Q. Tai. A. Mocellin/Ceramics International 25(1999)395-408 Microstructural observation of creep specimens gen 1400°c erally revealed more or less cavitation at interfaces grain boundaries and triple joint junctions. [ 43, 45- 苏 1200C 60, 52 Unaccommodated grain boundary sliding wa considered to be responsible for the formation of cav- ities. Higher stresses and higher temperatures often increased the amount of intergranular voids, cavities and cracks, and sometimes caused the separation of interfaces and grain boundaries [43, 48]. The Al2O3- SiC(w) composites exhibited higher number density and 言 smaller cavities than those of monolithic AlO3 [45] One study showed that 30 and 50 vol% SiC(w)comp sites exhibited cavity number density 2 orders of mag- nitude higher than that of Al2O3-20 vol%S 10 composite. The average cavity size of 30 and 50 vol% Appfied Stress(MPa) SiC(w) composites was 0.4 um, while that of 20 vol% composite was 0.05 um [47]. In the four-point bending Fig. 11. Influence of temperature on stress exponent in an AlOr tests, the cavity number density was approximately 1 to 2 orders of magnitude in the compressive surface region less than that in the tensile surface region [47], and at higher temperature, cracklike cavities tended to form increased with increasing whisker content [42, 46, 47] and extensive macroscopic tensile surface cracks, and the the values of n were lower [46, 47, but when the whisker number of surface cracks increased with the grain size of content exceeded 30 vol%, the creep rates and the values AlO3 [52] of n increased due to(1) the promotion of creep cavita Although a glassy phase was present in very limit tion and crack generation from the higher number den- amounts in some as-sintered composites, many authors sity of nucleation sites, and (2)more extensive [43, 46, 47, 49, 52] observed that after creep deformation formation of grain boundary glassy phase in air ambient a SiO2-rich glassy phase around the The effect of test ambient on creep behaviours is an whiskers had formed due to the oxidation of the latter important issue since the Sic whiskers are easily oxi- In most cases, the glassy phase penetrated along grain dized in air at elevated temperature Experiments [46, 48 boundaries and interfaces and accumulated at triple showed that although the stress exponents were almost grain junctions throughout the materials, which pre the same in different ambients, the creep rates were sumably facilitated grain boundary sliding and higher in air than in inert atmosphere. Lin et al. [52 increased the creep rate have investigated the effect of matrix grain size(varying Dislocation networks were observed by some authors from 1. 2 to 8.0 um)on the creep behaviours. At 1200C [42, 43, 46, 49, 51]. But in most cases, the dislocation den the creep rate exhibited an inverse grain size exponent of sity was very low [42, 43, 46, 49] and the dislocation net approximately l, but at 1300C the creep rate was not works were also observed in the as-sintered specimens sensitive to the grain size due to enhanced nucleation They probably formed as a result of the large residual and coalescence of creep cavities and the development stresses resulting from thermal expansion mismatch of macroscopic cracks as the grain size increased. In during the fabrication processing. Contrary to other addition, the creep rate of Al2O3-SiC(w) composites was authors, Xia et al. [51] observed extensive dislocation accelerated by the introduction of certain additives (i.e. networks in the crept specimens, and higher dislocation Y2O3)[47. The presence of an intergranular glassy densities were observed in specimens deformed to large phase introduced by the Y2O3 additive facilitated creep strains deformation resulting in an order of magnitude increase Some authors observed the grain offset and rotation in creep rate. Finally, it should be pointed out that of Al2O3 [45, 52. As far as the creep mechanism is con- the method of creep testing may also influence the cerned, in early studies, a dislocation-controlled creep experimental results. In four-point bending creep mechanism was proposed by some authors [(41, 42, 44 tests, the creep rate is strongly affected by the surface But no sufficient microstructural information was pre- tensile properties of the specimens. The growth of sented to support this assertion, and this mechanism for surface cracks can result in a creep exponent of 2 the deformed Al2O3-SiC(w) composites remains doubt or higher when the growth rate of microcrack obeys ful. A more recent study [51]revealed extensive disloca- a power-law dependence on the local normal stress. tion activity and very little evidence for the development This mode of testing can result in large errors in the of initial cavitation. The authors proposed an intra- stress exponent [49] granular dislocation mechanism controlled by latticeincreased with increasing whisker content [42,46,47] and the values of n were lower [46,47], but when the whisker content exceeded 30 vol%, the creep rates and the values of n increased due to (1) the promotion of creep cavita￾tion and crack generation from the higher number den￾sity of nucleation sites, and (2) more extensive formation of grain boundary glassy phase. The e€ect of test ambient on creep behaviours is an important issue since the SiC whiskers are easily oxi￾dized in air at elevated temperature. Experiments [46,48] showed that although the stress exponents were almost the same in di€erent ambients, the creep rates were higher in air than in inert atmosphere. Lin et al. [52] have investigated the e€ect of matrix grain size (varying from 1.2 to 8.0 m) on the creep behaviours. At 1200C the creep rate exhibited an inverse grain size exponent of approximately 1, but at 1300C the creep rate was not sensitive to the grain size due to enhanced nucleation and coalescence of creep cavities and the development of macroscopic cracks as the grain size increased. In addition, the creep rate of Al2O3±SiC(w) composites was accelerated by the introduction of certain additives (i.e. Y2O3) [47]. The presence of an intergranular glassy phase introduced by the Y2O3 additive facilitated creep deformation resulting in an order of magnitude increase in creep rate. Finally, it should be pointed out that the method of creep testing may also in¯uence the experimental results. In four-point bending creep tests, the creep rate is strongly a€ected by the surface tensile properties of the specimens. The growth of surface cracks can result in a creep exponent of 2 or higher when the growth rate of microcrack obeys a power-law dependence on the local normal stress. This mode of testing can result in large errors in the stress exponent [49]. Microstructural observation of creep specimens gen￾erally revealed more or less cavitation at interfaces, grain boundaries and triple joint junctions. [43,45± 50,52] Unaccommodated grain boundary sliding was considered to be responsible for the formation of cav￾ities. Higher stresses and higher temperatures often increased the amount of intergranular voids, cavities and cracks, and sometimes caused the separation of interfaces and grain boundaries [43,48]. The Al2O3± SiC(w) composites exhibited higher number density and smaller cavities than those of monolithic Al2O3 [45]. One study showed that 30 and 50 vol% SiC(w) compo￾sites exhibited cavity number density 2 orders of mag￾nitude higher than that of Al2O3-20 vol% SiC(w) composite. The average cavity size of 30 and 50 vol% SiC(w) composites was 0.4 m, while that of 20 vol% composite was 0.05 m [47]. In the four-point bending tests, the cavity number density was approximately 1 to 2 orders of magnitude in the compressive surface region less than that in the tensile surface region [47], and at higher temperature, cracklike cavities tended to form extensive macroscopic tensile surface cracks, and the number of surface cracks increased with the grain size of Al2O3 [52]. Although a glassy phase was present in very limited amounts in some as-sintered composites, many authors [43,46,47,49,52] observed that after creep deformation in air ambient a SiO2±rich glassy phase around the whiskers had formed due to the oxidation of the latter. In most cases, the glassy phase penetrated along grain boundaries and interfaces and accumulated at triple grain junctions throughout the materials, which pre￾sumably facilitated grain boundary sliding and increased the creep rate. Dislocation networks were observed by some authors [42,43,46,49,51]. But in most cases, the dislocation den￾sity was very low [42,43,46,49] and the dislocation net￾works were also observed in the as-sintered specimens. They probably formed as a result of the large residual stresses resulting from thermal expansion mismatch during the fabrication processing. Contrary to other authors, Xia et al. [51] observed extensive dislocation networks in the crept specimens, and higher dislocation densities were observed in specimens deformed to large strains. Some authors observed the grain o€set and rotation of Al2O3 [45,52]. As far as the creep mechanism is con￾cerned, in early studies, a dislocation-controlled creep mechanism was proposed by some authors [41,42,44]. But no sucient microstructural information was pre￾sented to support this assertion, and this mechanism for the deformed Al2O3±SiC(w) composites remains doubt￾ful. A more recent study [51] revealed extensive disloca￾tion activity and very little evidence for the development of initial cavitation. The authors proposed an intra￾granular dislocation mechanism controlled by lattice Fig. 11. In¯uence of temperature on stress exponent in an Al2O3± 33 vol% SiC(w) composite (at 1200C, n=1; at 1300, 1400C, n=3) [48]. 404 Q. Tai. A. Mocellin / Ceramics International 25 (1999) 395±408