Ambient and elevated temperature fracture and cyclic-fatigue properties in a series of Al-containing silicon carbides Page: 35 of 86
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Thus, by increasing the Al content in ABC-SiC to -5 wt.%, which leads to a
microstructure of high aspect-ratio u-SiC grains with both amorphous and crystalline
grain-boundary films, which in turn causes a change in the predominant crack-tip
shielding mechanism from frictional to uncracked-ligament bridging, optimal crack-
growth resistance at 250 and 1300 C can be obtained under both monotonic and cyclic
loading. Not only is the ligament bridging mechanism a more potent mode of toughening
in ABC silicon carbides, but it also is more resistant to degradation under cyclic loading
and thus leads to a reduced susceptibility to fatigue failure.
2.4 CONCLUSIONS
An experimental study has been made of the microstructural and mechanistic origins
of the fracture toughness and fatigue-crack growth resistance of a series of Al-containing
ABC-silicon carbides at 250 and 1300 C, the following conclusions can be made:
(1) Of the three silicon carbides examined here (3, 5, and 7ABC SiC), 5ABC-SiC
demonstrated the highest peak toughness of 8.9 MPaIm at 25 C and 7.1 MPaIm at
1300 C. These high toughness values were attributed to crack-tip shielding, primarily
from uncracked-ligament bridging in the crack wake. Corresponding toughening in
3ABC-SiC was primarily associated with frictional grain bridging; this structure
displayed roughly 15% lower toughness. No bridging was detected in 7ABC
microstructure, which accordingly exhibited the lowest toughness, comparable to that
of commercial SiC.25
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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/m1/35/?rotate=90: accessed July 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.