1720 R H. Jones, C.H. HenagerJr /Journal of the European Ceramic Sociery 25(2005)1717-1722 fiber stress rupture, 22,23. Fiber stress corrosion should be where K is the irradiation creep compliance for a given mate- accounted for by the SR mechanism since this data was ob- rial,p is the dose rate in dpa s-, and o is the applied stress tained in air. Since predicted fiber stresses in the crack wake For SiC a value of 4. 7 x 10- MPa-dpa"" is used for K. of a dynamic crack are on the order of 800 MPa or smaller, Then, in analogy with the thermal creep analysis but using a SR is difficult to achieve until temperatures above about linear stress relationship gives 1400K △ltee=Kφ△ tlf 2. 4. Irradiation induced creep crack growth(FIR for the free length of fiber and 2.4.1. Irradiation enhanced creep of sic fibers △ldb=Kφ The earliest studies of irradiation-enhanced creep of SiC were reported by Price24 who conducted bend stress relax- for the debonded length of fiber. These terms are then added ation of monolithic B-SiC strips. These strips were held at a using superposition to the thermal creep terms to give the total fixed strain and irradiated at 780, 950 and 1 130C with neu- elongation of the bridging fibers from simultaneous thermal trons in the ebr li fast reactor to a dose of 7.7 x>n/m and irradiation creep. The model can then be run to study the Price reported a substantial enhancement over thermal creep relative rates of each mechanism over this temperature range. More recent studies by Scholz et al. 5 have shown that a similar and even larger creep ef fect was found for p-SiC fibers(SCS-6)irradiated with light 3. Discussion ions. Over the same temperature range as that reported by Price24, the light ion irradiated SiC fibers exhibited a 100x's The materials included in the map(Fig 3)are 2D wo- faster creep rate. Relative to the thermal creep rate at 1000C, en SiC/SiC composites with Nicalon-CG fibers at -40% the irradiation creep rate for the neutron irradiated bulk sic by volume. Lewinsohn et al. 4 discuss the map concept, the ported by Price24 was 100x's faster while the light ion experimental data more fully. Ho owever irradiated fibers was 10.000 times faster The fr domin we have observed that temperature and oxygen concentra- tion regime in Fig. 3 results from thermal creep of the fibers. tion play the most important roles in determining the opera Clearly, the irradiation enhanced creep rate will also cause creep crack growth and must therefore be considered for nu- to include oxidation in the dynamic crack model in addition to the non-linear fiber creep. However, additional modeling remains and more experimentation is planned 2.4.2. Irradiation enhanced creep of siC/Sic composites Crack growth appears to be controlled by FR above the There is no existing irradiation creep data for SiC/Sic composite matrix-cracking threshold at elevated tempera composites as there is for fibers. Therefore, it is necessary to tures in inert environments pnnl and others have deter rely on model predictions of the effects of irradiation on creep mined composite deformation rates2,26-37 and activation crack growth. A dynamic crack growth model developed by Henager and Hoagland, 5 has been used to predict irradiation 0.003 enhanced creep effects on crack growth. We observe that ir- 373K 1373K Thermaic radiation creep of the fibers dominates the fiber deformation process for temperatures below 1273K(1000C)but ther mal creep dominates at higher test temperatures, Fig. 4. This 0.0026 at 800-900C(107.-1173K) at the end implies that we will need to further understand and model bench irradiation creep processes in SiC-based fibers since appar ently that process is important in applications that operate at 1273K or below1000°C 25. Fiber relaxation due to thermal and irradiation 0.002 0.0018051011031.1021032510331073.51074107 The time-dependent extension of a fiber bridge at elevated Time(s) mperatures still obeys the power-law creep as given by Eq (I)but now also has a portion that is proportional to the irra- Fig. 4. Irradiation creep as a function of temperature of SiC/SiC composi diation dose rate and stress Scholz showed that irradiation with Hi-Nicalon fibers using a fission damage spectrum and damage rate creep could be given as f 0.44 dpa/year. The curves shown include thermal and irradiation creep f the bridging fibers. At 1273 K the thermal crack velocity is equal to the E= Koo steady state velocities but continue to decrease with increasing time 3 irradiation-induced crack velocity. Note that the crack velocities are not1720 R.H. Jones, C.H. Henager Jr. / Journal of the European Ceramic Society 25 (2005) 1717–1722 fiber stress rupture16,22,23. Fiber stress corrosion should be accounted for by the SR mechanism since this data was ob￾tained in air. Since predicted fiber stresses in the crack wake of a dynamic crack are on the order of 800 MPa or smaller, SR is difficult to achieve until temperatures above about 1400 K. 2.4. Irradiation induced creep crack growth (FIR) 2.4.1. Irradiation enhanced creep of SiC fibers The earliest studies of irradiation-enhanced creep of SiC were reported by Price24 who conducted bend stress relax￾ation of monolithic
-SiC strips. These strips were held at a fixed strain and irradiated at 780, 950 and 1130 ◦C with neu￾trons in the EBR II fast reactor to a dose of 7.7 × 1025 n/m2. Price reported a substantial enhancement over thermal creep over this temperature range. More recent studies by Scholz et al.25 have shown that a similar and even larger creep ef￾fect was found for
-SiC fibers (SCS-6) irradiated with light ions. Over the same temperature range as that reported by Price24, the light ion irradiated SiC fibers exhibited a 100×’s faster creep rate. Relative to the thermal creep rate at 1000 ◦C, the irradiation creep rate for the neutron irradiated bulk SiC reported by Price24 was 100×’s faster while the light ion irradiated fibers was 10,000 times faster. The FR domina￾tion regime in Fig. 3 results from thermal creep of the fibers. Clearly, the irradiation enhanced creep rate will also cause creep crack growth and must therefore be considered for nu￾clear applications. 2.4.2. Irradiation enhanced creep of SiC/SiC composites There is no existing irradiation creep data for SiC/SiC composites as there is for fibers. Therefore, it is necessary to rely on model predictions of the effects of irradiation on creep crack growth. A dynamic crack growth model developed by Henager and Hoagland,15 has been used to predict irradiation enhanced creep effects on crack growth. We observe that ir￾radiation creep of the fibers dominates the fiber deformation process for temperatures below 1273 K (1000 ◦C) but ther￾mal creep dominates at higher test temperatures, Fig. 4. This implies that we will need to further understand and model irradiation creep processes in SiC-based fibers since appar￾ently that process is important in applications that operate at or below 1000 ◦C. 2.5. Fiber relaxation due to thermal and irradiation creep The time-dependent extension of a fiber bridge at elevated temperatures still obeys the power-law creep as given by Eq. (1) but now also has a portion that is proportional to the irra￾diation dose rate and stress. Scholz25 showed that irradiation creep could be given as: ε˙ = Kφσ˙ (8) where K is the irradiation creep compliance for a given mate￾rial, φ˙ is the dose rate in dpa s−1, and σ is the applied stress. For SiC a value of 4.7 × 10−6 MPa−1 dpa−1 is used for K. Then, in analogy with the thermal creep analysis but using a linear stress relationship gives: lfree = Kφ˙ Pb 2rf tlfree (9) for the free length of fiber and ∆ldeb = Kφ˙ Pb 4rf ∆tldeb (10) for the debonded length of fiber. These terms are then added using superposition to the thermal creep terms to give the total elongation of the bridging fibers from simultaneous thermal and irradiation creep. The model can then be run to study the relative rates of each mechanism. 3. Discussion The materials included in the map (Fig. 3) are 2D wo￾ven SiC/SiC composites with Nicalon-CG fibers at ∼40% by volume. Lewinsohn et al.4 discuss the map concept, the materials, and the experimental data more fully. However, we have observed that temperature and oxygen concentra￾tion play the most important roles in determining the opera￾ble crack growth mechanisms. Therefore, it was imperative to include oxidation in the dynamic crack model in addition to the non-linear fiber creep. However, additional modeling remains and more experimentation is planned. Crack growth appears to be controlled by FR above the composite matrix-cracking threshold at elevated tempera￾tures in inert environments. PNNL and others have deter￾mined composite deformation rates1,2,26–37 and activation Fig. 4. Irradiation creep as a function of temperature of SiC/SiC composite with Hi-Nicalon fibers using a fission damage spectrum and damage rate of 0.44 dpa/year. The curves shown include thermal and irradiation creep of the bridging fibers. At 1273 K the thermal crack velocity is equal to the irradiation-induced crack velocity. Note that the crack velocities are not steady state velocities but continue to decrease with increasing time