Microstructural evolution of VDM alloy 780 under isothermal heat treatment conditions

Abstract

VDM Alloy 780 is a novel, gamma prime (γ′) strengthened, wrought Ni-base superalloy that is getting wide attention in the aerospace industry for applications requiring high-temperature stability. The specially tailored composition offers a combination of the properties of Waspaloy and Inconel 718, two well-established Ni-base superalloys, eliminating their drawbacks regarding high-temperature instability and difficult-to-forge properties. The microstructure of VDM 780, like all other alloys, is highly dependent on its thermal history. Previous studies on this alloy show that a heat treatment method that enables the formation of γ′ phase and a high-temperature phase to tailor the microstructural properties has already been developed. However, the reasoning behind the heat treatment parameters was not elaborated upon. For this reason, an isothermal heat treatment study was performed under different conditions, and the findings were used to understand the standard heat treatment. The first objective of this thesis work was to perform isothermal heat treatments and evaluate the microstructural evolution. The heat treatments were performed under 48 different conditions, consisting of 2 initial material states, six isothermal temperatures ranging from (a) °C to (f) °C and four heat treatment durations ranging from 1 to 16 hours. The results of the isothermal heat treatments were based on different studies found in the literature, simulations, and various material characterization methods performed at GKN Aerospace. The second objective was to understand the necessity of the long duration of the standard heat treatment. Therefore, each step in the standard heat treatment is studied individually in the isothermal heat treatment study. Therefore, the findings of that study are used to understand the standard heat treatment steps. It has been concluded that the isothermal heat treatment conducted at (c) °C for 8 hours is the most efficient one to maximize the hardness. This is related to the size and volume fraction of the "gamma" phase precipitation in the same condition. Furthermore, each step of the standard heat treatment was confirmed to have a critical role in the precipitation of the high-temperature phase and γ′ phase.