Effect of rare earth additives on alumina fiber development in  Ti-Al intermetallic matrix composites

WANG Fen(王  芬)^{1, 2}, FAN Zhi-kang(范志康)¹, LIN Ying(林  营)²

1. School of Materials Science and Engineering, Xi’an University of Technology, Xi’an 710048, China;

 2. School of Materials Science and Engineering, Shaanxi University of Science and Technology,

Xianyang 712081, China

Received 28 July 2006; accepted 15 September 2006

Abstract: Al₂O_3(f)/TiAl composites were synthesized by an exothermic reaction method using Ti, Al and TiO₂ powders doped with Nb₂O₅ and La₂O₃. The effect of Nb₂O₅ and La₂O₃ additives on the growth and morphology of the fibers, the phases and microstructure of the composites were investigated by means of XRD and SEM. The result indicates that the in situ alumina fiber can be developed in Ti-Al matrix with the Ti/Al mole ratio of 1:2-1:7, and the addition of rare earth powders can improve the dispersion of the fibers in the matrix and increase the length-to-diameter ratio of the fibers.

Key words: rare earth; additives; alumina fiber; intermetallic composite; microstructure

1 Introduction

Reinforcing intermetallics with fiber is one of the effective methods to improve the fracture toughness, strength and fatigue resistance of intermetallics. Al₂O₃ fiber is one of the attractive candidates for this application due to its combination of high strength, high elastic modulus, good thermal stability and oxidation resistance. Al₂O₃ fibers are considered to be thermo- dynamically stable in many metal or intermetallic matrix for high temperature application. However, recent results have indicated that the fiber strength is nearly half reduced during processing [1-3]. Therefore, it is necessary to improve the process of fabrication.

In situ Al₂O₃ particles reinforced TiAl intermetallic matrix composites have been manufactured by self- propagating high-temperature synthesis (SHS) [4-5], by reactive infiltration of Al into porous Al₂O₃-TiO₂ preforms [6]. However, for reinforcing brittle matrix, whiskers or fibers have much more advantages than brittle ceramic particles [7]. The growth of alumina whisker in aluminum-based metal has been presented by displacement reaction method [8-9]. But there is no in situ synthesized whisker or fiber reinforced TiAl matrix reported in such a way yet.

In this study, we developed a simple potential method to synthesize TiAl intermetallic matrix composites reinforced with in-situ alumina fibers. It is found that the addition of rare earth (Nb₂O₅, La₂O₃) additives has a profound impact on the growth and morphology of the fibers.

2 Experimental

The Al₂O_3(f)/TiAl composites were prepared from the mixture of aluminum (99.3% purity, 75 μm), titanium (99.5% purity, 52 μm) and TiO₂ (99.5% purity, 50 μm) powders, which were doped with 0.8%-2% (mole fraction) Nb₂O₅ and 0.6% La₂O₃ (both 99.5% purity, 33 μm). The powders mixture was dry milled for 30 min with alumina balls and then sieved through a 200 mesh. The green samples of d30 mm×5 mm were prepared by uniaxial pressing at 30 MPa. The green samples were embedded in alumina and graphite powder (1:1), and were heated to 1 220 ℃ at a rate of about 5 ℃/min in an electric furnace and held for 30 min before cooling down.

The microstructure and phase composition of the as-prepared samples were analyzed by X-ray diffraction (XRD, D/max 2000PCX, Rigaku, Cu K_α, 40 kV, 40 mA, rate: 8(?)/min) and scanning electron microscopy (SEM, JSM-6460, JEOL).

3 Results and discussion

Fig.1 shows the XRD patterns of two typical samples. When the Ti/Al molar ratio in the green samples is close to 1:7, the synthesized materials mainly comprised TiAl₃, Al₂O₃ and Al as confirmed by XRD analysis (Fig.1(a)). If the additives of 2% Nb₂O₅ and 0.6% La₂O₃ are doped, the phases of composite consist of TiAl₃, NbAl₃ and Al₂O₃, and no Al element can be found by XRD analysis any more (Fig.1(b)). Under both conditions, the alumina fibers can be easily developed at a heat-rate below 5 ℃/min. However, when the Ti/Al molar ratio is in the range of 1:2-1:3, the major phases of the composites are TiAl and Al₂O₃ and the fibers are hardly developed unless the rare earth additives are added.

Fig.1 XRD patterns of samples: (a) x(Ti):x(Al)=1:7; (b) x(Ti): x(Al)=1:7

Rare earths like Nb₂O₅, La₂O₃ and Y₂O₃, are attractive as additives because they are aluminothermic agent which may assist the synthesis process, and the additions of Nb, La and Y are likely to improve the strength of TiAl and other intermetallics [10-11]. In this study, we found that rare earths have strong influence on the morphology and growth of in situ fibers.

In Ti-60Al-5TiO₂ reactants sample, doping of 0.8%-2% Nb₂O₅ and 0.6% La₂O₃ results in a synthesized material which consists mainly of γ-TiAl and α-Al₂O₃ plus a smaller amount of NbAl₃. No starting reagents materials are left. However, when Nb₂O₅ content is higher than 2%, XRD analysis indicates that a trace amount of NbO is identified. The additives of Nb₂O₅ and La₂O₃ promote the growth and uniform distribution of the in-situ formed fibers in the matrix (Fig.2). Without the additives, the fibers form mostly inside the pore areas in the high Al concentration samples (the content of Al is higher than 80%) (Fig.3).

Fig.2 Fibers morphology in composite with additives (x(Ti):x(Al)= 1:7)

Fig.3 Fibers formed mostly in pores in Al rich system without additives

Increasing the contents of both additives and Al in starting materials, the fibers will become shorter and thicker with good smooth surface (Fig.4). If only increasing the amounts of additives, the fiber becomes thinner and longer, namely, having a high length-to- diameter ratio (Fig.5). However, if the content of additives is too high as compared to that of aluminum (Nb₂O₅>2%, Al<60%), the fiber becomes knotty and the samples have a low compact density. In addition, some Ti₃Al phase will occur (Fig.6).

From the results discussed above, it is obvious that an additive and the molar ratio of starting elements makes possible changes in the morphology and growth of fiber. As we know, the reaction temperature, the liquid viscosity, the heating rate, etc will change the morphology of the fiber. Nb₂O₅ can react with Al and generate a large amount of heat energy, and La₂O₃ can react with liquid Al in high temperature [12], reduce the viscosity of liquid, and improve the wetting of liquid. Besides, Nb can improve the wetting of the liquid Al with Al₂O₃ for it can reduce the surface tension of liquid Al [13]. Furthermore, the oxygen released from the reaction of Nb₂O₅ with Al would help the forming of Al₂O₃. All of these make the system a lower viscosity and better wetting, which results in a lot of thin diameter whiskers distributed more evenly in the matrix [14].

Fig.4 Short and thick fibers in TiAl₃ matrix: (a) Low magnification; (b) High magnification

Fig.5 High length-to-diameter ratio fibers in TiAl matrix: (a) Low magnification; (b) High magnification

Fig.6 Knotty fibers in Ti₃Al matrix: (a) Low magnification; (b) High magnification

4 Conclusions

Al₂O_3(f)/TiAl₃ composites with uniformly dispersed in-situ formed fibers were synthesized by controlling the Ti/Al mole ratio close to 1:7 with about 2% Nb₂O₅ and La₂O₃ additives. And Al₂O_3(f) /TiAl composites with high length-to-diameter ratio in situ fibers were synthesized by controlling the Ti/Al mole ratio in the range of 1:2-1:3 with <2% Nb₂O₅ and La₂O₃ additives. It is found that the distribution and morphology of the formed fibers depend on the Al concentration and the amount of additives. An appropriate combination of Al and additives results in fibers with high length-to-diameter ratio and smooth surface. Whereas too much Nb₂O₅ (>2%) leads to knotty fibers and incompact samples.

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(Edited by YUAN Sai-qian)

Foundation item: Projects(50432010; 50372037) supported by the National Natural Science Foundation of China

Corresponding author: WANG Fen; Tel: +86-910-3571937; E-mail: wangf@sust.edu.cn