Gamma Prime Coarsening and Redistribution in Nimonic PE16

Abstract

Gamma prime precipitate microstructures have been examined in solution-treated Nimonic 2 PE16 following irradiation to 6 and 10 X 10 2 2 neutrons/cm 2 (E > 0.1 MeV) over the temperature range 425 to 650°C and their irradiation behavior has been compared with their thermal aging behavior at 750 and 850°C. The precipitate structures are deduced to result from the competing processes of irradiation-enhanced Ostwald coarsening and solute segregation by point-defect drag to point-defect sinks. Ostwald coarsening measurements clearly demonstrate an effect of irradiation-enhanced diffusion and indicate that irradiation-enhanced Ostwald coarsening behavior is controlled by point-defect recombination. Thermal behavior is found to dominate at irradiation temperatures above 600°C. Solute segregation measurements demonstrate a correlation of coating thickness with sink type in agreement with theoretical predictions. However, the temperature dependence of coating thickness is not consistent with theoretical predictions of solute segregation, and Ostwald coarsening response should therefore be included in precipitate stability models for alloys with significant amounts of solute in solution. On the basis of the foregoing results, it is now possible to quantitatively describe the response of precipitation-hardened alloys during fast neutron irradiation. Furthermore, these results, when extrapolated to other irradiation environments, are shown to provide good prediction of irradiation-enhanced coarsening when compared with available data.