Characterization of the microstructure evolution of the precipitation strengthened nickel-base superalloy René 65

Abstract

eng. René 65 is a new precipitation strengthened nickel-base superalloy, with γ’ precipitates being the main precipitates present in the microstructure. The primary material has been characterized regarding the grain size and the phase fraction of primary γ’ precipitates depending on the billet radius by the use of metallographic methods. It has been observed that the primary material has a very inhomogeneous microstructure and that one may strongly distinguish samples coming from the center from samples coming from the surface of the primary material ingot. Annealing and hot compression tests have also been performed in order to characterize the microstructure evolution. Metallographic methods have also been used for the observation of the grain growth, and a mathematical evaluation and modelling of the flow curves have been used for the data treatment of the hot compression tests. Due to the results obtained from the primary material characterization, these tests have been made for samples coming from different positions in the primary material ingot. It has been observed that the grain growth rate is very strongly bonded to the presence of primary γ’ precipitates, which slow down considerably the grain growth when the annealing temperature is under their solvus temperature. A proper constitutive modelling of the flow curves could not be made because of the particular shapes of the evaluated flow curves. It has been nevertheless possible to define 6 different types of these curves depending on the testing parameters during the hot compression. Further research opportunities have been also presented in this work.