Formation Of Dislocation Structure Research Articles

Blown-powder direct-energy-deposition of titanium-diboride-strengthened IN718 Ni-base superalloy

This paper reports on the adoption of TiB2 as an inoculant to fabricate IN718 via the direct energy deposition (DED) process. Effective grain refinement and low texture were achieved in IN718/TiB2 using a TiB2 powder size of d90 = 10 μm and mass fraction of 1.5 wt%. The use of low linear energy density (33.08 J/mm) produced IN718/TiB2 deposits free from large grains (>300 μm), however at the cost of the formation of interlayer defects. By comparison, the large grains were present in deposits made with a linear energy density of 78.74 J/mm and also in deposits manufactured without an included inoculant. Production of deposits at lower energies without interlayer defects was possible by reducing the powder flow rate to 7 g/min. However, this caused a moderate increase in grain size. The TiB2 inoculant reduced the Laves phase network by replacing it with homogeneously distributed Cr-, Mo-, Nb-, and B-enriched needle-shaped precipitates. Tensile strength increased by 300–500 MPa with TiB2 addition, but at the cost of significant ductility drop, regardless of the deposition conditions. The IN718 deposit displayed many micro-cracks at the network of Laves phase during tensile loading, whereas micro-cracks in the IN718/TiB2 occurred at the interface between the needle-shaped precipitates and the matrix. The strength enhancement in IN718/TiB2 was by a combination of strengthening mechanisms: grain boundary, dislocation structure formation, Orowan-type and load transfer related to the needle-shaped precipitates.

Modeling the evolution of representative dislocation structures under high thermo-mechanical conditions during Additive Manufacturing of Alloys

Mesoscale simulations of discrete defects in metals provide an ideal framework to investigate the micro-scale mechanisms governing the plastic deformation under high thermal and mechanical loading conditions. To bridge size and time-scale while limiting computational effort, typically the concept of representative volume elements (RVEs) is employed. This approach considers the microstructure evolution in a volume that is representative of the overall material behavior. However, in settings with complex thermal and mechanical loading histories careful consideration of the impact of modeling constraints in terms of time scale and simulation domain on predicted results is required. We address the representation of heterogeneous dislocation structure formation in simulation volumes using the example of residual stress formation during cool-down of laser powder-bed fusion (LPBF) of AISI 316L stainless steel. This is achieved by a series of large-scale three-dimensional discrete dislocation dynamics (DDD) simulations assisted by thermo-mechanical finite element modeling of the LPBF process. Our results show that insufficient size of periodic simulation domains can result in dislocation patterns that reflect the boundaries of the primary cell. More pronounced dislocation interaction observed for larger domains highlight the significance of simulation domain constraints for predicting mechanical properties. We formulate criteria that characterize representative volume elements by capturing the conformity of the dislocation structure to the bulk material. This work provides a basis for future investigations of heterogeneous microstructure formation in mesoscale simulations of bulk material behavior.

Inductive Determination of Rate-Reaction Equation Parameters for Dislocation Structure Formation Using Artificial Neural Network

The reaction-diffusion equation approach, which solves differential equations of the development of density distributions of mobile and immobile dislocations under mutual interactions, is a method widely used to model the dislocation structure formation. A challenge in the approach is the difficulty in the determination of appropriate parameters in the governing equations because deductive (bottom-up) determination for such a phenomenological model is problematic. To circumvent this problem, we propose an inductive approach utilizing the machine-learning method to search a parameter set that produces simulation results consistent with experiments. Using a thin film model, we performed numerical simulations based on the reaction-diffusion equations for various sets of input parameters to obtain dislocation patterns. The resulting patterns are represented by the following two parameters; the number of dislocation walls (p2), and the average width of the walls (p3). Then, we constructed an artificial neural network (ANN) model to map between the input parameters and the output dislocation patterns. The constructed ANN model was found to be able to predict dislocation patterns; i.e., average errors in p2 and p3 for test data having 10% deviation from the training data were within 7% of the average magnitude of p2 and p3. The proposed scheme enables us to find appropriate constitutive laws that lead to reasonable simulation results, once realistic observations of the phenomenon in question are provided. This approach provides a new scheme to bridge models for different length scales in the hierarchical multiscale simulation framework.

Dislocation structures formation induced by thermal stress in additive manufacturing: Multiscale crystal plasticity modeling of dislocation transport

The motion of dislocations governs the plastic deformation of crystalline materials, which in turn determines the mechanical properties. The complex thermal history, large temperature gradients and high cooling rates during the process of additive manufacturing (AM) can induce high dislocation density and unique dislocation structures in the material. The origin of these dislocation structures and their stability during mechanical loading are debated. A novel temperature dependent continuum dislocation dynamics (CDD) model is developed, in which four state variables are used for each slip system representing the total dislocation density, edge and screw geometrically necessary dislocation densities and dislocation curvature. The CDD model is fully coupled with a crystal plasticity solver, which captures the plastic deformation induced by the dislocation motion. A hybrid continuous and discontinuous Galerkin formulation is developed to accurately reproduce the dynamics of highly discontinuous dislocation density fields that are typical of dislocation structures. A multiscale modeling approach is used, in which the thermally induced deformation in specific grains of a polycrystal is extracted from larger scale crystal plasticity simulations of the laser powder-bed fusion process, and is then used for single crystal scale dislocation dynamics simulations. Simulation results reveal the dynamics of dislocation structure formation in grains at different positions during laser scanning and cooling stages. The effect of the cyclic thermal stress during multi-layer AM fabrication is also investigated. The simulations provide a new perspective on the specific conditions that should be satisfied during AM process for the formation of stable dislocation structures.

Effect of strain of cryorolling on structure and strength of nickel

The effect of isothermal rolling at liquid nitrogen temperature with reductions up to 90 % on the structural-mechanical behavior of coarse-grained nickel was evaluated by TEM, SEM-EBSD and X-ray analysis methods, and microhardness measurements. The highest hardening rate of the metal was found at the initial stage of rolling (up to about 50 %), after which it decreased slightly with further reductions. Detailed analyses of the evolution of dislocation density, grain size and grain boundary spectra have shown that the behavior found was attributed to an extremely low deformation temperature, resulted in a decreased rate of accumulation of crystalline defects, as well as the formation and rearrangement of dislocation structure. Due to the strong suppression of dynamic recovery and recrystallization, even when rolling to a reduction of 90 %, a relatively homogeneous coarse-fibered structure with a developed nanocellular substructure and less than 5 % fraction of nanoscale dynamically recrystallized grains was formed. It was concluded that the main structural strengthening factors in cryogenically rolled nickel was the formation of low-energy dislocation structures and their transformations into a well-developed substructure with nanosized crystallites separated predominantly by low-angle boundaries.

Effect of secondary austenite on fatigue behavior of S32750 super duplex stainless steel

Tension-tension fatigue tests were carried out to investigate the effect of secondary austenite (γ2) on the fatigue behavior of S32750 super duplex stainless steel (SDSS). Stress-life curves were fitted and analyzed in detail. It was demonstrated that γ2 deteriorates the fatigue resistance of SDSS. To further study the role of γ2 in fatigue deformation, fracture surfaces were examined. γ2 increased the probability of ferrite as a crack initiation source, which could be attributed to the combined variation of the ferrite/austenite interface relationship and alloying elemental distribution. γ2 increased the ratio of Kurdjumov-Sachs/Nishiyama-Wassermann crystallographic orientation, making the slip systems crossing the ferrite/austenite boundaries easier to affect the formation of fine-scaled dislocation structure in ferrite. Moreover, segregation of N enhanced the strength of γ2, thus leading to the earlier plastic deformation of partial ferrite in the fatigue deformation process.

Dislocation structure analysis in the strain gradient of torsion loading: a comparison between modelling and experiment

Complex stress states due to torsion lead to dislocation structures characteristic for the chosen torsion axis. The formation mechanism of these structures and the link to the overall plastic deformation are unclear. Experiments allow the analysis of cross sections only ex situ or are limited in spacial resolution which prohibits the identification of the substructures which form within the volume. Discrete dislocation dynamics simulations give full access to the dislocation structure and their evolution in time. By combining both approaches and comparing similar measures the dislocation structure formation in torsion loading of micro wires is explained. For the ⟨100⟩ torsion axis, slip traces spanning the entire sample in both simulation and experiment are observed. They are caused by collective motion of dislocations on adjacent slip planes. Thus these slip traces are not atomically sharp. Torsion loading around a ⟨111⟩ axis favors plasticity on the primary slip planes perpendicular to the torsion axis and dislocation storage through cross-slip and subsequent collinear junction formation. Resulting hexagonal dislocation networks patches are small angle grain boundaries. Both, experiments and discrete dislocation simulations show that dislocations cross the neutral fiber. This feature is discussed in light of the limits of continuum descriptions of plasticity.

Strain gradient induced grain refinement far below the size limit in a low carbon hypoeutectoid steel (0.19 wt% C) via pipe inner surface grinding treatment

A low carbon hypoeutectoid steel (0.19 wt% C) with proeutectoid ferrite and pearlite dual-components was subjected to surface plastic deformation via pipe inner surface grinding (PISG) at room temperature. The deformation microstructures for each component were systematically characterized along depth, and the patterns of structural evolution toward nanometer regime as well as the governing parameters were addressed. Proeutectoid ferrite grains were refined down to 17 nm, and the pattern covering a length scale of 4–5 orders of magnitude from micron- to nanometer-scale follows: formation of cellular dislocation structure (CDS), elongated dislocation structure (EDS), ultrafine lamellar structure (UFL) and finally the nanolaminated structure (NL). The pearlite experiences the deformation and refinement, and finally the transforming the ultrafine pearlite (UFP) into nanolaminated pearlite (NLP) with the ferrite lamellae as thin as 20 nm. Refinement for both UFL (UFP) and NL (NLP) can be realized via forming novel extended boundaries within ferrite lamellae. A critical lattice curvature of ∼2.8° is required for forming such extended boundary, corresponding to a minimum strain gradient of 0.25 μm−1 for a 100 nm-thick lamella. Refinement below size limit (expressed by lamellar thickness dT in nm) is correlated with the strain gradient (χ, in μm−1) by: dT=12.5χ. Refinement contributions from strain gradient caused by PISG processing and material heterogeneity were discussed.

Dislocation and grain structure formation in Nb filaments of different geometry at fabrication of Nb3Sn-based superconducting wires

The structure of Nb filaments of different geometry artificially doped with NbTi alloy in Nb/Cu-Sn composites after various regimes of deformation beginning from the very first stages of fabrication of thin multifilamentary wires has been studied by transmission electron microscopy, optical microscopy and X-ray analysis. The structure of Nb filaments in composites of different designs is compared, and the development of dislocation and grain structure and texture of niobium in monolithic and assembled filaments under deformation with intermediate annealings is analyzed.

Analysis of dislocation microstructure characteristics of surface grains under cyclic loading by discrete dislocation dynamics

The dislocation structure formation under low-amplitude fatigue in fcc metals for multislip loading conditions is investigated using three-dimensional discrete dislocation dynamics. Tools based on graph analysis, a statistical description of stable dislocation arrangements such as dislocation dipoles and prismatic loops are developed and applied. Upon decreasing the loading amplitude one order of magnitude below the persistent slip band threshold, although qualitative microstructural differences are seen, the elementary features of the investigated defects are the same. A critical number of cycles is required to produce sessile Lomer junctions that stabilize the structure and result in dislocation clustering around them. The crystallographic orientation of the crystal with respect to the loading axis results in different patterns strongly linked to sessile junctions, which are analyzed using spatial correlation functions. The increase in irreversible bulk dislocation arrangements results in roughening of the free surface and increase in surface step heights. Furthermore the crystallographic orientation with respect to the free surface is shown to control the dislocation density evolution combined with the macroscopic Schmid factor.

Influence of melting current pulse duration on microstructural features and mechanical properties of AA5083 alloy weldments

The formation of intermetallic particles and dislocation structure play a vital role in strengthening aspects of non-heat treatable aluminium alloy weldments. In this work, the effect of melting current pulse duration (McT) on grain boundary characteristics, dislocation structure, size and distribution of intermetallics on AA5083-H111 aluminium alloy weldments processed by pulsed gas tungsten arc welding are investigated. The mechanical responses of the weldments are evaluated by impression creep test, tensile test and microhardness measurements. The results suggest that at both lower and higher current level, the use of shorter McT (40%) had retained a considerable amount of Mg at fusion zone and assisted in nucleation of β-phase precipitates at various sites of grain boundaries whereas the change in McT from 40% to 60% produced grain boundaries free of β-phase precipitates. Also, the longer McT (60%) at a lower current level produced rod-like finer (~50–400 nm) Mn-rich intermetallics at grain boundaries and distributed uniformly at grain interiors. It caused a rise in activation energy which leads to an increase in pinning pressure during plastic deformation and resulted in higher tensile strength and creep resistance. In contrast, the longer McT (60%) at a higher current level encouraged the formation of plate-like coarser (~527 nm length and 419 nm width) Mn-rich intermetallics at random locations allowed the grain boundaries to slide at various points during deformation and resulted in lower mechanical properties. This study shows that the change in McT caused significant variations in mechanical properties by altering the microstructural features of weldments.

High-temperature low-cycle fatigue behavior and microstructural evolution of an improved austenitic ODS steel

Abstract

Влияние обработки в тлеющем разряде на формирование дислокационной структуры в молибдене

Влияние обработки в тлеющем разряде на формирование дислокационной структуры в молибдене

Effect of cryogenic deformation on microstructure and mechanical properties of 304 austenitic stainless steel

304 austenitic stainless steel plates have been deformed (10 to 40%) by multi-pass cold rolling incorporating soaking at 0°C and −196°C after each pass with an aim to correlate the microstructure and mechanical properties under cold/cryogenically deformed conditions. Characterisation of phase constituents, microstructure and mechanical properties of such steel specimens has been conducted after processing under different schedules. Rolling of the investigated steel at near cryogenic temperature results into the formation of extended stacking faults, ε-martensite and α′-martensite in contrast to the formation of homogeneous dislocation structure along with α′-martensite in the samples rolled at 0°C, which can be correlated with temperature dependent stacking fault energy. EBSD phase analysis reveals 46.3% and 69.2% α′-martensite in the austenitic matrix for 10% and 20% deformation at −196°C, respectively. Deformation twins are evident in all the samples rolled at 0°C as well as −196°C. 40% cold deformation at 0°C leads to high strength (1225MPa) and 13% total elongation, whereas comparatively lower 10–20% deformation at −196°C leads to higher level of strength (1306–1589MPa) with 15–9% elongation due to the formation of the higher volume fraction of strain induced martensite (ε/α′).

Distribution of Dislocations near the Interface in AlN Crystals Grown on Evaporated SiC Substrates

To exploit unique properties of thin films of group III-nitride semiconductors, the production of native substrates is to be developed. The best choice would be AlN; however, presently available templates on sapphire or SiC substrates are defective. The quality of AlN could be improved by eliminating the substrate during the layer growth. In this paper, we demonstrate freestanding AlN layers fabricated by an SiC substrate evaporation method. Such layers were used to investigate dislocation structures near the former AlN–SiC interface. Specimens were characterized by synchrotron radiation imaging, triple-axis diffractometry and transmission electron microscopy (TEM). We found that the evaporation process under non-optimal conditions affected the dislocation structure. When the growth had been optimized, AlN layers showed a uniform distribution of dislocations. The dislocations tended to constitute low-angle subgrain boundaries, which produced out-of-plane and in-plane tilt angles of about 2–3 arc-min. Similar broadening was observed in both symmetric and asymmetric rocking curves, which proved the presence of edge, screws as well as mixed dislocation content. TEM revealed arrays of edge threading dislocations, but their predominance over the other threading dislocations was not supported by present study. To explain the experimental observations, a theoretical model of the dislocation structure formation is proposed.

Specific features of the formation of dislocation structure in gallium arsenide single crystals obtained by the Czochralski method

The influence of the deviation of seed orientation from the [100] direction on the formation of a dislocation structure of gallium arsenide single crystals grown by the Czochralski method has been revealed. The intensive multiplication of dislocations and formation of a block structure occur at deviation by an angle of more than 3° in the region that is radially shifted to one of crystal sides. The linear density of dislocations in the walls changes from 1 × 104 cm–1 in low-angle boundaries to 6 × 104 cm–1 in subboundaries.

STRAIN HARDENING OF MONOCRYSTALS OF ALLOY FCC AT MESOLEVEL

The paper presents the analysis of strain hardening in monocrystals of single-phase disordered alloy Ni 3 Fe. These monocrystals are subjected to compression at room temperature. The compression axis is parallel to [001] crystallographic direction. The strain curve of monocrystals with [001] orientation is characterized by several stages conditioned by a certain sequence of substructural transformations. Ni 3 Fe alloy with monocrystals of atomic short-range order possesses an average value of stacking fault energy. Plastic deformation enables the low-energy evolutionary branch of substructure: plane dislocation clusters → knitted structure → striple structure. The linear stage of the alloy strain hardening is connected with the formation of non-homogeneous knitted dislocation structure. TEM images of this structure allow measuring the free distances between the different dislocation locks formed along the dislocation line due to the dislocation intersections of different slip systems. Using the parameters measured for the monocrystal knitted structure, the contribution of strain-hardening mechanisms to shear stress was evaluated. These mechanisms include dislocation intersection, threshold creep, formation and destruction of dislocation junctions, crossing of Lomer–Cottrell and Hirth dislocation barriers and spot defect generation. The formation laws for long-range stresses and elastic interaction between dislocations were studied and the static and dynamic stress contribution to the total stress was determined. To consider the non-homogeneity of knitted dislocation structure, the contributions are detected individually for its dense and loose areas. The estimation of partial contribution made by each mechanism indicates that the main impact to deformation resistance of monocrystals oriented for multiple slip is made by the dislocation hindering, caused by contact interaction between moving and forest dislocations. The deformation growth enables the density increase in the dislocation locks (thresholds and junctions) along the dislocation line, caused by strain hardening of alloy FCC having an atomic short-range order.

Formation of deformation substructures in FCC crystals under the influence of point defect fluxes

The article deals with sub-structural transformations in FCC crystals under the influence of point defect fluxes. Different relationships between accumulation of point defects in crystal and substructure transformations, in particular during the process of fragmented dislocation structure formation have been received.

Formation of grains and dislocation structure of geometrically necessary boundaries

A continuum dislocation model of formation of grains whose boundaries have a non-vanishing thickness is proposed. For a single crystal deforming in simple shear the lamellar structure of grains with thin layers containing dislocations as the geometrically necessary boundaries turn out to be energetically preferable. The thickness and the energy of this type of grain boundary are computed as functions of the misorientation angle.

A dislocation kinetic model of the dislocation structure formation in a nanocrystalline material under intense shock wave propagation

A dislocation kinetic model of the formation and propagation of plastic shock waves in nanocrystalline materials (with a grain size of 1–100 nm) at pressures ranging from 1 to 50 GPa has been discussed theoretically. The model is based on a nonlinear equation of the reaction-diffusion type for the dislocation density, which includes the processes of multiplication, annihilation, and diffusion of dislocations with a strong absorption of the dislocations by nanograin boundaries. The solution of this equation is obtained in the form of a traveling dislocation density wave propagating with a constant velocity. The dependences of the dislocation density and dislocation front width on the nanograin size and pressure in the wave are determined. A comparison of the obtained dependences with the available results of the experiments and molecular dynamics simulations of shock-deformed nanocrystalline materials demonstrates their good quantitative agreement.