July 2004 High-Temperature Fatigue Strength of Crack-Healed Al-O, Toughened by SiC Whiskers 1261 Mark () We determined the static fatigue limit(oo)of the heat-treated specimen to be about 800 MPa. The crack-healed specimens tested racked specimen(e=100um) under 700 and 800 MPa survived up to 10%s. One specimen leat-treated smooth specimen fractured under 750 MPa, however, the time to failure was quite long(5.6x 105 s). Thus, we determined the static fatigue limit 1400 (c,o) for the crack-healed specimen to be about 750 MPa. The cyclic fatigue test results are shown in Fig. 3(b). The pecimens that did not fracture up to N=2 x 10 cycles are marked by an arrow (-) The cyclic fatigue limit (oo) heat-treated specimen and the crack-healed specimen are about 800 and 850 MPa, respectively 会 The static and cyclic fatigue limits of the crack-healed speci- mens are similar to those of the heat-treated specimens, indicating that the room-temperature fatigue strength of the crack-healed zone is comparable to that of base material. The cyclic fatigue limi of crack-healed specimens is about 10% higher than that of the static fatigue limit. This reason will be discussed later in this paper The ratio of the static fatigue limit(ro) and the cyclic fatigue limit (Uo) to the average bending strength of crack-healed specimens (Un) is about 75% and 85%, respectively(see also Table I). Thus, fatigue limits of crack-healed specimens at room temperature are Healing condition: 1300C, Ih in air Bending tests for the specimens which survived the static and cyclic fatigue tests were also conducted at room temperature. The 10210410° o. for fatigue experimental results are shown on the right-hand sides of Figs. 3(a) and(b). The fatigue-tested specimens exhibited bending strengths Time to Failure, t (sec) similar to those of the monotonically tested specimens as shown on the left-hand side of Figs. 3(a)and(b). These experimental results (a) ndicate that significant crack growth from the crack-healed zone id not occu Figure 4 shows SEM micrographs of the fracture surface after static fatigue tests at room temperature. The slow crack growth can A As-cracked specimen((e=100um)RT be clearly observed, as shown in Fig. 4(a). Figure 4(b) shows the Heat-treated smooth specimen details of the crack-healed zone. The dark regions near SiC whiskers are oxidized, while the light ones not. Figure 4(c)shows 140 Crack-healed specimen the details of the slow crack growth zone. The fracture surface of the slow crack growth zone is quite rough and predominantly Cyclic fatigue test intergranular. Interactions between SiC whiskers and Al,O, ma- R=02,f=5Hz trix, such as bridging and/or pullout are clearly observed. Thus, Sic whiskers contribute to an increase in the crack growth resistance. As seen in Fig. 4(d), intergranular fracture cannot be 10 identified in the final fracture zone 切 In a previous study, we investigated the high-temperature bending strength of Al,,/SiC-W which was precracked(2c 100 um)and crack-healed at 1300C for I h in air. The bending strength of the crack-healed specimen was UB= 800 MPa and was almost constant up to 1100C. However, the UB showed a sudden decrease at 1.C and decreased with increasing testing temper Healing conditior ature up to 1300oC. Therefore, the heat-resistance limit tempera ture for bending strength of the crack-healed specimen was 00C. In the present study, the cyclic and static fatigue strengths 10 of crack-healed specimens were examined at 900"and 1100.C The results of static fatigue tests at high temperature are shown Number of cycles to failure, N(cycle) in Fig. 5. The monotonic bending strength of the heat-treated mooth specimens and the crack-healed specimens at 900% and 1100%C are shown on the left-hand side of Fig. 5. The average bending strength of the heat-treated smooth specimens at 900and Fig 3. Fatigue test results for Al20/SiC-w at room temperature: (a) 1100oC are about 850 MPa. However, the bending strengths of static fatigue test results for crack-healed Al,O,SiC-w,(b)cyclic fatigu crack-healed specimens are about 88% and 76% compared with test results for crack-healed Al2O /SiC-W. Data marked with an asterisk that of the heat-treated smooth specimen at 900 and 1100C, indicate that the fracture occurred outside of the crack-healed zone respectively, and decrease with increasing test temperature The values of the static fatigue limit (o,o) for the crack-healed pecimens at 900 and 1100.C are about 550 and 450 MPa tests are shown in Fig 3(a), respectively. The static fatigue limit of the crack-healed specimen th time to failure. The is about 70% in comparison to the bending strength at each testing specimens that did not fracture up to 10 s are marked by an arrow temperature(see also Table D)The results of the static fatigue tests are shown in Fig. 3(a), correlating applied constant stress with time to failure. The specimens that did not fracture up to 106 s are marked by an arrow (3). We determined the static fatigue limit (t0) of the heat-treated specimen to be about 800 MPa. The crack-healed specimens tested under 700 and 800 MPa survived up to 106 s. One specimen fractured under 750 MPa; however, the time to failure was quite long (5.6 105 s). Thus, we determined the static fatigue limit (t0) for the crack-healed specimen to be about 750 MPa. The cyclic fatigue test results are shown in Fig. 3(b). The specimens that did not fracture up to Nf  2 106 cycles are marked by an arrow (3). The cyclic fatigue limit (f0) for the heat-treated specimen and the crack-healed specimen are about 800 and 850 MPa, respectively. The static and cyclic fatigue limits of the crack-healed speci￾mens are similar to those of the heat-treated specimens, indicating that the room-temperature fatigue strength of the crack-healed zone is comparable to that of base material. The cyclic fatigue limit of crack-healed specimens is about 10% higher than that of the static fatigue limit. This reason will be discussed later in this paper. The ratio of the static fatigue limit (t0) and the cyclic fatigue limit (f0) to the average bending strength of crack-healed specimens (B) is about 75% and 85%, respectively (see also Table I). Thus, fatigue limits of crack-healed specimens at room temperature are quite high. Bending tests for the specimens which survived the static and cyclic fatigue tests were also conducted at room temperature. The experimental results are shown on the right-hand sides of Figs. 3(a) and (b). The fatigue-tested specimens exhibited bending strengths similar to those of the monotonically tested specimens as shown on the left-hand side of Figs. 3(a) and (b). These experimental results indicate that significant crack growth from the crack-healed zone did not occur. Figure 4 shows SEM micrographs of the fracture surface after static fatigue tests at room temperature. The slow crack growth can be clearly observed, as shown in Fig. 4(a). Figure 4(b) shows the details of the crack-healed zone. The dark regions near SiC whiskers are oxidized, while the light ones not. Figure 4(c) shows the details of the slow crack growth zone. The fracture surface of the slow crack growth zone is quite rough and predominantly intergranular. Interactions between SiC whiskers and Al2O3 ma￾trix, such as bridging and/or pullout are clearly observed. Thus, SiC whiskers contribute to an increase in the crack growth resistance. As seen in Fig. 4(d), intergranular fracture cannot be identified in the final fracture zone. (3) Static Fatigue Strength at High Temperature In a previous study,29 we investigated the high-temperature bending strength of Al2O3/SiC-W which was precracked (2c  100 m) and crack-healed at 1300°C for 1 h in air. The bending strength of the crack-healed specimen was B  800 MPa and was almost constant up to 1100°C. However, the B showed a sudden decrease at 1100°C and decreased with increasing testing temper￾ature up to 1300°C. Therefore, the heat-resistance limit tempera￾ture for bending strength of the crack-healed specimen was 1100°C. In the present study, the cyclic and static fatigue strengths of crack-healed specimens were examined at 900° and 1100°C. The results of static fatigue tests at high temperature are shown in Fig. 5. The monotonic bending strength of the heat-treated smooth specimens and the crack-healed specimens at 900° and 1100°C are shown on the left-hand side of Fig. 5. The average bending strength of the heat-treated smooth specimens at 900° and 1100°C are about 850 MPa. However, the bending strengths of crack-healed specimens are about 88% and 76% compared with that of the heat-treated smooth specimen at 900° and 1100°C, respectively, and decrease with increasing test temperature. The values of the static fatigue limit (t0) for the crack-healed specimens at 900° and 1100°C are about 550 and 450 MPa, respectively. The static fatigue limit of the crack-healed specimen is about 70% in comparison to the bending strength at each testing temperature (see also Table I). Fig. 3. Fatigue test results for Al2O3/SiC-W at room temperature: (a) static fatigue test results for crack-healed Al2O3/SiC-W, (b) cyclic fatigue test results for crack-healed Al2O3/SiC-W. Data marked with an asterisk indicate that the fracture occurred outside of the crack-healed zone. July 2004 High-Temperature Fatigue Strength of Crack-Healed Al2O3 Toughened by SiC Whiskers 1261