M B. Ruggles.Wrenn et al Composites: Part 4 37(2006)2029-2040 1.E02 given creep stress, creep-rupture life at 1330C is consider ■1200°c,Ar ably reduced compared to that at 1200C. With the creep 1E03 1330°c,Ar run-out condition defined as 100 h. 80 MPa was the run .E o a c steam N720 Fiber at v, =0. 2 out stress at 1200C. At 1330C the run-out was not 1200°c achieved. Even at a low creep stress of 50 MPa the rupture g1E05 time was 87h<100 h. The short creep lives produced at 1E06 1330C indicate that this oxideoxide Cmc should not be nder sustained loading at temperatures above 61E07 1200C. Presence of steam dramatically reduced creep lives N720 Fiber. 1200c Wilson. 2001 at both test temperatures. At 1200C, reduction in creep life 1.E08 due to steam was at least 90% for applied stress levels 1E0 >100 MPa, and 82% for the applied stress of 80 MPa. At 1000 1330C, presence of steam reduced creep lives by 96-98% Creep Stress(MPa) It is recognized that Nextel 720 fiber has the best creep Fig.13. Minimum creep rate as a function of applied stress at 1200 and performance of any commercially available polycrystalline 1330C in laboratory air and in steam environment Data for Nextel 720 oxide fiber. The superior high-temperature creep perfor fibers (Wilson [57])are also shown. mance of the Nextel720 fibers results from the high con tent of mullite, which has a much better creep resistance in air environment were close to the N720 fiber data than alumina [57]. Wannaparhum et al. [55] reported that adjusted for Vr=0. 22. For a given creep stress, creep rates exposure of the N720/A composite to water vapor at in steam were approximately an order of magnitude higher 1100 C could result in an increase in Al,O3 content of than those in air. Fitting the experimental results(obtained the fiber because of the loss of Sio, from its mullite phase in either air or steam) with a temperature-independent The loss of the mullite phase in the fiber may be the mech Norton-Bailey equation of the form anism behind the higher creep rates and reduced creep resistance observed in steam. However, further experiments ields the stress exponents that nsiderably higher quantifying the mullite loss from the fiber exposed to water than that reported for the N720 fibers alone [57]. It is be- vapor at high temperature would be required to ascertain lieved that the higher stress exponents are due to a contri- bution from the matrix. As expected, creep resistance 3. 4. Composite microstructure decreases dramatically with increasing temperature. Fc the creep stress of 100 MPa, creep rate at 1330C was al- most two orders of magnitude higher that at 1200C Fracture surface of a specimen tested in creep is shown The presence of steam further accelerates creep and de. in Fig. 15. It should be noted that the appearance of the fracture surface was not significantly affected by any of grades creep resistance. At 1330C, the presence of steam the following factors: test temperature(1200 vs 1330oC), increases the minimum creep rate by a factor of 100 Stress-rupture behavior is summarized in Fig test environment (air vs steam), test type(tension vs creep creep stress is plotted vs time to rupture at 1200 and 1330C vs tension-tension fatigue). Fracture surfaces of similar ere in air and in steam environments. At both temperatures appearance were produced in all tests. Micrographs in creep life decreases with increasing applied stress. For a ig. 15 are typical and representative of fracture surfaces obtained in all tests in this study. It is seen in Fig. 15(a) that the fracture plane is not well defined. The fibers in the 0o tows in each cloth layer exhibit UTS at1200°c 180 random failure producing fiber pull-out △1330°c,Ar shows that the 0 fiber tows break over a wide range of ▲1330°C. Steam axial locations, in general spanning the entire width of 1204 UTS at1330°c the specimen. The locations of the fiber breaks within an individual tow also exhibit a broad distribution, typically Ml mm in length, as seen in Fig. 15(c). It is important to note that no matrix holes were observed on the fracture surface. In conventional CFCCs with "weak"' interfaces the fiber pull-out results in formation of matrix holes, where broken fibers slide out of the matrix. However, in 1.E+00 1.E+01 1.E+02 1.E+03 1. E+04 1.E+05 1.E+06 1. E+07 leave matrix sockets but causes fragmentation of interven- Time(s) ing matrix in the region of strain localization. Some of the Fig. 14. Creep stress vs time to rupture at 1200 and 1330C in laborator matrix debris and matrix still bonded to the fibers are seen air and in steam environment in Fig. 15(d)in air environment were close to the N720 fiber data adjusted for Vf = 0.22. For a given creep stress, creep rates in steam were approximately an order of magnitude higher than those in air. Fitting the experimental results (obtained in either air or steam) with a temperature-independent Norton–Bailey equation of the form e_ ¼ Arn yields the stress exponents that are considerably higher than that reported for the N720 fibers alone [57]. It is be￾lieved that the higher stress exponents are due to a contri￾bution from the matrix. As expected, creep resistance decreases dramatically with increasing temperature. For the creep stress of 100 MPa, creep rate at 1330 C was al￾most two orders of magnitude higher that at 1200 C. The presence of steam further accelerates creep and de￾grades creep resistance. At 1330 C, the presence of steam increases the minimum creep rate by a factor of 100. Stress-rupture behavior is summarized in Fig. 14, where creep stress is plotted vs time to rupture at 1200 and 1330 C in air and in steam environments. At both temperatures, creep life decreases with increasing applied stress. For a given creep stress, creep-rupture life at 1330 C is consider￾ably reduced compared to that at 1200 C. With the creep run-out condition defined as 100 h, 80 MPa was the run￾out stress at 1200 C. At 1330 C the run-out was not achieved. Even at a low creep stress of 50 MPa the rupture time was 87 h < 100 h. The short creep lives produced at 1330 C indicate that this oxide/oxide CMC should not be used under sustained loading at temperatures above 1200 C. Presence of steam dramatically reduced creep lives at both test temperatures. At 1200 C, reduction in creep life due to steam was at least 90% for applied stress levels P100 MPa, and 82% for the applied stress of 80 MPa. At 1330 C, presence of steam reduced creep lives by 96–98%. It is recognized that NextelTM720 fiber has the best creep performance of any commercially available polycrystalline oxide fiber. The superior high-temperature creep perfor￾mance of the NextelTM720 fibers results from the high con￾tent of mullite, which has a much better creep resistance than alumina [57]. Wannaparhum et al. [55] reported that exposure of the N720/A composite to water vapor at 1100 C could result in an increase in Al2O3 content of the fiber because of the loss of SiO2 from its mullite phase. The loss of the mullite phase in the fiber may be the mech￾anism behind the higher creep rates and reduced creep resistance observed in steam. However, further experiments quantifying the mullite loss from the fiber exposed to water vapor at high temperature would be required to ascertain this. 3.4. Composite microstructure Fracture surface of a specimen tested in creep is shown in Fig. 15. It should be noted that the appearance of the fracture surface was not significantly affected by any of the following factors: test temperature (1200 vs 1330 C), test environment (air vs steam), test type (tension vs creep vs tension–tension fatigue). Fracture surfaces of similar appearance were produced in all tests. Micrographs in Fig. 15 are typical and representative of fracture surfaces obtained in all tests in this study. It is seen in Fig. 15(a) that the fracture plane is not well defined. The fibers in the 0 tows in each cloth layer exhibit random failure producing fiber ‘‘pull-out’’. Fig. 15(b) shows that the 0 fiber tows break over a wide range of axial locations, in general spanning the entire width of the specimen. The locations of the fiber breaks within an individual tow also exhibit a broad distribution, typically 1 mm in length, as seen in Fig. 15(c). It is important to note that no matrix holes were observed on the fracture surface. In conventional CFCCs with ‘‘weak’’ interfaces, the fiber pull-out results in formation of matrix holes, where broken fibers slide out of the matrix. However, in the present composite, the pull-out of the fibers does not leave matrix sockets but causes fragmentation of interven￾ing matrix in the region of strain localization. Some of the matrix debris and matrix still bonded to the fibers are seen in Fig. 15(d). 1.E-09 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 10 100 1000 Creep Stress (MPa) Creep Rate (s-1) 1200°C, Air 1200°C, Steam 1330°C, Air 1330°C, Steam N720 Fiber, 1200°C Wilson, 2001 N720 Fiber at Vf = 0.22 1200°C Fig. 13. Minimum creep rate as a function of applied stress at 1200 and 1330 C in laboratory air and in steam environment. Data for Nextel 720 fibers (Wilson [57]) are also shown. 0 20 40 60 80 100 120 140 160 180 200 1.E+00 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 Time (s) Stress (MPa) 1200°C, Air 1200°C, Steam 1330°C, Air 1330°C, Steam UTS at 1200°C UTS at 1330°C Fig. 14. Creep stress vs time to rupture at 1200 and 1330 C in laboratory air and in steam environment. M.B. Ruggles-Wrenn et al. / Composites: Part A 37 (2006) 2029–2040 2037