Dislocation Intersections Research Articles

Atomistic insight into the unique mechanism of helium management at FCC{111}//{111}FCC semi-coherent interfaces

Interfaces are effective traps for implanted helium (He), and research on the He interactions with interfaces contributes to our comprehension of the role of interface structure in controlling and mitigating the deleterious effects of He. In this study, we investigate He interactions with three types of face-centered cubic (FCC){111}//{111}FCC semi-coherent interfaces: Cu/Ni, Cu/Pd, and Cu/Ag {111} semi-coherent interfaces. Utilizing molecular dynamics simulations, we demonstrate that He atoms exhibit a tendency to segregate at these semi-coherent interfaces. Notably, the presence of new dislocation loops, accompanied by extrinsic stacking faults, is observed at these semi-coherent interfaces as the concentration of He atoms increases continuously. Moreover, this unique structural evolution of interfaces leads to the dissociation of misfit dislocation intersections, thereby significantly augmenting the nucleation sites for He atoms and improving the He managements at these interfaces. These findings provide insights into the interaction between He atoms and FCC{111}//{111}FCC semi-coherent interfaces, enriching our current understanding of He management at such interfaces.

Hot deformation behavior of commercial pure vanadium

The hot deformation behavior of pure vanadium was studied by uniaxial compression tests carried out in vacuum to a true strain of 0.6 in the temperature range of 800 to 1400 °C and strain rate range of 3 × 10−3 to 10 s−1. Strain rate sensitivity was calculated from the compression test data and mapped out in contour plots to determine the optimum regime of hot workability of vanadium. A high strain rate sensitivity of about 0.3 was obtained in the temperature and strain rate range of 1100 to 1400 °C and 3 × 10−3 to 1 s−1 for pure vanadium. Microstructures of the deformed samples showed features of dynamic recrystallization within the high strain rate sensitivity domain. Dynamic abnormal grain growth was observed at 1400 °C and strain rates of 0.1 and 1 s−1. The apparent activation energy of deformation was found to be 244 kJ·mol−1 and the stress exponent was about 3.4. The high values of activation volume suggest the barriers related to the dislocation intersection based process as the main rate controlling mechanism during the hot deformation of vanadium. The 〈100〉 and 〈111〉 fiber texture components evolved during the hot deformation of pure vanadium.

Kink bands promote exceptional fracture resistance in a NbTaTiHf refractory medium-entropy alloy.

Single-phase body-centered cubic (bcc) refractory medium- or high-entropy alloys can retain compressive strength at elevated temperatures but suffer from extremely low tensile ductility and fracture toughness. We examined the strength and fracture toughness of a bcc refractory alloy, NbTaTiHf, from 77 to 1473 kelvin. This alloy's behavior differed from that of comparable systems by having fracture toughness over 253 MPa·m1/2, which we attribute to a dynamic competition between screw and edge dislocations in controlling the plasticity at a crack tip. Whereas the glide and intersection of screw and mixed dislocations promotes strain hardening controlling uniform deformation, the coordinated slip of <111> edge dislocations with {110} and {112} glide planes prolongs nonuniform strain through formation of kink bands. These bands suppress strain hardening by reorienting microscale bands of the crystal along directions of higher resolved shear stress and continually nucleate to accommodate localized strain and distribute damage away from a crack tip.

Enhanced mobility of dislocation network nodes and its effect on dislocation multiplication and strain hardening

Understanding plastic deformation of crystals in terms of the fundamental physics of dislocations has remained a grand challenge in materials science for decades. To overcome this, the Discrete Dislocation Dynamics (DDD) method has been developed, but its lack of atomistic resolution leaves open the possibility that certain key mechanisms may be overlooked. By comparing large-scale Molecular Dynamics (MD) with DDD simulations performed under identical conditions we uncover significant discrepancies in the predicted strength and microstructure evolution in BCC crystals under high-strain rate conditions. These are traced to unexpected behaviors of dislocation network nodes forming at dislocation intersections, that can move in ways not previously anticipated as revealed by MD. Once these newfound freedoms of nodal motion are incorporated, DDD simulations begin to closely match plastic evolution observed in MD. This additional mechanism of motion whereby non-screw dislocations can change their glide plane profoundly affects fundamental processes of dislocation multiplication, recovery and storage that define strength of metals.

Decoding stress-strain parameters in FCC metals using digital constitutive analyses to devolve dynamic obstacle-strength factor and diffuse necking

Simulation codes use constitutive relations of work hardening to virtually predict shape change during metal forming. Recent analysis has shown that the modified Hollomon-type relation correlates to stress-aided thermal activation at obstacles for dislocation movement. The sweeping action of dislocation gives rise to strain and the dislocation intersections to strain rate sensitivity during work-hardening. The constitutive relation analyses (CRA) encompass fitting parameters which remain constant with strain and its validation is the precision to replicate the measured stress-strain diagram. In this study, digital constitutive analyses (DCA) are examined whereby the modelled-fit parameters are simultaneously numerically adjusted as strain proceeds. For bulk properties such as expended work, volume fraction of vacancy creation and mean slip velocity, CRA predictions have been validated. However, DCA enables the identification of defects being created with strain using derived obstacle-strength factor (αγ). The changes in mechanisms during work-hardening can be decoded using the αγ – γ plot whereby constant αγ indicate steady-state deformation and its rapid decrease, the start of diffuse necking. Thus, αγ is a composite factor of defects being continuously created whereas conventional α is a measure of the stored work up to that strain. The tensile data from polycrystalline super-pure aluminum tested at 78 K were used to validate the derived relations which were applied to the DCA of age-hardenable aluminum alloys tested at 298 K. The present work shows that integral replication of stress-strain diagram is essential, but a differential analysis is required to devolve the creation of crystal defects with strain.

Plasticity of solid solution Al-Mn and Al-Mn-Si alloys

The influence of Mn and Si on the mechanical properties of solid solution Al-Mn and Al-Mn-Si alloys have been studied at 4 K, 78 K, and 298 K. The solute strengthening potency of the yield strength increases with decreasing temperature. The addition of Si stimulates the precipitation of Mn-containing dispersoids and decreases the solubility of Mn in the Al matrix, reducing the strengthening effect. The alloys exhibit Stage III of declining work hardening rate with the flow stress during the plastic flow at three temperatures. The mean slip length Λ in the work hardening model of Nabarro, Basinski, and Holt (NBH) evaluated from σΘ curves correlates with the scale of the microstructure. The strain rate sensitivity (SRS) measurements conducted at 78 K show positive SRS of Al and the alloys. At 298 K, one observes a change in SRS due to the change of the mechanism from solute-dislocation to dislocation-dislocation dominated interactions. Mn and Si decrease SRS at 298 K. Al-Mn-Si alloys exhibit negative SRS and jerk flow at a sufficiently low strain rate at 298 K. Analysis of the apparent activation volume, distance, and work has been conducted at 78 K and 298 K to understand the rate-controlling mechanisms. The results indicate that dislocation intersections are rate-limiting processes, but the atomistic mechanism of dislocation cutting varies vastly depending on the temperature and alloy composition. It is suggested that at 78 K, forest obstacles are passed by the formation of jogs and sequential activation of individual partials without making constriction to the perfect defect. At 298 K, the cutting involves constriction, the creation of jogs, and the creation of point defects at each intersection site. The calculated activation work and distance provide signatures of these processes.

Insights on grain boundary effects on crack formation and propagation in Nb3Sn coatings at low temperature and high strain rates: a molecular dynamics simulation study

Mechanical behavior of Nb3Sn coatings is inherently correlated with the anti-interference ability of the superconducting cavity system and the amplitude and phase stability of the cavity field. We report on atomic-scale simulation and analysis of grain boundary effects on the crack formation and propagation in Nb3Sn coatings at low temperature and high strain rates. For tensile-deformed single crystal Nb3Sn, accompanied by the occurrence of slips, micro voids propagate along the () and (210) planes (stretching the crystal along the [100] direction), and subsequently coalesce, leading to the cracks formation. For tensile-deformed single-crystal Nb3Sn coating on Nb substrate, it fails by crack propagation in the coating before showing significant yielding plateaus. In the Nb3Sn coating, the slip bands on the (201) and () planes meet and merge, inducing the breaking of atomic bonds and the formation of voids at the slip band intersections, and the subsequent micro-crack initiation. The misfit dislocations of Nb3Sn/Nb heterostructure interface and the dislocations nucleated on slip systems control the plastic deformation of the Nb substrate, causing the BCC-to-FCC and FCC-to-HCP phase transformation in the substrate, for which, ductile fracture occurs after necking. For polycrystalline Nb3Sn coating on Nb substrate, the simulations demonstrate that cracks propagate entirely along grain boundaries, exhibiting the intergranular fracture characteristics. The composite exhibits a significant strain rate effect. At high strain rates, crack propagation across grain boundaries in Nb3Sn is hindered, dislocations are emitted into the both adjacent grains from the grain boundary facets, they glide across the grain, and voids are nucleated at the intersections of dislocations inside the grains, which leads to the formation of voids and ultimately the occurrence of trans-crystalline fracture. The findings provide important information about the crack evolution in Nb3Sn coatings, which are critical to the fabrication and performance analysis of Nb3Sn superconducting radio frequency cavities.

Work-hardening ability enhancement of lightweight high entropy alloys via adding yttrium

Lightweight high-entropy alloys (LHEAs) with body-centered-cubic (BCC) structure have attracted wide attention due to their considerable strength-ductility synergy. However, the lack of work hardening ability is a key weakness, limiting the engineering application. In this study, appropriate contents of rare-earth element yttrium (Y) were added to the as-cast Ti-Zr-V-Nb-Al LHEAs with single BCC structure, and the microstructure and work hardening behavior of these LHEAs were investigated. Benefiting from the addition of Y, these LHEAs generated slight of Y-rich and Y-O-rich phases both in the grains and on the grain boundaries. During plastic deformation, these precipitates can effectively prevent dislocation slip and promote the dislocation intersections and reactions, which can improve the work-hardening ability. Moreover, adding Y can promote the large constitutional supercooling of the LHEAs and refine the grain size of the LHEAs, which also improve the work-hardening ability. Finally, the work hardening of as-cast Ti-Zr-V-Nb-Al LHEAs were successfully turned from negative to positive with 0.2 at% Y addition, with only few ductility loss. Our work provides an effective means of optimizing the mechanical properties, especially the work hardening ability, of as-cast BCC LHEAs.

Deformation behaviors and the related high-temperature mechanical properties of Mg–11Gd–5Y–2Zn–0.7Zr via regulating extrusion temperatures

The effects of extrusion temperatures (430, 450, 470 and 490 °C) on the microstructures and mechanical properties of Mg–11Gd–5Y–2Zn–0.7Zr (GW) alloys at room temperature (RT), 250, 300, 350 °C were systematically investigated. The dynamic precipitation of blocky and lamellar long period stacking ordered (LPSO) phases was taken place during hot extrusion. The dominant texture changed from <10 1¯0 >//ED to <0001>//ED with increasing extrusion temperature. Tensile results indicated that the GW alloy extruded at 430 °C (GW430) exhibited the highest mechanical properties at both RT and elevated temperatures, which was ascribed to the fine dynamic recrystallized (DRXed) grains, un-DRXed grains with an orientation of <10 1¯0 >//ED, and the LPSO phase strengthening. When tensile tested below 300 °C, slips on the blocky LPSO phase and kinks of lamellar LPSO phase were the primary deformation modes, followed by grain boundary crack. At 350 °C, grain boundaries became the vulnerable sites to be activated. For GW alloys extruded at 450, 470 and 490 °C, an abnormal increase of mechanical properties was observed at 300 °C. This was due to the occurrence of multiple-slip, which showed the intersection and entanglement of dislocations and thus improving the strength of Mg matrix.

Low temperature plasticity of microcrystalline Al–Li alloy

The plastic deformation of Al-3.8 at.% Li polycrystals processed by the equal-channel angular hydroextrusion (ECAH) were investigated during tension in the temperature range of 4.2–400 K. The evolution of the microstructure during loading was studied by the electron backscatter diffraction (EBSD) method, including the orientation and kernel average misorientation (KAM) mappings. In as-prepared state the microstructure characterized by grains with small misorientation angles and a high density of dislocations. The deformation of a sample at 120 K leads to increase in the density of deformation defects while at 290 K to their decrease. The strong temperature sensitivity of the yield strength indicates the thermally activated nature of the plastic deformation. An increase in the plasticity and strength of polycrystals with decreasing temperature is explained by a decrease in the dislocation annihilation due to a decrease in the mobility of atoms, resulting in increase in the strain hardening rate. A nonmonotonic dependence of the activation volume Va(T) with a maximum at 180 K was established from stress relaxation data. The analysis of stress-strain curves and microstructure evolution indicate a dominance of thermally activated mechanism of intersection of the forest dislocations below 180 K and an increase in the activity of recovery processes at moderate temperatures.

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.

Work hardening behavior and fracture mechanisms of Fe-18Mn-1.3C-2Cr low-density steel castings with varying proportions of aluminum alloying

The alloying of high-manganese steel with aluminum produces wear-resistant casting components with greater yield strength and significantly reduced density, which facilitates considerable energy-savings and service life extension in cement, mining, and construction operations. However, the strengthening mechanism of Al alloying responsible for the increasing yield strength, and how the fracture mechanism influences the work hardening behavior of these castings require further clarification. The present work addresses these issues by evaluating the work hardening behavior and fracture mechanisms of casting Fe–18Mn-1.3C–2Cr steels with Al alloying concentrations of 0, 4, 7, and 11 wt%. The results reveal that work hardening is facilitated by twinning-dislocation cell intersection in the absence of Al, by deformation band-planar slipping dislocation intersections et al. concentrations of 4 and 7 wt%, and by the precipitation of κ carbides at an Al concentration of 11 wt%. Furthermore, the tensile fracture mode increasingly varied from ductile fracture at 0 Al wt.% to brittle rupture at 11 Al wt.%, which was also accompanied by large voids and cleavages.

Assessment of activation volume in 304L SS at high temperatures

The variation in apparent and true activation volumes of 304L SS with true plastic strain has been evaluated at three temperatures (973 K, 1073 K and 1123 K) over the strain rates of 3 × 10−3 s−1, 3 × 10−4 s−1 and 3 × 10−5 s−1. The instantaneous strain rate sensitivity parameter was derived from strain-rate jump tests across different strain levels to estimate apparent activation volume. The values of apparent activation volume were higher at 973 K than at other temperatures. In contrast to the apparent activation volume, the estimated values of true activation volume using internal-stress-based kinetic rate law systematically increase with the increasing temperature from 973 K to 1123 K. The higher flow stress with a larger strain hardening rate at 973 K leads to a lower true activation volume in the steel compared to other temperatures. The model outcomes revealed that the instantaneous strain rate jump-up or jump-down leads to the change in the internal and effective stresses. The observed variations in geometrically necessary dislocation density (GND) and intra-grain reference orientation deviation (GROD) in the substructure substantiated the predicted variation in the internal stress level upon strain rate jump up/down. The true activation volume was estimated for a fixed value of mobile dislocation density (ρm = 5 × 1011 m−2) and for varying mobile dislocation density. Irrespective of ρm values, the estimated true activation volume values for 304L SS were in the order of 102 b3 to 103 b3, thereby implying that the intersection of dislocations coupled with the climb of jogged dislocations is the dominant mechanism during tensile deformation at high temperatures. Based on the analysis using the kinetic rate law with a fixed value of mobile dislocation density, it has been realized that the multiplication of effective stress and true activation volume remains constant across the plastic strain levels for the given combination of strain rate and temperature.

Characterization of the hydroextruded oxygen-free copper via microindentation in the temperature range 77–300 K

The micromechanical properties of annealed coarse-grained (CG, average grain size of 1–10 μm) and ultrafine-grained (UFG) oxygen-free copper have been studied in the temperature range of 77–300 K. UFG samples were obtained in two ways: by direct hydroextrusion with a change in the diameter of the billet from 50 mm to 13 mm (DE, average grain size of 0.6 μm) and as a result of equal channel angular hydroextrusion of the billet with a diameter of 13 mm (ECAE, average grain size of 0.5 μm). The microhardness at the periphery of the extruded billets was 25% less than in the center. The maximum hardening as a result of extrusion expressed as the ratio of the microhardness of the UFG samples to the microhardness of the annealed CG sample was approximately 2 and 2.3 for the DE and ECAE cases, respectively. The temperature dependence of the microhardness of CG and UFG copper indicates that, upon indentation, plastic flow at temperatures of 77–300 K occurs without a change in the mechanism which is evidently the intersection of mobile dislocations with statistically stored (as forest) dislocations, and small values of the activation volume coincide with their high density. It is shown that the UFG structure of copper is stable; the micromechanical characteristics remain unchanged during long-term storage of extruded samples at room temperature. The results of indentation tests are compared with the data on the tensile deformation of similar samples. The correlation between microhardness and yield strength of DE and ECAE copper in the temperature range 77–300 K is considered.

Сейсмические исследования тектонического нарушения в зоне сочленения Кандалакшско-Двинского рифта с Архангельским выступом

The results of clarifying the deep Severodvinsk fault zone location are presented and its structure in the upper part of the section using shallow seismic survey by shear-wave reflection method and extensional reflected waves method using a system of multiple overlaps (CDP method) is considered. The site under study is located in the Kholmogorsky tectonic knot area (north of the EEP) and represents a zone of tectonic dislocation intersection. In the upper part of the section, a fracture zone was found accompanying this deep fault, in with water infiltration, discharge of reservoir gas fluids; karst was detected in carbonate rocks. This zone can be attributed to seismogenic, which implies a repetition of weak seismicity manifestation on the Arkhangelsk horst complicated by tectonic faults.

The strain rate sensitive flow stresses and constitutive equations of a selective-laser-melt and an annealed-rolled 316L stainless steel: A comparative study

The strain rate dependent compressive flow stresses of a Selective-Laser-Melt 316L (SLM-316L) alloy and a commercial (annealed-extruded) 316L (C-316L) alloy were determined, for comparison, between 1x10-4 and ∼2500 s-1 and between 1x10-4 and ∼2800 s-1, respectively. The Johnson and Cook flow stress material model parameters of both alloys were also determined. The microstructural examinations of the deformed cross-sections of tested specimens (interrupted tests) showed a twinning-induced-plasticity in SLM-316L alloy and a martensitic transformation-induced-plasticity in C-316L alloy. Twin and martensite formations were detected microscopically higher in the dynamically tested specimens until about 0.22 strain, while the twin and martensite formations decreased at increasing strains due to adiabatic heating. The rate sensitivity of SLM-316L was determined slightly higher than that of C-316L within the quasi-static strain rate range (1x10-4 and 1x10-2 s-1), while the rate sensitivities of both alloys were similar in the quasi-static-high strain rate range (1x10-4 and ∼2500-2800 s-1) at low strains. A more rapid decrease in the rate sensitivity of C-316L at increasing strains was found in the quasi-static-high strain rate range. The similar activation volumes of both alloys, corresponding to the dislocation intersections, indicated a similar thermally activated deformation process involvement in both alloys.

Dislocation re-emission induced staged work hardening in graphene-nanotwin reinforced Cu: A molecular dynamics simulation study

Graphene and nanotwins are two effective reinforced microstructural features to achieve improved mechanical properties of metallic composites, while the two features are generally applied separately. In this study, graphene/nano-twinned Cu nanocomposites models with different arrangement of the graphene and twin boundaries were designed by using molecular dynamics (MD) simulations, and the dislocation processes and the interactions between dislocation and graphene/twin were simulated and investigated. The simulation results indicated the arrangement of graphene and nanotwin affects the work hardening behaviors in the graphene/nano-twinned Cu composites, i.e., two staged work hardening behavior corresponded to cyclic process of dislocation hindrance-absorption-reemission in the model with relatively small twin spacing and twin-graphene spacing, while the work hardening dominated by dislocation intersection and multiplication occurred in the model with large twin-spacing. The simulation provided herein demonstrated that the special arrangement of graphene and nanotwins led a way to tailoring the mechanical properties of metallic composites with various work hardening behaviors. Graphical abstract Highlights 1. Dislocation reactions between twins and graphene were simulated and analyzed. 2. Twin-graphene distance and the twin distance play key roles in the reaction. 3. The mechanism corresponding to work hardening changes in the limited two distances.

Оценка вкладов различных механизмов в сопротивление деформированию монокристаллов сплава Cu–12 ат.% Al

The evolution of the structure in the process of plastic deformation by compression of Cu – 12 at.% Al single crystals is investigated. Data on the stress-strain curve, deformation stages and the relationship of stages with the types of forming substructures are presented. Quantitative measurements of various parameters characterizing the substructures were carried out. The results are presented in the form of a diagram of the dependence of the fraction of substructures on the density of dislocations. Dependences of the distances between non-reactive intersections of dislocations, inter-dislocation reactions, obstacles of different types, and dislocation lines on the degree of strain are shown. Particular attention is paid to the quantitative assessment of the contributions to the flow stress from local obstacles and long-range elastic stress fields, as well as the contributions to strain associated with the development of slip and twinning.

The smooth and notched three-point bending fatigue behavior of directionally solidified high-Nb TiAl alloy

To understand the fatigue behavior of directionally solidified (DS) TiAl alloys, the Ti46Al7Nb alloy fabricated by cold crucible DS method was investigated by microstructure characterization and three-point bending fatigue tests. Results show that the microstructure of the alloy is composed of α 2 /γ lamellae with β segregation, and the angle between lamellar arrangement and DS growth direction is 45°. Smooth and notched specimens were prepared to evaluate the fatigue performance of the TiAl alloy under pre-damage conditions. The finite element results indicate that the notched have more obvious stress concentration. The three-point bending fatigue limits of the smooth and notched specimens are 468 MPa and 213 MPa, respectively. The crack propagation path indicates that there are cleavage microcracks with different shapes around the main crack in the notched specimen, while the main crack in the smooth one is surrounded by many corrugated cracks perpendicular to the lamellar direction. The whole process of fatigue crack initiation and propagation under three-point loading is established. Fracture morphology shows that notched specimens are mostly brittle cleavage fracture along the lamellar facets, while the smooth are trans-lamellar fracture with plastic characteristics. The degree of fatigue deformation of the smooth specimen is greater than that of the notched one, which mainly reflects on high density dislocation and dislocation intersection, as well as more sub-grain boundaries and twins. • The high-Nb TiAl alloy with well-aligned lamellae was fabricated by CCDS. • The three-point bending fatigue limits of the smooth and notched specimens are 468 MPa and 213 MPa, respectively. • The corrugated secondary cracks formed around the main crack are conducive to the release of stress concentration. • The smooth specimen exhibits high-density dislocation intersection, as well as more sub-grain boundaries and twins.

Deformation Mechanisms of FCC-Structured Metallic Nanocrystal with Incoherent Twin Boundary

Incoherent twin boundaries (ITBs) can significantly affect the mechanical properties of twin-structured metals. However, most previous studies have focused on the deformation mechanism of the coherent twin boundary (CTB), and metals with ITB-accommodated plasticity still require further investigation. In this study, deformation mechanisms of FCC-structured nanocrystal metals with ITBs were investigated using molecular dynamic (MD) simulations. We revealed that three deformation mechanisms occur in metals with ITBs. The first type of deformation was observed in Au, where the plasticity is governed by partial dislocation intersections with CTBs or reactions with each other to form Lomer–Cottrell (L–C) locks. In the second type, found in Al, the deformation is governed by reversible ITB migration. The third type of deformation, in Ni and Cu, is governed by partial dislocations emitted from the ITB or the tips of the stacking faults (SFs). The observed L–C lock formation, as well as the reversible ITB migration and partial dislocation emission from the tips of SFs, have rarely been reported before.