Depinning Of Dislocations Research Articles

A probabilistic coupled hardening model of double irradiation defects in iron

Irradiation hardening effect is of significance to the design and life extension of nuclear materials. Although various types of defects coexist under in-pile condition, most irradiation hardening models like the dispersed barrier hardening (DBH) model ignore their strength difference and treat them as a single defect type. In this work, the barrier strengths in presence of combined 〈111〉 and 〈100〉 loops in bcc iron are investigated using discrete dislocation dynamics simulations. Considering the coupling effect, the non-periodic distributed effect is preliminarily analyzed. A probabilistic model accounting for their coupled hardening effect is established based on dislocation depinning mechanism from simulation results, which can be also integrated into the modified DBH model using an effective barrier strength factor. The model presents good agreements with experimental results of iron alloy, and exhibits potential applicability in predicting the coupling effects of other defect types, including the dislocation loops and voids.

Mechanism of DSA effect correlating to the macroscopic PLC banding in high-Mn austenitic steel

Mechanism of dynamic strain aging (DSA) effect in high-Mn austenitic steel, and its correlation with the Portevin-Le Chatelier (PLC) banding, was elucidated using the in-situ synchrotron XRD measurement and digital image correlation (DIC) method during tensile test. The average dislocation velocity beyond the PLC band ranged from 10−2 to 10−1 nm/s, while reached to the order of 10° nm/s within the PLC band. In comparison, carbon diffusivity was significantly lower, allowing carbon atoms to diffuse only from 10−7 to10−9 nm in one second. The findings indicate that dislocations beyond the advancing PLC band were pinned by nearby carbon atoms, while dislocations within the PLC band became de-pinned from the carbons. Such localized pinning and de-pinning of dislocations occurred sequentially during deformation, appearing as the propagation of PLC bands in macroscopic scale.

Strengthening from dislocation restructuring and local climb at platelet linear complexions in Al-Cu alloys

Stress-driven segregation at dislocations can lead to structural transitions between different linear complexion states. In this work, we examine how platelet array linear complexions affect dislocation motion and quantify the associated strengthening effect in Al-Cu alloys using atomistic simulations. The presence of platelet complexions leads to the faceting of the dislocations, with nanoscale segments climbing upwards along the platelet growth direction, resulting in a complex configuration that restricts subsequent dislocation motion. Upon deformation, the leading partial dislocation must climb down from the platelet complexions first, followed by a similar sequence at the trailing partial dislocation, in order to overcome the precipitates and commence plastic slip. The dislocation depinning mechanism of linear complexions is strikingly different from traditional precipitation-strengthened alloys, where dislocations overcome obstacles by either shearing through or looping around obstacles. The critical shear stress required to unpin dislocations from platelet complexions is found to be inversely proportional to precipitate spacing, which includes not just the open space (as observed in Orowan bowing) but also the region along the platelet particle where climb occurs. Thus, linear complexions provide a new way to modify dislocation structure directly and improve the mechanical properties of metal alloys.

Obtaining superior damping property of a Cu–Al–Mn shape memory alloy through ultra-high temperature aging

Ultra-high temperature aging was conducted on the Cu-11.9Al-2.5Mn (wt.%) shape memory alloy (SMA) with the aim of damping improvement. The martensite laths were refined, and the controllable precipitation of γ2 phase was realized. As the aging temperature decreased or the aging time prolonged, the amount of γ2 particles increased. The damping of the Cu–Al–Mn SMA measured in this study belongs to non-linear damping, and the damping plateau in the martensite state mainly arises from the sliding of the martensite/martensite interfaces and the stacking faults in martensitic variants. The increased density of the martensite/martensite interfaces caused by grain refinement, the de-pinning of dislocations, and the annihilation of quenched-in vacancies after aging contributed to the increase of damping, whereas the increased density of stacking faults and the pining of γ2 particles on various interfaces leaded to the decrease of damping. The specimen aged at 590 °C for 5 min has the highest damping plateau. Its damping capacity at room temperature is 3.5 times that of the quenched specimen. The effect of aging on the tensile properties was also studied, and based on in-depth microstructural observation, correlated mechanisms were discussed.

Edge dislocation depinning from hydrogen atmosphere in α-iron

Understanding the dislocation motion in hydrogen atmosphere is essential for revealing the hydrogen-related degradation in metallic materials. Atomic simulations were adopted to investigate the interaction between dislocations and hydrogen atoms, where the realistic hydrogen distribution in the vicinity of the dislocation core was emulated from the Grand Canonical Monte Carlo computations. The depinning of edge dislocations in α-Fe at different temperatures and hydrogen concentrations was then studied using Molecular Dynamics simulations. The results revealed that an increase in bulk hydrogen concentration increases the flow stress due to the pinning effect of solute hydrogen. The depinning stress was found to decrease due to the thermal activation of the edge dislocation at higher temperatures. In addition, prediction of the obtained results was performed by an elastic model that can correlate the bulk hydrogen concentration to depinning stress.

Spectrum of dislocation mobility resonances in NaCl crystals under ultralow magnetic fields in EPR scheme

Dislocation motions are studied in NaCl crystals exposed to an EPR impact under crossed ultralow magnetic fields, the Earth field BEarth ∼ 50 µT, and the AC pumping field B̃(ν) of the amplitude 2.5 µT in the frequency band of 6 kHz to 2.1 MHz. Mean dislocation paths form a spectrum of multiple peaks at definite resonance frequencies. Dislocation motions are supposed to be caused by spin-dependent transformation of impurity centers, which, in turn, provide depinning of dislocations and relaxation of their structure. The observed spectrum is attributed to specific features of the hyperfine EPR at the applied field BEarth small as compared with the crystalline local fields Bloc created by nuclei of Cl ligands surrounding the Ca pinning centers. The developed theory well describes peak positions in the observed frequency spectrum.

Decoupling electroplasticity by temporal coordination design of pulse current loading and straining

Multi-mechanism coupled electroplasticity (EP) has not been fully elucidated yet due to the difficulty in eliminating thermal effect, although isothermal, air-cooled, and cryogenic methods are usually used, there are different heating histories from electrically-assisted deformation. The 0–7% pre-strained Ti–6Al–4V alloy is taken as the case material. Based on the two different characteristics of the time-accumulation of thermal effect and the instantaneous nature of the athermal effect, a comparative study is conducted by designing two temporal coordination methods of pulse current and straining, viz., asynchronous loading with high-energy current (ALHC) and synchronous loading with low-energy current (SLLC), where the thermal and athermal effects are highlighted by different loading times, loading and unloading moments of pulse current of different magnitudes. Under ALHC condition, the decreasing yield strength and increasing elongation of the 7% pre-deformed material are observed with increasing current density and loading times, and stress reduction up to 20.7% for yield strength and ductility recovery of maximal 99.1% for elongation are obtained for 14.0 A/mm2 and loading three times. Moreover, stage B of the work hardening rate related to dislocation de-pinning disappears, and the TEM shows an annealing effect that the tangled dislocations are annihilated and rearranged by climbing. Thus, the Joule heat-based thermal effect dominates this condition. Under SLLC condition, the loading and unloading of pulse current of 2.81 A/mm2 in the plastic stage result in an instantaneous drop and rebound in flow stress and work hardening rate (stage B exists), respectively, and are accompanied by the negligible instantaneous temperature rise, while loading at the elastic stage, the overall flow stress and work hardening rate is decreased. It is concluded that the athermal effect can lower the dislocation phonon drag effect and the deformation activation energy, and change the athermal atomic state and dislocation slip mode, which dominates the varied flow stress and elongation. A dislocation dynamics-based physical model is developed by taking into account phonon drag, diffusion activation energy, and deformation activation energy to account for the thermal and athermal effects on dislocations, providing a self-consistent explanation of the above behaviors and mechanisms of the flow stress and ductility under ALHC and SLLC conditions.

Rate-Controlling Microplastic Processes during Plastic Flow in FCC Metals: Origin of the Variation of Strain Rate Sensitivity in Aluminum from 78 to 300 K

The thermodynamic response of dislocation intersections with forest dislocations and other deformation products is recorded using the Eyring rate relation wherein the application of shear stress increases the probability of activation at a given strain rate and temperature. The inverse activation volume, 1/ν, can be directly determined by instantaneous strain-rate change and its dependence on flow stress, τ, defines the strain-rate sensitivity, S, through the Haasen plot slope. A linear slope over a large strain interval is observed even for a heterogeneous distribution of obstacles that could be of more than one type of obstacles encountered by the gliding dislocation. It was deduced that ν and τ at each activation site are coordinated by the internal stress resulting in constant activation work (k/S). The stress changes from down-rate changes become larger than that from up-rate changes due to the formation of weaker obstacles, resulting in a composite S, whereas only forest dislocations are detected by the up-change. The additivity of 1/ν was used to separate obstacle species in specially prepared AA1100 and super-pure aluminum from 78 to 300 K. The deduction that repulsive intersection is the rate-controlling process and creates vacancies at each intersection site depending on temperature was validated by observing the pinning and depinning of dislocations via pipe diffusion above 125 K. A new method to separate S for dislocation-dislocation intersections from the intersections with other obstacles and their temperature dependence is presented and validated.

Lüders strain of the fine-grained material under the electric current

The Hall-Petch-like relationship between the Lüders strain and the average grain size of an Al-5.16Zn-2.2Mg-1.46Cu alloy under the pulsed electric current is investigated. It is found that the Lüders strain increases as the average grain size decreases. In addition, when the average grain size is the same, the current parameter has a significant effect on the absolute value of the slope in the Hall-Petch-like relationship. Based on the characteristics of interaction between the solute atoms and dislocations during the Lüders strain, the competition between pinning behavior of solute atoms and depinning behavior of dislocations under the electric current is explored, and a relevant mathematical model is established. According to the new mathematical model, the difference of the above competitive behavior under two different current densities is analyzed. When the current density is 5 A/mm2, the pinning behavior of solute atoms is dominant in the competition, and the length of the yield plateau of each grain size becomes shorter; however, when the current density is 55 A/mm2, the dislocation depinning behavior dominants the competition and the Lüders strain of each grain size increases. It is found that the current density can affect the competition between the pinning of solute atoms and the depinning of dislocations, thus affecting the Lüders strain.

Effect of high‐density current electropulsing on corrosion cracking of titanium aluminide intermetallic

AbstractThe effect of electropulsing on the corrosion cracking of titanium aluminide produced by self‐propagating high‐temperature synthesis has been investigated. The electropulsing treatment led to improved corrosion resistance in sodium fluoride solution and also eliminated corrosion cracking at the α2/γ interface during corrosion in a solution of nitric and hydrofluoric acids. This behavior was attributed to thermal and athermal effects resulting from electropulsing and leading to the interaction of conduction electrons with the defect structure. The effect of magnetic field accompanying electropusling on depinning of dislocations also has been discussed. Support for this is provided on the basis of X‐ray diffraction analysis and microhardness testing.

Quasi-Static Tensile Behaviors, Mechanisms, and Constitutive Descriptions of Commercially Pure Titanium at Diverse Strain Rates in Ambient Air and Liquid Nitrogen

The quasi-static tensile behaviors, mechanisms and the constitutive descriptions of commercially pure titanium over a strain rate range of 10−4–10−1 s−1 at 77 and 293 K have been investigated and compared by means of OM, SEM, EBSD, and TEM. The results show that the strain hardening rate at all strain rates generally shows a falling-increasing-falling changing tendency at 77 K. Due to the small twin’s density at 293 K, strain hardening rate shows monotone decreasing tendency and generally has much lower values at all strain levels than that at 77 K. Serration behavior can be found in the strain hardening rate curves at low strain rates. The repeated pinning and depinning of dislocations with interstitial atoms could be responsible for this. The elongation at 293 K is proportional to the twin volume fraction in microstructure, which is attributed to the twinning-induced plasticity effect. However, it shows the reverse changing tendency at 77 K. This means that excessive twins may exert detrimental effect on the plasticity of CP Ti. Based on the EBSD and TEM results at the uniformly deformed part of the tensile specimens, it is proved that excessive twins in microstructure could act as obstacles to the movement of dislocations. Won model has been used to describe the constitutive curves under different conditions. It shows good prediction accuracy at 293 K. By contrast, due to the lack of specific consideration of the twinning effect in Won model, its prediction performance at 77 K is ordinary.

Averaging in dislocation mediated plasticity

Starting from the assumption that all possible orientations of grains are equally probable, we evaluate the mean resolved shear stress and strains in polycrystalline bars subjected to axially symmetric tension or compression. The evaluated quantities, in combination with the kinetics of thermally activated dislocation depinning, are used to establish the equation for the stress rate. We then perform a large-scale least-square analysis to identify the physics based parameters of this theory and show that the simulated stress-strain curves for copper, iron and steel agree well with the experiments of Johnson and Cook.

Promoted diffusion mechanism of Fe2.7wt.%Si-Fe10wt.%Si couples under magnetic field by atomic-scale observations

Diffusion behavior of newly designed Fe2.7wt.%Si-Fe10wt.%Si couples at 1100 °C for up to 12 h has been investigated under the 0, 0.8 and 3 T magnetic fields. Diffusion thickness of solid solution layer and weight percent of Si on Fe2.7wt.%Si side increase significantly under a magnetic field. Application of a magnetic field promotes the diffusion of solid solution layer through the possible diffusion of vacancies mainly due to the appearance of defects, which has been demonstrated by the increased dislocation density and broadening of the typical XRD peaks. Replacement of Si sits by Fe atoms in the crystal structure leads to the appearance of Fe diffraction peaks, which has been confirmed by the increased interplanar spacings under a magnetic field. The magnetic field benefits the depinning of dislocations and leads to higher dislocation density because of the magnetoplastic effect which has been confirmed by the significantly reduced thickness of Fe2.7wt.%Si. Nano-sized Fe3Si particles precipitate in the matrix with an orientation relationship on Fe10wt.%Si side as {220}Fe3Si || {220}matrix & < 1–10 >Fe3Si || < 1–10 >matrix. Fe3Si particles pin dislocation moving and lead to higher dislocation density.

Effects of hydrogen on the mechanical response of X80 pipeline steel subject to high strain rate tensile tests

AbstractHydrogen‐induced degradation of X80 pipeline steel was investigated through a high strain rate tensile test (2 × 10−4/s) with interposed unloading, reloading, aging at 30°C, or annealing at 200°C with or without hydrogen charging. The results indicated that plasticity degradation does not occur in the hydrogen‐precharged specimens; however, hydrogen embrittlement occurs in the reloading stage when the specimens are charged with hydrogen in the unloading stage after applying a prestrain. Interposed aging at 30°C or annealing at 200°C can also increase the degradation. It indicates that the hydrogen traps caused by the strain along with hydrogen charging are the major source of dislocations. The formation of a hydrogen atmosphere around mobile dislocations, which is related to the rates of hydrogen diffusion and dislocation movement, plays an important role in the degradation process. Both pinning and depinning of dislocations affect plasticity degradation.

Understanding the mechanisms of electroplasticity from a crystal plasticity perspective

Electroplasticity is defined as the reduction in flow stress of a material undergoing deformation on passing an electrical pulse through it. The lowering of flow stress during electrical pulsing has been attributed to a combination of three mechanisms: softening due to Joule-heating of the material, de-pinning of dislocations from paramagnetic obstacles, and the electron-wind force acting on dislocations. However, there is no consensus in literature regarding the relative magnitudes of the reductions in flow stress resulting from each of these mechanisms. In this paper, we extend a dislocation density based crystal plasticity model to incorporate the mechanisms of electroplasticity and perform simulations where a single electrical pulse is applied during compressive deformation of a polycrystalline FCC material with random texture. We analyze the reductions in flow stress to understand the relative importance of the different mechanisms of electroplasticity and delineate their dependencies on the various parameters related to electrical pulsing and dislocation motion. Our study establishes that the reductions in flow stress are largely due to the mechanisms of de-pinning of dislocations from paramagnetic obstacles and Joule-heating, with their relative dominance determined by the specific choice of crystal plasticity parameters corresponding to the particular material of interest.

Effect of Alternating Magnetic Field on the Fatigue Behaviour of EN8 Steel and 2014-T6 Aluminium Alloy

The application of an alternating magnetic field (0.54 T) was observed to lead to an improvement in the fatigue endurance and an increase in Vickers microhardness and tensile strength of both EN8 steel and AA2014-T6 alloy. Fractography using scanning electron microscopy showed evidence of more ductile fracture features after treatment in contrast to untreated samples. The results of X-ray diffraction indicated formation of more compressive residual stresses following treatment; while examination by transmission electron microscopy showed evidence of fewer dislocations. In the case of the AA2014-T6 alloy; Guinier-Preston (GP) zones were also generated by the alternating magnetic field. However; the temperature increase during the treatment was too low to explain these observations. The results were attributed to the non-thermal effect of the alternating magnetic field treatment that led to depinning and movement of dislocations and secondary precipitation of copper.

Investigation of the electroplastic effect using nanoindentation

A promising approach to deform metallic-intermetallic composite materials is the application of electric current pulses during the deformation process to achieve a lower yield strength and enhanced elongation to fracture. This is known as the electroplastic effect. In this work, a novel setup to study the electroplastic effect during nanoindentation on individual phases and well-defined interfaces was developed. Using a eutectic Al-Al2Cu alloy as a model material, electroplastic nanoindentation results were directly compared with macroscopic electroplastic compression tests. The results of the micro- and macroscopic investigations reveal current induced displacement shifts and stress drops, respectively, with the first displacement shift/stress drop being higher than the subsequent ones. A higher current intensity, higher loading rate and larger pulsing interval all cause increased displacement shifts. This observation, in conjunction with the fact that the first displacement shift is highest, strongly indicates that de-pinning of dislocations from obstacles dominates the mechanical response, rather than solely thermal effects.

Effect of magnetic field on microstructure and mechanical properties of austempered 70Si3MnCr steel

The effect of magnetic field on bainite microstructure and mechanical properties of 70Si3MnCr steel was studied. Microstructure observation shows that introducing magnetic field during the austempering process, the microstructure was significantly refined and the volume fraction of the bainitic ferrite was increased. The high nucleation rate for bainitic ferrite caused by the high magnetostatic energy should be responsible for this positive effect. As compared with samples without magnetic field treatment, the magnetic samples also show a simultaneously increased strength and ductility. The ultra-fine bainitic ferrite with higher dislocation density, in magnetic samples increased the tensile strength, and carbon-riched filmy retained austenite and small blocky retained austenite in magnetic samples have a tremendous effect on ductility. High magnetic strength in magnetic samples increases the strength of the bainitic ferrite, which acts as a stress shielding source to prevent the retained austenitic phase from higher stress and increase the plasticity. Moreover, high magnetic strength can alter the spins of the free radicals between the dislocations and obstacles from S state to T state which had weak bonding with dislocations. Hence, depinning of dislocations is facilitated which results in increased plasticity. Therefore, due to the magnetoplasicty, high magnetic properties, and ultra-fine microstructure, magnetic samples show a more excellent strength and ductility as compared with the normal samples.

The anisotropic kinetics of strain aging in a martensitic steel

ABSTRACTAn experimental study has been conducted to evaluate the effect of static strain aging (bake hardening) on the yield strength and yield strength asymmetry in torsion and torsion/reverse torsion tests on a low-carbon martensitic steel. It is shown that vastly different strain aging responses are observed in the forward and reverse portions of the torsion test following strain aging. While the yield stresses can be described phenomenologically by classic segregation kinetics, the observed asymmetry in the kinetics and magnitude of strain aging call into question the simple interpretation of dislocation de-pinning. This is further emphasised by the fact that strain aging modifies the apparent work hardening rate in the reverse loading tests.

Molecular dynamics investigations of the strengthening of Al-Cu alloys during thermal ageing

Classical molecular dynamics simulations of the interaction of edge dislocations with solid soluted copper atoms and Guinier-Preston zones (I and II) in aluminium are performed using embedded atom method potentials. Hereby, the strengthening mechanism and its modulus are identified for different stages of thermally aged Al-Cu alloys. Critical resolved shear stresses are calculated for different concentrations of solid soluted copper. In case of precipitate strengthening, the Guinier-Preston zone size, its orientation and offset from the dislocation plane are taken as simulation parameters. It is found that in case of solid soluted copper, the critical resolved shear stress is proportional to the copper concentration. In case of the two subsequent aging stages both the dislocation depinning mechanism as well as the depinning stress are highly dependent on the Guinier-Preston zone orientation and to a lesser degree to its size.