Title: Multi-class modeling of grain growth and delta phase dissolution kinetics of alloy 718 with in-situ HT-EBSD measurements

Abstract

In the aerospace industry, the microstructure evolution of alloy 718 during forging and heat treatment and the resulting mechanical properties are decisive in view of the high quality requirements of aircraft components. During thermo mechanical processing, the temperature control and adiabatic heating leads to grain growth and, if δ-solvus temperature is exceeded, to the dissolution of the δ-phase, which further results in accelerated grain growth. To describe the history of the microstructure in terms of grain size during and after forging or heat treatment, an existing multi-class microstructure model was optimized with focus on grain growth kinetics and parameterized by experimental results. In the model the distribution of various grain fractions is simultaneously monitored and the growth of fractions in favor of thermodynamically disadvantaged classes is calculated. To verify and optimize the thermodynamic and kinetic parameters in this multi-class grain growth model, two in-situ HT-EBSD experiments with different initial microstructures (with and without δ-phase) were performed at 1045°C with a holding time of 90 minutes. The experiments were supplemented with a series of annealing treatments where temperatures and holding times were varied and results documented with light microscopy. All image files from the different experiments were evaluated for grain size distribution and δ-phase area fraction by the use of proper image analysis techniques. Furthermore, the δ-phase dissolution kinetics was evaluated with an in-situ SEM video and the influence of the δ-phase on grain growth inhibition was visualized. After complete δ-dissolution, the re-building of Ni3Nb precipitates was investigated with focus on kinetics and morphology. The multi-class model describes the microstructure and the coarsening during processing more precisely in terms of the grain size distribution than previously used single-class models. This is crucial in order to be able to predict mechanical properties such as tensile strength, fracture toughness and creep resistance.

Biography

Christian Gruber completed his Bachelor degree - Metallurgical engineering, University of Leoben, Austria 2011 – 2017. He completed his Master degree - Metallurgical engineering, University of Leoben, Austria 2017 – 2018. He did PhD in Metallurgical engineering, University of Leoben, Austria. Currently he is an Innovation manager at Voestalpine BÖHLER aerospace GmbH & Co KG, Austria since 2021 and since 2018 he is working as a Junior Scientist at Materials Center Leoben Forschung GmbH, Austria.