Unstable Behavior of Nano-Polycrystalline and Amorphous Metals under Uniaxial Tension : Local Lattice Instability Analysis
Kisaragi YASHIRO, Masaomi NISHIMURA and Yoshihiro TOMITA
Abstract:Toward the challenging question, ``What dominates an unstable behavior in the stress--strain curve of in-homogeneous system?'', we have performed again molecular dynamics simulations on pseudo-polycrystalline and amorphous nickels under uniaxial tension. Here the nano-polycrystals are made by dividing a cuboidal periodic cell and rotating the crystal orientations of each cubic grain randomly. Stress drops are observed in the stress--strain curves of pseudo-polycrstals with the grain size of 8.76, 5.28 and 3.52nm, while the amorphous doesn't show distinct peak stress under the strain control of ezz=1.0x10-7 or 1.0x10-6[1/fs]. It is also true for the stress control of szz=1.0x10-5 [Gpa/fs] that the polycrystals show flexion points in the stress-strain curves as the onset of unstable strain burst, while the amorphous comes up to the strain burst without showing determinable flexion point in the continuous nonlinear curve. Then the positiveness of the elastic stiffness coefficients for whole the system (global stability) and those for each atom (local stability) are investigated in detail, but we still couldn't find out common tendency for the prediction of the unstable behavior under uniaxial tension. Nevertheless, we have separately evaluated the stress acting on the unstable and stable atoms according to the local stabilities, finding out the following facts: (1) the unstable atoms in the polycrystals, or mainly grain boundary atoms, largely deform and show higher stress than the stable atoms, (2) the stress increase on the unstable atoms seems to saturate at the first transition point in the stress--strain curves of the polycrystals, (3) the unstable atoms in the amorphous show negative residual stress at the initial equilibrium, and (4) the stress increase on the unstable atoms catches up with that on the stable atoms, at the point where the plateau region begins in the stress--strain curve of the amorphous. Key Words:Molecular dynamics, Local lattice instability analysis, Nano-polycrystals, Amorphous, Peak stress