Superplastic compression, microstructural analysis and mechanical properties of a fine grain three-phase alumina zirconia mullite ceramic composite

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1 Materials Science and Engineering A (2005) Superplastic compression, microstructural analysis and mechanical properties of a fine grain three-phase alumina zirconia mullite ceramic composite Tiandan Chen, Martha L. Mecartney University of California, Irvine, Department of Materials Science and Chemical Engineering, Irvine, CA , USA Received in revised form 17 March 2005 Abstract The creep behavior, microstructure and mechanical properties of a three-phase 40 vol.% alumina 30 vol.% zirconia 30 vol.% mullite ceramic composite were investigated. This material was made from zirconia (3Y-TZP) and the reaction of nanoscale powders of alumina and a silica sol to form mullite. Only a few small (10 15 nm) silica pockets remained after sintering, although the original amount of silica was 5.9 wt.%. The material demonstrated a steady-state strain rate and showed superplastic flow under compression. Most grains were equiaxed and sharply facetted. Samples quenched from 1450 C indicated that the high temperature microstructure is similar to the as-sintered state. Dislocations formed in both mullite and zirconia grains during deformation, suggesting that a dislocation mechanism plays a role during superplastic deformation. The hardness was 15 GPa and the fracture toughness was 6.5 MPa m 1/2, with no degradation in properties with 100% deformation Elsevier B.V. All rights reserved. Keywords: Superplasticity; Microstructure; Dislocations; Mechanical properties 1. Introduction High temperature deformation in fine-grained pure alumina is limited in both tension and compression, due to extensive dynamic grain growth during high temperature deformation [1,2]. To limit the rapid grain growth in alumina, dispersions of second-phase zirconia have been used to increase the high temperature ductility of alumina [3,4]. These alumina zirconia materials met with only limited success. However, a fine grain three-phase zirconia alumina spinel ceramic composite demonstrated a strain of more than 1050% in tension and a strain rate of 0.4 s 1 at 1650 C [5]. The high strain rate and extensive strain to failure was attributed to a combination of limited grain growth in the three-phase structure and dislocation-induced plasticity in the zirconia grains [5]. Three-phase composites in other ceramic systems have also shown superior high temperature ductility compared to twophase systems [6]. A codispersion of 15 vol.% zirconia and 15 vol.% mullite in alumina more effectively limited the grain Corresponding author. Tel.: ; fax: address: martham@uci.edu (M.L. Mecartney). growth and enhanced the strain rate at high temperature than a dispersion of 30 vol.% zirconia only in alumina [6]. In this present paper, the creep behavior and microstructure of a threephase alumina zirconia mullite ceramic composite with a similar volume fraction for each phase is investigated. In order to prepare 40 vol.% alumina 30 vol.% zirconia 30 vol.% mullite (AZ30M30) materials, a silica sol combined with nanoscale alumina and nanoscale zirconia powders was used. The silica reacts with alumina and forms mullite during sintering. If significant amounts of amorphous silica remain, the volume fraction and distribution of this glassy phase will have an important effect on creep behavior [7,8]. Transmission election microscopy (TEM) studies are required to detect small amounts of amorphous second phases. Most of the reported microstructural observations in the literature of superplastically deformed materials have been conducted at room temperature. The microstructure during high deformation temperatures is rarely reported and could be significantly different from that at room temperature. Quenched specimens are expected to exhibit a microstructure close to the equilibrium state at high temperature. In order to evaluate the microstructure at high temperature when it is deformed rather than room temperature microstructure, quenching experiments /$ see front matter 2005 Elsevier B.V. All rights reserved. doi: /j.msea

2 T. Chen, M.L. Mecartney / Materials Science and Engineering A (2005) were conducted to study the high temperature microstructure by TEM. In addition, although many previous TEM analyses of the microstructure of superplastically deformed ceramics have indicated that these materials were free of dislocations [9,10], recent observations of dislocations in superplastically deformed 3Y- TZP [5,11] and spinel [12] have been reported. The role of dislocations in ceramics during superplastic deformation is currently under debate [13,14], but may be important in multiphase ceramics. Evidence of dislocations activity was also investigated in this current work. The introduction of mullite and zirconia into alumina ceramic can improve its mechanical properties and thermal shock behavior [15,16]. The significant increase of fracture toughness was attributed to the presence of crack-closure compressive strains and transformation toughing [17]. However, mechanical properties, such as hardness and fracture toughness, are rarely reported in the literature for superplastically deformed specimens [18]. Micro-mechanical properties, i.e., Vicker hardness and fracture toughness, for deformed three-phase alumina zirconia mullite are also presented in this paper. 2. Experimental procedures The starting materials were a 40 nm high purity alumina powder (Baikowski Inter. Corp.), a 26 nm tetragonal zirconia containing 3 mol% Y 2 O 3 (3Y-TZP) (Tosoh Co, Ltd.) and 15 nm silica sol (Nissan Chemical Industries Ltd.). Once sintered, assuming that the silica completely reacted with the alumina, the composition should be 40 vol.% alumina, 30 vol.% zirconia and 30 vol.% mullite (AZ30M30 designation). The powders of alumina and zirconia and silica sol were mechanically mixed, dried, sieved to <80 m, and then isostatically pressed into cylinders of 2 mm diameter and 5 mm height and sintered at 1723 K for 1 h. Bulk density of the sintered specimens was measured using the Archimedes method in distilled water. Compressive tests were carried out in air using a commercial Applied Test System (ATS) apparatus. After the temperature reached the set temperature, the sample was kept at that temperature for 10 min before creep testing. By calculating the change of the cross-sectional area of the sample with the strain, the load was adjusted to keep the true stress constant during the test. From the displacement versus time curves, the strain rate was calculated. The mechanical properties of the as-sintered and deformed samples (30 MPa, 1698 K) were characterized by Vicker hardness and fracture toughness using Zwick microhardness tester (Zwick 3210) with a load of 8 kg mass. The microindentation toughness K IC was calculated using the below-given Eq. (1) developed by Shetty et al. [19]: TEM specimens were obtained using a standard technique of grinding to 100 m, dimpling 15 m and then ion milling to electron transparency. A dimpled sample (about 15 m thick in the center and 100 m at the edge) was heated at 1723 K for 30 min and then quenched to room temperature, and subsequently ion milled for TEM observation. TEM thin disks of deformed samples were cut from the plane normal to the compression stress. TEM observations were performed using a Philips CM20 (200 kv). 3. Results and discussion 3.1. Creep behavior Graphs of the true strain rate versus true strain curves under compression at different temperatures and stresses are shown in Fig. 1. The material shows superplastic flow under compression. Two noteworthy points are the high strain rate and the steady-strain rate demonstrated after the primary creep. At 1723 K and a stress of 15 MPa, the steady-state strain rate for AZ30M30 is about s 1, which compares favorably with the strain rate of s 1 for 3Y-TZP (d 0 = 0.5 m) at 1723 K and 10 MPa under compression [20]. However, 3Y- TZP never achieves a steady-state strain rate and shows apparent strain hardening due to grain growth. In contrast, for AZ30M30, the steady-state strain rate indicates there is no significant grain growth under such deformation conditions [20]. Fig. 2 shows the dependence of true stress and inverse temperature on the strain rate for AZ30M30 under compression. At 1500 C and under 50 MPa, the material demonstrates a strain rate of 0.01 s 1, which is regarded as characteristic of a high strain rate material. Mullite is reported to be a good creep resistant material due to the elongated grain shape in single-phase mullite [21]. The interlocking elongated grains hinder grain boundary sliding, and thus, decrease the strain rate. However, in alumina matrix composites, mullite does not develop an elongated grain shape [22]. In addition, the mullite is formed by the reaction of isolated pockets of silica with nearby alumina. This ensures that the mullite grains are isolated by alumina and zirconia grains. Thus, grain boundary sliding is not controlled by ( ) Hv P 1/2 K IC = (1) 4l where P is the load, H v the Vicker hardness and l is the crack length from an indentation corner to its corresponding crack tip. Fig. 1. True strain rate vs. true strain profiles for AZ30M30 under compression.

3 136 T. Chen, M.L. Mecartney / Materials Science and Engineering A (2005) Fig. 2. The dependence of (a) true stress and (b) inverse temperature on true strain rate for AZ30M30 deformed under compression in air. mullite/mullite grain boundaries. The stress exponent was close to 2 and the activation energy calculated to be 870 ± 40 kj/mol. This value of n = 2 is broadly consistent with the values reported in previous studies on alumina materials [23,24]. However, the calculated activation energy is higher than that of pure alumina (about 450 kj/mol [24]) and alumina zirconia composites ( kj/mol [25,26]). The increased activation energy is attributed to the much higher activation energy for deformation of mullite [22] Microstructure As-sintered specimens Fig. 3 shows bright-field transmission electron micrographs (TEM) of AZ30M30 in the as-sintered state. The grains are nearly equiaxed, and most of the grain boundaries are sharply facetted (Fig. 3(a)). Another feature of the as-sintered materials is that some round zirconia particles are trapped in the mullite grains formed by the reaction between alumina and amorphous silica (Fig. 3(b)), a feature also seen in other zirconia/mullite composites [27]. Fig. 3. (a) Typical TEM micrograph of AZ30M30, (b) small zirconia grain trapped in mullite and (c) a small glassy pocket at mullite grain junction (left grain is mullite).

4 T. Chen, M.L. Mecartney / Materials Science and Engineering A (2005) The observation of pockets of silica at multiphase grain boundary junctions is typical for silica-doped zirconia ceramics, even when the doped amount of silica is as small as 0.5 wt.% [28]. Although the total amount of doped silica was 5.9 wt.% in the starting material prepared here, only a few very small glass pockets (10 15 nm) at grain junctions were found after the formation of mullite (Fig. 3(c)). EDS analysis showed that these few nanometer scale glass pockets were primarily silica. The limited number of small isolated silica pockets indicates that nearly all the silica reacted with alumina and formed the mullite zirconia alumina three-phase microstructure. No detectable amorphous phase was found along grain boundaries, but high resolution TEM studies were not conducted Quenched specimens TEM images of AZ30M30 specimens quenched from 1723 K are shown in Fig. 4. The microstructure shows sharp grain boundary junctions and is similar to that observed for room temperature samples. Even fewer glass pockets were present in the quenched specimens than in the as-sintered specimens. This is in contrast to Clarke s work [29] which found that amorphous silicate pockets in quenched Si 3 N 4 were significantly larger in size and presented in larger volume fraction than in the same material cooled slowly to room temperature. In addition, the quenched microstructure of Si 3 N 4 had a more rounded grain morphology and a thicker intergranular phase. In AZ30M30, mullite is stable at 1723 K and so the volume fraction of residual silica should not increase. Fewer glass pockets of amorphous silica are probably the result of the further reaction of alumina and silica to form mullite or possibly due to higher solubility of silica dissolved into zirconia [30] or grain boundaries [31] at high temperatures. Fig. 4. TEM micrographs of AZ30M30 after quenching from 1723 K with sharply facetted grain boundaries Deformed specimens TEM observations of 50% deformed specimens show that the grains primarily remain equiaxed with sharply facetted grain boundaries. Although significant cavitation has been reported for iron-doped alumina after limited deformation under compression [32], no sign of cavitation was observed in the deformed AZ30M30 samples. The lack of significant cavitation was confirmed by density measurements. The absence of cavitation in the deformed specimens indicates that grain boundary sliding is well accommodated during deformation. Similar to the assintered state, only a few small silica pockets at multiple grain junctions were found. The average grain size after deformation (0.43 m) is similar to that before deformation (0.39 m), indicating limited dynamic grain growth during high temperature deformation. The three-phase structure with a similar volume fraction for each phase increases the separation distance between same phases, and thus, increases the diffusion path length and suppresses the grain growth. Strain contrast and bend contours are more pronounced in deformed specimens than in the as-sintered material, suggesting a high amount of strain energy accumulated during high temperature deformation. A higher density of dislocations was found in AZ30M30 after deformation, than in sintered or quenched samples that were cooled from the same temperatures, indicating that the increased strain energy and increased dislocation density are not simply due to thermal expansion mismatch. Dislocations are primarily present in both mullite and zirconia grains (Fig. 5(a and b)). In Fig. 5(a), dislocations (shown by arrow) traverse the mullite grains from one side to another side; in Fig. 5(b), a pile-up of dislocations is observed in zirconia. The dislocation density is the highest in mullite grains and lowest in alumina grains, indicating that dislocations tend to form in the softer phases with lower elastic modulus values. Dislocations in zirconia in three-phase alumina zirconia spinel have also been observed [5]. A difference in dislocation density for different phases was also reported for a deformed Al 2 O 3 YAG composite, i.e., many linear dislocations in alumina and few dislocations in YAG [33]. Grain boundary sliding is well accepted as the primary deformation mechanism for both metals and ceramics. The contribution of grain boundary sliding to total strain has been measured to be 80% [34,35] and direct evidence of grain boundary sliding has been reported using SEM [36,37]. However, to prevent void formation and maintain coherency between grains during deformation, grain boundary sliding requires an accommodation process at grain boundary triple points or ledges. There are several possible accommodation processes including boundary migration, dislocation motion, diffusion-related flow and cavitation. Although grain boundary sliding accommodated by dislocation motion is well accepted as the main deformation mechanism for superplastic metals [38,39], accommodation involving dislocations in ceramics has not been considered feasible for several reasons. Firstly, the theoretically calculated shear stress to nucleate dislocations is much higher than the applied stress [13]. In addition, most of the superplastically deformed ceramic materials are found to be free of dislocations when viewed after deformation [9,10].

5 138 T. Chen, M.L. Mecartney / Materials Science and Engineering A (2005) materials suggests that the dislocations are also an accommodation process for grain boundary sliding in ceramics, especially in multiphase ceramics. Stress concentrations at multiphase grain boundaries can cause the nucleation and propagation of dislocations in the lower elastic modulus (softer) materials [14] Micro-mechanical properties If the material is to be shape-formed into useful components, good mechanical properties for the deformed/shaped specimen are desired. For AZ30M30, the fracture toughness and hardness values did not change from 25 to 100% deformation. Similar to Flacher s work [40], initial strains up to 25% helped further densify the sample and improve the mechanical properties as the samples were originally 96 97% dense. The hardness of AZ30M30 was 15GPa and the fracture toughness was 6.5 MPa m 1/2. While the strength of the AZ30M30 material was not tested, it is expected that the fine grain size with a multiphase structure will provide superior strength [41]. 4. Conclusions (1) A three-phase alumina (40 vol.%) zirconia (30 vol.%) mullite (30 vol.%) ceramic composite was prepared by reaction sintering. This material demonstrated a steady-state strain after primary creep with limited grain growth during high temperature deformation. (2) Under compression, this material demonstrated a stress exponent of 2 and an activation energy of 870 ± 40 kj/mol. (3) Doped silica reacted with alumina and formed the mullite phase, leaving very little residual silica. TEM observations showed that grains were mainly equiaxed and the most grain boundaries were sharply faceted. The microstructure of quenched specimens is similar to that at room temperature. (4) No cavitation was found in deformed materials, but there was a higher density of dislocations in both zirconia and mullite grains after deformation. This observation indicates that dislocation motion may also be a critical accommodation process for grain boundary sliding in ceramics. Acknowledgement Fig. 5. TEM micrographs of deformed AZ30M30 (a) dislocations in mullite and (b) dislocations in zirconia (arrow A points to grain boundary dislocations and arrow B to dislocations inside the zirconia grain). In this present work, the limited grain growth and equiaxed grain shape rule out an accommodation mechanism of grain migration. The absence of cavitation in the deformed specimens also excludes cavitation as the accommodation mechanism. Diffusion-related flow is more difficult when the material is not a single phase. However, a higher density of dislocations was found in deformed three-phase AZ30M30 materials and significantly fewer dislocations were found in the as-sintered materials. The observance of a high density of dislocations in the deformed This research work is supported by the Division of Materials Research of the National Science Foundation under Grant No References [1] Y. Yoshizawa, T. Sakuma, Acta. Metall. Mater. 40 (1992) [2] R. Venkatachari, R. Raj, J. Am. Ceram. Soc. 69 (1986) [3] F. Wakai, H. Kato, Adv. Ceram. Mater. 3 (1988) [4] K. Okada, T. Sakuma, J. Am. Ceram. Soc. 79 (1996) [5] B.N. Kim, K. Hiraga, K. Morita, Y. Sakka, Nature 413 (2001) [6] T. Chen, M.L. Mecartney, J. Am. Ceram. Soc. 88 (2005) [7] K. Kajihara, Y. Yoshizawa, T. Sakuma, Acta. Metall. Mater. 43 (1995) [8] R.P. Dillon, M.L. Mecartney, Scr. Mater. 50 (2004) [9] S. Tekeli, T.J. Davis, J. Mater. Sci. 33 (1998)

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