April 2007 SiC-Based Fibers and Low Oxygen Conten 1151 Hi-Nicalot sA3(1) wii E=9.7×10-9s-1 80.5+- E=3.8×10s 15 4 T=1200°C T=1200°c g=850 MPa d= 500 MPa 04896144192240288336 04896144192240288336 Time(h) sA3(2) Hi-Nicalon S experiment 0.6 0.5 0.8 0.4 E=13×10-8s-1 0.6 --=325×10-8s-1 T=1350°C d=850 MP 0.1 d=850 MPa 0.0 Time(h) Time(h) Fig. 6. Creep behavior of (a)Hi-Nicalon. (b) Tyranno SA3(1),(c)SA3(2), and(d) Hi-Nicalon S fibers The Hi-Nicalon fiber contains a significant amount o (4) Secondary Creep: Mechanism nsity SiC. This SiC not stable. as a result of the duration of pyrolysi during fiber pi rowth thus starts at I in the Hi-Nicalon fiber. It causes primary creep stage. Extensive investigation of Hi-Nicalon a decrease in creep rate. This phenomenon may explain the im- would have required long experiments, as indicated above perfect fit of the experimental creep curve by Eqs.(1H4) Creep rate in the steady state is expressed by the following (Fig. 6(a)) general relationship: 8= Dde o"with D=D。exp (是) where is a material constant, D is the diffusivity, Q is the ap- parent activation energy, R is the universal gas constant, Tis temperature in Kelvin, de is the grain size o is the applied stress, and m and n are exponents. Different values of m and n corres 1650°c pond to different mechanisms. Values of n and o may be ob- ;1550°c;16 tained experimentally and used to infer a rate-controlling 145c1500c: The creep rate depends on temperature through an Arrheniu xponential term(Eq. (5). Figure 8 shows typical Arrhenius creep rate plots obtained for the sA3(2)fiber Activation energy 0 12 24 36 48 60 72 84 96 was determined by fitting Eq(5)to creep rates determined at vanous temperatures 7. Creep of SA3 (2) fiber under a stress of 150 MPa and in the 0 -1700C temperature range 6. 3 The largest apparent activation energy was determined for Nicalon S fibers (Table ID). Lower similar values were ob- ned for both SA3(1)and SA3(2)fibers. Knowing that theThe Hi-Nicalon fiber contains a significant amount of low￾density SiC. This SiC phase is not stable, as a result of the short duration of pyrolysis step during fiber processing.5 Grain growth thus starts at 12001C in the Hi-Nicalon fiber. It causes a decrease in creep rate. This phenomenon may explain the im￾perfect fit of the experimental creep curve by Eqs. (1)–(4) (Fig. 6(a)). (4) Secondary Creep: Mechanism Stationary creep was investigated essentially on Hi-Nicalon S and Tyranno SA3 fibers, which displayed a reasonably short primary creep stage. Extensive investigation of Hi-Nicalon would have required long experiments, as indicated above. Creep rate in the steady state is expressed by the following general relationship: e : ¼ fDdm g snwith D ¼ Do exp
Q RT
(5) where f is a material constant, D is the diffusivity, Q is the ap￾parent activation energy, R is the universal gas constant, T is temperature in Kelvin, dg is the grain size, s is the applied stress, and m and n are exponents. Different values of m and n corres￾pond to different mechanisms. Values of n and Q may be ob￾tained experimentally and used to infer a rate-controlling mechanism for creep. The creep rate depends on temperature through an Arrhenius exponential term (Eq. (5)). Figure 8 shows typical Arrhenius creep rate plots obtained for the SA3 (2) fiber. Activation energy was determined by fitting Eq. (5) to creep rates determined at various temperatures. The largest apparent activation energy was determined for Hi-Nicalon S fibers (Table II). Lower similar values were ob￾tained for both SA3 (1) and SA3 (2) fibers. Knowing that the Hi-Nicalon (a) (b) (c) (d) SA3(1) SA3 (2) Hi-Nicalon S 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0 48 96 144 192 240 288 336 Time (h) Strain (%) experiment prediction · 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0 12 24 36 48 60 Time (h) Strain (%) experiment prediction · 0 0.5 1 1.5 2 2.5 3 3.5 0 48 96 144 192 240 288 336 Time (h) Strain (%) experiment prediction · T = 1200°C 0.0 0.2 0.4 0.6 0.8 1.0 1.2 0 12 24 36 48 60 Time (h) Strain (%) experiment prediction ·
= 3.25 × 10–8 s–1
= 1.3 × 10–8 s–1
= 9.7 × 10–9 s–1
= 3.8 × 10–9 s–1 T = 1250°C σ = 850 MPa T = 1350°C σ = 850 MPa σ = 850 MPa T = 1200°C σ = 500 MPa Fig. 6. Creep behavior of (a) Hi-Nicalon, (b) Tyranno SA3 (1), (c) SA3 (2), and (d) Hi-Nicalon S fibers. 0 1 2 3 4 5 6 7 8 0 12 24 36 48 60 72 84 96 Time (h) Strain (%) 1350°C 1400°C 1450°C 1500°C 1550°C 1600°C 1650°C 1700°C Fig. 7. Creep of SA3 (2) fiber under a stress of 150 MPa and in the 13501–17001C temperature range. April 2007 SiC-Based Fibers and Low Oxygen Content 1151