Effect of long-term exposure at elevated temperatures on the structure and properties of a nimonic PE 16 superalloy

Abstract

The influence of long-term exposure in the temperature range 973-1123 K on the microstructure and room temperature mechanical properties of a Nimonic PE 16 superalloy was investigated. The data obtained on the coarsening kinetics of the γ' (Ni₃(Al, Ti)) precipitates during aging and on the activation energy of the coarsening process lead to the conclusion that the coarsening of γ' in this alloy is controlled by the volume diffusion of aluminium or titanium in the matrix. While chromium-rich M₂₃C₆ carbides formed predominantly when the samples were aged at 973 K, a mixture of M₂₃C₆ and MC carbides was found to precipitate when the samples were aged at higher temperatures. Aging of the solution-treated alloy at 973 K for up to 1000 h led to a monotonic gradual increase in the yield and ultimate tensile strengths at room temperature. In contrast, aging for more than 100 h at 1073 K resulted in a rapid decrease in the yield strength of the alloy. When the mean radius of the γ' precipitates was below 10 nm, the γ' precipitates were sheared by superdislocation pairs during plastic deformation, and the strengthening contribution due to γ' was found to be due mainly to order hardening. When the mean radius of γ' exceeded 50 nm, the γ' precipitates were invariably by passed by moving dislocations by the Orowan mechanism. However, when the radius of γ' precipitates was in the range 10-50 nm, both the shearing and Orowan looping of precipitates were observed.