Phase Field Crystal Study Of Dislocaton Configuration Evolution And Crystal Structure Transition

The plastic deformation of a crystalline material is mainly affected by its dislocation motion and crystal structure transformation.Recent studies have found many new dislocation extension configurations,which have attracted extensive attention in the academic community,and related research and exploration have been carried out.The Phase Field Crystal（PFC）method based on density generalized function theory is an advanced numerical simulation method for materials with atomic-level resolution scale.Currently,the PFC method has been widely used to simulate the structural evolution of material nanoscopic defects.In this paper,the PFC method is applied to study the dislocation configurations evolution of small-angle bicrystal materials with Body-Centered Cubic（BCC）and its effect on the structural transition of BCC to Faceted-Centered Cubic（FCC）under strain.By studying its dislocation extension configuration evolution,dislocation strain field distribution,stress-strain relationship and free energy change,the mechanism of dislocation extension and dislocation proliferation and the mechanism of dislocation proliferation-induced BCC-FCC structural transformation are revealed.The main conclusions are as follows:1.Under the tensile strain,the grain boundary dislocation a[100]extends,and its extension width increases with the increase of external strain.Once the dislocation extension width reaches a critical value,the grain boundary extension dislocation a[100]dissociates and proliferates:(α[100]→(α[100]+(α[（?）00]+(α[100].Under compressive strain,vacancy plates and new dislocations are nucleated at grain boundary dislocations to relax the compressive strain and coordinate the deformation between grains.The proliferating dislocations form subgrain boundaries due to collective slip of dislocations and grain boundary migration occurs.The new grains grow by engulfing the old grains as the subgrain boundaries migrate,and the growth rate is consistent with the Orowan relationship.2.The energy model of the system is established by the dislocation linear elasticity theory,and the energy changeΔE of the dislocation configuration evolution and the strain energyΔUs compete with each other.Under the action of external strain,when the dislocation configuration extends,the energy consumptionΔE is less than the increase ofΔUs,and the total energy W of the system increases.When the energy of the system reaches a maximum value,the energy of the system is destabilized and dislocation proliferation and dislocation slip annihilation occur.This process consumes more energyΔE than the increase ofΔUs,releases the accumulated strain energy,and the energy of the system W decreases.3.Under the action of y-axis tensile and x-axis compressive strain,grain boundary dislocations of BCC bicrystal extend and dissociation.The FCC phase nucleates on the dislocation slip band due to strain instability caused by the proliferation of new dislocations,and grows with the slip of those.The external strain causes the BCC lattice to generate the FCC lattice through Bain distortion,and the shear force generated by the slip of dislocations causes the lattice to rotate and generate the habit plane.The orientation relationship of this crystal structure transition is consistent with the Nishiyama-Wassermann（N-W）orientation relationship:[001]_BCC//[011]_FCC,（110）_BCC//（11（?））_FCC.4.The BCC-FCC structure transition process of the bicrystal system is divided into four stages.The first stage is the elastic deformation stage of the BCC phase.The BCC phase structure remains unchanged,and the free energy of the system rises rapidly.The second stage is the plastic deformation stage dominated by dislocation slip.The dislocation expands,decomposes and slips,and the number of randomly arranged atoms around the defect increases,causing the strain energy of the system to rise.The third stage is the plastic deformation stage dominated by the structural transformation of BCC-FCC.After dislocation proliferation,the FCC phase grows rapidly as the dislocation slip motion engulfs the BCC phase and defect atoms,releasing the accumulated strain energy.The dislocation proliferation has a facilitating effect on the structural transformation.The fourth stage is the FCC phase elastic stage,where the BCC phase structure disappears and the system transforms into the FCC phase bicrystal structure.In this paper,the continuum theory and the atomic-scale PFC method are applied,and the above results and conclusions are consistent with the relevant theoretical and experimental results.This has a certain guiding role in the study of dislocation structure configuration evolution and crystal structure transformation,and is also of great significance for understanding the plastic deformation behavior of materials and the design of material structures.