Molecular Dynamics Simulation On Void Evolution Of Nano-Cu/Al Bilayer Films Under Cyclic Loading And Uniaxial Tension

Cu/Al bilayer films is favored due to high corrosion resistance,excellent electrical and thermal conductivity and high strength.It has broad application prospects in micro electro-mechanical system/nano electro-mechanical system.The void defects often occur during Cu/Al dissimilar metal joining or service.The void defects have a serious impact on the properties of the material.In addition,the growth and aggregation of the void may cause crack initiation and lead to premature fracture failure of the material.The study of the internal void of Cu/Al bilayer films contributes to the better performance of Cu/Al bilayer films in practical applications.In this paper,Molecular dynamics method were used to study the void nucleation,growth and closure evolution under cyclic loading.The evolution process of internal void growth to material fracture failure under uniaxial tension without void and voidcontaining Cu/Al bilayer films is compared.The effects of different size void on the mechanical properties of Cu/Al bilayer films is also investigated.The following main conclusions are drawn:(1)Cyclic loading simulation experiments on Cu/Al bilayer films under cyclic loading conditions with strain ratio R=-1: It is found that under cyclic loading conditions,the void mainly nucleates inside the Al side of Cu/Al bilayer films,and two kinds evolution modes of voids I and II are found.The void I nucleates at the position of the gap defects produced by the Kirkendall effect when the Cu-Al diffuses to form the bilayer films.Under this nucleation mode,after the gap defects have become void,the void moves to the area where Cu atoms are relatively dense inside the OTHER structure on the Al side.When gap accumulate to form void,the void grow at a fixed position.The void II on the Al side nucleates at the position of the gap defects formed by overcoming of the stair-rod dislocation and then remains motionless in the process of nucleation,growth and closure.Compared with the void I,the stress corresponding to the nucleation of void II is larger,the growth speed of the void II is faster and the size of the void II is slightly larger in the process of strain loading.The void II closure speed is also faster in the strain unloading stage.(2)The two kinds of voids have two common characteristics in the process of nucleation,growth and closure.Both kinds of voids nucleate at the position of the gap defects inside OTHER structure on the Al side.During the process of voids growth and closure,both kinds of voids have the same shape changes.In the void growth stage,both kinds of voids firstly grow along the strain loading direction,then expand in the direction perpendicular to the strain loading direction,and finally,the shapes of two kinds of voids tend to become spherical.In the stage of void closure,the shapes of two kinds of voids are firstly compressed into ellipsoidal shape along the strain loading direction,and then disappears from both ends of the void to the center of the void in the direction perpendicular to the strain loading direction.In the subsequent cyclic loading process,none of new voids appears again at the disappearing position of the voids,but the nucleation of voids at other the position of gap defects form inside the OTHER structure located on the Al side.(3)Tensile comparison of Cu/Al bilayer films void-containing and without void: It is found that when Cu/Al bilayer films without voids reaches the maximum tensile strength,the void nucleates in the Al side OTHER structure of the gap defects region,and the nucleation and emission of dislocations near the void increase the size of the void.When the stress of Cu/Al bilayer films containing the void reaches the yield strength,the interface position of the void first has dislocation nucleation and emission.The void expand toward the Al side during the growth process,and the void size of the Cu side decreases with the increase of the strain.Finally,the void expands to the material boundary on the Al side and causes the Cu/Al bilayer films to break.The common feature of both is that the cavity expands and grows under the stacking fault of the {111} plane.In addition,the void expands to the Al side in the direction of strain stretching in the process of growth and expansion to the material fracture failure.The strain stretching phase extends perpendicular to the film boundary.(4)Uniaxial tensile study of Cu/Al bilayer films with different size void: The results show that Cu/Al bilayer films with different size void have similar elastic stages,but in the plastic deformation stage,Cu/Al bilayer films with void radius less than 1.45 nm has more complex flow stress.In addition,the elastic modulus,yield strength and tensile strength of Cu/Al bilayer films tend to decrease with increasing void size.