J M. Ehrman et al Composites Science and Technology 67(2007)1425-1438 b 23p% 10 V mm too, 2 OK d 1.0mm Fig 13. Fracture surfaces of the N720/A CMC tested at 1200C in steam with Omax=125 MPa: (a)fatigue, (b) fatigue with 10-s hold, (c) fatigue with 100-s hold, and(d) creep hold time are also dominated by regions of planar fracture. high-temperature creep performance of the Nextel720 The hold duration appears to have little effect on the frac- fibers is due to the high content of mullite, which has a ture topography. Once a 10-s hold is introduced, planar much better creep resistance than alumina [35, 36]. Creep fracture prevails. Increasing the hold to 100 s has little resistance of NextelmM720 fibers is also due in part to the lar dditional effect. Note that the specimen tested in fatigue ger grain size of this fiber relative to other oxide fibers such ig 13a), which produced the largest amount of fiber pull- as Nextel 610 [35, 36]. A recent study [37]concluded that out, also achieved a run-out. On the contrary, specimens at 1100C in water-vapor environment, Sioz could be lea- with nearly planar fracture surfaces shown in Figs. 13b- ched from NextelTM720 fiber. Wannaparhun et al. [37]sug- d,exhibited short lives of 0. 22-0. h In air as well as in gested that exposure of the N720/A CMC to water vapor steam,planar fracture surface accompanies short time to at 1100C could cause an increase in Al2O3 content of failure, while brushy fracture surface is indicative of longer the fiber due to the loss of Sio2 from its mullite phase fe. For both air and steam environments, the fracture sur- Campbell et al. [38]observed a M15% reduction in strength faces obtained in fatigue are readily distinguished from of the N720/A composite after a 1000-h exposure to water those produced in creep. In addition, the fracture surface vapor environment at 1200C appearance may be correlated with time to failure a qualitative EDS analysis was performed to assess whether the loss of the mullite phase in the fiber may be 3.6. Energy dispersive X-ray spectroscopy(EDS) the mechanism behind the reduced creep resistance in steam. Specimens subjected to fatigue with 10-s hold at Results of the present study as well as the prior work 125 MPa in air and in steam as well as the as-processed [27]reveal the degrading effect of steam on fatigue as well material were examined. The electron beam spot mode as on creep performance of the N720/A composite at was employed in order to precisely control the beam place- 1200C. Mechanical behavior of a composite with 0/90 ment. Due to the uncertainties associated with the size and fiber orientation under tensile loading is fiber-dominated. shape of the reaction volume, the EDS results are inter Therefore, fiber degradation represents a likely source of preted as qualitative. All data were normalized to a CPS composite degradation. It is recognized that the superior of 1.0 for aluminum.hold time are also dominated by regions of planar fracture. The hold duration appears to have little effect on the frac￾ture topography. Once a 10-s hold is introduced, planar fracture prevails. Increasing the hold to 100 s has little additional effect. Note that the specimen tested in fatigue (Fig. 13a), which produced the largest amount of fiber pull￾out, also achieved a run-out. On the contrary, specimens with nearly planar fracture surfaces shown in Figs. 13b– d, exhibited short lives of 0.22–0.31 h. In air as well as in steam, planar fracture surface accompanies short time to failure, while brushy fracture surface is indicative of longer life. For both air and steam environments, the fracture sur￾faces obtained in fatigue are readily distinguished from those produced in creep. In addition, the fracture surface appearance may be correlated with time to failure. 3.6. Energy dispersive X-ray spectroscopy (EDS) Results of the present study as well as the prior work [27] reveal the degrading effect of steam on fatigue as well as on creep performance of the N720/A composite at 1200 C. Mechanical behavior of a composite with 0/90 fiber orientation under tensile loading is fiber-dominated. Therefore, fiber degradation represents a likely source of composite degradation. It is recognized that the superior high-temperature creep performance of the NextelTM720 fibers is due to the high content of mullite, which has a much better creep resistance than alumina [35,36]. Creep resistance of NextelTM720 fibers is also due in part to the lar￾ger grain size of this fiber relative to other oxide fibers such as NextelTM610 [35,36]. A recent study [37] concluded that at 1100 C in water-vapor environment, SiO2 could be lea￾ched from NextelTM720 fiber. Wannaparhun et al. [37] sug￾gested that exposure of the N720/A CMC to water vapor at 1100 C could cause an increase in Al2O3 content of the fiber due to the loss of SiO2 from its mullite phase. Campbell et al. [38] observed a 15% reduction in strength of the N720/A composite after a 1000-h exposure to water￾vapor environment at 1200 C. A qualitative EDS analysis was performed to assess whether the loss of the mullite phase in the fiber may be the mechanism behind the reduced creep resistance in steam. Specimens subjected to fatigue with 10-s hold at 125 MPa in air and in steam as well as the as-processed material were examined. The electron beam spot mode was employed in order to precisely control the beam place￾ment. Due to the uncertainties associated with the size and shape of the reaction volume, the EDS results are inter￾preted as qualitative. All data were normalized to a CPS of 1.0 for aluminum. Fig. 13. Fracture surfaces of the N720/A CMC tested at 1200 C in steam with rmax = 125 MPa: (a) fatigue, (b) fatigue with 10-s hold, (c) fatigue with 100-s hold, and (d) creep. 1434 J.M. Mehrman et al. / Composites Science and Technology 67 (2007) 1425–1438