Full Dislocation Research Articles

Simulations of plasticity in diamond nanoparticles showing ultrahigh strength

We use molecular dynamics (MD) simulations to deform single crystal spherical carbon nanoparticles (NP), 4–45 nm diameter, with a hard, flat indenter, compressing along the [001] direction. There is no clear amorphization nor phase change in the NP, but there is significant deformation, with bent crystalline planes, and many atoms that retain sp3 coordination, but are no longer recognized as having diamond structure by different structure-identification methods. Machine-learning is used to improve diamond-structure identification. The NP deforms laterally, and volumetric strain is ~0.1 when the uniaxial strain is ~0.5. Poisson's ratio increases with strain, and the elastic limit is reached at 0.2–0.3 strain, at a contact pressure of ~150 GPa. For NPs above 5 nm, dislocations appear and are mostly (1/2)<110>{111} full dislocations, with a few partial dislocations for larger nanoparticles, without twinning. These results agree with the recent observation of plastic deformation in diamond nanopillars. Small NP display elastic modulus, yield stress and hardness increasing with NP size, but NPs with diameter larger than 25 nm display an approximately constant dislocation and dislocation junction density, which leads to a plateau in the hardness versus NP size, at ~150 GPa, close to bulk diamond. Diamond nanoparticles could provide high strength thin coatings, lighter than full-density nanotwinned diamond but with nearly the same strength.

Generalized pole figures from post-processing whole Debye-Scherrer patterns for microstructural analysis on deformed materials.

Debye-Scherrer patterns, obtained from X-ray diffraction experiments using synchrotron light in transmission geometry, were analysed to construct generalized pole figures, and further used as input for an orientation distribution function inversion algorithm. By using Langford's method for separating strain and size contributions to peak broadening, it was possible, for the first time, to obtain full domain size and dislocation density generalized distribution functions (GDFs). This method was applied to cold-rolled and annealed interstitial-free steel. The predictions made using GDFs were corroborated by electron backscatter diffraction measurements and were also consistent with what was previously known for this kind of material under these conditions.

Bound states at partial dislocation defects in multipole higher-order topological insulators

The bulk-boundary correspondence, which links a bulk topological property of a material to the existence of robust boundary states, is a hallmark of topological insulators. However, in crystalline topological materials the presence of boundary states in the insulating gap is not always necessary since they can be hidden in the bulk energy bands, obscured by boundary artifacts of non-topological origin, or, in the case of higher-order topology, they can be gapped altogether. Recently, exotic defects of translation symmetry called partial dislocations have been proposed to trap gapless topological modes in some materials. Here we present experimental observations of partial-dislocation-induced topological modes in 2D and 3D insulators. We particularly focus on multipole higher-order topological insulators built from circuit-based resonator arrays, since crucially they are not sensitive to full dislocation defects, and they have a sublattice structure allowing for stacking faults and partial dislocations.

A pediatric physiatrist's approach to neuromuscular hip dysplasia in cerebral palsy.

Cerebral palsy (CP) encompasses a group of disorders pertaining to abnormalities in movement, tone, and/or posture due to a nonprogressive lesion to an immature brain. Hip dysplasia is the second most common orthopedic deformity seen in CP, and its severity can range from a hip at risk for subluxation to full hip dislocation with degenerative changes. The purpose of this article is to review the hip pathologies that occur in CP focusing on their pathogenesis, physical exam findings, impact on function, and conservative treatment. Through a review of the medical literature, it is demonstrated that early, aggressive, and comprehensive care led by a pediatric physiatrist is essential to mitigate progression to complete hip dislocation and preserve range of motion, prevent contracture, and promote maximum functional ability in all children with CP.

Study of deformation mechanism of structural anisotropy in 4H–SiC film by nanoindentation

4H–SiC film is strongly anisotropic in terms of deformation behavior. This study systematically investigated the deformation mechanism of structural anisotropy in 4H–SiC film on three indentation planes (i.e. base plane, a-plane and m-plane) by nanoindentation. The results of molecular dynamics (MD) study show that the hardness and Young’s modulus of 4H–SiC film are different on three indentation planes. The morphology of the defective microstructures and the nucleation and extension of dislocations are strongly dependent on the indentation planes. The deformation behavior of 4H–SiC film is related to the crystal direction, atomic displacement, atomic strain and stress distribution. Further analysis reveals that the formation of prismatic dislocation half-loops on the basal plane (0001) can be attributed to the interaction of one full dislocation loop on the basal plane (0001) and two full dislocation loops on the prismatic plane {11‾00}. Whereas the prismatic dislocation loops on the a-plane (112‾0) and m-plane (11‾00) are formed by the “lasso"-like mechanism.

Size effects and plastic deformation mechanisms in single-crystalline CoCrFeNi micro/nanopillars

As emerging and revolutionary alloy materials, high entropy alloys (HEAs) have been extensively studied because of their unique composition, microstructures and excellent mechanical properties and performances. However, there have been limited studies on the deformation behaviors and mechanisms of HEA single crystals at the micro/nanoscale. Here, we fabricated single-crystalline CoCrFeNi HEA pillars with typical orientations of <100>, <110> and <111> and diameters ranging from 272 nm to 1,253 nm and then conducted in situ uniaxial compression tests on these fabricated micro/nanopillars inside a scanning electron microscope. The in situ compression tests showed pronounced size effects on yield and flow stresses in the <100>-, <110>- and <111>-oriented micro/nanopillars, i.e., both yield and flow stresses follow a scaling law with pillar diameter. The scaling exponents of <110>- and <111>-oriented micro/nanopillars are approximately -0.60, which is close to those of face-centered cubic (FCC) pure metallic micro/nanopillars. However, the <100>-oriented pillars, having a scaling exponent of -0.32∼-0.17, exhibit weaker size effects on yield and flow stresses compared with the <110>- and <111>-oriented micro/nanopillars. Combining observations and analyses using transmission electron microscopy, and large-scale atomistic simulations, we revealed that the nucleation and slip of full dislocations dominate the plastic deformation in the <110>- and <111>-oriented micro/nanopillars, while deformation twinning is a governing mechanism in the <100>-oriented micro/nanopillars. These underlying deformation mechanisms are responsible for the size effects of single-crystalline HEA micro/nanopillars. Large-scale atomistic simulations further reveal that twin nucleation in <100>-oriented pillars is initiated by the slip of pre-existing partial dislocations from an internal source. By considering the free energy change during partial slip, we used a theoretical model to predict the scaling law for the size effects of <100>-oriented micro/nanopillars. The predicted scaling exponent for the size effect of <100>-oriented micro/nanopillars is consistent with our experimental results. Our current study sheds light on the underlying deformation mechanisms in FCC HEA single crystals at the micro/nanoscale, which provides a guide for the design and fabrication of HEAs with high strength and remarkable plasticity.

Synergetic deformation mechanism in hierarchical twinned high-entropy alloys

The mechanical properties of crystalline materials can be efficiently optimized using a hierarchical twinned structure. Conventional deformation mechanisms for coherent Σ3 boundaries generally involve three basic models: cross-slip, partial dislocation step, and full dislocation step. In this study, we report a novel deformation mechanism that allows the co-existence of twin-separation, phase transformations, grain rotation, and cracking, around a triple junction of twin boundaries in a hierarchical twinned high-entropy alloy. The deformation mechanisms in the reference high-entropy alloy (Fe-30Mn-10Co-10Cr at.%) were investigated using LAADF-STEM. The triple junction of the hierarchical twinned structure gradually deformed during in-situ strain and showed mechanisms significantly different from that observed in the purely twinned structures. These new mechanisms are referred to as “novel synergetic deformation mechanisms of hierarchical twin boundaries”. Understanding the fundamental mechanisms of the hierarchical twin boundaries under deformation could assist the design of strong and ductile bulk materials with hierarchical twinned structure.

Twin thickness and dislocation interactions affect the incoherent-twin boundary phase in face-centered cubic metals

Incoherent-twin boundaries (ITBs) can significantly affect the mechanical properties exhibited by metals. Although numerous studies have been conducted to date, the atomic structures of such ITBs remain unclear, owing to difficulties in imaging their structure. In this study, high-angle annular dark-field imaging was used to reveal the atomic structure of the ITBs present in Pt. We discovered that both the twin thickness and the dislocation-ITB interaction can affect the ITB phase structure. In thin twins, the {111} planes between the ITB remain flat without any obvious displacement along the <111> direction, whereas in thicker twins, the {111} planes between the ITB exhibit clear displacement along the <111> direction, with this displacement increasing as the twin thickness increases. The ITBs frequently absorb full dislocations, which leads to the formation of dislocation-misaligned ITBs. This twin-thickness effect and dislocation-ITB interaction, which resulted ITB-phase variation, has rarely been reported.

Deformation-Induced Phase Transformations in Gold Nanoribbons with the 4H Phase.

The mechanical stability of metallic nanomaterials has been intensively studied due to their unique structures and promising applications. Although extensive investigations have been carried out on the deformation behaviors of metallic nanomaterials, the atomic-scale deformation mechanism of metallic nanomaterials with unconventional hexagonal structures remains unclear because of the lack of direct experimental observation. Here, we conduct an atomic-resolution in situ tensile-straining transmission electron microscopy investigation on the deformation mechanism of gold nanoribbons with the 4H (hexagonal) phase. Our results reveal that plastic deformation in the 4H gold nanoribbons comprises three stages, in which both full and partial dislocations are involved. At the early deformation stage, plastic deformation is governed by full dislocation activities. Partial dislocations are subsequently activated in regions that have undergone full dislocation gliding, leading to phase transformation from the 4H phase to the face-centered cubic (FCC) phase. At the last stage of the deformation process, the volume fraction of the FCC phase increases, and full dislocation activities in the FCC regions also play an important role.

Three-level model based on physical theories of plasticity: Formulation, implementation algorithms, results of application to the study of cyclic loading

The physical and mechanical properties of metals and alloys, as well as, the operational characteristics of products made of them, are known to be significantly determined by the meso- and microstructure of materials. Therefore, physically oriented models that can be used to analyze the material structure evolution have been intensively developed and widely applied in recent decades for the study of thermomechanical processing of metals and alloys. These models are based on the introduction of internal variables, theories of crystal plasticity (elastoviscous plasticity), and a multilevel approach. In this paper, we consider the structure, mathematical formulation and algorithm for implementing a three-level (macro-, meso-1, and meso-2 levels) dislocation-oriented model designed to study the behavior of a representative macrovolume (macrosample) of mono- and polycrystalline alloys under cyclic deformation paths. The loading is given, and the Voigt (Taylor) hypothesis is used to connect the macro- and mesolevel-1. The mesolevel-2 submodel operates with the densities and velocities of full and split edge dislocations on slip systems. Interactions of dislocations of various systems, such as annihilation, hardening due to forest dislocations, formation of barriers of a dislocation nature (Lomer–Cottrell, Hirt) are taken into account. At mesolevel-1, the description is carried out in terms of shear stresses and shear rates along slip systems, determined by the Orowan equation using the mesolevel-2 data. A rigid moving coordinate system associated with a crystal lattice is introduced to evaluate the rotation of crystallites. The response of the material at the macro level is determined by averaging the stresses in crystallites. The results of applying the model to the study of deformation of alloy macrosamples with different values of stacking fault energy along simple and complex deformation trajectories are presented. It is shown that the materials with low stacking fault energy demonstrate the effect of additional cyclic hardening when loaded along complex deformation trajectories.

Generalized stacking fault energy surface mismatch and dislocation transformation

Even though the fundamental rules governing dislocation activities have been well established in the past century, we report a phenomenon, dislocation transformation, governed by the generalized-stacking-fault energy surface mismatch (GSF mismatch for short) between two co-existing phases. By carrying out ab-initio-informed microscopic phase-field simulations, we demonstrate that the GSF mismatch between a high symmetry matrix phase and a low symmetry precipitate phase can transform an array of identical full dislocations in the matrix into an array of two different types of full dislocations when they shear through the precipitates. The precipitates serve as a passive Shockley partial source, creating new Shockley partial dislocations that are neither the ones from the dissociation of the full dislocation. This phenomenon enriches our fundamental understanding of partial dislocation nucleation and dislocation-precipitate interactions, offering additional opportunities to tailor work-hardening and twinning processes in alloys strengthened by low-symmetry precipitate phases.

Influence of lamellar and equiaxed microstructural morphologies on yielding behaviour of a medium Mn steel

Additional austenitization followed by quenching was introduced before the intercritical annealing (IA) of cold rolled medium Mn steel to produce the duplex microstructures having both lamellar and equiaxed morphologies, whose respective fractions vary with austenitization temperature. When it increased from 850°C to 1050°C, more martensite was formed during quenching and then transformed to the lamellar grains after the IA. At the same time, the higher austenitization temperature resulted in the greater microplasticity before macro-yielding, the decreases in both upper yield point and yield drop and the shorter yield point elongation. The relationship between all the yielding behavior and the microstructural morphologies were discussed on the basis that the lamellar and the equiaxed phase interfaces could have the different capability of emitting either partial or full dislocations during deformation.

Atomic scale observation of FCC twin, FCC → 9R and 9R → 12R’ transformations in cold-rolled Hafnium

The face-centered cubic (FCC) twin, FCC →9R and 9R → 12R’ transformations were observed simultaneously in cold-rolled pure Hf. The formation of FCC twin was due to the Shockley partial dislocations extended in the different basal planes. For FCC →9R transformation, the collective gliding of Shockley partial dislocations along (111) planes with every three atomic layers eventually lead to the stacking sequence of close-packed structure, i.e. ACBACBACBA changed to ABACACBCBA. The 9R → 12R’ transformation was related to the formation of the full dislocations at every three atomic layers.

Effect of heterointerface on the indentation behavior of nano-laminated c-BN/diamond composites

Heterointerface is generally accepted to tune the mechanical properties of metallic nano-laminated composites (MNLCs). Recent experiments have shown that the ceramic NLCs composed of c-BN and diamond can also possess unprecedented comprehensive performance. However, the physical image of the heterointerface formed by such two hard ceramics is still unclear, and how the interface can affect the mechanical behavior of such CNLCs is unknown. In this work, atomistic simulations are adopted to explore the above problems. The heterointerface is found to be semi-coherent interface composed of periodic parallelogram networks. Each network is further composed of coherent region, stacking faults, Shockley partial dislocations and nodes. Then, the effect of semi-coherent interface on the mechanical property of the CNLCs is subsequently studied. It is found that the growth and transition of interfacial defects dominate the mechanical behavior when the indentation depth is small, while at large depth, the nucleation of full dislocations from the semi-coherent interface instead of the location below the indenter plays a leading role and the mechanical behavior of the CNLCs is governed by the interaction between the semi-coherent interface and the indentation-induced dislocations. The present results should be helpful for the deep understanding of the mechanical behavior of CNLCs.

Grain size dependent microstructure and texture evolution during dynamic deformation of nanocrystalline face-centered cubic materials

Evolution of microstructure and texture in nanocrystalline (nc) face-centered cubic materials subjected to high-strain-rate compression are investigated. Three nc materials, differing in grain size or stacking fault energy, namely, 27 nm Ni, 70 nm Ni, and a 27 nm NiCo alloy are deformed under high strain rates (8000–23000 s−1) using the split-Hopkinson pressure bar technique. The transmission Kikuchi diffraction technique and discrete-crystal plasticity finite element (D-CPFE) simulations are used to evaluate structural evolution. Experimental results show that grain growth and grain refinement occurred in samples with small and large initial grain sizes, respectively, while all deformed materials evolve to a (110) texture. The D-CPFE simulations, built for grain-size dependent nc deformation with no fitting parameters, reveal that partial dislocation slip caused faster texture evolution than full dislocation slip. Comparison between experimental and simulation results suggests that the (110) texture is mainly generated by combined full and partial dislocation slip in the 27 nm and 70 nm Ni samples, and by partial dislocation slip alone in the NiCo alloy. Deformation twinning made almost no contributions to the observed texture evolution. Grain-boundary-mediated deformation facilitates grain coarsening, while partial dislocation activities help to promote dynamic stabilization and further refinement of the nano-grains. The highly coupled evolution of microstructure and texture during dynamic deformation is controlled by a synergy of grain size and strain rate effects.

In situ atomic-scale observation of AuCu alloy nanowire with superplasticity and high strength at room temperature

Metallic nanowires usually exhibit ultrahigh strength but suffered low ductility. Previous studies on pure metals suggested this strength-ductility trade-off results from limiting the dislocation activities. However, it is unclear whether such deformation model is valid for a solid solution alloy as well. Here, for the first time, the atomic-scale deformation process of AuCu nanowires with size of ∼16 nm was investigated in situ. The results show the NWs exhibit superplasticity (∼185%) and high strengths (∼2.98 GPa) at room temperature. It was discovered that superplasticity originates from continuous full dislocation nucleation and disappearance, as well as dislocation dipole formation and annihilation etc., which differ from the previous studies in pure metals. The observed full dislocation activities, also different from the ones in the previous studies, suggested that, as the size of the metals is below ∼100 nm, their deformation should be governed by partial dislocation and twinning.

Synergetic Deformation Mechanism in Hierarchical Twinned High-Entropy Alloys

The mechanical properties of crystalline materials can be efficiently optimized using a hierarchical twinned structure. Conventional deformation mechanisms for coherent Σ3 boundaries generally involve three basic models: cross-slip, partial dislocation step, and full dislocation step. In this study, we report a novel deformation mechanism that allows the co-existence of de-twinning, phase transformations, grain rotation, and cracking, around a triple junction of twin boundaries in a hierarchical twinned high-entropy alloy. The deformation mechanisms in the reference high-entropy alloy (Fe-30Mn-10Co-10Cr at. %) were investigated using LAADF-STEM. The triple junction of the hierarchical twinned structure gradually deformed during in-situ strain and showed mechanisms significantly different from that observed in the purely twinned structures. These new mechanisms are referred to as “novel synergetic deformation mechanisms of hierarchical twin boundaries.” Understanding the fundamental mechanisms of the hierarchical twin boundaries under deformation could assist the design of strong and ductile bulk materials with hierarchical twinned structure.

Numerical Study and Experimental Validation of Deformation of &lt;111&gt; FCC CuAl Single Crystal Obtained by Additive Manufacturing

The importance of taking into account directional solidification of grains formed during 3D printing is determined by a substantial influence of their crystallographic orientation on the mechanical properties of a loaded material. This issue is studied in the present study using molecular dynamics simulations. The compression of an FCC single crystal of aluminum bronze was performed along the &lt;111&gt; axis. A Ni single crystal, which is characterized by higher stacking fault energy (SFE) than aluminum bronze, was also considered. It was found that the first dislocations started to move earlier in the material with lower SFE, in which the slip of two Shockley partials was observed. In the case of the material with higher SFE, the slip of a full dislocation occurred via successive splitting of its segments into partial dislocations. Regardless of the SFE value, the deformation was primarily occurred by means of the formation of dislocation complexes involved stair-rod dislocations and partial dislocations on adjacent slip planes. Hardening and softening segments of the calculated stress–strain curve were shown to correspond to the periods of hindering of dislocations at dislocation pileups and dislocation movement between them. The simulation results well agree with the experimental findings.

The quantitative influence of current treatment options on patellofemoral stability in patients with trochlear dysplasia and symptomatic patellofemoral instability - a finite element simulation

BackgroundTrochlear dysplasia is highly associated with patellofemoral instability. The goal of conservative and surgical treatment is to stabilize the patella while minimizing adverse effects. However, there is no literature investigating the quantitative influence of different treatment options on patellofemoral stability in knees with trochlear dysplasia. We created and exploited a range of finite element models to address this gap in knowledge. MethodsMRI data of 5 knees with trochlear dysplasia and symptomatic patellofemoral instability were adapted into this previously established model. Vastus medialis obliquus strengthening as well as double-bundle medial patellofemoral ligament reconstruction and the combination of medial patellofemoral ligament reconstruction and trochleoplasty were simulated. The force necessary to dislocate the patella by 10 mm and fully dislocate the patella was calculated in different flexion angles. FindingsOur model predicts a significant increase of patellofemoral stability at the investigated flexion angles (0°-45°) for a dislocation of 10 mm and a full dislocation after medial patellofemoral ligament reconstruction and the combination of medial patellofemoral ligament reconstruction and trochleoplasty compared to trochleodysplastic (P = 0.01) and healthy knees (P = 0.01–0.02). Vastus medialis obliquus strengthening has a negligible effect on patellofemoral stability. InterpretationsThis is the first objective quantitative biomechanical evidence supporting the place of medial patellofemoral ligament reconstruction and medial patellofemoral ligament reconstruction combined with trochleoplasty in patients with symptomatic patellofemoral instability and trochlear dysplasia type B. Vastus medialis obliquus strengthening has a negligible effect on patellar stability at a low total quadriceps load of 175 N.

Stacking fault and transformation-induced plasticity in nanocrystalline high-entropy alloys

In this work, the plastic deformation in a model nanocrystalline high entropy alloy (HEA), CoNiCrFeMn, is studied by using molecular dynamics simulations. It is found that the plastic deformation of nanocrystalline CoNiCrFeMn HEAs is dominated by a partially reversible face-centered cubic (FCC) to hexagonal close-packed (HCP) transformation mediated by stacking faults and partial dislocations, which is dramatically different from the full dislocation and deformation twinning-dominated plasticity in conventional FCC metals. This mechanism is strongly associated with the metastable nature of CoNiCrFeMn. Furthermore, although the transformed HCP structures can hinder the migration of the subsequent partial dislocations, they can penetrate each other to form a complicated stacking fault network, which is consistent with the recent experimental observations. Nevertheless, the nanocrystalline CoNiCrFeMn HEAs still show the conventional Hall–Petch breakdown when the grain sizes are reduced below a critical value. It is hoped that this study provides an atomistic insight into the plasticity of metastable HEAs and sheds some light on the design of novel HEAs for ultrahigh strength and plasticity.