Trans. Nonferrous Met. Soc. China 26(2016) 1878-1884

Microstructure and evolution of (TiB₂+Al₂O₃)/NiAl composites prepared by self-propagation high-temperature synthesis

Xiao-jie SONG¹, Hong-zhi CUI¹, Li-li CAO¹, P. Y. GULYAEV²

1. School of Materials Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China;

2. Polytechnic Institute, Yugra State University, Rhanty-Mansiysk 628012, Russian Federation

Received 5 June 2015; accepted 3 March 2016

Abstract: (TiB₂+Al₂O₃)/NiAl composites were synthesized by self-propagation high-temperature synthesis, and their phase compositions, microstructures and evolution modes were studied. The microstructures and shapes vary with the TiB₂+Al₂O₃ content in the NiAl matrix. TiB₂ particles take a great variety of elementary shapes such as white bars, plates, herringbones, regular cubes and cuboids. These results outline a strategy of self-assembly processes in real time to build diversified microstructures. Some TiB₂ grains in sizes of 2-5 μm are embeded in Al₂O₃ clusters, while a small number of TiB₂ particles disperse in the NiAl matrix. It is believed that the higher the TiB₂+Al₂O₃ content is, the more the regular shapes and homogeneous distributions of TiB₂ and Al₂O₃ will be present in the NiAl matrix.

Key words: (TiB₂+Al₂O₃)/NiAl composites; self-propagation high-temperature synthesis; microstructure; evolution mechanism

1 Introduction

Due to its low density (5.59 g/cm³), high melting point (1911 K), good thermal conductivity and good corrosion resistance [1], the NiAl intermetallic compounds have a wide application for components in high-temperature and corrosive environments, such as aero-engines, blades of gas turbine, thermal barrier coatings, and corrosion-resisting coatings. However, the low plasticity at room temperature and low strength at high temperature limit their applications in industry and production [2,3].

Up to now, many publications have involved in improving the room temperature plasticity and high temperature strength of Ni-Al intermetallic compounds. Adding different alloy elements such as Mo [4], Ti [5], Cr [6], and rare-earth metals [7] in NiAl is an efficient way to improve its properties. By adding or forming ceramics particles, such as Al₂O₃ [8,9], TiB₂ [10], NbB₂ [11] and TiC [12,13], the strength and creep properties of Ni-Al composite material can also be greatly improved. MICHALSKI et al [14] fabricated a NiAl-Al₂O₃ composite with different amounts of Al₂O₃ in NiAl through spark plasma sintering and self- propagation high-temperature synthesis (SHS) of Ni and Al, and the results showed that the hardness of NiAl-Al₂O₃ increased and the fracture toughness of it was almost twice that of NiAl. The fabrication of NiAl matrix composites was carried out by a variety of novel processes such as combustion synthesis [11,14], mechanical alloying [13] and high pressure reaction sintering. It has been proved that combustion synthesis has significant advantages, such as low energy consumption, inexpensive fabrication equipment, simplicity of operation and in situ synthesis of composite components, compared with other conventional methods. Via combustion synthesis procedure, a series of NiAl matrix composites reinforced by TiB₂, NbB₂, TiC, Al₂O₃, ZrB₂ [15] and TiN [16] were obtained conveniently.

At present, many researches focus on TiB₂ and Al₂O₃ ceramics as a duplex reinforced phase in different matrices because of their similar thermal expansion coefficient, chemical and physical compatibility [17]. By adding TiB₂ into Al₂O₃ ceramics, the growth of Al₂O₃ grains and the propagation of cracks in the matrix can be prevented. Consequently, the fine and well-distributed TiB₂ phase would help to enhance the strength and fracture toughness of the composite material. Thus, it would contribute to improving the abrasion resistance and fracture toughness in cutting tool materials [18,19].

The study on the previous work indicates that the fabrication of TiB₂+Al₂O₃/NiAl composite materials through combustion synthesis has not been fully investigated. The fundamental objective of the present study is to investigate the microstructures and their evolution modes when different TiB₂+Al₂O₃ contents were formed in the NiAl matrix, using Ni, Al, B₂O₃ and TiO₂ as raw materials, via combustion synthesis. Also, the correlation between the TiB₂+Al₂O₃ content, microstructure and properties of the composites was investigated.

2 Experimental

The raw materials are commercial pure Ni powder (99.9%, 38-50 μm), Al powder (99.9%, 75 μm), B₂O₃ powder (98%, 75 μm), and TiO₂ powder (98%, 45 μm). The powder mixtures were considered as two parts: one was Ni+Al in which the mole ratio was fixed to n(Ni):n(Al)=1:1, the other was Al+TiO₂+B₂O₃ in which the mole ratio was fixed to n(Al):n(TiO₂):n(B₂O₃)= 10:3:3, and the latter was added into the former in mass fractions of 0, 5%, 10%, 15%, 20%, 25% and 30%, respectively. In order to produce a homogeneous mixture, all the powders were mixed in a three– dimension blender for about 6 h. Then, the mixtures were poured into metal molds with dimensions of d20 mm × 20 mm and pressed at a pressure of 150 MPa into cylindrical compacts. Seven typical green compacts were fabricated with different contents of Al+TiO₂+B₂O₃. In order to make the reactants react completely, the green compacts were preheated in an electric furnace at 300 °C for 30 min. After that, the compacts together with the molds were taken out and a small amount of Mg powders was poured onto the surface as the initiating combustion agent. At last the compacts were ignited by live tungsten wire immediately. The reactions between powders in raw materials were as follows:

Ni+Al=NiAl                               (1)

10Al+3TiO₂+3B₂O₃=3TiB₂+5Al₂O₃         (2)

Under the present condition, the reaction in green compact could self-sustain due to its high exothermic heat (SHS), the process is shown in Fig. 1.

The crystalline phases of the products were characterized by X-ray diffraction (XRD, Model D/Max 2500PC Rigaku, Japan). The morphologies of the fracture and microstructure were observed by scanning electron microscopy (SEM, Model KYKY2800B) and field emission scanning electron microscopy (FESEM, NOVA NANOSEM 450, FEI). The compositions of samples were analyzed using electron probe microanalysis (EPMA, JXA-8230). The microhardness was measured by FM-700 with the load of 1 N and sustaining for 10 s. For each compound, five indentations were taken in the microhardness test and made an average of these points with a consideration of relative errors.

Fig. 1 Schematic diagrams of SHS reaction initiated by electrifying tungsten wire (a) and SHS reaction (b)

3 Results

In the sample without Al+TiO₂+B₂O₃, the Ni+Al reaction wave propagated steadily, and the speed was about 25 mm/s. When adding Al+TiO₂+B₂O₃ into Ni+Al, the igniting delay time prolonged. Once the reaction was initiated, the reaction wave spread rapidly. When the content of Al+TiO₂+B₂O₃ increased to 15%, the spreading speed reached up to 40 mm/s. Thus, the whole reaction in the compacts almost took place at the same time, similar to thermal explosion. After the reactions finishing, the samples were slowly cooled down.

3.1 Phase analysis

Figure 2 shows the XRD pattern of the raw materials. It illustrates that the powder mixture is composed of Ni, Al, TiO₂ and B₂O₃. Figure 3 shows the XRD patterns of products obtained by adding different contents of Al+TiO₂+B₂O₃.

Fig. 2 XRD pattern of reactants

Fig. 3 XRD patterns of NiAl products with different contents of Al+TiO₂+B₂O₃

The reaction products simply consist of NiAl when the mixture contains only Ni and Al, which is in accordance with Reaction (1). After adding Al+TiO₂+ B₂O₃, TiB₂ and Al₂O₃ ceramic phases appear besides NiAl phase. The higher the reactant content of Al+TiO₂+B₂O₃ is, the more the TiB₂ and Al₂O₃ ceramic phases can be generated. Moreover, there are hardly any unreacted raw materials. Thus, it can be deduced that the reactions were completely carried out, and the phases which are expected according to Reaction (2) are obtained.

Fig. 4 SEM images of NiAl products with different contents of Al+TiO₂+B₂O₃

3.2 Microstructure and hardness

The microstructures on polished surface of samples with different contents of ceramics are shown in Fig. 4. In the samples only containing Ni+Al, the NiAl grains are coarse with a lot of needles and parallel plates, as shown in Figs. 4(a) and (b), and this is in accordance with Ref. [20]. After adding Al+TiO₂+B₂O₃, some white bars and plates (~5 μm) distributed along the boundary of the matrix appear, and the grains of NiAl matrix are refined remarkably, as shown in Figs. 4(c) and (d). Furthermore, some plates (10-15 μm) are alternatively distributed with a kind of grey phase, and their distributional patterns are very similar to herringbone structure. When the content of Al+TiO₂+B₂O₃ increases to 20% , the white phase turns to be fine regular particles (smaller than 5 μm), mainly in the shape of cubes or cuboids gathering together as clusters on an irregular dark grey conglomerations, as shown in Fig. 4(e). Also the aggregation (60-70 μm) of white particle clusters on the irregular conglomerations and some white particles still distribute along the grains boundary of the matrix. The white particles seem to be inlaid in the dark grey conglomerations. With further increasing the content of Al+TiO₂+B₂O₃, the amount of white particles increases significantly. But the size of aggregations of white particles, becomes smaller. In addition, some white particles disappear in the matrix, as shown in Fig. 4(f).

The microhardness test results are listed in Table 1. The average microhardnesses of the matrix in NiAl products without and with 10% (Al+TiO₂+B₂O₃) are HV₁₀₀ 398 and HV₁₀₀ 501, respectively, which are close to the hardness of pure NiAl [20]. The microhardness of herringbone structure in the product with 10% (Al+ TiO₂+B₂O₃) is HV₁₀₀ (1160-1730), which fluctuates remarkably. Compared with the product with 10% (Al+TiO₂+B₂O₃), the hardness of white cube or cuboids gathering clusters in the products with 20% and 30% (Al+TiO₂+B₂O₃) is higher, and the highest hardness is up to HV₁₀₀ 2700. The hardness of dark grey irregular conglomerations which look like being pinned under the cube or cuboid particles is HV₁₀₀ (1260-1480). The hardness of the products with 20% and 30% (Al+TiO₂+ B₂O₃) also varies from HV₁₀₀ 570 to HV₁₀₀ 1420, and the highest hardness is close to that of dark grey conglomerations.

Table 1 Microhardness and adiabatic temperature of samples with different contents of Al+TiO₂+B₂O₃

3.3 Composition

The SEM images of typical samples and the results of EDS composition analysis are shown in Fig. 5 and Tables 2 and 3, respectively. Combining the results of XRD analysis, microhardness test and EDS composition analysis, we can deduce that the white bars and plates which distribute along the boundary of the matrix or in herringbone structure at Spectrum 1 in Fig. 5(a) and the white particles in regular cube or cuboids shapes shown in Figs. 4(e) and (f) and Spectrum 3 in Fig. 5(b) are all TiB₂ grains. The gray matrix shown in Figs. 4(a) and (c) and Spectrum 2 in Figs. 5(a) and (b) is NiAl intermetallic compound. The irregular dark grey clusters which are around or under the cube or cuboids particles in Figs. 4(e) and (f) and Spectrum 3 in Figs. 5(a) and (b) are Al₂O₃.

Fig. 5 SEM images of products with different contents of Al+TiO₂+B₂O₃

Table 2 EDS compositions of composite with 5% (Al+ TiO₂+B₂O₃)

Table 3 EDS compositions of composite with 15% (Al+ TiO₂+B₂O₃)

4 Discussion

The mixture of reactants could be considered as two parts: one is Ni+Al which follows Reaction (1), the other is Al+TiO₂+B₂O₃ which follows Reaction (2). If the reaction systems are initiated at room temperature, the adiabatic temperature of Reaction (1) would be T_ad=T_m,_NiAl=1911 K [21], while the adiabatic temperature of Reaction (2) would be T_ad=2448 K [22]. When preheating the samples at 300 °C, the adiabatic temperatures of Reactions (1) and (2) at 300 °C are much higher than their T_ad at room temperature. The T_ad of Ni+Al+TiO₂+B₂O₃ hybrid system can be calculated according to the thermodynamic data provided in Ref. [23]. With the increase of Al+TiO₂+B₂O₃ content, the T_ad of the mixtures rises to above 1911 K. The adiabatic temperatures of samples with different mass fractions of Al+TiO₂+B₂O₃ are listed in Table 1.

The reaction processes and microstructure evolution mechanisms of the system are shown in Fig. 6. As the melting points of Al and B₂O₃ are 660 and 445 °C, respectively, Al and B₂O₃ in the green powder (Fig. 6(a)) melt firstly during the reaction proces. The melted Al contacts with liquid B₂O₃ and infiltrates into the surface of TiO₂ particles, as shown in Fig. 6(b). Thus, B and Ti elements are displaced from B₂O₃ and TiO₂, respectively [24,25]. At the same time, Al₂O₃ and TiB₂ are produced rapidly through Reaction (2). Moreover, the surplus molten Al and solid Ni react to from liquid NiAl, as the melting point of NiAl is lower than the adiabatic temperature of reaction system, as shown in Fig. 6(c). The sequences of the whole reaction are as follows:

Ni+Al→NiAl                               (3)

2Al+B₂O₃→Al₂O₃+2B                        (4)

4Al+3TiO₂→2Al₂O₃+3Ti                     (5)

Ti+2B→TiB₂                               (6)

Fig. 6 Sketch of reaction processes and structure evolution modes

When the content of Al+TiO₂+B₂O₃ is less than 10%, both the amounts of TiB₂ and Al₂O₃, and the temperature of the reaction system are not sufficient. Therefore, the TiB₂ grains grow as irregular bars and plates [26,27] (Figs. 4(d) and 6(d)). TiB₂ and Al₂O₃ grow into herringbone alternative structure through “self- assembly” mode. When the reaction system cools down below the NiAl meting point, NiAl grains begin to nuclear and grow to form the matrix of composite gradually.

With increasing the content of Al+TiO₂+B₂O₃, the T_ad of reaction system increases and a great amount of TiB₂ and Al₂O₃ is produced. The abundant TiB₂ resources and higher temperature are better for the regularization of TiB₂ [26]. Thus, TiB₂ particles grow freely, develop fully and take shapes of cube or cuboids finally. As Al₂O₃ grains form around the TiB₂ grains almost simultaneously, it is possible for them to attach together to form large irregular conglomerations, as shown in Figs. 4(e) and (f) and 6(e) and (f). During the grow process of NiAl grains, the conglomerations are pushed to the corner of grains or distribute along the boundaries. Also, this kind of distribution impedes the boundary’s movement and restricts the grain’s growth of NiAl matrix.

Fig. 7 SEM images of regular TiB₂ particles (a) and Al₂O₃ conglomerations (b)

When the Al+TiO₂+B₂O₃ content increases to 30%, much higher T_ad can be reached and more TiB₂ and Al₂O₃ are produced. All the above contribute a lot to TiB₂ grains to grow as regular cubes cuboids or hexagons, as shown in Figs. 4(e) and (f) and 7(a). In addition, a large amount of Al₂O₃ adheres to TiB₂ grains to nuclear, as shown in Fig. 7(b), and grows around them. Because there are no plates to subdivide the spaces between TiB₂ grains, as shown in Fig. 4(b), the Al₂O₃ grains develop more freely until touching each other to form conglomerations, as shown in Figs. 4(c) and (d). In the meantime, there are also some single TiB₂ cubes or cuboids dispersed in the NiAl liquid and some Al₂O₃ grains adhere to the impurities in NiAl liquid to nuclear. Finally, when NiAl begins to solidify, almost all the above solid phases that form before NiAl will be pushed to the grain boundaries in the similar way shown in Figs. 4(b) and 6(e), which makes contribution to the formation of refine NiAl grains. What is more, the Al₂O₃ grains which nucleate on the impurities may remain in the NiAl matrix to help to improve the hardness. These Al₂O₃ grains may flock together and become big conglomerations because of the self-diffusion of Al₂O₃. Therefore, the hardness of matrix in Fig. 4(c) varies greatly, and the highest hardness is close to that of pure conglomerations.

5 Conclusions

1) (TiB₂+Al₂O₃)/NiAl composites were synthesized by combustion synthesis using Ni, Al, TiO₂ and B₂O₃ mixed powders. With increasing the content of Al+TiO₂+B₂O₃, more TiB₂ and Al₂O₃ ceramic phases are generated in the NiAl matrix. When the reactant content of Al+TiO₂+B₂O₃ is less than 10%, TiB₂ and Al₂O₃ grow into herringbone alternative structure through self-assembly mode.

2) When more TiB₂+Al₂O₃ form, TiB₂ particles would grow freely and develop fully and take shapes of cube, cuboids or hexagons finally. At the same time, more Al₂O₃ which forms around TiB₂ grains tends to attach together to form large irregular conglomerations.

3) The microhardness of NiAl matrix composite increases by 50% from HV (398±19) to HV (608±31), due to grain refining with increasing the content of TiB₂+Al₂O₃ to 25%.

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自蔓延高温合成(TiB₂+Al₂O₃)/NiAl复合材料的显微组织及演化机制

宋晓杰¹，崔洪芝¹，曹丽丽¹，P. Y. GULYAEV²

1. 山东科技大学 材料科学与工程学院，青岛 266590；

2. Polytechnic Institute, Yugra State University, Rhanty-Mansiysk 628012, Russian Federation

摘  要：采用自蔓延高温合成技术制备(TiB₂+Al₂O₃)增强NiAl基复合材料，并研究其相组成、显微组织和演化机制。结果表明：复合材料的组织及陶瓷相的形貌随添加(TiB₂+Al₂O₃)含量的不同而变化，TiB₂在短时间内以自组装的模式形成长条状、片状、鱼骨状以及规则的四方体和六棱柱等不同形态。TiB₂的尺寸在2~5 μm之间，与不规则的Al₂O₃伴生存在，少量TiB₂分布于NiAl基体中。随着陶瓷相含量的增多，TiB₂的形状趋于规则，且在NiAl基体中的分布更加均匀。

关键词：(TiB₂+Al₂O₃)/NiAl复合材料；自蔓延高温合成；显微组织；演化机制

(Edited by Mu-lan QIN)

Foundation item: Project (51272141) supported by the National Natural Science Foundation of China; Project (ts20110828) supported by the Taishan Scholars Project of Shandong Province, China; Project (2015AA034404) supported by the Ministry of Science and Technology of China

Corresponding author: Hong-zhi CUI; Tel: +86-532-86057929; E-mail: hzhcui@163.com

DOI: 10.1016/S1003-6326(16)64265-6