Influence of initial microstructure on creep deformation behaviors and fracture characteristics of Haynes 230 superalloy at 900 °C

The effect of initial microstructure on creep properties at 900 °C has been investigated in a solid-solution-strengthened Haynes 230 superalloy. The proprietary direct aging combined with controlled cooling rate led successfully to the transition of serrated grain boundaries. The serrated grain boundaries were found to have plane normal terminated by low-index planes. The lamellar carbides formed by discontinuous precipitation process became more significant in the serrated specimen, as the aging temperature to which the specimen was slow-cooled from 1280 °C decreased. The fine intragranular carbides which precipitate during initial stage of creep are responsible for the strong hardening effect by pinning the dislocations in the standard specimen. The serrated grain boundaries with stable planar M23C6 carbides enhanced highly creep ductility due to their high resistance to cavitation cracking. However, the serrated specimens exhibited a higher creep rate, which is attributed to the promotion of intragranular deformation due to the lack of fine intragranular carbides, as well as cavitation at the lamellar carbides. Based on the relationships between microstructural factors and creep behaviors at 900 °C, the optimal microstructure is discussed.