Simulation Study On Tensile Deformation Mechanism Of Nanopolycrystalline Ni-Co Alloy

Nanocrystalline materials have attracted much attention in manufacturing,aerospace and medical devices field because of their excellent mechanical properties and practicability,but their service life is often limited by different factors during service,or by their own structural defects and other characteristics.Therefore,it is very important to explore these different influencing factors and deformation process,and it has become a key step to design the next generation of nanostructured materials from the atomic point of view.The effect of various factors on the mechanical properties of nanocrystalline Ni-Co alloys was deliberated by large-scale molecular dynamics simulation,then the tensile deformation mechanism of gradient crystal NiCo alloys with exclusive nanostructures is simulated and calculated.(1)Nano-polycrystalline Ni-Co alloy has stable structure and good mechanical properties when cobalt content is about 30%;When the systems with different grain size are stretched,it is found that nanocrystals with 4.3 nm diameter are the critical point of Hall-Petch relationship.When d>4.3 nm,the existence of twins and the movement and proliferation of dislocations play an important role.When d<4.3 nm,the reverse Hall-Petch behavior is presented,and the grain rotation and grain boundary migration dominate the plastic deformation;With the increase of loading temperature,the density of partial dislocation decrease,which reduces the strengthening effect caused by dislocation entanglement,and after tensile fracture,the number of holes in the fracture section of the system at high temperature is less but the size of holes is larger,it shows that the resistance of the system to grain growth at low temperature is greater than that at high temperature;The number of twins enhances the strength of nanocrystalline Ni-Co alloys,and twins are also a dislocation source,around which Shockley dislocations and Frank dislocations can be produced.(2)After stretching the nanocrystalline Ni-Co alloy with a gradient of cobalt content at different temperatures,it was found that with the increase of temperature,the yield strength and young's modulus decreased respectively,and with the increase of strain,the dislocations first appear in the gradient area with a large Co content,and then gradually expands to the pure Ni area.Different loading directions have great influence on the deformation of the material.When loading in the vertical gradient direction,the distribution of stress is uniform,however,when loading along the gradient direction,the region with large cobalt content is subjected to greater stress;After stretching the nanocrystalline Ni-Co alloy with grain size gradient at different temperatures,it is found that the failure of the material at high temperature is gradually fracture along the grain boundary,and the failure of the material at low temperature is the crack beginning from the inside of the grain.When loading along the gradient direction,the stress gradually shift from small grain to large grain,while the distribution of stress is more uniform when loading along the vertical gradient direction;After stretching the nanocrystalline Ni-Co alloy with twin gradient at different temperatures along the gradient direction,it is found that the increase of temperature has little effect on plasticity.At high temperature,the ability of the material to resist crack growth increases and the stress concentration is reduced by producing multiple cracks.After stretching along the vertical gradient at different temperatures,it is found that different loading directions do not affect the strength,but the temperature has great influence on plasticity and young's modulus when stretching in the vertical gradient direction.The whole stress distribution in the system is uniform and there is no local stress concentration.