HIGH-TEMPERATURE LOW-CYCLE FATIGUE AND THERMAL-MECHANICAL FATIGUE BEHAVIOR OF AN OXIDE-DISPERSION-STRENGTHENED NICKEL-BASE SUPERALLOY

The high temperature low cycle fatigue (LCF) and thermal-mechanical fa tigue (TMF) behaviour of the oxide-dispersion-strengthened (ODS) nicke l-base superalloy MA 760 was investigated. LCF tests were carried out at 950 and 1050-degrees-C. TMF tests were performed using two differen t temperature ranges, 550-1050-degrees-C and 600-1100-degrees-C. Mater ial was tested parallel to the extrusion direction. Experimental resul ts indicate the influence of strain rate, temperature and shape of the TMF cycle on cyclic stress-strain behaviour, fatigue life, damage and microstructure stability. TMF lives were compared with LCF lives. Met allographic observations were used to study the initiation sites and c rack paths as well as the stability of the gamma' strengthening phase. At very high temperature, MA 760 exhibits delamination at longitudina l grain boundaries parallel to the applied stress and reduction in siz e of the gamma' precipitates within a factor of 10-30. The effects of such a microstructure change are discussed in terms of a cyclic stress -strain behaviour and lifetime. Finally, a comparison between the ODS alloy and the directionally solidified alloy CM 247-DS is undertaken.