Physics of Equilibrium and Non-equilibrium Deformation Processes in the Nickel Surface Layer
Abstract
Based on the Hamilton’s principle, invariants are proposed to describe the processes of formation and evolution of the structure of the metal interface under friction. These invariants can be used, among other things, in the creation, evolution and destruction of nanomaterials.
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Introduction
The mechanisms of hardening and destruction of the surface layer of metals, as well as the conditions for its self-organization under equilibrium deformation are well studied [1, 2], which cannot be said about the self-organization of processes under non-equilibrium deformation under conditions of phase instability of the crystal lattice [3]. In accordance with nonlinear mechanics and mesomechanics, plastic flow in a loaded solid is a multi-level process and is associated with the loss of shear stability at the nano, micro, meso, and macro-scale levels [4]. The intensity of external influence determines the structure, properties and mechanisms of destruction of surface layers of metals. The physics of processes occurring on the metal surface under multi-cycle, low-amplitude and alternating tribo-loading is described in [5-8]. The mechanisms of formation of elements of the defective structure for each of the structural levels of deformation (nano, micro, meso and macro) of the surface layer of Nickel under multicycle low-amplitude and alternating deformation, where the high density of dislocations and the local orientation gradient of structural elements in the Nickel crystal lattice plays a fundamental role at each of the scale levels of deformation [3]. Using various model representations, namely, extinction contours, local curvature of the Nickel crystal lattice, and dislocation contributions to the hardening mechanisms, a quantitative assessment of the values of internal stresses and parameters of the defective structure of the Nickel surface layer was carried out under conditions of its phase instability, where the value of internal stresses is comparable and exceeds the value of the elastic modulus of Nickel ≈ 2∙1011 Pa. The literature sources contain limited data on such highly dispersed materials and their physical and chemical properties [4, 5]. It is generally accepted that the formation of nanoobjects occurs under high-energy influence or intense plastic deformation and / or equal-channel angular compression in the top-down direction, when the material is fragmented from macro, meso, micro to nanoscale as a result of external influence at contact pressures in the GPa [5]. Data on the formation of nanostructures at contact pressures of ≈ 0.1÷0.2 kPa in the presence of chemically active substances under conditions of non-equilibrium deformation are extremely limited.
The question of determining the mechanisms and dominant factors that determine the physical, chemical and mechanical properties (amorphous, superplasticity, catalytic activity, etc.) of the metal interface under non-equilibrium deformation remains open. Determination of the basic laws in the field of equilibrium and nonequilibrium deformation is an urgent problem in the creation of nanomaterials with unique properties.
The aim of this work is to study the mechanisms of plastic deformation at various structural-scale levels under conditions of phase instability of the surface layer of nickel under tribo-loading and to determine the basic laws describing the kinetics of hardening and destruction of the surface layer of metals.
This work presents the results of electron microscopical investigation of surface layer microstructures resulted from sliding friction according to dislocation structure study using the method of ferromagnetic resonance (FMR). It analyzes the micro structural destruction mechanism of nickel on the basis of complex data.
Conclusion
Analysis of the kinetics of the processes of deformation and destruction of scale levels: nano, submicro, micro, meso and macro using boundary conditions applied to the wave equation shows that:
In the area of the equilibrium deformation in the absence of a gradient flow structural defects (fig. 1, area A, region II): the flow of the dislocation density is a constant value directed inward from the surface; the change of local gradient of the orientation of structural elements is a constant value that decreases in accordance with an inversely proportional relationship with increasing distance from the surface;
The intensity of external influence determines the duration of the cycle of changes in strength characteristics, the amount of deformation energy accumulation and the degree of fragmentation of the crystal lattice of metals, and, accordingly, the local gradient of orientation of the boundaries of structural elements, where their geometric size, quantity, density, and interaction determine the dominant role of one or another scale level of plastic deformation at a given time of the kinetics of structure formation, and the mechanism of its destruction in accordance with the minimum potential energy of interaction of the formed structure in the area of equilibrium deformation in the presence of a flow gradient of structural defects: in the area of equilibrium deformation in the presence of a flow gradient of structural defects: the velocity of the dislocation density movement (fig. 1, area B) deep from the surface is a constant value equal to ≈ 1 (m·s) -1 ; the flow of the dislocation density gradient through a unit of area per unit of time is a constant value of ≈ 1 m-4 ;
In non-equilibrium deformation ((fig. 1, region III)): the flow gradient of dislocation density changes its direction; the flow gradient of dislocation density ( X J ) is oscillating in time; an increase in the intensity of the gradient of ascending and descending flows of dislocation density causes an increase in the lower limit of the change in the wear intensity, at least ≈ 7 times, and the upper limit of the change in the intensity of wear by three orders of magnitude at the avalanche selective mechanism of destruction of surface layer; the gradient of the flow of dislocation density (J2) at decrease in the stress value exceeds the gradient of the dislocation density flow (J1) at increase in the stress value by ≈ 1. 7 times.
The principle of least action is implemented, where the selective mechanism of destruction of the porous and amorphous layer determines the removal of foci of discontinuity of the material (microcracks, pores, duplicates, packaging defects, etc.), which determines the preservation of the integrity and continuity of the material.
It has been established that the kinetics of structure formation and evolution of the interface between metals under triboloading proceeds in accordance with the following provisions of nonequilibrium thermodynamics:
Each stable state of the metal interface will have its own structure with a certain value of free energy and, accordingly, with the types of its redistribution between the elements of the boundaries and within the structural formation;
The system tends to occupy a position or form such a structure of the interface, which corresponds to the minimum thermodynamic Gibbs potential;
If the action of load-speed parameters or external influence exceeds some critical value of the energy supplied to the system, then it passes into a new structural state characterized by a lower value of free energy;
The intensity of external influence determines the duration of the cycle of changes in strength characteristics, the amount of deformation energy accumulation and the degree of fragmentation of the crystal lattice of metals, and, accordingly, the local gradient of orientation of the boundaries of structural elements, where their geometric size, quantity, density, and interaction determine the dominant role of one or another scale level of plastic deformation at a given time of the kinetics of structure formation, and the mechanism of its destruction in accordance with the minimum potential energy of interaction of the formed structure.