Acta Materialia 54 (2006) 4415–4426 www.actamat-journals.com Threshold stress superplastic behavior and dislocation activity in a three-phase alumina–zirconia–mullite composite Tiandan Chen, Farghalli A. Mohamed, Martha L. Mecartney * Department of Chemical Engineering and Materials Science, University of California, Irvine, CA 92697-2575, USA Received 27 April 2006; accepted 1 May 2006 Available online 22 August 2006 Abstract Creep tests were performed in compression on a three-phase alumina–zirconia–mullite composite in the temperature range 1673– 1773 K. The data show steady-state deformation behavior with a high-strain rate. Plots of strain rate versus applied stress on a logarith- mic scale exhibit a sigmoidal relationship, i.e., an increase of stress exponent with a decrease in applied stress. It is shown that such an increase in the stress exponent with increasing stress can be correlated with the existence of a threshold stress, which decreases with increasing temperature. Transmission electron microscopy studies reveal dislocation activity in some grains of the deformed material, which was not present in the as-sintered materials, indicating that dislocations were generated during deformation. These results, which are similar in trend to those reported for superplastic metallic alloys, lead to the conclusion that dislocation motion plays a role in accom- modating grain boundary sliding in this material. Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Transmission electron microscopy; Ceramics; Dislocations; Superplasticity; High-temperature deformation 1. Introduction more recent study of deformation behavior in ceramics has benefited from the prior achievements in micrograin Since superplastic flow in ceramic materials was superplasticity in metals. However, several common char- reported by Wakai et al. [1,2], significant advances in the acteristics of superplasticity in metals including the occur- development of superplastic ceramics have been made, rence of dislocation motion as an accommodation including extensive deformations of over 1000% for a mechanism for sliding and the presence of a sigmoidal rela- 5 wt.% SiO2-doped tetragonal zirconia (3Y-TZP) at tionship between strain rate and stress are neither often 1673 K [3] and over 500% for 5 wt.% SiO2-doped cubic zir- reported nor accepted in ceramics, especially for alumina- conia (8Y-CSZ) at 1703 K [4]. More recently, a three-phase based ceramics. zirconia–alumina–spinel achieved more than 1000% defor- By contrast, grain boundary sliding accommodated by mation with a high-strain rate of 0.4 sÀ1 at 1923 K [5]. dislocation motion is accepted as a primary deformation However, despite many advances in superplastic ceramic mechanism in metals, although the details and assumptions materials development, there is no consistent agreement of the accommodation process in various models are differ- on the deformation mechanism for superplasticity in ent [7–9]. Dislocation motion has often been ruled out as ceramics. an accommodation mechanism in ceramics for two main Superplasticity in ceramics was widely reported years reasons [10,11]. First, most transmission electron micros- after the discovery of superplasticity in metals [1,6]. The copy (TEM) studies have reported that the deformed microstructure in superplastic ceramics, including 3Y- * Corresponding author. TZP [12], 8Y-CSZ [13], and alumina [14] was dislocation E-mail address: [email protected] (M.L. Mecartney). free or contained very limited isolated dislocations, and 1359-6454/$30.00 Ó 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2006.05.002 4416 T. Chen et al. / Acta Materialia 54 (2006) 4415–4426 many TEM investigations did not mention whether there the concept of a threshold stress, speculating that the inter- were dislocations present in the deformed materials. Sec- action between boundary dislocations and obstacles in the ond, the stress to generate dislocations based on theoretical grain boundary (ledges and precipitates) could serve as the calculations [10] and single-crystal creep experiments [11] origin of the threshold stress. The threshold stresses pre- was much higher than the applied stress in most cases. dicted by these models exhibit a very weak temperature However, the absence of evidence regarding dislocations dependence; the temperature dependence of the threshold in ceramics after deformation is not evidence of the absence stress was expected to be the same as that of the shear mod- of dislocation activity during high-temperature deforma- ulus. As a result, these earlier models failed to explain the tion. Xun and Mohamed [15] have shown that for metal strong dependence of the threshold stress on temperature superplasticity, dislocations can rapidly migrate into the in superplastic metals as documented by experiments. grain boundaries, and only the presence of pinning particles Mohamed [27,28] was the first to attribute the threshold effectively traps dislocations so they can be seen during sub- stress to the segregation of impurities at boundaries, a sequent TEM studies. In addition, it is not clear whether the model that fits the exponential temperature dependence experimental procedures employed in previous studies in observed in experiments in superplastic metal alloys. ceramics are valid for retaining dislocations. Dislocation However, low stress creep data in ceramics are not often features and the dislocation density revealed under TEM reported, especially for alumina-based materials; thus the can be influenced by the deformation technique and TEM sigmoidal relation was rarely reported in the past for sample preparation methods. To preserve the deformed ceramics. One exception is for yttria-stabilized tetragonal microstructure and dislocation activity, the deformed mate- zirconia (TZP), where the presence of a threshold stress rial needs to be cooled under load [16]. This step was not has been documented [29]. The threshold stress was often taken. observed to be dependent on the purity of the TZP mate- Recently, dislocation activity in deformed ceramics has rial. More recent publications suggest that segregation been reported in several studies. A high-strain rate and (excess yttrium ions) at the grain boundaries is the cause extensive strain in a three-phase zirconia–alumina–spinel of the threshold stress in TZP, similar to metallic systems were attributed to both limited grain growth and plastic [30,31]. The concept of a threshold stress in ceramics, how- deformation within zirconia grains [5]. TEM micrographs ever, has not been universally accepted. Calculations by of this material showed developed dislocation structures other researchers using experimental data for TZP have in the zirconia phase. Additional observations of disloca- yielded a negative threshold stress [32,33]. A negative value tions have been reported in deformed 3Y-TZP by Morita for the threshold stress causes the exclusion of the thresh- et al. [16,17] and Duclos and Crampon [18]. Morita et al. old stress approach in ceramics. [19] also reported dislocation activity in deformed poly- It is clear from the preceding summary that guiding crystal MgAl2O4 and proposed that the dislocation motion information on dislocation activity and creep characteris- could relax the stress concentration generated from bound- tics at low stresses in ceramics under superplastic condi- ary sliding. Strain hardening and a subsequent softening tions is needed to shed light on the origin and details of phenomenon were ascribed to an increase and decrease of the mechanism of superplasticity in these materials. In an the dislocation density. Dislocations were observed in both attempt to obtain such information, a detailed investiga- TZP and spinel in a deformed ZrO2–30 vol.% MgAl2O4 tion was undertaken in which the creep and substructure superplastic composite, and the yield drop was attributed of a three-phase alumina (40 vol.%)–zirconia (30 vol.%)– to a sharp increase in mobile dislocation in the spinel mullite (30 vol.%) ceramic composite at low stresses are [20]. Dislocation sub-boundaries, formed from a shot blast- examined for the first time. It is the purpose of the present ing technique at room temperature, can significantly paper to report and discuss the results of this investigation improve the fracture toughness of ceramics [21]. All these with a focus on the role of dislocations in accommodating studies suggest that the role of dislocations should not be grain boundary sliding. ignored in ceramics. A sigmoidal relationship between strain rate and stress is 2. Experimental another common characteristic in superplastic metals [22]. The increase of stress exponent in the low stress region is 2.1. Material thought to be due to the existence of threshold stress. The concept of a threshold stress in superplastic metals The material used in this investigation is a three-phase was first introduced by Johnson [23] followed by experi- alumina (40 vol.%)–zirconia (30 vol.%)–mullite (30 vol.%). mental verification by Burton [24] in a superplastic Pb–Sn For the sake of brevity, this material will be referred to as system. These authors were the first to suggest that the dif- AZ30M30. AZ30M30 was selected for several reasons. ference between regions I and II was not due to different First, it has been shown that the three-phase AZ30M30 deformation processes, but caused by the presence of a exhibits steady-state deformation under constant compres- threshold stress