Structure Of Dislocation Networks Research Articles

Improve the microstructure and properties of Al0.5CoCrFeNi compositionally complex alloy fabricated by selective laser melting with electropulsing treatment

In order to improve the microstructure of the Al0.5CoCrFeNi compositionally complex alloy (CCA) prepared by selective laser melting (SLM), the electropulsing treatment (EPT) has been used as a post processing in this study. The strengthening and softening mechanism was analyzed by comparing the microstructure of SLM-Al0.5CoCrFeNi CCAs under different EPT parameters. At a 10 s power on time, the substructure size of CCA decreases continuously with increasing voltage. Under 90V/10s EPT, the yield strength increases from 658 ± 10 MPa in the printed state to 748 ± 17 MPa, and the microhardness increases from 265.1 ± 5.6 HV0.1 to 318.9 ± 5.0 HV0.1. At 90V/20s EPT, the cellular substructure disappears, and a large number of uniformly distribute BCC phases precipitates inside the FCC matrix. At 90V/30s EPT, recrystallization occurs and the BCC phase coarsen. Dislocation network is the main factor that affects the strength of SLM-CCA. EPT can change the dislocation network structure to enhance the strength of SLM-CCA. Simultaneously, the EPT effectively promotes the transition from FCC phase to BCC phase in a short time. Joule heat and electronic wind force are the main reasons for changing structure and properties of SLM-Al0.5CoCrFeNi CCA. This work provides a post-processing method named as EPT to effectively control the microstructure in additive manufacturing CCA products.

Atomistic simulation for initiation of crystal slip deformation from surface of nanoscale copper single-crystal nanowires

In nano/micrometer-scale materials, dislocations accounting for crystal slip deformation are introduced from the surface because of lack of pre-existing dislocations inside the materials. The dislocation emission from the surface must play an important role in the formation of dislocation network structures in the nano/micrometer-scale materials under cyclic loading, which should differ from that in macroscale materials. The purpose of this study is to investigate the effect of the atomic-level structure of surfaces on the initiation of crystal slip deformation. We create copper single-crystal nanowire models with different surface structures by varying the cross-section shape and crystal orientation. To observe the initiation of the slip deformation from the surface, we conduct atomistic simulations of tensile deformation for these models. We also discuss the effect of the surface structure on the critical shear stress for slip occurrence from the surface. Our analyses reveal that the orientation of surface steps and the normal stress on the active slip plane are important factors affecting the slip occurrence. Thus, the slip occurs in a slip system so as to reduce the height of a surface step. The active slip system is affected by the cross-section shape and crystal orientation because the slip system is dependent on the relationship between slip planes and the orientation of the surface step. Our results also suggest that the cross-section shape and crystal orientation affect the critical shear stress because of the difference of the normal stress on the active slip plane according to these factors.

Nanoparticles induced intragranular and dislocation substructures in powder bed fusion for strengthening of high-entropy-alloy

In recent years, high entropy alloys (HEAs) have attracted considerable attention because of their excellent mechanical and functional properties. Despite this, as-cast high entropy alloys cannot meet the requirements for structural parts with high strength. Powder bed fusion (PBF) technology and nanoparticle additives are expected to enhance the overall properties of HEAs. We present here an AlN nanoparticle-reinforced high-entropy alloy matrix nanocomposite fabricated by powder bed fusion with excellent mechanical properties. Experimental results demonstrate that the addition of 1 wt% AlN nanoparticles results in an increase in the density of dislocations and subgrain boundaries within the HEA, in turn increasing its yield strength, ultimate tensile strength by 27.7%, 23.9% respectively, while elongation decreases slightly. In addition, the fracture toughness of the AlN/FeNiCrCo composite is greater than that of FeNiCrCo HEA because of the well-balanced combination of strength and ductility. The formation of the substructure of HEA grains was investigated using molecular dynamics (MD) by investigating the effects of rapid melting and solidification during SLM of AlN/FeNiCrCo composite. It is found that during the rapid solidification process, a large number of subgrain boundaries and dense dislocation network structures are formed in the grains due to the heat transfer and the difference in lattice structure between nanoparticles and HEA. These findings agree with the experimental results. The substructures contribute significantly to the composite's strength while maintaining its ductility, which allows it to be more suitable for practical applications.

Static Unified Inelastic Model: pre- and post-yield dislocation-mediated deformation

Modelling dislocation glide over the initial part of a stress–strain curve of metals received little attention up to now. However, dislocation glide is essential to ones understanding of the fundamental relationship between inelastic deformation and the evolution of the dislocation network structure. Therefore, we present a model of dislocation-driven deformation under static loading conditions.We reproduce repeated cyclic uniaxial tensile tests on Interstitial-Free and Low-Alloy steels. The elastic mechanical behaviour is described by isotropic linear elasticity, pre-yield anelastic mechanical behaviour by a dislocation bow-out model with dissipation, and the post-yield evolution of dislocation network structure by a statistical storage model. We hypothesise that when the local anelastic compliance is lower than the global plastic compliance, deformation is mechanically recoverable, and vice versa. This hypothesis is corroborated with the classical Taylor relation. We report the relation between stable and unstable dislocation glide using this prototypical modelling framework.We find four structural variables, that are based on dislocation physics, to describe the stress–strain curve: total dislocation density, average dislocation segment length, dislocation junction formation rate, and average dislocation junction length. Firstly, we quantify the dislocation network evolution during uniaxial monotonic loading, and verify work-hardening by dislocation junction formation and a Taylor-type equation for flow. Finally, we present a semi-empirical relation for the evolution of the dislocation network structure. Which allows us to: refine the physical interpretation of the Taylor relationship, and rationalise experimental observations on apparent modulus degradation by thermomechanical processing. Both these findings circumvent the limitations of current, physics-based hardening models.

Microstructural evolution of (FeCoNi)85.84Al7.07Ti7.09 high-entropy alloy fabricated by an optimized selective laser melting process

High-entropy alloy (HEA) FeCoNiAlTi (FCNAT) systems have been demonstrated to provide high strength while sustaining good ductility, which is of particular interest for the fabrication of structural components with complex geometries. Meanwhile, selective laser melting (SLM) technology that fabricated metallic material using layer-by-layer melting strategy can meet this demand. However, SLM fabrication technology has not yet been applied in conjunction with FCNAT-HEA systems, and the optimal SLM processing parameters and the microstructural characteristics of the resulting components remain unknown. The present study addresses this issue by fabricating highly dense (>99.9 %) cubic (FeCoNi)85.84Al7.09Ti7.07 FCNAT-HEA samples. The results demonstrate that the as-fabricated FCNAT-HEA samples possess highly non-equilibrium microstructures that form under the rapid heating and cooling cycles of the SLM process, and include typical overlapping between semi-elliptical melt pools, widely varying grain morphologies with coarse columnar grains and typical equiaxed grains with 〈001〉 growth orientation, dislocation network structures with elemental Ti and Al segregation, high-density L21-phase precipitates with sizes on the order of 200–300 nm, and near-spherical nano-sized Al-oxide particles. This research confirms the feasibility of fabricating FCNAT-HEA components by SLM, and therefore provides guidance for fabricating such HEA components with complex geometries.

Investigation on the deformation mechanisms and size-dependent hardening effect of He bubbles in 84 dpa neutron irradiated Inconel X-750

Microstructural characterization and calculation of how several intrinsic features contribute to the mechanical properties of highly strained Inconel X-750 alloy specimens irradiated to 84 dpa at two distinct temperature ranges (300 °C–330 °C and 120 °C–280 °C) were performed. The individual contributions of different microstructural features to the critical resolved shear stress (CRSS) of the material, including the γ matrix, γ′ precipitates, irradiation induced defects, and helium bubbles, were calculated. The obstacle strength of He bubbles was found to be size-dependent and a critical size was determined. For bubbles < 2.4 nm, the obstacle strength is only approximately 0.03 while it can be as high as 0.55 for bubbles > 6.6 nm. In the high temperature range (300–330 °C) irradiated specimen, localised dislocation slip was found responsible for the failure of the specimen during micro-tensile testing, associated with significant helium bubble elongation. Elongation of helium bubbles on both the primary and adjacent secondary {1 1 1} planes was likely induced by the cross slip of screw dislocations. Nanotwins were also found adjacent to the shear failure surface in the high temperature specimen, but no elongated bubbles were found within the nano-twinned region. In the low temperature range (120–280 °C) irradiated highly strained specimen, a dislocation network structure was revealed.

The Effect of a Composite Nanostructure on the Mechanical Properties of a Novel Al-Cu-Mn Alloy through Multipass Cold Rolling and Aging

An effective approach composed of solution treatment, multipass cold rolling and aging was developed to improve the strength and ductility of novel Al-Cu-Mn alloys. This approach increased the yield strength by 214 MPa over that of the conventional peak-aged samples while maintaining a good elongation of 8.7%. The microstructure evolution was examined by confocal laser scanning microscopy (CLSM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). During postaging, deformed structures underwent a considerable decrease in dislocation density and typical dislocation network structures were formed. At the same time, highly dispersed nanoprecipitates and extensive ultrafine grains and nanograins were generated. These nanoprecipitations enabled effective dislocation pinning and accumulation during tension deformation. Therefore, composite nanostructures containing ultrafine grains, nanograins, dislocation network structures and nanoprecipitates were responsible for the simultaneous increases in strength and ductility. This paper provides a new understanding of designing composite nanostructure materials for achieving high strength and good ductility that is expected to be used for other age-hardenable alloys and steels.

Role of Symmetry, Geometry, and Termination Chemistry on Misfit Dislocation Patterns at Semicoherent Heterointerfaces

<h2>Summary</h2> Interfaces in semicoherent ceramic heterostructures are characterized by a network of misfit dislocations that plays a key role in dictating structural and functional properties in this important class of materials. Prediction of a misfit dislocation network structure, given the chemistries forming the heterointerface, remains a challenging problem that requires highly specialized experimental synthesis and characterization or computation-intensive atomistic simulations. Here, we present a heuristic approach that allows us to qualitatively predict the structure of misfit dislocation networks in semicoherent ionic heterostructures. The predictions within the proposed and validated approach are based entirely on elastic effects and the nature of electrostatic interactions across the interface and, therefore, do not require any input from explicit simulations. In addition to providing an intuitive and rational justification of misfit dislocation network structure in semicoherent ionic heterostructures, our approach is expected to complement experimental and simulation-based efforts targeted at determining and quantifying misfit dislocation patterns.

Dislocation and node states in bilayer graphene systems

We investigate the electronic structure of realistic partial dislocation networks in bilayer graphene that feature annihilating, wandering, and intersecting partial lines. We find charge accumulation states at partials that are sensitive to Fermi energy and partial Burgers vector but not to the screw versus edge character of the partial. These states are shown to be current carrying, with the current density executing a spiral motion along the dislocation line with a strong interlayer component to the current. Close to the Dirac point localization on partials switches to localization on intersections of partials, with a corresponding complex current flow around the nodes.

The relaxed structure of intrinsic dislocation networks in semicoherent interfaces: predictions from anisotropic elasticity theory and comparison with atomistic simulations

We investigate the effect of nodal relaxation on the structure and energy of interfacial dislocation networks predicted by a dislocation-based simulation method. To assess the accuracy of these predictions, we compare them to corresponding atomistic simulations. Two types of interfaces are investigated: pure twist grain boundaries along {110}-type planes in niobium as well as heterophase interfaces between {111}-type planes in silver and {110}-type interfaces in vanadium. We find that dislocation core energies play a major role in the former, leading to significant discrepancies between the dislocation-based model and atomistic simulations. By contrast, core contributions do not appear to be significant in the latter, giving rise to excellent agreement between the atomistic and dislocation-based models.

Molecular dynamics study on the evolution of interfacial dislocation network and mechanical properties of Ni-based single crystal superalloys

The evolution of misfit dislocation network at γ/γ′ phase interface and tensile mechanical properties of Ni-based single crystal superalloys at various temperatures and strain rates are studied by using molecular dynamics (MD) simulations. From the simulations, it is found that with the increase of loading, the dislocation network effectively inhibits dislocations emitted in the γ matrix cutting into the γ′ phase and absorbs the matrix dislocations to strengthen itself which increases the stability of structure. Under the influence of the temperature, the initial mosaic structure of dislocation network gradually becomes irregular, and the initial misfit stress and the elastic modulus slowly decline as temperature increasing. On the other hand, with the increase of the strain rate, it almost has no effect on the elastic modulus and the way of evolution of dislocation network, but contributes to the increases of the yield stress and tensile strength. Moreover, tension–compression asymmetry of Ni-based single crystal superalloys is also presented based on MD simulations.

Mechanical behaviour and microstructural evolution of Ni-based single crystal alloys under shock loading.

Engine turbine blades are subjected to high-temperature and high-speed fragment impacts during use, and the ability of the blades to resist shocks affects their reliability. At present, there are few studies on the ability to withstand shocks of Ni-based single crystal alloys, especially with regards to their mechanical behaviour and microstructural evolution under shock loading. The solutions to the above problems can further help us understand the mechanisms of the mechanical responses and microstructural evolution of Ni-based single crystal alloys under shock loading. Thus, we study the mechanical behaviour and microstructural evolution characteristics of Ni-based single crystal alloys with different crystal orientations under shock loading using molecular dynamics simulations. We find that the (001) phase interface has the strongest impediment ability due to its dislocation network structure and the expansion of dislocations, which lead to the greatest reinforcing effect on the matrix. The penetration force of the (001) phase interface is the greatest with fragment penetration. Moreover, the energy dissipation capacity of the (001) phase interface is the highest with fragment penetration because it has the strongest resistance to shock loading. The second highest is the (110) phase interface, and the minimum dissipation capacity comes from the (111) phase interface. This study has an important theoretical significance for the in-depth understanding of the failure mechanisms of Ni-based single crystal alloys under shock loading.

Phase field crystal simulation of grain boundary motion, grain rotation and dislocation reactions in a BCC bicrystal

We investigate grain boundary motion and grain rotation in a body-centered cubic bicrystal composed of a spherical grain embedded in a single crystal matrix by three-dimensional phase-field crystal simulations. Structure and time evolution of dislocation networks formed on the grain boundary during the capillarity-driven grain shrinkage are examined. The results for initially spherical grains rotated about the [110] or [111] axes of the matrix grain reveal the formation of hexagonal dislocation networks (HDNs) on the grain boundary. We demonstrate that the anisotropic distribution of the HDNs is responsible for asymmetric shrinkage of the embedded grain. Through a detailed analysis of the HDNs, we clarify the mechanisms of dislocation reactions during the grain shrinkage in three dimensions, which include dissociation and recombination of a/2<111> and a<100> dislocations. The configuration of the HDNs is strongly affected by the rotation axis of the embedded grain. For large misorientations, the high density of the HDNs accelerates dislocation reactions and leads to very small grain rotations. However, if the misorientation is small and the dislocations are further apart, the lack of dislocation reactions on grain shrinkage results in grain rotation. Even though the rotation axis and the misorientation strongly affect the grain shape and the grain rotation, the kinetics of the grain shrinkage associated with the grain rotation follow the classical theory for grain growth: area of the embedded grain shrinks linearly with time. We also show that the stagnation of the grain rotation slows the shrinkage of the embedded grain.

A modification on Brook formula in calculating the misfit of Ni-based superalloys

The validity of the Brooke formula in estimating the lattice misfit based on dislocation network structure in Ni-based superalloys is investigated. The misfit value of a sample subjected to prolonged thermal exposure as derived from the popular Brooke formula is below that determined by X-ray diffraction (XRD) method, whereas the misfit estimated using the Brooke formula for a sample subjected to creep is greater than that determined by XRD method. These discrepancies imply the invalidity of the popular Brooke formula in two aspects: that a dislocation network is not always mature to fully relax the lattice misfit leading underestimate the misfit and that using the full magnitude of the Burgers vector overestimates the misfit. To overcome this problem, a modified Brook formula using the {100} projection of the edge component of a Burgers vector as the effective magnitude to relieve the misfit is proposed. The misfit derived from the modified Brook formula is in good agreement with the values determined by XRD method for a number of Ni-based superalloys studied.

Influences of strain rate on the low cycle fatigue behavior of gravity casting Al alloys

The strain-controlled low cycle fatigue properties were evaluated on specimens of 333 aluminum alloy at different strain rates. The material exhibited initial cyclic hardening followed by cyclic stabilization at the strain rates of 0.15min−1, 0.20min−1, 0.30min−1 and 0.32min−1, while the material showed continuous cyclic hardening at the strain rate of 0.25min−1. The observed phenomena in hysteresis loops were in accord with the cyclic hardening and stability characteristics. At the strain rates of 0.15min−1 and 0.32min−1, the cyclic hardening characteristics observed were mainly due to the existence of dislocation network structures, as well as the accumulation of dislocations around some obstacles. The cyclic stabilization observed in both samples was correlated closely with the cross-slip during cycling at the strain rates of 0.15min−1 and 0.32min−1. At the strain rate of 0.25min−1, the continued cyclic hardening observed in the specimen was bound up with the evolution of dislocation loops. The fracture surface for the alloy exhibited some dimples and tear ridges at the strain rates of 0.15min−1 and 0.25min−1, whereas some fatigue striations were observed at the strain rate of 0.32min−1. The damage process of the 333 aluminum alloy composed of several mixed events: α-Fe/silicon particle cracking, short crack initiation and propagation, and local linkage of short cracks.

Structure and energy of (1 1 1) low-angle twist boundaries in Al, Cu and Ni

We study the structure and energy of (111) low-angle twist boundaries in face-centered cubic Al, Cu and Ni, using a generalized Peierls–Nabarro model incorporating the full disregistry vector in the slip plane and the associated stacking fault energy. It is found that dislocation network structures on these twist boundaries can be determined by a single dimensionless parameter, with two extreme cases of a hexagonal network of perfect dislocations and triangular network of partial dislocations enclosing stacking faults. We construct a simple model of these networks based upon straight partial dislocation segments. Based on this structural model, we derive an analytical expression for the twist boundary energy as a function of the twist angle θ, intrinsic stacking fault energy and parameters describing the isolated dislocation core. In addition to the θ and θlogθ terms [3] and the θ2 term [17] in the boundary energy, our new energy expression also includes additional terms in θ that represent the partial dissociation of the dislocation nodes and the effect of the stacking fault energy. The analytical predictions of boundary structure and energy are shown to be in excellent agreement with the Peierls–Nabarro model simulations.

Molecular Dynamics Studies of Dislocations in CdTe Crystals from a New Bond Order Potential

Cd1-xZnxTe (CZT) crystals are the leading semiconductors for radiation detection, but their application is limited by the high cost of detector-grade materials. High crystal costs primarily result from property nonuniformity that causes low manufacturing yield. Although tremendous efforts have been made in the past to reduce Te inclusions/precipitates in CZT, this has not resulted in an anticipated improvement in material property uniformity. Moreover, it is recognized that in addition to Te particles, dislocation cells can also cause electric field perturbations and the associated property nonuniformities. Further improvement of the material, therefore, requires that dislocations in CZT crystals be understood and controlled. Here, we use a recently developed CZT bond order potential to perform representative molecular dynamics simulations to study configurations, energies, and mobilities of 29 different types of possible dislocations in CdTe (i.e., x = 1) crystals. An efficient method to derive activation free energies and activation volumes of thermally activated dislocation motion will be explored. Our focus gives insight into understanding important dislocations in the material and gives guidance toward experimental efforts for improving dislocation network structures in CZT crystals.

Evolution of misfit dislocation network and tensile properties in Ni-based superalloys: a molecular dynamics simulation

The structural evolution of dislocation network is closely related to ′ rafting and tensile properties. In this work, the effects of strain rate and temperature on the structural evolution of interface dislocation network in Ni-based superalloys are studied by molecular dynamics simulations. The correlation between the evolution of dislocation network and tensile properties is also explored. The results indicate that the dislocation network shows different degrees of deformation and damage at various strain rates and temperatures. The ′ rafting depends on the damage structure of dislocation network at various strain rates and temperatures. Moreover, the tensile properties of interface in Ni-based superalloys are closely related to the evolution of dislocation network and dislocation motion mechanisms. Ni-based superalloys, tensile properties, dislocation network, rafting, molecular dynamics simulation

Theoretical study of (Ag,Au and Cu)/Al2O3 interfaces

Interface structure is a basic problem in interface and surface science. It is usuallyindicated by atomic positions for an ideal interface. But this way is sometimesunsuitable for a mismatch interface, because there are too many atoms underconsideration, whose coordinates may confuse our mind in understanding the interfacestructure. In this case, a ‘dislocation representation’ is introduced. A misfit dislocationnetwork is used as an effective representation of the interface structure. However,there are two questions on this topic. How to determine the dislocation networkstructure? And how to relate it to interface dynamics? In this paper, we work on thefirst question and make an effort to build up the ‘dislocation representation’ formetal/Al2O3 interfaces.

Phase equilibria, crystal growth and characterization of the novel ferroelectric tungsten bronzes (CBN) and (CSBN)

Abstract Phase diagram analyses and crystal growth experiments of the novel ferroelectric tetragonal tungsten bronzes Ca x Ba 1 - x Nb 2 O 6 (calcium barium niobate, CBN) and Ca x Sr y Ba 1 - x - y Nb 2 O 6 (calcium strontium barium niobate, CSBN) were performed in this study. The quasi-binary phase diagram BaNb 2 O 6 – CaNb 2 O 6 was found to be similar to the related system BaNb 2 O 6 – SrNb 2 O 6 , but with a remarkably reduced existence region. In the quasi-ternary phase diagram CaNb 2 O 6 – SrNb 2 O 6 – BaNb 2 O 6 the stability field of Ca x Sr y Ba 1 - x - y Nb 2 O 6 mixed crystals was determined by X-ray powder diffraction of sintered and annealed samples. Large, transparent, and nearly colorless CBN and CSBN single crystals of high quality were grown by the Czochralski method. The dislocation network structure of the core region in the as-grown crystals was analyzed by X-ray topography employing white beam synchrotron radiation. Precise measurements of the refractive index dispersion showed a composition dependance only for the extraordinary beam. The Curie temperature of the ferroelectric phase transition 4 m mm → 4 mm determinated by calorimetric measurements exhibited a clear composition dependance. It is expected that in the complete mixed crystal system Ca x Sr y Ba 1 - x - y Nb 2 O 6 the Curie temperature could be varied between about 80 ∘ C for strontium barium niobate with x Sr = 0.61 and about 300 ∘ C for CBN with x Ca ≈ 0.20 , the lowest possible Ca content.