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)101020A 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 microstruc￾ture 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 ceram￾ics, 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 bridg￾ing and pullout, microcracking and crack deflection are operative depending on microstructural factors and ex￾perimental conditions [1-4]. The improvements ob￾tained 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 struc￾tural 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 be￾tween 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 amor￾phous grain boundary phase [14-24]. Reinforcement of alumina with SiC-whiskers is ex￾pected 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