a r de arellano-lLopez et al /Internattonal Journal of Refractory Metals Hard Maternals 16(1998)337-341 3.3. Creep plastic defc remore considered as rigid inclusions A standard creep equation(17 was used to analyze A stress exponent of x1 for lower stresses, the results structural observations that cavities formed plastic deformation and absence of dislocation activity Q can be related to a mechanism that involves grain 8=Ad∞-Rr boundary sliding, but for which the sliding was not fully accommodated by diffusion [191. Creep resistance for where g is the strain rate, g is the stress r and t have the AISiTi composite might be improved by inhibiting their usual meanings and A is constant. The param grain-boundary sliding, which could be achieved by eters the stress exponent and @, the activation adding more of a rigid reinforcing phase or by energy,are related to plastic deformation mechanisms preserving more of the initial SiC whisker length through various models (14, 171 The creep resistance of the AlsiTi composite is Figure 5 shows a log-log plot of strain rate vs stress comparable to that of other whisker-reinforced ceramic for five different samples. At 1400.C, the maximum composites with similar grain size(Al2O3 grain size 1 stress in the CL tests was a80 MPa; stresses in the um). A comparison with creep of Al, O3-30 vol% SiC CSR tests were 150-540 MPa. The low-stress regime whisker [20] and a Al2O3-55 vol %o ZrO2 particle-28 vol. SiC whisker [21 composites is shown in Fig (<80 MPa)can be characterized by a stress exponent At lower stresses, in the ns1 region, all composites N1.0+0.3, which is typical for diffusional creep of monolithic fine-grained polycrystalline ceramics [18. Creep at about the same rate, whereas the AISiTi shows somewhat more creep resistance and damage tolerance On the other hand, the high-stress regime showed ar important degree of sample to sample variability, due at the higher stresses to the formation of macroscopic damage [10]. The activation energy was determined to be a470 kJ/mole at 23 MPa and 1350-1450.C. The values of the creep parameters, n and Q, in the steady-state region are consistent with that of fine-grained polycrystalline Al2O3, which is the only plastic phase in this system s The strength of an Al2O.9 vol. SiC(whiskers)- vol. TiC(particles) was independent of tempera- under the test conditions ture to 1000.C, but decreased slightly at 1200.C. The fracture mode, a combination of transgranular and especially at the higher stresses. Little evidence of intergranular, was unaffected by temperature. At samples before or after deformation 10]. The TiC chieved. At stresses below 80 MPa, creep occurred by rticles appeared to remain intact throughout the partially unaccommodated grain-boundary sliding, with Al O 10 5.5 voL% Zro 28 vol. sic AlO 30 vol, % SIC 010 1000 10 1000 Stress(MPa) Stress(MPa) Fig 6 Creep data at 1400 C from this work(triangles)compared wIth creep of Al_O330 vol SIC whisker (dashed line)and sents a dnfe. p results from five AISITi samples, each symbol repre- Al2O3 55 vol %o ZrO, particle-28 vol %o SiC whisker (sold lne)340 A R de Arellano-L@ez et al /Internattonal Journal of Refractory Metals & Hard Materials 16 (1998) 337-341 3.3. Creep A standard creep equation [17] was used to analyze the results: where + is the strain rate, a is the stress, R and T have their usual meanings and A is constant. The param￾eters n, the stress exponent and Q, the activation energy, are related to plastic deformation mechanisms through various models [14,17]. Figure 5 shows a log-log plot of strain rate vs stress for five different samples. At 1400°C, the maximum stress in the CL tests was ~80 MPa; stresses in the CSR tests were 150-540 MPa. The low-stress regime (< 80 MPa) can be characterized by a stress exponent ~1.0+0.3, which is typical for diffusional creep of monolithic fine-grained polycrystalline ceramics [18]. On the other hand, the high-stress regime showed an important degree of sample to sample variability, due to the formation of macroscopic damage [10]. The activation energy was determined to be ~470 kJ/mole at 23 MPa and 1350-1450°C. The values of the creep parameters, n and Q, in the steady-state region are consistent with that of fine-grained polycrystalline A1203, which is the only plastic phase in this system under the test conditions. SEM and TEM revealed that cavitation occurred, especially at the higher stresses. Little evidence of dislocation activity was observed using TEM in the samples before or after deformation [10]. The TiC particles appeared to remain intact throughout the plastic deformation process and can therefore be considered as rigid inclusions. A stress exponent of ~ 1 for lower stresses, micro￾structural observations that cavities formed during plastic deformation and absence of dislocation activity can be related to a mechanism that involves grain￾boundary sliding, but for which the sliding was not fully accommodated by diffusion [19]. Creep resistance for the A1SiTi composite might be improved by inhibiting grain-boundary sliding, which could be achieved by adding more of a rigid reinforcing phase or by preserving more of the initial SiC whisker length. The creep resistance of the A1SiTi composite is comparable to that of other whisker-reinforced ceramic composites with similar grain size (A1203 grain size ~ 1 pro). A comparison with creep of A1203-30 vol.% SiC whisker [20] and a A1203-5.5 vol.% ZrO2 particle-28 vol.% SiC whisker [21] composites is shown in Fig. 6. At lower stresses, in the n ~ 1 region, all composites creep at about the same rate, whereas the A1SiTi shows somewhat more creep resistance and damage tolerance at the higher stresses. 4. Summary The strength of an A1203-30.9 vol.% SiC (whiskers)- 23 vol.% TiC (particles) was independent of tempera￾ture to 1000°C, but decreased slightly at 1200°C. The fracture mode, a combination of transgranular and intergranular, was unaffected by temperature. At temperatures > 1350°C, steady-state creep was achieved. At stresses below 80 MPa, creep occurred by partially unaccommodated grain-boundary sliding, with 10 "5 V ¢) 10 .0 ¢- .m 10-7 A ~7 V V / I0 -8 , , ~ , , ,,,I , L ~ , r ,, 10 100 1000 Stress (MPa) Fig. 5. Creep results from five A1S~TI samples, each symbol repre￾sents a different sample , , , , , ''4 I ' ' ' , , AlaO 3- ," - // 5.5 vol.% Zr_ aO ,' A /x,, 10 s 28 vol.% SiCk/ ,z~ A '~ A ,' A 10-6 ,' "~ /t I .~- .., AlaO £ ~ - " 30 vol.°/o SiC 10 .7 10 .8 ....... ,~ ....... I0 I00 I ~00 Stress (MPa) Fig 6 Creep data at 1400°C from this work (triangles) compared with creep of A1203-30 vol.% S1C wh]sker (dashed line) and A1203-5 5 vol % ZrO2 particle-28 vol % SIC whisker (sohd hne) composites