Microstructure and mechanical properties evolutions of alloy 718 during isothermal and thermal cycling over-aging
Résumé
Direct aged alloy 718 was subjected to isothermal and non-isothermal aging at temperatures above 650 °C to assess its potential use at higher temperatures. Non-isothermal aging conditions were achieved by thermal cycling to represent the operating conditions of aero-engine components. The effect of different aging treatments/conditions on mechanical properties was understood by performing Brinell hardness and hot tensile tests at 650 °C. Brinell hardness and hot tensile properties were found to decrease with increasing aging time and higher temperature. Additionally, age related mechanical properties degradation was found to accelerate after thermal cycling. According to High Resolution Transmission Electron Microscopy, all aged samples showed coarsening of γ′, γ″ and δ phases, in addition to the transformation of γ″ into ordered orthorhombic phase δ by ledge growing mechanism. This transformation, which was accelerated by thermal cycling is controlled by the orientation of γ″ and/or δ precipitates with respect to γ matrix and also by the diffusion of solute atoms assisted by dislocations at the precipitate-matrix interface. This study elucidates the mechanism of γ″ to δ transformation and suggests this mechanism is responsible for the accelerated loss of mechanical properties of alloy 718 under non-isothermal conditions.