Instability Of Ideal Crystal Lattice As A Reason Of Solids Plasticity

L.Kozak

Thermodynamics Department, Institute of Mechanical Engineering, Ivano-Frankivsk National Technical Oil and Gas University, 15 Carpathian st., 76019, Ivano-Frankivsk, (UKRAINE)

PAGE NO: 91-100

ABSTRACT – DOI : https://dx.doi.org/10.47204/EMSR.1.2.2020.091-100

The paper proposes to consider the phenomenon of plasticity, as a result of the instability of a crystal ideal lattice relative to shear deformations. According to the classical model, a ideal crystal lattice is stable, and its plasticity is due to the presence of local unstable areas – dislocations, in it.

During crystallization a stable crystal lattice is formed. When temperature decreases the crystal lattice of plastic bodies becomes unstable. Instability is the cause of both polymorphic transformations and plasticity.

Instability is due to compression of the crystal lattice with decreasing of temperature. It is shown that the reason of compression is a spherically symmetric long-range potential of interatomic interaction. Under such a potential, there are forces of attraction between the distant atoms, which are balanced by the forces of repulsion between the near atoms. Such a lattice is unstable.

The instability of the crystal lattice means that the atoms are in a position that does not correspond to their minimal energy. In this position atoms are kept by a surface layer which structure and properties differ from the crystal’s internal areas. Such crystal is in a state of unstable equilibrium.

In this crystal, the shear of the atomic planes takes place at very low stresses and in the absence of dislocations. The low shear stresses are stipulated by the fact that internal atoms shear to a position with lower potential energy. Energy is spent only on the displacement of surface atoms.

For the verification of the proposed model, given are experimental data and the result of theoretical studies carried out earlier on a two-dimensional crystal lattice by using the method of molecular mechanics.