Solution Heat-Treatment of the Nb-Modified MAR-M247 Superalloy

MAR-M247 superalloy has excellent mechanical and oxidation/corrosion properties at elevated temperatures. Niobium is a refractory element normally present in the composition of nickel-based superalloys, found mainly in γ’ phase and MC carbides [1]. In general, solution heat-treatments are applied before an aging heat-treatment. The objectives of these treatments is to dissolve the γ’ phase in γ matrix and minimize segregation from non-equilibrium solidification, in order to produce a γ’ controlled re-precipitation during aging heat-treatment, an uniform and isotropic material with better properties [2,3]. The aim of this work was to find the best condition for solution heat-treatment of a MAR-M247 modified Ni-base superalloy through microstructural characterization of as-cast and heat-treated materials, comparing the results with experiments of differential thermal analysis (DTA) and Thermocalc simulations.

The conventional MAR-M247 superalloy was modified by replacing the amount of tantalum by niobium (in atomic %) and then the nominal composition of Nb-modified MAR-M247 superalloy is: 10.2 wt % Co, 10.2W, 8.5Cr, 5.6Al, 1.6Nb, 1.4Hf, 1.1Ti, 0.7Mo, 0.15C, 0.06Zr, 0.015B, Ni balance. This superalloy was produced in a vacuum induction melting furnace (VIM) via investment casting (lost-wax technique) at Açotécnica (Jandira, São Paulo, Brazil).

The samples selected to solution heat-treatments were encapsulated in quartz tube in argon atmosphere and heat-treated in a ceramic tube furnace with MoSi₂ resistive element followed by air cooling. The samples were heat-treated at 1185, 1240, 1260, 1280 and 1300 ^oC for times of 2, 4 and 8 hours. The as-cast and heat-treated samples were prepared following conventional metallographic techniques, etched with a glyceregia solution and characterized in conventional SEM and FEG-SEM, both with EDS detector. All samples were submitted to microhardness measurements.

According to Thermocalc simulations, the window of solution heat-treatment for MAR-M247(Nb) superalloy is between 1212 ^oC (γ’ solvus temperature) and 1315 oC (solidus temperature).  Besides, in DTA experiments, the γ’ solvus temperature was 1238 ^oC and the incipient melting temperature was 1281 ^oC, limiting temperature of heat-treatment because segregation. The sample heat-treated at 1280 ^oC showed incipient melting. Therefore, Thermocalc simulations have shown a good agreement with DTA experiments.