G Model ARTICLE IN PRESS M.B. Ruggles-Wrenn er al/ Materials Science and Engineering A xxx(2008)xxx-XXx Summary of creep-rupture results for the N720 A ceramic composite at 1200.C in T=1200° Creep stress(MPa) Creep strain (%) Time to rupture(s) 100 h at 1 MPa a Run-out Ennu2 As-Processed T=1200° 0.1 STRAIN (% e0.1 Fig. 7. Effect of prior creep at stress levels in the Ooc ge on tensile (shrinkage )for the Nextel M720 fibers subjected to creep at 1200oC at applied stresses up to 80 MPa(equivalent to the stress of 18 MPa or a crossply composite with the fiber volume of 45%). In addition, recent studies 54-56]of the effects of thermal aging Time(h) on the physical and mechanical properties of a composite consist- ing of NextelM720 fibers and a porous alumina matrix, reported Fig.6. Creep strain vs time curves for N720/A ceramic composite at 1200.C Creep a porosity reduction of -6% after a 10-min exposure at 1200C. stress=1.6. 20 and 26 MPa. The loss of porosity was attributed to additional sintering of the Al2O3 matrix during the aging treatments. It is likely that additional secondary creep(41). for creep stresses in the MPa range tran- sintering of the alumina matrix occurred during the 100-h creep sition from primary to secondary creep occurs much later in creep tests performed in this effort. Shrinkage of the n720 fibers occur- life. Primary creep persists during the first 30-50 h of the creep test. ring simultaneously with the loss of porosity in the alumina matrix The creep curves in Fig. b indicate that secondary creep is likely to tion exhibited by the n720/ A composite at applied stresses up to reached in all tests. All steady-state creep rate magnitudes were below 10-95-1. Creep run-out of 100 h was reached in all tests Retained tensile strength and modulus of the run-out specimens 43. Composite porosity and density were measured in tensile tests conducted at 25 MPa s at 1200C. The retained properties are summarized in Table 2. Representative The density as well as the porosity and average pore size of tensile stress-strain curves obtained for the specimens subjected t the specimens subjected to prior creep at 1, 20 and 26 MPa was rior creep at 1200 C are presented in Fig. 7 together with the measured. The results are summarized in table 3 where the mea- sile stress-strain curves for the as-processed material. The run-out surements obtained for the as-processed material are included for pecimens retained 100% of their tensile strength. A slight increase comparison. Differential plots of pore size distribution for the as- in tensile modulus was also observed. Tensile stress-strain behav- processed material and specimens subjected to prior creep at 1, 20, lor of the specimens subjected to prior creep remained qualitatively and 25 MPa are shown in Fig 8. An increase in bulk density and a milar to that of the as-processed material. decrease in porosity observed for the pre-crept specimens are con- The results of this effort are consistent with those found for the sistent with the negative creep deformation(shrinkage)produced Nextel M720 fibers alone [52, 53]. Wilson et al. [52]reported that at 1093Cthe Nextel720 fibers exhibited negative creep at applied Table 3 stresses below 138 MPa(equivalent to the stress of 31 MPa for a Density and porosity of the N720/A specimens subjected to prior thermo- crossply composite with the fiber volume of 45%). Fiber shrink- mechanical loading age continued at a decreasing rate for the entire duration of these Bulk density (g/mL Porosity (% tests (257 h at 69 MPa and 130 h at 138 MPa). At 69 MPa(equivalent to the stress of 15. 5 MPa for a crossply composite with the fiber As-proces volume of 45%), the total shrinkage was 0. 12%. The shrinkage of As-processee 22.0375 the Nextel M720 fibers during creep tests was attributed to crys- 100h at 1 MPa at tallization of a-Al2O3 from mullite supersaturated with alumina. 00h at 20 MPa at Likewise, Deleglise et al. [53] reported only negative deformation 100h at 26 MPa at 1200C 20259 Table 2 Retained properties of the n720/A specimens subjected to prior creep at 1200C Creep stress(MPa) Retained strength(MPa) Strain at failure (% 198 0.36 0.34 Please cite this article in press as: M.B. Ruggles-Wrenn, et al., Mater. Sci. Eng. A(2008). doi: 10. 1016/j. msea. 2008.03.006Please cite this article in press as: M.B. Ruggles-Wrenn, et al., Mater. Sci. Eng. A (2008), doi:10.1016/j.msea.2008.03.006 ARTICLE IN PRESS G Model MSA-24026; No. of Pages 7 M.B. Ruggles-Wrenn et al. / Materials Science and Engineering A xxx (2008) xxx–xxx 5 Table 1 Summary of creep-rupture results for the N720/A ceramic composite at 1200 ◦C in laboratory air Creep stress (MPa) Creep strain (%) Time to rupture (s) 1 −0.23 360,000a 6 −0.20 360,000a 20 −0.17 360,000a 26 −0.11 360,000a a Run-out. Fig. 6. Creep strain vs. time curves for N720/A ceramic composite at 1200 ◦C. Creep stress = 1, 6, 20 and 26 MPa. secondary creep [41], for creep stresses in the 0–30 MPa range tran￾sition from primary to secondary creep occurs much later in creep life. Primary creep persists during the first 30–50 h of the creep test. The creep curves in Fig. 6 indicate that secondary creep is likely to continue for the duration of the creep life. Minimum creep rate was reached in all tests. All steady-state creep rate magnitudes were below 10−9 s−1. Creep run-out of 100 h was reached in all tests. Retained tensile strength and modulus of the run-out specimens were measured in tensile tests conducted at 25 MPa/s at 1200 ◦C. The retained properties are summarized in Table 2. Representative tensile stress–strain curves obtained for the specimens subjected to prior creep at 1200 ◦C are presented in Fig. 7 together with the ten￾sile stress–strain curves for the as-processed material. The run-out specimens retained 100% of their tensile strength. A slight increase in tensile modulus was also observed. Tensile stress–strain behav￾ior of the specimens subjected to prior creep remained qualitatively similar to that of the as-processed material. The results of this effort are consistent with those found for the NextelTM720 fibers alone [52,53]. Wilson et al. [52] reported that at 1093 ◦C the NextelTM720 fibers exhibited negative creep at applied stresses below 138 MPa (equivalent to the stress of 31 MPa for a crossply composite with the fiber volume of 45%). Fiber shrink￾age continued at a decreasing rate for the entire duration of these tests (257 h at 69 MPa and 130 h at 138 MPa). At 69 MPa (equivalent to the stress of 15.5 MPa for a crossply composite with the fiber volume of 45%), the total shrinkage was 0.12%. The shrinkage of the NextelTM720 fibers during creep tests was attributed to crys￾tallization of -Al2O3 from mullite supersaturated with alumina. Likewise, Deleglise et al. [53] reported only negative deformation Fig. 7. Effect of prior creep at stress levels in the 0–30 MPa range on tensile stress–strain behavior of N720/A ceramic composite at 1200 ◦C. (shrinkage) for the NextelTM720 fibers subjected to creep at 1200 ◦C at applied stresses up to 80 MPa (equivalent to the stress of 18 MPa for a crossply composite with the fiber volume of 45%). In addition, recent studies [54–56] of the effects of thermal aging on the physical and mechanical properties of a composite consist￾ing of NextelTM720 fibers and a porous alumina matrix, reported a porosity reduction of ∼6% after a 10-min exposure at 1200 ◦C. The loss of porosity was attributed to additional sintering of the Al2O3 matrix during the aging treatments. It is likely that additional sintering of the alumina matrix occurred during the 100-h creep tests performed in this effort. Shrinkage of the N720 fibers occur￾ring simultaneously with the loss of porosity in the alumina matrix are the probable mechanisms behind the negative creep deforma￾tion exhibited by the N720/A composite at applied stresses up to 26 MPa. 4.3. Composite porosity and density The density as well as the porosity and average pore size of the specimens subjected to prior creep at 1, 20 and 26 MPa was measured. The results are summarized in Table 3 where the mea￾surements obtained for the as-processed material are included for comparison. Differential plots of pore size distribution for the as￾processed material and specimens subjected to prior creep at 1, 20, and 25 MPa are shown in Fig. 8. An increase in bulk density and a decrease in porosity observed for the pre-crept specimens are con￾sistent with the negative creep deformation (shrinkage) produced Table 3 Density and porosity of the N720/A specimens subjected to prior thermo￾mechanical loading Specimen Bulk density (g/mL) Porosity (%) As-processed 2.8544 21.8081 As-processed 2.8432 21.8431 As-processed 2.8544 22.0375 100 h at 1 MPa at 1200 ◦C 2.8952 20.0421 100 h at 20 MPa at 1200 ◦C 2.9237 20.0412 100 h at 26 MPa at 1200 ◦C 2.8843 20.2592 Table 2 Retained properties of the N720/A specimens subjected to prior creep at 1200 ◦C Creep stress (MPa) Retained strength (MPa) Retained modulus (GPa) Strain at failure (%) 1 198 76 0.36 6 196 72 0.34 20 201 74 0.35 26 202 72 0.36