Microstructural stability and mechanical properties of GH742 Ni-based wrought superalloy for turbine disk applications

Abstract

Microstructural stability and mechanical properties were investigated in a Ni-based wrought superalloy during long-term thermal exposure at 650 °C and 700 °C for up to 7500h. The results demonstrate that the morphology of primary γ′ precipitate has no significant change after long-term thermal exposure, and it exhibits excellent thermal stability both at 650 °C and 700 °C. However, the morphology of secondary γ′ precipitates transformed from complex flower-like to rounded cubic during long-term exposure at 700 °C. The coarsening of tertiary spherical γ′ precipitates occurs with increasing thermal exposure time, and its coarsening behavior is consistent with Ostwald ripening theory. M23C6 carbides coarsen at grain boundaries and form continuous film with increasing time, resulting in the embrittlement of grain boundary. The multimodal distribution of γ′ precipitates results in high strength of the alloy after long-term thermal exposure. Both the strength of the alloy after exposure at 650 °C and 700 °C for 7500 h are not less than that of the alloy after standard heat treatment. The stress rupture lifetime tests at 650 °C/873 MPa increases firstly and then decreases with increasing exposure time at 650 °C and 700 °C. The increase of stress rupture lifetime is attributed to the supplementary precipitation and growth of finest tertiary γ′ precipitates, while the decrease of it is attributed to the γ′ evolution and the coarsen of M23C6 carbides.