抄録
We have so far discussed the deformation behavior of inhomogeneous systems, e.g. nano-polycrystalline and amorphous metals, from the unique viewpoint of “local lattice stability” or the positive definiteness of atomic elastic stiffness coefficients, Bαij. In the present study, the physical meaning of the “unstable atoms”, or the atoms of det Bαij < 0, is clarified in the molecular dynamics simulations of a monatomic bulk amorphous of nickel. The criterion of det Bαij < 0 definitely points out the existence of “defects” in the homogeneous random structure. They show hydrostatic compression at the initial equilibrium under σxx = σyy = σzz = 0 ; that is, they find more surrounding atoms than the stable ones. In other words, the unstable atoms have less free volume than stable ones. The ratio of unstable atoms doesn't change in the linear stress–strain response at the early stage of uniaxial tension and compression. The initial negative stress on unstable atoms works as deformation buffer under the early stage of uniaxial tension ; the unstable atoms show higher stress increase than the stable ones. The onset of blunting and plateau region in the stress–strain curve coincides with the point where the stress difference between the stable and unstable atoms vanishes in the tensile direction, and the ratio of unstable atoms begins to increase. Here, the unstable atoms always feel compressive stress in the lateral direction while the stable ones almost zero stress in the plateau region. Thus we can deduce that the stable local configuration crushes in the lateral direction to absorb the elongation in the plateau or steady flow deformation. This mechanism corresponds to the conventional picture of the free volume. It is also true for compression that the ratio of unstable atoms begins to increase at the point where the stress–strain curve deviate from the initial linearity. In the case of compression, the local configuration crushes in the loading direction by the stable → unstable transition, since the unstable atoms always feel higher compression than the stable ones despite of the increase in the number of unstable atoms.
