Cyclic fatigue and resistance-curve behavior of an in situ toughened silicon carbide with Al-B-C additions
- Lawrence Berkeley Lab., CA (United States). Materials Science Div.
The room-temperature crack-growth properties of an in situ toughened, monolithic silicon carbide are reported. Hot pressing was performed at 1900 C with 3 wt.% Al, 2 wt.% C and 0.6 wt.% B additions. Compared to a commercial SiC (Hexoloy SA), significant improvements in both the fracture toughness and cyclic fatigue-crack propagation resistance have been achieved through control of the {beta} to {alpha} transformation. Using fatigue-precracked, disk-shaped compact-tension specimens, marked rising resistance-curve behavior was measured over the first {approximately}600 {micro}m of crack extension, leading to a plateau fracture toughness of K{sub c} {approximately} 9.1 MPa{radical}m; this represents more than a threefold increase over the toughness of Hexoloy, where a K{sub c} value of 2.5 MPa{radical}m was measured with no evidence of a resistance curve. Cyclic fatigue-crack growth rates in the toughened SiC were found to be faster than those under sustained loads (static fatigue) at the same stress-intensity level. The cyclic fatigue-crack growth resistance was found to be far superior to that of Hexoloy. Whereas cracking in the commercial SiC became unstable when the maximum stress intensity K{sub max} exceeded {approximately}2 MPa{radical}m, thresholds for fatigue-crack growth in the in situ toughened material exceeded a K{sub max} of 7 MPa{radical}m. Such dramatic improvements in the crack-growth resistance of SiC are attributed to a microstructure consisting of a network of interlocking, plate-like predominantly {alpha}-phase grains, which combine to both bridge and deflect the crack. These results represent the first reported evidence of cyclic fatigue behavior in a monolithic silicon carbide and the first direct measurement of the resistance curve properties in this ceramic.
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 367327
- Journal Information:
- Acta Materialia, Vol. 44, Issue 8; Other Information: PBD: Aug 1996
- Country of Publication:
- United States
- Language:
- English
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