Microstructure and mechanical properties of a nimonic PE 16 superalloy subjected to double-aging treatments

Abstract

Solution-treated Nimonic PE 16 superalloy was subjected to double-aging treatments involving intermediate aging at 1073 or 1173 K and final aging at 973 or 1023 K. The microstructures developed at different stages of these treatments were characterized by optical microscopy and scanning and transmission electron microscopy. The mechanical properties of the alloy were evaluated at room temperature and correlated with the microstructure. Intermediate aging at 1173 K led to the formation of intergranular titanium-rich MC particles alone, while aging at the 1073 K resulted in the precipitation of M₂₃C₆ particles and fine γ' precipitates. In general, the strength properties of the double-aged alloy were superior when intermediate aging was carried out at 1073 K. The optimum combination of the mechanical properties for this alloy was obtained by adopting the following sequence: solution annealing at 1313 K for 4 h, intermediate aging at 1073 K for 2 h and final aging at 973 K for 16 h. A transmission electron microscopy study showed that the size distribution of the γ' precipitates observed at the end of this two-step aging treatment was uniform rather than duplex. Inferior strength properties resulting from the intermediate aging step at 1173 K could be attributed to the depletion of titanium from the matrix during aging at 1173 K and the consequent reductions in the volume fraction of the γ' precipitates as well as in the inherent strength of the γ' phase formed during final aging. Further, it is shown that the experimental value of the contribution to the yield strength of the alloy arising from the γ' precipitates is in good agreement with the theoretical estimate based on the shearing of the γ' precipitates during plastic deformation.