January 1999 Creep and Fatigue Behavior in Hi-Nicalon/SiC Composites at High Temperatures 300 250 200 9150 1300ˇc 100 Standard SiC/SiC, Ar Enhanced SiC/SiC, Air a Hi-Nicalon SiC, Air 0002 0.004 0006 0.008 Tensile strai Fig. 2. Monotonic tensile stress versus strain in the Hi-Nicalon TM/SiC, enhanced SiC/SiC, and standard SiC/SiC composites at 1300C under a constant stress rate of 50 MPa/s composite indicates linear elastic behavior up to the propor- is lower than that in air (n =9.4). The effect of the environ- tional limit of 70 MPa; this stress is -30% of the ultimate ment on the creep resistance in the Hi-Nicalon TM/SiC compos- tensile strength(UTS). The UTS of the Hi-NicalonTM/SiC com- ite is the same as that in the enhanced SiC/SiC composite. 1 posite is similar to that of the standard SiC/SiC and enhanced Nicalon M/SiC and enhanced SiC/SiC composites are much Enhanced SiC/SiC Compositer nd SiC/SiC composites; however, the strains at UTS of the Hi- (3) Comparison with Standard In argon, the creep rate of the Hi-Nicalon TM/SiC composite <i The modulus calculated from the linear portion of the curve at 1300C(Fig 8)is lower than that of the enhanced SiC/SiC 140 GPa. This value is higher than that for the enhanced composite, and the time to rupture of the Hi-Nicalon TM/SiC SiC/SiC composite(90 GPa) and lower than that for the stan- composite is longer than that of the enhanced SiC/Sic com- dard SiC/SiC composite(200 GPa)at 1300oC osite(Fig. 9). However, although the creep rate of both the (2) Creep and Fatigue e enhance Creep strains versus time at different maximum stresses in 1300oC is higher than that of the standard SiC/SiC composite air at 1300oC are shown in Fig 3. Only transient creep strai Fig 8), the time to rupture for both the Hi-Nicalon TM/SiC and exists at stresses >105 MPa, and the tertiary creep strains ap. the enhanced SiC/SiC composites is longer than that of the pear at 90 MPa for static loads. However, the curves under standard SiC/SiC composite cyclic loads show primary, secondary, and tertiary stages at In air, the creep rate of the hi-Nicalon M/SiC composite at stresses of 120 and 90 MPa 1300%C is much lower than that of the standard SiC/SiC com- (Fig. 3), the steady-state or minimum strain rates of cyclic creep(fatigue)are always slightly lower than those of stati composite but is also similar to that of the enhanced SiC/SiC composite(Fig. I1) In air, the cyclic-fatigue life versus the maximum stress of The creep strain rate(e)can be described by the power law the Hi-Nicalon TM /SiC composite is almost the same as that the enhanced SiC/SiC composite; however, this lifetime is ∈=Ao"e much longer than that of the standard sic/Sic 1300°C(Fg.12) where A is a constant, o the stress, n the stress exponent for In summary, the creep and fatigue resistance of the Hi- creep, Q the activation energy for creep, R the gas constant, and NicalonTMSiC composite is similar to that of the enhanced T the absolute temperature. The stress exponent for cyclic SiC/SiC composite but is much better than that of the standard creep(n =9.8)is similar to that for static creep(n = 9.4) SiC/SiC composite in However, in argon, the The time to rupture under creep loads is slightly shorter than deformation resistance is not consistent with the creep rupture that under fatigue loads at a given maximum stress other than resistance. The creep rates, from lowest to highest, are as fol- results of the minimum creep rates(Fig. 4) ite, enhanced SiC/SiC composite. However, the time to rupture The creep strain rates of the Hi-Nicalon TM/SiC composite in of the standard SiC/SiC composite is the shortest, and the Hi- ir compared with those in argon in Fig. 6. The creep rates in Nicalon TM/SiC composite has the longest life al argon are evidently higher than those in air. Consequently, the For the standard SiC/SiC composite, the creep rates at a time to rupture at a given stress in air is longer than that in given stress in argon are lower than those in air(Fig. 13) argon(Fig. 7). The stress exponent for creep in argon(n=5 ly, the time to rupture at a given stress in argon iscomposite indicates linear elastic behavior up to the propor￾tional limit of 70 MPa; this stress is ∼30% of the ultimate tensile strength (UTS). The UTS of the Hi-Nicalon™/SiC com￾posite is similar to that of the standard SiC/SiC and enhanced SiC/SiC composites; however, the strains at UTS of the Hi￾Nicalon™/SiC and enhanced SiC/SiC composites are much higher than that of the standard SiC/SiC composite. The modulus calculated from the linear portion of the curve is ∼140 GPa. This value is higher than that for the enhanced SiC/SiC composite (90 GPa) and lower than that for the stan￾dard SiC/SiC composite (200 GPa) at 1300°C. (2) Creep and Fatigue Creep strains versus time at different maximum stresses in air at 1300°C are shown in Fig. 3. Only transient creep strain exists at stresses >105 MPa, and the tertiary creep strains ap￾pear at 90 MPa for static loads. However, the curves under cyclic loads show primary, secondary, and tertiary stages at stresses of 120 and 90 MPa. Regardless of whether cyclic creep strain is larger (at high stress) or smaller (at low stresses) than the static creep strain (Fig. 3), the steady-state or minimum strain rates of cyclic creep (fatigue) are always slightly lower than those of static creep (Fig. 4). The creep strain rate (e . ) can be described by the power law e . = Asn expS− Q RTD (1) where A is a constant, s the stress, n the stress exponent for creep, Q the activation energy for creep, R the gas constant, and T the absolute temperature. The stress exponent for cyclic creep (n 4 9.8) is similar to that for static creep (n 4 9.4). The time to rupture under creep loads is slightly shorter than that under fatigue loads at a given maximum stress other than 105 MPa (Fig. 5). This result is qualitatively consistent with the results of the minimum creep rates (Fig. 4). The creep strain rates of the Hi-Nicalon™/SiC composite in air compared with those in argon in Fig. 6. The creep rates in argon are evidently higher than those in air. Consequently, the time to rupture at a given stress in air is longer than that in argon (Fig. 7). The stress exponent for creep in argon (n 4 5) is lower than that in air (n 4 9.4). The effect of the environ￾ment on the creep resistance in the Hi-Nicalon™/SiC compos￾ite is the same as that in the enhanced SiC/SiC composite.11 (3) Comparison with Standard and Enhanced SiC/SiC Composites In argon, the creep rate of the Hi-Nicalon™/SiC composite at 1300°C (Fig. 8) is lower than that of the enhanced SiC/SiC composite,11 and the time to rupture of the Hi-Nicalon™/SiC composite is longer than that of the enhanced SiC/SiC com￾posite (Fig. 9). However, although the creep rate of both the Hi-Nicalon™/SiC and the enhanced SiC/SiC composites at 1300°C is higher than that of the standard SiC/SiC composite (Fig. 8), the time to rupture for both the Hi-Nicalon™/SiC and the enhanced SiC/SiC composites is longer than that of the standard SiC/SiC composite (Fig. 9). In air, the creep rate of the Hi-Nicalon™/SiC composite at 1300°C is much lower than that of the standard SiC/SiC com￾posite but is similar to that of the enhanced SiC/SiC compos￾ite11 (Fig. 10). The time to rupture of the Hi-Nicalon™/SiC composite is much longer than that of the standard SiC/SiC composite but is also similar to that of the enhanced SiC/SiC composite11 (Fig. 11). In air, the cyclic-fatigue life versus the maximum stress of the Hi-Nicalon™/SiC composite is almost the same as that of the enhanced SiC/SiC composite; however, this lifetime is much longer than that of the standard SiC/SiC composite at 1300°C (Fig. 12). In summary, the creep and fatigue resistance of the Hi￾Nicalon™/SiC composite is similar to that of the enhanced SiC/SiC composite but is much better than that of the standard SiC/SiC composite in air. However, in argon, the creep￾deformation resistance is not consistent with the creep rupture resistance. The creep rates, from lowest to highest, are as fol￾lows: standard SiC/SiC composite, Hi-Nicalon™/SiC compos￾ite, enhanced SiC/SiC composite. However, the time to rupture of the standard SiC/SiC composite is the shortest, and the Hi￾Nicalon™/SiC composite has the longest life. For the standard SiC/SiC composite, the creep rates at a given stress in argon are lower than those in air (Fig. 13). Consequently, the time to rupture at a given stress in argon is Fig. 2. Monotonic tensile stress versus strain in the Hi-Nicalon™/SiC, enhanced SiC/SiC, and standard SiC/SiC composites at 1300°C under a constant stress rate of 50 MPa/s. January 1999 Creep and Fatigue Behavior in Hi-Nicalon™/SiC Composites at High Temperatures 119