Ambient and elevated temperature fracture and cyclic-fatigue properties in a series of Al-containing silicon carbides

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A series of in situ toughened, Al, B and C containing, silicon carbide ceramics (ABC-SiC) has been examined with Al contents varying from 3 to 7 wt%. With increasing Al additions, the grain morphology in the as-processed microstructures varied from elongated to bimodal to equiaxed, with a change in the nature of the grain-boundary film from amorphous to partially crystalline to fully crystalline. Fracture toughness and cyclic fatigue tests on these microstructures revealed that although the 7 wt.% Al containing material (7ABC) was extremely brittle, the 3 and particularly 5 wt.% Al materials (3ABC and 5ABC, respectively) displayed excellent crack-growth ... continued below

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86 pages

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Yuan, Rong August 30, 2004.

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A series of in situ toughened, Al, B and C containing, silicon carbide ceramics (ABC-SiC) has been examined with Al contents varying from 3 to 7 wt%. With increasing Al additions, the grain morphology in the as-processed microstructures varied from elongated to bimodal to equiaxed, with a change in the nature of the grain-boundary film from amorphous to partially crystalline to fully crystalline. Fracture toughness and cyclic fatigue tests on these microstructures revealed that although the 7 wt.% Al containing material (7ABC) was extremely brittle, the 3 and particularly 5 wt.% Al materials (3ABC and 5ABC, respectively) displayed excellent crack-growth resistance at both ambient (25 C) and elevated (1300 C) temperatures. Indeed, no evidence of creep damage, in the form of grain-boundary cavitation, was seen at temperatures at 1300 C or below. The enhanced toughness of the higher Al-containing materials was associated with extensive crack bridging from both interlocking grains (in 3ABC) and uncracked ligaments (in 5ABC); in contrast, the 7ABC SiC showed no such bridging, concomitant with a marked reduction in the volume fraction of elongated grains. Mechanistically, cyclic fatigue-crack growth in 3ABC and 5ABC SiC involved the progressive degradation of such bridging ligaments in the crack wake, with the difference in the degree of elastic vs. frictional bridging affecting the slope, i.e., Paris law exponent, of the crack-growth curve. In addition an investigation of fracture resistance in non-transforming ceramics toughened by grain bridging mechanism is presented using linear elastic fracture mechanics (LEFM). Linear superposition theorems are used for the superposition of crack opening displacements, as well as stress intensity factors, resulting from the external tractions and the internal compressive bridging stresses. Specifically weight functions are used to relate the CODs, stress intensity factors, and tractions and the bridging stress. Expressions are derived for apparent material resistance, the bridging resistance and the intrinsic toughness, and an experimental procedure is proposed by which these predictions can be verified.

Physical Description

86 pages

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OSTI as DE00834276

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  • Other Information: TH: Thesis (M.S.); Master of Science, Engineering: Materials Science and Engineering, submitted to the University of California, Berkeley, Berkeley, CA (US)

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  • Report No.: LBNL--56229
  • Grant Number: AC03-76SF00098
  • Office of Scientific & Technical Information Report Number: 834276
  • Archival Resource Key: ark:/67531/metadc787712

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Office of Scientific & Technical Information Technical Reports

Reports, articles and other documents harvested from the Office of Scientific and Technical Information.

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  • August 30, 2004

Added to The UNT Digital Library

  • Dec. 3, 2015, 9:30 a.m.

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  • Sept. 21, 2017, 6:01 p.m.

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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. (digital.library.unt.edu/ark:/67531/metadc787712/: accessed December 15, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.