3ABC-SiC doped with 1at% Yttrium. The important in-situ toughened microstructure is retained at this doping level.
D. CHEN, C. J. GILBERT, X. F. ZHANG and R. O. RITCHIE
Materials Sciences Division, Lawrence Berkeley National Laboratory
and Department of Materials Science and Mineral Engineering
University of California, Berkeley, CA 94720, USA
Abstract -- The growth of fatigue cracks at elevated temperatures (25–1300oC) is examined under cyclic loading in an in situ toughened, monolithic silicon carbide with Al-B-C additions (termed ABC–SiC), with specific emphasis on the roles of temperature, load ratio, cyclic frequency, and loading mode (static vs cyclic). Extensive crack-growth data are presented, based on measurements from an electrical potential-drop crack-monitoring technique, adapted for use on ceramics at high temperatures. It was found that at equi-valent stress-intensity levels, crack velocities under cyclic loads were significantly faster than those under static loads. Fatigue thresholds were found to decrease with increasing temperature up to 1200oC; behavior at 1300oC, however, was similar to that at 1200oC. Moreover, no effect of frequency was detected (between 3 and 1000 Hz), nor evidence of creep cavitation or crack bridging by viscous ligaments or grain-boundary glassy phases in the crack wake. Indeed, fractography and crack-path sectioning revealed a fracture mode at 1200–1300oC that was essentially identical to that at room temperature, i.e. predominantly intergranular cracking with evidence of grain bridging in the crack wake. Such excellent crack-growth resistance is attrib-uted to a process of grain-boundary microstructural evolution at elevated temperatures, specifically invol-ving crystallization of the amorphous grain-boundary films/phases.
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3ABC-SiC doped with 1at% Yttrium. The important in-situ toughened microstructure is retained at this doping level. |
Full text in pdf form: Acta
Materialia, 40 (2000) 659-674
TMS
Conference
Presentation, Fall 2000
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