Molecular Dynamics Simulation Of Micro-crack Propagation In Single Crystal And Poly-crystal Titanium

Titanium metal has excellent mechanical properties and is mainly used in the manufacture of bearing components.Therefore,it is particularly important to prevent component failure,and crack propagation is an important link in the occurrence of fracture.In the process of material preparation,micro-cracks will occur on the surface or in the interior of titanium metal.These defects will grow and grow during the service of parts,burying hidden dangers for Parts' fracture.However,the pre-propagation of cracks is very micro,so it is difficult to predict and analyze them effectively by traditional simulation and experimental methods.In this paper,molecular dynamics numerical simulation is used to study the propagation of different types of cracks under different environmental factors and loads.The propagation mechanism of microcracks in single crystal and polycrystalline titanium is revealed at nanoscale.From the microscopic point of view,the atoms at the crack tip will become active and disordered under the action of stress concentration.The spacing between atoms at the crack tip increases,the bond elongation and the energy increase.When the energy absorbed by the atom bond breaks through the critical state,the interaction between atoms at the crack tip will be broken,leading to the unstable propagation of the crack.Crack propagation and dislocation emission occur alternately at the crack tip.Atoms around the dislocation have higher potential energy.There is a competitive relationship between dislocation emission and propagation and crack growth.In the study of cracks in single crystal titanium,it is found that the direction of loading will determine whether the crack propagates into voids or necking healing;the crack propagates earlier under high strain rate loading,and when the model reaches the same strain value,the lattice phase transformation ratio caused by high strain rate is higher and the material damage is more serious;the ambient temperature has obvious influence on the frequency of crack emission dislocation and the growth density of dislocation.The symmetrical load of tension and compression is very friendly to material protection,and the lower the frequency,the more stable the crack morphology.In the simulation study of polycrystalline titanium,it is found that crack brittle cleavage occurs at low temperature.The main reason for material brittleness at low temperature is the decrease of atomic bonding force at grain boundaries,which leads to the crack path.At high temperature,the toughness of polycrystalline titanium is enhanced,the grain boundary effect is weakened,and the crack plastic propagation occurs,and the crack evolves into a hole.The passivation hindered crack tip will turn through the emission dislocation and continue to cleavage along the crystal defects caused by the dislocation.Based on molecular dynamics,the effects of loading properties,strain rate,frequency and direction,and ambient temperature on crack growth are discussed.The internal relationship between crystal behavior such as dislocation,grain boundary and lattice phase transition and crack growth is also discussed.At the same time,the effect of cracks on the internal energy,volume strain and other physical quantities of materials is revealed,and the propagation mechanism of cracks in titanium crystals is summarized,so as to improve the understanding of the mechanical behavior of titanium materials with cracks from the micro-scale.