Reinforcing and toughening of TiAl composites by doping Sm₂O₃

WANG Xiao-feng, WANG Fen, ZHU Jian-feng, XIANG Liu-yi

Key Laboratory of Auxiliary Chemistry and Technology for Chemical Industry of Ministry of Education,

School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi’an 710021, China

Received 18 October 2010; accepted 28 November 2010

Abstract: Al₂O₃/TiAl composites were fabricated by in-situ reaction synthesis using Ti, Al, TiO₂ and Sm₂O₃ as starting materials. Effect of the doping Sm₂O₃ on the microstructures and properties of the Al₂O₃/TiAl composites was analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and universal properties tests. The results show that the phases of the Sm₂O₃-doped composites are mainly composed of γ-TiAl/α₂-Ti₃Al matrix and reinforcing phases of Al₂O₃ and SmAl. The microstructures of Al₂O₃/TiAl composites are significantly refined with doping Sm₂O₃. Doping Sm₂O₃ has a positive effect on improving the mechanical properties of Al₂O₃/TiAl composites. When the Sm content is 5% (mass fraction), the flexural strength and fracture toughness reach the maximum values of 658.9 MPa and 10.13 MPa·m^1/2, respectively.

Key words: TiAl; Sm₂O₃; grain size; mechanical properties

1 Introduction

Titanium aluminides exhibit numerous attractive properties for high-temperature structural applications due to their high elastic modulus, low density, and good creep resistance to high temperatures [1]. A further increase of application may be accomplished through microstructural control and alloy modi?cation [2-3]. Generally, coarse grain fully lamellar microstructures exhibit relatively poor tensile ductility and strength, but relative grain-refining near-γ or duplex microstructures show better tensile properties [4]. Therefore, controlling and re?ning microstructure is one way to attain ideal mechanical properties. Several routes are commonly used for this purpose, such as activation of selected phase transformations, wrought processing or addition of grain growth inhibitors [5].

There are different second-phase particles, such as TiB₂, SiC, TiC, Ti₂AlC and Al₂O₃. Al₂O₃ has been chosen as a ceramic reinforcement because of its advantageous thermo-mechanical behavior, inclusive of wear resistance, environmental stability, high temperature strength, and so on [6]. The ductility and strength can be improved by reducing grain size and lamellar spacing without compromising the fracture toughness and creep resistance. Rare earths (RE) are widely used in metallurgical fields, due to improved performance of sintering, heat-machining, mechanics, oxidation/corrosion and wear resistance by means of purifying, transformation and alloying processes [7].

Previous researches on TiAl intermetallic alloys were focused mainly on RE addition, but the effects of doping RE₂O₃ on TiAl were not studied in detail. In this work, Sm₂O₃ was added to the starting materials of Ti, Al, TiO₂ to modify the Al₂O₃/TiAl in situ composites. The effects of Sm₂O₃ addition on the microstructure and mechanical properties of the composites were investigated. Furthermore, the refined mechanism was also discussed.

2 Experimental

For the production of the Al₂O₃/TiAl composite, reactant samples were prepared from the mixture of Ti (Strem chemicals, 53 μm, 99.3% of purity), Al (Showa Chemical Co, 75 μm, 99% of purity), TiO₂ (0.5 μm, 99% of purity), and Sm₂O₃ (30 μm, 99.9% of purity) powders by the stoichiometry according to the following  reaction:

xTi+(x+13)Al +3TiO₂+Sm₂O₃=

(x+3)TiAl+3Al₂O₃+2SmAl₂                 (1)

where the stoichiometric parameter x is 16.97, which represents the Al₂O₃ content is 12% (mass fraction) in the target TiAl-matrix composites, and the Sm mass fraction varies from 0 to 6%. Eight samples were designed in Table 1.

Table 1 Composition of samples (mass fraction, %)

The desired mixtures were ball milled in ethanol for 1 h. The mass ratio of ball to powder was 3:1, and the rotational velocity was kept at 800 r/min. Further processing included drying at 60 °C for 5 h and sieving through 75 μm-sieve. The reactive sintering of the as-milled powders was performed in a sintering furnace under vacuum less than 10^-2 MPa. The compacted powder mixture was heated at a rate of 10 °C/min from room temperature to 500, 700, 900, 1 100, 1 200, 1 250 and 1 300 °C and held for 2 h, respectively, wherein the pressure was gradually increased to 35 MPa. Finally, the sample was cooled down in furnace to room temperature.

The phase composition of the fabricated samples was analyzed by an X-ray diffractmeter (Rigaku D/max 2000P) with Cu K_α radiation operating at 40 kV. The microstructure, fracture surfaces and crack propagation of the samples were investigated by scanning electron microscopy (SEM) (JSM-6700F SEM).

The Vickers hardness of Al₂O₃/TiAl composites was measured at room temperature on a HXD-1000 tester with a diamond indenter under 10 N for 15 s. The density measurements were performed via Archimedes principle. The samples were cut and ground into strip specimens with dimensions of 25 mm×4 mm×3 mm for measuring the three-point bending strength on a universal testing machine with a span of 25 mm at a cross-head speed of 5 mm/min at room temperature. The bending strength was calculated by the following equation:

σ=3PL/(2bh²)                                (2)

where P is the breaking load of the specimen, and L, b, and h denote the span, width, and height, respectively.

The fracture toughness was measured on the universal testing machine by using the single-edge notch beam (SEPB) method with specimen dimensions of 36 mm×4 mm×8 mm. The notch depth was 4 mm with ~0.15 mm in width. The crosshead speed was 0.05 mm/min, with a loading span of 30 mm. The fracture toughness, K_IC, was calculated by the following equation:

K_IC=Y×3PL a^1/2/(2bh²)                        (3)

where a is the notch length, and Y is a geometrical factor.

3 Results and discussion

Figure 1 shows the X-ray diffraction patterns of Al₂O₃/TiAl composites with 5% Sm after being hot pressed at various temperatures for 2 h. According to XRD analysis, there is no reaction among Ti, Al, TiO₂ and Sm₂O₃ powders when the mixed powders are sintered at 500 °C for 2 h. When the temperatures is increased to 700 °C (higher than the melting points of Al), the as-sintered samples mainly consist of Al₃Ti and Al₂O₃ as two major phases together with a significantly smaller amount of Al₂Ti, SmAl and unreacted Ti phase.

Fig. 1 XRD patterns of composites at various temperatures

When the temperatures is 800 °C, the values of free-energy change, of reactions among Al，Sm and O₂ are as follows [8]:

=-1 829.69-(-1 098.15×402.38×10^-3)

 =-1 387.82 kJ/mol                    (4)

=-1 583.99-(-319.23×10^-3)×(800+298.15)

  = -1 233.43 kJ/mol                   (5)

According to the Gibbs free-energy change, there is no thermite reaction between Sm₂O₃ and Al, which indicates that the formation of SmAl is thermodynamically infeasible. However, Sm₂O₃ can decompose into [Sm] and [O] because of active effect of the massive reaction heat released by Ti and molten aluminum, which can affect the kinetics or the mechanism of reaction between Sm₂O₃ and Al, which leads to the formation of SmAl.

When the temperature is increased to 900 °C, the composite mainly consists of γ-TiAl, α₂-Ti₃Al, Al₂O₃ and SmAl phases. The presence of the above-mentioned phases in the composites samples confirms the feasibility of the following in-situ reactions:

3TiO₂+4Al=2Al₂O₃+3Ti                      (6)

Sm₂O₃+4Al=Al₂O₃+2SmAl                    (7)

4Ti+2Al=TiAl+Ti₃Al                         (8)

When the temperature was increased from 900 to  1 250 °C, it can be found that there were no much changes in phase composition, but the TiAl diffraction peaks were weakened and broadened gradually; on the contrary, the Ti₃Al diffraction peaks were increased because some TiAl phase transformed to Ti₃Al phase and formed the duplex microstructures, which is beneficial to increasing the combination properties of Al₂O₃/TiAl composites [9].

Figure 2 presents the XRD patterns of the in-situ composite samples with different contents of Sm₂O₃ hot-pressed at 1 250 °C for 2 h. Compared with Sm₂O₃ free samples, the samples with Sm₂O₃ addition consisted of the same three major phases of γ-TiAl, α₂-Ti₃Al and Al₂O₃. But a significant smaller amount of SmAl phase was produced. Although it is hard to quantify the individual phases in the products, it is clear that the intensity of the SmAl diffraction peaks increases with the increase of the content of Sm₂O₃.

Fig. 2 XRD patterns of in-situ composites with various Sm₂O₃ contents

Figure 3 shows the typical fracture microstructures of the Al₂O₃/TiAl composites with various Sm₂O₃ contents after being hot-pressed at 1 250 °C for 2 h. The fracture microstructure of the composites is characterized by numerous brighter reinforcing phase of Al₂O₃ and black matrix phases of TiAl, Ti₃Al and SmAl. The fracture surface is uneven and few pores exist. The brighter areas present interpenetrating network structure. The Sm₂O₃ doping results in finer microstructure and more  uniform distribution of Al₂O₃ particles without obvious agglomeration. It is attributed to Sm₂O₃ that the wettability between Al₂O₃ and TiAl increases because of its surface-active action [10].

Figure 3 shows that composites consist of single phase regions of γ-TiAl, lamellar regions of γ-TiAl+α₂-Ti₃Al with a dispersion of randomly oriented Al₂O₃ particles and SmAl distributing at grain boundaries, where most of the Al₂O₃ particles have smaller average grain size. These observations are found to be in agreement with the inferences drawn from the XRD studies. The average grain size of the composites with Sm₂O₃ addition was 0.3-0.5 ?m. According to Ref. [11], the additions of other beta-stabilizing elements are beneficial to preventing peritectic growth. In this work, the addition of Sm₂O₃ hinders the growth of the TiAl matrix and Al₂O₃ grains and re?nes the grain size.

Figure 4 shows the crack propagation paths of the Al₂O₃/TiAl composites. Microstructures of the representative crack propagation paths reveal that variously complex toughening mechanisms can be found in the as-synthesized composite including extensive crack deflection, crack bridging, particle pullout, which mostly results from particles toughening effect [12].

Fig. 3 Fracture microstructures of Al₂O₃/TiAl composites with various Sm₂O₃ contents: (a), (a′) Without Sm; (b), (b′) With 5%Sm

Fig. 4 Crack propagation paths of Al₂O₃/TiAl composites

Figure 5 shows the density of the Al₂O₃/TiAl composite sintered at 1 250 °C for 2 h as a function of Sm₂O₃ content measured by the Archimedes method. The densities of Al₂O₃/TiAl increased steadily from 3.65 to 3.9 g/cm³ with the Sm content increasing from 0.2% to 6%. Samples with the addition of Sm₂O₃ have higher densities due to their lower melting points [13], which increases densities of the TiAl-based composites. The second reason must be the higher densities of Sm₂O₃ itself, which also improves the densities of the composites. Finally, the densities of the composites increase due to the fine Al₂O₃ particles filling the pores.

Figure 6 presents the Vickers hardness of samples as a function of Sm₂O₃ content. A closer observation of the Vickers hardness data reveals that the hardness of the as-sintered composites goes up from HV₁₀ 470 to HV₁₀ 655.2 with the increase of Sm mass fraction from 0.2% to 6%. The increase of hardness can be correlated with the amount of SmAl reinforced phase. Meanwhile, the SmAl phases have a higher melting point and hardness than γ or α₂ phase [14], filling a part of the pores, which significantly increases the hardness of the composites.

Fig. 5 Density of Al₂O₃/TiAl composite as function of Sm₂O₃ content

As shown in Fig. 7, the variation trend of the fracture toughness is similar with that of the flexural strength. The fracture toughness and flexural strength increase with the increase of Sm mass fraction from 0.2% to 5%. With 5% Sm addition, the fracture toughness and flexural strength reach the maximum values of 10 MPa·m^1/2 and 659 MPa, respectively. When the mass fraction of Sm₂O₃ is further continuously increased over 5%, both the flexural strength and fracture toughness decrease. Referred to previous work [15], the strength of the composites generally increases with decreasing size of the reinforcements. The addition of Sm₂O₃ decreases the size of the reinforcements, which is beneficial for the improvement of the mechanical properties of the Al₂O₃/TiAl composites [16]. However, the fracture toughness and the flexural strength decrease rapidly with excess Sm₂O₃ addition due to the excessive formation of the brittle SmAl phase.

Fig. 6 Vickers hardness of products as function of Sm₂O₃ content

Fig. 7 Flexural strength and fracture toughness of products as function of Sm₂O₃ content

4 Conclusions

1) Al₂O₃/TiAl composites were fabricated by in-situ reaction synthesis using Ti, Al, TiO₂ and Sm₂O₃ as starting powders. The as-sintered samples mainly consist of α₂-Ti₃Al, γ-TiAl, Al₂O₃ and SmAl phases.

2) The grain size of TiAl matrix decreases with the addition of Sm₂O₃ due to the uniform distribution of the in situ formed fine Al₂O₃ particles. The densities of the composites increase gradually from 3.5 g/cm³ to 3.9 g/cm³ and Vickers hardness increases from 340 to 655 kg/mm² with increasing Sm content from 0 to 6%. Both the flexural strength and fracture toughness are modified and reach the maximum values of 659 MPa and 10 MPa·m^1/2 respectively when Sm₂O₃ mass fraction is 5%.

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Sm₂O₃掺杂强韧化TiAl复合材料

王晓凤, 王 芬, 朱建锋, 向六一

陕西科技大学 材料科学与工程学院，教育部轻化工助剂化学与技术重点实验室，西安710021

摘  要：以Ti，Al，TiO₂和Sm₂O₃为原料，利用原位合成法制备Al₂O₃/TiAl复合材料；并借助XRD、SEM和力学性能测试，研究Sm₂O₃掺杂对Al₂O₃/TiAl复合材料微观结构和力学性能的影响。结果表明：掺杂Sm₂O₃的Al₂O₃/TiAl复合材料由γ-TiAl/α₂-Ti₃Al基体相以及Al₂O₃、SmAl增强相组成；掺杂Sm₂O₃细化了复合材料的微观结构，改善了TiAl复合材料的力学性能；当Sm含量为5%(质量分数)时，该复合材料的弯曲强度和断裂韧性达到最大，分别为658.9 MPa 和10.13 MPa·m^1/2。

关键词：TiAl；Sm₂O₃；晶粒尺寸；力学性能

(Edited by LI Xiang-qun)

Foundation item: Project (51072109) supported by the National Natural Science Foundation of China; Project (XK06010-1) supported by the Foundation of Xianyang, China; Project supported by the Graduate Innovation Fund of Shaanxi University of Science and Technology, China; Project (BJ08-14) supported by the Education Foundation of Shaanxi University of Science and Technology, China

Corresponding author: WANG Fen; Tel: +86-15114805183; E-mail: wangf@sust.edu.cn

DOI: 10.1016/S1003-6326(11)60851-0