M.B. Ruggles-Wrenn, CL. Genelin/Composites Science and Technology 69(2009)663-669 200H O N720/AN MPa, Argon 73 MPa, Argon 91 MPa. Air 73 MPa Air E031E-021.E-011E+001E+011E+021E+03 Time(h) 08 Fig. 5. Creep stress vs time to rupture for N720/AM and N720 A CMCs at 1200" in T=1200° team. Data for N720/A from Ruggles-Wrenn et al. [34 All data are 114 MPa, Argon 136 MPa, Argon adjusted for V,=0.40. 14 MPa Steam is nearly two orders of magnitude higher than that in air. In steam, the n20/ AM creep rates are slightly lower than those observed for N720/A, especially for applied stress levels <114 MPa. It is note- worthy that the presence of argon also accelerates the creep rates ates obtained to those produced in steam for a given applied stress. In contrast, the presence of argon did not increase the n720 /A creep rates. In argon, creep rates of n720 A were somewhat lower than those ob- tained in air. Time(h) Stress-rupture behavior is summarized in Fig. 5, where results for N720/A from prior work 34 are also included. As expected. Fig. 3. Creep strain Vs time curves for N720/alur composite at 1200c creep life decreases with increasing applied stress. In air, the creep in air, steam and argon (a) at 73 and 91 MPa run-out stress for N720/AM was 91 MPa. The presence of steam drastically reduced the creep lifetimes of N720/ AM. The reduction in creep life due to steam was >95% for applied stress levels of 5, while the presence of steam increases creep strain by nearly a >91 MPa, and 63% for the applied stress of 73 MPa In steam, creep actor of 3 run-out was not achieved In argon, specimen tested at 73 MPa sur Minimum creep rate was reached in all tests. Creep rate as a vived 92.8 h, almost achieving creep run-out of 100h.However,at function of applied stress is presented in Fig. 4, where results for stresses >91 MPa, the presence of argon degraded creep lifetimes N720/A from prior work [34] are included for comparison. In air, by at least 80%. It is notable that in air and in steam the creep life- 73 MPa to 136 MPa In ai, thie the creep stress incretoxim mom times of N720) AM were similar to those of N720/A.Conversely, ar- secondary creep rate of N720/AM the two composites. The presence of had a beneficial effect is approximately an order of magnitude lower than that of N720/ on creep performance of N720/A, increasing the creep lifetimes A for a given creep stress. The N720/AM creep rates increase dra- at least twofold matically in steam. For a given creep stress, the creep rate in steam Retained strength and modulus of the specimens that achieved creep run-out in air are summarized in Table 2. Tensile stress- strain curves obtained for the specimens subjected to prior creep 1.0E-04 are presented in Fig. 6 together with the tensile stress-strain curve M,Ar■N720A.Ai for the as-processed material. Prior creep appears to have a bene- 10E05△N720/ AM, Argon ficial effect on tensile strength. The strength of the N720 /AM spec- imens subjected to 100 h of prior creep in air was x10% higher than the uts of the untested material. Nevertheless a reduction g10E06 in modulus was observed. Modulus loss due to prior creep a 73 MPa was 6%, and modulus loss due to creep at 91 MPa was 9%. Results of the present study reveal that at 1200C the presence 西1.0E07 of steam dramatically reduces creep lifetimes of N720/AM. Because the creep performance of the composite with 0/90 orientation is 1.0E08 dominated by the fibers, fiber degradation is a likely source of the T=1200°c omposite degradation. It is possible that environmentally-assisted subcritical crack growth in the n720 fibers is the mechanism be- 405060708090100 00 hind reduced creep resistance of N720 /AM composite at 1200C in steam. In this case, subcritical(slow crack growth in the fiber Max Stress(MPa) is caused by a chemical interaction of water molecules with Fig 4. Minimum creep mechanically strained Si-o bonds at the crack ramic composites at 1200.C in air, argon and steam. Data for N720/A from of chemical reaction increasing exponentially with applied stress Ruggles-Wrenn et al. [34]. All data are adjusted for Vr=0. 40. 27,35-42of 5, while the presence of steam increases creep strain by nearly a factor of 3. Minimum creep rate was reached in all tests. Creep rate as a function of applied stress is presented in Fig. 4, where results for N720/A from prior work [34] are included for comparison. In air, the minimum creep rate of N720/AM increases by approximately two orders of magnitude as the creep stress increases from 73 MPa to 136 MPa. In air, the secondary creep rate of N720/AM is approximately an order of magnitude lower than that of N720/ A for a given creep stress. The N720/AM creep rates increase dra￾matically in steam. For a given creep stress, the creep rate in steam is nearly two orders of magnitude higher than that in air. In steam, the N720/AM creep rates are slightly lower than those observed for N720/A, especially for applied stress levels 6114 MPa. It is note￾worthy that the presence of argon also accelerates the creep rates of N720/AM. The N720/AM creep rates obtained in argon are close to those produced in steam for a given applied stress. In contrast, the presence of argon did not increase the N720/A creep rates. In argon, creep rates of N720/A were somewhat lower than those ob￾tained in air. Stress–rupture behavior is summarized in Fig. 5, where results for N720/A from prior work [34] are also included. As expected, creep life decreases with increasing applied stress. In air, the creep run-out stress for N720/AM was 91 MPa. The presence of steam drastically reduced the creep lifetimes of N720/AM. The reduction in creep life due to steam was P95% for applied stress levels P91 MPa, and 63% for the applied stress of 73 MPa. In steam, creep run-out was not achieved. In argon, specimen tested at 73 MPa sur￾vived 92.8 h, almost achieving creep run-out of 100 h. However, at stresses P91 MPa, the presence of argon degraded creep lifetimes by at least 80%. It is notable that in air and in steam the creep life￾times of N720/AM were similar to those of N720/A. Conversely, ar￾gon environment had opposing effects on the creep performance of the two composites. The presence of argon had a beneficial effect on creep performance of N720/A, increasing the creep lifetimes at least twofold. Retained strength and modulus of the specimens that achieved creep run-out in air are summarized in Table 2. Tensile stress– strain curves obtained for the specimens subjected to prior creep are presented in Fig. 6 together with the tensile stress–strain curve for the as-processed material. Prior creep appears to have a bene- ficial effect on tensile strength. The strength of the N720/AM spec￾imens subjected to 100 h of prior creep in air was 10% higher than the UTS of the untested material. Nevertheless, a reduction in modulus was observed. Modulus loss due to prior creep at 73 MPa was 6%, and modulus loss due to creep at 91 MPa was 9%. Results of the present study reveal that at 1200 C the presence of steam dramatically reduces creep lifetimes of N720/AM. Because the creep performance of the composite with 0/90 orientation is dominated by the fibers, fiber degradation is a likely source of the composite degradation. It is possible that environmentally-assisted subcritical crack growth in the N720 fibers is the mechanism be￾hind reduced creep resistance of N720/AM composite at 1200 C in steam. In this case, subcritical (slow) crack growth in the fiber is caused by a chemical interaction of water molecules with mechanically strained Si–O bonds at the crack tip, with the rate of chemical reaction increasing exponentially with applied stress [27,35–42]. 0 1 2 3 4 5 0 10 20 30 40 50 Strain (%) Time (h) 73 MPa, Air T = 1200 ºC 91 MPa, Steam 73 MPa, Argon 91 MPa, Argon 91 MPa, Air 73 MPa, Steam 0.0 0.2 0.4 0.6 0.8 0.0 0.2 0.4 0.6 0.8 Strain (%) Time (h) 114 MPa, Air 136 MPa, Air 114 MPa, Argon 114 MPa, Steam T = 1200 ºC 136 MPa, Argon 136 MPa Steam a b Fig. 3. Creep strain vs. time curves for N720/alumina–mullite composite at 1200 C in air, steam and argon: (a) at 73 and 91 MPa and (b) at 114 and 136 MPa. 1.0E-09 1.0E-08 1.0E-07 1.0E-06 1.0E-05 1.0E-04 40 Strain Rate (s-1) Max Stress (MPa) N720/AM, Air N720/A, Air N720/AM, Steam N720/A, Steam N720/AM, Argon N720/A, Argon T = 1200 ºC 50 60 70 80 90 100 200 Fig. 4. Minimum creep rate as a function of applied stress for N720/AM and N720/A ceramic composites at 1200 C in air, argon and steam. Data for N720/A from Ruggles-Wrenn et al. [34]. All data are adjusted for Vf = 0.40. 0 50 100 150 200 250 1.E-03 1.E-02 1.E-01 1.E+00 1.E+01 1.E+02 1.E+03 Stress (MPa) Time (h) N720/AM, Air N720/A, Air N720/AM, Argon N720/A, Argon N720/AM, Steam N720/A, Steam N720/AM, Steam-Prediction N720/A, Steam-Prediction T = 1200 ºC Fig. 5. Creep stress vs. time to rupture for N720/AM and N720/A CMCs at 1200 C in air, argon and steam. Data for N720/A from Ruggles-Wrenn et al. [34]. All data are adjusted for Vf = 0.40. 666 M.B. Ruggles-Wrenn, C.L. Genelin / Composites Science and Technology 69 (2009) 663–669