Ambient and elevated temperature fracture and cyclic-fatigue properties in a series of Al-containing silicon carbides Page: 26 of 86
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2.2.2 Fracture Toughness and Strength
2.2.2.1 Fracture Toughness
The variation in fracture toughness at 250 and 1300 C with Al wt.% is summarized in
Table 2.2. For both as-hot-pressed and annealed 3ABC and 5ABC structures at 25 C, R-
curves were determined and are shown in Fig. 2.3. Peak values from the R-curves
indicate that in as-hot-pressed condition, 5ABC is some 17% tougher than 3ABC; both
these structures are essentially twice as tough as 7ABC, which displayed a fracture
toughness K, value no greater than 4.0 MPaim, comparable to that of commercial SiC
(e.g., Hexaloy SA). Results for 3ABC and 5ABC at elevated temperatures showed a
reduction in the fracture toughness at 1300 C of roughly 20 to 40%, while 7ABC had an
~20% increase. Finally, experiments using samples pre-exposed for 72 hr at 1300 C
prior to room temperature testing showed that although grain-boundary crystallization
occurred, there was no significant effect on the toughness (Table 2.2); the toughness of
3ABC and 5ABC showed a 16% increase and 26% decrease, respectively.
Table 2.2. Comparison of Mechanical Properties of ABC-SiC
3ABC-SiC 5ABC-SiC 7ABC-SiC
Strength at 25 C, (Ref. [8]) 4- 691 12 480 30 533 58
pt bending (MPa)
Toughness K, at 25 C 7.6 8.9 4.0*
(MPaim) (upper bound)
Toughness K, at 1300 C 4.5 7.4 49*
(MPaim) (upper bound)
Toughness K, of pre-exposed 6.6
samples at 25 C (MParm) 8.8
*Toughness evaluated from a razor micro-notch.
2.2.2.2 Strength16
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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/m1/26/: accessed July 17, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.