ELSEVIER Materials Science and Engineering A209( 1996)251-259 A microstructural investigation of the mechanisms of tensile creep deformation in an Al,O3/SiCw composite C. O'Mearaa, T. Suihkonena. T. Hansson,, R. Warren Department of Physics, Chalmers University of Technology, Goteborg S-412 96, Sweden Department of Mechanical Engineering, Nagaoka University of Technology, Nagaoka, Japan Department of Materials Science and Production Technology, Luled University of Technology, Luled, Sweden Abstract The tensile creep behaviour of an SiCw (25%)reinforced alumina composite was investigated using scanning electron microscopy(SEM) and transmission electron microscopy and automatic image analysis in SEM. The creep tests were carried out in air in the ranges 1100-1300C and 11-67 MPa Each creep test was performed at a constant temperature. The material had a stress exponent of about three for all temperatures and an approximate activation energy of 650 kJ mol, The creep resistance of this composite is poorer than that of similar composites studied earlier. Microstructural examination revealed the microstruc- ture to be extremely inhomogeneous consisting of spherical whisker-rich clusters(20-100 um)surrounded/separated by AlO, rich rims(10 um). The secondary is dominated by a damage accumulation process namely cavitation and crack growth in both the Sic clusters and the Al,O, rims. Final fracture seems to occur through the alumina rich regions. The lower creep resistance of this composite compared to that of similar composites is attributed primarily to the inhomogeneity of the as-received Keywords: Tensile creep deformation; Alumina composites; Microstructure 1. Introduction whiskers into alumina is also expected to improve the creep resistance and this has largely been confirmed in Monolithic alumina exhibits only moderate strength bend and compression tests [5-12 but not in tension and creep resistance and, like most monolithic ceram- [13] ics, is extremely brittle. SiC whisker reinforcement of Observed creep mechanisms in monolithic alumina alumina(SICw/Al,O3) has been employed primarily include basal slip, diffusional creep and grain boundary and successfully to improve fracture toughness [1-4]. sliding(GBS)[14, 15. Grain boundary cavitation has various toughening mechanisms such as whisker bridg- also been observed in association with GBS [16, 17]. The ng and pullout, microcracking and crack deflection are stress exponent has generally been found to vary be- operative depending on microstructural factors and ex- tween I and 2 and the activation energy between 400 erimental conditions [1-4]. The improvements ob- 650 kJ mol". The creep resistance has been found to ained in the composite in both fracture toughness and increase with grain size and there is a general agreement strength as compared to monolithic alumina has led to that aluminas with"clean"grain boundaries exhibit s application as, for example, cutting tool inserts and higher creep resistance than aluminas with an amor extrusion valves, and give it potential for use in struc- phous grain boundary phase [14-24 tural applications at high temperatures. Ho Reinforcement of alumina with SiC-whiskers is ex- use of ceramic materials in high temperature structural pected to improve the creep resistance primarily by applications is inevitably dependent on their time interlocking/pinning of grains which then limits grain eep and boundary sliding [8]. However, several factors can be oxidation resistance. The incorporation of SiC- identified which will affect the mechanical response of 0921-509396S1500c 1996- Elsevier Science S.A. All rights reserved SSD09215093(95)101020
A E L S E V I E R Materials Science and Engineering A209 (1996) 251 259 A microstructural investigation of the mechanisms of tensile creep deformation in an AI203/SiC w composite C. O'Meara a, T. Suihkonen a, T. Hansson b, R. Warren c aDepartment of Physics, Chalmers University of Technology, G6teborg S-412 96, Sweden bDepartment of Mechanical Engineering, Nagaoka University of Technology, Nagaoka, Japan ~Department of Materials Science and Production Technology, Lule~ University of Technology, Lule~, Sweden Abstract The tensile creep behaviour of an SiCw (25%) reinforced alumina composite was investigated using scanning electron microscopy (SEM) and transmission electron microscopy and automatic image analysis in SEM. 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 temperature. The material had a stress exponent of about three for all temperatures and an approximate activation energy of 650 kJ mol 1. The creep resistance of this composite is poorer than that of similar composites studied earlier. Microstructural examination revealed the microstructure to be extremely inhomogeneous consisting of spherical whisker-rich clusters (20-100/~ m) surrounded/separated by A1203 rich rims (10 ~tm). The secondary creep rate is dominated by a damage accumulation process namely cavitation and crack growth in both the SiC clusters and the A1203 rims. Final fracture seems to occur through the alumina rich regions. The lower creep resistance of this composite compared to that of similar composites is attributed primarily to the inhomogeneity of the as-received material. Keywords: Tensile creep deformation; Alumina composites; Microstructure 1. Introduction Monolithic alumina exhibits only moderate strength and creep resistance and, like most monolithic ceramics, is extremely brittle. SiC whisker reinforcement of alumina (SiCw/A1203) has been employed primarily and successfully to improve fracture toughness [1-4]. Various toughening mechanisms such as whisker bridging and pullout, microcracking and crack deflection are operative depending on microstructural factors and experimental conditions [1-4]. The improvements obtained in the composite in both fracture toughness and strength as compared to monolithic alumina has led to its application as, for example, cutting tool inserts and extrusion valves, and give it potential for use in structural applications at high temperatures. However the use of ceramic materials in high temperature structural applications is inevitably dependent on their time dependent mechanical properties such as creep and oxidation resistance. The incorporation of SiC- 0921-5093/96/$15.00 © 1996 - Elsevier Science S.A. All rights reserved SSDI 0921-5093(95)10102-0 whiskers into alumina is also expected to improve the creep resistance and this has largely been confirmed in bend and compression tests [5-12] but not in tension [13]. Observed creep mechanisms in monolithic alumina include basal slip, diffusional creep and grain boundary sliding (GBS) [14,15]. Grain boundary cavitation has also been observed in association with GBS [16,17]. The stress exponent has generally been found to vary between 1 and 2 and the activation energy between 400- 650 kJ mol-1. The creep resistance has been found to increase with grain size and there is a general agreement that aluminas with "clean" grain boundaries exhibit higher creep resistance than aluminas with an amorphous grain boundary phase [14-24]. Reinforcement of alumina with SiC-whiskers is expected to improve the creep resistance primarily by interlocking/pinning of grains which then limits grain boundary sliding [8]. However, several factors can be identified which will affect the mechanical response of
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 software
252 C. O'Meara et al. / Materials Science and Engineering A209 (1996) 251-259 the composite under creep conditions: the volume fraction 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 accommodated mechanisms and above which a damage accumulation process involving cavitation and microcracking 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 decrease 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 composite 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 investigations 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 deformation 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 temperature 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 materials 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 semiautomatic polishing apparatus. The specimens were examined 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
C. O'Meara et al. Materials Science and Engineering A209(1996)251-259 253 Tensile creep test conditions and results Sample Temp Stress Time to fracture Strain to fracture Secondary creep Creep exponent Preheated at 1300C (%) (s-) 3.4 9×1 3.25 11001-35728 3.7×10-10 24×10 8×10-6 2.94 1300 252 20×10-9 .7 1300 19 4.1×10 !30 0.17 2.3×10 1275 28×10 1300 1.3 6.6×10-7 0 i Load increased during the test. ted before failur Defect in the sample 2.3.3.1. Cavity analysis. Two sets of measurements were 3. Results made for each fractured specimen, one at the fracture surface and one 3 mm further into the bulk. At a 3. 1. Creep results magnification of 5000 x, 50 fields were examined in ach measurement with a total frame area of 10 x 5 A summary of the creep results is given in Table 1 327. 12 um2=16356 um2. Cavities/pores in the size The composite exhibited limited regions of primary range 0. 1-10 um were measured by AIA. The parame- creep, relatively well defined secondary creep and en ter measured was the area of the pore. This was con- tered a tertiary creep region just before fracture for all erted to the equivalent diameter of that area by conditions of stress and temperature. The material had DCIRCLE-2/4 a stress exponent of approximately three for all temper (1) atures and an approximate activation energy of 650 kJ mol-. The creep resistance of this composite in ten Where dCirClE is the average diameter of the pore sion is poorer than that of similar composites studie and a is its measured area earlier in bend or compression. However, the creep resistance improved significantly following high temper 2.3.3. 2. Grain size analysis. For the alumina grain size ature pre-heat treatment analysis the specimens were first etched in argon at 1300C for 15 min For each specimen 10 micrographs t a time were taken in a line close to and parallel with the fracture surface. At least 500 alumina grains were analysed in each specimen. The parameter measured in Ala was the area of the grain which was converted to average diameter by Eq (1) 2.3.3.3. Volume fraction of phases. As will be discussed in the results, considerable inhomogeneity in the matrix was observed dividing the microstructure into whisker /rich”and“ alumina rich” areas. A qualitative AIA analysis was carried out simply by marking he whisker/rich"areas in one colour and the" alu- mina rich"areas in another in order to get an indica- tion of the extent of inhomogeneity. In addition ar estimation of the whisker fraction in the whisker /rich and"alumina rich"areas was carried out using EDX ing the inhomogeneity in the pherical whisker- rich clusters surrounded/ separated by Al, O, rich
C. O'Meara et al. / Materials Science and Engineering A209 (1996) 251 259 253 Table 1 Tensile creep test conditions and results Sample Temp Stress Time to fracture Strain to fracture Secondary creep Creep exponent Preheated at 1300 °C (°C) (MPa) (h) (%) rate (s- i ) (h) 1 1200 t9 42.1 3.4 1.9 x 10 7 3.25 2 ~ 1100 11-35 728 2.3 3.7x I0 m 3.25 1.6 x 10 8 3 1200 35 13.9 1.7 2.5 x 10 7 3.28 - 4 1200 67 1.2 1.2 2.4x 10 -6 3.28 - 5 1300 19 1.7 2.5 3.8 X 10 -6 2.94 - 6 b 1300 11 252 3.4 2.0 X 10 9 3.7 - 7 1300 19 13.7 3.0 4.1 × 10 7 3.08 61 8 1300 35 0.17 1.5 2.3× 10 -5 2.94 .- 9 1275 35 2.2 2.5 2.8 × 10 -6 3.08 72 10 ~ 1300 1 t 4.9 1.3 6.6 x 10 -v 3.08 72 Load increased during the test. b Test aborted before failure. Defect in the sample. 2.3.3. I. Cavity analysis. Two sets of measurements were made for each fractured specimen, one at the fracture surface and one 3 mm further into the bulk. At a magnification of 5000 x, 50 fields were examined in each measurement with a total frame area of 10 x 5 x 327.12 pm2= 16356 pm 2. Cavities/pores in the size range 0.1-10 pm were measured by AIA. The parameter measured was the area of the pore. This was converted to the equivalent diameter of that area by ?.-,- DCIRCLE = 2 k/A (1) Where DCIRCLE is the average diameter of the pore and A is its measured area. 2.3.3.2. Grain size analysis. For the alumina grain size analysis the specimens were first etched in argon at 1300 °C for 15 min. For each specimen 10 micrographs at a time were taken in a line close to and parallel with the fracture surface. At least 500 alumina grains were analysed in each specimen. The parameter measured in AIA was the area of the grain which was converted to average diameter by Eq. (1). 2.3.3.3. Volume fraction of phases. As will be discussed in the results, considerable inhomogeneity in the matrix was observed dividing the microstructure into "whisker/rich" and "alumina rich" areas. A qualitative AIA analysis was carried out simply by marking the "whisker/rich" areas in one colour and the "alumina rich" areas in another in order to get an indication of the extent of inhomogeneity. In addition an estimation of the whisker fraction in the "whisker/rich" and "alumina rich" areas was carried out using EDX analysis. 3. Results 3.1. Creep results A summary of the creep results is given in Table 1. The composite exhibited limited regions of primary creep, relatively well defined secondary creep and entered a tertiary creep region just before fracture for all conditions of stress and temperature. The material had a stress exponent of approximately three for all temperatures and an approximate activation energy of 650 kJ mol-t. The creep resistance of this composite in tension is poorer than that of similar composites studied earlier in bend or compression. However, the creep resistance improved significantly following high temperature pre-heat treatment. Fig. 1. SEM image showing the inhomogeneity in the microstructure, spherical whisker-rich clusters surrounded/separated by AI203 rich rims
C. O'Meara et al. Materials Science and Engineering A209(1996)251-259 The cavity size measurements of cavities between 0. 1 and 10 um Mean cavity avity nucleation Area fraction of location cavities(1 m diameter (um) 50 115 163 161 041 0.45 1214.9 0 stands for as-received sample 3. 2. Microstructure AlO3 SiC and SiC/SiC contacts. The extent of cavita tion decreased somewhat with distance from the frac- 3. 2. 1. As-received material ture surface but was evident the entire length of th As is shown in Fig. I microstructural examination specimens (a 5 um). Cavitation between the AlO revealed that the as-received material had an inhomoge grains was observed to be very frequent and to occur neous microstructure consisting of spherical whisker two grain junctions. Ca Al2O3 rich rims(< 10 um). The inhomogeneity is prob- the grain facets were separated the contacts and here rich clusters(= 20-100 um) surrounded/ separated curred most commonly at two gr entire length of the bly inherited from spray drying used during processing grains forming lath like cavities( Fig. 2). The nucleatic of the composite powders which resulted in a clustering of smaller cavities along two-grain facets was also of the SiC phase. The results of AlA and EDX analysis fre requently observed in TEM. These cavities were ap- indicate that approximately 40% of the microstructure as a sic content of less than 15 vol while about and encroached on both grains as is shown in Fig. 3 60% has a SiC content of w 30 vol % The microstruc- No grain growth was detected following creep even for ture Is the preheated samples and few dislocations were ob- addition the alumina grain size was small(sub-micron (sub-micron) served. Indication of possible GBS was found both in nd bi-modal in distribution with the larger grains in TEM and in SEM examination of etched specimens the rims and very small grains in the whisker clusters In TEM very little(< I nm)intergranular amorphous SiC/SiC contacts. As is shown in Fig. 5, Sic debonding and pullout occurred frequently. When these debonded alumina grain morphology was variable with no evi- contacts lay close together a small isolated intergranu- dence of preferred orientation. Very few dislocations lar cavity formed. Fracture of long SiC whiskers was were observed but small pores within the Al,O, grains also observed. Fig. 6 shows the presence of an amor ere detected. Whisker lengths of up to 10 um were observed. No coating could be detected on the Sic Whiskers but individual whiskers were uneven in diame- ter with micro-roughness resulting from the stacking of Sic polytypes. A small amount of porosity, up to 0.5 um in diamter at AlO, triple grain junctions and at Al2O3 SiC and SiC/SiC interfaces was observed and corresponds with the 0.4% porosity estimated in SEM by AlA(see Table 2) 3. 2. 2. Crept material SEM and TEM examination of the crept materials revealed that all samples contained two distinct families f creep damage (i)a large volume of sub-micron and micron size(0. 1-10 um) pores and cavities and (ii) cracks of> 10 um in length (i) The cavities/pores were observed at Al2 O3/ Al2O3 Al O3 grains following creep testing at 35 MPae cavity between two Fig. 2. TEM bright field image showing a lat
254 C. O'Meara et al. / Materials Science and Engineering A209 (1996) 251-259 Table 2 The cavity size measurements of cavities between 0.1 and 10/~m Sample and Area density of Median cavity Mean cavity Cavity nucleation Area fraction of location cavities (l/ram -~) diameter (jzm) diameter (/zm) rate (l/h) cavities (%) 0 22 0.42 0.50 0.42 1 115 0.50 0.60 45 3.26 2 170 0.52 0.60 4.0 5.0 3 178 0.40 0.50 210 3.10 4 96 0.30 0.38 1306 1.1 5 200 0.51 0.60 1980 5.6 6 163 0.50 0.56 10.57 4.0 7 126 0.5I 0.61 150 4.0 8 198 0.53 0.57 19317 5.1 9 161 0.41 0.45 1214.9 2.6 0 stands for as-received sample. 3.2. Microstructure 3.2.1. As-received material As is shown in Fig. 1 microstructural examination revealed that the as-received material had an inhomogeneous microstructure consisting of spherical whiskerrich clusters (~ 20-100 /~m) surrounded/separated by A1203 rich rims ( ~ 10/lm). The inhomogeneity is probably inherited from spray drying used during processing of the composite powders which resulted in a clustering of the SiC phase. The results of AIA and EDX analysis indicate that approximately 40% of the microstructure has a SiC content of less than 15 vol.% while about 60% has a SiC content of ,-~ 30 vol.%. The microstructure is in effect a composite within a composite. In addition the alumina grain size was small (sub-micron) and bi-modal in distribution with the larger grains in the rims and very small grains in the whisker clusters. In TEM very little ( < 1 nm) intergranular amorphous phase could be detected in the microstructure. The alumina grain morphology was variable with no evidence of preferred orientation. Very few dislocations were observed but small pores within the A1203 grains were detected. Whisker lengths of up to 10 /~m were observed. No coating could be detected on the SiC whiskers but individual whiskers were uneven in diameter with micro-roughness resulting from the stacking of SiC polytypes. A small amount of porosity, up to 0.5 /lm in diamter at ~ AI203 triple grain junctions and at A1203/SiC and SiC/SiC interfaces was observed and corresponds With the 0.4% porosity estimated in SEM by AIA (see Table 2). 3.2.2. Crept material SEM and TEM examination of the crept materials revealed that all samples contained two distinct families of creep damage (i) a large volume of sub-micron and micron size (0.1-10 pm) pores and cavities and (ii) cracks of>_- 10 pm in length. (i) The cavities/pores were observed at AI203/A1203, A1203/SiC and SiC/SiC contacts. The extent of cavitation decreased somewhat with distance from the fracture surface but was evident the entire length of the specimens (~ 5 /zm). Cavitation between the A1203 grains was observed to be very frequent and to occur both at triple and two grain junctions. Cavitation occurred most commonly at two grain contacts and here the grain facets were separated the entire length of the grains forming lath like cavities (Fig. 2). The nucleation of smaller cavities along two-grain facets was also frequently observed in TEM. These cavities were approximately 100 nm in length and lenticular in shape and encroached on both grains as is shown in Fig. 3. No grain growth was detected following creep even for the preheated samples and few dislocations were observed. Indication of possible GBS was found both in TEM and in SEM examination of etched specimens (see Fig. 4). Cavities also formed at AI203/SiC and SiC/SiC contacts. As is shown in Fig. 5, SiC debonding and pullout occurred frequently. When these debonded contacts lay close together a small isolated intergranular cavity formed. Fracture of long SiC whiskers was also observed. Fig. 6 shows the presence of an amorFig. 2. TEM bright field image showing a lath like cavity between two AI203 grains following creep testing at 35 MPa and 1200 °C
C. O'Meara et al./ Materials Science and Engineering A209(1996)251-259 tested at 1300oC and 35 MPa (TEM), lacets in a specimen creep Fig. 3. Cavities along twograin Al phous coating(< 2 nm) observed in TEM on the surface of some Sic whiskers in cavitated areas. This phenomena was observed in all specimens examined indicating that these interfaces were oxidised during creep exposure. It is pointed out that the TEM foils were taken from the centre of the specimens where it assumed that the effects of oxidation are at a minimum nonetheless, some qualification must be made on these observations: (1)the amorphous film was not observed at all Al,O3/SiC and SiC/Sic cavitated contacts in the Fig. 5. TEM images showing whisker debonding(a) and pullout(b) same specimen;(2)no significant increase could be detected in the volume of intergranular amorphous close to the fracture surfaces are given in Table 2.All phase in non-cavitated regions of the specimen or at the the specimens showed a similar pore/ cavity size distri ALO3/Al,O3 cavities as compared to as-received mate bution with a mean cavity size of approx 0.5 um. In all rial; (3)Fig. 6, a typical oxidised cavity shows a cleanly cases 95% of the cavities were under 1.5 um in length fractured Sic whisker surrounded by an even layer of which agrees well with TEM observations. Even with amorphous phase, no cavitation or glassy ligaments are the limited number of test points a linear-stress rela observed in the ar morphous phase which appear tionship at temperature is observed between both the "close off "the cavities. The authors therefore feel that area density of cavities and the cavity nucleation rate oxidation of the whiskers may equally well have oc- (total number of cavities/ time to fracture)(Fig. 7).The curred after cavitation as much as being the facilitator pre-heat treated specimens show the same dependence of cavitation as suggested in other works. Quantitative but at lower values indicating some positive effect of alysis was undertaken to investigate differences in the heat treatment. These results do imply that cavita- cavitation between the samples. The results for the area tion is the main deformation mechanism occurring during creep and that it was operative under all test conditions. However the results also imply that creep rupture does not occur when a similar level of cavita- tolerable cavitation is highly stress temperature depen- dent due to an interplay of additional microstructural factors as will be discussed later As is shown in Fig 8, more severe creep damage in the form of cracks(>10 um) was observed: (i) through the Al,O3 in the rims around the clusters originating at the edge of the specimen and;(ii)in the interior of the clusters associated predominantly with the SiC whiskers. In both cases the cracking is mainly perpen Fig 4. Grain boundary sliding of alumina grains observed in SEM. dicular to the tensile direction and is intergranular. The
C. O'Meara et al. / Materials Science and Engineering A209 (1996) 251-259 IIIII 255 Fig. 3. Cavities along two-grain A1203 facets in a specimen creep tested at 1300 °C and 35 MPa (TEM). phous coating (~2 nm) observed in TEM on the surface of some SiC whiskers in cavitated areas. This phenomena was observed in all specimens examined indicating that these interfaces were oxidised during creep exposure. It is pointed out that the TEM foils were taken from the centre of the specimens where it is assumed that the effects of oxidation are at a minimum; nonetheless, some qualification must be made on these observations: (1) the amorphous film was not observed at all AI203/SiC and SiC/SiC cavitated contacts in the same specimen; (2) no significant increase could be detected in the volume of intergranular amorphous phase in non-cavitated regions of the specimen or at the A1203/A1203 cavities as compared to as-received material; (3) Fig. 6, a typical oxidised cavity shows a cleanly fractured SiC whisker surrounded by an even layer of amorphous phase, no cavitation or glassy ligaments are observed in the amorphous phase which appears to "close off" the cavities. The authors therefore feel that oxidation of the whiskers may equally well have occurred after cavitation as much as being the facilitator of cavitation as suggested in other works. Quantitative analysis was undertaken to investigate differences in cavitation between the samples. The results for the area Fig. 4. Grain boundary sliding of alumina grains observed in SEM. Fig. 5. TEM images showing whisker debonding (a) and pullout (b) in crept material. close to the fracture surfaces are given in Table 2. All the specimens showed a similar pore/cavity size distribution with a mean cavity size of approx. 0.5/z m. In all cases 95% of the cavities were under 1.5 /~m in length which agrees well with TEM observations. Even with the limited number of test points a linear-stress relationship at temperature is observed between both the area density of cavities and the cavity nucleation rate (total number of cavities/time to fracture) (Fig. 7). The pre-heat treated specimens show the same dependence but at lower values indicating some positive effect of the heat treatment. These results do imply that cavitation is the main deformation mechanism occurring during creep and that it was operative under all test conditions. However the results also imply that creep rupture does not occur when a similar level of cavitation has been attained but rather that the level of tolerable cavitation is highly stress/temperature dependent due to an interplay of additional microstructural factors as will be discussed later. As is shown in Fig. 8, more severe creep damage in the form of cracks ( ~> 10/tm) was observed: (i) through the AI20 3 in the rims around the clusters originating at the edge of the specimen and; (ii) in the interior of the clusters associated predominantly with the SiC whiskers. In both cases the cracking is mainly perpendicular to the tensile direction and is intergranular. The