2272 Journal of the American Ceramic Society-Zhu et al. Vol 81. No 9 eep,n=8 1300°c,Air 如d 后 6 1300“c Standard. air Maximum stress, MP Stress, MPa Fig. 4. Minimum creep strain rate versus stress of the enhanced ig. 6. Minimum creep strain rate versus stress of the standard and SiC/SiC composite under constant load (creep) and cyclic loading enhanced SiC/SiC composites under constant load in air and in argon (fatigue)in air at 1300C atl300°C naximum strain, i.e, without considering the strain of the un- loading cycle one hand, is much improved. On the other hand, this condition We assume that creep strain rates(e)of the composites can is partly because the creep rates of Sic fibers in argon are be described by the power law igher than those in air. 27 Figure 6 also shows that the creep rates of the enhanced SiC/SiC composite in argon are higher than those of the stan- dard SiC/SiC composite, because of the low creep resistance of he enhanced SiC matrix. However, in air, the creep rates in the where A is a constant, n the stress exponent fo 2 the enhanced SiC/SiC composite are substantially lower than those activation energy for creep, R the gas constar in the standard SiC/SiC composite olute temperature, The stress exponent for cycl 10, All the stress exponents(n) for the creep of the enhanced and which is slightly higher than that for static andard SiC/SiC composites in air and argon are much higher (Fg.4). han the stress exponent(n= 1-2.5) for the creep of Nicalon Although there is an obvious difference in creep rates be- fibers.27-29 However, in a severe-matrix-cracking AL,O,/SiC a given maximum stress are almost the same(Fig. 5). If a consistent with that of the Al2 O, fiber ip of the composite is In Nicalon TM-fiber- shorter time at the maximum stress under fatigue is considered, reinforced glass-ceramics, n for creep of the composite is also a longer time to rupture under fatigue should be expected, the same as that of NicalonTM fibers. The creep rate is con- ompared to that for creep. The same time to rupture under trolled by the creep of fibers in either severe- matrix-cracking fatigue and creep means that the real behavior of fatigue is Al2O,/SiC composites or glass-ceramic-matrix SiC/calcium more complicated, which will be discussed in the last section. aluminosilicate(SiC/CAS) composites, because of their weak Figure 6 shows the creep rates of the enhanced and standard matrix. In SiC/SiC composites, the Sic SiC/SiC composites in air and argon. For the standard SiC/Sic resistance. Therefore, the creep-rate-co of the composite, the creep rates in air are much higher than those in enhanced SiC/SiC composite should be argon, because of the oxidation effects on creep. Conversely, of the fibers constrained by the matrix, an creep of free for the enhanced SiC/SiC composite, which implies that the Figure 7 shows that the time to rupture in air is longer than oxidation resistance of the enhanced SiC/SiC composite that in argon at a given stress in the enhanced SiC/SiC com- 1300°c Cree ague Strength Standard. Ai 50 10210310410 102 TIme to Rupture (s) TIme to Rupture, s Fig. 5. under constant in air at I300°C.maximum strain, i.e., without considering the strain of the un￾loading cycle. We assume that creep strain rates (e . ) of the composites can be described by the power law e˙ = Asn expS− Q RTD (2) where A is a constant, n the stress exponent for creep, Q the activation energy for creep, R the gas constant, and T the ab￾solute temperature. The stress exponent for cyclic creep is 10, which is slightly higher than that for static creep (n 4 8) (Fig. 4). Although there is an obvious difference in creep rates be￾tween creep and fatigue (Fig. 4), their time-to-rupture values at a given maximum stress are almost the same (Fig. 5). If a shorter time at the maximum stress under fatigue is considered, a longer time to rupture under fatigue should be expected, compared to that for creep. The same time to rupture under fatigue and creep means that the real behavior of fatigue is more complicated, which will be discussed in the last section. Figure 6 shows the creep rates of the enhanced and standard SiC/SiC composites in air and argon. For the standard SiC/SiC composite, the creep rates in air are much higher than those in argon, because of the oxidation effects on creep. Conversely, the creep rates in argon are evidently higher than those in air for the enhanced SiC/SiC composite, which implies that the oxidation resistance of the enhanced SiC/SiC composite, on one hand, is much improved. On the other hand, this condition is partly because the creep rates of SiC fibers in argon are higher than those in air.27 Figure 6 also shows that the creep rates of the enhanced SiC/SiC composite in argon are higher than those of the stan￾dard SiC/SiC composite, because of the low creep resistance of the enhanced SiC matrix. However, in air, the creep rates in the enhanced SiC/SiC composite are substantially lower than those in the standard SiC/SiC composite. All the stress exponents (n) for the creep of the enhanced and standard SiC/SiC composites in air and argon are much higher than the stress exponent (n 4 1–2.5) for the creep of Nicalon™ fibers.27–29 However, in a severe-matrix-cracking Al2O3/SiC composite, the stress exponent n for creep of the composite is consistent with that of the Al2O3 fiber.11 In Nicalon™-fiber￾reinforced glass-ceramics, n for creep of the composite is also the same as that of Nicalon™ fibers. The creep rate is con￾trolled by the creep of fibers in either severe-matrix-cracking Al2O3/SiC composites or glass-ceramic-matrix SiC/calcium aluminosilicate (SiC/CAS) composites, because of their weak matrix. In SiC/SiC composites, the SiC matrix has high creep resistance. Therefore, the creep-rate-controlling process of the enhanced SiC/SiC composite should be considered to be creep of the fibers constrained by the matrix, rather than creep of free fibers. Figure 7 shows that the time to rupture in air is longer than that in argon at a given stress in the enhanced SiC/SiC com￾Fig. 4. Minimum creep strain rate versus stress of the enhanced SiC/SiC composite under constant load (creep) and cyclic loading (fatigue) in air at 1300°C. Fig. 5. Maximum stress versus time to rupture of the enhanced SiC/ SiC composite under constant load (creep) and cyclic loading (fatigue) in air at 1300°C. Fig. 6. Minimum creep strain rate versus stress of the standard and enhanced SiC/SiC composites under constant load in air and in argon at 1300°C. Fig. 7. Stress versus time to rupture of the standard and enhanced SiC/SiC composites under constant load in air and in argon at 1300°C. 2272 Journal of the American Ceramic Society—Zhu et al. Vol. 81, No. 9