Ambient and elevated temperature fracture and cyclic-fatigue properties in a series of Al-containing silicon carbides Page: 7 of 86
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LIST OF FIGURES
Figure 1.1: Figure 1-1: (a) Crystal structures and (b)stacking sequence for various SiC
polytypes [16].
Fig. 2.1. SEM micrographs of etched surfaces of ABC-silicon carbides, showing the
characteristic grain structures: (a) uniformly elongated grains in 3ABC, (b) bimodal
structure of elongated (a-SiC) and equiaxed (13-SiC) in 5ABC, and (c) equiaxed grains
with sparsely distributed needle-like grains in 7ABC. (Courtesy of Qing Yang)
Fig. 2.2. High-resolution TEM micrographs showing the nature of the grain-boundary
films in hot-pressed SiC with different Al content, showing (a) amorphous films in 3ABC,
(b) fully crystallized films in 7ABC, and (c) amorphous, (d) partially crystallized and (e)
fully crystallized films in 5ABC. (Courtesy of Dr. Xiao-Feng Zhang)
Fig. 2.3. Crack growth resistance, KR, plotted as a function of crack extension, ?a, for
as-hot-pressed and annealed xABC-SiC at room temperature.
Fig. 2.4. Cyclic fatigue-crack growth rates, da/dN, at R = 0.1 in ABC-SiC, as a function
of the applied stress-intensity range, ?K, at temperatures of 250 and 1300 C. Note that
7ABC is sufficiently brittle that it exhibits no subcritical crack growth, i.e., it fails
catastrophically once Kmax > K,; the dashed curve for this structure is drawn where Kthmax
= Kc.
Fig. 2.5. SEM micrographs of the overload fracture surfaces of (a) 3ABC-, (b) 5ABC-
and (c) 7ABC-SiC. Fracture in 3ABC and 5ABC is intergranular, whereas it is
predominantly transgranular in 7ABC.
Fig. 2.6. SEM micrographs of metallographic sections taken perpendicular to the fracture
surface showing the crack paths during fatigue-crack growth at 25 C and 1300 C for
3ABC (a,b) and 5ABC-SiC (c-f). Frictional bridging (a,b,f) was most common in 3ABC
while uncracked ligament bridges (c,d,e) were predominant in 5ABC. The horizontal
arrows indicate the general direction of crack growth.
Fig. 2.7. Transmission electron micrographs near the crack tip in (a) 3ABC, and (b, c)
5ABC grown under cyclic loading at 1300 C (R = 0.1, ? = 25 Hz). Note in (b) the
equiaxed grains also fracture intergranularly and in (c) a failure of uncracked ligament.
Large arrows indicate the general direction of crack propagation, and small arrows point
to the primary cracks near the crack tip. (Courtesy of Dr. Xiao-Feng Zhang)
Fig. 2.8. Comparison of the crack-opening profiles for a monotonically- loaded crack in
the 3ABC and 5ABC microstructure after R-curve testing. Samples were loaded in situ
in a field-emission FESEM to an applied driving force of 6.2 MPa/m. The inset shows
the curve fit for the first 70 pm behind the crack tip, where the solid line is for 5ABC
(Ktip=1.6 MPavm), and the broken line for 3ABC (Ktip=2.5 MPavm).iv
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Yuan, Rong. Ambient and elevated temperature fracture and cyclic-fatigue properties in a series of Al-containing silicon carbides, thesis or dissertation, August 30, 2004; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc787712/m1/7/: accessed July 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.