252 C O Meara et al. Materials Science and Engineering A209(1996)251-259 the composite under creep conditions: the volume frac- The mixture was cold pressed to 20 mm diameter rods tion of whiskers; the strength of the interfacial bond and then HIPped (1600C, 160 MPa, I h). Cylindrical between the fibre and the matrix; the grain size of the test specimens were produced by precision machining matrix grains; the volume of intergranular amorphous from the rods. Each specimen had a total length of 150 phase and; the oxidation susceptibilty of the material mm and a diameter of 10 mm reducing to 4 mm over a which causes the formation of glass at the whisker 20 mm long gauge length matrix interface. These factors will vary from material to material and will complicate both the interpretation 2.2. Creep testing of the creep behaviour and the comparison of different The creep equipment was specially designed for the Studies on bending and compression of the testing of brittle materials and details of the test system composite indicate that there exists a transition stress e described in Ref. [13]. The creep tests we below which the creep is dominated by diffusion ac commodated mechanisms and above which a damage Each creep test was performed at a constant stress and accumulation process involving cavitation and microc- temperature. Two specimens were pre-heat treated in racking become increasingly important. In the low air at 1300oC prior to creep testing. The high tempera- stress regime the stress exponent is 1-2 while above the ture heat treatment was used to investigate the effect of energies similar to those in monolithic alumina are non heat treated specimens subjected to he aring with transition it increases to values of 5[6-10. Activation oxidation on the creep behaviour by cor same creep bserved. No direct evidence of dislocation activity has conditions een found In general whisker reinforcement increases the creep resistance of alumina, however in the high 2.3. Microstructural examination not provide further improvement and may even de crease the creep resistance [8]. Grain boundary and he microstructure of the as-received and crept mate rials were studied using both scanning and transmission interfacial amorphous phases are detrimental to creep electron microscopy (SEM/TEM)and quantitative esistance and may promote transition to a damage accumulation process [9]. This is consistent with the SEM using automatic image analysis(AIA) observation that creep resistance is lower in air than in inert atmospheres since the composite is sensitive to 3. TEM xidation Thin sections for TEM analysis were taken from the Work by the authors on tensile creep of the com- centre of the gauge section directly above the fracture posite has shown that in tension even in the low stress surface and were cut in the longitudinal direction regime damage accumulation was the dominant creep parallel to the stress axis. Thinned sections were dimple mechanism and a stress exponent of three was obtained ground followed by ion-beam thinning to perforation or all temperatures and stresses [13]. Previous investi TEM examination was carried out using a JEOL gations on ceramic materials tested in tension and 2000FX TEM/STEM instrument equipped with a Link flexure have shown stress exponents of three to arise Systems AN 10 000 EDX spectrometer from creep cavitation [25]. However for composite and multiphase ceramics because of the complex interaction 2. 3.2. SEM between the microstructural constituents, creep defor- SEM specimens were cut from the gauge section in mation mechanisms cannot be reliably deduced from the longitudinal direction from the fracture surface to a creep data alone [9] but require direct observation of distance of about 0. 5 cm along the gauge length. The the deformed microstructures specimens were mounted in transoptic plastic, ground This work presents a microstructural examination of and polished down to 0. 25 um using a Struers semiau the tensile creep behaviour of a SiCw(25%)reinforced tomatic polishing apparatus. The specimens were exam- alumina composite. Electron microscopy was used to ined in a CAM Scan S-4 80DV instrument equipped obtain information on the possible creep mechanisms. with a Link Systems AN 10000 EDX spectrometer The backscattered electron mode 2.E 2. Material (AIA)was used for cavity, alumina grain size and phase volume fraction estimation. A Jeol JXA/8600 The composite was produced from a powder mixture Electron Probe Micro Analyser(EPMA) was used with of alumina and whiskers without sintering additives. Kantron software252 C. O'Meara et al. / Materials Science and Engineering A209 (1996) 251-259 the composite under creep conditions: the volume frac￾tion of whiskers; the strength of the interfacial bond between the fibre and the matrix; the grain size of the matrix grains; the volume of intergranular amorphous phase and; the oxidation susceptibilty of the material which causes the formation of glass at the whisker matrix interface. These factors will vary from material to material and will complicate both the interpretation of the creep behaviour and the comparison of different works. Studies on bending and compression creep of the composite indicate that there exists a transition stress below which the creep is dominated by diffusion ac￾commodated mechanisms and above which a damage accumulation process involving cavitation and microc￾racking become increasingly important. In the low stress regime the stress exponent is 1-2 while above the transition it increases to values of 5 [6-10]. Activation energies similar to those in monolithic alumina are observed. No direct evidence of dislocation activity has been found. In general whisker reinforcement increases the creep resistance of alumina, however in the high stress regime whisker volume fractions above 20% do not provide further improvement and may even de￾crease the creep resistance [8]. Grain boundary and interfacial amorphous phases are detrimental to creep resistance and may promote transition to a damage accumulation process [9]. This is consistent with the observation that creep resistance is lower in air than in inert atmospheres since the composite is sensitive to oxidation. Work by the authors on tensile creep of the com￾posite has shown that in tension even in the low stress regime damage accumulation was the dominant creep mechanism and a stress exponent of three was obtained for all temperatures and stresses [13]. Previous investi￾gations on ceramic materials tested in tension and flexure have shown stress exponents of three to arise from creep cavitation [25]. However for composite and multiphase ceramics because of the complex interaction between the microstructural constituents, creep defor￾mation mechanisms cannot be reliably deduced from creep data alone [9] but require direct observation of the deformed microstructures. This work presents a microstructural examination of the tensile creep behaviour of a SiCw (25%) reinforced alumina composite. Electron microscopy was used to obtain information on the possible creep mechanisms. 2. Experimental 2.1. Material The composite was produced from a powder mixture of alumina and whiskers without sintering additives. The mixture was cold pressed to 20 mm diameter rods and then HIPped (1600 °C, 160 MPa, 1 h). Cylindrical test specimens were produced by precision machining from the rods. Each specimen had a total length of 150 mm and a diameter of 10 mm reducing to 4 mm over a 20 mm long gauge length. 2.2. Creep testing The creep equipment was specially designed for the testing of brittle materials and details of the test system are described in Ref. [13]. The creep tests were carried out in air in the ranges 1100-1300 °C and 11 67 MPa. Each creep test was performed at a constant stress and temperature. Two specimens were pre-heat treated in air at 1300 °C prior to creep testing. The high tempera￾ture heat treatment was used to investigate the effect of oxidation on the creep behaviour by comparing with non heat treated specimens subjected to the same creep conditions. 2.3. Microstructural examination The microstructure of the as-received and crept mate￾rials were studied using both scanning and transmission electron microscopy (SEM/TEM) and quantitative SEM using automatic image analysis (AIA). 2.3.1. TEM Thin sections for TEM analysis were taken from the centre of the gauge section directly above the fracture surface and were cut in the longitudinal direction, parallel to the stress axis. Thinned sections were dimple ground followed by ion-beam thinning to perforation. TEM examination was carried out using a JEOL 2000FX TEM/STEM instrument equipped with a Link Systems AN 10 000 EDX spectrometer. 2.3.2. SEM SEM specimens were cut from the gauge section in the longitudinal direction from the fracture surface to a distance of about 0.5 cm along the gauge length. The specimens were mounted in transoptic plastic, ground and polished down to 0.25/zm using a Struers semiau￾tomatic polishing apparatus. The specimens were exam￾ined in a CAM Scan S-4 80DV instrument equipped with a Link Systems AN 10000 EDX spectrometer. The specimens were examined in secondary and backscattered electron mode. 2.3.3. Quantitative microscopy (AIA) was used for cavity, alumina grain size and phase volume fraction estimation. A Jeol JXA/8600 Electron Probe Micro Analyser (EPMA) was used with Kantron software