Microstructural Evolution and Formation Mechanism of the Interface
Between Titanium and Zirconia Annealed at 15501C
Kun-Lin Lin and Chien-Cheng Lin*
,wDepartment of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30050, Taiwan The diffusional reaction between titanium and zirconia was
car-ried out isothermally at 15501C in argon. The distinct reaction layers in the reaction-affected zone between Ti and ZrO2were investigated using analytical scanning electron microscopy, an-alytical transmission electron microscopy, and electron probe microanalyses. In the metal side, there existed five reaction lay-ers in a sequence ofa-Ti(O), Ti2ZrO1a-Ti(O, Zr), Ti2 ZrO1a-Ti(O, Zr)1b0-Ti (O, Zr),a-Ti (O, Zr)1b0-Ti (O, Zr), andb0-Ti (Zr, O) after cooling. In the zirconia side, two reaction layers were found: near the original interface,b0-Ti coexisted with fine spherical c-ZrO2x and Chinese-script-like c-ZrO2x, which dissolved a significant amount of Y2O3in solid solution; further away from the original interface, the coarsened intergranular a-Zr was excluded from metastable a-ZrO2x, resulting in the len-ticular t-ZrO2xand ordered c-ZrO2x. An attempt was made to determine and propose the microstructural evolution and for-mation mechanism of the reaction layers between titanium and zirconia isothermally annealed at 15501C.
I. Introduction
E
XTENSIVEstudies have been carried out on the interface re-action between titanium and zirconia in the last few dec-ades.1–3Weber et al.1indicated that zirconia was blackened by a limited solid solution of titanium in zirconia, when the titanium melt reacted with the ZrO2crucible. In discussing the effect ofthe Ti additive on the microstructure of zirconia at 20001C/3 h, Ruh et al.2found that the liquid titanium diffused into zirconia to form various reaction layers. Zhu et al.3 investigated the wettability and the interaction between pure liquid titanium and yttria-stabilized zirconia by the sessile drop method in an Ar atmosphere at 17001C. They found that there existed two dis-tinct chemical reaction layers in the interface. Recently, Lin and his coworkers4–6conducted an intense investigation on the
dif-fusional reaction between titanium and zirconia. Using trans-mission electron microscopy/energy dispersive spectroscopy (TEM/EDS) analyses, Lin and Lin4indicated that an ordered titanium suboxide (Ti3O) and the orthorhombic lamellae
Ti2ZrO were formed in the solid solution of a-Ti(O) during
cooling from 17001C. In addition to the lamellar Ti2ZrO and
a-Ti(O), Lin and Lin5observed the orthorhombic b0-Ti(Zr, O) and a spherical-ordered Ti2ZrO phase in the metal side after
anneal-ing at 15501C. By focusanneal-ing upon the zirconia side far away from the original interface of Ti and ZrO2, Lin and Lin6also observed
twinned t0-ZrO2x, lenticular t-ZrO2x, ordered c-ZrO2x, and
the intergranular a-Zr after the Ti/ZrO2diffusion couple was
annealed at 15501C using analytical TEM.
In order to shed light on the microstructural evolution of the various distinct reaction layers between titanium and zirconia, the Ti/ZrO2diffusion couple was isothermally annealed in argon
at 15501C for various periods in the present study, and the microstructures were characterized using analytical scanning electron microscopy (SEM), analytical transmission electron mi-croscopy (TEM), and an electron probe microanalyzer (EPMA). Finally, the formation mechanisms, as well as microstructural evolution, of the reaction layers between titanium and zirconia were proposed.
II. Experimental Procedures
Bulk ZrO2specimens used in this study were prepared from the
powder of 3 mol% Y2O3partially stabilized zirconia (with a
nominal composition of 494 wt% ZrO21HfO2(HfO2accounts
for approximately 2%–3% of this total), 5.4 wt% Y2O3,
o 0.001 wt% Fe2O3,o 0.01 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., Santa Rosa, CA). The bulk ZrO2and commercially available titanium billets (with a
nom-inal 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; Kobe Steel, Ltd., To-kyo, Japan) were cut and machined into dimensions of 14 mm 14 mm 5 mm. The sandwiched samples, one Ti in be-tween two ZrO2specimens, were slightly pressed and annealed
at 15501C for 0.5, 3, and 6 h in an atmosphere of Ar. The hot-pressing procedures of bulk ZrO2and the sandwiched samples
have been described in detail elsewhere.5
The cross-sectional SEM, TEM, and EPMA specimens per-pendicular to the interface of titanium and zirconia were cut, ground, and polished by standard procedures. The microstruc-tures at the interface were observed using an analytical SEM (Model JSM 6500F, JEOL Ltd., Tokyo, Japan) and an analyt-ical TEM (Model JEM 2000Fx, JEOL Ltd.). The quantitative composition analyses were carried out based on the principle of the Cliff–Lorimer standardless technique7 by an EDS (Mode ISIS300, Oxford Instrument Inc., London, U.K.) attached to the TEM. The backscattered electron images (BEI) had been taken on the SEM. The compositions of those phases in the re-action layers at the interface between Ti and ZrO2were
quan-titatively measured by an EPMA (JXA-8800M, JEOL, Tokyo, Japan) with the aid of an atomic number, absorption, and flu-orescence corrections (ZAF) program.8The measurement con-ditions for EPMA were as follows: the accelerating voltage was 15 kV, the probe current was 1.5 108A, and the beam di-ameter was 1 mm.
III. Results and Discussion
(1) Formation Mechanism of Various Reaction Layers Figures 1(a), (b), and (c) display the backscattered electron im-ages of the cross section normal to the interface between Ti and ZrO2after reaction at 15501C for 0.5, 3, and 6 h, respectively.
Titanium was on the left-hand side, while zirconia was on the
Journal
J. Am. Ceram. Soc., 89 [4] 1400–1408 (2006) DOI: 10.1111/j.1551-2916.2005.00877.x r2006 The American Ceramic Society
1400 T. Mitchell—contributing editor
Research supported by National Science Council of Taiwan under Contract No. NSC 92-2216-E-009-027.
*Member, American Ceramic Society. w
Author to whom correspondence should be addressed. e-mail: chienlin@faculty. nctu.edu.tw
other side. The arrows in the upper middle of these figures in-dicated the original interface of Ti and ZrO2. There were five
reaction layers, designated as ‘‘A,’’ ‘‘B,’’ ‘‘C,’’ ‘‘D,’’ and ‘‘E,’’ on the metal side, while only two distinct reaction layers, designated as ‘‘F’’ and ‘‘G,’’ were found on the zirconia side. The thickness of the individual reaction layers increased with the annealing time. Moreover, in the reaction layer ‘‘G,’’ zirconia grains grew obviously and a-Zr (brighter) was coarsened and became iso-lated after reaction at 15501C for 6 h (Fig. 1(c)). The existence of the pores in the ceramic side was attributed to the Kirkendall effect, as zirconium and oxygen diffused to the titanium side much faster than titanium diffused toward the zirconia side.
Figure 2(a) displays the backscattered electron image of the cross section normal to the interface between Ti and ZrO2after
reaction at 15501C/6 h. The distributions of Y, Ti, Zr, and O elements in the interface are demonstrated by X-ray mappings in Figs. 2(b)–(e), respectively. The original interface (the arrows) was deliberately located according to the results of the charac-teristic Ka X-ray map of yttrium (Fig. 2(b)), which was relatively immobile compared with the elements Zr, O, and Ti. Because Ti and Zr are isomorphous elements and the differences in their atomic radii are very small,9they are readily substituted for each other. Figures 2(c) and (d) confirm that the interdiffusion of Ti and Zr was intense and the diffusion distance of Ti and Zr was nearly equal. Figure 2(e) shows that oxygen underwent a long-range diffusion into titanium, leading to the formation of the oxygen-containing a-Ti.
Figure 3 displays the backscattered electron image of the reaction layers ‘‘A,’’ ‘‘B,’’ and ‘‘C’’ between Ti and ZrO2after
reaction at 15501C/6 h. The reaction layer ‘‘A’’ was a-Ti, which dissolved a large amount of oxygen and a small amount of zir-conium. The quantitative analyses by the EPMA showed that it contained 71.30 at.% Ti, 0.97 at.% Zr, and 27.72 at.% O, Original corresponding to a-Ti(O). a-Ti(O) was formed by the oxidation–reduction reaction of titanium and zirconia, and can be expressed as follows:
ZrO2þ Ti ! heating
ZrO2xþ b-TiðOÞ (1)
b-TiðOÞ !coolinga-TiðOÞ (2)
The reaction layer ‘‘B’’ with a continuous lamellar morphology consisted of a-Ti (gray) and needle-like Ti2ZrO (bright). The
thin continuous layer, consisting of lamellar a-Ti and Ti2ZrO,
was first found in the present study. Unlike these continuous lamellar phases found in the present study, Lin and Lin5 report-ed that the Ti2ZrO lamellae were precipitated from plate-like
a-Ti (O, Zr) by a eutectoid reaction during cooling. The forma-tion of lamellar a-Ti (O) and Ti2ZrO in the thin continuous layer
(the reaction layer ‘‘B’’) can be expressed as follows: ZrO2þ Ti !
heating
ZrO2xþ b-TiðZr; OÞ (3)
b-TiðZr; OÞ !coolinga-TiðZr; OÞ !coolinga-TiðZr; OÞ þ Ti2ZrO (4)
Fig. 1. (a), (b), and (c) Scanning electron micrographs (backscattered electron image, BEI) of the cross section between Ti/ZrO2after reaction at
15501C/0.5, 3, and 6 h, respectively. The arrows indicate the original interface between Ti/ZrO2.
The reaction layer ‘‘C’’ consisted of b0-Ti (bright) and the la-mellae of Ti2ZrO and a-Ti (gray). At high temperatures, the
primary a-Ti dissolved a large amount of zirconium and oxygen, forming metastable a-Ti(Zr, O), and thus resulted in the pre-cipitation of the lamellae Ti2ZrO during cooling. Meanwhile,
the b-Ti, which dissolved a large amount of Zr and O, trans-formed into orthorhombic b0-Ti(Zr, O) solid solution during cooling. The coexistence of a-Ti (O), Ti2ZrO, and b0-Ti (O, Zr)
can be expressed by the following reactions: ZrO2þ Ti !
heating
ZrO2xþ a-TiðZr; OÞ þ b-TiðZr; OÞ (5) a-TiðZr; OÞ þ b- TiðZr; OÞ
! cooling
a-TiðZr; OÞ þ Ti2ZrOþ b0-TiðZr; OÞ
(6)
Figure 4 displays the backscattered electron image of the reaction layer ‘‘D’’ in the interface between Ti and ZrO2after
reaction at 15501C/6 h. The original interface of titanium and zirconia was beyond the right-hand side of the micrograph. When the concentration of zirconium is like that in the region on the left-hand side of Fig. 4, the reaction layer ‘‘D’’ would consist of the acicular a-Ti (dark) in the b-Ti matrix (gray). The pre-cipitation of the acicular a-Ti(O, Zr) can be expressed as follows:
ZrO2þ Ti ! heating
ZrO2xþ b-TiðZr; OÞ (3)
b-TiðZr; OÞ !coolinga-TiðZr; OÞ þ b0-TiðZr; OÞ (7) As zirconium was the stabilizer of b-Ti, fewer acicular a-Ti was observed on the right-hand side of Fig. 4, where a higher con-centration of Zr existed in titanium. Also shown in Fig. 4, the amount of the acicular a-Ti increased with the distance away from interface. The reaction layer ‘‘D,’’ which dissolved a sig-nificant amount of zirconium (b stabilizer) and oxygen (a sta-bilizer), was composed of a1b titanium with Zr and O in solid
Fig. 2. (a) Scanning electron micrograph (backscattered electron image, BEI) of the cross section between Ti/ZrO2after reaction at 15501C/6 h; (b)–(e)
X-ray maps of Y, Ti, Zr, and O, respectively.
solution. For comparison, it is worth noting that the acicular-like a-Ti was also found in a1b titanium alloys such as Ti–6Al– 4V or Ti–8Al–1Mo–1 alloys.10
The orientation relationship of the acicular a-Ti and the b0-Ti has been subjected to an intense investigation. Figure 5(a) shows the TEM micrograph (bright-field image, BFI) of the reaction layer ‘‘D’’ viewed in the direction normal to the interface of Ti and ZrO2after reaction at 15501C/6 h. The acicular a-Ti and the
b0-Ti were identified as hexagonal and orthorhombic crystal structures, respectively, from the superimposed selected area diffraction patterns (SADPs), as shown in Fig. 5(b). The SADPs in Fig. 5(b) were schematically redrawn in Fig. 5(c), with the diffraction spots being indexed. The orientation relations be-tween a-Ti and b0-Ti were thus identified as follows:½2110
a-Ti|| ½001b0-Tiandð0001Þa-Ti|| ð100Þb0-Ti. Moreover, the lattice con-stants of the b0-Ti orthorhombic unit cell were calculated as fol-lows: ao50.58 nm, bo50.84 nm, and co50.61 nm, and those
of the a-Ti hexagonal unit cell were ah5 bh50.30 nm, and
ch50.46 nm. Figure 5(d) shows the EDS spectrum of the
acicular a-Ti, revealing that it comprised 54.0 at.% Ti, 13.4 at.% Zr, and 32.8 at.% O. Figure 5(e) shows the EDS spectrum of the b0-Ti, consisting of 58.1 at.% Ti, 30.7 at.% Zr, and 11.2 at.% O. As Zr is a stabilizer of b-titanium, it is not surprising that b0-Ti(Zr, O) contains much more Zr than a-Ti(Zr, O) does.
Figure 6(a) also shows the TEM micrograph (BFI) of the reaction layer ‘‘D’’ viewed in the direction normal to the inter-face of Ti and ZrO2after reaction at 15501C/6 h. Another
ori-entation relationship of the acicular a-Ti and the b0-Ti was identified by the superimposed SADPs (Fig. 6(b)). The SADPs were redrawn and indexed in Fig. 6(c). The orientation relations were recognized as follows:½2110a-Ti||½021b0-Tiandð0001Þa-Ti ||ð112Þb0-Ti. Figure 6(d) displays the image, taken from the high-resolution transmission electron microscopy (HRTEM), of the acicular a-Ti and the b0-Ti. The d-spacing of the planeð0110Þ for a-Ti was equal to 0.50 nm, while that of theð312Þ plane for b0-Ti was equal to 0.28 nm. The high-resolution image, as shown in Fig. 6(e), of the marked area in Fig. 6(d) was simulated by a computer program (Digital Micrograph 3.3, Gatan Inc., Pleasan-ton, CA). It indicated that the ledge mechanism (labeled as ‘‘L’’) was probably the favorable precipitation mechanism of a-Ti from the matrix of b0-Ti. The ledge mechanism was frequently encoun-tered in the partially coherent interface, consisting of many sets of edge dislocations or misfit dislocations. When the crystal struc-tures of the matrix and the precipitate were different to a
signif-Fig. 4. Scanning electron micrograph (backscattered electron image, BEI) of the reaction layer ‘‘D’’ in the interface between Ti/ZrO2after
reaction at 15501C/6 h.
Fig. 3. Scanning electron micrograph (backscattered electron image, BEI) of the reaction layers ‘‘A,’’ ‘‘B,’’ and ‘‘C’’ in the interface between Ti/ZrO2after reaction at 15501C/6 h.
Fig. 5. (a) Transmission electron micrograph (bright-field image, BFI) of the reaction layer ‘‘D’’ after reaction at 15501C/6 h; (b) and (c) se-lected area diffraction patterns of the a-Ti and b0-Ti, Z¼
½2 1 1 0a-Ti==½0 0 1b0
-Ti and its schematic diagram (D, a-Ti; J, b 0-Ti),
respectively; (d) and (e) energy-dispersive spectra of a-Ti and b’-Ti.
icant degree, the interface boundaries had to migrate by the ledge mechanism.11The ledge mechanism was reported, for instance, to be the growth mechanism of a and b interface boundaries in a Ti-based alloy.12It was likely that the acicular a-Ti was also precip-itated from a b0-Ti matrix by means of the ledge mechanism.
Figure 7 shows the backscattered electron image of the reac-tion layers ‘‘E’’ and ‘‘F,’’ together with ‘‘D’’ between Ti and ZrO2 after reaction at 15501C/6 h. The arrows in the upper
middle indicate the original interface of Ti and ZrO2. Abutting
the zirconia side, the precipitate-free reaction layer ‘‘E’’ (gray), which dissolved a large amount of zirconium and oxygen, was b0-Ti in the solid solution, consisting of 35.33 at.% Ti, 25.85 at.% Zr, and 38.82 at.% O measured by the EPMA. The for-mation of b0-Ti in the solid solution, designated as b0-Ti(Zr, O), can be expressed as follows:
ZrO2þ Ti ! heating
ZrO2xþ b-TiðZr; OÞ (3)
b -TiðZr; OÞ !coolingb0- TiðZr; OÞ (8)
In contrast, b0-Ti (gray) and c-ZrO
2x(bright, either rounded
or Chinese script like) coexisted in the reaction layer ‘‘F.’’ On performing EPMA analyses, it was found that this b0-Ti con-tained 46.72 at.% Ti, 27.00 at.% Zr and 26.28 at.% O; mean-while, the Chinese-script-like c-ZrO2xcontained 2.36 at.% Ti,
20.92 at.% Zr, 66.87 at.% O, and 9.84 at.% Y, displaying a large amount of yttrium, an effective stabilizer of c-ZrO2x. This
is the reason why the zirconia was cubic symmetric in crystal structure. A few small rounded c-ZrO2xgrains were scattered
near the interface, as c-ZrO2xgrains were extensively dissolved
into titanium. On the right-hand side of Fig. 7, a Chinese-script-like c-zirconia phase (resembling the stroke of a calligraphic brush) was surrounded by the b0-Ti. The reason why c-ZrO
2x
became Chinese script like was still unclear at present. It was believed that zirconium was excluded from the metastable ox-ygen-deficient zirconia during cooling. However, no a-Zr was found in the reaction layer ‘‘F’’ like in the reaction layer ‘‘G,’’ as zirconium went into the solid solution in b0-Ti (O, Zr). The for-mation mechanisms in the reaction layer ‘‘F’’ can be expressed as follows:
ZrO2þ Ti ! heating
ZrO2xþ b-TiðZr; OÞ ð3Þ
ZrO2x !
cooling
a-Zrþ c-ZrO2yðy < xÞ (9)
a-Zrþ c-ZrO2yðy < xÞþ b-TiðZr; OÞ !
cooling
c-ZrO2yðy < xÞþ b0-TiðZr; OÞ
(10)
Figure 8 displays the backscattered electron image and various characteristic X-ray maps of the reaction layer ‘‘F’’ after reac-tion at 15501C/6 h. The X-ray mapping of Y in Fig. 8(b) indi-cates that yttrium was hardly dissolved in titanium and then remained in both small spherical and Chinese-script-like zircon-ia. This is consistent with the results reported by Zhu et al.,3who
found that the yttrium element congregated and remained at the interface to form a high Y2O3content of ZrO2when ZrO2
re-acted with molten titanium. The distributions of Ti, Zr, and O elements in the reaction layer ‘‘F’’ (Figs. 8 (c)–(e)) indicated that a significant amount of titanium diffused into the ceramic side and reacted with zirconia to form b0-Ti(Zr, O).
Figure 9(a) displays the backscattered electron image of the reaction layer ‘‘G’’ in the zirconia side far away from the inter-face after reaction at 15501C/6 h. The distributions of Y, Ti, Zr, and O elements in the interface are demonstrated by individual characteristic X-ray mappings in Figs. 9(b)–(e), respectively. It
Fig. 7. Scanning electron micrograph (backscattered electron image, BEI) of the reaction layers ‘‘D,’’ ‘‘E,’’ and ‘‘F’’ in the interface between Ti/ZrO2after reaction at 15501C/6 h.
Fig. 6. (a) Transmission electron micrograph (bright-field image, BFI) of the reaction layer ‘‘D’’ after reaction at 15501C/6 h; (b) and (c) se-lected area diffraction patterns of the a-Ti and b0-Ti, Z¼ ½2 1 1 0a-Ti==½0 2 1b0-Tiand its schematic diagram (D, a-Ti; J, b
0-Ti),
respectively; (d) images taken from the high-resolution transmission electron microscopy of acicular a-Ti and b0-Ti; (e) the computer
simu-lation of the marked area in Fig. 6(d).
indicates that a significant amount of oxygen was dissolved in a-Zr (bright), with both yttrium and titanium being hardly dissolved in a-Zr. From the EMPA analyses, the a-Zr in the reaction layer ‘‘G’’ contained 1.05 at.% Ti, 57.5 4 at.% Zr, 40.73 at.% O, and 0.68 at.% Y, corresponding with a-Zr (O). Because the content of Zr in c- or t-ZrO2xdeclined as the temperature
decreased, as shown in the Zr–O phase diagram,13the a-Zr was segregated on grain boundaries during cooling by the exsolution of zirconium from ZrO2x, causing an increase in the O/Zr ratio
of oxygen-deficient zirconia.
The c-ZrO2xand the lenticular t-ZrO2xwere observed in
the gray region (Fig. 9(a)). It was believed that the specimen was cooled down from the two-phase (c1t) region in the ZrO2–Y2O3
phase diagram. The fact that the lenticular t-ZrO2 with three
variants was transformed in the c-ZrO2xhad been reported by
several previous studies.6,14–17Lin and Lin6further found that the lenticular t-ZrO2xwas formed in an ordered c-ZrO2x. It
was worth noting that zirconia grains with a concentrated
yttrium (Fig. 9(b)) had a cubic symmetry in crystal structure. The formation mechanisms in the reaction layer ‘‘G’’ can be expressed as follows: ZrO2þ Ti ! heating ZrO2xþ b-TiðOÞ (1) ZrO2x ! cooling
a-ZrðOÞ þ c-ZrO2yðy < xÞþ t- ZrO2yðy < xÞ (11) Based upon the foregoing results and discussion, the individ-ual phases and formation mechanisms of the distinct reaction layers in the reaction-affected zone between titanium and zir-conia have been summarized in Table I.
(2) Proposed Model of Microstructural Evolution
Even though extensive studies were carried out on the interface reaction between titanium and zirconia, the microstructure
Fig. 8. (a) Scanning electron micrograph (backscattered electron image, BEI) of the reaction layer ‘‘F’’ in the interface between Ti/ZrO2after reaction at
15501C/6 h; (b)–(e) X-ray maps of Y, Ti, Zr, and O, respectively.
evolution has not yet been elucidated to date. The information with regard to the relationships between the ternary constitution and the microstructures produced by isothermal diffusion among three elements is very sparse.18In the present study, an
attempt has been made to infer the microstructure resulting from isothermal diffusion among titanium and zirconia with the aid of the Ti–Zr–O ternary phase diagram. However, only the isothermal Ti–Zr–O ternary phase diagram at 14501C (not 15501C) has been found in the literature.19Assuming that the compositions of the solid phases are approximated as constant between 14501 and 15501C, the interpretation of the microstruc-ture at the interface between Ti and ZrO2in terms of the Ti–Zr–
O phase diagram can be reasonably determined. The Ti–Zr–O phase diagram and the diffusion couple are somewhat different chemically. The ZrO2contains 3 mol% Y2O3, so it is out of the
Ti–Zr–O ternary system. However, this probably does not have much effect on the results, and the effect may be negligible in this study.
Figure 10 shows the microstructural evolution at the interface between a titanium and zirconia diffusion couple after annealing at 15501C based upon the observation and analyses mentioned above. Figure 10(a) illustrates the titanium and zirconia diffu-sion couple prior to the anneal heat treatment, indicating the original interface and finely distributed ZrO2grains. When
tita-nium was held in contact with zirconia at high temperatures, various diffusion layers developed.
By cross examining the experimental results and the ternary phase diagram, it was proposed that all gross compositions, in various vertical slices along the longitudinal direction perpen-dicular to the interface, lay upon the line a–b–c–d–e, or the so-called diffusion path, in the Ti–Zr–O ternary system at 14501C.19 As shown in Fig. 10(b), the diffusion path crosses the fields of b-Ti, a-Ti1b-Ti, b-Ti, b-Ti1t-ZrO2, a-Zr1b-Ti1
t-ZrO2, and a-Zr1t-ZrO2.
It is well known that the divariant or three-phase regions in the ternary phase diagram are simply in correspondence to the
Fig. 9. (a) Scanning electron micrograph (backscattered electron image, BEI) of the reaction layer ‘‘G’’ in the zirconia side away from the interface after reaction at 15501C/6 h; (b)–(e) X-ray maps of Y, Ti, Zr, and O, respectively.
interfaces between layers in the diffusion couple, which is iso-thermally annealed at high temperatures. Conversely, ternary isothermal diffusion can result in the formation of both one-phase and two-one-phase layers in the diffusion couple, but not three-phase layers.18 As a result, in the Ti–ZrO2 diffusion
couple, the layers of b-Ti, a-Ti1b-Ti, b-Ti, b-Ti1t-ZrO2, and
a-Zr1t-ZrO2would have been observed in sequence from Ti to
ZrO2, but not the layers of a-Zr1b-Ti1t-ZrO2. The region of
a-Zr1b-Ti1t-ZrO2in the Ti–Zr–O ternary phase diagram was
represented in the diffusion couple by the interface between the layers of b-Ti1c-ZrO2and a-Zr1c-ZrO2as shown in Fig. 10(c).
Furthermore, as illustrated in Fig. 10(b), the b-Ti layer, corre-sponding to the layer ‘‘A,’’ varied in composition from Ti to ba.
The a in the a-Ti1b-Ti layer should vary from aato aband the b
from bato bb, while the b-Ti, corresponding to layers ‘‘D’’ and
‘‘E,’’ varied from bbto bc.
In the case of a Ti-ZrO2diffusion couple where the reaction
was predominant in the initial stage and the diffusion velocities of the three components were markedly different, the diffusion path deviated from the direct path between the compositions of the end points, i.e., Ti and ZrO2in the present study. It was
believed that the oxidation–reduction reaction in the initial stage shifted the end point in the ceramic side to the two-phase region of a-Zr1t-ZrO2, presumably to point e. Besides, it was unlikely
that the diffusion path passed the region of a-Ti1b-Ti1t-ZrO2
as it was impossible for such an interface to exist in between the layers of a-Ti1b-Ti and b-Ti1t-ZrO2. It was also noted that the
layer of a-Zr1b-Ti was not observed in the present study. As a consequence, the ternary diffusion path a–b–c–d–e provided the best approximation that could be made at present.
In reality, the high chemical affinity of titanium to oxygen led to the formation of oxygen-deficient zirconia (ZrO2x), indicat-ing that the oxidation–reduction reaction predominated in the initial stage before the diffusion prevailed. A significant increase in oxygen vacancies, as a consequence of the oxidation–reduc-tion reacoxidation–reduc-tion between titanium and zirconia, triggered the stab-ilization effect of zirconia. The zirconia in the Ti–Zr–O ternary phase diagram, shown in Fig. 10(b), was in the cubic phase in-stead of tetragonal phase when the Ti/ZrO2diffusion couple was
isothermally annealed at 15501C.6
The various layers formed during the cooling stage are dis-played in Fig. 10(d). The b-Ti layer abutting the reaction-unaf-fected Ti, with a small amount of Zr in solid solution, was transformed to a-Ti (designated as layer ‘‘A’’) on cooling, and the continuous lamellar Ti2ZrO1a-Ti (designated as layer ‘‘B’’)
is attributed to the precipitation of Ti2ZrO in the metastably
transformed a-Ti matrix. Ti2ZrO was precipitated from a-Ti in
the a-Ti1b-Ti layer during cooling, leading to the existence of
the lamellar (Ti2ZrO1a-Ti)1b0-Ti region (designated as layer
‘‘C’’). In the layer ‘‘D,’’ the b-Ti was transformed to b0-Ti where the acicular a-Ti was precipitated; meanwhile, a continuous b0 -Ti(Zr, O) with a large amount of Zr in solid solution was found near the interface (designated as layer ‘‘E’’). On the zirconia side, the dissolution of zirconia in titanium led to the formation of b-Ti(Zr, O) and the dissolution of titanium in zirconia sup-pressed the grain growth of zirconia near the interface. During cooling, b0-Ti(Zr, O) and cubic ZrO2x, with high yttrium, were
formed in the reaction layer ‘‘F.’’ Because of very low solubility in titanium, almost all of the yttrium remained in zirconia, re-sulting in the stabilization of c-ZrO2xduring cooling down to room temperature. Away from the original metal–ceramic in-terface, ZrO2was dramatically reduced to ZrO2xby titanium,
accompanied by a significant grain growth of ZrO2xon
heat-ing. The intergranular a-Zr was excluded from the metastable oxygen-deficient zirconia, while the lenticular t-ZrO2 and
or-dered c-ZrO2xwere formed from the c-ZrO2xmatrix during
cooling (the reaction layer ‘‘G’’). IV. Conclusions
1. The diffusional reaction between titanium and zirconia was carried out at 15501C in argon. On the metal side, there existed five reaction layers in a sequence of a-Ti(O), Ti2
ZrO1a-Ti(O, Zr), Ti2ZrO1a-Ti(O, Zr)1b0-Ti (O, Zr), a-Ti (O, Zr)1b0
-Ti (O, Zr), and b0-Ti (Zr, O) after cooling. Table I. Microstructural Features and Formation
Mechanisms of Various Phases at the Interface of Ti and ZrO2at 15501C
Ti/ZrO2
diffusion
couple Reaction layer Phases Formation mechanisms
Metal side ‘‘A’’ a-Ti(O) Eqs. (1) and (2)
‘‘B’’ a-Ti(Zr, O) Eqs. (3) and (4)
Ti2ZrO
‘‘C’’ a-Ti(Zr, O) Eqs. (5) and (6)
Ti2ZrO b0-Ti(Zr, O)
‘‘D’’ a-Ti(Zr, O) Eqs. (3) and (7)
b0-Ti
‘‘E’’ b0-Ti(Zr, O) Eqs. (3) and (8) Zirconia side ‘‘F’’ b0-Ti(Zr, O) Eqs. (3), (9), and (10)
c-ZrO2x
‘‘G’’ a-Zr(O) Eqs. (1) and (11)
t-ZrO2x c-ZrO2x
Fig. 10. Schematic diagrams showing the microstructural evolution of the Ti/ZrO2diffusion couple annealed at 15501C. (a) As hot pressed; (b)
the diffusion path drawn in the Ti–Zr–O ternary phase diagram at 14501C19; (c) the structure of the Ti/ZrO
2diffusion couple on annealing
at 15501C, and (d) the structure on cooling.
2. On the zirconia side, two reaction layers were found: near the original interface, b0-Ti coexisted with fine spherical c-ZrO2xand Chinese-script-like c-ZrO2x; further away from the original interface, coarsened intergranular a-Zr, lenticular t-ZrO2x, and ordered c-ZrO2xwere found.
3. The acicular a-Ti and the b0-Ti showed two different ori-entation relations. One of the oriori-entation relations was deter-mined to be ½2110a-Ti || ½001b0-Tiandð0001ÞaTi || ð100Þb0-Ti and the other was ½2110a-Ti ||½021b0-Tiandð0001Þa-Ti|| ð112Þb0-Ti.
4. Based upon the experimental results in this study, the diffusion path, connecting phases formed by the reaction be-tween Ti and ZrO2, was drawn on the Ti–Zr–O ternary phase
diagram.
5. The information with regard to the relationships between the Ti–Zr–O ternary phase diagram and the microstructures produced by isothermal diffusion between Ti and ZrO2 at
15501C has been provided.
Acknowledgments
The authors would like to thank Mr. Chi-Ming Wen at the Chung-Shan Institute of Science and Technology for preparing the hot-pressed specimens.
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