Phase Field Crystal Modeling Of Interphase Boundary Structure And Migration Mechanism During Diffusional Solid Phase Transformation

Interphase boundaries are the frontier of diffusional solid phase transformations.Interphase boundary structures and migration mechanisms during solid phase transformations can significantly affect the phase transformation processes,and thus the microstructures and the final performances of the product phases.Therefore,the interphase boundary structures and migration mechanisms are key to the solid transformation mechanisms.However,there are many insufficiencies in the field of atomic structures and migration mechanisms of interphase boundaries.On the one hand,based on the perfect lattice model,the crystallographic models have some shortcomings in describing the real atomic structures of interphase boundaries due to the lack of the relaxation process.On the other hand,the detailed interphase boundary structures and migration mechanisms are still poorly understood.The knowledge of the relationship between the atomic structures and migration mechanisms remains unknown.The phase field crystal model,which has advantages in describing phenomena occurring at atomic length scales and diffusional time scales,has emerged as an efficient and mathematically accessible option in the field of phase transformations.In this dissertation,by using the phase field crystal model,we investigated the atomic structures and migration mechanisms of the coherent,semi-coherent and incoherent interphase boundaries.By analyzing the atomic attachments,we tried to give an insight into the interphase boundary migration mechanisms and kinetics.We also studied the influence of grain boundaries on the migration of interphase boundaries.The main conclusions are as follows:(1)We have developed a phase field crystal model to describe order-disorder phase transitions by adding an order parameter to reflect the atomic potential distribution.The atomic potential distribution can represent the basis crystal lattices and superlattices,so it can describe the order-disorder transitions.By constructing the direct correlation function reflecting the effects of basis crystal lattices and superlattices and taking the effect of composition into account,we constructed the free energy functional for the phase field crystal model.(2)We have demonstrated the migration mechanism of coherent interphase boundaries,and decomposition processes of antiphase domain boundaries in the order-disorder transitions.The coherent interphase boundaries migrate based on the ledge mechanism.Because the ledge height is only one lattice parameter,the hindrance from ledge formation is small,which results that the migration of coherent interphase boundaries is controlled by the diffusion of solute atoms.The antiphase domain boundary will decompose into the order/disorder interphase boundary at a high temperature.The decomposition of antiphase domain boundary is by the rearrangement of atoms and diffusion of solute atoms.(3)The microscopic characteristics of the semi-coherent interphase boundary migrations and the influence of interface dislocations on the interphase boundary migration are clarified.Semi-coherent interphase boundaries migrate based on the ledge mechanism with ledge forming and extending.The ledge forming processes are realized by the movements of original dislocations and nucleation of new dislocations,while the ledge extending processes are realized by the motions of new dislocations.Therefore,the barrier of ledge nucleation should be described by the hindrance from the dislocation movements and nucleation.Because the ledge height during the migration process of the N-W orientation relationship is smaller than that of K-S one,resulting in the longer distance of dislocation movements and the larger corresponding hindrances,the migration rate of the semi-coherent interphase boundary under the N-W orientation relationship is slower than that of the K-S one.(4)The existence of 1-dimensional ledge and the 1-dimensional ledge mechanism have been proved from the viewpoint of numerical simulation.On some incoherent but edge-to-edge matching interphase boundaries,the 1-dimensional ledges are regular stripy atom clusters overhanging the IPB with the height of 2 or 3 atomic spacings.The 1-dimensional ledges only exist on some interphase boundaries owning high atomic densities.The interphase boundaries with 1-dimensional ledges migrate based on a 1-dimensional ledge mechanism,while the interphase boundaries without 1-dimensional ledges migrate based on a continuous migration mechanism.(5)By comparing the migration mechanisms of coherent/semi-coherent/incoherent interphase boundaries,we have summarized how interphase boundary structures influence the microstructure and migration mechanisms of interphase boundaries.Interphase boundaries locate on the crystallographic planes owning the lowest interface energy,so the orientations of interphase boundaries are influenced by the interface energy.The migration mechanisms of interphase boundaries can be divided into the ledge mechanism and the continuous mechanism.The energy barriers of interphase boundary migrations can be sorted as the ledge forming of semi-coherent interphase boundary> the ledge forming of coherent interphase boundary> the 1-dimensional ledge forming of incoherent interphase boundary>the continuous migration of incoherent interphase boundary.The atomic structures of interphase boundary affect the interphase boundary migration mechanisms remarkably.The interphase boundaries migrating based on the ledge mechanism own the periodic smallmismatch distributions while those migrating based on the continuous migration lack periodic small-mismatch distributions.(6)The influence of grain boundary on the migration of interphase boundary has been studied.The grain boundary can hinder the migration process of interphase boundary.When the interphase boundary meets the grain boundary,the ledge formation is hindered,and then the migration velocity decreases.When the interphase boundary transfers across some lowangle grain boundaries,the newly generated phase can change its orientation,which results to the vanishing of hindrance.There are large numbers of low-energy atomic clusters around the low-angle grain boundaries,which can reduce the energy barrier of ledge nucleation.