Molecular dynamics simulation of void nucleation, growth and closure of nano-Cu/Al films under cyclic loading

In this paper, molecular dynamics method is used to simulate the evolution mechanism of void nucleation, growth and closure of diffusion-welded copper/aluminum bilayer film under cyclic loading condition with a strain-to-width ratio of <i>R</i> = –1. It is found that under cyclic loading condition, the voids mainly nucleate inside the aluminum side of the copper/aluminum bilayer film, and two kinds of evolution modes of voids I and II are found. The void I nucleates at the position of the gap defect produced by the Kirkendall effect when the copper-aluminum diffuses to form the bilayer film. Under this nucleation mode, after the gap defects have become void, the void moves into the area where copper atoms are relatively dense inside the OTHER structure on the aluminum side. When gaps accumulate to form voids, the voids grow at a fixed position. The void II on the aluminum side nucleates at the position of the gap defect formed by overcoming the stair-rod dislocation and then remains motionless in the process of nucleation, growth and closure. Comparing with the void I, the stress corresponding to the nucleation of void II is large, the growth speed of the void II is fast and the size of the void II is slightly large in the process of strain loading. The void II closure speed is also faster in the strain unloading stage. The two kinds of voids have two common characteristics in the process of nucleation, growth and closure. 1) Both kinds of voids nucleate at the position of the gap defect inside OTHER structure on the aluminum side. 2) In 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 first 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 two kinds of voids are first compressed into ellipsoidal shape along the strain loading direction, and then disappear 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 position where the voids disappearred, but the nucleation of voids at other position of gap defect forms inside the other structure located on the aluminum side.