PR Jackson et al. /Materials Science and Engineering A 454-455(2007)590-601 slightly lower than the UTS. Note that at 1100C, in contrast to Table 4 the mostly linear compressive behavior, the tensile stress-strain Summary of compressive creep-rupture results for the NIO/M/A and the curve departs from linearity. Such non-linear behavior is indica- N610/A composites tive of progressive matrix cracking and crack deflection, the Specimen Temperature Creep stress Creep Time to mechanisms likely responsible for a considerably larger tensile rupture(s) failure strain and a somewhat higher tensile strength N610/monazite/alumina composite is seen in Fig. 2(b) that at 900C, B1461100 360.0003 stress-strain behavior of the uncoated fiber composite is also B14 360.0003 nearly linear to failure. However, at 1100C, the compres- B14-41100 360.0003 sive stress-strain curve of N610/A contains two parts. The first is a linear part which extends to a stress of approxi- mately-157 MPa The second part is also approximately linear with a noticeably lower slope. This stress-strain behavior indi- B19.2 1100 cates probable composite damage at the change in slope. It is B19-5 900 500650万 0.03 180.0003 3600004 -2.65 3600004 360.0002 0.03 180.000 likely that as the compressive stress approaches -157 MPa shear B196 0.01 180.000 cracking in the 90 fiber bundles and ply delamination take place. 0.05 180.000 The damage relieves constraints acting on the 0o bundles allow Run-out ing them to deform more readily by buckling, and leading to a lower slope in the stress-strain curve. When the compressive °C( see Fig.3(b)) stress reaches -240 MPa, the composite uckling and exhibit primary and secondary creep regimes. Unlike in the shear fracture of the 0o bundles. At 900 and 1100C, the com- case of tension, in compression transition from primary to sec pressive modulus and strength of N610/A considerably exceed ondary creep occurs much later in creep life. For N610/M/A the corresponding tensile values. It appears that the processing- composite, primary creep transitions into secondary creep after related cracks in both matrix and 0 fiber bundles close up during 5h at the stress of-50MPa. At stresses <-65 MPa, primary compression producing a higher modulus. It can be further con- jectured that in tension the same processing-related cracks would start propagating at lower stress levels, leading to lower UTS It is noteworthy that while the use of the monazite coat- N610/Monazite/Alumina T=1100°c ing served to improve the high-temperature tensile strength, the 4.0 gth of the monazite -contai considerably lower than that of the uncoated fiber CMC. The 3.0 addition of the monazite coating resulted in v54% loss in com- o pressive strength at 900C, and in -60% loss in compressive u2.0 100 MP strength at 1100°C 20 MPa It is important to note that in all monotonic tension and com- 61.0 pression tests, as well as in all other tests reported herein, the 40 MPa failure occurred within the gage section of the extensometer. 25050075010001250150017502000 4.2. Creep-rupture 10.0 Results of the compressive creep-rupture tests are summa- N610/Monazite/Alumina T=1100°c rized in Table 4, where creep strain accumulation and rupture 9 N610/Alumina time are shown for each test temperature and creep stress z level. Creep curves obtained at 1100 and 900C are shown in Figs. 3 and 4, respectively. Tensile creep data from prior work [53]is included in Figs. 3 and 4 for comparison Results of the u 65 MP. Tensile creep curves obtained for the n610/M/A at 1100%C 54.0 tensile creep tests of the two composites appear in Ref. [ 53] (see Fig 3(a)) exhibit primary, secondary and tertiary creep regimes. Primary creep rapidly transitions to secondary creep. For stresses 2 100 MPa, transition from secondary to tertiary creep occurs during the first third of the creep life. Creep strain 100000200000300000400000500000 and creep life decrease with increasing applied stress. Note that (b) TIME (s) the tensile creep strains accumulated in all tests conducted at 1100oC significantly exceed the failure strain obtained in the Fig. 3. Creep curves for NlO/MA and Nlo/A composites at 1100C: (a) tension test. The tensile creep run-out(set to 100h) was not Time scale in(a)is selected to clearly show creep curves obtained at stress levels above 100 MPa594 P.R. Jackson et al. / Materials Science and Engineering A 454–455 (2007) 590–601 slightly lower than the UTS. Note that at 1100 ◦C, in contrast to the mostly linear compressive behavior, the tensile stress–strain curve departs from linearity. Such non-linear behavior is indica￾tive of progressive matrix cracking and crack deflection, the mechanisms likely responsible for a considerably larger tensile failure strain and a somewhat higher tensile strength. It is seen in Fig. 2(b) that at 900 ◦C, the compressive stress–strain behavior of the uncoated fiber composite is also nearly linear to failure. However, at 1100 ◦C, the compres￾sive stress–strain curve of N610/A contains two parts. The first is a linear part which extends to a stress of approxi￾mately −157 MPa. The second part is also approximately linear with a noticeably lower slope. This stress–strain behavior indi￾cates probable composite damage at the change in slope. It is likely that as the compressive stress approaches −157 MPa shear cracking in the 90◦ fiber bundles and ply delamination take place. The damage relieves constraints acting on the 0◦ bundles allow￾ing them to deform more readily by buckling, and leading to a lower slope in the stress–strain curve. When the compressive stress reaches −240 MPa, the composite fails by buckling and shear fracture of the 0◦ bundles. At 900 and 1100 ◦C, the com￾pressive modulus and strength of N610/A considerably exceed the corresponding tensile values. It appears that the processing￾related cracks in both matrix and 0◦ fiber bundles close up during compression producing a higher modulus. It can be further con￾jectured that in tension the same processing-related cracks would start propagating at lower stress levels, leading to lower UTS. It is noteworthy that while the use of the monazite coat￾ing served to improve the high-temperature tensile strength, the compressive strength of the monazite-containing composite was considerably lower than that of the uncoated fiber CMC. The addition of the monazite coating resulted in ∼54% loss in com￾pressive strength at 900 ◦C, and in ∼60% loss in compressive strength at 1100 ◦C. It is important to note that in all monotonic tension and com￾pression tests, as well as in all other tests reported herein, the failure occurred within the gage section of the extensometer. 4.2. Creep–rupture Results of the compressive creep–rupture tests are summa￾rized in Table 4, where creep strain accumulation and rupture time are shown for each test temperature and creep stress level. Creep curves obtained at 1100 and 900 ◦C are shown in Figs. 3 and 4, respectively. Tensile creep data from prior work [53] is included in Figs. 3 and 4 for comparison. Results of the tensile creep tests of the two composites appear in Ref. [53]. Tensile creep curves obtained for the N610/M/A at 1100 ◦C (see Fig. 3(a)) exhibit primary, secondary and tertiary creep regimes. Primary creep rapidly transitions to secondary creep. For stresses ≥ 100 MPa, transition from secondary to tertiary creep occurs during the first third of the creep life. Creep strain and creep life decrease with increasing applied stress. Note that the tensile creep strains accumulated in all tests conducted at 1100 ◦C significantly exceed the failure strain obtained in the tension test. The tensile creep run-out (set to 100 h) was not achieved. Table 4 Summary of compressive creep–rupture results for the N610/M/A and the N610/A composites Specimen Temperature ( ◦C) Creep stress (MPa) Creep strain (%) Time to rupture (s) N610/monazite/alumina composite B14-6 1100 −50 −1.05 360,000a B14-5 1100 −65 −4.53 360,000a B14-4 1100 −75 −7.79 360,000a B15-2 900 −50 −0.03 180,000a N610/alumina composite B19-4 1100 −50 −1.53 360,000a B19-3 1100 −65 −2.65 360,000a B19-2 1100 −75 −6.95 360,000a B19-5 900 −50 −0.03 180,000a B19-6 900 −75 −0.01 180,000a B19-7 900 −95 −0.05 180,000a a Run-out. Compressive creep curves obtained at 1100 ◦C (see Fig. 3(b)) exhibit primary and secondary creep regimes. Unlike in the case of tension, in compression transition from primary to sec￾ondary creep occurs much later in creep life. For N610/M/A composite, primary creep transitions into secondary creep after ∼5 h at the stress of −50 MPa. At stresses ≤ −65 MPa, primary Fig. 3. Creep curves for N610/M/A and N610/A composites at 1100 ◦C: (a) tensile creep, data from Ruggles-Wrenn et al. [53] and (b) compressive creep. Time scale in (a) is selected to clearly show creep curves obtained at stress levels above 100 MPa