EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

A physical-based model is developed to describe the microstructure transformations that occur during the processing of titanium alloys. The evolution of the microstructure during hot deformation is fully coupled with the phenomena of static recrystallisation and grain coarsening that occur during annealing. Static and dynamic recovery and continuous dynamic recrystallisation are considered as restoration mechanisms for β-phase. At the same time, α-phase deforms via crystal rotation, resulting in dynamic globularisation when the α-phase presents an initial lamellar structure. The microstructure evolves via static recovery or static recrystallisation by strain-induced boundary migration during annealing. The starting condition of the substructure plays a vital role in its evolution during deformation. Larger initial β-grain sizes lead to a fast evolution of the fraction of high angle grain boundaries for a given temperature and strain rate. Finer initial β-grains lead to faster annihilation of dislocations due to high angle grain boundary movement and a finer substructure formation. Thus, higher strains are required to achieve a steady-state condition. The microstructure evolves faster via continuous dynamic recrystallisation if the deformation occurs in a fully recrystallised microstructure considering the same initial grain size and the same initial mean boundary misorientation angle. Deformation at higher strain rates leads to higher stored energy, thus higher nucleation rate during static recrystallisation is achieved. A refined microstructure is not easily achieved because higher stored energy also leads to higher growth rates of the recrystallised grains. During heating and cooling, phase transformation is integrated into the model approach using a simple diffusion-based one-dimensional model. Small variations in the cooling rate or the size of primary α-phase lead to significant differences in the final fraction of the different α-morphologies. The integrated modelling tools enable a comprehensive understanding of the several thermomechanical processing steps of typical Ti alloys.