On the kinetics of gamma prime (γ’) precipitation and its strengthening mechanism in Alloy 617 during a long-term thermal aging

The changes in the mechanical properties and the microstructure of solid-solution-strengthened Ni-Cr-Co-Mo Alloy 617 during an extended thermal aging at 750 °C from 200 h up to 32,000 h are investigated. It is shown that thermal aging leads to the formation of nanoscale γ’ precipitates, which affect the strengthening of the γ-matrix and thus overall mechanical properties of Alloy 617. Small-Angle Neutron Scattering (SANS) measurements indicate that about 3 vol.% of nanoscale γ’ precipitates with an average radius below 20 nm is formed after thermal aging for only 200 h. The volume fraction of these precipitates does not increase dramatically reaching about 6% after 32,000 h, however, their size does change significantly reaching the average of about 100 nm after 32,000 h. The kinetics of the coarsening of the present nanoscale γ’ precipitates is shown to follow the classical Lifshitz–Slyozov–Wagner (LSW) theory for diffusion-controlled precipitate Ostwald ripening. Furthermore, SANS measurements of the microstructure-averaged volume fraction and size of present nanoscale γ’ precipitates allow for an investigation of the strengthening mechanisms of the formed nanoscale γ’ precipitates. It is found that the Jackson–Reed order strengthening model, which accounts for shearing of precipitates by the strongly coupled dislocation pairs agrees with the experimental observations.