A Nanoscale Study Of Nucleation And Propagation Of The Zener Cracks:Phase Field Crystal Model

Fracture failure of materials may lead to severe component damage and even major life safety accidents.Material fracture is often caused by nucleation,expansion,and connectivity of nanocracks.Therefore,it is very necessary to study the mechanism of nanocrack nucleation in materials.At present,the research on crack nucleation and propagation mainly focuses on the macro and micro scales.The reason is that it is difficult to capture the dynamic evolution process of nanoscopic defects in the experimental process.Therefore,an advanced phase field crystal(PFC)model is used to study the nucleation evolution of Zener cracks in the dislocation region.The innovative work of this paper is mainly reflected in:(1)Based on the PFC model,a Multiple Peak algorithm(MPA)for PFC samples is proposed.This method can overcome the calculation mode that the traditional strain calculation method needs to track the trajectory of the atom,so it is very suitable for the calculation of the strain field of nano-cracks in PFC model samples;(2)The PFC model is used to observe the new mechanism of the effect of disclination-dislocation structure on nanocrack nucleation.The main results of the study are as follows:(1)Under the effect of external strain,nucleation and initiation of Zener cracks will occur at the grain boundary dislocations;(2)The propagation of Zener cracks is accompanied by the formation of disclination and disclination dipole,the intensity of the disclination is approximately equal to the angle of the angle of crack tip;(3)The tensile strain caused by the negative disclination in the region around the crack is conducive to the nucleation and propagation of the Zener crack;(4)The crystal columns passing through the Zener crack is deflected at the surfaces of the crack under the action of strain,and the deflection angle in the middle of the crack is the largest;(5)The MPA method is used to calculate the local strain field of the two-dimensional PFC sample containing single Zener crack and double Zener cracks,the complete strain field of the two-dimensional PFC sample containing grain boundary dislocations or grain boundary cracks.It is found that there is a strong tensile strain at the tip of the Zener crack,while the atomic area on the surface of the crack has a strong compressive strain due to the extrusion of surrounding atoms.The complete strain field of the two-dimensional PFC sample containing grain boundary dislocations or grain boundary cracks is also calculated.It is found that the strain field at the grain boundary dislocation pair is butterfly-shaped.The direction of the half atomic planes of the dislocations determines the orientations of the butterfly-shaped strain field.The MPA method proposed in this paper has very good adaptability and accuracy,and provides a new and reliable method for the strain field calculation of PFC samples.The PFC simulation experiment in this paper studies the causes,influencing factors and behavior characteristics of Zener cracks nucleation and propagation at the grain boundary dislocations.On the basis of conforming to the nucleation mechanism of the original Zener crack,a new mechanism of nucleation and propagation of the Zener nanocrack at grain boundary is found.Through strain field analysis,the effect form and mechanism of dislocations,disclinations and other defects on the nucleation and propagation of Zener cracks are explained.