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 orientations. The cyclic hardening curves at different strain amplitudes were determined at 650, 900, and 950-degrees-C, and the stress-strain response of the material was substantiated by detailed fractographical scanning electron microscopy (SEM) and microstructural/optical microscopy, STEM) observations. During cycling, MA 760 did not exhibit marked hardening or softening, and also the final fracture occurred very rapidly without a significant decrease in the cyclic strength of the material. The ''brittle'' nature of deformation behavior of MA 760, especially at lower test temperatures, as well as oxidation-induced secondary cracking along grain boundaries, was clearly revealed by microscopical 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 gamma' 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 particles, was frequently observed.