The elastic limit of a crystal can be evaluated by the positiveness of elastic stiffness coefficients, B ijkl. We had demonstrated that the nucleation of lattice defects such as a dislocation and cleavage cracking can be predicted by the atomic B ijkl at each atom point. The amorphous metals and bulk metallic glasses draw intense interest whether the criteria is applicable or not since they are regarded as " ultimate " of lattice defects. In the present study, an amorphous Ni-Al binary alloy is made by a usual melt-quench simulation and subjected to tension by means of molecular dynamics simulation. During the simulations, the positiveness of atomic B ijkl is discussed for all atoms. Contrary to a Ni-Al crystal, many atoms show negative value even in the initial equilibrium of the amorphous before loading. These" unstable "atoms turn out to be the non-clustered atom or the outer-shell of the local cluster such as 12(0,0,12,0) icosahedron. On the other hand, the center atoms of the local clusters show high stability resulting in the positive B ijkl of the whole system. It is also demonstrated that the change in the atomic B ijkl can reveal the collapse and re-configuration of local clusters during the deformation.
In order to discuss the origin of the buckling of carbon nanotubes from the atomic level, we have performed the compressive simulation of non-defective and defective triplewalled carbon nanotubes (TWCNT) by the molecular dynamics method using the adaptive intermolecular reactive empirical bond order potential, and observed changes in atomic stresses until the buckling. In the non-defective TWCNT, standard deviations of atomic axial stresses rise drastically before the buckling. The transition from homogeneous stress distributions to inhomogeneous ones play an important role in the occurrence of the buckling. In TWCNTs with a vacancy-type defect or a Stone-Wales defect, buckling stresses differ according to location of the defect. Repulsive interlayer interactions caused by the constriction of the outer layer including a defect reduce significantly the buckling strength. On the other hand, constrictions of the middle or inner layer including a defect produce slightly attractive interaction with the outer layer. Therefore, whole layers is buckled at the same time as the buckling of the outer non-defective layers. TWCNTs including many defects that are generated by the heat treatment simulation show smaller buckling stresses than that of TWCNTs including a defect. Defect configurations have a significant influence on distribution of atomic stresses until the buckling. The buckling occurs from constriction parts located close to defects.
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