some basal slip could easily occur. Furthermore, when composite theory: isostrain and isostress model [15,16 grain size of Al_O3 was between 50 and 500 um and the (Fig. 6). Isostrain and isostress prediction diagrams can imposed stress was large enough, then dislocation climb be constructed by calculation. By comparing the and glide could occur. But in practice, the theory and experimental data with the isostrain and isostress pre- the experimental data used to construct the maps are diction, the model fitting the creep deformation can be poor or insufficient for many ceramic materials, thus determined. Since the isostress model is dominated by limiting their use in applications he least creep resistant phase and highest creep rate and At present, the main method to identify the possible the isostrain model is dominated by the most creep ate controlling mechanism is to compare the values of resistant phase and lowest creep rate, the phase con np and o obtained from experiments with theoretical trolling the creep behaviour can be determined predictions. Chokshi et al. [12 indicated that many For the two phase composites in which both phases ceramics exhibit stress exponents of N5, N3 or l deform inelastically, a self-consistent model was devel which appeared to be associated with dislocation glide oped [17, 18], which predicts the deformation behaviour and climb, climb from Bardeen-Herring sources, and of the composites when the viscoplastic laws of diffusion creep, respectively. The stress exponent of 2 phase are known. By self-consistent calculations, effec- might be due to the presence of a partially wetting grain tive strain rate sensitivity parameter and effective pre- boundary glassy phase or to control by an interface factor which are characteristic of the composite reaction[ 12] Besides, the stress exponent between I and behaviour can be obtained. Stress and strain rates in might be associated with grain boundary sliding and each phase are also attainable. From the comparison grain rearrangement, and a higher stress exponent between the model and the experiments, the possible (n>3)can also be associated with cavitation creep and deformation mechanisms of each phase can be deter microcracking. The inverse grain size exponent points to mined and the phase controlling the creep behaviour either a boundary mechanism (p>1)or a lattice can also be qualitatively determined mechanism (p=0). In general, a direct observation and analysis of the microstructure of the specimens after deformation is necessary to check preliminary conclu- 3. Plastic deformation behaviours of Al2O3-based sions drawn from the experimentally determined stress- ceramic composites train rate relationship For two phase composites, especially for those with In studies of the plastic deformation behaviours of duplex microstructures, authors are not only interested Al2O3-based ceramic composites, great attention is in the dominant mechanism of deformation, but also try concentrated on three aspects: strain rates, micro- to find out which phase controls the creep behaviour. structural changes and deformation mechanisms Creep of composites can be modelled by using standard As concerns the first of these items one of the main aims is to investigate the relationship between the creep rates and operating variables(imposed stress, grain size, temperature)and to evaluate the creep parameters(n, P, Q). The deformation behaviours are critically dependent Nabarro Climb, Dp DIffusional Crae :88: 3. MOCELILIOEmGEAY STRESS, MPa Fig. 6. Idealized composite microstructures:(a) isostress and(b)iso- Fig. 5. Deformation map for MgO-doped Al2O3 at 1500C [9]. strain orientationssome basal slip could easily occur. Furthermore, when grain size of Al2O3 was between 50 and 500 mm and the imposed stress was large enough, then dislocation climb and glide could occur. But in practice, the theory and the experimental data used to construct the maps are poor or insucient for many ceramic materials, thus limiting their use in applications. At present, the main method to identify the possible rate controlling mechanism is to compare the values of n,p and Q obtained from experiments with theoretical predictions. Chokshi et al. [12] indicated that many ceramics exhibit stress exponents of 5, 3 or 1, which appeared to be associated with dislocation glide and climb, climb from Bardeen±Herring sources, and di€usion creep, respectively. The stress exponent of 2 might be due to the presence of a partially wetting grain boundary glassy phase or to control by an interface reaction. [12] Besides, the stress exponent between 1 and 3 might be associated with grain boundary sliding and grain rearrangement, and a higher stress exponent (n53) can also be associated with cavitation creep and microcracking. The inverse grain size exponent points to either a boundary mechanism (p51) or a lattice mechanism (p=0). In general, a direct observation and analysis of the microstructure of the specimens after deformation is necessary to check preliminary conclu￾sions drawn from the experimentally determined stress± strain rate relationships. For two phase composites, especially for those with duplex microstructures, authors are not only interested in the dominant mechanism of deformation, but also try to ®nd out which phase controls the creep behaviour. Creep of composites can be modelled by using standard composite theory: isostrain and isostress model [15,16] (Fig. 6). Isostrain and isostress prediction diagrams can be constructed by calculation. By comparing the experimental data with the isostrain and isostress pre￾diction, the model ®tting the creep deformation can be determined. Since the isostress model is dominated by the least creep resistant phase and highest creep rate and the isostrain model is dominated by the most creep resistant phase and lowest creep rate, the phase con￾trolling the creep behaviour can be determined. For the two phase composites in which both phases deform inelastically, a self-consistent model was devel￾oped [17,18], which predicts the deformation behaviour of the composites when the viscoplastic laws of each phase are known. By self-consistent calculations, e€ec￾tive strain rate sensitivity parameter and e€ective pre￾factor which are characteristic of the composite behaviour can be obtained. Stress and strain rates in each phase are also attainable. From the comparison between the model and the experiments, the possible deformation mechanisms of each phase can be deter￾mined and the phase controlling the creep behaviour can also be qualitatively determined. 3. Plastic deformation behaviours of Al2O3-based ceramic composites In studies of the plastic deformation behaviours of Al2O3-based ceramic composites, great attention is concentrated on three aspects: strain rates, micro￾structural changes and deformation mechanisms. As concerns the ®rst of these items, one of the main aims is to investigate the relationship between the creep rates and operating variables (imposed stress, grain size, temperature) and to evaluate the creep parameters (n, p, Q). The deformation behaviours are critically dependent Fig. 5. Deformation map for MgO-doped Al2O3 at 1500C [9]. Fig. 6. Idealized composite microstructures: (a) isostress and (b) iso￾strain orientations. 398 Q. Tai. A. Mocellin / Ceramics International 25 (1999) 395±408