Synthesis of Ti2AlC by hot pressing and its mechanical and electrical properties
WANG Ping(王 苹)1, 2, MEI Bing-chu(梅炳初)2, HONG Xiao-lin(洪小林)2, ZHOU Wei-bing(周卫兵)2
1. School of Science, Wuhan University of Technology, Wuhan 430070, China;
2. State Key Laboratory of Advanced Technology for Materials Synthesis and Processing,Wuhan University of Technology, Wuhan 430070, China
Received 20 November 2006; accepted 10 May 2007
Abstract: Ti2AlC bulk material was synthesized by hot pressing of mixture powders of TiC, Ti, Al and active carbon. The phase compositions of resultant product at different temperature were detected by X-ray diffractometer. The microstructures of the samples were observed by SEM. Finally, the mechanical properties and thermal properties of the sample at 1 400 ℃ were measured. The results show that high purity Ti2AlC material with little Ti3AlC2 can be synthesized by hot pressing 0.5TiC/1.5Ti/1.0Al/0.5C at 1 400 ℃. Ti2AlC exhibits high mechanical properties and metallic electrical properties. Its fracture toughness is 7.0 MPa?m1/2, its flexural strength is 384 MPa at room temperature, and its electrical conductivity is 2.56×106 Ω-1?m-1 at room temperature.
Key words: hot pressing; Ti2AlC; mechanical properties; electrical properties
1 Introduction
Layered ternary carbide Ti2AlC has attracted increasing attention because it is a remarkable material that combines many of the merits of both metals and ceramics[1-12]. Like ceramics it has high mechanical properties, and like metals it is an excellent electrical conductor at room and high temperatures. Recently, the synthesis methods of Ti2AlC developed its mechanical properties and electrical properties. According to previous literatures[13-14] TiC replacing a part of Ti and C elemental powder as a raw material can accelerate the reactions and was helpful for forming Ti2AlC. In this study, Ti2AlC bulk material with fantastic mechanical properties and electrical properties is synthesized by hot pressing 0.5TiC/1.5Ti/1.0Al/0.5C powder mixture. Active carbon rather than graphite is used as a raw material due to its high activity.
2 Experimental
Commercially available TiC (99.0% in purity, 11.8 μm), Ti (99.0% in purity, 10.6 μm), Al (99.8% in purity, 12.8 μm) and active carbon (99% in purity, 13.2 μm) (all from the General Research Institute of Nonferrous Metals, Beijing, China) were used as raw materials to synthesize Ti2AlC. The mixture of 0.5TiC/1.5Ti/1.0Al/ 0.5C was firstly mixed in ethanol for 24 h, and was filled into graphite crucibles, then hot pressed at 1 300, 1 400, 1 450 and 1 500 ℃ for 60 min under a pressure of 30 MPa in flowing Ar atmosphere. After being hot pressed, the compacts were heated according to the following procedures: heating at a rate of 5 ℃/min to 300 ℃, then at 60 ℃/min to the final temperature. The soaking time was 60 min.
Phase identification of the material prepared at different temperatures was conducted via a X-ray diffractometer (D/MAX-RB, RIGAKU Corporation, Japan) after the graphite layer of the samples surface was removed. The microstructure evolution of the hot pressed material was investigated by scanning electron micro- scope (JSM-5610LV, JEOL Ltd., Japan).
The mechanical properties of Ti2AlC were measured by using omnipotence ceramic material test machine (MTS-810 Ceramic Test System, USA). Moreover, the Vicker’s hardness of Ti2AlC was measured using Leitz Micro-hardness Tester (Leitz Wetzlar, Germany) at 0.3 mm/s and a load of 0.98 N.
The electrical conductivity of Ti2AlC at room temperature and high temperature was measured using standard four-probe method (Thermo Electricity Test System, AEM-I, Japan) in Ar atmosphere.
3 Results and discussion
3.1 XRD patterns of Ti2AlC
Fig.1 displays the XRD patterns of samples prepared by hot pressing 0.5TiC/1.5Ti/1.0Al/0.5C at different temperatures. It can be seen from Fig.1(a) that Ti2AlC is the main crystal phase at 1 300 ℃, and the samples contains quite a large amount of TiC and Ti-Al intermetallics. The XRD result of the sample prepared at 1 400 ℃ (Fig.1(b)) shows the peaks of Ti2AlC with little Ti3AlC2. At the same time, other peaks disappear. The XRD pattern of sample prepared at 1 450 ℃ (Fig.1(c)) is similar to that at 1 400 ℃. With further increasing the temperature, other new phase Ti3AlC2 is formed obviously (Fig.1(d)) although the main phase is also Ti2AlC at 1 500 ℃. Fig.1 indicates that Ti2AlC with little Ti3AlC2 can be synthesized at 1 400 ℃ and 1 450 ℃ by hot pressing TiC, Ti, Al and active carbon.
Fig.1 XRD patterns of samples synthesized by hot pressing 0.5TiC/1.5Ti/1.0Al/0.5C powder mixtures at 1 300 ℃ (a), 1 400 (b), 1 450 ℃ (c) and 1 500 ℃ (d)
3.2 Microstructure of Ti2AlC
Fig.2 shows the microstructures of Ti2AlC prepared by hot pressing at different temperatures. For the sample prepared at 1 300 ℃, a part of product has a tendency to form layered structure. But the majority is small crystal grains, and the size of them is between 2 and 8 μm. According to the XRD patterns of Fig.1(a), they are TiC and Ti-Al intermetallics. In the sample prepared at 1 400 ℃, the layered Ti2AlC develops perfectly and layered characteristics become more obvious than that at 1300 ℃. The mean size of layered structure is 15 μm or so. As the synthesis temperature is elevated to 1 500 ℃, however, the edge of layered structure becomes unclear like dissolution and the size (below 10 μm) is smaller than that at 1 400 ℃.
Fig.2 Microstructures of samples fabricated by hot pressing powder mixture at 1 300 ℃ (a), 1 400 ℃ (b) and 1 500 ℃ (c)
The synthesis procedure of Ti2AlC by hot pressing can be described as the following possible reactions:
Al(s)→Al(l) (1)
Al(l)+Ti(s)→Al-Ti intermetallics(s) (2)
Ti(s)+C(s)→TiC(s) (3)
TiC(s)+Al-Ti intermetallics→Ti2AlC(s) (4)
2Ti2AlC→Ti3AlC2 (s)+Al-Ti intermetallics(s) (5)
According to previous literature[14], TiC is a key intermediate matter during the synthesis of Ti2AlC. Ti and C must react into TiC to form Ti2AlC with Al-Ti intermetallics if there is no TiC in raw materials (reaction (1)-(4)). However, because the formation of TiC from Ti and C is a strongly exothermic reaction[15], the temperature of green sample is much higher than that by hot pressing. Furthermore, the vapor pressure of Al in Ti-Al-C system is very high, so the loss of Al increases. If TiC is doped, the heat of reaction released between Ti and C decreases, so the temperature of green sample goes down. Due to the loss of Al decreasing, the synthesizing speed and the purity of Ti2AlC are improved. But doping too much TiC cannot improve the synthesizing speed of Ti2AlC when the amount of TiC reaches a value that is enough to form Ti2AlC. As the temperature reaches about 1 500 ℃, a part of Ti2AlC will react as reaction (5), and new phase Ti3AlC2 appears. Thus, doping TiC will help to improve the synthesizing speed of Ti2AlC.
3.3 Mechanical properties of Ti2AlC
As mentioned above the sample prepared at 1 400 ℃ is highly pure and dense Ti2AlC. Its density is 4.10 g/cm3 measured by Archimedes method, which is close to the theoretical density of 4.11 g/cm3. Fig.3 shows the micrograph of fracture surfaces of Ti2AlC. Rectangular bars for mechanical properties testing were cut from the hot pressed bulk by wire cutting, and the sizes of samples are listed in Table 1.
Fig.3 Fractographs of Ti2AlC samples
Table 1 Size of samples for measurement of mechanical properties
The test results of mechanical properties can be seen in Table 2. The fracture toughness of Ti2AlC reaches 7.0 MPa?m1/2 and its flexural strength is 384 MPa. Moreover, its Vicker’s hardness is between 4.2 GPa and 5.7 GPa.
Table 2 Mechanical properties of Ti2AlC
3.4 Electrical properties of Ti2AlC
Fig.4 shows the measured electrical resistivity of Ti2AlC as a function of temperature. It is seen from the figure that, the electrical resistivity of Ti2AlC increases linearly with increasing temperature, which is typical of metallic conductors. According to the fitting line of electrical resistivity in Fig.4, the temperature coefficient of electrical resistance of Ti2AlC in the temperature range of 300-500 K is 2.3×10-3 K-1. The electrical resistivity of Ti2AlC is 0.39 μΩ?m which is corresponded with electrical conductivity of 2.56×106 Ω-1?m-1 at 300 K. They become 0.55 μΩ?m and 1.83×106 Ω-1?m-1 at 480 K respectively. This fact proves that Ti2AlC exhibits a good electrical conductivity.
Fig.4 Temperature dependence of electrical resistivity of Ti2AlC at 300-500 K
4 Conclusions
1) Bulk material of Ti2AlC containing quite little Ti3AlC2 with density of 4.10 g/cm3 is synthesized by hot pressing TiC, Al, Ti and active carbon at 1 400 ℃ for 60 min under a pressure of 30 MPa. Doping TiC will help to improve the synthesizing speed of Ti2AlC.
2) The fracture toughness of Ti2AlC synthesized at 1 400 ℃ is 7.0 MPa?m1/2, the flexural strength is 384 MPa at room temperature, and the compressive strength is 670 MPa. It is an excellent electrical conductor at 300- 500 K and the electrical conductivity is 2.56×106 Ω-1?m-1 at room temperature.
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Foundation item: Project(50572080) supported by the National Natural Science Foundation of China
Corresponding author: WANG Ping; Tel: +86-27-63178509; E-mail: wangping0904@whut.edu.cn
(Edited by YUAN Sai-qian)
