Depinning Of Dislocations Research Articles

Local dynamics of topological magnetic defects in the itinerant helimagnet FeGe

Chiral magnetic interactions induce complex spin textures including helical and conical spin spirals, as well as particle-like objects such as magnetic skyrmions and merons. These spin textures are the basis for innovative device paradigms and give rise to exotic topological phenomena, thus being of interest for both applied and fundamental sciences. Present key questions address the dynamics of the spin system and emergent topological defects. Here we analyse the micromagnetic dynamics in the helimagnetic phase of FeGe. By combining magnetic force microscopy, single-spin magnetometry and Landau–Lifschitz–Gilbert simulations we show that the nanoscale dynamics are governed by the depinning and subsequent motion of magnetic edge dislocations. The motion of these topologically stable objects triggers perturbations that can propagate over mesoscopic length scales. The observation of stochastic instabilities in the micromagnetic structure provides insight to the spatio-temporal dynamics of itinerant helimagnets and topological defects, and discloses open challenges regarding their technological usage.

A deformation mechanism based crystal plasticity model of ultrafine-grained/nanocrystalline FCC polycrystals

A new crystal plasticity model based on the deformation mechanism for ultrafine-grained/nanocrystalline face-centered cubic (FCC) metals was developed. The deformation mechanism was that dislocations glide from grain boundary to grain boundary (GB). Constitutive equations on the slip system level were developed based on dislocation glide and all stages of dislocation activities were considered especially their interactions with GB. An Arrhenius type rate equation was established based on the thermally activated depinning of dislocations from GB obstacles. The new constitutive equations were incorporated into a 3D crystal plasticity formulation, and this crystal plasticity model was implemented into a UMAT subroutine of the ABAQUS finite element program. The uniaxial deformation responses of the two ufg/nc materials were simulated. Crystal plasticity finite element method (CPFEM) simulations gave flow stress predictions that were very close to the experimental results. The dislocation mechanism-based crystal plasticity UMAT is ready to be used for more advanced simulation studies.

Quantum Mechanical Tunneling of Dislocations: Quantization and Depinning from Peierls Barrier

Theories of Mott and Weertmann pertaining to quantum mechanical tunneling of dislocations from Peierls barrier in cubic crystals are revisited. Their mathematical calculations about logarithmic creep rate and lattice vibrations as a manifestation of Debye temperature for quantized thermal energy are found correct but they can not ascertain to choose the mass of phonon or “quanta” of lattice vibrations. The quantum mechanical yielding in metals at relatively low temperatures, where Debye temperatures operate, is resolved and the mathematical formulas are presented. The crystal plasticity is studied with stress relaxation curves instead of logarithmic creep rate. With creep rate formulas of Mott and Weertmann, a new formula based on logarithmic profile of stress relaxation curves is proposed which suggests simultaneous quantization of dislocations with their stress, i.e., and depinning of dislocations, i.e., , where is quantum action, σ is the stress, N is the number of dislocations, A is the area and t is the time. The two different interpretations of “quantum length of Peierls barrier”, one based on curvature of space, i.e., yields quantization of Burgers vector and the other based on the curvature of time, i.e., yields depinning of dislocations from Peierls barrier in cubic crystals, are presented. , i.e., the unitary operator on shear modulus yields the variations in the curvature of time due to which simultaneous quantization, and depinning of dislocations occur from Peierls barrier in cubic crystals.

Solute segregation kinetics and dislocation depinning in a binary alloy

Solute segregation kinetics and dislocation depinning in a binary alloy

Mechanism of high pulsed magnetic field treatment of the plasticity of aluminum matrix composites

For aluminum matrix composite, the introduced particles will strengthen the matrix, but as the obstacles, the heterogeneous particles will hinder the dislocation movement, generate uneven material structure, and may become a source of stress concentration. Therefore, they are detrimental severely to the elongation and plasticity of composite. It is known that dislocations exhibit a paramagnetic behavior because they contain paramagnetic centers including localized electrons, holes, triplet excitons, ion radicals, etc. The initial radical pair of the dislocation-obstacle S (spin angular momentum) = ± 1/2 is in a singlet state, and the total spin of the radical pair is 0 and in the antiparallel spin direction, offsetting a magnetism of the radical pair. The magnetic field can change the spin direction from singlet state to triplet state. In the triplet state the electron spin is 1 and in the same spin direction. A strong bond of the dislocation-obstacle is formed only in the singlet state when the spins of the two electrons are antiparallel. So an obstacle is able to pin a dislocation only if the radical pair is in the singlet state. Under the condition of high pulsed magnetic field treatment (HPMFT) the conversion of electronic spin will be a fundamental cause of dislocation motion along a glide plane. The movement of pinned dislocations will change the material microstructure and influence the performance of material. By comparing the microstructural evolutions and the residual stresses of samples subjected to HPMFT with different values of magnetic induced density (B), the positive influence of magnetoplastic effect on the plasticity of aluminum matrix composite is investigated in this paper. The results show that the dislocation density is significantly increased when B changes from 2 T to 4 T. When B=4 T the dislocation density is enhanced by 3.1 times compared with that of the sample without HPMFT. Moreover, the residual stress is reduced apparently from 41 MPa (B=0) to -1 MPa (B=3 T). In the view of atomic scale, the high magnetic field leads to a magnetoplastic effect which contributes to the dislocation movement and promotes the dislocation depinning, thereafter, the number of movable dislocations increases up. From the viewing of the internal structure of composite, the magnetic field accelerates the releasing rate of internal stress and lowers the residual stress in material, which is beneficial to improving the plasticity of aluminum matrix composite.

Size effects in dislocation depinning models for plastic yield

Typically, the plastic yield stress of a sample is determined from a stress–strain curve by defining a yield strain and reading off the stress required to attain it. However, it is not a priori clear that yield strengths of microscale samples measured this way should display the correct finite size scaling. Here we study plastic yield as a depinning transition of a 1 + 1 dimensional interface, and consider how finite size effects depend on the choice of yield strain, as well as the presence of hardening and the strength of elastic coupling. Our results indicate the existence of a crossover length that depends on the yield strain. It is only above this length scale that standard finite size scaling is expected to hold. These results are also expected to be particularly relevant for simulations of single dislocations, such as those used to study strengthening due to included particles.

Inelastic deformation of nanocrystalline Au thin films as a function of temperature and strain rate

The mechanical behavior of nanocrystalline Au thin films with average grain size of 64nm was investigated at strain rates 10−5–10s−1, and temperatures between 298 and 383K. The yield strength was highly sensitive to both temperature and strain rate: at room temperature it increased by ∼100% within the range of applied strain rates, while it decreased by as much as 50% in the given temperature range at each strain rate. The ductility and activation volume trends pointed to two distinct regimes of plastic deformation: namely, creep-driven and dislocation-mediated plasticity, with the transition occurring at increasing strain rate for increasing temperature. The activation volume for creep-influenced deformation increased monotonically from 6.4b3 to 29.5b3 between 298 and 383K, signifying grain boundary (GB) diffusion processes and dislocation-mediated creep, respectively. Dislocation climb, as an accommodation mechanism for GB sliding, provided an explanation for the increased activation volume at higher temperatures. The activation volumes calculated at high strain rates decreased from 19.7b3 to 11.4b3 between 298 and 383K. A model for thermally activated dislocation depinning was applied to explain this abnormal decreasing trend in the activation volume, resulting in activation energy of 1.2eV.

Dislocation pinning by substitutional impurities in an atomic-scale model for Al(Mg) solid solutions

We report our atomic-scale computations for the static depinning threshold of dislocations in Al(Mg) solid solutions. The interaction between the dislocations and the isolated obstacles is studied for different types of obstacle, i.e. single solute atoms situated at different positions, and solute dimers with different bond directions. Part of this work is used to apply different standard analytical theories for solid solution hardening, the predictions of which are finally compared with our direct atomic-scale simulations (AS) for dislocation depinning in random Al(Mg) solid solutions. According to our comparisons, the dislocation statistics in our AS is qualitatively well described by the Mott–Nabarro–Labusch theory. In agreement with earlier results about a different system, namely Ni(Al), the depinning thresholds are similar for the edge and for the screw dislocations.

Magnetoplasticity and magnetic memory in diamagnetic solids

A mechanism for the depinning of dislocations pinned by a stopper is formulated. This mechanism includes the transfer of an electron from a dislocation to the stopper and the appearance of a spin two-electron nanoreactor that has no Coulomb interaction that would hold the dislocation at the stopper in the initial state. The spin dynamics in the nanoreactor is controlled by a magnetic field; therefore, it causes magnetoplasticity and short-term magnetic memory. Another origin of magnetoplasticity is the aggregation of diffusing paramagnetic ions (stoppers) into dimers, trimers, and clusters; this aggregation is also spin-selective and magnetically sensitive. The magnetic-field dependence of the structural evolution of the stoppers provides long-term magnetic memory in diamagnetic solids. Both mechanisms of magnetoplasticity and magnetic memory can coexist and be independent of or dependent on each other.

Thermal dislocation depinning in a deformed crystal

Abstract The temperature dependence of the dislocation mobility threshold is investigated on the basis of a dislocation model suggested by Frenkel–Kontorova. The critical value is obtained for the stress/temperature, corresponding to the dislocation depinning from its equilibrium position. The universal behaviour of a barrier height at finite stress and temperature is revealed and investigated.

Spin effects in nonmagnetic crystals in a magnetic field

The effect of a weak magnetic field on the interaction of a dislocation with a paramagnetic center in nonmagnetic crystals is considered. An alternative mechanism of the transition between the bonding and antibonding states of the valence electrons for the description of the depinning of dislocations in these systems is proposed. It is demonstrated that a significant role in this process is played by the spin-orbit interaction.

Effect of magnetic field on mechanics of nonmagnetic crystals: The nature of magnetoplasticity

The magnetoplastic effect in mechanics of nonmagnetic crystals is attributed to spin evolution in the spin-selective nanoscale reactor created by electron transfer from a dislocation to a stopper. In this “dislocation + stopper” system, dislocation depinning is facilitated because the Coulomb attraction between the dislocation and the stopper is switched off. Since magnetic field stimulates the singlet-to-triplet conversion of the nanoscale reactor (the reverse electron transfer is forbidden), the nanoscale reactor with switched-off Coulomb interaction has a longer lifetime. The resulting increase in depinning rate and dislocation mobility provides a physical explanation for magnetoplasticity.

Mechanochemical reactions between structural defects in magnetic fields

An influence of steady, pulse and microwave magnetic fields on mechanochemical reactions between structural defects in doped ionic crystals has been studied. The effects of dislocation depinning, dislocation mobility increase, macroplastic unhardening and magnetoresonant softening in magnetic fields have been revealed. It is shown that the magnetosensitive spin-dependent reactions between structural defects is the reason of magnetoplastic effects in nonmagnetic crystals. It means that the electron and nuclear spin states of the paramagnetic defects play an important role in the formation of mechanical properties of crystals in magnetic fields and without them.

Edge dislocations in crystal structures considered as traveling waves in discrete models.

The static stress needed to depin a 2D edge dislocation, the lower dynamic stress needed to keep it moving, its velocity, and displacement vector profile are calculated from first principles. We use a simplified discrete model whose far field distortion tensor decays algebraically with distance as in the usual elasticity. Dislocation depinning in the strongly overdamped case (including the effect of fluctuations) is analytically described. N parallel edge dislocations whose average interdislocation distance divided by the Burgers vector of a single dislocation is L>>1 can depin a given one if N=O(L). Then a limiting dislocation density can be defined and calculated in simple cases.

Selective Effect of Weak Magnetic Field on Triglycine Sulfate Crystal

Selective effect of weak permanent magnetic field on nominally pure ferroelectric triglycine sulfate crystal has been observed for the first time. The prolonged changes of permittivity and Curie point were observed after short-term treatment of the crystal in a narrow interval of magnetic field ∼0.1 T . The effect is explained by magnetic field assisted depinning of domain walls from point defect clusters by spin-depended mechanism similar to depinning of dislocations from paramagnetic obstacles in magnetoplasticity. The point defect clusters are assumed to be H-bonded ones.

Amplitude dependent damping in HgI 2 wide band gap semiconductor

Amplitude dependent damping in crystals is considered as one of the methods of mechanical spectroscopy for investigating different mechanisms of dislocation microplasticity. A short review of the amplitude dependent damping studies in semiconductors is presented. A new acousto–optic experimental technique for studying a dislocation point defect interaction developed over the last several years is described. Experimental data of photoacoustic investigations of HgI 2 single crystals obtained in a wide amplitude range at frequencies of longitudinal vibrations of about 100 kHz are reviewed. More recent results are presented as well. Besides, acousto–electric and acousto–optic (for a band of photoluminescence spectrum) effects which result from a high amplitude ultrasonic treatment (HAUT) with the amplitudes from the amplitude dependent damping range illustrate the possibilities of the technique. A model of an electron or hole trapping center which transforms in a dislocation pinning point during the sample illumination is assumed to be the most convenient to explain the experimental data. Under the influence of HAUT, one can destroy (due to dislocation depinning) the center which consists of the dislocation, point defect (it is, surely, a lattice, iodine or mercury, vacancy or interstitial) and the trapped electron or hole. A binding energy between the dislocation and the photosensitive pinning point defect has been estimated by displacement under the HAUT of specific points in the luminescence excitation spectrum. A possible practical use of HAUT for a restoration of performance of γ-ray HgI 2 detectors is briefly discussed.

Effect of a magnetic field on the yield point of NaCl crystals

A constant magnetic field is found to have a substantial effect on the macroplasticity of NaCl crystals when they are being actively strained at a constant rate \(\dot \varepsilon = const\) during magnetic treatment. We have measured the dependence of the yield point σ y on the magnetic induction B=0–0.48 T and the strain rate \(\dot \varepsilon = 1.4 \times 10^{ - 5} - 1.4 \times 10^{ - 3} s^{ - 1}\). It is shown that this magnetic effect has a threshold character and is observed only for B>B c , where B c grows with increasing \(\dot \varepsilon\) as \(\sqrt {\dot \varepsilon }\). The lower the strain rate \(\dot \varepsilon\), the larger the relative decrease in the yield point σ y (B)/σ y (0) at fixed field B>B c . At small enough strain rates \(\dot \varepsilon\) the threshold field B c ceases to depend on \(\dot \varepsilon\) and goes constant. A theoretical model is proposed which is in good agreement with the observed regularities. The model is based on the competition between thermally activated and magnetically stimulated depinning of dislocations from paramagnetic impurity centers.

Creep at low temperatures: unzipping of dislocations, inertia, and criticality processes

We consider creep processes in plastically deformed materials as a critical process such as proposed by Zaitsev (Physica A 189 (1992) 411). We give arguments that the low-temperature creep rate necessarily has two different modes. During the first (fast) stage of stress relaxation, dislocations surmount many barriers simultaneously due to their inertia and a low viscosity of the electron liquid. In the second (slow) stage the depinning of dislocations is mostly due to thermal activation. Our experiments on the mechanical relaxation process in lead alloys and zinc are consistent with the proposed mechanism of depinning of dislocations.

On the nature of the influence of an electric current on the magnetically stimulated microplasticity of Al single crystals

A large increase in dislocation mobility in Al single crystals in a static magnetic field in the absence of mechanical loading of the samples is observed when a dc electric current of low density (105–106 A/m2)is additionally passed through the samples. Apparently, the role of the current reduces to depinning of dislocations from strong pinning centers on the surface of the crystal as a result of surface electromigration of defects. This interpretation is supported by the fact that in samples whose surface is insulated by a layer of lacquer the passage of a current through the volume of the crystal does not change the ordinary dislocation mobility level in a magnetic field. It is hypothesized that surface electromigration of defects, which frees dislocations and unblocks dislocation sources, also plays a key role in the physical mechanism of the long-ago discovered macroplastification of metals upon the passage of an electric current through them.

Influence of the concentration of Ca impurity on the magnetic threshold of the magnetoplastic effect in NaCl crystals

It is found experimentally that the threshold magnetic field Bc for the magnetoplastic effect, i.e., the field at which the depinning of dislocations from paramagnetic impurities in an external magnetic field begins to be observed, increases with increasing concentration C of Ca impurity in NaCl crystals in the range C=(0.5–100) ppm. It is shown that the dependence Bc(C) exhibits a distinct tendency toward saturation. The physical interpretation of the observed dependence rests on the notion that as the impurity concentration C increases, the average size of the impurity complexes increases and, accordingly, the local atomic configuration around the impurity atoms changes according to a definite pattern. In particular, the average number \(\bar n_v \) of cation vacancies among the nearest neighbors increases from 1 to 6 as the number N of Ca atoms in the complex increases, and this trend, in turn, should cause the thermal vibration amplitude of the Ca atoms to increase. In other words, the phenomenon in question appears to be physically analogous in its microscopic mechanisms to the previously observed increase of Bc with increasing temperature. The proposed interpretation is further supported by good correlation of the experimental dependence Bc(C) with the calculated function \(\bar n_c (N)\).