Dynamic fatigue behavior of two SiC and a SiCp reinforced Al2O3 at elevated temperatures

Dynamic fatigue behavior of a siliconized SiC, a sintered beta-SiC, and a SiCp (silicon carbide particle)-reinforced Al2O3 formed by directed oxidation of Al metal, which are considered for use in heat exchangers in coal-fired power plants, were evaluated at 1100 and 1400 degrees C in air. Four-point flexure specimens were tested at Jive stressing rates from 37 MPa s(-1) to 0.0001 MPa s(-1) resulting in total times to failure up to 1200 h. Thirty specimens of each material were rested at the fast-fracture condition and 10 specimens were tested at the four dynamic fatigue conditions at each temperature. At 1100 degrees C none of the materials exhibited any loss of strength as a function of stressing rate and very little tendency to creep was observed At 1400 degrees C the sintered beta-SiC exhibited no strength toss, while the siliconized SiC showed a significant loss of strength and some signs of creep at stressing rates less than 0.01 MPa s(-1). The SiCp reinforced Al2O3 exhibited extensive creep at stressing rates ranging from 0.01 to 0.0001 MPa s(-1) at 1400 degrees C, in fact at the slower stressing rates the creep was dominant and the specimens could not be brought to fracture in the four point flexure fixtures. Extensive fractography showed that the failure mode for the sintered beta-SiC was indeed a fast-fracture mode at all temperatures and stressing rates, the specimens mostly failing from pores in the microstructure. The siliconized SiC failed partly from pores and partly from metal inclusions at 1100 degrees C and fast stressing rates, and at 1400 degrees C at slower stressing rates slow crack growth was observed to occur with the Si-metal inclusions as starting points. The failure modes in SiCp reinforced Al2O3 changed from fast fracture from residual Al-alloy rich areas to a creep failure at intermediate stressing rates at 1400 degrees C. (C) 1997 Elsevier Science Limited.