Simulation Of Mechanical Behavior Of Gradient Nanocrystalline Nickel-cobalt Alloys

Gradient nanostructured metals have become one of t he research hotspots in the field of material science because of their excellent mechanical properties and unique deformation behavior.At present,most gradient nanostructured metals are obtained by surface plastic deformation,whose microstructure size c hanges from surface nanoscale to core micron,and shows high strength,good plasticity and work-hardening properties.In this study,the gradient structure was introduced into the nano-twin structure,and a series of nano-gradient structure models with different structural gradients and different gradient sequences were designed and prepared.The effects of gradient structures on the shear properties,macro and micro deformation behavior of polycrystalline Ni-Co alloys were studied,and the strengthening mechanism was revealed.(1)In the nanocrystalline Ni-Co alloy with concentration gradient,the deformation mechanism of each region is different with the increase of Co content.The results show that the mechanism of grain boundary diffusion and dislocation slip is dominant in the cobalt-free region,while there is a synergistic effect of solid solution strengthening in other regions.In addition,the migration of GB atoms to form new small grains is conducive to the deformation of large grains,and the allo y shows a gentle and stable plastic deformation.At the same time,although the dislocation density is different in different regions,the dislocation density remains stable before and after shear.Compared with the uniform structure,the flow stress fluctuation of the concentration gradient structure during plastic deformation is smaller,indicating that the structure is more stable.(2)The grain gradient structure will improve the strength of the alloy to a certain extent while maintaining its toughness.The microscopic deformation process and dislocation motion are also obviously different from the uniform structure.Through the zoning study of each grain size part of the gradient structure,it is found that the deformation mechanism of different grain p arts is different,and the phenomena in the shear process are also different.The mutual influence and cooperation of various parts in the deformation process is shown macroscopically to improve the strength while ensuring the toughness and stability of th e material.At low temperature,the higher the dislocation density,the higher the dislocation entanglement and more defects such as stacking faults.Although there are fewer atoms in the grain boundary,the diffusion kinetic energy is low,and the dislocation movement is hindered by the grain boundary.On the contrary,at high temperature,the dislocations and defects in the crystal are greatly reduced,and the disordered diffusion of GB atoms in the crystal reduces the grain boundary strength,which is beneficial to the slip of dislocations.In addition,the number of grain boundary atoms increases and diffuses in the crystal,which indirectly destroys the internal stability of the grain and increases the deformation ability of the material.(3)The gradient nano-twin Ni-Co alloy has elastic deformation at the initial stage of shear,and there is no obvious yield process,then work-hardening occurs,and the stress decreases after reaching the stress limit,and the uniform plastic deformation is maintained at the later stage.The excellent stability of the ultra-fine eutectic layer and its effective hindrance to the dislocation nucleation and movement can improve the strength of the material.The moderate spacing twin structure dominates the plasticity and toughness of the material to prevent obvious work hardening to increase the brittleness of the material.In addition,at low temperature,the nano-twin gradient Ni-Co alloy is prone to yield phenomenon and work hardening,and the stress-strain curve fluctuates obviously.The dislocation density at low temperature is much higher than that at high temperature,and the number of dislocations increases greatly in the later stage of shear,and the dislocation lines become longer and become entangled gradually.The phenomenon of work hardening is more obvious at low temperature,and with the increase of strain,the dislocation network structure near the grain boundary gradually disappears and forms chaotic entanglement.The alloy is prone to plastic deformation at h igh temperature,and the strength decreases macroscopically.The higher the temperature,the more difficult it is to yield,which shows a uniform plastic deformation process as a whole.In addition,the high temperature will aggravate the disappearance of the twin structure and destroy the length of the twin,and the grain boundary atoms diffuse into the grain,but there is no phase transition.