4606 B. wilshire, M.R. Bache /Journal of the European Ceramic Society 27(2007)4603-4611 o与gL9g日 HNSiCf-Al2O3 ccc vHNSICr-SiBC NSIC:-Az0 5075100 104 Stress(MPa) Minimum Creep Rate x Time to Fracture Fig. 4. Comparisons of the stress/creep life relationships for SiCr-Al2O,, and Fig. 6. The relationship between the product, Emf, and the creep ductility (eD HNSiCf-Al2O3, as well as for SiCr-SiC,6.9 SiCr-SiBC, 10 HNSiCe-Sicll and for SiCr-Al2O3'and HNSiCr-Al2O3, as well as for SiCr-SiC, 6. 9 SiCr-SiBC,10 HNSiCr-SiBC samples tested in air at 1300oC. HNSiCr-Sicll and HNSiCe-SiBC samples tested in air at 1300oC. As shown in Fig. 2, with the AlzO3-matrix composites, the As the applied stress is decreased over the ranges cov creep rates at a given stress are reduced by a factor of about 5 ered at 1300C, Emff increases from 0.0002 to 0.002 by replacing NicalonM NLM202 with stronger Hi-Nicalon TM with the SiCr SiC samples, 6.9 from -0.002 to -0.04 with fibres. Similarly, this change in fibre type results in an equiv- the SiCr-Al2O3, SiCr-SiBCiO and HNSiCr-Sicll and from alent enhancement in creep resistance with composites having 0.008 to -0.04 with the present HNSiCr-AlO3 and either sic or sibc matrices. as evident from Fig. 3. Moreover. HNSiCr-SiBC products(Fig. 5). Moreover, these increases in comparisons of the data sets in Figs. 3 and 4 demonstrate that Emf are matched by increases in creep ductility(ef)as the stress fibre-matrix combinations which improve creep resistance also is reduced(Fig. 6), indicating that lead to substantially longer creep lives. This result would be expected because the creep life is often inversely proportional Emlf=XEf to the minimum creep rate(Fig. 5), such that with x increasing from0. 4 to 0. 7 with increasing test duration Emtf and Ef, as well as the resu em签 constant ( presented in Figs. 2-6, can then be explained& by reference to showing that the rates at which creep damage develops to cause the e/t trajectories included in Fig. 7 fracture are determined by the rates of creep strain accumula tion, i.e. creep failure is strain controlled. Yet, with the present 3.3. Variations in creep curve shape CFCMC'S, the magnitude of Emtr is both material and test con- dition sensitive Under uniaxial tension, the creep rupture life(tr) can be defined conveniently as the time taken for the accumulated creep strain(E)to become equal to the limiting creep ductility, speci 0.012 t=843 ks 0013 SiCr-Al2O3 0.004 10-7 HNSiCp-SiBc oocoooo SiCcf-SiC 0.002 HNSICE. SBC 10110210310410510610 0.000 100 200 Time to Fracture(s) Time (ks) SiCr-Al2O3 and HNSiCr-Al2O3, as well as for SiC-Sic.9 and SiCp-SiBC, o Fig. 7. Creep strain-time curves recorded for SiCp-SiC, SiCr-SiBCIOat HNSiCr-Sic and HNSiCt-siBC samples tested in air at 1300C. 90 MPa and a HNSiCr-SiBC specimen at 100 MPa for tests in air at 1300C.4606 B. Wilshire, M.R. Bache / Journal of the European Ceramic Society 27 (2007) 4603–4611 Fig. 4. Comparisons of the stress/creep life relationships for SiCf–Al2O3 7 and HNSiCf–Al2O3, as well as for SiCf–SiC,6,9 SiCf–SiBC,10 HNSiCf–SiC11 and HNSiCf–SiBC samples tested in air at 1300 ◦C. As shown in Fig. 2, with the Al2O3–matrix composites, the creep rates at a given stress are reduced by a factor of about 5 by replacing NicalonTM NLM202 with stronger Hi-NicalonTM fibres. Similarly, this change in fibre type results in an equiv￾alent enhancement in creep resistance with composites having either SiC or SiBC matrices, as evident from Fig. 3. Moreover, comparisons of the data sets in Figs. 3 and 4 demonstrate that fibre–matrix combinations which improve creep resistance also lead to substantially longer creep lives. This result would be expected because the creep life is often inversely proportional to the minimum creep rate (Fig. 5), such that ˙mtf ∼= constant (1) showing that the rates at which creep damage develops to cause fracture are determined by the rates of creep strain accumula￾tion, i.e. creep failure is strain controlled. Yet, with the present CFCMC’s, the magnitude of ε˙mtf is both material and test con￾dition sensitive. Fig. 5. The dependences of the rupture life on the minimum creep rate for SiCf–Al2O3 7 and HNSiCf–Al2O3, as well as for SiCf–SiC6,9 and SiCf–SiBC,10 HNSiCf–SiC11 and HNSiCf–SiBC samples tested in air at 1300 ◦C. Fig. 6. The relationship between the product, ε˙mtf, and the creep ductility (εf) for SiCf–Al2O3 7 and HNSiCf–Al2O3, as well as for SiCf–SiC,6,9 SiCf–SiBC,10 HNSiCf–SiC11 and HNSiCf–SiBC samples tested in air at 1300 ◦C. As the applied stress is decreased over the ranges cov￾ered at 1300 ◦C, ε˙mtf increases from ∼0.0002 to 0.002 with the SiCf–SiC samples,6,9 from ∼0.002 to ∼0.04 with the SiCf–Al2O3, 7 SiCf–SiBC10 and HNSiCf–SiC11 and from ∼0.008 to ∼0.04 with the present HNSiCf–Al2O3 and HNSiCf–SiBC products (Fig. 5). Moreover, these increases in ε˙mtf are matched by increases in creep ductility (εf) as the stress is reduced (Fig. 6), indicating that ε˙mtf = χεf (2) with χ increasing from ∼0.4 to 0.7 with increasing test duration. This relationship between ε˙mtf and εf, as well as the results presented in Figs. 2–6, can then be explained8 by reference to the ε/t trajectories included in Fig. 7. 3.3. Variations in creep curve shape Under uniaxial tension, the creep rupture life (tf) can be defined conveniently as the time taken for the accumulated creep strain (ε) to become equal to the limiting creep ductility, speci￾Fig. 7. Creep strain–time curves recorded for SiCf–SiC,6 SiCf–SiBC10 at 90 MPa and a HNSiCf–SiBC specimen at 100 MPa for tests in air at 1300 ◦C.