Q. Tai. A. Mocellin/Ceramics International 25(1999)395-408 containing Sio2 which decreases the creep resistance. phase developing at grain boundaries led to increased Impurities such as Y2O3 favour the formation of the strain rates. The creep deformation of the Al_ -TiC grain boundary glassy phase as well. Unaccommodated composite was mainly attributed to grain boundary slid grain boundary sliding often causes cavitation and ing although the diffusion of Al,O3 and dislocation microcracks In some conditions, dislocation creep cau- motion controlled by the diffusion of carbon of TiC ses dislocation networks. The main creep mechanism of played a small part in the deformation of the composite the Al2O3-SiC composites is grain boundary sliding The deformation in Al2O3-54 vol% TiCo.s No5 com some accommodated by diffusion and some unac- posite was studied by Katsumura et al. [58]. This mate- commodated in most instances But in some instances. a rial showed higher strength at elevated temperature. It creep cavitation and microcracking mechanism or a deformed above 1300C and exhibited a superplastic dislocation creep mechanism may control the deforma- behaviour. The values of n were almost unchanged with tion of the composites the temperature, 2.2(at 1300 C)and 2.0(at 1400C) respectively. The apparent activation energy was rather 3.2.3. Al2OrTiCKNI-x composites low (281.6KJ mol-). The observation of micro- siea t lthough the processing of Al2O3-TiC Ni-x compo- structure of the deformed specimens showed that the (x=O-l)has been known for some time and their grain size was nearly the same as that before deforma- ambient temperature mechanical properties were repor- tion and the grains remained equiaxed. No new phase ted, their high temperature mechanical properties, espe- was formed. Furthermore, the authors observed that the lly the creep behaviours, remain but poorly amount of TiCo.s Nos grains in the deformed section documented was slightly less than that of TiCo.s.s grains in the Nagano et al. [57] studied the deformation of Al_O3- undeformed section. They suggested the migration of 6 vol% TiC composite. Their experiments showed that Al2O3 against TiCo. s Nos during the deformation and this Al2O3-TiC composite exhibited a ductility of 66% proposed a 'heterogeneous grain boundary sliding in tension. It deformed at a faster strain rate than Al2O3 mechanism in this composite. This conclusion however in the high stress region. The values of the stress expo- should be verified by more microstructural information nent increased with increasing temperature, from 3. 2(at Jiao et al. [56] evaluated the interfacial fracture 1450C)to 4.2(at 1550.C), respectively(Fig. 13). The energy and its relation to mechanical behaviour in authors suggested that the difference in stress exponent Al2O3-5 wt% TiN nanocomposite. They pointed out might be affected by the stoichiometry of TiC itself that according to a theoretical calculation the interfacial shape after deformation was changed. The grain aspect higher than grain boundary fracture energy of Alo s modified by reaction between TiC and Al2O3. The grain fracture energy between TiN and Al2O3 was mu ratio was 1.26 in the direction of tensile stress. Cavities but in fact, this composite was found to fracture inter were also observed at grain boundaries. No new phase granularly in a similar manner to monolithic Al2O3 and was found at temperatures lower than 1500C, but Ti203 its strength was 35% less than that of Al_O3. This is formed by the reaction between Al2O3 and TiC was presumably due to the formation of TiO2 at AlO3/TIN observed in the specimens tested at 1550C. The Ti2O3 interfaces, resulting in a lower interfacial fracture energy and weaker grain boundaries of AlO3, which deterio rate the high temperature mechanical properties includ ing creep resistance of the Al2O3-TiN composite Since Al2O3-based ceramic composites possess excellent Temp mechanical properties at moderate temperatures, their plastic deformation behaviours are also investigated in the purpose to evaluate their practical applications at high temperature. In this paper, creep mechanisms of cera- mic materials are first briefly reviewed. Then the studies of plastic deformation behaviours of Al2O3-based cera- mic particle or whisker composites since the mid 1980s are reviewed. The influences of various factors on the Strain rate【s-) creep behaviours, the changes of the microstructure of the deformed specimens, and the creep mechanisms of Fig. 13.. Influence of temperature on stress exponent in an Al,O the Al2O3-based ceramic composites are summarised TiC composite [56]. and analysedcontaining SiO2 which decreases the creep resistance. Impurities such as Y2O3 favour the formation of the grain boundary glassy phase as well. Unaccommodated grain boundary sliding often causes cavitation and microcracks. In some conditions, dislocation creep cau￾ses dislocation networks. The main creep mechanism of the Al2O3±SiC composites is grain boundary sliding some accommodated by di€usion and some unac￾commodated in most instances. But in some instances, a creep cavitation and microcracking mechanism or a dislocation creep mechanism may control the deforma￾tion of the composites. 3.2.3. Al2O3±TiCxN1-x composites Although the processing of Al2O3±TiCxN1-x compo￾sites (x=0±1) has been known for some time and their ambient temperature mechanical properties were repor￾ted, their high temperature mechanical properties, espe￾cially the creep behaviours, remain but poorly documented. Nagano et al. [57] studied the deformation of Al2O3- 26 vol% TiC composite. Their experiments showed that this Al2O3±TiC composite exhibited a ductility of 66% in tension. It deformed at a faster strain rate than Al2O3 in the high stress region. The values of the stress expo￾nent increased with increasing temperature, from 3.2 (at 1450C) to 4.2 (at 1550C), respectively (Fig. 13). The authors suggested that the di€erence in stress exponent might be a€ected by the stoichiometry of TiC itself modi®ed by reaction between TiC and Al2O3. The grain shape after deformation was changed. The grain aspect ratio was 1.26 in the direction of tensile stress. Cavities were also observed at grain boundaries. No new phase was found at temperatures lower than 1500C, but Ti2O3 formed by the reaction between Al2O3 and TiC was observed in the specimens tested at 1550C. The Ti2O3 phase developing at grain boundaries led to increased strain rates. The creep deformation of the Al2O3±TiC composite was mainly attributed to grain boundary slid￾ing although the di€usion of Al2O3 and dislocation motion controlled by the di€usion of carbon of TiC played a small part in the deformation of the composite. The deformation in Al2O3-54 vol% TiC0.5N0.5 com￾posite was studied by Katsumura et al. [58]. This mate￾rial showed higher strength at elevated temperature. It deformed above 1300C and exhibited a superplastic behaviour. The values of n were almost unchanged with the temperature, 2.2 (at 1300C) and 2.0 (at 1400C), respectively. The apparent activation energy was rather low (281.6 KJ molÿ1 ). The observation of micro￾structure of the deformed specimens showed that the grain size was nearly the same as that before deforma￾tion and the grains remained equiaxed. No new phase was formed. Furthermore, the authors observed that the amount of TiC0.5N0.5 grains in the deformed section was slightly less than that of TiC0.5N0.5 grains in the undeformed section. They suggested the migration of Al2O3 against TiC0.5N0.5 during the deformation and proposed a `heterogeneous grain boundary sliding' mechanism in this composite. This conclusion however should be veri®ed by more microstructural information. Jiao et al. [56] evaluated the interfacial fracture energy and its relation to mechanical behaviour in Al2O3-5 wt% TiN nanocomposite. They pointed out that according to a theoretical calculation the interfacial fracture energy between TiN and Al2O3 was much higher than grain boundary fracture energy of Al2O3, but in fact, this composite was found to fracture inter￾granularly in a similar manner to monolithic Al2O3 and its strength was 35% less than that of Al2O3. This is presumably due to the formation of TiO2 at Al2O3/TiN interfaces, resulting in a lower interfacial fracture energy and weaker grain boundaries of Al2O3, which deterio￾rate the high temperature mechanical properties includ￾ing creep resistance of the Al2O3±TiN composite. 4. Summary Since Al2O3-based ceramic composites possess excellent mechanical properties at moderate temperatures, their plastic deformation behaviours are also investigated in the purpose to evaluate their practical applications at high temperature. In this paper, creep mechanisms of cera￾mic materials are ®rst brie¯y reviewed. Then the studies of plastic deformation behaviours of Al2O3-based cera￾mic particle or whisker composites since the mid 1980s are reviewed. The in¯uences of various factors on the creep behaviours, the changes of the microstructure of the deformed specimens, and the creep mechanisms of the Al2O3-based ceramic composites are summarised and analysed. Fig. 13. . In¯uence of temperature on stress exponent in an Al2O3± TiC composite [56]. 406 Q. Tai. A. Mocellin / Ceramics International 25 (1999) 395±408