HIGH-TEMPERATURE LOW-CYCLE FATIGUE BEHAVIOR OF SUPERALLOY MA-760

The high-temperature cyclic behavior of oxide dispersion-strengthened superalloy MA 760 was studied at different temperatures and specimen o rientations. The cyclic hardening curves at different strain amplitude s were determined at 650, 900, and 950-degrees-C, and the stress-strai n response of the material was substantiated by detailed fractographic al scanning electron microscopy (SEM) and microstructural/optical micr oscopy, STEM) observations. During cycling, MA 760 did not exhibit mar ked hardening or softening, and also the final fracture occurred very rapidly without a significant decrease in the cyclic strength of the m aterial. The ''brittle'' nature of deformation behavior of MA 760, esp ecially at lower test temperatures, as well as oxidation-induced secon dary cracking along grain boundaries, was clearly revealed by microsco pical studies of the fracture surfaces. At 650-degrees-C, evidence of dislocation cutting of gamma' was observed in transmission micrographs , but at higher temperatures, dislocation climb and by-passing of gamm a' particles were found to become more prominent features. At all test temperatures, dislocation climb over yttria particles, as well as the departure side pinning effect of dislocations at nonshearable particl es, was frequently observed.