High temperature deformation of non-directionally solidified Al2O3/YAG/ZrO2 eutectic bulk ceramic
Introduction
Melt-grown Al2O3-based eutectic oxides prepared by directional solidification methods present high strengths and excellent thermal stabilities at high temperatures. Thus, eutectic ceramic oxides have the potential to be used as ultra-high temperature materials in aerospace applications, jet aircraft engines, and gas turbine systems [1], [2], [3]. For example, the flexural strength of the melt-grown Al2O3/YAG eutectic ceramic is maintained in the range of 350–400 MPa between room temperature and 1800 °C, which is just below its melting point of about 1830 °C [4]. The excellent mechanical properties of melt-grown eutectic ceramics arise from the clean, strong interfaces between the eutectic domains [5], [6]. In comparison, the strength of traditionally sintered oxide ceramics deteriorates dramatically when the temperature increases above 800 °C [5] because of the existence of cavities and amorphous phases at the grain boundaries, which cause high temperature deformation by means of grain boundary sliding [7], [8]. Researchers also believe that the high-temperature characteristics of melt-grown eutectic ceramics are linked to the unique microstructures of entangled single crystal phases [5], [9], [10]. As a result, most researchers have employed directional solidification methods to prepare melt-grown eutectic ceramics composed of single crystals, such as the Bridgman [11], micro-pulling down (µ-PD) [12], edge-defined film-fed growth (EFG) [13], and laser float zone methods [14], [15], [16]. In contrast, it is difficult to produce single crystals using non-directional solidification methods without a stable high temperature gradient, but it is easy to form shrinkage porosity and cavity defects that are detrimental to the mechanical properties of the crystals. To the best of our knowledge, the microstructure characteristics and high-temperature properties of non-directional solidified samples have not yet been reported.
In the present work, we used a traditional casting method to prepare melt-grown Al2O3/YAG/ZrO2 bulk ceramics, and examined the temperature dependence of their compression deformation and microstructure characteristics.
Section snippets
Material and methods
Al2O3/ZrO2/Y2O3 nanocomposite powders with eutectic compositions were prepared by the liquid-phase co-precipitation method. Al(NO3)3·9H2O, ZrOCl2·8H2O, and Y(NO3)3·6H2O were used as starting materials, and NH3·H2O solution was used as the precipitant agent. Al2O3/YAG/ZrO2 nanocomposite powders were obtained after calcination at 1200 °C and had an average particle size of approximately 50 nm. The composition of the Al2O3/ZrO2/YAG ternary eutectic with a melting point of 1715 °C was 65 mol% Al2O3, 19
Results and discussion
Fig. 2 shows X-ray diffraction patterns of the sintered and solidified Al2O3/YAG/ZrO2 ternary eutectic ceramics. Both the sintered and solidified samples have the identical components, and are composed of α-Al2O3, YAG, and cubic ZrO2 phases. It is noted that there are as many diffraction peaks from the solidified sample as there are from the sintered sample, which indicates that the melt-grown Al2O3/YAG/ZrO2 eutectic bulk ceramic was polycrystalline. Fig. 3 shows the microstructure
Conclusions
An Al2O3/YAG/ZrO2 eutectic prepared by a non-directional solidification method is polycrystalline and has a colony structure. The colony microstructure is not uniform, and micro-pore and crack defects frequently appear in the intercolony region. The non-directionally solidified eutectic ceramic does not have a satisfactory thermal stability. The compressive strength decreases from 320 MPa to about 100 MPa as the temperature increases from 1500 °C to 1650 °C. The non-directionally solidified
Acknowledgements
This work was supported by the National Natural Science Foundation of China (Nos. 51675078 and 51175059), Aeronautical Science Foundation of China (20153663010), Foundation of Liaoning Educational Committee (LZ2015014), and the Fundamental Research Funds for the Central Universities (DUT16RC(4)33).
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