M B. Ruggles-Wrenn et al / Composites Science and Technology 68(2008)1588-1595 Stress-rupture behavior is summarized in Fig. 6, where Table 2 results for N720/A composite with 0/90 fiber orientation Retained properties of the N720/A specimens with*45fiber orientation from prior work [20] are also included. As expected, creep subjected to prior creep at 1200C in laboratory air, steam and argon life decreases with increasing applied stress for both fiber orientations. In the case of the 0/90 orientation, the pres Environment Creep etained Retained Strain at ence of steam dramatically reduced creep lifetimes. The ength modulu failure(%) (MPa) GPa) reduction in creep life due to steam was at least 90% for applied stress levels >100 MPa, and 82% for the applied Air 0.15 stress of 80 MPa. Because the creep performance of the 0%/Steam 63 90 orientation is dominated by the fibers, fiber degrada- Steam tion is a likely source of the composite degradation. Recent Argon 73.0 68.8 studies [27, 28] suggest that the loss of mullite from the fiber may be the mechanism behind the degraded creep perfor mance in steam. Alternatively, poor creep resistance in steam may be due to a stress-corrosion mechanism. In this case,crack growth in the fiber could be caused by a chem- 100 ical interaction of water molecules with mechanically 100 h at 15 MPa strained Si-o bonds at the crack tip with the rate of chem- Steam ical reaction increasing exponentially with applied stress 75 As-Processed 29-34]. In the case of the +45 orientation, environment has little effect on the creep lifetimes (up to 100 h) for applied stresses <35 MPa. For stresses >40 MPa, creep lifetimes can be reduced by as much as an order of magni 100 h at 15 MPa tude in the presence of steam. An even greater reduction in creep life is seen in the presence of argon. Further experi- ments would be required to understand the cause of such T=1200 drastic degradation of the creep performance in argon Retained strength and modulus of the specimens that Strain (%) achieved a run-out are summarized in Table 2. Tensile stress-strain curves obtained for the specimens subjected prior creep are presented in Fig. 7 together with the te 100h at 35 MPa sile stress-strain curve for the as-processed material. while 75 the specimens subjected to prior creep in all environments 100h at 35 MPa As-Processed in Steam exhibit increased tensile strength and stifness, their capac- y for inelastic straining appears to be considerably reduced. The pre-crept specimens produced higher propor tional limits and much lower failure strains than the as-pro- cessed material. Since the tensile properties of the T=1200°c 0.1 0.2 Strain T=1200°c ■0°/90 ▲0°90°, Steam Effects of prior creep at 1200C in laboratory air, steam and argon 口±45°,Air nsile stress-strain behavior of N720/A with +45 fiber orientation △±45° Stean creep stress:(a)15 MPa and (b)35 MPa. a150 ◇±45°, Argon composite with the +45 fiber orientation are largely dom- inated by the matrix, results indicate that matrix strength ening(most likely due to additional sintering) may be 50fUTSxs taking place during the 100 h of creep 3.3. Composite microstructure E+001E+011.E+021.E+031.E+041.E+051.E+061E+07 (s) Fracture surfaces of the N720/A specimens with +45 Fig. 6. Creep stress vs time to rupture for n720/A ceramic composite at fiber orientation tested in creep at 45 MPa are presented 1200C in laboratory air, steam and argon. Data for 0/900 fiber in Figs. 8 and 9. It is seen that the fracture occurred along orientation from Ruggles-Wrenn et al. [20] the plane at 45 to the loading direction. The failureStress-rupture behavior is summarized in Fig. 6, where results for N720/A composite with 0/90 fiber orientation from prior work [20] are also included. As expected, creep life decreases with increasing applied stress for both fiber orientations. In the case of the 0/90 orientation, the pres￾ence of steam dramatically reduced creep lifetimes. The 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. Because the creep performance of the 0/ 90 orientation is dominated by the fibers, fiber degrada￾tion is a likely source of the composite degradation. Recent studies [27,28] suggest that the loss of mullite from the fiber may be the mechanism behind the degraded creep perfor￾mance in steam. Alternatively, poor creep resistance in steam may be due to a stress-corrosion mechanism. In this case, crack growth in the fiber could be caused by a chem￾ical interaction of water molecules with mechanically strained Si–O bonds at the crack tip with the rate of chem￾ical reaction increasing exponentially with applied stress [29–34]. In the case of the ±45 orientation, environment has little effect on the creep lifetimes (up to 100 h) for applied stresses 635 MPa. For stresses P40 MPa, creep lifetimes can be reduced by as much as an order of magni￾tude in the presence of steam. An even greater reduction in creep life is seen in the presence of argon. Further experi￾ments would be required to understand the cause of such drastic degradation of the creep performance in argon. Retained strength and modulus of the specimens that achieved a run-out are summarized in Table 2. Tensile stress–strain curves obtained for the specimens subjected to prior creep are presented in Fig. 7 together with the ten￾sile stress–strain curve for the as-processed material. While the specimens subjected to prior creep in all environments exhibit increased tensile strength and stiffness, their capac￾ity for inelastic straining appears to be considerably reduced. The pre-crept specimens produced higher propor￾tional limits and much lower failure strains than the as-pro￾cessed material. Since the tensile properties of the composite with the ±45 fiber orientation are largely dom￾inated by the matrix, results indicate that matrix strength￾ening (most likely due to additional sintering) may be taking place during the 100 h of creep. 3.3. Composite microstructure Fracture surfaces of the N720/A specimens with ±45 fiber orientation tested in creep at 45 MPa are presented in Figs. 8 and 9. It is seen that the fracture occurred along the plane at 45 to the loading direction. The failure 0 50 100 150 200 250 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) 0º/90º, Air 0º/90º, Steam ±45º, Air ±45º, Steam ±45º, Argon UTS0/90 UTS±45 T = 1200 ºC Fig. 6. Creep stress vs time to rupture 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]. Table 2 Retained properties of the N720/A specimens with ±45 fiber orientation subjected to prior creep at 1200 C in laboratory air, steam and argon environments Environment Creep stress (MPa) Retained strength (MPa) Retained modulus (GPa) Strain at failure (%) Air 15 61.1 64.5 0.15 Air 35 58.3 48.4 0.14 Steam 15 67.0 63.4 0.17 Steam 35 53.4 58.3 0.07 Argon 15 73.0 68.8 0.14 Argon 35 49.4 55.3 0.09 25 50 75 100 Strain (%) Strain (%) Stress (MPa) 100 h at 15 MPa in Steam 100 h at 15 MPa in Air 100 h at 15 MPa in Argon As-Processed T = 1200 ºC 0 0 25 50 75 100 0 0.1 0.2 0.3 0 0.1 0.2 0.3 Stress (MPa) As-Processed T = 1200 ºC 100 h at 35 MPa in Air 100 h at 35 MPa in Steam 100 h at 35 MPa in Argon Fig. 7. Effects of prior creep at 1200 C in laboratory air, steam and argon on tensile stress–strain behavior of N720/A with ±45 fiber orientation. Prior creep stress: (a) 15 MPa and (b) 35 MPa. 1592 M.B. Ruggles-Wrenn et al. / Composites Science and Technology 68 (2008) 1588–1595