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, richC. 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 parame￾ter measured was the area of the pore. This was con￾verted 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 "alu￾mina rich" areas in another in order to get an indica￾tion 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 en￾tered 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 temper￾atures and an approximate activation energy of 650 kJ mol-t. The creep resistance of this composite in ten￾sion is poorer than that of similar composites studied earlier in bend or compression. However, the creep resistance improved significantly following high temper￾ature pre-heat treatment. Fig. 1. SEM image showing the inhomogeneity in the microstructure, spherical whisker-rich clusters surrounded/separated by AI203 rich rims