| During the manufacturing and serving process of materials, crystal structuresand its transformations determine the mechanical, physical and chemical propertiesof the materials to a large extent. Phase field crystal (PFC) model not only inherits itsmerit of describing the atoms, but also elongates the time scale to diffusive scale viatime-averaging technique, making it a suitable tool in the study of grain boundaryand interphase interfaces.And the O-lattice theory, as a comprehensiecrystallographic model, can provide us a great support on revealing thecrystallographic features associated with the crystal structures and its transformation.In this paper, studies on the crystal structure and its transformation are carried outfrom both the static structure analysis and dynamic microstrucuture evolution byintegrating the PFC model and O lattice theory.Based on the PFC model, multiple aspects of interfaces, including the grainboundary evolution process under the application of external strain, the interactionsbetween spinodal decomposition and grain boundaries, the preferred orientationselection process during solidification and the corresponding morphologicalevolution, and the nucleation and growth process occuring in the hexagonal to squarephase transformation, are investigated.According to our results, the symmetric small angle grain boundarycomposedby hexogonal crystalscan be regarded as one set of dislocations whose Burgersvectors are perpendicular to the grain boundary. The mechanism of grain boundarymovement can be described as: dislocation climbing along grain boundary,dislocation dissociation and dislocation annihilation with opposite signs. Thedecrease of temperature, increase of misorientation and application of external stressstrain along the grain boundary will delay grain boundary movement.In order to lower the total strain energy arising from the mismatch along thecoherent interfaces during spinodal decomposition, grain boundaries will move correspondingly. As the misorientation increasing, the stability of grain boundary isrelatively improved, and the distribution of phase domains becomes more uniform;As the composition of alloy increasing, the stress field around the interphaseboundary becomes stronger, and so is the interaction with the grain boundary, whichresults in that the stability of grain boundary during phase transformation isweakened, and the distribution of phase domains becomes irregular; As thetemperature increasing, the interphase boundary becomes wider and the strength ofits stress field is lowered, which results in that the stability of grain boundary isrelatively increased, and the distribution of phase domains becomes regular.Under different driving forces of phase transformation, the morphologyevolution of hexognal and square crystals differs, but their interface migrationmechanisms are in a good accordance with each other. As the driving force of phasetransformation increases, the interface migration mechanisms are both changed fromlayer-by-layer growth mode to muti-layer growth mode, but the morphologyevolutions have their own trends: for square phase crystal, its morphology changesfrom quadrilaterals to four-fold symmetry dendrites, and for hexagonal phase crystal,its morphology changes from hexagon to six-fold symmetry dendrites.For the nucleation process in the structural transformation from hexagonal tosquare phase, we have developed a new method by combining the PFC model andfree-end nudged elastic band approach, which is capable of conducting a quatitativeanalysis of critical nucleation. For the growth process in the phase transformation,the orientation relationship and the inclination of habit planes have been identified byO lattice theory and simulation results, characterized as [10]S//[10]T,15°and15°regarding to [10]Tdirection, respectively. In the last, based on the interface structureanalysis and its evolution process, an interface migration mechanism regarding tostructural ledge structure is proposed. |
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