The effect of the solution and aging treatments on the microstructures and microhardness of nickel-based superalloy

Abstract

Due to their high thermal stability under severe working conditions and harsh environment, nickel (Ni)-based superalloys, such as Inconel 718, Inconel 738, GTD11, and Rene 40, are largely used in gas turbine industry. The gamma prime (γʹ) precipitates, that are coherent with the gamma (γ) matrix in these superalloys, provide excellent mechanical properties at high operating temperatures. However, after a long time of functioning, (γʹ) precipitates coarsen and lose their coherency with the matrix, which causes damages in gas turbine blades. The main purpose of this paper is to study the effect of both solid solution and aging heat treatments temperatures on the microstructural evolution in an Inconel 738 superalloy blade, which was received after few tens of thousands of service hours. The resulting microstructures were analyzed by using scanning electron microscopy and optical microscopy along with Vickers microhardness measurements. The obtained results show that the Vickers microhardness of the studied superalloy is related to size, shape, volume fraction, and distribution of the hardening phase (γʹ) [Ni3(Al, Ti)], and to its carbides as well. It was found that the microstructural stability is related to the working temperature.