Elevated temperature low cycle fatigue behaviour of Haynes 282 and its correlation with microstructure – Effect of ageing conditions

Abstract

In this paper, total strain-controlled high temperature (760 °C) low cycle fatigue (HTLCF) behaviour (at 0.5% strain amplitude) of a new γ′-strengthened wrought Ni based superalloy (Haynes 282) for various ageing conditions is presented for the first time, emphasizing the microstructural effects. Haynes 282 is a promising material for hot gas path components of ultra-supercritical steam turbines due to its adequate fatigue property at elevated temperatures, along with high creep and oxidation resistance. Various microstructures were generated by varying ageing time (1 day, 6 days and 9 days) and temperature (650 °C and 760 °C). While continuous cyclic hardening behaviour was witnessed for lower aging temperature (650 °C), predominantly cyclic softening after initial hardening through few cycles was observed for higher aging temperature (760 °C), due to formation of more number of larger size voids and secondary cracks in the later specimen during the testing (as characterized by X-ray tomography). Underlying deformation mechanisms (characterized through TEM) and fracture modes (characterized through SEM, EDS) were correlated to alloy's microstructures under various HTLCF conditions; distinct differences were observed between the two ageing temperatures. While 650 °C, 9 days ageing condition showed primarily signatures of γ′ shearing, 760 °C, 9 days ageing condition showed primarily the Orowan mechanism of deformation. For 650 °C ageing condition, predominantly intergranular fracture mode has been observed, which is attributed to the presence of brittle primary MC particles on the boundaries. Whereas for the 760 °C ageing condition, the fracture mode remained predominantly transgranular because it involves less crack deflection through its path. Plastic strain energy density based life prediction model were used to predict fatigue life. Close agreement between calculated and measured life indicates validity of these models for the present material.