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 Kα 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:
HVC=HVM(1－φV)+HVf ·φV (1)
where HVC, HVM and HVf are the hardness of the composites, matrix and the second phase particles, respectively; φV 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:
?σYM≈kD-1/2≈kd-1/2{(1-φp1/6)/ φp} (2)
where ?σYM is the increment of yield strength, k is a dislocation unpinning parameter and its canonical value is 0.1 MPa·m1/2, D is the grain dimension, d is the dimension of the second phase particles, and φp 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=(σSV-σSL)/σLV (3)
where θ is wetting angle of fluid-solid surface, σSV is solid-gas surface tension, σSL is solid-fluid surface tension, and σLV is fluid-gas surface tention. It can be seen from Eq.(3) that the alloying element that can reduce the surface energy (σLV) and the interface energy (σSL) 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 σM is less than σAl. 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 σLV and σSL, 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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Corresponding author: AI Tao-tao; Tel: + 86-910-3571937; E-mail: taoatao8183@tom.com
