Internal Vibrations of Edge Dislocation Dipoles

A high density of edge dislocation dipolar loops is formed during plastic deformation of crystalline materials, especially under cycling loading [1–3]. During deformation, the loops are clustered becoming main building blocks of the dipolar deformation microstructure (tangles, veins, dipolar walls). The size of the prismatic dipolar loops is typically of the order of 10 nm (in Ni crystals deformed cyclically at room temperatures, the averaged loop height is 4 nm, the averaged length is 60 nm [2, 3]). The leading factor, which influences the rate of clustering and hence the degradation process, is the loop mobility expressed through the drift coefficient. In principle, this coefficient can be decreased by internal vibration of the loops, in analogy to the effect of temperature increase. There is experimental evidence that higher temperature accelerates the clustering process [4]. In this note, small internal vibrations in the slip planes of two long elastically interacting dislocation arms forming a dipole will be studied as forced vibrations due to an oscillating external shear stress. The effect of damping will also be briefly discussed. This approach is similar to the study of vibrations of two partials forming a split dislocation in f.c.c. metals [5] and of internal vibrations of the core of screw dislocations in b.c.c. metals modeled by four partials in [6].


INTRODUCTION
A high density of edge dislocation dipolar loops is formed during plastic deformation of crystalline materials, especially under cycling loading [1][2][3].During deformation, the loops are clustered becoming main building blocks of the dipolar deformation microstructure (tangles, veins, dipolar walls).The size of the prismatic dipolar loops is typically of the order of 10 nm (in Ni crystals deformed cyclically at room temperatures, the averaged loop height is 4 nm, the averaged length is 60 nm [2,3]).The leading factor, which influences the rate of clustering and hence the degradation process, is the loop mobility expressed through the drift coefficient.In principle, this coefficient can be decreased by internal vibration of the loops, in analogy to the effect of temperature increase.There is experimental evidence that higher temperature accelerates the clustering process [4].
In this note, small internal vibrations in the slip planes of two long elastically interacting dislocation arms forming a dipole will be studied as forced vibrations due to an oscillating external shear stress.The effect of damping will also be briefly discussed.This approach is similar to the study of vibrations of two partials forming a split dislocation in f.c.c.metals [5] and of internal vibrations of the core of screw dislocations in b.c.c.metals modeled by four partials in [6].

FORCED VIBRATIONS
An edge dislocation dipole with the dislocation lines in the z-direction (Figure 1) is formed by dislocation 1 in the slip plane y = h/2 with the Burgers vector of length b in the x direction and by dislocation 2 in the slip plane y = −h/2 with Burgers vector b = −b.Length l of the dipole in the z direction will be assumed much larger than the dipole height h.In the equilibrium configuration of the dipole, x = h/2 and x = −h/2, the force components in the slip planes are zero [6].The force components in the direction perpendicular to the slip planes, which may induce the climbing of dislocations, will not be considered.
For changed positions x and x , the interaction force F int from dislocation 2 on 1 is equal to F int = bτ (2,1)   xy and that from 1 on 2 F int = −bτ (2,1)   xy = −F int .The stress component τ xy for an edge dislocation lying in the z-axis is well known [6], 2 , where G is the shear modulus and ν the Poisson's ratio, so that for the displaced dislocations (forces will be given on unit lengths of dislocations), , where Only the vibrations symmetrical to the central y-z plane will be considered (which may be called optical mode), so that x = −x.The deviation of dislocation 1 from the equilibrium position x = h/2 will be denoted as u, x = u + h/2, so that the width of the dipole L is L = h + 2u (Figure 1).For small deviations, u h/2, the equation for force vibrations is reduced to the equation of a standard linear undamped oscillator.For oscillating external shear stress τ ext xy = τ 0 cos ωt, the equation of motion reads as follows:

Research Letters in Materials Science
The amplitude A of harmonic solution u = A cos ωt is The assumption of small deviations is violated when the frequency of external oscillations ω approaches the frequency of self-vibrations ω S : In the last equation the effective mass m of a dislocation per unit length can be estimated [7] as m = ρb 2 ,where ρ is the mass density.For example, for copper, G = 45.5 GPa, Therefore, for the expected values of the typical dipolar loop height, h∼10 nm, the circular self-frequencies are very high, ω S ∼10 11 s −1 , that is, ω S is in the range of 100 GHz, that is, the frequencies f in the range of 10 GHz.

FORCED VIBRATIONS WITH DAMPING
The damping decreases the amplitude accompanied by a phase shift.The main part of the damping force can be taken proportional to the dislocation velocity, where R is the damping constant.The equation of motion modifies to with the solution u = Bcos(ωt − α), where B is the amplitude of damped vibrations and α is the phase shift, The value of the damping constant R for metals like Al and Pb was estimated in [5] to be of the order R ∼ = 10 −6 Nm −2 s (the estimate corresponds to the temperature independent electronic damping, for higher temperature R increases due to the phonon damping).Amplitude B reaches maximum for ω M = (ω 2 S − R 2 /2m 2 ) 1/2 , however, as R/m ≈ 10 9 s −1 , we have ω M ≈ ω S and B max ≈ bτ 0 /Rω S .For the validity of the linearized model, the vibrations have to be small, that is, B max h/2, hence τ 0 hRω S /2b ≈ 2 MPa.

DISCUSSION
According to the presented estimate, the mobility of a typical dipolar loop approximated as the edge dipole of the height ∼10 nm can be facilitated by ultrasound waves in the 100 GHz range of frequencies.The commercial apparatus works with the frequencies up to 10 GHz, using special equipment, the ultrasound frequency 600 GHz was reached in thin foils [7].The main problem is a strong attenuation increase at higher frequencies, especially in metals.Therefore, the effect of ultrasound in the GHz range on dislocation dipole vibrations can be reached in principle, however, only in thin specimens; for example, in fatigued specimens exposed to GHz ultrasound waves, the degradation process could be accelerated.To our knowledge, no experiments of that kind have been done, as a verification of this effect seems to be rather difficult.The contribution of dislocations dipoles to distortion of ultrasound waves was analyzed and measured in [8] at frequency 5 MHz, that is, far below the resonance frequency estimated in the present note.