Study On Crack Propagation Behavior And Mechanism Of NiTi Alloy Based On Multiscale Method

NiTi alloys are widely accepted to be one of the most promising structural materials due to its unique superelasticity and shape memory properties.To meet the safety and durability requirements in practical applications of NiTi alloys and their structures,it is of important realistic significance for deepening the understanding of the deformation and failure mechanisms and mechanical response of such materials.However,material damage and fracture belong to systematic scientific issues with multiple spatiotemporal scale coupled,and thus it is far from enough to reveal the complexity of material damage evolution behavior from a single scale alone.Consequently,developing a novel method that enables cross-scale characterization and prediction of material properties has become a hot point in the field of fracture mechanics.In this work,the fracture behavior of polycrystalline NiTi alloys under uniaxial tensile load was numerically simulated based on a multiscale method combining molecular dynamics（MD）and cohesive zone model（CZM）.Firstly,MD simulations were performed to reveal the microscopic mechanism of crack propagation from an atomic perspective,and to achieve the traction–separation（T–S）relationship describing the dynamic characteristics of damage evolution in the local region ahead of the crack tip.It needs to be mentioned that the polycrystalline structures were characterized by closed Voronoi tessellation,and cohesive elements were embedded along grain boundaries or within grains as potential crack paths.Finally,the characteristic parameters extracted from the T–S curve were assigned to the cohesive elements to reproduce the transgranular and intergranular fractures of NiTi alloys by finite element method.In this paper,grain size（GS）dependence of cracking performance in polycrystalline NiTi alloys was firstly investigated.MD results showed that the micro pre-crack propagated with brittle characteristics along the[001]symmetric tilt grain boundary.Stress concentration in the front of the crack tip induced martensite transformation,and the HCP and FCC phases were alternately stacked to form a lath-like structure.Interestingly,the martensite laths preferred to the[100]orientation in the original austenite structure.The T–S curve for local fracture region near the crack surface conformed to the bilinear law,and the maximum traction force corresponded to unstable crack growth.Based on the characteristic T–S parameters describing interfacial debonding,the critical stress intensity factors(K_IC)of compact tension specimens with average grain size from 17 to 45μm were predicted through finite element simulations.The K_IC was taken as a characterization of material fracture toughness.It was found that the K_ICof polycrystalline models with different grain sizes was 109.4–130.2 MPa·m^1/2,providing a reasonable agreement with the experimental values.The calculation results presented that the values of K_IC and average energy release rate were positively correlated with grain size.Importantly,the internal toughening mechanism was elucidated from the views of the dependencies of stress-induced phase transformation and crack-path configuration on grain size.According to the strain field analysis,the specimens with larger grain size were found to have wider transformation zones compared with the fine-grained specimens.This result,combined with the relatively high K_IC of NiTi alloy with coarse grain,indicated a phase transformation induced toughening mechanism.In addition,as grain size decreased,the tortuous cracking path became straight,causing the reduction of the threshold driving force for crack propagation.For crystals,the periodicity and density of atomic arrangement vary along different lattice directions,leading to the anisotropy of material properties.Therefore,the effect of initial crystal orientation on the mechanical properties and fracture behavior of NiTi was explored from multiple scales in this work.It was discovered that the situation of micro-cracks propagating along grain boundaries was similar to that of transgranular extension.The stress concentration ahead of the crack tip induced the formation of martensite with a herringbone twin or mixed HCP/OTHER atom morphology,while reverse martensite transformation occurred behind the crack tip due to stress relaxation.Interestingly,typical brittle fracture by cleaving along the high-energy domain boundary,quasi-brittle fracture as a result of the transformation-induced plasticity,as well as ductile fracture caused by a combination of phase transition,stacking fault,and atomic slip were observed in the samples with（^<sub>100）[010],（110）[1^<sub>10],and（^<sub>1^<sub>12）[111]crack orientations,respectively.The distinctive microscopic cracking mechanism for each orientation was revealed in depth by combining the information of fracture characteristics,surface energy,and microstructure evolution.Taking the orientation-dependent T–S parameters as a bridge,the transgranular propagation behavior for macro-crack was reproduced by finite element simulation.The values of K_IC and the damage dissipation energy for each orientation satisfied the relationship of（110）[1^<sub>10]>（^<sub>100）[010]>（^<sub>1^<sub>12）[111].This was consistent with the rule of the critical traction force for unstable crack growth in various orientation at the atomic scale,indicating that the microscopic and macroscopic crack growth behavior were conformable in some ways.The K_IC,which corresponded to the unstable crack growth resistance,i.e.,material fracture toughness,was found to have no direct relationship with the microscale brittle/ductile cracking.The present study reproduced the multiscale fracture performance of NiTi alloys using an atomic-based cohesive zone model,and the influences of grain size and crystal orientation on crack propagation and corresponding microstructure evolution were discussed.The numerical results were in good agreement with published experimental data,highlighting the adequacy of this cross-scale method for studying the crack evolution and predicting material fracture properties.This work are expected to provide a theoretical foundation for deeply understanding the damage and failure laws of NiTi,and further promote material practical engineering application.