Q. Tai. A. Mocellin/Ceramics International 25(1999)395-408 almost the same results as those obtained by Wakai et Microstructural characterization of the deformed al. But the results obtained by Clarisse et al. [37] showed specimens generally revealed remarkable structural sta that when Zro2 content exceeded that of Al2O3, the bility as long as the second phase was present at a small creep rate of the composites was somewhat higher than volume fraction, typically no more than 10%[2]. Chen that of single-phase AlO3 due to a more ductile Zro2 attributed this to the particle pinning effect [2]. The phase which played a role in the deformation of the com- measurements showed that there was no or very limited posites. The values of n obtained by Calderon-Moreno concurrent grain growth during deformation [35]showed a slight increase with the ZrOz content [15, 28, 29, 32, 33] and there was no significant change in Wang et al. [31] and Owen et al. [32] studied the the aspect ratio of the grains, which essentially retained influence of grain size on the creep behaviours of their equiaxed shapes [29, 33]. Wakai et al. [28] and Al2O, composites. The values of p obtained by Owen et al. [32] observed that in 3Y20A there was very Wang et al. was about 3, while that obtained by Owen little increase in the aspect ratio of the two phases fol- et al. was 2. 1. The difference of the values of p is mainly lowing deformation, the grains were elongated slightly in caused by the content of ZrOz. In the experiment of the tensile direction. The grain aspect ratio of ZrO2 Wang et al. the ZrO2 content was 5 vol%, but in the grains was 1. 15-1. 16, and that of Al2O3 grains was 1.3- experiment of Owen et al. the ZrO2 content was 1.5. This implies that the intragranular strain of ZrO2 72.7 vol%, and in this two phase composite the values grains is smaller than that of Al_O3 grains. In some of p of ZrO,, AlO3 phase and their volume average experiments, neither significant intragranular dislocation obtained by Owen et al. were 2.0, 2.6 and 2. 1 respec- activity nor significant cavitation was observed [15, 3 tively(Fig. 8). So, the addition of Zro, decreases the but in most experiments, cavitation was noted [28, 33, 34] values of p of Al_O - ZrO2 composites. The influence of Cavities tended to nucleate at triple point junctions grain size on the creep behaviours was also investigated associated with an alumina grain and then they grew by other authors [33, 34, 37]. The impurity content of quite quickly along those Al2O3-Al2O3, ZrO2-ZrOz and AlO3 and ZrO? and the level of segregation at grain Al2O3-ZrO2 interfaces normal to the tensile axis. the boundaries may affect strongly the creep behaviours of cavity volume fraction and size were dependent on the he composites. Generally, the higher the content of Si, strain rate, both decreasing with decreasing strain rate Fe, Na impurity, the higher the creep rate of the com- posite. Although French et al. [15 indicated that o Many authors ruled out the deformation mechanisms intragranular dislocation or significant contributions Al2O3/c-ZrO2(8 mol%Y2O3)composite the segregation of y+ to the Al2O3 grain boundaries slowed the creep te of this composite the results obtained by Chevalier et al. [34] showed that magnesia-stabilized ZrO2-Al2O3 composite decreased the creep rate while yttria-stabi lised zirconia was not favourable for creep resistance of composite. The authors sium-containing grain boundary glassy phase was favourable for creep resistance of the composite. The role of the y+ and Mg2+ at grain boundaries needs to be more systematically investigated rrw ▲3TG Llm) l Fig. 8. Variation in creep rate with grain size for Al2Or-727 vol% ZrO2 composite. The grain size may be defined in terms of the ZrO2 Fig. 7. Influence of ZrO2 content on the strain rate in Al2O3-zrO2 or Al2O3 phases or their volume average. p:(0)ZrO2, 2.0:(A)Al2O3, composites [38]. 6:(o)Av.2.1[2Jalmost the same results as those obtained by Wakai et al. But the results obtained by Clarisse et al. [37] showed that when ZrO2 content exceeded that of Al2O3, the creep rate of the composites was somewhat higher than that of single-phase Al2O3 due to a more ductile ZrO2 phase which played a role in the deformation of the com￾posites. The values of n obtained by Calderon±Moreno [35] showed a slight increase with the ZrO2 content. Wang et al. [31] and Owen et al. [32] studied the in¯uence of grain size on the creep behaviours of Al2O3±ZrO2 composites. The values of p obtained by Wang et al. was about 3, while that obtained by Owen et al. was 2.1. The di€erence of the values of p is mainly caused by the content of ZrO2. In the experiment of Wang et al. the ZrO2 content was 5 vol%, but in the experiment of Owen et al. the ZrO2 content was 72.7 vol%, and in this two phase composite the values of p of ZrO2, Al2O3 phase and their volume average obtained by Owen et al. were 2.0, 2.6 and 2.1 respec￾tively (Fig. 8). So, the addition of ZrO2 decreases the values of p of Al2O3±ZrO2 composites. The in¯uence of grain size on the creep behaviours was also investigated by other authors [33,34,37]. The impurity content of Al2O3 and ZrO2 and the level of segregation at grain boundaries may a€ect strongly the creep behaviours of the composites. Generally, the higher the content of Si, Fe, Na impurity, the higher the creep rate of the com￾posite. Although French et al. [15] indicated that in Al2O3/c-ZrO2 (8 mol% Y2O3) composite the segregation of Y3+ to the Al2O3 grain boundaries slowed the creep rate of this composite, the results obtained by Chevalier et al. [34] showed that magnesia-stabilized ZrO2-Al2O3 composite decreased the creep rate while yttria-stabi￾lised zirconia was not favourable for creep resistance of the composite. The authors suggested that a magne￾sium-containing grain boundary glassy phase was favourable for creep resistance of the composite. The role of the Y3+ and Mg2+ at grain boundaries needs to be more systematically investigated. Microstructural characterization of the deformed specimens generally revealed remarkable structural sta￾bility as long as the second phase was present at a small volume fraction, typically no more than 10% [2]. Chen attributed this to the particle pinning e€ect [2]. The measurements showed that there was no or very limited concurrent grain growth during deformation [15,28,29,32,33] and there was no signi®cant change in the aspect ratio of the grains, which essentially retained their equiaxed shapes [29,33]. Wakai et al. [28] and Owen et al. [32] observed that in 3Y20A there was very little increase in the aspect ratio of the two phases fol￾lowing deformation, the grains were elongated slightly in the tensile direction. The grain aspect ratio of ZrO2 grains was 1.15±1.16, and that of Al2O3 grains was 1.3± 1.5. This implies that the intragranular strain of ZrO2 grains is smaller than that of Al2O3 grains. In some experiments, neither signi®cant intragranular dislocation activity nor signi®cant cavitation was observed [15,32], but in most experiments, cavitation was noted [28,33,34]. Cavities tended to nucleate at triple point junctions associated with an alumina grain and then they grew quite quickly along those Al2O3±Al2O3, ZrO2±ZrO2 and Al2O3±ZrO2 interfaces normal to the tensile axis. The cavity volume fraction and size were dependent on the strain rate, both decreasing with decreasing strain rate. Many authors ruled out the deformation mechanisms of intragranular dislocation or signi®cant contributions Fig. 7. In¯uence of ZrO2 content on the strain rate in Al2O3-ZrO2 composites [38]. Fig. 8. Variation in creep rate with grain size for Al2O3±72.7 vol% ZrO2 composite. The grain size may be de®ned in terms of the ZrO2, or Al2O3 phases or their volume average. p: (&) ZrO2, 2.0; (
) Al2O3, 2.6; (o) Av. 2.1 [32]. Q. Tai. A. Mocellin / Ceramics International 25 (1999) 395±408 401