Establishment Of Center Atom Model And Study Of Nanovoid Healing Mechanism

The study of nanovoid healing in materials is of great importance in the field of material damage and fracture.In the current study,the atomic-level microscopic details of three-dimensional body-centered cubic(BCC)crystalline materials during void healing are not well observed due to the severe experimental environment,while the existing strain characterization methods still have some limitations when used for void healing samples,such as the peak pair approach(PPA)and the geometric phase analysis(GPA)which calculate extremely high strain distributions in the void region that do not match the actual conditions in the region.Therefore,to address these problems and limitations,this paper uses atomic-level simulations to simulate void healing in nanoscale BCC samples by the phase crystal field(PFC)method to observe the evolution process and reveal the healing mechanism.The main work carried out in this paper and the main results and conclusions obtained are as follows:(1)In response to the shortcomings of PPA and GPA,a new method for calculating the elastic strain field,namely center atom model(CAM),is proposed in this paper.This method considers the effect of distortions of the surrounding nearest neighbor atoms on the strain of the center atom by defining standard position vectors in the complete atomic lattices region for strain calculation.The advantage is the ability to correctly reflect the strain fields of polycrystalline and samples with larger missing atomic lattices by rotating the standard position vectors and filling in the ideal atoms,respectively.The strain distribution of defective strain fields of atomic lattices including grain boundary dislocations,polycrystalline boundary orientation differences,void cracks,and 3D lattice dislocations simulated by using CAM for PFC and quantitatively compared with GPA and continuum elasticity(CE)shows that CAM is realistic and reliable for strain calculations,especially in the crack and void regions containing large missing atomic lattices,and reflects the strain fields in the defective regions realistic situation.(2)By using PFC,the void healing process of a BCC structural sample containing a single rectangular void is simulated.It is observed that the voids continuously form a step structure through the raised surface at both ends and generate cavities,resulting in dislocation nucleation and dislocation emission.As the dislocations are emitted and climb to the outer surface of the sample and disappear,and the vacancies are filled with diffuse atoms,the void healing is finally completed.The strain field characterization of the defect area by CAM reveals that the accumulation and release of compressive strain occurs several times on both sides of the voids with the emission of dislocations.The system’s free energy change curve and stress curve reflect the transition from elastic to plastic deformation of the sample after the first dislocation emission of the voids.For the dislocations emitted by the voids,a void healing mechanism model is established.This model reflects that under the action of compressive strain,both sides of the voids,which are regarded as blunt cracks,will change into sharp crack tips through expansion,which makes the stress intensity factor increase on both sides of all voids and reach the critical value required for dislocations emission.(3)PFC was used to simulate the healing process of porous samples.The void healing rate was obtained by comparing the speed of dislocation emission from the voids,and it was found that the central void healed faster than the surrounding voids in the experiment.The strain field analysis using CAM reveals that under vertical compressive strain,the tensile strain region generated by void collapse dissipates from the center of the upper and lower surfaces of the void in the vertical direction to the surrounding area.Due to the dispersion of the strain field,the accumulation of compressive strain in other voids is disturbed,and therefore the dislocation emission rate is reduced.The tensile strain regions generated by the collapse of porous are interconnected and attract the emitted dislocations.The above simulation results have important reference value for the study of void healing.The research results are consistent with the experimental results of molecular dynamics simulation.CAM solves the problem that the existing strain characterization methods cannot correctly reflect the strain field in the defect region,and can be widely used for strain field characterization of complex defects in two and three dimensions.