Srikanth Vedantam

The influence of the mode of deformation on recrystallisation behaviour of Ti was studied by experiments and modelling. Ti samples were deformed through torsion and rolling to the same equivalent strain of 0.5. The deformed samples were annealed at different temperatures for different time durations and the recrystallisation kinetics were compared. Recrystallisation is found to be faster in the rolled samples compared to the torsion deformed samples. This is attributed to the differences in stored energy and number of nuclei per unit area in the two modes of deformation. Considering decay in stored energy during recrystallisation, the grain boundary mobility was estimated through a mean field model. The activation energy for recrystallisation obtained from experiments matched with the activation energy for grain boundary migration obtained from mobility calculation. A multi-phase field model (with mobility estimated from the mean field model as a constitutive input) was used to simulate the kinetics, microstructure and texture evolution. The recrystallisation kinetics and grain size distributions obtained from experiments matched reasonably well with the phase field simulations. The recrystallisation texture predicted through phase field simulations compares well with experiments though few additional texture components are present in simulations. This is attributed to the anisotropy in grain boundary mobility, which is not accounted for in the present study.

An anomalous feature in polycrystalline grain growth in metals and ceramics is the rapid abnormal growth of a few grains relative to the surrounding matrix grains. In certain systems, this phenomenon is reported to be associated with the presence of first order phase transitions in the grain boundaries often referred to as complexion transitions. The fundamental question of how the growth advantage of abnormally growing grains persists in this context is not yet well understood. We present a thermodynamically consistent mechanism by which abnormal grain growth may occur. The presence of two complexion transitioned boundaries at a triple junction provides a driving force for partial wetting at the triple junction leading to the formation of a quadruple junction. The subsequent dissociation of the quadruple junction into two partial wetting triple junctions favors the persistence of abnormal grain growth. We incorporate this dissociation process into a multi-order parameter phase field model and demonstrate the occurrence of abnormal grain growth without the need for mobility advantage or additional driving forces. The results from simulations of abnormal grain growth capture experimentally observed microstructural features such as peninsular and island grains. Through a systematic study we show that the initial fraction of complexion boundaries plays a significant role in abnormal grain growth and that maximum abnormal grain growth is observed when the fraction of complexion boundaries is less than about 2%.

Grain boundary engineering involves manipulation of grain boundary character and network to improve the properties of materials. Twin boundaries which form during annealing of low and medium stacking fault energy face centered cubic materials play an important role in grain boundary engineering. In this work, we study the formation, growth, interactions and annihilation of annealing twins using a phase field model. The model incorporates twin formation through growth accidents on grain boundaries and triple junctions. The results indicate that higher stored energy gradients in the microstructure enhance the twin formation and their rate of growth. Also, recovery with a slower rate of stored energy decay increases twin formation events and delays the annihilation of the twins. The influence of the variant selection (based on the axes of misorientation of the twin boundaries with the parent grains) on the nature of their interactions with other twins and coincident site lattice boundaries is examined. The variant selection is found to affect the interactions of twins with other coincident site lattice boundaries thereby influencing the grain boundary character distribution.