Evolution of microstructure and mechanical properties of Ti modified superalloy Nimonic 80A

Nickel based superalloy Nimonic 80A modified with various Ti contents has been developed. Microstructure evolutions were investigated by optical microscope, X-ray diffraction, scanning electron microscope and transmission electron microscope in relation to the room temperature tensile properties and stress-rupture properties at 750 °C/310 MPa. After full heat treatment, the precipitate changed from γ′ phase to γ′ + η phases, the volume fraction of γ′ phase, lattice misfit of γ′/γ and room temperature tensile strength increased with the increase of Ti content. With the increase of lattice misfit, the orderly {1 1 1} atomic planes in the γ′/γ interface changed to slight distorted atomic planes and edge dislocations with Burgers vector of a/3 〈1 1 1〉 were identified. The stress-rupture life first increased and then decreased with the increase of Ti content, and the decrease was primarily due to the precipitate of intermetallic η phase at grain boundaries. After stress-rupture tests, the precipitate of Cr23C6 carbide exhibited an orientation relationship with the γ′ and γ phases, the spherical morphology of γ′ phase transformed to cubic shape for the alloy with longest stress-rupture life and the morphology transformation process of γ′ phase was schematically suggested.