G Model MSA-24026: No of Pages 7 ARTICLE IN PRESS M.B. Ruggles-Wrenn et al Materials Science and Engineering A xxx(2008)xxx-Xxx -10 mm 10 mm Fig 3. Fracture surfaces of the N720 / A specimens obtained in tensile tests conducted at 1200C with the stress rate of: (a)25 MPa/s and(b)0.0025 MPa/s. ical flaws and formation of new cracks and flaws, finally causing reduction in tensile strength and an increase in failure strain. At Stress Rate =0.0025 MPa/s T=1200° fast loading rates, the time-dependent diffusion creep is minimal and has little influence on tensile behavior and tensile strength Because tensile behavior of the N720/A composite is dominated by the tensile response of the N720 fibers, the uts of the compos- g ite at 1200C is likely to be strongly influenced by the diffusion creep damage mechanism operating in N720 fibers at loading rates ≤100MPa/s Additional tests were carried out in order to determine t 12hat1200° observed at 0.0025 MPa/s is an artifact of the incomplete proces 0100hat1200° ng of the n720 fibers, which could be eliminated by additional heat treatment. Several specimens were heat-treated at 1200 Cfor 12 and 100 h and then tested at 0.0025 MPa/s. Results presented STRAIN (% in Fig. 4 demonstrate that the stress-strain response of the spe ens subjected to additional sintering was essentially the same Fig 4. Tensile stress-strain cur treatments obtained at 1200oc with the stress 0025 MPa/. The influence s of the specimens subjected to additional sintering(see Fig 5) have the same appearance as those of the specimens tested in the as-processed condition. Apparently, the N720 /A composite stresses in the 0-30 MPa range. Because in several potential appli- exhibits shrinking when exposed to low and slowly varying load at cations this Mc could be subjected to sustained loading at such 1200Cat the beginning of the 0.0025 MPa/s test. Note that all spec- fairly low stress levels, tensile creep behavior for applied stresses imens tested at 0.0025 MPa/s, regardless of prior heat treatment. in this range was investigated. Results are summarized in Table 1 produced longer damage zones (8-9 mm)than the specimens where creep strain accumulation and rupture time are shown sted at 25 MPa/s. As expected the specimens which produced each creep stress level. Creep curves are shown in Fig. 6. larger strains also exhibit longer damage zones. It is noteworthy that at applied stresses up to 26 MPa at 1200.C the N720/A composite exhibited only negative deformation. The negative creep strain accumulations ranged from-0 23% at 1 MPa 4.2. Creep-rupture at stress levels in 0-30 MPa range to-0.11% at 26 MPa. As expected, creep strain increases with applied stress Creep curves presented in Fig. 6 exhibit primary 10m Fig. 5. Fracture surfaces obtained in tensile tests conducted at 0.0025 MPa/s at 1200 C on N720 /A specimens with different prior heat treatment: (a)12 h at 1200.C and (b 0hat1200° 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 4 M.B. Ruggles-Wrenn et al. / Materials Science and Engineering A xxx (2008) xxx–xxx Fig. 3. Fracture surfaces of the N720/A specimens obtained in tensile tests conducted at 1200 ◦C with the stress rate of: (a) 25 MPa/s and (b) 0.0025 MPa/s. ical flaws and formation of new cracks and flaws, finally causing reduction in tensile strength and an increase in failure strain. At fast loading rates, the time-dependent diffusion creep is minimal and has little influence on tensile behavior and tensile strength. Because tensile behavior of the N720/A composite is dominated by the tensile response of the N720 fibers, the UTS of the compos￾ite at 1200 ◦C is likely to be strongly influenced by the diffusion creep damage mechanism operating in N720 fibers at loading rates ≤100 MPa/s. Additional tests were carried out in order to determine whether the atypical stress–strain behavior (i.e. strain rate reversal) observed at 0.0025 MPa/s is an artifact of the incomplete process￾ing of the N720 fibers, which could be eliminated by additional heat treatment. Several specimens were heat-treated at 1200 ◦C for 12 and 100 h and then tested at 0.0025 MPa/s. Results presented in Fig. 4 demonstrate that the stress–strain response of the spec￾imens subjected to additional sintering was essentially the same as that of the as-processed specimens. Furthermore, the fracture surfaces of the specimens subjected to additional sintering (see Fig. 5) have the same appearance as those of the specimens tested in the as-processed condition. Apparently, the N720/A composite exhibits shrinking when exposed to low and slowly varying load at 1200 ◦C at the beginning of the 0.0025 MPa/s test. Note that all spec￾imens tested at 0.0025 MPa/s, regardless of prior heat treatment, produced longer damage zones (∼8–9 mm) than the specimens tested at 25 MPa/s. As expected, the specimens which produced larger strains also exhibit longer damage zones. 4.2. Creep-rupture at stress levels in 0–30 MPa range The negative strains observed in the 0.0025 MPa/s tensile tests at stresses <30 MPa suggest that negative creep may occur for creep Fig. 4. Tensile stress–strain curves for N720/A specimens with different prior heat treatments obtained at 1200 ◦C with the stress rate of 0.0025 MPa/s. The influence of prior heat treatment on stress–strain behavior is negligible. stresses in the 0–30 MPa range. Because in several potential appli￾cations this CMC could be subjected to sustained loading at such fairly low stress levels, tensile creep behavior for applied stresses in this range was investigated. Results are summarized in Table 1, where creep strain accumulation and rupture time are shown for each creep stress level. Creep curves are shown in Fig. 6. It is noteworthy that at applied stresses up to 26 MPa at 1200 ◦C the N720/A composite exhibited only negative deformation. The negative creep strain accumulations ranged from −0.23% at 1 MPa to −0.11% at 26 MPa. As expected, creep strain increases with applied stress. Creep curves presented in Fig. 6 exhibit primary and secondary creep regimes. Unlike in the case of tensile creep at stresses ≥80 MPa where primary creep rapidly transitions into Fig. 5. Fracture surfaces obtained in tensile tests conducted at 0.0025 MPa/s at 1200 ◦C on N720/A specimens with different prior heat treatment: (a) 12 h at 1200 ◦C and (b) 100 h at 1200 ◦C