Difference between Ni and Al Unstable Atoms

Abstract

We have attempted to reveal the deformation mechanism of amorphous metal from the unique viewpoint of “local lattice instability” or the positive definiteness of atomic elastic stiffness coefficients, Bαij. In the previous report, we discussed the physical meaning of the “unstable atoms”, or the atoms of detBαij < 0, in the molecular dynamics simulations of a monatomic Ni amorphous. Unstable atoms of Ni show hydrostatic compression at the initial equilibrium. The ratio of detBαij < 0 atoms increases in the later stage of tension/compression, as if the “defect” increase in crystalline metals. We have then compared the mechanical condition of unstable atoms in amorphous Ni and Al. The unstable atoms in Al show the opposite aspect ; they show hydrostatic tension at the initial equilibrium. That is, they have more free volume than stable ones. We have carefully discussed the difference by the stress distribution, Voronoi polyhedra analysis and change in the pair distribution function. Contrary to Ni amorphous, the unstable atoms never increase both under tension and compression, but decrease in the later stage of inelastic deformation. That is, in the Al amorphous, the inelastic deformation is absorbed by unstable → stable transitions : This phenomena may correspond to common image of usage of free volume.