Highly porous open cellular TiAl-based intermetallics fabricated by thermal explosion with space holder process
Graphical abstract
Introduction
Inorganic porous materials have been widely investigated attributed to their outstanding performances such as large surface area, low density, mechanical stability and good permeability [1], [2], [3]. In general, porous ceramics possess inherent advantages of high melting points, high corrosion and wear resistance, low thermal mass and thermal conductivity, thus they find applications in catalyst supports, wastewater treatment, heat insulation and metal filtration [4], [5], [6]. However, the brittleness and poor weldability have limited their applications due to difficulties in fabrication of large components by joining. Though porous metals have advantages over ceramics, like superior toughness and weldability, they exhibit poor corrosion resistance in acid/alkali environment. Porous TiAl-based intermetallic compounds with a mixture of metallic and covalent bonds combine advantages of ceramics and metals, exhibiting excellent performances such as superior mechanical properties, low density, good acid/alkali corrosion resistance and oxidation resistance at elevated temperatures [7], [8], [9], [10], [11], which enable them good candidates for industrial application.
Jiang et al. [12] have prepared porous TiAl membranes through reactive synthesis from Ti and Al powders based on Kirkendall effect, and the sintering process was completed in more than 20 h with an average heating rate of 0.8 °C/min. Liang et al. [13] have fabricated porous TiAl–Nb alloys based on Kirkendall effect through a 16.25 h sintering process accompanied with a low heating rate and six temperature holding platforms. It is important to mention that these researches employ long-time sintering processes for preparing porous TiAl-based materials by Kirkendall effect, which indicates a high-energy consumption fabrication route. However, combustion synthesis (CS) offers an attractive method for preparing intermetallic compounds with high melting point in short time. The CS reaction is highly exothermic in nature, and it can be ignited at relatively low temperature. Once ignited, it completes in a self-sustaining mode, indicting a simple and low-energy consumption method [14], [15]. CS involves two reaction modes, I) self-propagating high-temperature synthesis (SHS) and II) thermal explosion (TE). SHS is ignited by heating one end of the reactant compact and typically results in deformation of compact [16], [17], which is not preferable for preparing final products with desirable shapes in one step. Contrary to SHS, TE is ignited by heating the whole reactant compact, and the specimens keep original shape and integrity before and after the reaction. The fabrication of Fe-Al [17], TiC–Cu [18] and Ti–Al [19] intermetallics through TE has been reported.
Up to now, various technological routes have been proposed, like sacrificial template [20], extrusion [21] and space holder process [22] to fabricate porous ceramic, intermetallic and metallic materials. Extrusion is limited to prepare compacts with directional pores. The sacrificial template method is not environment-friendly, since templates are usually made from polymeride, and the decomposition of polymeride is harmful to environment. Space holder process involves production of products with tailored pores where space holder particles present at first, followed by the removal of space holder particles. This method represents an environment-friendly and economical route with ease of fabrication and capable of direct control of pore size and porosity [22]. Kobashi et al. [23] fabricated porous Ti–Al materials by SHS using NaCl (melting point 801 °C) as space holder, and NaCl was removed by water leaching after SHS. However, the porosity, pore shape and size were restricted due to melting of NaCl during SHS. The same problem has been observed in fabrication of other porous materials with high sintering temperature above 801 °C e.g., Ti [24] and NiTi [25]. Besides, the remains of corrosive NaCl in TiAl monoliths, fabricated by Kobashi et al., [23], accelerate oxidation of TiAl and have a negative effect on the furnace [26]. Thus finding an improved method to remove NaCl particles properly is necessary to guarantee porous materials with a tailored pore size and structure.
In present work, a novel method of combining TE with space holder process is proposed for fabricating highly open cellular porous TiAl-based intermetallics, for the purpose of expanding the range of their structures and applications. NaCl is removed absolutely before TE and sintering. The effects of different volume fraction of NaCl particles on porosity and pore structure have been investigated. Additionally, the process of the TE has been analyzed.
Section snippets
Experimental
Elemental Ti (<50 μm, 99.6% purity, WODETAI (Beijing) Co. Ltd., China, AR) and Al (<50 μm, 99.0% purity, QIANGSHENG (Jiangsu) chemical Co. Ltd., China, AR) powders were used as reactant materials. The NaCl (200–500 μm, 99.5% purity, SUYI (Shanghai) chemical Co. Ltd., China, AR) particles were used as space holder particles. The whole experimental process was illustrated in Fig. 1. Ti and Al powders mixture of 1:1 ratio was homogenized in a planetary ball mill (QM-ISP 2CL, Nanjing NanDa
Results and discussion
Fig. 2 illustrates the sintering temperature as a function of time, in which two temperature holding platforms are indicated, and the heating rate to holding temperatures adopted is 5 °C/min. The sintering process is completed within 6 h, which suggests a lower-energy consumption route compared to Jiang et al. (>20 h) [12] and Liang et al. (16.25 h) [13]. Fig. 3 shows the temperature profile of specimen during the holding stage at 600 °C in tube furnace. It clearly shows that TE occurs in the
Conclusions
- (1)
A time and energy saving sintering process with heating rate of 5 °C/min and the maximum sintering temperature of 1100 °C, was successfully conducted for fabricating TiAl-based intermetallic compounds within 6 h. A TE occurs at 600 °C with TiAl3, TiAl2, Ti3Al and TiAl synthesized. By increasing temperature up to 1100 °C, TiAl is the dominated phase in the final products.
- (2)
The porous TiAl-based intermetallics possess a hierarchical pore structure and a high open porosity of 72–84% when 60–80 vol.%
Acknowledgment
This work was supported by the Fundamental Research Funds for the Central Universities (2013RC18) and National Natural Science Foundation of China (51574241).
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