Microstructure and property models of alloy 718 applicable for simulation of manufacturing processes

Sammanfattning: Metallic materials experience a wide range of thermo-mechanical loads during manufacturing. These conditions influence the final mechanical and microstructural state of the work piece, which in turn de- termine the efficiency and service life of the structural component. Traditionally, engineering models are used to simulate manufacturing of a material and predict its final mechanical state. These models are based on curve-fitting procedure using stress-strain curves obtained from mechanical tests. They do not include the underlying microstructural phenomena taking place during deformation, thus limiting their range of validity. In order to improve the accuracy of simulation and prediction capability of not only one process, but a chain of manufacturing processes, physically-based models are developed. These models consist of equations describing the microstructure evolution and its effect on the mechanical behaviour of a material. The present study is part of a project which aims to simulate the deformation of a nickel-based alloy during manufacturing processes, employing a physically-based model which uses the evolution of dislocation density as an internal state variable. The research work presented in this thesis consists of an experimental characterisation of the deformation behaviour of alloy 718, including both mechanical tests and microstructural observations, and the formulation of a physically-based material model for this alloy. In order to investigate the mechanical behaviour of alloy 718, compression tests were performed at high strain rates (103 s-1) using a Split-Hopkinson bar set-up, for temperatures from 20 °C to 1100 °C. The microstructure of the deformed sample is observed using optical and scanning-electron microscopy, coupled with electron back-scattered diffraction technique, and its evolution according to the deformation conditions is characterised. For high deformation temperatures (1000 °C and above), recrystallisation was identified as the main deformation mechanism. Knowledge about the deformation mechanisms of alloy 718, acquired experimentally and from the literature, enables to formulate mathematically a physics-based material model for this alloy. This model includes several parameters which are calibrated using the data obtained from the mechanical tests, as well as values captured by the microstructural analysis. The material model is then validated using additional experimental results from mechanical tests.