HIGH-TEMPERATURE FRACTURE-TOUGHNESS OF SINGLE-CRYSTAL YTTRIUM-ALUMINUM-GARNET

Y3Al5O12 (YAG) is the most creep-resistant single crystal oxide known and is therefore an attractive candidate for very high temperature app lications. The fracture toughness, Ki,, was measured as a function of temperature using the single edge precracked beam (SEPB) method and wa s compared to notched beam method results in the literature. The fract ure toughness of annealed SEPB specimens was found to be independent o f both temperature from 20 degrees C to at least 1700 degrees C and lo ading rate over two orders of magnitude. Thus the brittle-to-ductile t ransition does not occur before 1700 degrees C. Previous reports of re markable increases in the fracture toughness below 1700 degrees C usin g notched beam methods are considered erroneous due to microcrack heal ing and crack blunting effects. The SEPB fracture toughness method avo ids these problems since a long, sharp crack exists in the specimen pr ior to testing and can be effectively preserved at high temperatures u sing a preloading procedure. (C) 1997 Acta Metallurgica Inc.