1426 XIA and LANGDON: DEFORMATION OF AN ALUMINA COMPOSITE lower than in the present experiments by about three fiber stress of 61.5 MPa(Fig. 7)whereas the exper- orders of magnitude. iments of Lin and Becher [17], conducted at the lower The procedure employed by Lin and Becher [17] temperature of 1573 K, covered estimated outer fiber was similar to that used in the present experiments. stresses in the range from 100 to 234 MPa. It is he samples were obtained by hot-pressing, tests probable that these higher stress levels account for were conducted in air using a four-point bending the markedly different creep behavior in the two sets rig and the stresses and strains were calculated of experiments using equations (1)and(2). The average grain size was x8 um in the as-fabricated samples, but this larger size should have no effect on the measured 5. SUMMARY AND CONCLUSIONS train rates if an intragranular deformation process (Creep tests were conducted in air on an is dominant alumina composite reinforced with 9.3 vol. of A significant difference lies in the overall creep silicon carbide whiskers behavior of the two sets of samples. Lin and Becher (2)The creep curves exhibited a well-defined [17 tested their specimens at a temperature of 1573 K steady-state region with strains to failure in excess of and obtained total strains to failure of <0.5%, 5% and >10% under whereas the present experiments were conducted at conditions temperatures in the range from 1673 to 1823 K and (3)The creep data gave a stress exponent of yielded strains to failure which were consistently + 3.8 and an activation energy for creep >5% and even up to >10%. Further, Lin and + mol cher[17] reported the development of very exten-(4)The microstructure after deformation revealed sive internal cavitation, primarily in the form of small extensive dislocation activity and with very little polyhedral and penny-shaped cavities lying along the evidence for the development of internal cavitation grain boundaries and cracking along boundaries (5)It is concluded that the presence of many where no whiskers were present, whereas very little Sic whiskers in the grain boundaries inhibits the cavitation was visible in the present experiments after occurrence of Lifshitz grain boundary sliding and failure therefore diffusion creep is suppressed. Instead, the The occurrence of extensive cavitation in the expe composite deforms by an intragranular dislocatic ments of Lin and Becher [17] may account for the process apparent discrepancy in the creep data as recorded in anions controlled by lattice diffusion of the oxygen Fig. 10. In four-point bending experiments where equations(1)and (2)necessitates that the neutral axis the Advanced Composite Materl au to Dr J F. Rhodes of samples deform by power-law creep, the use of Acknowledgements-We are gr mains along the central line of the specimen with ing the material used in this investigation. This work was the tensile and compressive behavior obeying the supported in part by the U.S. Army Research Office under Grant No. DAALO3-91-G-0230 same constitutive law. When the stress exponent, n, is greater than s2, the development of microcracking and cavitation shifts the neutral axis away from this REFERENCES central line and this will lead to a significant overes- timation of the outer fiber stress and a consequent L. P. F. Becher W. H underestimation of the equivalent tensile creep rate Warwick, in Fracture Mechanics of Ceramics, Vol. 7 [30, 31]. In view of the large amounts of cavitation Hasselman and F, F. Lange), p. 61. Plenum Press reported by Lin and Becher [17), it is possible that New York(1986) heir estimates of the outer fiber stresses and strains 2. K. Niihara, A. Nadahira, T. Uchiyama and T. Hirai, may be in error. R. C. Bradt, A. G. Evans, D. P. H. Hasselman Finally, it is necessary to address the apparent and F. F. Lange), p 103. Plenum Press, New York dichotomy between the experiments Becher [17] where cavitation was extensive and the L. Vaughn and M. K. Ferber, Am. present experiments where cavity development wa very limited. In the work of de Arellano-Lopez 4.P D. Shalek, JJ. Petrovic, G F. Hurley and F D Gac, et al. [18] on alumina composites containing up 5. R Lundberg, L. Kahlman, R Pompe and RCarlsson to 30 vol. of Sic whiskers, it was reported that, pon the fabrication pro sed to 6. M. Ruhle, N. Clausson and A. H. Heuer. J. Am. ceran Soc.69,195(1986 duce the composites, there is a critical stress below 7. s. C. Samanta and S. Musikant, Ceram. Engng Sci hich the development of cavitation damage is essen- tially negligible. Although insufficient information 8. G. C. Wei and P F Becher, J. Am. Ceram. Soc. 67, 571 is currently available to determine the magnitude of the critical stress under any selected experimental conditions 1(1983) nevertheless noted that the present 10. W.R. Cannon and T.G. Langdon, J Mater. Sci. 23, experiments were performed up to a maximum outer 1(1988)1426 XIA and LANGDON: DEFORMATION OF AN ALUMINA COMPOSITE lower than in the present experiments by about three orders of magnitude. The procedure employed by Lin and Becher [17] was similar to that used in the present experiments. The samples were obtained by hot-pressing, tests were conducted in air using a four-point bending rig and the stresses and strains were calculated using equations (1) and (2). The average grain size was ~8/~m in the as-fabricated samples, but this larger size should have no effect on the measured strain rates if an intragranular deformation process is dominant. A significant difference lies in the overall creep behavior of the two sets of samples. Lin and Becher [17] tested their specimens at a temperature of 1573 K and obtained total strains to failure of ~<0.5%, whereas the present experiments were conducted at temperatures in the range from 1673 to 1823 K and yielded strains to failure which were consistently >5% and even up to >10%. Further, Lin and Becher [17] reported the development of very exten￾sive internal cavitation, primarily in the form of small polyhedral and penny-shaped cavities lying along the grain boundaries and cracking along boundaries where no whiskers were present, whereas very little cavitation was visible in the present experiments after failure. The occurrence of extensive cavitation in the exper￾iments of Lin and Becher [17] may account for the apparent discrepancy in the creep data as recorded in Fig. 10. In four-point bending experiments where samples deform by power-law creep, the use of equations (1) and (2) necessitates that the neutral axis remains along the central line of the specimen with the tensile and compressive behavior obeying the same constitutive law. When the stress exponent, n, is greater than ~ 2, the development of microcracking and cavitation shifts the neutral axis away from this central line and this will lead to a significant overes￾timation of the outer fiber stress and a consequent underestimation of the equivalent tensile creep rate [30, 31]. In view of the large amounts of cavitation reported by Lin and Becher [17], it is possible that their estimates of the outer fiber stresses and strains may be in error. Finally, it is necessary to address the apparent dichotomy between the experiments of Lin and Becher [17] where cavitation was extensive and the present experiments where cavity development was very limited. In the work of de Arellano-L6pez et al. [18] on alumina composites containing up to 30 vol.% of SiC whiskers, it was reported that, depending upon the fabrication procedure used to produce the composites, there is a critical stress below which the development of cavitation damage is essen￾tially negligible. Although insufficient information is currently available to determine the magnitude of the critical stress under any selected experimental conditions, it is nevertheless noted that the present experiments were performed up to a maximum outer fiber stress of 61.5 MPa (Fig. 7) whereas the exper￾iments of Lin and Becher [17], conducted at the lower temperature of 1573 K, covered estimated outer fiber stresses in the range from ~ 100 to ~234 MPa. It is probable that these higher stress levels account for the markedly different creep behavior in the two sets of experiments. 5. SUMMARY AND CONCLUSIONS (1) Creep tests were conducted in air on an alumina composite reinforced with 9.3vo!.% of silicon carbide whiskers. (2) The creep curves exhibited a well-defined steady-state region with strains to failure in excess of 5% and up to > 10% under some experimental conditions. (3) The creep data gave a stress exponent of ~3.8 and an activation energy for creep of 820-830 kJ mol- i. (4) The microstructure after deformation revealed extensive dislocation activity and with very little evidence for the development of internal cavitation. (5) It is concluded that the presence of many SiC whiskers in the grain boundaries inhibits the occurrence of Lifshitz grain boundary sliding and therefore diffusion creep is suppressed. Instead, the composite deforms by an intragranular dislocation process controlled by lattice diffusion of the oxygen anions. Acknowledgements--We are grateful to Dr J. F. Rhodes of the Advanced Composite Materials Corporation for supply￾ing the material used in this investigation. This work was supported in part by the U.S. Army Research Office under Grant No. DAAL03-91-G-0230. REFERENCES 1. P. F. Becher, T. N. Tiegs, J. C. Ogle and W. H. Warwick, in Fracture Mechanics of Ceramics, Vol. 7 (edited by R. C. Bradt, A. G. Evans, D. P. H. Hasselman and F. F. Lange), p. 61. Plenum Press, New York (1986). 2. K. Niihara, A. Nadahira, T. Uchiyama and T. Hirai, in Fracture Mechanics of Ceramics, Vol. 7 (edited by R. C. Bradt, A. G. Evans, D. P. H. Hasselman and F. F. Lange), p. 103. Plenum Press, New York (1986). 3. J. Homeny, W. L. Vaughn and M. K. Ferber, Am. Ceram. Soc. Bull. 67, 333 (1987). 4. P. D. Shalek, J. J. Petrovic, G. F. Hurley and F. D. Gac, Am. Ceram. Soc. Bull. 65, 351 (1986). 5. R. Lundberg, L. Kahlman, R. Pompe and R. Carlsson, Am. Ceram. Soc. Bull. 66, 330 (1987). 6. M. Riihle, N. Clausson and A. H. Heuer, 3. Am. Ceram. Soc. 69, 195 (1986). 7. S. C. Samanta and S. Musikant, Ceram. Engng Sci. Proc. 6, 663 (1985). 8. G. C. Wei and P. F. Becher, J. Am. Ceram. Soc. 67, 571 (1984). 9. W. R. Cannon and T. G. Langdon, J. Mater. Sci. 18, I (1983). 10. W. R. Cannon and T. G. Langdon, J. Mater. Sci. 23, I (1988)