Temperature dependence of mechanical properties of aluminum titanate ceramics
Introduction
Aluminum titanate ceramics (Al2TiO5: AT) are synthetic materials of potential interest for many structural applications owing to their high melting point, low thermal conductivity and excellent thermal shock resistance. Recently, AT ceramics have been used as refractories in aluminum alloy casting systems because they have superior thermal shock resistance and better non-wettability compared to molten aluminum alloys.1 However, the mechanical properties of polycrystalline AT are greatly limited by grain-boundary microcracking that occurs as a result of the large thermal anisotropy of individual grains during cooling after sintering. At RT, the thermal expansion coefficients of AT ceramics along the three crystalline axes are αa = 9.8 × 10−6 K−1, αb = 20.6 × 10−6 K−1, αc = −1.4 × 10−6 K−1.2 Grain-boundary microcracking due to the large thermal expansion anisotropy in noncubic polycrystalline ceramics including AT ceramics has been studied extensively.3, 4, 5, 6, 7 The occurrence of microcracking is influenced by the microstructure. The cyclic and static fatigue of AT ceramics was investigated in Al alloy casting systems and the fatigue life was linked to the morphological change of the microcracks.1, 8 The mechanical properties of AT ceramics strongly depend on the microstructure, which in turn is directly related to the degree of microcracking and the stress redistribution in the matrix. The morphology of microcracks in AT ceramics shows a remarkable temperature dependence. As the temperature increases up to the actual casting temperature of 702 °C, the microcracks at the grain boundaries close up, and the AT grains are bonded together by the glassy phase added as sintering aid.1
In the present study, the temperature dependence of the fracture strength, fracture toughness and critical frontal process zone size were investigated at actual casting temperatures (from RT to 702 °C). Furthermore, mechanical properties of AT ceramics were examined in relation to the repeated thermal treatment and the microstructure.
Section snippets
Experimental procedure
AT ceramics (TM-20, Marusu Glaze Co. Ltd.) were sintered at 1582 °C in the presence of gairome clay. ICP analysis shows crystalline components after sintering as Al2O3: 52.82, TiO2: 38.47, SiO2: 6.31, Fe2O3: 1.91, Nb2O5: 0.16, K2O: 0.17, CaO: 0.09 and MgO: 0.06. Mechanical testing was performed using 3 mm × 4 mm × 20 mm rectangular specimens. To reduce the risk of sample failure initiating from an edge, samples were chamfered at 45° using a 10-μm polishing wheel. Four samples of each kind were loaded
Microstructural observation
The SEM micrograph of the polished surface of a specimen is shown in Fig. 1. The gray areas are the AT grains with an average length of about 13 μm, and the dark areas are the pores distributed uniformly throughout the matrix.
Fig. 2 shows SEM micrographs of fracture surfaces of the specimens at RT: (a) without thermal treatment, (b) after the first thermal treatment, and (c) after the second thermal treatment. The white arrows indicate the microcracks, and the dotted red circles show the glassy
Summary
Three-point bending tests on AT ceramics were carried out from ambient temperature to 702 °C. The temperature dependence of physical and mechanical properties was examined, and the microstructure was characterized. The sintered samples were subjected to repeated thermal treatment to investigate the change in mechanical properties of AT ceramics. Both the fracture strength and fracture toughness increased considerably with increasing temperature due to the difference in the microcrack morphology.
References (16)
- et al.
Temperature dependence of static and cyclic fatigue behavior of Al2TiO5 ceramics
J. Ceram. Soc. Jpn.
(2004) - et al.
Structure studies on Al2TiO5 at room temperature and at 600 °C
Acta Cryst. B
(1972) - et al.
Grain-boundary microcracking due to thermal expansion anisotropy in aluminum titanate ceramics
J. Am. Ceram. Soc.
(1987) Alumina fiber FP
Philos. Trans. R. Soc. Lond. A
(1980)- et al.
Alumina fibers: preparation, properties and applications
The preparation and properties of aolycrystalline aluminum oxide fibers
Trans. J. Br. Ceram. Soc.
(1983)- et al.
Measurement of crack volume due to thermal expansion anisotropy in aluminum titanate ceramics
J. Mater. Sci.
(1996) - Mathumura, Y., Characterization of Al2TiO5 ceramics. Master Thesis. Nagoya Institute of Technology,...