Abstract
For Ni-Fe-base superalloy 706, a stabilizing treatment between solution-annealing and age- hardening treatments has been proposed to improve its creep rupture life. The precipitation behavior of Alloy 706 was investigated in the present study and related with creep rupture properties. Samples taken from a gas turbine disk forging were solution-treated at 980°C for 3 h and stabilizing-treated in a range of 780 to 900°C for 1.5 h, followed by the double-aging at 720°C for 8 h and at 620°C for 8 h. Precipitation behavior of these samples was examined by SEM and TEM, and creep rupture tests were conducted at three conditions. Four kinds of intra-granular precipitates were identified : fine γ'-γ" coprecipitates having the core of γ' being overlaid with γ" on its top and/or bottom, large γ'-γ" co-precipitates having the core of γ' being completely covered with γ", large γ' precipitates and fine γ" precipitates. At the grain boundary η was identified and found to be accompanied by a serrated grain boundary and denuded zone. Such precipitation behavior was significantly affected by stabilizing temperature, especially below 840°C, and so was creep rupture property accordingly. It is concluded that the best creep rupture properties are obtained when an optimum combination is established among intra-granular fineγ '-γ" co-precipitates, intra-granular large γ'-γ" co-precipitates, and the inter-granular η phase.
