrO2xphase lies in the range
of 1.64oO/Zro1.70 between 18151 and 20651C, while that for t-ZrO2xwas 1.925 below 13001C.27,28It was not surprising that
the measured O/Zr ratio in the present study was as low as 1.41 because of the intense oxidation–reduction reaction on anneal-ing and sluggish diffusion duranneal-ing coolanneal-ing. The SADPs of the c-ZrO2xmatrix, with the zone axes being [110] and [310], are
shown in Figs. 6(d) and (e), respectively. The fact that (112) re-flections were absent in Fig. 6(d) confirmed that the matrix was c-symmetric in structure. The ordered structure was character-ized by1
5{113} superlattice reflections for zirconia unit cell in the mean time.
According to the Ellingham diagram, Ti cannot reduce the ZrO2. If the following reactions were applicable in the present
study:
Tiþ O2! TiO2 (3)
Zrþ O2! ZrO2 (4)
Then the oxidation–reduction reaction can be written as follows:
ZrO2þ Ti ! Zr þ TiO2 (5)
The Gibbs-free energy for the oxidation–reduction reaction (5) is DG¼ DGoxidðTiÞ DGoxidðZrÞ, where DGoxidðTiÞand DGoxidðZrÞ are the Gibbs-free energies of oxidation of Ti and Zr, respec-tively. Based on the Ellingham diagram, the Gibbs-free energy
Fig. 4. (a) Scanning electron microscopy (SEM) micrograph (backscattered electron image) of the zirconia side in the ZrO2/Ti diffusion couple after
reaction at 15501C/6 h, indicating the coarsening of intergranular a-Zr (marked as ‘‘A’’) and t-ZrO2x(marked as ‘‘B’’) in the c-ZrO2xmatrix (marked
as ‘‘C’’); (b) SEM micrograph (secondary electron image) of as hot-pressed zirconia after annealing at 15501C/6 h in Ar.
Fig. 5. (a) Transmission electron microscopy micrograph (bright field image) of zirconia in the ZrO2/Ti diffusion couple after reaction at
15501C/6 h, indicating the twinned t0-ZrO
2xin t0-ZrO2xmatrix; (b)
and (c) show microdiffraction patterns from the twinned t0-ZrO
2xand
the t0-ZrO
for the oxidation–reduction reaction (5) DG is positive, indicat-ing that Ti cannot reduce ZrO2. However, this is not the case in
the present study. Based on the experimental results, the fol-lowing dissolution reaction should be applied:
1=2O2ðgÞ! O00
iðin titaniumÞþ 2hðin titaniumÞ (6) The Gibbs-free energy of oxygen dissolution in solid titanium DGdissis related to the temperature by the following equation:29
DGdiss¼ RT ln Kdiss¼ 609 þ 0:126T ðkJ=molÞ (7) where Kdiss is the equilibrium constant of reaction (6), R is
the gas constant ( 8.3144 J/mol/K), and T is the
abso-lute temperature. When T 515501C 5 1823 K, DGdiss¼
379:30ðkJ=molÞ. On the other hand, the reduction reaction of zirconia can be expressed as follows:
OxOðin zirconiaÞ! VOðin zirconiaÞþ 2e0ðin zirconiaÞþ 1=2O2ðgÞ (8) The Gibbs-free energy of the reduction reaction (8) DGredcan be calculated from the following equation:30
DGred¼ RT ln Kred¼ RT 2 68 000
T
(9)
where Kredis the equilibrium constant of the reduction reaction
(8), of which the nature logarithm is equal to (2–68 000/T). When T 5 15501C 5 1823 K, DGred¼ 0:54 ðkJ=molÞ. The re-sultant reaction of reactions (6) and (8) is written using the fol-lowing equation:
OXOðin zirconiaÞ!VOðin zirconiaÞ þ 2e0ðin zirconiaÞ þ O00iðin titaniumÞþ 2hðin titaniumÞ
(10)
As DG¼ DGredþ DGdiss¼ 0:54 379:30 ¼ 378:76ðkJ=molÞ at 15501C, reaction (10) is thermodynamically favorable, con-sistent with the results observed in the present study.
(4) Proposed Model of Phase Development at 13001C The proposed model of microstructural evolution at the zirconia side of ZrO2/Ti diffusion couple annealed at 13001C is
schema-tically displayed in Fig. 7. In as hot-pressed zirconia, the grain size was about 0.3–0.4 mm on average (Fig. 7(a)). On heating up to 13001C, because titanium had a much higher affinity with oxygen than zirconium, ZrO2was dramatically reduced to
Fig. 6. (a) Transmission electron microscopy micrograph (bright field image) of zirconia in the ZrO2/Ti diffusion couple after reaction at 15501C/6 h,
displaying {100}-type variants of the lenticular t-ZrO2xin c-ZrO2xmatrix; (b) a magnified image of the c-ZrO2xmatrix in (a), showing the ordered
structure; (c) energy-dispersive spectrum of the ordered c-ZrO2x; (d) and (e) show selected area diffraction patterns of the ordered c-ZrO2xmatrix with
the zone axes being [110] and [310], respectively.
Fig. 7. Schematic diagrams showing the microstructural evolution of the zirconia side in the ZrO2/Ti diffusion couple annealed at 13001C/6 h.
(a) As hot pressed; (b) grain growth on heating to 13001C; (c) exclusion of a-Zr from ZrO2xduring cooling.
ZrO2xby titanium and an insignificant grain growth occurred (Fig. 7(b)). During cooling, Zr was excluded from the unstable oxygen-deficient zirconia to the grain boundaries (Fig. 7(c)). (5) Proposed Model of Phase Development at 15501C Figure 8 displays the schematic diagrams of microstructural evo-lution of the zirconia side in a hot-pressed ZrO2/Ti diffusion
couple annealed at 15501C. In the heating stage, ZrO2x was
initially formed following the intense oxidation–reduction reac-tion between zirconia and titanium as menreac-tioned above. Then ZrO2xgrains would grow rapidly because the vacancy
diffu-sion was drastically enhanced (Fig. 8(b)). In the cooling stage, the a-Zr segregated on the grain boundaries of ZrO2x
(Fig. 8(c)), causing the suppression of grain growth. The micro-structure would be varied depending upon the distance from the interface. At the position close to the interface, the oxygen-deficient zirconia, with more concentrated oxygen vacancies, would be located in the single c-phase region and experience the displasive diffusionless c-t0 transformation, resulting in twinned t0-ZrO
2x(Fig. 8(d)). At a position further away from
the interface, it was believed that the oxygen-deficient zirconia, with less oxygen vacancies, was in the (c1t) dual-phase region and the lenticular t-ZrO2with three variants precipitated in the
ordered c-ZrO2x(Fig. 8(e)).
IV. Conclusions
1. The diffusion couple of zirconia and titanium was iso-thermally annealed at temperatures ranging from 11001 to 15501C. Three distinct microstructures were found depending upon the annealing temperature.
2. In the zirconia/titanium diffusion couple annealed at 11001C/6 h, the t-ZrO2grain did not grow conspicuously and
only a trace of oxygen evolved, yielding t-ZrO2xwithout a-Zr.
3. After annealing at 13001C/6 h, more oxygen was evolved from zirconia, resulting in a decreasing O/Zr ratio. The a-Zr was excluded from t-ZrO2x, leading the O/Zr ratio to be
close to 2.
4. When held at 15501C/6 h, zirconia grains grew rapidly in addition to the intense oxidation–reduction reaction between zirconia and titanium. The a-Zr was segregated on grain bound-aries during cooling by the exsolution of zirconium from ZrO2x, while twinned t0-ZrO2xor lenticular t-ZrO2x, which
was embedded in ordered c-ZrO2x, was found. The ordered
c-ZrO2xwas identified by the15{113} superlattice reflections of
its electron diffraction patterns.
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Fig. 8. Schematic diagrams showing the microstructural evolution of the zirconia side in ZrO2/Ti diffusion couple annealed at 15501C/6 h. (a) As hot
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