Microstructural evolution of VDM alloy 780 under isothermal heat treatment conditions
Typ
Examensarbete för masterexamen
Program
Publicerad
2022
Författare
Baytaroglu, Arda
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
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.
Beskrivning
Ämne/nyckelord
VDM 780 , isothermal heat treatments , gamma prime phase , high temperature phase , microstural evolution