Investigation of the interactions between titanium and calcium zirconium oxide ( CaZrO 3 ) ceramics modified with alumina

Powdered mixtures of CaO, ZrO2 and Al2O3 in various ratios were hot pressed. The mixtures reacted with titanium at 1600 °C for 30 min in argon to evaluate the suitable ceramic crucibles for casting of titanium. The interfacial microstructures between titanium and ceramic composites were characterized using X-ray diffractometer, scanning and transmission electron microscope. The produced hot pressed mixtures that were chemically bonded together, contained calcium aluminate (CaAl2O4), calcium dialuminate (CaAl4O7), cubic zirconia (c-ZrO2), and calcium zirconium oxide (CaZrO3). The increase in Al2O3 amount led to the decrease in CaZrO3 amount and an increase in the amount of CaAl2O4, CaAl4O7 and c-ZrO2 due to the reaction of CaO and Al2O3. At the end of the reaction of the ceramic mixtures with Ti at 1600 °C for 30 min, the acicular α-Ti and β-Ti were formed at the interface of Ti and the composites containing up to 10 vol.% Al2O3. In composites containing more than 20 vol.% Al2O3, Ti3Al5 was found at the interface instead of α-Ti and β -Ti. Furthermore, CaZrO3, ZrO2 and Ca3Al2O6 existed on the sides of the ceramic far away from the interface. CaZrO3/Al2O3 composites with less than 20 vol.% Al2O3 could be a potential crucible or mould material for productive applications in titanium casting.


I. Introduction
Titanium and its associated alloys possess high tensile strength, excellent toughness and light weight [1].There are some advantages of titanium over other metals: titanium has very high corrosion resistance and it is widely used in aerospace, jet engines, medical devices, computer industry and in the manufacturing of consumable parts, such as bicycle parts and the head of a golf club [2,3].Titanium tends to react with ceramic crucible in the vacuum through induction melting [4].Hence, titanium alloys are subjected to melting in a water-cooled copper crucible by the consumable electrode vacuum arc melting (VAR) technique [5].However, there are some disadvantages of the VAR process, such as the high cost of VAR equipment, challenges associated with the recycling of scrap, and long cycle time.Furthermore, an abundant oxygen layer on the surface of titanium, called "α-case" occurs as a result of the reaction between the oxide on the surface of the crucibles and titanium during casting.The α-case will lead to degradation of titanium's surface, hence, causing a deterioration in the mechanical properties of titanium.Therefore, the concept of determining how to control the interfacial reactions between molten titanium and some ceramic materials is of great interest.
Refractory materials, such as Y 2 O 3 , CaO and ZrO 2 , have been investigated as crucible materials for melting titanium alloys.In the past few decades, extensive studies have been conducted with regard to interfacial reactions between molten titanium and zirconia moulds and/or crucibles [6][7][8][9][10][11][12][13].Saha and Jacob [9] indicated that a brittle α-case was formed on the surface of titanium, thereby adversely affecting the mechanical properties of titanium.Economos and Kingery [6] discovered that molten titanium could penetrate through the grain boundaries of ZrO 2 to form black oxygendeficient zirconia.Recently, Lin et al. [14][15][16][17][18][19][20] thoroughly investigated the phase formation mechanisms and the microstructures formed at the interface be-tween titanium (or titanium alloys) and 3Y-ZrO 2 (or varying Y 2 O 3 /ZrO 2 ratios) using analytical electron microscopy.Both lamellar orthorhombic Ti 2 ZrO and spherical hexagonal Ti 2 ZrO were found in α-Ti(Zr,O) after the reaction at 1550 °C [17,19,20].Lin and Lin [18] also found intergranular α-Zr, twinned t'-ZrO 2-x , lenticular t-ZrO 2-x , and ordered c-ZrO 2-x on the zirconia side far from the interface between Ti and 3Y-ZrO 2 , after annealing at 1550 °C.The ZrO 2 was dissolved into Ti on the zirconia side near the original interfaces; Y 2 O 3 precipitates in the samples containing 30-70 vol.%Y 2 O 3 [21].Furthermore, the Y 2 O 3 /ZrO 2 samples became more stable with increasing Y 2 O 3 because the solubility of Y 2 O 3 in Ti was very low.
As for CaO-ZrO 2 system, Kim et al. [22,23] demonstrated that the surface coating of CaZrO 3 on to the crucibles is suitable for Ti alloys casting.The CaZrO 3 possesses comparable chemical properties to ZrO 2 because CaZrO 3 is chemically inert to ZrO 2 and has lower cost compared to yttria-based systems.Thus, CaZrO 3 could be a potential refractory material for titanium casting.According to Chang and Lin [24], a diffusion zone featuring columnar CaZrO 3 was formed after reaction of Ti and CaO/ZrO 2 composites at 1550 °C.The CaZrO 3 was formed due to the outward diffusion of oxides and zirconia away from fully stabilized ZrO 2 .This result indicates that CaZrO 3 was in a stable phase and it was not significantly dissolved in Ti.Furthermore, Lin et al. [25] studied CaZrO 3 as a crucible for Ti-6Al-4V (or TC4) and TiNi-alloy based melts material.Small amount of Ca, Zr, Ti and Ni diffused to the original interfacial reaction layer, indicating that CaZrO 3 has a promising performance and a very good refractory resistance [25].In Schafföner et al. study [26], CaZrO 3 crucible was able to withstand Ti-6Al-4V and TiAl melts, thereby not exhibiting any form of cracking due to the low thermal shock resistance.
The CaZrO 3 mixed with Al 2 O 3 , Y 2 O 3 , and MgO possess high chemical stability and a very good tolerance against thermal shock [27].The CaZrO 3 /Al 2 O 3 samples contained Al 2 O 3 in various quantities, i.e. 10, 20, 30, and/or 40 vol.% of Al 2 O 3 and the equimolar ratio (1 : 1) of CaO and ZrO 2 powders in a balanced reaction with commercially pure titanium (Cp-Ti) melt at 1600 °C in 30 min.The macrostructures were characterized using X-ray diffractometer (XRD), scanning electron microscope (SEM), and transmission electron microscope (TEM) with an attached energy-dispersive Xray spectroscope (EDS).The phase formation mechanism and microstructural evolution between Cp-Ti and CaZrO 3 /Al 2 O 3 composites were elucidated.

II. Experimental procedure
Ceramic composites were prepared from powdered zirconia (> 99.9 wt.% ZrO 2 ; the average particle size was smaller than 0.5 µm; Toyo Soda Mfg.Co., Tokyo, Japan), calcia (> 99.9 wt.% CaO 0.5 µm average parti-cle size; Sigma Aidrich, Missouri, United States), and α-alumina (> 99.9 wt.% Al 2 O 3 ; ≤ 10 µm average particle size; Sigma Aidrich, Missouri, United States).Powder mixtures of ZrO 2 and CaO containing 10, 20, 30, and 40 vol.%Al 2 O 3 were prepared and designated as CZA10, CZA20, CZA30, CZA40, respectively.Ceramic powder mixtures were dispersed in ethanol and the pH of the suspension was adjusted to 11 by adding 0.2-0.7 wt.% ammonium hydroxide (NH 4 OH).The suspension was subjected to ultrasonic vibration for 10 min, dried in an oven at 150 °C, ground with an agate mortar and pestle, and screened through an 80 mesh (sieve size is 0.177 mm).Bulk ceramic composites were fabricated by hot pressing in a graphite furnace at 1 atm argon.Then, the composite specimens were heated at 300 °C and held for 3 min under 5 MPa at a heating rate of 30 °C/min, followed by heating to 1300 °C and holding for 30 min under 30 MPa at a heating rate of 25 °C/min.During cooling, pressure was released at 1100 °C.The phase identification of CaZrO 3 /Al 2 O 3 composites was performed by XRD.
The apparent densities and bulk densities of hot pressed CaZrO 3 /Al 2 O 3 samples were determined by the Archimedes principle using water as an immersion medium.The hot press conditions, compositions, relative densities, and designations of CaZrO 3 /Al 2 O 3 samples are listed in Table 1.The relative densities of the CZ (pure CaZrO 3 ), CZA10, CZA20, CZA30 and CZA40 were 98.0, 98.5, 98.2, 98.3 and 98.6% TD, respectively.
Hot pressed ceramic composites were cut and machined to the dimensions of about 10 mm × 10 mm × 5 mm.The composite CaZrO 3 /Al 2 O 3 was placed into the graphite crucible and tightly packed with commercially pure titanium powder.The crucible was placed in an electric resistance furnace having its heating elements made of tungsten.The chamber was evacuated at vacuum of about 10 −4 torr and refilled with argon.The cycle of vacuum evacuation and purging with argon was repeated three times.The temperature was increased to 1600 °C at a heating rate of 10 °C/min, where the composites were held at 1600 °C for 30 min and cooled to room temperature in the furnace.
The phase identification was performed using an XRD Model D8 DISCOVER, Bruker AXS, Germany).The operating conditions of X-ray diffraction were CuKα radiation at 40 kV and 40 mA, and a scanning rate of 1.2°/min.SEM (Model JSM 6500F, JEOL Ltd., Japan) was used for the microstructural observation of the interfaces between Ti and CaZrO 3 /Al 2 O 3 samples.SEM specimens were cut and ground using a diamond matted disc, and polished using diamond pastes of sizes 6, 3, and 1 µm in sequence.The TEM specimens were prepared by focused ion beam (FIB) so that the thickness is less than 100 nm.Prior to the FIB milling, a layer of platinum approximately 1 µm thick was deposited on the specimen by ion beam chemical deposition using Trimethyl-methylcyclopentadienyl-platinum(IV) (C 9 H 16 Pt) as a precursor gas.The Pt layer served as a marker and prevented the outer surface of the sample from being directly exposed to the gallium ion beam implantation during subsequent ion milling operations.The FIB milling was performed with a Ga + ion beam at 30 keV.After rough milling (7-1 nA), polishing (0.5-0.1 nA), and final polishing (50-10 pA), a thin foil of (17 µm × 2 µm × 0.05 µm) was cut off and transferred to a TEM grid (Formvar/Carbon Coated-Copper 200 mesh) using a micromanipulator for subsequent TEM analyses.
The interfacial microstructures were then characterized using a TEM (Model JEM 2010F, JEOL Ltd.) equipped with an EDS (OXFORD INCA, X-Max N Silicon Drift Detector, Oxford Instrument Inc.).The analyses of atomic configurations in various phases were performed using a computer simulation software for crystallography (CaRIne Crystallography 3.1, Divergent S.A., Compiegne, France).Chemical quantitative analyses for various phases were conducted by the Cliff-Lorimer standardless technique [28].A conventional ZAF correction was done using the LINK ISIS software.
Figure 2 shows the backscattered electron images (BEI) of various CaZrO 3 /Al 2 O 3 samples after they have been hot pressed at 1300 °C for 30 min.The EDS analyses showed that the bright area was identified as c-ZrO 2 , the grey area was identified as CaZrO 3 , and the dark portion of the grains was identified as CaAl 2 O 4 .Obvi-   [17,21,24].As for calcium zirconate (CaZrO 3 ), the CaO reacts with ZrO 2 to form CaZrO 3 as demonstrated by the negative free energy in Eq. 1 [30]: The From the calculation of the Gibbs free energies of calcium aluminate phases, the most negative free energy was that of Ca  cause zirconium and oxygen must have diffused to the titanium side more rapidly than the rate of diffusion of titanium into the ceramic side.
Reaction layer "I" -at the titanium side Reaction layer "I" contains dark needle-like regions and bright regions near the interfaces of Ti/CZ and Ti/CZA10, as shown in Figs.3a,b.The needle-like regions should be α-Ti(Zr,O) phase and the bright regions should be the β ′ -Ti(Zr,O) phase, which conforms to the previous reports by Lin and Lin studies [17,19,20].The needle-like α-Ti(O) phase and the β ′ -Ti(Zr,O) phase are usually found in the titanium side of the micrograph because of the interfacial reactions between Ti and 3Y-ZrO 2 .An α-Ti(O) phase with a small amount of oxygen in solid solution and β ′ -Ti(Zr,O) dissolved a significant amount of zirconium (β stabilizer) and oxygen (α stabilizer) in solid solution [17,19,20].Moreover, the needlelike α-Ti(O) and β ′ -Ti(Zr,O) in reaction layer "I" were effectively suppressed at the titanium side of Ti/CZA20, Ti/CZA30 and Ti/CZA40.
Reaction Layer "II" -near the interface BEI images of reaction layer "II" in the ceramic side of Ti/CZ, Ti/CZA10, Ti/CZA20, Ti/CZA30 and Ti/CZA40 after reaction at 1600 °C for 30 min are shown in Fig. 4.After reaction at 1600 °C for 30 min, a large amount of titanium diffused to the region occupied by ZrO 2 and CaZrO 3 .The diffused titanium was also involved in a reaction with both ZrO 2 and CaZrO 3 of Ti/CZ and Ti/CZA10 composites to form acicular α-Ti(Zr,O), β ′ -Ti(Zr,O) and residual CaZrO 3 , as shown in Figs.4a,b.Furthermore, the precipitation of the needle-like α-Ti(Zr,O) solid solution on the β ′ -Ti(Zr,O) substrate during the cooling process was very common at the interfacial reactions between Ti and ZrO 2 [17,19,21,24].However, two phases of α-Ti(Zr,O) and β ′ -Ti(Zr,O) in reaction layer "II" were found when Al 2 O 3 content increased above 20 vol.%.Titanium was involved in the reaction with Al from liquid CaAl 2 O 4 (melting point ∼1604 °C, which is close to the reacting temperature of 1600 °C) to form Ti 3 Al 5 .The formation mechanism for Ti 3 Al 5 is shown in the equation below: 3 Ti + 5 Al → Ti 3 Al 5 (8) Meanwhile, the ZrO 2 at reaction layer "II" was dissolved in Ti  The phase identification for Ca 3 Al 2 O 6 is discussed in the TEM analysis below.
Figure 5 shows the bright field images (BFI), selected area diffraction patterns (SADPs), and EDS of CaZrO 3 and Ti 3 Al 5 in reaction layer "II" of Ti/CZA40 after the reaction at 1600 °C for 30 min.Figures 5b,c show the SADPs of CaZrO  Figure 6a shows the bright field image between reaction layer "II" and "III" of Ti/CZA40 after reaction at 1600 °C for 30 min.To evaluate the effect of titanium on the composites, the composites were annealed at 1600 °C for 30 min without reacting with titanium.Figure 8 shows the BEI images of the investigated composites after annealing at 1600 °C for 30 min.By the EDS analyses of SEM (not shown), the bright area was identified as c-ZrO 2 , the grey area was CaZrO 3 , and the dark portion of the grains was CaAl 4 O 7 .Obviously, there were lots of pores in the CZ and CZA10 samples (Figs.8a,b) (arrows in figure).When the content of Al 2 O 3 in the composites increased, the pores gradually disappeared and were filled with CaAl 4 O 7 , supported by CZA20, CZA30, and CZA40 specimens (Figs.8c,d).Thus, the addition of Al 2 O 3 contributed to the increase in the density of the composites due to the formation of CaAl 4 O 7 at 1600 °C.
Figure 10 shows the XRD spectra of CZA40 after annealing at 1300 °C, CZA40 after reacting with Ti at 1600 °C, and CZA40 after annealing at 1600 °C in a sequence from the bottom to the top.Figure 11a displays the proposed model of microstructural evolution in reaction layers II and III in Ti/CZ at 1600 °C for 30 min.Before hot pressing, the composite is composed of ZrO 2 , CaZrO 3 and minor pores, but after hot pressing near the interface of reaction layer "II", titanium diffuses into the composite and dissolves a relatively large amount of O and Zr to form β-Ti(Zr,O) solid solution.As the solubility of Ca in Ti was quite limited, Ca was retained in the original CaZrO 3 .After cooling, the precipitation of the needle-like α-Ti(Zr,O) solid solution occurred on the β ′ -Ti(Zr,O) substrate.As for reaction layer "III", which is the ceramic side located slightly away from the interface, the phases were ZrO 2 , CaZrO 3 , and minor pores formed after cooling.Figure 11b

Figure 5 .Figure 6 .
Figure 5.The bright field image of transmission electron microscopy of the reaction layers II at the Ti/CZA40 after reaction at 1600 °C for 30 min (a); SADPs of CaZrO 3 along the [ 101] (b) and [ 201] zone axes (c); an energy-dispersive spectrum of CaZrO 3 (d); an SADP of Ti 3 Al 5 along the [0 10] (e) and [0 12] zone axes (f); an energy-dispersive spectrum of Ti 3 Al 5 (g) Fig. 6d shows that Ca 3 Al 2 O 6 is composed of 22.41 at.%Ca, 23.16 at.%Al, 49.67 at.%O and 4.76 at.%Zr.Reaction Layer "III" -at the composites side Figures 7a-e display the backscattering electron image of the reaction layer "III" in the Ti/composites joint after reaction at 1600 °C for 30 min.It was observed that the shape of CaZrO 3 was changed to spherical form when amount of Al 2 O 3 increased above 20 vol.% due to the formation of the liquid phase, i.e.Ca 3 Al 2 O 6 .The liquid phase, Ca 3 Al 2 O 6 , would penetrate between CaZrO 3 grains, exerting an attractive force, thereby pulling the grains together.Furthermore, the content of Ca 3 Al 2 O 6 increases with an increase in the amount of Al 2 O 3 on the ceramic side.A large amount of liquid Ca 3 Al 2 O 6 tended to enhance the shrinkage and increase in density of CaZrO 3 , resulting in the reduction of the pores and grain growth in reaction layer "III".During the reaction, liquid CaAlO 2 dissolved CaZrO 3 and ZrO 2 at 1600 °C.When the solution reached the saturation concentration, CaZrO 3 and ZrO 2 precipitated again in the solution during cooling.To evaluate the effect of titanium on the composites, the composites were annealed at 1600 °C for 30 min without reacting with titanium.Figure8shows the BEI images of the investigated composites after annealing at 1600 °C for 30 min.By the EDS analyses of SEM (not shown), the bright area was identified as c-ZrO 2 , the grey area was CaZrO 3 , and the dark portion of the grains was CaAl 4 O 7 .Obviously, there were lots of pores in the CZ and CZA10 samples (Figs.8a,b) (arrows in figure).When the content of Al 2 O 3 in the composites increased, the pores gradually disappeared and were filled with CaAl 4 O 7 , supported by CZA20, CZA30, and CZA40 specimens (Figs.8c,d).Thus, the addition of Al 2 O 3 contributed to the increase in the density of the composites due to the formation of CaAl 4 O 7 at 1600 °C.Figures9shows the BFI, SADPs, and EDS of CZA40 after annealing at 1600 °C for 30 min.Figure9a shows

Figure 7 .
Figure 7. BEI of a scanning electron microscopy of the reaction layer "III" in the ceramic side at the interface between Ti and (a) CZ, (b) CZA10, (c) CZA20, (d) CZA30, and (e) CZA40 after reaction at 1600 °C for 30 min Four phases, CaZrO (o), c-ZrO 2 (c), CaAl 2 O 4 (m), and CaAl 4 O 7 (#) were found both in the CZA40 at 1300 °C and 1600 °C.The amount of CaAl 4 O 7 increased as indicated by the peak at 21°of XRD spectra and the CaAl 2 O 4 decreased as indicated by the peak at 37°XRD peak for CZA40 annealed at 1600 °C.The formation mechanism for the CaAl 4 O 7 could be described as the Eq.10: 2 CaAl 2 O 4 + ZrO 2 → CaAl 4 O 7 + CaZrO 3(10)

Figure 8 .Figure 9 .
Figure 8. BEI of a scanning electron microscopy of the hot pressed: a) CZ, b) CZA10, c) CZA20, d) CZA30 and e) CZA40 after the heat treatment at 1600 °C for 30 min shows the formation mechanism for reaction layers II and III in the Ti/CZ-Al 2 O 3 additive at 1600 °C for 30 min.Before hot pressing, the composite is made of ZrO 2 , CaZrO 3 , CaAl 2 O 4 , CaAl 4 O 7 and minor pores after hot pressing.When the volume percentage of Al 2 O 3 increased above 20 vol.%, Ti tends to diffuse toward the composites and a reaction occurs between titanium and aluminium present in the liquid CaAl 2 O 4 to form Ti 3 Al 5 close to the reaction layer "II".After cooling, the formation of Ca 3 Al 2 O 6 occurs due to CaZrO 3 reacting with liquid CaAl 2 O 4 .At some distance slightly away from the ceramic side (reaction layers "III") ZrO 2 , CaZrO 3 and Ca 3 Al 2 O 6 were found and no Ti 3 Al 5 was formed.The reaction layers of each of the samples and their corresponding crystal phases are all marked in Table 2. IV. Conclusions 1. Ceramic composites of CaO, ZrO 2 and Al 2 O 3 in various ratios were used with titanium at 1600 °C for 30 min in Ar.The interfacial microstructures between titanium and ceramic composites were characterized by using XRD, SEM and TEM. 2. Before the addition of titanium, calcium aluminate (CaAl 2 O 4 ) or calcium dialuminate (CaAl 4 O 7 ), c-
(9)5 and CaZrO 3 changed into smooth and spherical shapes due to the formation of a large amount of liquid CaAl 2 O 4 .In addition to the Ti 3 Al 5 phase, the Ca 3 Al 2 O 6 phase was also found as shown in Figs.4b-e.The formation mechanism of Ca 3 Al 2 O 6 phase could be described as follows: CaAl 2 O 4 + 2 CaZrO 3 → Ca 3 Al 2 O 6 + 2 ZrO 2(9)