Microstructure and mechanical properties of Al₂O₃/TiAl composite

AI Tao-tao(艾桃桃), WANG Fen(王  芬), ZHU Jian-feng(朱建锋)

School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xianyang 712081, China

Received 28 July 2006; accepted 15 September 2006

Abstract: Al₂O₃/TiAl composites were fabricated by PAXD (pressure-assisted exothermic dispersion) method. The effects of Nb₂O₅ content on the microstructure and mechanical properties of the composites were investigated. The results show that the ultimate phases of the composite consist of TiAl, Ti₃Al, Al₂O₃ and a small amount of NbAl₃. SEM reveals that a submicron γ+(α₂/γ) dual phases structure can be presented after sintered at 1 200 ℃. Furthermore, with the increase of Nb₂O₅ content, the ratio of TiAl to Ti₃Al phase decreases correspondingly, the grains of the composites are remarkably refined, and the produced Al₂O₃ particles are uniformly dispersed. When 6% Nb₂O₅ is added, the composite has the best comprehensive properties. It exhibits a Vickers hardness of 4.77 GPa and a bending strength of 642 MPa. Grain-refinement and dispersion-strengthening are the main strengthening mechanisms.

Key words: exothermic dispersion; Al₂O₃/TiAl composite; microstructure; mechanical properties

1 Introduction

TiAl intermetallic compounds have received considerable attention due to their merits of low density, high specific modulus, and relatively good strength retention at elevated temperatures. Therefore, they are considered prospective structural materials for applications in aerospace and automobile industries[1-3]. However, their practical applications are still hindered by the relatively poor room temperature mechanical properties, machinability and insufficient oxidation resistance above 800 ℃. Ceramic particles reinforced TiAl matrix composite is a promising material to cope with these problems. Especially, Al₂O₃ can be regarded as a good choice in virtue of its high elastic modulus, hardness, isotropic property, amenability of component forming and stability at high temperature. Moreover, a certain amount of niobium has been shown to be the most effective element for improving the oxidation resistance of TiAl alloys, because Nb could not only suppress the growth of rutile but also lead to the formation of protective Al₂O₃ scales[4, 5]. Moreover, proper amount of Nb could significantly improve the high-temperature strengths of TiAl alloys[6-8] and alloying with Nb element would often lead to a refinement of the microstructure[6]. A novel exothermic dispersion (XD)[9-10] method for the low-cost manufacturing of alumina-aluminide alloys has been developed, which is a liquid-solid reaction method of producing ceramic reinforcing in situ particulate in a matrix.

In situ TiAl matrix composites were based on Ti-Al-TiO₂-Nb₂O₅ system. Nb₂O₅ took place of Nb powders to reduce production cost. The main purpose of the current work is to investigate the microstructures and mechanical properties of Al₂O₃/TiAl in situ composites fabricated through pressure-assisted exothermic dispersion (PAXD) method.

2 Experimental

Powders of titanium (＜50 μm, ＞99.3% in purity), aluminum (＜75 μm, ＞99.5% in purity) and titanium dioxide (0.5 μm,＞99% in purity) and niobium pentaoxide (＜33 μm, ＞99.5% in purity) were used as the additives for the synthesis of Al₂O₃/TiAl in situ composites. The 43.9%Ti-38.6%Al-17.5%TiO₂ powders mixed with 0, 6%, 10% and 20%Nb₂O₅ respectively were attrition milled for 2 h in ethyl alcohol using 3 mm diameter alundum balls. The ball to powder mass ratio was 3∶1, and the rotational velocity was kept at 800 r/min. Further processing included drying at 80 ℃ for  5 h and sieving through a 200 mesh. Then the as-milled powders were performed in a sintering furnace under a vacuum of less than 5.3×10^-2 Pa. The sintering was performed following a two-step procedure, i.e., pre-sintering at 600 ℃ for 1 h and subsequent sintering at 1 200 ℃ for 2 h under a specific pressure of 35 MPa. Then, the composites were held at the maximum temperature for 30 min with the pressure maintained.

Phase identification was performed using Cu K_α irradiation on a X-ray diffractometer with at a scan rate of 3 (?)/min. Fracture patterns of the samples were conducted in a scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectrometer (EDS). The polished surface of the product was etched in a modified Kroll’s reagent of 5% HF, 10% HNO₃ and 85% H₂O (mole fraction) before microstructure test observed on optical microscope (OM).

Hardness tests were conducted with a standard Vickers apparatus (HXD-1000) using a load of 300 N applied for 15 s. The samples were cut into strip specimens with 25 mm×4 mm×3 mm and used for the measurement of the three-point bending strength by a universal material testing machine with a span of 25 mm and a cross-head speed of 5 mm/min at room temperature.

3 Results and discussion

3.1 Phase composition and microstructure

Fig.1 shows the X-ray patterns of the in situ composites with different contents of Nb₂O₅. According to XRD analysis, the composites comprise TiAl, Ti₃Al and Al₂O₃ as three major phases, as well as a significantly smaller amount of NbAl₃.

Fig.1 X-ray diffraction patterns of Al₂O₃/TiAl in situ composites with different contents of Nb₂O₅: (a) 0; (b) 6%;   (c) 10%; (d) 20%

The cross-section SEM micrographs of the composites are shown in Fig.2. It can be seen that the composites consist of a brighter Al₂O₃ phase and dark γ+(γ+α₂) grains. Seen from Fig.2(a), Al₂O₃ particles tend to distribute on grain boundaries and the average size of Al₂O₃ and grain is 0.7 μm and 6 μm, respectively. With increasing Nb₂O₅ content, the mass fraction of TiAl phase slightly decreases and Ti₃Al phase increases. On the other hand, the mass fraction of Al₂O₃ and NbAl₃ phase increases gradually. A part of Al react with Nb₂O₅ resulting in the decreasing concentration of Al solution. Simultaneously, the grains of the composites are remarkably refined with an average grain size of 2.5 μm. The distribution of Al₂O₃ particles are more uniform, and the average size of Al₂O₃ decreases to 0.4 μm. The brighter area changes from an agglomerating state to an interpenetrating network structure.

Fig.3 represents the optical micrographs (OM) of Al₂O₃-TiAl in situ composites. With increasing Nb₂O₅ content, the mass fraction of Al₂O₃ phase increases judged from the increasing of black section. It significantly indicates the distribution of Al₂O₃ particles improves from high glomeration (Fig.3(a)) to diffusion distribution (Figs.3(c) and (d)). And the number of spherical Al₂O₃ particles dispersing in the matrix has increased significantly attributed to the evolution of more amounts of reinforcements in the matrix, which is due to the more favorable kinetics of the in situ reaction at higher temperatures. The matrix exhibits a duplex structure consisting of the lamellar phases containing α₂/γ and colonies equiaxed γ grain. Seen from Fig.3(a), it presents a colony structure. When the Nb₂O₅ content is higher than 6%, the flake-like colony structure vanishes and the composites have a fine-grained microstructure with an inhomogeneous size distribution.

3.2 Mechanical properties

The relative density measured by the Archimedes method increases steadily with increasing Nb₂O₅ content, as illustrated in Table 1. It increases from 94.6% to 96% as Nb₂O₅ content increases from 0 to 20%. Vickers hardness also increases steadily with increasing Nb₂O₅ content, as presented in Table 1. With increasing Nb₂O₅ content from 0 to 20%, Vickers hardness increases from 3.53 GPa to 5.07 GPa. The increase in relative density is correlated with the formation of NbAl₃ phases that fill up the pores. The hardness of the composites depends on the content of Al₂O₃, which can be expressed by mixture rule as follows:

HV_C=HV_M(1－φ_V)+HV_f ·φ_V                     (1)                                                   

where  HV_C, HV_M and HV_f are the hardness of the composites, matrix and the second phase particles, respectively; φ_V is the volume fraction of the second phase particles.

Bending strength obviously increases with increasing Nb₂O₅ content, as seen in Table 1. The composite with 6% Nb₂O₅ content exhibits a higher strength, which is 642 MPa and 118.2% higher than the sample without Nb₂O₅. Subsequently, the bending strength of the composites decreases gradually, but it is still higher than that without Nb₂O₅ content.

Fig.2 SEM photographs of Al₂O₃/TiAl in situ composites with different contents of Nb₂O₅: (a) 0; (b) 6%; (c) 10%; (d) 20%

Fig.3 Optical micrographs of Al₂O₃/TiAl in situ composites with different contents of Nb₂O₅: (a) 0; (b) 6%; (c) 10%; (d) 20%

Table 1 Relative density, bending strength and Vickers hardness of Al₂O₃/TiAl in situ composites

The strengthening mechanism of Nb₂O₅ is attributed to the superposition of the second phase strengthening by Al₂O₃ particles, the dispersion strengthening by fine precipitated Al₂O₃ and the fine colony grain size strengthening. The fine colony grain strengthening plays an important role in increasing the strength of the composites due to the fact that yield strengths exhibit inverse square root dependence on the average grain and the dispersion of Al₂O₃ particles. The fine colony grain strengthening can be estimated by Hall-Petch Equation:

?σ_YM≈kD^-1/2≈kd^-1/2{(1-φ_p^1/6)/ φ_p}                (2)                                      

where  ?σ_YM is the increment of yield strength, k is a dislocation unpinning parameter and its canonical value is 0.1 MPa·m^1/2, D is the grain dimension, d is the dimension of the second phase particles, and φ_p is the volume fraction of the second phase particles. Eq.(2) can be used to explain the increase in yield strength with a corresponding decrease in grain size. On the other hand, Nb₂O₅ additive remarkably improves the wettability between Al₂O₃ particles and Al liquid. According to Young’s equation, the relationship of liquid metal on the behavior of impregnation between solid surface can be expressed as follows:

cosθ=(σ_SV-σ_SL)/σ_LV                            (3)                                                           

where  θ is wetting angle of fluid-solid surface, σ_SV is solid-gas surface tension, σ_SL is solid-fluid surface tension, and σ_LV is fluid-gas surface tention. It can be seen from Eq.(3) that the alloying element that can reduce the surface energy (σ_LV) and the interface energy (σ_SL) of the fluid metal can improve the wettability of solid-fluid. In order to reduce the surface energy, the alloying element must conform to the condition that σ_M is less than σ_Al. S.Z.BEER has concluded Nb element conforms the condition from the relationship between surface tension of the fluid metallic and atomic volume. In other words, Nb element can reduce σ_LV and σ_SL, and consequently improve the wettability between Al₂O₃ particles and Al liquid.

4 Conclusions

1) Al₂O₃/TiAl in situ composites were fabricated by pressure-assisted exothermic dispersion method in Al-Ti-TiO₂-Nb₂O₅ system. The ultimate phases of the composite consist of TiAl, Ti₃Al, Al₂O₃ and a small amount of NbAl₃. SEM reveals that a submicron γ+(α₂/γ) dual phases structure can be obtained. With increasing Nb₂O₅ content, the grains of the composites are remarkably refined and the Al₂O₃ particles are uniformly dispersed.

2) The application of a moderate external pressure of 35 MPa during sintering at 1 200 ℃ yields high relative densities in the range of 94.6%-96%. When 6% Nb₂O₅ is added, the composite has the best comprehensive properties. It exhibits a Vickers hardness of 4.77 GPa and a bending strength of 642 MPa.

3) Nb₂O₅ in Al₂O₃/TiAl in situ composites leads to a slight refinement of the microstructure and improves the wettability of liquid aluminum to alumina, which makes Al₂O₃ particles disperse more evenly and the composite be more compactable.

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(Edited by PENG Chao-qun)

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

Corresponding author: AI Tao-tao; Tel: + 86-910-3571937; E-mail: taoatao8183@tom.com