Molecular Dynamics Simulations Of Deformation Mechanism For Nanocrystalline Cu/Ni Structure

Cu/Ni materials have attracted considerable attention owing to their outstandingmechanical properties such as corrosion resistance, anti-fouling performance, excellentelectrocatalytic activity, good wear resistance and high tensile strength. In fact, in recent yearsmany researchers performed intensive studies on the magnetic properties, hardness, Young’smodulus of nanocrystalline Cu/Ni systems, which can be tailored by varying the percentage ofNi composition. However, owing to the difficult to identify and characterize the process ofimpact loading, the ductile failure micromechanisms of nanocrystalline metals resulting fromgrain boundary sliding, dislocation pileups and void nucleation are not clearly understood. Inthis manuscript, molecular dynamics simulations are carried out to investigate the propertiesof void evolution, the deformation behaviors and mechanical properties of nanocrystalline Cuand Cu/Ni materials under conditions of tensile strain for the purpose of predicting anddesigning material features. The following main researches can be drawn:Firstly, the investigation of nanocrystalline Cu samples with and without a pre-existingvoid under both uniaxial and triaxial tensile straining were performed to study theirmechanical properties. The nucleation and aggregation of voids in a simulation systemwithout a pre-existing void was similar to the formation and gathering of clouds; whereas, inthe case of a sample with a pre-existing void, few voids emerged beside the pre-existing void,at which dislocations were preferentially nucleated. In the process of deformation bycontinuous straining, the tip at the pre-existing void became blunt, however, no nucleation andpropagation of crack could be observed even though the specimens were highly stressed. Ithas been found that the presence of a pre-existing void greatly influenced the mechanicalproperties of nanocrystalline materials.Secondly, the method of molecular dynamics has been carried out to simulate thedeformation behavior and mechanical properties of nanocrystalline Cu/Ni films under thecondition of tensile strain at different strain rates. The results indicate that Cu/Ni films arecharacterized by relatively high yield strength and strain rate sensitivity (m) at high strainrates. The nucleation of voids in Cu/Ni films is observed at the strain rate of108s-1, whereasthe spallation in nanocrystalline Cu films is observed at the strain rate of1010s-1. Thesignificant changes of the FCC, HCP and OTHER atomic groups occur in both Cu and Nifilms at higher strain rates. However, obvious structural changes can only be observed in theCu films under the condition of tensile strain at low strain rates. The simulation results alsoshow that the increase of strain rates contributes to the formation of HCP structure, but that the increase of the number of disordered atomic groups may impede the growth of HCPatomic groups if the strain rate crosses a certain threshold.Thirdly, the method of molecular dynamics has been carried out to simulate thedeformation behavior and mechanical properties of homogeneous and gradient nanocrystalCu/Ni alloys under the condition of tensile strain. The results indicate that the increase of Niwould lead to the high Young’s modulus of nanocrystal Cu/Ni alloy. For the monocrystalCu/Ni alloy, the strength decrease slightly with the low level of Ni content and increasegradually as the content of Ni increasing. While the strength of nanocrystalline Cu/Ni alloysincreasing with the content of Ni increasing. The simulation results also show that theincrease of the gradient of Ni contributes to the obvious low strength of monocrystal Cu/Nialloy and indistinctive high strength of nanocrystalline Cu/Ni alloy when the models underconditions of uniaxial tensile strain in the gradient direction. The increase of the gradientvalue of Ni content contributes to the strength decrease of monocrystal and nanocrystallineCu/Ni alloys when the strain loading is perpendicular to the gradient direction, but thestrength will increase if the gradient value crosses a certain threshold.