Local Lattice Instability of Amorphous Metals under Tension: A Molecular Dynamics Study
Kisaragi YASHIRO, Masaomi NISHIMURA and Yoshihiro TOMITA
Abstract:In order to clarify the deformation behavior ofbulk metallic glasses from atomistic viewpoint, an amorphous Ni-Al binary alloy is made and subjected to tension by means of molecular dynamics simulation. The initial equilibrium is made by a melt-quench simulation of a single crystal in which Ni or Al atoms are randomly arranged on the fcc lattice point. Both the radial distribution and Voronoi analysis attests that the initial equilibrium is in amorphous state. The amorphous metal is then subjected to tension with the strain rate of 1.0x109/s under periodic boundary condition, showing nonlinear stress-strain curve from the initial stage of loading. The curve shows the peak stress of about 3.0GPa at the strain of 0.07. After the peak the stress slightly decreases and the metal deforms with constant stress of about 2.4GPa. There is no remarkable change in the radial distribution function and Voronoi polyhedron during the deformation so that the metal maintains the short-range order of amorphous structure. Then we have calculated the elastic stiffness matrix, which is evaluated by the stress and elastic coefficients, at each atom point during tension to evaluate the local stability. Contrary to the crystal, many atoms show negative value even in the initial equilibrium of amorphous metal. These "unstable" atoms turn out to be in the outer-shell of the local cluster such as i0,0,12,0j icosahedron or in the boundary of the local clusters. On the other hand, the center atoms of the local clusters show high stability, resulting in the system stability of about 2x1012GPa6. It is also demonstrated that the change in the local stability reveals the collapse or reconfiguration of local clusters. Key Words:Amorphous metal, Bulk metallic glasses, Local lattice instability, Elastic stiffness coefficients, Molecular dynamics