Molecular Dynamics Simulation Study On Microstructure Optimization And Deformation Mechanism Of Gradient Nano-metal Ni With High Strength And Toughness

With the widespread application of nano-materials in aerospace,micro-nano electronic devices,and intelligent medical equipment,the "inverted" relationship between strength and plasticity in traditional nano-crystalline metals severely restricts their application and development.Many studies have found that the plastic deformation of metallic materials can be significantly improved by constructing gradient nanostructures.Therefore,studying relationship between the mechanical behavior and deformation mechanism in gradient nanostructure metal materials is of great practical significance for the development and application of new materials with high strength and high toughness.However,the molecular dynamics research on gradient nanostructured materials at present stage has not fully discussed the effects on grain size and twin spacing brought by the transition zone,and there are few studies for indepth analysis of the micro-mechanisms within the gradient structure.In this paper,molecular dynamics simulation methods were used to analyze in depth the effects of factors such as grain size gradient,twin spacing gradient on the mechanical behavior and deformation mechanism of gradient nano-structure metal Ni.The main conclusions are as follows:(1)The spatial distribution gradient of grain size affects the mechanical behavior and deformation mechanism of gradient nano-grained(GNG)Ni.When the distribution of grain size gradient satisfies a linear relationship(i.e.gradient rate n=1),the GNG structure exhibits the optimal strength-plasticity synergy effect.In addition,the toughening mechanism in the optimal GNG structure was further explored.The results show that in different gradient structures,the stress gradient and strain gradient of GNG Ni with a gradient rate of n=1 are the highest,and the optimal synergy effect of strength-plasticity is attributed to the significant enhancement of dislocations and the reduction of grain boundary activity.(2)The spatial distribution of twin spacing affects the mechanical behavior and deformation mechanism of gradient nano-twinned(GNT)Ni.As the average twin spacing decreases,the mechanical behavior shifts from strengthening to softening.When the average twin spacing is λ=1.88 nm,the GNT Ni exhibits the weakest strain localization phenomenon,and grain growth and boundary migration are significantly inhibited,resulting in additional strengthening.It is also found that during the deformation process of the GNT structure,the stress and strain show a weak gradient distribution,which limits the improvement of the strength and plasticity compared to uniform nano-twinned.(3)The spatial distribution gradient of both grain size and twin spacing gradients affects the mechanical behavior and deformation mechanism of double gradient nano-twinned(DGNT)Ni.The DGNT Ni with dual gradients exhibits higher plasticity than the GNG and GNT structures.The toughening mechanism in the double gradient structure was further explored,and the results showed that the stress and strain gradients in DGNT Ni were larger than those in the other two structures.During deformation,a large number of cross slip dislocations appeared in the DGNT Ni,which stimulated multiple slip systems and caused dislocation pileup.In addition,the phenomenon of twinning was continuously weakened in small twin spacing,leading to a strengthening effect.This study elucidates the relationship between gradient distribution,mechanical properties,and deformation mechanisms,providing a theoretical basis for the design,development,and optimization of novel nano-structured materials.