Abstract
The high temperature low cycle fatigue (LCF) and thermal-mechanical fatigue (TMF) behaviour of the oxide-dispersion-strengthened (ODS) nickel-base superalloy MA 760 was investigated. LCF tests were carried out at 950 and 1050 °C. TMF tests were performed using two different temperature ranges, 550–1050 °C and 600–1100 °C. Material was tested parallel to the extrusion direction. Experimental results 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. Metallographic observations were used to study the initiation sites and crack paths as well as the stability of the γ′ strengthening phase. At very high temperature, MA 760 exhibits delamination at longitudinal grain boundaries parallel to the applied stress and reduction in size of the γ′ 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.
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