Activation energy calculations for discontinuous yielding in Inconel 718SPF

The kinetics of yield point return and of dynamic strain aging (DSA) were investigated for a commercial heat of Inconel 718SPF. During uniaxial tensile tests, the alloy exhibited serrated flow on the load–elongation curves at intermediate temperatures. Types A, B and C serrations were observed depending upon the test temperature and strain rate. For type A and B serrations, apparent activation energies ranging from 97 to 133 kJ mol−1 were measured, which suggests that type A and B serrated flow results from the diffusion of an interstitial solute, most likely carbon. For type C serrations, apparent activation energies in the range ∼210 to ∼272 kJ mol−1 were measured, which suggests that type C serrations arise from the diffusion of a substitutional solute, most likely Cr. Yield points induced during strain aging under stress experiments increased with a tn relationship, where the exponent n ranged from 0.3 to 0.5 before reaching a plateau. This suggests that the yield points were a combined result of bulk lattice diffusion and dislocation pipe diffusion of solutes. The apparent activation energies, ranging from 42 to 50 kJ mol−1, are consistent with prior observations, but cannot be explained by any classic models for static strain aging. It is suggested that the yield points result from the reorientation of carbon-vacancy pairs in the strain fields of dislocations.