Micropillar Compression Of Single Crystal Cu With Dislocation Density Gradient Structure:Discrete Dislocation Dynamics Simulation

In recent years,a variety of micro-electro-mechanical systems(MEMS)have emerged in an endless stream,and the demand is also increasing,the components of such systems are usually small to micron or even nanometer level.By introducing gradient structure into metal materials,the original single structure can be diversified,which can make a good combination of various properties of materials and provide an excellent way to improve the overall performance of materials.The gradient structures of metal materials include grain size gradient,dislocation density gradient,twin lamellar thickness gradient,solid solution concentration gradient,phase gradient and mixed gradient.The existence of the dislocation density gradient structure was observed in pure Ni torsional deformation experiment,but the study on the deformation mechanism of dislocation density gradient structure was seldom reported.Therefore,it has great significance to study the mechanical properties of materials with a dislocation density gradient structure in academia and engineering.In this paper,the following aspects of research for small-scale Cu single crystal Microcolumns are carried out:(1)A three-dimensional discrete dislocation dynamics simulation method is used to systematically study the mechanical response of Cu single crystal Microcolumns with inhomogeneous dislocation density in this paper,and the deformation mechanism is Taylor hardening.Three gradient models are considered: Uniform dislocation density model,lower dislocation density gradient model and higher dislocation density gradient model.The results show that the stress-strain curve with the dislocation density gradient model has a steeper slope in the elastic stage;dislocation loops generated by dislocation activation quickly reach the sample surface and escape,resulting in no significant increase in dislocation density;In the model with the dislocation density gradient,plastic flow takes place preferentially in the region with the lowest dislocation density;In the plastic deformation stage,stress redistributes from the region with low dislocation density(low local flow stress)to the region with high density(high local flow stress),resulting in a stepped increase of stress in the yield stage,The higher the dislocation density gradient,the more obvious the stepped increase of the stress is.(2)Different dislocation density gradient models under dislocation source activation mechanism are established,and the effects of dislocation density gradient under different plastic deformation mechanisms are compared.It is found that when the plastic deformation mechanism is activated by dislocation source,there is no significant correlation between the dislocation density gradient structure and the critical stress of the model from elasticity to plasticity,and the hardening rate of the three different gradient models is almost 0.It was also observed that only a few or even one dislocation source was activated in all three gradient models.It can be concluded that the effect of dislocation density gradient structure on mechanical properties of materials needs to be considered only when plastic deformation meets Taylor hardening mechanism.When plastic deformation is dislocation source activation mechanism,the effect of dislocation density gradient in materials can be neglected.(3)In order to study the orientation effect of the dislocation density gradient,the mechanical response of the structure model with dislocation density gradient under different loading directions is studied by using the discrete dislocation dynamics simulation method.It is revealed that the mechanical properties of the structure with dislocation density gradient are obviously different under different loading directions.Because the main slip system of the model is different under different loading directions,the model loaded parallel to the dislocation density gradient has higher critical yield stress.It is also found that: when the loading direction is perpendicular to the direction of the dislocation density gradient,the dislocation initiation sequence develops from the lowest dislocation density layer to the higher dislocation density layer one by one,and multiple slip zones will be generated,and the overall plastic deformation will be more uniform.When the loading direction is parallel to the direction of the dislocation density gradient,the dislocation initiation mainly occurs in the middle layer with higher dislocation density,and gradually expands to the adjacent layers.Dislocation slip mainly occurs on a slip surface.