Precipitation kinetics in N18 P/M superalloy : experimental study and numerical modelling

Heat treatments of aeronautical turbine disks produced by powder metallurgy include y' solutionning, quenching and subsequent annealing. In the present work, the intragranular y' precipitation kinetics during the quench has been followed by dilatometry and transmission electron microscopy. The precipitation steps are studied on specimens "hyperquenched" from selected temperatures during a simulated industrial quench. Experimental results are compared with a pseudo-binary model based on classical homogeneous nucleation and coarsening theories. While the precipitate volume fraction is lower than 20%, a good quantitative agreement is observed. INTRODUCTION The excellent high temperature strength of the superalloys used in aeronautical gas turbines is derived from the precipitation of ordered and coherent y' phase Nig(A1,Ti) within Ni solid solution. The mechanical properties of these materials are strongly dependent upon the microstructure produced during thermal treatments. This work is focussed on the experimental study and the modelling of intragranular y' precipitation (also called secondary precipitation) during the oil quench corresponding to a 200Klmin (3Kls) quench rate. A previous experimental study has been discussed in [I]. Further work is done to improve the comparison with the model, through the evaluation of volume fraction (from dilatometry experiments) and the determination of the precipitate size by TEM (from dilatometry and high quench rate experiments). The alloy we study is the N18, a complex commercial grade used by SNECMA for turbine disks [2]. The characterization of the y + y + y' transformation through dilatometric experiments and TEM has also been published by HAZOTTE and SIMON but on the single crystal superalloy CMSX-2 [3].