Dislocation Sources Research Articles

Molecular dynamics study on mechanical properties and plastic deformation mechanism of GNPs/Mg composites

This study utilizes molecular dynamics methods to embedded GNPs (graphene nanosheets) of uniform size and thickness into single crystal Mg, establishing a GNPs/Mg composite model. The mechanical properties and plastic deformation behavior of the composite under various compression conditions (volume fraction of GNPs, strain rate, and temperature) are investigated. The results indicate that uniformly distributed GNPs enhance load transfer and provide dispersion strengthening, effectively improving the composite's mechanical properties. As the volume fraction of GNPs increases, the yield stress of the composite first increases and then decreases. When the volume fraction of GNPs is 3.9%, both the yield stress and Young's modulus of the composite reach their maximum values of 9.16GPa and 108.83GPa, respectively. The uniformly dispersed GNPs constrain dislocations on prismatic and pyramidal planes, thereby suppressing twinning deformation. Consequently, dislocation slip predominantly occurs as 1/3<-1100> partial dislocations on the (0001) basal plane. Young's modulus is not sensitive to strain rate; as the strain rate increases from 0.0025Å·ps⁻¹ to 0.0125Å·ps⁻¹, the yield stress rises from 8.50GPa to 10.42GPa. At high strain rates, only some dislocation sources are activated, and the number of 1/3<-1100> partial dislocations decreases with increasing strain rate. Within the temperature range of 100K to 500K, both the elastic modulus and yield stress of the GNPs/Mg composite decrease with increasing temperature.

Optimal Arrangements and Local Anisotropy of {100} Guinier-Preston (GP) Zones by Parametric Dislocation Dynamics (PDD) Simulations.

Stress-oriented precipitation and the resulting mechanical anisotropy have been widely studied over the decades. However, the local anisotropy of precipitates with specific orientations has been less thoroughly investigated. This study models the interaction between an edge dislocation source and {100} variants of Guinier-Preston (GP) zones in Al-Cu alloys using the parametric dislocation dynamics (PDD) method. Concentric geometrically necessary dislocation (GND) loops were employed to construct a line integral model for thin platelets. The simulations, conducted with our self-developed code based on Green's function method and Eshelby inclusion theory revealed distinct strengthening behavior along the strong and weak directions for 60° GP zones, demonstrating anisotropic strengthening from the perspective of elastic interactions. Furthermore, the optimal inclined arrangement of the GP zone array was determined through elastic energy calculations, and these results were corroborated by TEM observations.

Multi-scale ceramic TiC solves the strength-plasticity equilibrium problem of high entropy alloy

High entropy alloys (HEAs) with single FCC structure exhibits unique structure and properties. However, lack of strength hinds the engineering application ability seriously. Therefore, it is urgent to propose effective methods and theories to enhance the strength while maintaining favorable plasticity. Introducing reinforced phases is one crucial research approach for improving the mechanical properties of HEAs. However, the resulting reduction in ductility has been neglected. In this work, we propose a novel multi-scale TiC coupling reinforced alloy which preserves the high plasticity while increasing the strength. The results demonstrate that TiC (μm) hinders the dislocations movement and improves strength through the second phase strengthening mechanism. Apart from acting as barriers to dislocation motion, the TiC (nm) also provides more dislocation sources in the distortion area at the junction with the matrix, which increasing the number of movable dislocations and enhancing the plastic strain capacity. Compared with the Al0.4 alloy, the tensile yield strength of the Al0.4-TiC (μm + nm) alloy is increased by 145 %, the ultimate tensile strength is up to 574 MPa, while maintaining a high plastic strain by 30.1 %. The addition of multi-scale ceramic phase TiC provides a novel approach to obtain high strength and high plasticity HEAs.

Microstructure, mechanical and wear behavior of AlxTi(1−x)CoCrFeNi (x = 0.875, 0.75) high-entropy coatings fabricated via HVOF and post-annealing

AlxTi(1−x)CoCrFeNi (x = 0.875, 0.75) high-entropy coatings (HECs) were prepared using High-Velocity Oxygen-Fuel technology, and then underwent vacuum annealing at 500 ℃, 700 ℃, 900 ℃, and 1100 ℃, respectively. The sprayed coatings exhibited BCC phase, with the hardness increasing as the Ti content added. The wear mechanism of the HECs included abrasive, adhesion and oxidation wear. The microstructure and phase structure of the HECs annealed at 500 ℃ were basically consistent with the as-sprayed coatings. The Al0.75Ti0.25CoCrFeNi HEC nanohardness reached the peak (19.6 ± 1.1 GPa) and had the best wear resistance (1.8 ×10−5 mm3 N−1 m−1) due to the grain boundary dislocation source strengthening. With the annealing temperature increasing, the BCC to FCC phase transition occurred, the grains coarsened, the hardness decreased, and oxidation wear were intensified.

Transcrystalline Mechanism of Banded Spherulites Development in Melt-Crystallized Semicrystalline Polymers.

The decades-long paradigm of continuous and perpetual lamellar twisting constituting banded spherulites has been found to be inconsistent with several recent studies showing discontinuity regions between consecutive bands, for which, however, no explanation has been found. The present research demonstrates, in three different semicrystalline polymers (HDPE, PEG10000 and Pluronic F-127), that sequential transcrystallinity is the predominant mechanism of banded spherulite formation, heterogeneously nucleated on intermittent self-shear-oriented amorphous layers excluded during the crystals' growth. It is hereby demonstrated that a transcrystalline layer can be nucleated on amorphous self-shear-oriented polymer chains in the melt, by a local melt flow in the bulk or in contact with any interface-even in contact with the interface with air, e.g., in contact with an entrapped air bubble or at the edges of the sample-or nucleated following the multiple directions and orientations induced by a turbulent flow. The bilateral excessive local exclusion of amorphous non-crystallizable material, following a short period of initial non-banded growth, is found to be the source of dislocations leading to spirally banded spherulites, through the transcrystalline layers' nucleation thereon. The present research reveals and demonstrates the sequential transcrystalline morphology of banded spherulites and the mechanism of its formation, which may lead to new insights in the understanding and design of polymer processing for specific applications.

An order-disorder core-shell strategy for enhanced work-hardening capability and ductility in nanostructured alloys

Nanocrystalline metallic materials have the merit of high strength but usually suffer from poor ductility and rapid grain coarsening, limiting their practical application. Here, we introduce a core-shell nanostructure in a multicomponent alloy to address these challenges simultaneously, achieving a high tensile strength of 2.65 GPa, a large uniform elongation of 17%, and a high thermal stability of 1173 K. Our strategy relies on an ordered superlattice structure that excels in dislocation accumulation, encased by a ≈3 nm disordered face-centered-cubic nanolayer acting as dislocation sources. The ordered superlattice with high anti-phase boundary energy retards dislocation motions, promoting their interaction and storage within the nanograins. The disordered interfacial nanolayer promotes dislocation emission and effectively accommodates the plastic strain at grain boundaries, preventing intergranular cracking. Consequently, the order-disorder core-shell nanostructure exhibits enhanced work-hardening capability and large ductility. Moreover, such core-shell nanostructure exhibits high coarsening resistance at elevated temperatures, enabling it high thermal stability. Such a design strategy holds promise for developing high-performance materials.

Gradient microstructures and mechanical properties of WAAMed and aged Al–Cu alloy after laser shock peening

The integration of wire-arc additive manufacture (WAAM) and laser shock peening (LSP) facilitated the study of the gradient microstructure in T6-treated WAAM Al–Cu alloy and the synergistic strengthening mechanisms involving the precipitation phase and LSP parameters. The results indicate that pore closures, increases in kernel average misorientation (KAM), and low angle grain boundary (LAGB) are positively associated with LSP energy density. Residual stress positively correlates with LSP energy density and inversely with spot diameter, where larger spots exacerbate non-uniformity. The values of dislocation density, microhardness and residual stress of the post-LSP specimen gradually increase with the increase of depth, and reach the maximum value at ~100 μm. As the depth continues to increase, the value begins to decrease gradually. Dislocation accumulation near grain boundaries, interacting with θ’-Al2Cu, contributes to strength enhancement. The main strengthening mechanisms enhanced by LSP involve creating a gradient microstructure through significant plastic deformation, pore closure, and the presence of high-density dislocations. Post-T6 treatment, θ’-Al2Cu significantly restricts dislocation slip, pins dislocations, and encourages the emergence of new dislocation sources, thereby enhancing the strengthening effect and stability induced by LSP.

The effect of dislocation pile-up on microcrack initiation in microcrystalline materials in the hydrogen environment

In this paper, hydrogen (H) induced microcrack initiation in the first stage of crack propagation is studied. Based on the knowledge of fracture mechanics and discrete distributed dislocation method, the interaction model between hydrogen atoms at crack tip and dislocation source, the grain boundary (GB) penetration model and microcrack initiation criterion are established, and the microcrack initiation mechanism caused by dislocation pile-up near the grain boundary 2 (GB2) is studied. Meanwhile, the mechanism of dislocation synthesis (DS) caused by dislocation stacking is studied. The results show that hydrogen atoms promote the emission of dislocations at the crack tip. The distribution of H atoms at the crack tip (CT) leads to an increase in the number of dislocations penetrating the grain boundary, and the number of dislocation pile-up in the second phase grain increases. In the hydrogen environment, the number of dislocation pile-up near GB2 increases, resulting in a large stress concentration near GB2, which makes microcracks more likely to occur. At the same time, H can decrease the surface energy of materials, making materials more prone to brittle fracture. The study of hydrogen induced crack initiation is helpful to understand and control the impact of H on the performance and reliability of metal materials, which plays a crucial role in improving the working efficiency and service life of metal materials.

Elastic strain mapping of plastically deformed materials by TEM

A method for mapping elastic strains by TEM in plastically deformed materials is presented. A characteristic feature of plastically deformed materials, which cannot be handled by standard strain measurement method, is the presence of orientation gradients. To circumvent this issue, we couple orientation and strain maps obtained from scanning precession electron diffraction datasets. More specifically, orientation gradients are taken into account by 1) identifying the diffraction spot positions in a reference pattern, 2) measuring the disorientation between the diffraction patterns in the map and the reference pattern, 3) rotating the coordinate system following the measured disorientation at each position in the map, 4) calculating strains in the rotated coordinate system. At present, only azimuthal rotations of the crystal are handled. The method is illustrated on a Cr2AlC monocrystal micropilar deformed in near simple flexion during a nanomechanical test. After plastic deformation, the sample contains dislocations arranged in pile-ups and walls. The strain-field around each dislocation is consistent with theory, and a clear difference is observed between the strain fields around pile-ups and walls. It is further remarked that strain maps allow for the orientation of the Burgers vector to be identified. Since the loading undergone by the sample is known, this also allows for the position of the dislocation sources to be estimated. Perspectives for the study of deformed materials are finally discussed.

Microstructures, mechanical properties, and strengthening mechanisms of the (NbMoTa)100−xCx refractory medium-entropy alloys

Refractory high/medium-entropy alloys (RH/MEAs) are known for their outstanding performance at elevated temperatures; however, they usually exhibit poor room-temperature plasticity, which can be attributed to the non-uniform deformation that occurs at room temperature. Once cracks nucleate, they will rapidly propagate into vertical splitting cracks. Here, we introduce multiple phases including FCC and HCP phases into the NbMoTa RMEA via appropriate addition of carbon. The results show that multiple-phase synergy effectively suppresses non-uniform deformation, thereby delaying the onset of vertical splitting cracks. An optimal combination of compressive strength-plasticity is achieved by the (NbMoTa)92.5C7.5 alloy. The significant improvement in room-temperature mechanical properties can be attributed to its hierarchical microstructure: in the mesoscale, the BCC matrix is divided by eutectic structures; while at the microscale, the BCC matrix is further refined by abundant lath-like FCC precipitates. The FCC precipitates contain high-density stacking faults, acting as a dislocation source under compressive loading. The HCP phase in the eutectic microstructures, in turn, acts as a strong barrier to dislocation movement and simultaneously increases the dislocation storage capacity. These findings open a new route to tailor the microstructure and mechanical properties of RH/MEAs.

Mechanical behavior and underlying mechanism of nickel-based superalloy during coupling electrical pulse and ultrasonic treatment

Coupling electric pulse and ultrasonic treatment (CEPUT) is a multi-energy field-assisted forming technology that can improve the forming efficiency of materials. The current research focused on the surface modification of materials by CEPUT, lacking the explanation of its coupling mechanism. In this study, Inconel 718 was adopted as the research object to observe the mechanical behavior and microstructure after CEPUT uniaxial tension experiment. The deformation tendency of grain under coupling multi-energy field was described by analyzing the texture evolution and dislocation morphology. The coupling mechanism of ultrasonic vibration and pulsed current was revealed in CEPUT. The experimental results revealed that there was a nonlinear relationship between the increase of flow stress softening efficiency and the increase of ultrasonic energy density and current density. This phenomenon was the macroscopic manifestation of interference between initial ultrasonic vibration and vibration after energy attenuation, which was related to dislocation slip and recrystallization under coupled multi-field. In addition, more efficient path of dislocation slip was demonstrated through the CEPUT. Among them, ultrasonic vibration caused local resistance to rise at the dislocation source and dislocation pile-up, which strengthened the local Joule heating effect; while the pulsed current reduced the activation energy required for dislocation slip near the waning points. This effectively reduced the resistance of dislocation motion near the pinning point, promoting the chance of dislocation crossing and annihilation. Meanwhile, the high temperature caused by pulsed current and local high strain caused by ultrasonic vibration were more favorable to dynamic recrystallization, and the change of grain orientation was more active. The texture strength and grain orientation of the matrix can be effectively controlled by adjusting the input power of ultrasonic vibration and pulsed current.

The effect of annealing on micro-hardness of molybdenum single crystals

Molybdenum (Mo) crystals exhibit excellent mechanical properties such as high yield strength, high elastic modulus, very high melting point, and excellent corrosion resistance, making them a suitable choice for various industrial applications. The mechanical behaviour of these crystals has been investigated through tension, compression, and indentation experiments. The indentation experiments on annealed Mo single-crystal thin film show an increase in hardness with increase in annealing temperature from 400 °C–650 °C. However, it is not clear whether this behaviour is specific to the Mo thin films or these are applicable to the bulk samples of single crystals also. In order to address these issues, the Vickers indentation experiments are performed on as-received and annealed samples of (100) -, (110) -, and (111) -oriented Mo single crystals. The results show that the low-temperature annealing leads to an increase in micro-hardness by 9% for (100) oriented crystals. The x-ray diffraction (XRD), electron backscatter diffraction (EBSD) and energy dispersive x-ray spectroscopy (EDS) studies have shown that there is a drop in the number of dislocation sources or dislocation density ( ρ ) with no high-angle grain boundary formation or any microstructural changes or any oxide formation. This necessitates to enhance the applied stress required for significant yielding (or yield strength) which results in an increase in the hardness. Based on experimental observations, a hardening mechanism governed by dislocation annihilation during annealing has been proposed in the present study.

High-cycle and low-cycle fatigue characteristics of multilayered dissimilar titanium alloys

Multilayered structures of dissimilar titanium alloys can achieve excellent fracture ductility and strength, while their fatigue characteristics especially dislocation networks and twin formation are rarely reported. Heterogeneous microstructures are observed in the multilayered TC4/TB8 alloys, including fine acicular α grains, continuous α layer at prior β grain boundaries (αGB) and β matrix on the TB8 layer, together with equiaxed α grains on the TC4 layer. High-cycle fatigue (HCF) and low-cycle fatigue (LCF) tests show that the initial fatigue damage appears at the αGB/β matrix interfaces on the TB8 layer instead of the boned TC4/TB8 interfaces. Since the stress concentration induced by dislocation pile-up is prone to micro-void formation and crack propagation at the αGB/β interfaces. For LCF, the αGB/β interfaces can not only act as impenetrable barriers and sources of lattice dislocations, but also allow the dislocations cross boundaries during cyclic tension and compression because of the high boundary energy. The formation characteristic of deformation twins that is beneficial for the plastic deformation of α grains in TC4 layer during cyclic strain is investigated. Furthermore, the hexagonal dislocation networks are also found within the equiaxed α grains of TC4 layer after LCF, and the role between interface barrier and slip direction in the formation mechanism is analyzed.

Influence of magnetic field on growth kinetics of sodium chlorate crystals from aqueous solution

The results of the study of the influence of a static magnetic field of 55 ± 3 mT on the face growth kinetics of sodium chlorate crystals in the supersaturation range of (0.89–1.78) % are presented. The supersaturation dependence of growth rate was analyzed using different theoretical models and the kinetic parameters of crystal growth in zero and in the applied magnetic field were estimated. It is shown that in both types of experiments the observed crystals grew by a complex dislocation source. Analysis of the growth rate distribution by using the Weibull distribution function was performed to obtain the maximum growth rate Rmax depending on their corresponding initial length lmax. The linear relationship between these quantities is derived from the model of growth by a group of cooperating screw dislocations. It was observed that both the initial size threshold l*max and the slope dRmax/dlmax exhibit higher values in the presence of a magnetic field.

Optimizing misfit dislocation glide kinetics for enhanced threading dislocation density reduction in Si1−xGex/Si(001) layers through dynamic growth rate control

Relaxed Si1−xGex layers on Si(001) serve as virtual substrates for strained Si or Ge layers. However, plastically relaxed layers inevitably contain misfit and threading dislocations, negatively affecting devices. Deposition of a SiGe layer on the backside of the substrate introduces a dislocation reservoir at the wafer edge that can reduce the threading dislocation density (TDD) of Si0.98Ge0.02/Si layers, as these preexisting dislocations start gliding toward the wafer center upon reaching the critical thickness. Here, we show that this low-strain system can be used effectively to study dislocation glide kinetics. In agreement with the literature, dislocation glide is a thermally activated process with an activation energy of 2.12–2.16 eV. Near the critical thickness, relaxation is sluggish and inefficient due to the linear dependence of the glide velocity on excess stress. At lower growth rates, dislocations from the edge reservoir are activated in a lower density due to the increase in the critical thickness through partial strain relaxation by already activated dislocations. Contrary to common models, here, the lowest possible growth rate is not essential for minimizing the TDD. Instead, a careful balance between low and high growth rates is beneficial. Overcoming the initial sluggish and inefficient relaxation phase is critical while also avoiding accumulation of strain energy, and, therefore, the activation of dislocation sources. Only in a later stage of buffer growth, the growth rate should be reduced to a minimum. With this method, the TDD of strain relaxed Si0.84Ge0.16 layers is reduced to 7 × 104 cm−2.

Calculating the grain size effect during strain hardening through a probabilistic analysis of the mean slip distance in polycrystals

Grain refinement is an important mechanism to produce stronger alloys. Strain hardening is an essential phenomenon in metal forming processes. The interaction between grain size and strain hardening is evident: a decrease in grain size (dg)causes an increase in ultimate tensile strength but a decrease in uniform elongation. The Kocks-Mecking (KM) model for strain hardening is based on the relationship between shear strain and the path length for dislocation slip. It provides good general estimates for stress-strain curves, and empirical modifications have been made to include dg. Here, the empirical approach is substituted by theoretical probability calculations, accounting for the fact that the grain size imposes a bound on the mean slip distance, while strain compatibility defines a relationship between grain boundary-dislocation interaction and bulk storage and annihilation. The resulting differential only uses the two parameters inherent to KM. Fitting to published tensile curves for Al, Cu, and Ni produces excellent results. The fitting parameters allow to predict the tensile strength as a function of dgto good approximation, for dg>1μm. Below this limit, fundamental changes in dislocation statistics impose the activation of grain boundary dislocation sources and may induce dislocation density gradients, which seem to determine the flow stress in the sub-μm range.

The origin of exceptionally large ductility in molybdenum alloys dispersed with irregular-shaped La2O3 nano-particles

Molybdenum and its alloys are known for their superior strength among body-centered cubic materials. However, their widespread application is hindered by a significant decrease in ductility at lower temperatures. In this study, we demonstrate the achievement of exceptional ductility in a Mo alloy containing rare-earth La2O3 nanoparticles through rotary-swaging, a rarity in Mo-based materials. Our analysis reveals that the large ductility originates from substantial variations in the electronic density of states, a characteristic intrinsic to rare-earth elements. This characteristic can accelerate the generation of oxygen vacancies, facilitating the amorphization of the oxide-matrix interface. This process promotes vacancy absorption and modification of dislocation configurations. Furthermore, by inducing irregular shapes in the La2O3 nanoparticles through rotary-swaging, incoming dislocations interact with them, creating multiple dislocation sources near the interface. These dislocation sources act as potent initiators at even reduced temperatures, fostering diverse dislocation types and intricate networks, ultimately enhancing dislocation plasticity.

Molecular dynamics simulation of fatigue damage formation in single-crystal/polycrystalline aluminum

The fatigue plasticity mechanisms and local defect characteristics of engineering materials are critical for addressing fatigue damage. This study aims to identify the formation mechanisms of fatigue damage and delineate the microstructural characteristics that resisted or facilitated fatigue-induced plastic accumulation. The fatigue mechanical properties and microstructural evolution processes of single-crystal/polycrystalline aluminum were investigated through strain-controlled fatigue testing, considering three distinct strain amplitudes. The initiation and cumulative occurrence of local plasticity in single-crystal aluminum occur at the intersections of slip planes, which serve as sources of dislocations. The cumulative formation of irreversible surface steps results in local stress concentration, ultimately leading to fatigue damage at the free surface. Polycrystalline aluminum undergoes grain rotation and merging during cyclic loading, leading to a gradual increase in grain size during plastic deformation. The initiation and cumulative occurrence of local plasticity occur at grain boundaries, ultimately leading to fatigue damage at the grain boundaries.

Mechanism of solute hardening and dislocation debris-mediated ductilization in Nb-Si alloy

Niobium (Nb) is sensitive to even minute quantities of silicon (Si) solutes, which are known to induce pronounced hardening. However, the underlying mechanism for hardening remains elusive since the effect of Si solutes on dislocation behavior is unclear. Here, using tensile testing, in-situ microscopy and nanomechanical testing, the behavior of dislocations in dilute Nb-Si alloys, containing from 0 at.% to 0.8 at.% Si, is investigated. We show that the hardness, strength and strain hardening rate increase from two to four times, while the uniform elongation in tension only reduces 50 % as the Si content increases. Dislocations evolve from complex entangled patterns in Nb to parallel long-straight screw dislocation-dominated structures in Nb-Si alloys. In-situ indentation reveals that the origins of the marked hardening in Nb-Si alloy are the reduction of dislocation mobility and cross-slip propensity. Large densities of dislocation debris-superjogs and loops introduced throughout the sample during warm rolling and annealing are found to provide active internal dislocation sources, which explain the minimal ductility loss seen in these Nb-Si alloys. These findings can help guide the alloy design of high-performance refractory materials for extreme temperature applications.

High friction stress and weak grain-refinement strengthening effect induced by severe lattice distortion in ZrTiNb medium entropy alloys

In this work, we have studied the grain-refinement strengthening effect of body-centered cubic Zr50Ti35Nb15 medium entropy alloy. The results suggest that the yield strength increases as grain size decreases from 530 μm to 0.51 μm, reaching the maximum value of 651 MPa, and still having decent ductility. Moreover, it is found that the alloys exhibit a high Hall–Petch intercept of 564 MPa and a low Hall–Petch slope, which means strong lattice friction stress and weak grain-refinement strengthening. The high lattice friction stress originates from the obstruction of dislocation motion caused by atomic size and modulus misfit. Dislocation interaction and multiplication in grain interior is difficult due to the high dislocation slip resistance, leading to a weak dislocation pile-up capability at grain boundary. Instead, dislocations nucleation and emission become the dominant deformation process since low shear modulus promotes dislocation source activation.