ONERA, the French aerospace laboratory based in Chatillon, reported at the recent EuroSuperalloys 2014 conference held in Giens, France, that researchers have successfully optimised and upgraded the microstructure and mechanical properties of a patented nickel-base Powder Metallurgy (PM) superalloy designated N19. The composition of the PM superalloy is shown in Table 1. The enhanced mechanical properties which have been obtained confirms the alloy’s potential for aeroengine turbine disk applications.
Didier Locq and colleagues from ONERA, along with colleagues from MinesParisTech and SNECMA, reported in their paper ‘Metallurgical Optimization of PM Superalloy N19’ that they investigated different heat treatment regimes (Table 2) which were used to reach the best compromise between static strength and cyclic resistance for the N19 superalloy.
The alloy was produced by Hot Isostatic Pressing (HIP) argon atomised powder (<53 micron) at 1160°C and 100 MPa for 3 hrs. The HIPed billets were then extruded at 1070°C with a 5.5 extrusion ratio and cut sections were isothermally forged into pancake shapes about 190 mm in diameter and 28 mm thick.
The researchers found whilst the grain size in PM superalloys is usually limited to 25 microns, a coarser grain microstructure obtained after solutioning heat treatment exhibited better creep and crack propagation resistance as well as a reduced propensity for crack nucleation from ceramic inclusions under fatigue loading. This was due to the distribution, sizes and morphologies of secondary and tertary y’ being significantly modified by the cooling path after solution heat treatment.
The route enabling grain coarsening up to 50-70 micron for the N19 superalloy is to be investigated in further research.