Abstract
Alloy AD730TM is a recently developed Nickel base superalloy for application as turbine disk in modern gas turbines with improved thermal efficiency. Ingot casting followed by open die-forging and then heat treatments are the main manufacturing steps for the production of parts made of this alloy. Solution heat treatment operations are applied at different stages of the manufacturing in order to ease the deformation processing and/or prepare the microstructure for final heat treatment. In this research, the influence of various solution heat treatment schedules on morphology and distribution of the γ′ phase are investigated and documented. The obtained results will contribute to a better understanding of microstructure evolution of AD730TM during solution heat treatments. Differential Thermal Analysis (DTA) is used for the purposes of measurement of temperatures of phase transformations of the alloy. Based on DTA results, three solutionizing temperatures and three holding times were selected for performing and assessing the solution heat treatment process. Optical and electron microscopy were used to study the morphological evolution as well as the coarsening and dissolution of secondary phases at solvus and subsolvus temperatures. The results indicated that precipitate agglomeration and Ostwald ripening are the governing mechanisms during the initial stages and splitting and partial dissolution of γ′ precipitates takes place during subsolvus solution treatments.
Highlights
The study of microstructural changes for different solution and precipitation treatments in various nickelbased superalloys had been the subject of several research works
The back-scattered electrons (BSE) images are recorded from polished cross sections by use of a backscattered electron detector attached to scanning electron microscopy (SEM)
The microstructure of AD730TM consists of Ni-based austenitic matrix γ, primary γ precipitates most of them present at the grain boundaries, secondary and tertiary γ and two types of carbides
Summary
The study of microstructural changes for different solution and precipitation treatments in various nickelbased superalloys had been the subject of several research works. No data is available about γ evolution during solution heat treatments for AD730TM. The effects of heat treatment with the microstructural features of AD730TM are studied to determine the range of γ dissolution temperature. Ni Fe Co Cr Mo W Al Ti Nb Base 4 8.5 15.7 3.1 2.7 2.25 3.4 1.1 treatments on microstructural evolution of AD730TM is considered. The dissolution behaviour of γ under different experimental conditions is determined and the operating mechanisms are discussed
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