Transmission electron microscopy of single and double aged 718Plus superalloy

The effects of single and double aging upon the ATI 718Plus Nickel superalloy are investigated. Double aging is found to produce an increase in hardness as compared to single aging, thus an extensive micro-structural study is performed to identify and understand the physical reasons for this. Precipitation of the γ′ phase is of particular interest in this case. Using transmission electron microscopy and energy dispersive X-ray spectroscopy, the γ′ shape, size, number density and elemental constitution are investigated and correlated to the aging conditions. The γ/γ′ lattice misfit is also measured. Only one main population of γ′ precipitates is observed after double aging: simply the population produced during the first aging continues to evolve. Using a theoretical model for precipitation hardening, based on the shearing of precipitates, the hardness is related to the size and phase fraction of the γ′ and a value for the anti-phase boundary energy of the γ′ is calculated. Measurements are compared with thermo-kinetic simulation using the software package MatCalc. A dense dispersion of 1–2 nm size γ′ phase clusters is additionally observed after both single and double aging, and this is correlated through simulation with the final cooling stage of the heat treatment.