Influence of microstructural instabilities on elevated temperature creep deformation of Inconel* 690

Abstract

The elevated temperature creep behaviour of Inconel 690 has been examined from 900 to 1200°C (0·7–0·9T_m) at constant stresses between 6 and 33 MPa (σ/μ = 1·4 × 10^-4 to 9·5 × 10^-4). Two complementary strain–time responses were observed. A normal strain–time response, characterised by decreasing creep rate with increasing time until steady state, was observed under all normalised stresses at 900°C. Under these conditions creep deformation was associated with intragranular dislocation climb and glide. A second periodic oscillatory strain–time response was observed at higher temperatures (1050–1200°C) and intermediate to low stresses (σ/μ = 1·65 × 10^-4 to 4·5 × 10^-4). Under these conditions repetitive grain boundary sliding and migration controlled creep deformation.

The creep behaviour at 1000°C exhibited both strain–time responses. While intermediate stress exposure (σ/μ = 3·3 × 10^-4 to 4·2 × 10^-4) resulted in an oscillatory response, exposure at either lower or higher normalised stresses resulted in a normal strain–time response. Metallographic evidence indicates that this transitory behaviour is controlled by the interaction between the operative stress, the exposure time, and the rate of grain boundary carbide coarsening. At high stresses and short times, deformation is similar to that observed at 900°C and is controlled by intragranular processes. At intermediate stresses and times, some carbide coarsening occurs with grain boundary sliding and migration becoming feasible. Finally at longer times and lower stresses, grain growth occurred and intragranular processes again predominate.