Creep behavior in the new AD730TM nickel-based disk superalloy – Influence of aging heat treatment and local chemical fluctuations

Abstract

Transmission electron microscopy (TEM) experiments combining conventional and weak beam observations, energy dispersive X-ray and electron energy loss spectroscopies have been used to identify and quantify, in the polycrystalline AD730TM Ni-Based superalloy, the effect of isothermal aging after solution treatment in the range 730–790 °C on the creep behavior at 700 °C – 500 MPa. The microstructure of different aged crept specimens has been first characterized as the creep behavior is directly influenced by the microstructural features (size of the γ′ precipitates and of the γ-channel). The analyzes show similar results whatever the heat treatment. Then, the investigation of the dislocation configurations indicate that the deformation occurs by the propagation of straight perfect dislocations, which shear the γ-matrix and the precipitates at 730 °C and 760 °C, whereas a combination of perfect and partial dislocations is observed after aging at 790 °C. The investigation of the local chemical composition has revealed some strong local variations in Cr content within the γ-phase and smaller ones in Ti content within the γ′-phase after aging at 790 °C. These elemental concentration quantifications have been used to evaluate the antiphase boundary and stacking fault energies. This allows to interpret the presence of partial dislocations after aging at 790 °C and thus the increase in the creep strain rate when the specimen is previously aged at 790 °C.