M B. Ruggles-Wrenn et al/ Composites Science and Technology 68(2008)1588-1595 Table l during the first third of the creep life At the intermediate ry of creep-rupture results for the N720/A ceramic composite with stress of 40 MPa, only primary and secondary creep are +450fiber orientation at 1200 C in laboratory air, steam and argon observed in air and in steam, but all three creep regimes are seen In argon. Environment Creep stress Creep strain Time to rupture (%) Minimum creep rate was reached in all tests. Creep rate as a function of applied stress is presented in Fig. 5, where 13.3 results for N720/A composite with 00/90 fiber orientation l1.9 75,569 from prior work [20] are included for comparison. It is seen 19 that in air the secondary creep rate of the +45 orientation can be as high as 10 times that of the 0%/90%orientation.In 17.8 steam, the +45 creep rate can be as high as 10 times the 2,615 Steam 0.6 0/90 rate. This result is hardly surprising, considering Argon 360000 that the creep-rupture of the 0 /90 orientation is likely Argon 550555055500 dominated by creep-rupture of the Nextel720 fibers It Argon is recognized that Nextel720 fiber has the best creep per Argon Argon 3.55 17 formance of any commercially available polycrystalline Argon oxide fiber. The superior high-temperature creep perfor mance of the Nextel720 fibers results from the high con- tent of mullite, which has a much better creep resistance than alumina [26]. Conversely, the creep-rupture of the +45 orientation is largely dominated by an exceptionally weak porous alumina matrix. For both fiber orientations, T=1200° the minimum creep rates increase with increasing applied stress. In the case of the 0/90 orientation, the secondary creep rate increases by two orders of magnitude as the creep stress increases from 80 to 154 MPa. For a given creep stress, creep rate in steam is approximately an order of magnitude higher than that in air. In the case of the +45 fiber orientation, for stresses <40 MPa creep rate is relatively unaffected by environment. Creep rates obtained in all tests at 15 and 35 MPa are <10s. As the creep stress increases to 45 MPa, the creep rate in air increases by a3 orders of magnitude. The creep rate obtained at 100000200000300000400000500000 Time(s) 45 MPa in steam remains close to that obtained in air while in argon the creep rate increases even more dramat ically. The creep rate in argon is at least one order of mag nitude higher than the rates obtained in air and in steam at Pa, Argon 45 MPa 40 MPa, Argon 1E+00 1.E-01 0e90°,Ar ▲0°0°, Steam 1E22 △ 1.E04 100 Time(s) 9 1.E-0 Fig 4. Creep curves for N720/A composite with #45 fiber orientation at 1E07 1200C in air, steam and argon: (a) time scale chosen to shor 1E08 T=1200 rains accumulated at 15-35 MPa and (b) time scale reduced to show the reep curves obtained at 45 MPa. 1E-09 how primary, secondary and tertiary creep. Transition from primary to secon creep occurs almost immedi- Fig. 5. Minimum creep rate as a function of applied stress for N720/A ceramic composite at 1200C in laboratory air, steam and argon. Data for ately, and secondary transitions to tertiary creep 0/90 fiber orientation from Ruggles-Wrenn et al [20] are also shownshow primary, secondary and tertiary creep. Transition from primary to secondary creep occurs almost immedi￾ately, and secondary creep transitions to tertiary creep during the first third of the creep life. At the intermediate stress of 40 MPa, only primary and secondary creep are observed in air and in steam, but all three creep regimes are seen in argon. Minimum creep rate was reached in all tests. Creep rate as a function of applied stress is presented in Fig. 5, where results for N720/A composite with 0/90 fiber orientation from prior work [20] are included for comparison. It is seen that in air the secondary creep rate of the ±45 orientation can be as high as 106 times that of the 0/90 orientation. In steam, the ±45 creep rate can be as high as 105 times the 0/90 rate. This result is hardly surprising, considering that the creep-rupture of the 0/90 orientation is likely dominated by creep-rupture of the NextelTM720 fibers. It is recognized that NextelTM720 fiber has the best creep per￾formance 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 [26]. Conversely, the creep-rupture of the ±45 orientation is largely dominated by an exceptionally weak porous alumina matrix. For both fiber orientations, the minimum creep rates increase with increasing applied stress. In the case of the 0/90 orientation, the secondary creep rate increases by two orders of magnitude as the creep stress increases from 80 to 154 MPa. For a given creep stress, creep rate in steam is approximately an order of magnitude higher than that in air. In the case of the ±45 fiber orientation, for stresses <40 MPa creep rate is relatively unaffected by environment. Creep rates obtained in all tests at 15 and 35 MPa are 6 105 s 1 . As the creep stress increases to 45 MPa, the creep rate in air increases by 3 orders of magnitude. The creep rate obtained at 45 MPa in steam remains close to that obtained in air, while in argon the creep rate increases even more dramat￾ically. The creep rate in argon is at least one order of mag￾nitude higher than the rates obtained in air and in steam at 45 MPa. 0 5 10 15 20 25 0 100000 200000 300000 400000 500000 Time (s) Strain (%) 15 MPa, Steam 35 MPa, Steam T = 1200 ºC 15 MPa, Air 15 MPa, Argon 35 MPa, Argon 40 MPa, Air 35 MPa, Air 0.0 1.0 2.0 3.0 4.0 5.0 0 50 100 150 200 Time (s) Strain (%) 45 MPa, Argon T = 1200 ºC 40 MPa, Air 40 MPa, Steam 45 MPa, Steam 45 MPa, Air 40 MPa, Argon Fig. 4. Creep curves for N720/A composite with ±45 fiber orientation at 1200 C in air, steam and argon: (a) time scale chosen to show creep strains accumulated at 15–35 MPa and (b) time scale reduced to show the creep curves obtained at 45 MPa. 1.E-09 1.E-08 1.E-07 1.E-06 1.E-05 1.E-04 1.E-03 1.E-02 1.E-01 1.E+00 10 100 1000 Creep Stress (MPa) Creep Strain Rate (s-1) 0º/90º, Air 0º/90º, Steam ±45º, Air ±45º, Steam ±45º, Argon T = 1200 ºC Fig. 5. Minimum creep rate as a function of applied stress for N720/A ceramic composite at 1200 C in laboratory air, steam and argon. Data for 0/90 fiber orientation from Ruggles-Wrenn et al [20] are also shown. Table 1 Summary of creep-rupture results for the N720/A ceramic composite with ±45 fiber orientation at 1200 C in laboratory air, steam and argon environments Environment Creep stress (MPa) Creep strain (%) Time to rupture (s) Air 15 3.38 360,000* Air 35 13.3 360,000* Air 40 11.9 75,569 Air 45 1.48 119 Steam 15 4.80 360,000* Steam 35 17.8 360,000* Steam 40 4.92 2,615 Steam 45 0.65 59 Argon 15 7.05 360000* Argon 35 20.5 360000* Argon 40 3.58 64 Argon 40 3.36 49 Argon 45 3.55 17 Argon 45 4.12 22 * Run-out. M.B. Ruggles-Wrenn et al. / Composites Science and Technology 68 (2008) 1588–1595 1591