The influence of different alloying elements on the development of the γ/γ′ microstructure of nickel-base superalloys during high-temperature annealing and deformation

The development of the gamma/gamma' microstructure during high-temperature annealing and deformation is investigated in a series of experimental nickel-base superalloys, with varying contents of alloying elements, in particular thenium, ruthenium and iridium. Some samples were creep-deformed in compression at 950 degrees C to different plastic strains epsilon(pl), while others were aged at 1000 degrees C. By investigating the resulting microstructures one can deduce that the development of the gamma' microstructure is influenced in two ways by the alloying elements. The lattice misfit and thereby the coherency stresses between gamma' phase and gamma matrix are altered but also the rate of diffusion in the gamma matrix. While coherency stresses are the driving force for changes of the gamma' shape at elevated temperatures, the development of the gamma' structure itself is diffusion-controlled. To understand the effects of alloying elements on the development of the gamma' structure, the modifications of both the lattice misfit and the modifications of diffusion rate caused by the alloying elements have to be taken into account. (c) 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.