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 degrees C at stresses ranging from 115 to 320 MPa. At 650 degrees C, a significant steady-state secondary creep is observed, while at 700 and 750 degrees C limited secondary creep and prolonged tertiary creep are observed. Creep data analysis using power-law creep approach estimates high-stress exponents (n approximate to 13.4 at 650 degrees C, approximate to 11.5 at 700 degrees C, and approximate to 9.7 at 750 degrees C) and activation energy (Q approximate to 530.6 kJ mole-1). After incorporating threshold stress, the activation energy for creep is found to be approximate to 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. Ni3(Al, Ti) (gamma ') precipitates and M23C6 carbides evolve during exposure at operating temperatures. These are observed to lead to enhanced creep resistance at 700 degrees C. Significant coarsening rate of gamma ' precipitates are noted during exposure at 750 degrees 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.