Abstract
Understanding the interaction between complex thermal fields and metallic structures at the meso-scale is crucial for the prediction of microstructural evolution during thermomechanical processing. The competitive growth of crystal grains, driven by thermodynamic forces at the grain boundaries, is one of the most fundamental phenomena in metallurgy and solid state physics. The presence of second phase particles, which act as pinning sites for boundaries, drastically alters the coarsening behaviour of the system; particularly when considering that these particles have different thermal properties to the primary phase. In this work a multi-phase field model, incorporating thermal gradient and curvature driving forces, is used to predict grain growth in a Ti6Al4V alloy system with second phase particle inclusions representative of oxide and carbide precipitates. The multi-phase field framework is fully coupled to the heat equation. The incorporation of the thermal gradient driving force enables the detailed behaviour of the grain boundaries around the particles to be predicted. It is shown that the inclusion of particles with a lower thermal conductivity has a significant influence on the coarsening behaviour of various systems of grains, due to the combined effects of thermal shielding and the generation of thermal gradient driving forces between the boundaries and pinning particles.
Highlights
It is known that strong thermal gradients drive extended defects, such as grain boundaries and voids, to migrate in preferential directions[13,14,15]
Www.nature.com/scientificreports driving force is an important factor in determining the coarsening behaviour in the vicinity of high energy density sources of heat, such as electron-beam, electric-arc and laser welding heat sources, all of which are utilised in advanced manufacturing processes[2]
Given that the temperature field induced in a material is the primary mechanism by which competitive crystal growth is initiated, and that the thermal gradient driving force is an important contribution in determining boundary migration kinetics, it is clear that second phase particles present in the microstructure with heterogeneous thermal properties relative to the primary phase will strongly perturb the induced local temperature field and generate additional thermal gradients influencing the microstructural evolution behaviour
Summary
It is known that strong thermal gradients drive extended defects, such as grain boundaries and voids, to migrate in preferential directions[13,14,15]. Given that the temperature field induced in a material is the primary mechanism by which competitive crystal growth is initiated, and that the thermal gradient driving force is an important contribution in determining boundary migration kinetics, it is clear that second phase particles present in the microstructure with heterogeneous thermal properties relative to the primary phase will strongly perturb the induced local temperature field and generate additional thermal gradients influencing the microstructural evolution behaviour. In this work a multi-phase field formulation for the prediction of grain coarsening due to local boundary curvature and thermal gradient driving forces, is fully coupled to the heat equation This framework permits the prediction of grain boundary evolution, in the vicinity of high energy density sources of heat, with various distributions of second phase particles with homogeneous and heterogeneous thermal properties relative to the primary metallic phase. Various scenarios are investigated using the model, with key insights gleaned from the framework regarding the migration kinetics of single grain boundaries, and grain boundary networks, over second phase particles with heterogeneous thermal properties
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