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.

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

Additional publications on SiC:

LBNL , MSD * Ritchie Group * Dept of MSME , UC Berkeley

Last updated 08/03