Evolution of Microstructure during Creep Deformation in a Forged 617 Superalloy

Creep tests are conducted on forged 617 M superalloy samples at 650, 700, and 750 °C at stresses ranging from 115 to 320 MPa. At 650 °C, a significant steady‐state secondary creep is observed, while at 700 and 750 °C limited secondary creep and prolonged tertiary creep are observed. Creep data analysis using power‐law creep approach estimates high‐stress exponents ( n ≈ 13.4 at 650 °C, ≈11.5 at 700 °C, and ≈9.7 at 750 °C) and activation energy ( Q ≈ 530.6 kJ mole −1 ). After incorporating threshold stress, the activation energy for creep is found to be ≈261 kJ mole −1 . This is consistent with the activation energy for lattice self‐diffusion in the alloy. Transmission electron microscopy provides evidences affirming that climb‐controlled dislocation creep may be the operative creep mechanism. Ni 3 (Al, Ti) (γ′) precipitates and M 23 C 6 carbides evolve during exposure at operating temperatures. These are observed to lead to enhanced creep resistance at 700 °C. Significant coarsening rate of γ′ precipitates are noted during exposure at 750 °C. This intensifies recovery processes and lowers creep threshold stress drastically. Extended tertiary creep prevails near the service conditions of this alloy, which is ascribed to instability in microstructure during creep.