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Interaction between �����- TiAl alloy and

zirconia*Liu Aihui1, Li Bangsheng1, Nan Hai2, Sui Yanwei1, Guo Jingjie1, Fu Hengzhi1

(1. School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China; 2. Beijing Institute of

Aeronautical Materials, Beijing 100095, China)

TiAl alloys are promising high-temperature structuralmaterials for a wide range of applications such as aerospace,

marine and automobile parts owing to their low density, highstrength and creep resistance [1-3]. A range of manufacturingprocesses have been successfully developed for utilization ofthese alloys. Among these processes, investment casting is theoptimal process because of its low cost, mass-production andhigh efficiency. However, the main limitation of investmentcasting for titanium alloys is the chemical reaction betweenmolten titanium and molding materials at high temperature. Manyresearchers have investigated the mechanisms of the interfacialreactions. Saha [4] and Suzuki [5] proposed that the interfacialreaction is characterized by the exchange of oxygen betweenmolten titanium and mold materials and the generation of gaseousoxides of titanium. Zhou Yanbang[6], Degawa[7] and Holcombe[8]

found that the substitution reaction takes place between titaniummelt and ceramic crucible by analyzing the reaction betweenmolten titanium and ceramic crucible. Although extensive studieshave been reported, the interfacial reaction between TiAl andzirconia has not been fully elucidated to date.

In this paper, to understand the mechanism of the interfacialreactions, the microstructure of the interface between TiAl alloyand Zirconia is investigated using SEM and EDS.

1 Experimental procedure

The calcia partially stabilized ZrO2 (5 mol% CaO-ZrO

2) was

machined into U-type crucible of � 10 mm � 6 mm insidediameter and � 20 mm �=10 mm outside diameter. The TiAl

1. TiAl alloy; 2. induction coil; 3. ZrO2 crucible; 4. support

Fig.1 Schematic diagram of experimental apparatus

Abstract: A study on the interaction between TiAl alloy and zirconia was carried out in argon atmosphere. The

micrographic observations of the cross-section perpendicular to the interface were obtained using a scanning electron

microscope with a dedicated energy dispersive spectrometer (SEM/EDS). The results showed that firstly TiAl alloy

spreads on the ceramic surface, and then infiltrates into the pores between ceramic particles accompanied by a chemical

reaction. The whole ceramic mold is broken into tiny blocks. A multiple fission reaction mechanism was developed to

explain the interaction between TiAl alloy and zirconia.

Key words: ceramics; microstructure; Y-TiAl alloy; interfacial reaction

CLC number: TG146.2 Document Code: A Article ID: 1672-6421(2008)01-044-03

alloy used in this study had the nominal composition of Ti-48Al-2Cr-2Nb (at.%), its size �=8 mm � 100 mm. The experimentwas carried out using the self-designed induction melting tester,as shown in Fig.1. The chamber of tester was first evacuated to0.01Pa by an oil diffusion pump at room temperature, and thenbackfilled with argon to atmospheric pressure. The cycle ofevacuation and purge with argon was repeated two times. TheTiAl alloy was melted by the high frequency induction coil, andthen dripped into the ZrO

2 crucible. The interface of TiAl alloy

and zirconia was observed using a scanning electron microscope(SEM) with energy dispersion spectrometry system (EDS). Thecross-sectional SEM specimen perpendicular to the interface ofTiAl and zirconia was prepared including sectioning, SiC papersgrinding, diamond paste polishing, ultrasonic cleaning, and etching

Female, born in 1976, doctoral candidate. Research interests:

investment casting technology, interfacial reaction between Ti

based alloys and oxide ceramic materials.

E-mail: hlglah@tom.com

Received: 2007-09-12; Accepted:2008-01-30

*Liu Aihui

2 Results and discussion

Figure 2 shows a SEM micrograph, from which the interfacebetween the TiAl alloy and zirconia can be noted: the left grayzone corresponding to TiAl alloy, and the right white block zonecorresponding to zirconia. For TiAl alloy cast specimen, the as-cast microstructure mainly included two phases, a dendritic phase

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Fig.2 SEM micrograph showing the interfacial reaction

between TiAl alloy and ZrO2

Fig.3 SEM micrograph of TiAl alloy after reaction

(a) black phase

(b) white phase

Fig.5 SEM micrograph of ZrO2 (CaO stabilized) after reaction

Fig.4 XRD analysis of TiAl alloyFig.6 Interfacial SEM micrograph of ZrO

2 (CaO stabilized)

after reaction

�2+� and an interdendritic phase�[9] (Fig.3). From Fig.3, it

can be still noted that to the left of the interface, new phases areformed at the grain boundary in TiAl alloy: black block phasessurrounded by the white plate phases. The EDS spectra methodis used to determine the nature of the above two phases (Fig.4).

It can be found that the oxygen and Zr in zirconia were activated,and then diffused into TiAl alloy melt, respectively. At the sametime, the diffusion distance of oxygen atoms was larger owingto the smaller atom radius. The black oxide blocks at phaseboundary consist of the oxygen from zirconia and Al from TiAlalloys. In general, Ti with high reactivity can react with oxygenfrom zirconia and form any Ti oxide at high temperature.However the EDS spectrum suggests not a titanium oxide but analuminum oxide. It was shown that Al reduced the activity of Tiand suppressed the reaction between Ti and oxygen. The whitephases around the black blocks were oxygen-deficient zirconia.

The black alumina formed and distributed non-uniformly atthe left of the interface between TiAl alloy melt and zirconiadue to the discrete contact of TiAl alloy with zirconia. At theclose contact zone, in terms of the purity and crystal orientationof ceramic particles, ceramic particles were more likely to act asnucleating center favoring the reaction process.

Figure 5 displays the SEM micrograph with respect to zirconiamold side. As shown in Figs.2 and 5, the microstructures amongthe blocks inside the ceramic mold were identical to that of TiAlalloy matrix, which suggests that the interfacial reaction betweenTiAl alloys melt and zirconia undergoes physical wetting as wellas chemical wetting. Chemical wetting decreases greatly thecontact angle of TiAl and zirconia, promoting the penetration ofmolten TiAl alloy into the pores among the mold, and thenbreaking up the ceramic particles. Fig.6 gives the SEMmicrograph with respect to the above result.

From the above analysis, the interaction mechanism betweenTiAl alloy melt and zirconia was deduced. In the TiAl alloy melt

AlxO

y γγγγγ

ααααα22222+++++γγγγγ

ZrO2-x

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This project (No. 50434030) was supported by the National Natural Science Foundation of China.

filling process, because the surface of zirconia is not sufficientlywetted by the TiAl alloy melt at the initial physical wetting stage,advancing front of TiAl alloy melt holds the plane morphology.Therefore, no chemical reaction occurs at the mold surface zonewhich is non-contacting with the alloy melt. However, becauseof the high chemical activity of Ti at high temperature, chemicalreactions would occur at the close contact zones between TiAlalloy melt and zirconia. This indicates that the surface of zirconiamold is chemically wetted by TiAl alloy melt to different extentowing to the difference in the chemical reactivity. Under suchconditions as TiAl alloy melt has a good contact with the zirconiagrains, or the zirconia grains contacting with TiAl alloy have apreferred orientation, the chemical reactions take placepreferentially at these zones meeting the conditions ofcomposition, temperature and energy fluctuations. The chemicalreaction greatly decreases the contact angle of TiAl alloy meltand zirconia, promoting the penetration of molten TiAl into thezirconia mold. Meanwhile, since the penetrating process increasesthe effective contact area between TiAl alloy melt and zirconia mold,the interfacial reaction is facilitated. The interaction between thepenetration and chemical reaction makes the zirconia mold near theinterface fragment completely during a short period of time.

3 Conclusions

In summary, the interfacial reaction between TiAl alloy meltand zirconia was facilitated by the penetration of TiAl alloy melt

into the zirconia mold. The penetration and the chemical reactionpromote each other. In addition, the chemical reactions do notproceed uniformly, but take place preferentially at the closecontact zone meeting the conditions of concentration, temperatureand energy fluctuations.

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