Ti
2ZrO Phases Formed in the Titanium and Zirconia Interface After
Reaction at 15501C
Kun-Lin Lin and Chien-Cheng Lin*
,wDepartment of Materials Science and Engineering, National Chaio Tung University, Hsinchu 30050, Taiwan Hot-pressed 3 mol% Y2O3partially stabilized ZrO2was
react-ed with titanium at 15501C/30 min. The interface was charac-terized by analytical transmission microscopy (transmis-sion electron microscopy/energy-dispersive spectroscopy). The lamellar and the spherical Ti2ZrO as well as the orthorhombic
b0-Ti were found to exist in the titanium side after cooling down
to room temperature. The crystal structures of the lamellar and the spherical Ti2ZrO were orthorhombic and hexagonal,
respec-tively. On heating, the dissolution of a large amount of zirco-nium and oxygen into titazirco-nium gave rise to the metastably supersaturated disordereda-Ti(Zr, O) solid solution where two different Ti2ZrO phases subsequently precipitated, while the
b-Ti coexisting with a-Ti at high temperatures was transformed to the orthorhombicb0-Ti during cooling. The spherical hexag-onal Ti2ZrO was an ordered structure, with Zr and O occupying
substitutional and interstitial sites, respectively. The orientation relations betweena-Ti and the lamellae orthorhombic Ti2ZrO
were determined to be [0001]a-Ti//½110Ti2ZrO and (1010)a-Ti//
ð110ÞTi2ZrO; meanwhile, those between thea-Ti and the spherical hexagonal Ti2ZrO were [0001]a-Ti//½0001Ti2ZrOand (1010)a-Ti// ð1010ÞTi2ZrO.
I. Introduction
T
ITANIUMalloys have excellent properties such as high specificstrength and corrosion resistance. However, they are very active and tend to dissolve some interstitial elements (e.g., C, N, O, H) of ceramic molds during casting. Saha and Jacob1showed that there was an oxidation–reduction reaction between titani-um alloys and various oxide molds, and thus, the so-called a-case was formed at the surface of titanium casting, resulting in a hardening and brittle structure.2–6That is to say, the oxygen was
readily dissolved into the titanium to form a-Ti at the surface of titanium alloys and caused the deterioration of mechanical properties.
The reactions between titanium and zirconia have been sub-jected to an intensive investigation, indicating that zirconia became oxygen-deficient because of the interfacial reaction. Economos and Kingery7 revealed that titanium penetrated along the grain boundaries of ZrO2with the formation of the
oxygen-deficient zirconia. Ruh8reported that up to 4 at.% of titanium was retained in zirconia at room temperature, while up to approximately 10 mol% zirconia would be dissolved in tita-nium. Zirconium entered into the titanium lattice substitution-ally and oxygen entered interstitial positions, but no evidence of other compounds except oxygen-deficient zirconia at the inter-face was provided.
Some researchers have studied the existence of interfacial phases and the effects of titanium on the stabilization and me-chanical properties of zirconia. Weber et al.9 indicated that
zirconia, with Ti exceeding the solubility limit, showed good strength and thermal shock resistance. Lin et al.10stated that
zirconia was stabilized because of the dissolution of TiO, formed by the reaction between titanium and residual trace oxygen in the vacuum furnace. The improvement in the strength and the thermal shock resistance of zirconia was attributed to the dis-solution of TiO as well. According to the pseudobinary diagram of Ti–ZrO2proposed by Domagala et al.,11 (Ti, Zr)3O could
precipitate from the supersaturated solid solution of a-Ti(Zr, O) during cooling. However, Weber et al.9and Lin et al.10found neither (Ti, Zr)3O nor a-Ti (Zr, O).
Recently, Lin and Lin12reported that the interfacial reactions between zirconia and titanium melt resulted in titanium oxides such as Ti3O or TiO2after reaction at 17501C/7 min.
Mean-while, the lamellae of Ti2ZrO and a-Ti(Zr, O) were also found.
In the present study, the titanium and zirconia diffusion couple was annealed at 15501C/30min. The interfacial microstructures were characterized by transmission electron microscopy/energy-dispersive spectroscopy (TEM/EDS).13 The orientation
rela-tions between a-Ti and Ti2ZrO were determined using
stereo-graphic projection analyses.
II. Experimental Procedures
Bulk zirconia specimens used in this study were prepared from the powder of 3 mol% Y2O3partially stabilized zirconia (Toyo
Soda Mfg. Co., Tokyo, Japan) by hot-pressing in a graphite furnace in vacuum (Model HP50-MTG-7010, Thermal Techno, Inc., Santa 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. The as-hot pressed samples were an-nealed at 12001C/4 h so that the stoichiometric ZrO2 samples
were obtained. On the other hand, billets of commercially pure titanium (Cp-Ti) were used in this study. Nominal compositions of the starting materials, i.e., zirconia powder and titanium bil-lets, were listed in Table I.
Both zirconia and titanium specimens were cut and machined to dimensions of 14 mm 14 mm 5 mm. Their surfaces were ground and polished to 0.5 mm with a diamond paste, and then ultrasonically cleaned in acetone. One titanium specimen was in-serted in between two ZrO2specimens to produce a sandwiched
sample, and then put in the graphite furnace mentioned above, preparatorily pressed under 5 MPa, evacuated to 2 104Torr, and filled with argon to one atmospheric pressure. This cycle of evacuation and purging was repeated at least three times. The temperature was raised to 10001C at a raising rate of 301C/min, to 15501C at 251C/min, and then held at 15501C for 30 min. There-after, the temperature was lowered to 10001C at a cooling rate of 251C/min, and then furnace cooled down to room temperature.
The microstructures at the interface between zirconia and titanium were characterized using an analytical transmission electron microscope (Model JEM 2000FX, JEOL Ltd., Tokyo,
Journal
1268
D. J. Green—contributing editor
Supported by the National Science Council of Taiwan 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
Japan). Cross-sectional TEM specimens perpendicular to the interface of zirconia and titanium were cut into about 3 mm 2 mm 0.5 mm. They were ground, polished, and dimpled to 50 mm in thickness. The TEM specimens were then ion milled by a precision ion miller (Model 691, Gatan Inc., Pleasanton, CA). The quantitative composition analyses were carried out based on the principle of the Cliff–Lorimer standardless technique13
by an EDS (Mode ISIS300, Oxford Instrument Inc., London, UK) attached to the TEM.
III. Results and Discussion
The lamellae of Ti2ZrO and a-Ti as well as the orthorhombic b0
-Ti were found to exist in the titanium side after cooling down to
room temperature, as shown in Fig. 1(a). Figure 1(b) shows a magnified micrograph of the inset area in Fig. 1(a), showing the elongated b0-Ti in the middle of the bright field image and the Ti2ZrO lamellae precipitated in the a-Ti matrix. The spherical
ordered Ti2ZrO phase, showing high strain-field contrast
be-cause of the lattice distortion, precipitated in a-Ti matrix in the right lower corner (arrowed). It is believed that the ordering of Zr and O in a-Ti caused the lattice strain owing to the difference in atomic sizes.
Figures 2(a) and (b) display the selected area diffraction patterns (SADPs) of b0-Ti, with zone axes of [110] and [111],
re-spectively, being identified as orthorhombic with lattice param-eters a 5 0.585 nm, b 5 0.847 nm, and c 5 0.606 nm. The EDS spectrum in Fig. 2(c) indicates that b0-Ti(Zr, O) contained
Fig. 1. (a) The transmission electron microscopy micrograph showing the interface between Cp-Ti and ZrO2(3Y) after reaction at 15501C/30 min; (b) A
magnified micrograph of the marked region in (a), indicating the elongated b0-Ti and the lamellar structure of a-Ti and Ti
2ZrO on both sides of b0-Ti.
The arrows in the lower right region indicate the spherical Ti2ZrO.
Table I. Compositions of the Starting Materials
Material Composition (wt%)
Ti 99.31 Ti, 0.3 Fe, 0.25 O, 0.1 C, 0.03 N, 0.01 H
3Y-ZrO2 494 ZrO21HfO2, w
5.4 Y2O3,o0.001 Fe2O3,o0.01 SiO2,o0.005 Na2O,o0.005 TiO2,o0.02 Cl, o0.005 SO42
w
Accounts for approximately 2B3% of this total.
57.9 at.% Ti, 30.7 at.% Zr, and 11.4 at.% O. From the Ti–ZrO2
phase diagram,11it is known that hexagonal a-Ti and body-cen-tered cubic b-Ti coexist in Cp-Ti at 15501C, and the b-Ti will transform into a-Ti during cooling. Oxygen raises the transfor-mation temperature of a-b,14causing the a-phase to be stabi-lized. Despite the dissolution of a large amount of zirconium, which is a b-stabilizer, b-Ti did not survive but rather trans-formed to an orthorhombic b0-Ti(Zr, O) solid solution during
cooling. The transformation of b-Ti to b0-Ti has been
investi-gated in Ti–Al–Nb systems.15However, the formation of b0-Ti in
the titanium and zirconia system was first found in this study. At high temperatures, a-Ti dissolved a large amount of zir-conium and oxygen, forming a metastably supersaturated disordered solid solution a-Ti(Zr, O), thus resulting in the precipitation of the lamellae Ti2ZrO during cooling. The
inter-lacing lamellar phases were identified as orthorhombic Ti2ZrO
and hexagonal disordered a-Ti from the superimposed SADPs of these two phases shown in Fig. 3(a). The streaking of the diffraction spots was caused by the lamellar-shaped effect. The orientation relations were identified as [0001]a-Ti//½110Ti2ZrOand (1010)a-Ti//ð110ÞTi2ZrO. Moreover, the lattice constants of Ti2ZrO orthorhombic unit cell were calculated as follows:
ao50.494 nm, bo50.817 nm, and co50.309 nm, and those
of a-Ti hexagonal unit cell are ah5 bh50.301 nm, ch50.468 nm.
Figure 3(b) shows the EDS spectrum of the a-Ti, revealing that it comprised 80.7 at.% Ti, 8.5 at.% Zr, and 10.8 at.% O. Figure 3(c) shows the EDS spectrum of the lamellar Ti2ZrO, consisting
of 56.1 at.% Ti, 22.9 at.% Zr, and 21.0 at.% O. Figure 3(d) displays the standard stereographic projection corresponding to Fig. 3(a) with [0001]a-Ti//½110Ti2ZrO. It indicates that the (1010) plane of a-Ti is parallel to the (110) plane of Ti2ZrO, in
agree-ment with the result presented by Lin and Lin.12
The crystallographic relation between a-Ti (solid line) and the lamellar Ti2ZrO (dash line) inferred from Fig. 3(a) is
schemat-ically shown in Fig. 4. The habit planes of Ti2ZrO and a-Ti are
(1010)a-Tiandð110ÞTi2ZrO, respectively. The stacking sequence of a-Ti is ABABAB. It reveals that the crystal structure of the or-thorhombic Ti2ZrO is based on that of hexagonal a-Ti with a
stacking sequence of ABACABAC.
Figure 5(a) shows the superimposed SADPs of the spherical ordered Ti2ZrO phase and the a-Ti matrix with [0001]a-Ti//
½0001Ti2ZrO and (1010)a-Ti//ð3030ÞTi2ZrO. From superimposed
diffraction patterns, the spherical ordered phase was identified as hexagonal Ti2ZrO, in agreement with the result presented by
Fykin et al.16The crystal structure of the hexagonal Ti2ZrO was
proposed as shown in Fig. 5(b). It indicates that the hexagonal Ti2ZrO unit cell with space group D6h1-P6/mmm had one Zr
atom at 0, 0, 0, and two Ti atoms at 1/3, 2/3, 1/2, and 2/3, 1/3, 1/2. Additionally, one O atom was statistically positioned at 1/2, 0, 0; 0, 1/2, 0; 1/2, 1/2, 0.
In order to understand the reflection planes of Ti2ZrO
struc-ture, its structure factor of Ti2ZrO must be calculated. From the
atomic positions of Ti2ZrO by Fykin et al.,16the structure factor
was calculated as follows:
F ¼fZrþ fTi½e2piðh=3þ2k=3þl=2Þþ e2pið2h=3þk=3þl=2Þ
þ fO½e2piðh=2Þþ e2piðk=2Þþ e2piðh=2þk=2Þ
Multiplication by the complex conjugate, however, would give the square of the absolute value of the resultant wave amplitude F. F j j2¼ ½ fZrþ fTicos 2p hþ 2k 3 þ l=2 þ fTicos 2p 2hþ k 3 þ l=2 þ fOcos 2pðh=2Þ þ fOcos 2pðk=2Þ þ fOcos 2pðh þ k=2Þ2 þ ½ fTisin 2p hþ 2k 3 þ l=2 þ fTisin 2p 2hþ k 3 þ l=2 þ fOsin 2pðh=2Þ þ fOsin 2pðk=2Þ þ fOsin 2pðh þ k=2Þ2
When all possible values of h, k, l were considered, the results were summarized as in Table II. The relative intensities of the diffraction spots in Fig. 5(a) were consistent with the results of calculated structure factors for Ti2ZrO. The structure factors of
the reflectionsð4220Þ and ð2110Þ were Fj j2¼ ½ fZrþ 2fTiþ 3fO2
and Fj j2¼ ½ fZrþ 2fTi fO2, respectively. The reflection spot of
theð4220Þ plane was thus brighter than that of the ð2110Þ plane as shown in Fig. 5(a). Furthermore, the reflection spot of the ð3030Þ plane with Fj j2¼ ½ fZrþ 2fTi fO2 was visible. The
ð2020Þ and ð1010Þ reflections were absent, because its structure factor j jF2¼ ½ fZr fTi fO2 was comparatively small and
negligible.
The lattice constants of hexagonal Ti2ZrO unit cell were
cal-culated as follows: a 5 b 5 0.902 nm and c 5 0.455 nm, while those of the a-Ti hexagonal unit cell are a 5 b 5 0.301 nm and c 50.468 nm. Figure 5(c) displays the EDS spectrum of spher-ical ordered Ti2ZrO consisting of 54.2 at.% Ti, 22.7 at.% Zr,
and 23.1 at.% O. Figure 5(d) shows the standard stereographic projection of the spherical ordered Ti2ZrO and a-Ti
correspond-ing with Fig. 5(a), showcorrespond-ing that theð1010Þ plane of a-Ti was parallel to theð3030Þ plane of the spherical ordered Ti2ZrO.
Zirconia was highly soluble in titanium,11allowing the for-mation of a disordered solid solution of a-Ti(Zr, O) at high temperatures. The lamellar and spherical Ti2ZrO phases with
different crystal structures were precipitated from the supersat-urated a-Ti(Zr, O) during cooling. In contrast, Lin and Lin12 reported only one of the Ti2ZrO phases in their previous study.
Fig. 2. (a) and (b) selected area diffraction patterns of b0-Ti, Z 5 [110]
b0-Ti
According to the Ti–ZrO2 phase diagram,11 the solubility of
ZrO2in titanium could exceed 20 at.%. In the present study,
a-Ti dissolved a large amount of Zr and O at 15501C, forming a supersaturated solid solution a-Ti(Zr, O), whereby Ti2ZrO
pre-cipitated. However, a wide two-phase region (23–75 wt% ZrO2)
of a-Ti(Zr, O) and (Ti, Zr)3O in the Ti–ZrO2phase diagram is
inconsistent with the coexistence of a-Ti(Zr, O), Ti2ZrO, and b0
-Ti in this study.
Lin and Lin reported12that the reaction at 17501C/7 min be-tween zirconia and titanium melt caused the formation of the lamellae Ti2ZrO and a-Ti(Zr, O). However, whether Ti2ZrO was
formed during solidification or by precipitation from a-Ti(Zr, O) during cooling could not be confirmed. Because no liquid a-Ti(Zr, O) existed in this study, it is concluded that the lamellae Ti2ZrO phase was not formed in the Ti melt but rather
precip-itated from a-Ti(Zr, O). Furthermore, Lin and Lin’s report12did not find the spherical ordered Ti2ZrO phase and the b0-Ti in the
interface of zirconia and titanium reaction.
IV. Conclusions
1. In the zirconia/titanium diffusion couple annealed at 15501C/30 min, titanium readily dissolved a large amount of zirconium and oxygen, resulting in a metastably supersaturated solid solution.
2. The lamellar and the spherical Ti2ZrO phases were
pre-cipitated from the supersaturated a-Ti solid solution, while the b-Ti coexisting with a-Ti at high temperatures was transformed to the orthorhombic b0-Ti during cooling.
3. The lamellar Ti2ZrO had an orthorhombic crystal
struc-ture with the (110) planes being stacked in the ABACABAC sequence. In contrast, the spherical Ti2ZrO had a hexagonal
crystal structure, with Zr and O orderly occupying substitutional and interstitial sites, respectively.
4. The orientation relations between a-Ti and the lamellae orthorhombic Ti2ZrO were determined to be [0001]a-Ti//
½110Ti
2ZrO and ð1010Þa-Ti//ð110ÞTi2ZrO; meanwhile, those
be-Fig. 3. (a) Selected area diffraction patterns of the lamellar Ti2ZrO and a-Ti, Z 5 [0001]a-Ti//½110Ti2ZrO; (b) an energy-dispersive spectrum of
a-Ti; (c) an energy-dispersive spectrum of the lamellar Ti2ZrO; (d) the standard stereographic projection with [0001]a-Ti//½110Ti2ZrO.
Fig. 4. Lattice relation of the orthorhombic Ti2ZrO (dash line) and the
hexagonal a-Ti (soild line).
tween the a-Ti and the spherical hexagonal Ti2ZrO were [0001] a-Ti//½0001Ti2ZrOandð1010Þa-Ti//ð1010ÞTi2ZrO.
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16
L. E. Fykin, V. V. Glazova, I. I. Kornilov, R. P. Ozerov, V. P. Smirnov, and S. P. Solov’ev, ‘‘Crystal Structure of the Suboxide Ti2ZrO,’’ Crystallography, 13 [9]
845–8 (1969). &
Fig. 5. (a) Selected area diffraction patterns of the spherical ordered Ti2ZrO and the a-Ti, Z 5 [0001]a-Ti//½0001Ti2ZrO; (b) the hexagonal Ti2ZrO unit
cell;16(c) an energy-dispersive spectrum of the spherical ordered Ti
2ZrO; (d) the standard stereographic projection with [0001]a-Ti//½0001Ti2ZrO.
Table II. Calculated Results of the Hexagonal Ti2ZrO
Structure Factor
h12k (or 2h1k) hand k l jFj2
3m, m 5 even Even Even (fZr12fTi13fO)2
3m, m 5 even Even Odd (fZr2fTi13fO)2
3m, m 5 even Mixed Even (fZr12fTifO)2
3m, m 5 even Mixed Odd (fZr2fTifO)2
3m, m 5 odd Odd or mixed Even (fZr12fTifO)2
3m, m 5 odd Odd or mixed Odd (fZr2fTifO)2
3m71, m 5 even Odd or mixed Even (fZrfTifO)2
3m71, m 5 even Odd or mixed Odd (fZr1fTifO)2
3m71, m 5 odd Mixed Even (fZrfTifO)2
3m71, m 5 odd Mixed Odd (fZr1fTifO)2
3m71, m 5 odd Even Even (fZrfTi13fO)2
3m71, m 5 odd Even Odd (fZr1fTi13fO)2
![Fig. 5. (a) Selected area diffraction patterns of the spherical ordered Ti 2 ZrO and the a-Ti, Z 5 [0001] a-Ti //½0001 Ti 2 ZrO ; (b) the hexagonal Ti 2 ZrO unit](https://thumb-ap.123doks.com/thumbv2/9libinfo/7963215.158227/5.891.137.718.60.583/fig-selected-diffraction-patterns-spherical-ordered-hexagonal-zro.webp)
