Q. Tai. A. Mocellin/Ceramics International 25(1999)393-408 from diffusion creep by comparison of the stress expo-- nent with theoretical predictions and the observation of 1250°c microstructures of Al,OxZrO composites. They attributed the deformation to some form of grain 10 boundary sliding and grain rearrangement process [27 YAG 29, 32-37. Owen et al. indicated that in their study of 3Y20A the grain boundary sliding was mostly of the 10 AY50 Rachinger type [32]. Calderon-Moreno et al. pointed F out that grain boundary sliding with a mix of irc d (intertface-reaction controlled) and TMc (transport of 10 ism in their study [33]. Wakai et al. [30] indicated that 5070100 200 when the content of Zro, was very small and the size of Stress. MPa ZrO2 grains was much smaller than that of Al2O3 grains, the interface-reaction controlled diffusion creep Fig. 9. Variation in strain rate with stress for AlO3, Y3Al5O12 and could be the main creep mechanism. Clarisse et al. pro- Al203-50vol% Y3AlsOi2 composite [51 posed that the main mechanism was grain boundary sliding accommodated by either grain boundary difft sion at high stress or an interface reaction at low stress 695 KJ mol-, respectively. While those obtained by in their study [37]. While in the study of Chevalier et al. Duong et al. were 1. 1, 612 KJ mol-, respectively. The [34]. it is proposed that cavitation and microcracking by difference is possibly caused by different test types and grain boundary sliding was the main mechanism. This is range of applied stress. The formers performed their possibly related to their type of test (bending) and tests under tension at higher stress(25-75 MPa), while higher effective stresses he latters tests under uniaxial compression at lower From those investigations mentioned above, it can stress(3-20 MPa). The experiments of Duong et al be seen that the introduction of ZrO, in Al,O3 can showed that the grain size rather than Y3AlsO1? content improve the creep resistance of Al2O3 matrix either by played an important role in the creep behaviour, and the suppression of interface reactions controlled difft the value of the stress exponent was independent of sion creep of Al2O3 or by the hindrance of grain- tempera boundary movement. In some conditions, the creep rate The observation of microstructure of the AlO3- increases but the value of n remains almost unchanged Y3AlsO12 composites showed that no dynamic grain or increases slightly with increasing the ZrO2 content. growth occurred as a result of the deformation [15,16] The addition of ZrO2 can decrease the values of p of The grains remained fairly equiaxed after deformation Al2O, composites. The impurities of Si, Fe, Na at The evidence of cavitation was observed by Duong et grain boundaries favour grain boundary sliding, a al, the amount of cavitation was estimated to be higher level of such impurities results in a higher creep between 2 and 5%[16]. While French et al. did not rate of the composite. But the effect of Y3+ or Mg2+ on observe significant cavitation. The former authors per- the creep rate of the composites is ambiguous, further formed their tests at more elevated temperatures Investigation is needed. Microstructures of the temperature may favour the nucleation and sites show considerable stability due to the ZrO2 particle growth of cavities pinning effect and there is no or very limited concurrent As far as the deformation mechanism is concerned grain growth during deformation. Cavitation caused by Duong et al. suggested that the creep behaviour was unaccommodated grain boundary sliding often occurs controlled by a diffusional Nabarro-Herring mechan during deformation. The main creep mechanism is grain ism [16. French et al. proposed a diffusional creep con- boundary sliding and grain rearrangement trolled by an interface reaction (source-controlled) mechanism [15] The creep data correlated well with the 3.1.2. A72O3-Y3AlsO12 composites predicted behaviour based on the isostress model [16] The studies of the behaviour of the Al2O3- Thus, the authors suggested that the creep behaviour Y3AlsO12 system showed that the strain rate of the was most probably controlled by Y3Al5O12 in their composites was lower than for pure AlO3 or Y3Al5O12 conditions [15, 16(Fig. 9). French et al. attributed it to the strong segregation of Y3+ to interfaces hindering interface 3. 1.3. A120rTiO2 composite reaction controlling diffusional rate [15]. Duong et al A study of the creep behaviour of Al2O3-TiO2 com indicated that the creep rate was controlled by the posite was reported by Wang et al. [31] The addition of Y3AlsO12 phase which is more creep-resistant. [16] The 5 vol% TiO2 into Al2O3 matrix obviously enhanced values of n and obtained by French et al. were 2.6, its creep rate. This composite exhibited excellentfrom di€usion creep by comparison of the stress expo￾nent with theoretical predictions and the observation of microstructures of Al2O3±ZrO2 composites. They attributed the deformation to some form of grain boundary sliding and grain rearrangement process [27± 29,32±37]. Owen et al. indicated that in their study of 3Y20A the grain boundary sliding was mostly of the Rachinger type [32]. Calderon±Moreno et al. pointed out that grain boundary sliding with a mix of IRC (interface±reaction controlled) and TMC (transport of matter controlled) was the main deformation mechan￾ism in their study [33]. Wakai et al. [30] indicated that when the content of ZrO2 was very small and the size of ZrO2 grains was much smaller than that of Al2O3 grains, the interface±reaction controlled di€usion creep could be the main creep mechanism. Clarisse et al. pro￾posed that the main mechanism was grain boundary sliding accommodated by either grain boundary di€u￾sion at high stress or an interface reaction at low stress in their study [37]. While in the study of Chevalier et al. [34], it is proposed that cavitation and microcracking by grain boundary sliding was the main mechanism. This is possibly related to their type of test (bending) and higher e€ective stresses. From those investigations mentioned above, it can be seen that the introduction of ZrO2 in Al2O3 can improve the creep resistance of Al2O3 matrix either by the suppression of interface reactions controlled di€u￾sion creep of Al2O3 or by the hindrance of grain￾boundary movement. In some conditions, the creep rate increases but the value of n remains almost unchanged or increases slightly with increasing the ZrO2 content. The addition of ZrO2 can decrease the values of p of Al2O3±ZrO2 composites. The impurities of Si, Fe, Na at grain boundaries favour grain boundary sliding, a higher level of such impurities results in a higher creep rate of the composite. But the e€ect of Y3+ or Mg2+ on the creep rate of the composites is ambiguous, further investigation is needed. Microstructures of the compo￾sites show considerable stability due to the ZrO2 particle pinning e€ect and there is no or very limited concurrent grain growth during deformation. Cavitation caused by unaccommodated grain boundary sliding often occurs during deformation. The main creep mechanism is grain boundary sliding and grain rearrangement. 3.1.2. Al2O3-Y3Al5O12 composites The studies of the creep behaviour of the Al2O3± Y3Al5O12 system showed that the strain rate of the composites was lower than for pure Al2O3 or Y3Al5O12 [15,16] (Fig. 9). French et al. attributed it to the strong segregation of Y3+ to interfaces hindering interface reaction controlling di€usional rate [15]. Duong et al. indicated that the creep rate was controlled by the Y3Al5O12 phase which is more creep-resistant. [16] The values of n and Q obtained by French et al. were 2.6, 695 KJ molÿ1 , respectively. While those obtained by Duong et al. were 1.1, 612 KJ molÿ1 , respectively. The di€erence is possibly caused by di€erent test types and range of applied stress. The formers performed their tests under tension at higher stress (25±75 MPa), while the latter's tests under uniaxial compression at lower stress (3±20 MPa). The experiments of Duong et al. showed that the grain size rather than Y3Al5O12 content played an important role in the creep behaviour, and the value of the stress exponent was independent of temperature. The observation of microstructure of the Al2O3± Y3Al5O12 composites showed that no dynamic grain growth occurred as a result of the deformation. [15,16] The grains remained fairly equiaxed after deformation. The evidence of cavitation was observed by Duong et al., the amount of cavitation was estimated to be between 2 and 5% [16]. While French et al. did not observe signi®cant cavitation. The former authors per￾formed their tests at more elevated temperatures. Higher temperature may favour the nucleation and growth of cavities. As far as the deformation mechanism is concerned, Duong et al. suggested that the creep behaviour was controlled by a di€usional Nabarro±Herring mechan￾ism [16]. French et al. proposed a di€usional creep con￾trolled by an interface reaction (source±controlled) mechanism. [15] The creep data correlated well with the predicted behaviour based on the isostress model [16]. Thus, the authors suggested that the creep behaviour was most probably controlled by Y3Al5O12 in their conditions. 3.1.3. Al2O3±TiO2 composite A study of the creep behaviour of Al2O3±TiO2 com￾posite was reported by Wang et al. [31] The addition of 5 vol% TiO2 into Al2O3 matrix obviously enhanced its creep rate. This composite exhibited excellent Fig. 9. Variation in strain rate with stress for Al2O3, Y3Al5O12 and Al2O3-50 vol% Y3Al5O12 composite [15]. 402 Q. Tai. A. Mocellin / Ceramics International 25 (1999) 395±408