258 C. O Meara et al. Materials Science and Engineering A209 (1996)251-259 damage accumulation will be a summation of damage 100 um) surrounded/separated by al2O3 rich rims(10 effects in the two different elements of the microstruc- um). Stress exponents of approximately three ure. The clusters have a high SiC content and can be spond well with the microstructural analysis expected to cavitate easily as observed. The skeleton of indicated that the secondary creep rate is dominated by Al2O3 with low whisker should be more creep resistant a damage accumulation process namely cavitation and and will act as the reinforcing phase encapsulating the crack growth in both the Sic clusters and the Al2O SiC damage and preventing it from linking up. Cavita- rims. Final fracture seems to occur through the alumina tion and crack formation in the clusters would then be rich regions. The poorer creep resistance of this com te and Sic cluster cracks are observed to be more frequent in attributed primarily to the inhomogeneity of the as-re- specimens with a high secondary creep rate. Cavitation ceived material also occurs in the Al,O3 rich rims phase but not as ly as in the Sic clusters. Final fracture seems to originate from the surface in the alumina regions and then to propagate through these regions intergranular either when the fracture stress is reached for alumina of References this particular grain size(specimen 4)or when a suffi cient number of contiguous cavities have been built up [] P F. Becher and G.C. Wei, Toughening behaviour in SiC such that the crack is subject to a stress intensity whisker- reinforced alumina, J, Am, Ceram, Soc,, 67C (1984)267. [2]GC. Wei and P F. Becher, Development of SiC-whisker-rein- K>Kh [26(specimens 1-3, 5-9). This interpretation forced ceramics, Am. Ceram Soc. Bull., 64(1985)298 is consistent with and can explain the differences in [3] J. Homeny and W.L. Vaughn, Silicon carbide whisker/alumina crept microstructures, times to failure and secondary atrix composites effect of whisker surface treatment on frac- creep rates of the tested samples re toughness, J. Am. Ceram. Soc., 73(1990)394. The effect of oxidation on the creep behaviour was 4 T. Hansson, R. Warren and J. Wasen, Fracture toughness ifficult to evaluate as all the samples had been sub anisotropy and toughening mechanisms of a hot-pressed alumina reinforced with silicon carbide whiskers, J. Am. Ceram. Soc.76 jected to high temperature air exposure. Although other (1993)841 microstructural factors are responsible for creep failure [5]A H Chokshi and J.R. Porter, Creep deformation of an alumina ith reference to other work it is highly likely that matrix composite reinforced with silicon carbide whiskers, J. oxidation during creep exposure does facilitate cavita- Am. Ceram. Soc., 68C(1985)144 tion in this work some oxidation of the sic whiskers [6]J. R. Porter, FF. Lange and A H. Chokshi, Processing and creep rformance of SiC-whisker-reinforced Al,O3, Am. Ceram. Soc in the matrix was observed in TEM although no signifi- Bu.,66(1987)343. cant increase could be detected in the volume of inter- [7] A.R. de arellano-Lopez, F L. Cumbrera, A. Domingues-Ro granular amorphous phase in non-cavitated regions of drigues, K C. Goretta and J. L. Routbort, Compressive creep of the microstructure as compared to as- received material SiC-whisker-reinforced Al,O,, J. Am. Ceram. Soc., 73(1990) The TEM evidence suggests that oxidation of the [8]H -T Lin and P F Becher, High-temperature creep deformation whiskers had occurred after cavitation and may bond of alumina-SiC-whisker composites, J. Am. Ceram. Soc. the whiskers or the whiskers and the matrix grains gether. In addition the specimens which were pre-heat [9]P, Lipetzky, S.R. Nutt, D.A. Koester and R F. Davis, Atmo- treated had a lower density of cavities, lower cavity spheric effects on compressive creep of Sic-whisker-reinforced nucleation rates, fewer Sic cluster cavities and lower (10) A R. de Arellano-Lopez, A.D. Domingues-Rodrigues, KC secondary creep rates than specimens subjected to the Goretta and J.L. Routbort Plastic deformation in SiC-whisker same conditions without pre-heat treatment. In general reinforced alumina, J. Am. Ceram. Soc., 76(1993)1425 the specimens with the lowest secondary creep rates 11]A H. Swan, M. V Swain and G L. Dunlop, Compressive creep of were also those which were the most severely oxidised SiC-whisker-reinforced alumina, J. Eur. Ceram. Soc., 10( 1992) At this stage it is therefore difficult to accurately iden- [12)J. L. Routbort, K.C. Goretta, A. D. Domingues-Rodrigues and tify the role of oxidation, whether positive or negative, A R. de Arellano- Lopez, Creep of whisker-reinforced ceramics. on the creep deformation process(es) J. Hard [13] T. Hansson, C. OMeara, K. Rundgren, P. Svensson and Warren, Tensile creep of alumina and Sic whisker reinforced 5. Conclusions alumina, Proc. Plastic Deformation of Ceramics, Engineering Foundation Conf, Snowbird, Utah, August 7-12, 1994 [14] W.R. Cannon and T.G. Langdon, Review: Creep of ceramics- The tensile creep behaviour of a Sic(25%)reinforced Part 1. Mechanical characteristics, J. Mater. Sci, 18(1983) alumina composite was investigated in air in the ranges 5]A H. Chokshi and J. R. Porter, High temperature mechanic 1100-1300 C and 11-67 MPa. The as-fabricated properties of single phase alumina, J. Mater. Sci., 21(1986)705 microstructure was found to be extremely inhomoge [16] A.G. Robertson, D.S. Wilkinson and C H. Caceres, Creep and reep fracture in hot-pressed alumina, J. Am. Ceram. Soc., 74 neous consisting of spherical whisker-rich clusters(20 (1991)915258 C. O'Meara et al. / Materials Science and Engineering A209 (1996) 251-259 damage accumulation will be a summation of damage effects in the two different elements of the microstruc￾ture. The clusters have a high SiC content and can be expected to cavitate easily as observed. The skeleton of A1203 with low whisker should be more creep resistant and will act as the reinforcing phase encapsulating the SiC damage and preventing it from linking up. Cavita￾tion and crack formation in the clusters would then be a major contributor to the secondary creep rate and SiC cluster cracks are observed to be more frequent in specimens with a high secondary creep rate. Cavitation also occurs in the A1203 rich rims phase but not as easily as in the SiC clusters. Final fracture seems to originate from the surface in the alumina regions and then to propagate through these regions intergranularly either when the fracture stress is reached for alumina of this particular grain size (specimen 4) or when a suffi￾cient number of contiguous cavities have been built up such that the crack is subject to a stress intensity K~> Kth [26] (specimens 1-3, 5-9). This interpretation is consistent with and can explain the differences in crept microstructures, times to failure and secondary creep rates of the tested samples. The effect of oxidation on the creep behaviour was difficult to evaluate as all the samples had been sub￾jected to high temperature air exposure. Although other microstructural factors are responsible for creep failure, with reference to other work it is highly likely that oxidation during creep exposure does facilitate cavita￾tion. In this work some oxidation of the SiC whiskers in the matrix was observed in TEM although no signifi￾cant increase could be detected in the volume of inter￾granular amorphous phase in non-cavitated regions of the microstructure as compared to as-received material. The TEM evidence suggests that oxidation of the whiskers had occurred after cavitation and may bond the whiskers or the whiskers and the matrix grains together. In addition the specimens which were pre-heat treated had a lower density of cavities, lower cavity nucleation rates, fewer SiC cluster cavities and lower secondary creep rates than specimens subjected to the same conditions without pre-heat treatment. In general the specimens with the lowest secondary creep rates were also those which were the most severely oxidised. At this stage it is therefore difficult to accurately iden￾tify the role of oxidation, whether positive or negative, on the creep deformation process(es). 5. Conclusions The tensile creep behaviour of a SiC (25%) reinforced alumina composite was investigated in air in the ranges 1100-1300 °C and 11-67 MPa. The as-fabricated microstructure was found to be extremely inhomoge￾neous consisting of spherical whisker-rich clusters (20- 100/tm) surrounded/separated by A1203 rich rims (10 /~m). Stress exponents of approximately three corre￾spond well with the microstructural analysis which indicated that the secondary creep rate is dominated by a damage accumulation process namely cavitation and crack growth in both the SiC clusters and the AlzO 3 rims. Final fracture seems to occur through the alumina rich regions. The poorer creep resistance of this com￾posite compared with that of similar composites is attributed primarily to the inhomogeneity of the as-re￾ceived material. References [1] P.F. Becher and G.C. Wei, Toughening behaviour in SiC￾whisker-reinforced alumina, J. Am. Ceram. Soc., 67C (1984) 267. [2] G.C. Wei and P.F. Becher, Development of SiC-whisker-rein￾forced ceramics, Am. Ceram. Soc. Bull., 64 (1985) 298. [3] J. Homeny and W.L. Vaughn, Silicon carbide whisker/alumina matrix composites: effect of whisker surface treatment on frac￾ture toughness, J. Am. Ceram. Soc., 73 (1990) 394. [4] T. Hansson, R. Warren and J. Was6n, Fracture toughness anisotropy and toughening mechanisms of a hot-pressed alumina reinforced with silicon carbide whiskers, J. Am. Ceram. Soc., 76 (1993) 841. [5] A.H. Chokshi and J.R. Porter, Creep deformation of an alumina matrix composite reinforced with silicon carbide whiskers, J. Am. Ceram. Soc., 68C (1985) 144. [6] J.R. Porter, F.F. Lange and A.H. Chokshi, Processing and creep performance of SiC-whisker-reinforced A1203, Am. Ceram. Soc. Bull., 66 (1987) 343. [7] A.R. de Arellano-L6pez, F.L. Cumbrera, A. Domingues-Ro￾drigues, K.C. Goretta and J.L. Routbort, Compressive creep of SiC-whisker-reinforced A1203, J. Am. Ceram. Soc., 73 (1990) 1297. [8] H.-T. Lin and P.F. Becher, High-temperature creep deformation of alumina-SiC-whisker composites, J. Am. Ceram. Soc., 74 (1991) 1886. [9] P. Lipetzky, S.R. Nutt, D.A. Koester and R.F. Davis, Atmo￾spheric effects on compressive creep of SiC-whisker-reinforced alumina, J. Am. Ceram. Soc., 74 (1991) 1240. [10] A.R. de Arellano-L6pez, A.D. Domingues-Rodrigues, K.C. Goretta and J.L. Routbort, Plastic deformation in SiC-whisker￾reinforced alumina, J. Am. Ceram. Soc., 76 (1993) 1425. [11] A.H. Swan, M.V. Swain and G.L. Dunlop, Compressive creep of SiC-whisker-reinforced alumina, J. Eur. Ceram. Soc., I0 (1992) 317. [12] J.L. Routbort, K.C. Goretta, A.D. Domingues-Rodrigues and A.R. de AreUano-L6pez, Creep of whisker-reinforced ceramics, J. Hard Mater., I (1990) 221. [13] T. Hansson, C. O'Meara, K. Rundgren, P. Svensson and R. Warren, Tensile creep of alumina and SiC whisker reinforced alumina, Proc. Plastic Deformation of Ceramics, Engineering Foundation Conf., Snowbird, Utah, August 7-12, 1994. [14] W.R. Cannon and T.G. Langdon, Review: Creep of ceramics-- Part 1. Mechanical characteristics, J. Mater. Sci., 18 (1983) 1. [15] A.H. Chokshi and J.R. Porter, High temperature mechanical properties of single phase alumina, J. Mater. Sci., 21 (1986) 705. [16] A.G. Robertson, D.S. Wilkinson and C.H. C~tceres, Creep and creep fracture in hot-pressed alumina, J. Am. Ceram. Soc., 74 (1991) 915