April 2007 Sic-Based Fibers and Low Oxygen Conten 1149 (b) Hi-Nicalon S Hi-Nicalon S at 1350C. The creep results reported by most au- thors were obtained during much shorter tests(<48 h). Thus, it nay be anticipated that their tests were not sufficiently long, so that true secondary creep stage was probably not reached Figure 7 shows a typical creep curve obtained at incremental temperature steps. It can be noted that creep accelerated at tem- peratures>1600.C(tertiary creep). Creep curves were fitted by the following well-accepted equations of deformations in the primary and in the secondary stages. Tertiary creep is examined in a subsequent section (1) Ep=oA[1-exp(pr) Es= Bot where subscripts e, p, and s refer, respectively, to elastic regime primary, and secondary creep. o is the stress on the fiber, Ec the initial fiber Youngs modulus, I is time, and A, B, n, and p re constants Figure 6 shows that an excellent agreement was obtained for all the fibers. Note that Hi-Nicalon s and sa3 fibers are less 500nm 50nm Based on microstructure analysis, the fibers can be considered SC(251A) to be a mixture of wrinkled carbon-layer packets and Sic grains a possible controlling creep mechanism may involve grain- boundary sliding, carbon diffusion, dewrinkling, deform and sliding of carbon crystallites.6 (3) Creep Mechanisms-Primary Creep Primary creep can be attributed to viscoelastic deformation of carbon at grain boundaries. The viscoelasticity of carbon has SiCu(131A) been discussed by Kelly20 and it has been observed by Sauder et al. on various carbon fibers at high temperatures. Because Fig 3. Microstructure and electron diffraction pattern of (a) Tyranno of the very weak interaction between layer planes, each basal SA3 (2)and(b) Hi-Nicalon S fiber(effective beam size =2. 15 um). plane can deform as a separate unit in two dimensions, which Hi-Nicalon S sA3(2) ig. 4. Lattice fringe images showing the presence of turbostratic carbon at the Sic grain boundary for(a) Hi-Nicalon S and(b) Tyranno SA3(2)fiberHi-Nicalon S at 13501C. The creep results reported by most au￾thors were obtained during much shorter tests (o48 h). Thus, it may be anticipated that their tests were not sufficiently long, so that true secondary creep stage was probably not reached. Figure 7 shows a typical creep curve obtained at incremental temperature steps. It can be noted that creep accelerated at tem￾peratures 416001C (tertiary creep). Creep curves were fitted by the following well-accepted equations of deformations in the primary and in the secondary stages. Tertiary creep is examined in a subsequent section: ee ¼ s Eo (1) ep ¼ sA½1
exp ð
ptÞ (2) es ¼ Bsn t (3) e ¼ ee þ ep þ es (4) where subscripts e, p, and s refer, respectively, to elastic regime, primary, and secondary creep. s is the stress on the fiber, Eo is the initial fiber Young’s modulus, t is time, and A, B, n, and p are constants. Figure 6 shows that an excellent agreement was obtained for all the fibers. Note that Hi-Nicalon S and SA3 fibers are less sensitive to creep than Hi-Nicalon. Based on microstructure analysis, the fibers can be considered to be a mixture of wrinkled carbon-layer packets and SiC grains. A possible controlling creep mechanism may involve grain￾boundary sliding, carbon diffusion, dewrinkling, deformation, and sliding of carbon crystallites.6 (3) Creep Mechanisms—Primary Creep Primary creep can be attributed to viscoelastic deformation of carbon at grain boundaries. The viscoelasticity of carbon has been discussed by Kelly20 and it has been observed by Sauder et al. 21 on various carbon fibers at high temperatures. Because of the very weak interaction between layer planes, each basal plane can deform as a separate unit in two dimensions, which carbon carbon SiC 10 nm 7 nm SiC SiC SiC SiC SA3(2) (a) (b) Hi-Nicalon S Fig. 4. Lattice fringe images showing the presence of turbostratic carbon at the SiC grain boundary for (a) Hi-Nicalon S and (b) Tyranno SA3 (2) fiber. Fig. 3. Microstructure and electron diffraction pattern of (a) Tyranno SA3 (2) and (b) Hi-Nicalon S fiber (effective beam size 5 2.15 mm). April 2007 SiC-Based Fibers and Low Oxygen Content 1149