Redistribution Of Dislocations Research Articles

Deformation mechanisms of additively manufactured TiNbTaZrMo refractory high-entropy alloy: The role of cellular structure

Additive manufacturing of refractory high-entropy alloys (RHEA) is challenging, limited information is available on the microstructure and mechanical properties of as-built RHEA. In this work, Ti1.5Nb1Ta0.5Zr1Mo0.5 (TNTZM; at%) alloy is successfully fabricated by additive manufacturing. It is characterized by distinctive cellular structures, exhibited outstanding plasticity of 50 %, and a yield strength of 904 MPa. The cellular structure hindered dislocations transfer between adjacent cells and limited dislocation slip inside the cells, thereby significantly improving strength and uniform deformation. Moreover, the cellular structure facilitated the redistribution of dislocations during later deformation to avoid strain concentrations within the grains. The regulated dislocation initiation on demand during the evolution of the cellular structure at the different deformation stages is proposed. This work provides new insights improving the mechanical properties of additive manufactured RHEA.

Porosity and microstructure of L-PBF printed AlSi10Mg thin tubes in laser shock peening

Laser powder bed fusion (L-PBF) has emerged as one of the most promising technologies for producing complex geometries that are difficult to achieve with other methods. However, its widespread adoption is hindered by issues such as deleterious microstructure, tensile residual stresses, and porous structure, mainly while working with aluminum alloys. To address these challenges, laser shock peening (LSP) offers a potential solution by mitigating the negative effects associated with aluminum L-PBF. This study investigates the impact of the important LSP parameters, namely energy, spot size, and overlap on L-PBF printed thin AlSi10Mg tubes. A total of 17 specimens were examined by varying the mentioned parameters at three levels each. The outcome of the study was evaluated in terms of residual stresses, porosity, microstructure and surface roughness. The results have shown significant improvements in residual stresses, where a maximum improvement of over 200% was observed and a decrease in porosity by 70%. Furthermore, the microstructure analysis revealed grain refinement and dislocation redistribution as material reactions, aligning with the observed microhardness increase. These findings demonstrate the viability of LSP as a post-processing method for demanding applications, effectively addressing the limitations of the L-PBF process.

A Lie-algebra-based description of disclination densities and the quantification of partial disclinations in deformed polycrystalline metals

Crystalline defects (e.g., dislocations, disclinations and grain boundaries) play a critical role in determining the mechanical and functional properties of metallic materials. From a physical point of view, dislocations and disclinations are the two fundamental types of topological defects in crystals, which originate from the breaking of translational symmetry and rotational symmetry, respectively. In spite of extensive studies on dislocations in the literature, the effects of disclinations on mechanical properties of metals have not been well recognized, partially due to the lack of an appropriate description for the rotational nature of disclinations. Here we suggest a Lie-algebra-based method to quantify the rotational properties of disclinations, which leads to a convenient way to determine disclination density distribution directly from Electron Backscatter Diffraction data. Through quasi-in-situ Electron Backscatter Diffraction characterizations, three major formation mechanisms of disclinations have been identified in deformed polycrystalline Mg alloys, i.e., dislocation-grain boundary interaction, dislocation redistribution and twin-twin reaction, all of which can be treated as topological reactions among various types of defects under a general framework. Our work not only suggests a new mathematical tool to investigate the interactions and reactions among multiple types of crystalline defects, but also provides a new insight to understand the deformation behaviors of metals and alloys based on dislocation/disclination theory.

Mechanism of Microstructural Alterations of M50 Bearing Steel during Rolling Contact Fatigue under High Loads

Rolling contact fatigue (RCF) of vacuum induction melted–vacuum arc remelted (VIM-VAR) M50 bearing steel under high loads was carried out, using a three-ball-rod RCF tester. Dark etching regions (DER) and butterflies were found in the subsurface region below the raceway of the RCF-tested sample. The DER appeared in the region of maximum shear stress located at a depth of 30 μm to 170 μm below the raceway. Carbon atoms migrated through high-density dislocations, and part of the martensite plates was transformed into cellular ferrites, due to the redistribution of dislocations during the deformation of martensite under the action of cyclic shear stress. Butterflies appeared in the region of maximum shear stress located at a depth of 20 μm to 314 μm below the raceway. Butterflies were initiated in the primary carbides, with length values ranging from 5 μm to 15 μm. The plate martensite in the butterfly wings was transformed into nanocrystalline ferrites, due to the increase in the dislocation density and rearrangement of dislocations during the extension of fatigue cracks from the primary carbides to the matrix under cyclic shear stress.

Magnetic Field‐Assisted Laser Shock Peening of Ti6Al4V Alloy

Laser shock peening is widely used in mechanical property improvements of metallic materials. One concerning factor is the nonuniformity of mechanical properties and microstructures, such as surface hardness and grain refinement. Herein, a new strengthening strategy that integrates femtosecond laser shock peening and pulse magnetic field that can uniformize the performance of metallic materials is proposed. With the magnetic field‐assisted laser shock peening (MFLSP), the reduction of preferred‐clustering points shows uniform grain refinement with the maximum area fraction of small grains. The fabrication of uniform grain refinement is attributed to the dislocation redistribution by the release of pinning effect. With magnetic field stimulation, dislocation lines are observed to disperse from low‐angle grain boundary to the intragranular area. Dislocation behaviors stimulated by magnetic field are attributed to atomic migration. The dispersed dislocations serve as potential glide sources for grain refinement during laser‐induced plastic deformation. The enhanced grain refinement results in uniform surface hardening of metallic materials. The application of this integrated strategy with laser processing and external energy field can be used for adjusting mechanical properties of metallic materials with uniform microstructures and surface hardness.

Geometrically necessary dislocations and related kinematic hardening in gradient grained materials: A nonlocal crystal plasticity study

Gradient grained metals whose microstructure is characterized by a spatially graded grain size distribution show a better strength-ductility combination than their homogeneous counterparts. Kinematic hardening associated with geometrically necessary dislocations (GNDs) is considered to be a dominant strengthening mechanism in gradient grained metals. However, the precise kinematics of GND accumulation and the nature of the back stress fields remain unclear, restricting the understanding of their deformation mechanisms. In this work, a nonlocal crystal plasticity model which explicitly accounts for the interaction between dislocations and grain boundaries is developed. The nonlocal feature is achieved by introducing a flux term to account for the spatial redistribution of dislocations due to their motion. In addition, back stress produced by the spatial variation of GND density introduces an explicit internal length scale into the model. The nonlocal nature of the model on the slip system level enables the direct investigation of strain gradient effects caused by internal deformation heterogeneities. Furthermore, the interaction between dislocations and grain boundaries leads to the formation of pileups near grain boundaries, which is key to studying the grain size effects in polycrystals. Finite element implementation of the model for polycrystals with different grain sizes quantitatively captures the grain size effect. Simulation results of gradient grained materials and their homogeneous counterparts demonstrate that smaller grains lead to higher GND density and enhanced back stress. Small grains significantly contribute to the GND-induced isotropic hardening and GND-induced kinematic hardening in gradient grained metals. This investigation helps to understand the underlying strengthening mechanisms of gradient grained metals, and the model can be readily applied to other kinds of heterogeneous materials.

Significant improvement in fatigue life of titanium alloy induced by superlattice coating

The purpose of this paper is to investigate the effect of the TiN coating and the TiN/AlN superlattice coating on the fatigue performance of titanium alloys. The effect of the coating on the fatigue property of the titanium alloy is studied through the coating-substrate interaction. The results show that the TiN/AlN superlattice coating with high hardness and fracture toughness inhibits strain localization on the surface, changes the location of fatigue crack initiation and improves the fatigue strength at 107 cycles by 23.7%. Dislocation redistribution and “secondary slip” are found in the titanium alloy to eliminate stress concentration.

Regularities of changing the dimensions of the main bore of the cylinder of TMZ-450D diesel engine during the technological process

The article deals with the problems arising during the mechanical and thermal treatment of the TMZ-450D diesel engine cylinder, which is the base part of the engines of small-sized generators and compressors, which are widely used for mobile units in the oil and gas and mining industries. It was found that the metal in the casting has a non-uniform structure, the density of which ranges from 6.75 to 7.25 g/cm3. Redistribution of dislocations and residual stresses in the casting leads to significant changes in the size and shape of the main bore. In addition to the successive changes in size specified by the technology due to the removal of the designated allowance, the dimensions and shape change arbitrarily, uncontrollably in the course of the technological process. It is shown that artificial aging by a thermal method does not provide the desired dimensional stability; therefore, it is proposed to supplement it with natural aging after rough boring for six months. It was revealed that the use of morally and physically outdated equipment makes it necessary to increase the number of finishing operations of honing and, accordingly, to increase the labor intensity of cylinder manufacturing. The use of a two-position boring machine is substantiated, on which the transitions of semi-finishing and fine boring are combined. This completely eliminates the copying of errors that arose when changing the base on previous operations. The use of a two-position modular boring machine ARS-4/Ts of increased accuracy and rigidity significantly increases the accuracy of the bore hole, which makes it possible to reduce the number of honing operations. A variant of the technological process of mechanical and heat treatment is proposed, including natural aging, the use of double boring on a modular boring machine, which will reduce the number of honing operations to one, including rough and finish transitions.

Study on the Dislocation Density and Deformation Mechanism of Superalloy IC10 at 700 °C

Superalloy IC10 is one of the materials used for hot section components of new generation aero engine. High temperature tensile tests are carried out for IC10 under different strains at 700 °C. The dislocation density increases with the increase of the strain. At early stage of deformation (strain <3%), the motion of dislocation is mainly described as climbing and shearing. When the deformation further increases (strain <6%), cross slip of the screw dislocation is found and various microstructures, such as stacking faults, K-W locks and dislocation dipoles are observed in IC10 superalloy. At last stage of deformation (fracture), many dislocations pile up at phase boundary, leading to the reorganization and redistribution of dislocations. The mechanical properties of IC10 alloy at 700 °C are mainly featured by strain hardening, which is concerned to the increase of dislocation density, and to the microstructure of dislocation cell, stacking fault and K-W lock. Finally, the dislocation density is used in the constitutive model to describe the flow behavior of IC10 alloy at 700 °C.

Deformation characteristics of ultrafine grained and nanocrystalline iron and pearlitic steel - An in situ synchrotron investigation

The deformation characteristics of ultrafine grained (UFG) iron, ultrafine lamellar (UFL) pearlite and nanolamellar (NL) pearlite were investigated using in situ synchrotron compression tests. The direct measurement of the peak position and peak width of the transmitted diffraction rings provided insights into the correlation between the strain hardening and the evolution of coherent domain size and dislocation density. The results showed that the hard cementite supported the initial hardening in UFL pearlite, while the dislocation density in the ferrite lamellae gradually increased. At higher stresses, the cementite lamellae deformed plastically or fractured, thus transferring the hardening process to the ferrite lamellae. On the other hand, in UFG iron and NL pearlite an initial high strain hardening was observed due to strain path change and redistribution of dislocations. The nanolamellar structure of pearlite further introduced a strong anisotropy in yield behavior, without affecting the hardening characteristics of ferrite lamellae. The physical origin of the anisotropy in flow behavior of NL pearlite was identified as the importance of shear stress on dislocation motion within the ferrite nanolamellae.

Impact Toughness of Ultrafine‐Grained Commercially Pure Titanium for Medical Application

This study aims at achieving the best combination of strength, ductility, and impact toughness in ultrafine‐grained (UFG) Ti Grade 4 produced by equal‐channel angular pressing via Conform scheme (ECAP‐C) with subsequent cold drawing. UFG structures with various parameters (e.g., size and shape of grains, dislocation density, conditions of boundaries) are formed by varying the treatment procedures (deformation temperature and speed at drawing, annealing temperature). The tensile and impact toughness tests were performed on samples with a V‐shaped notch and different structures of commercially pure Ti Grade 4 in the coarse‐grained and UFG states. The results demonstrated that grain refinement, higher dislocation density, and their elongated shape were obtained as a result of drawing at 200 °С, which led to a decrease in both the uniform elongation at tension and the impact toughness of Ti Grade 4. Short‐term annealing at 400–450 °C could improve the impact toughness of UFG Ti with a non‐significant decrease in strength. This short‐term annealing contributes to the dislocation density decrease without considerable grain growth as a result of the recovery and redistribution of dislocations. The dependence of impact toughness on the strain hardening ability of UFG Ti was discussed.

Characterization of double aluminium alloy specimens after ECAP

The Equal Channel Angular Processing or pressing, i.e. the ECAP, allows to modify the properties of materials at the microstructure level. It consists in the induction of a high amount of shear deformation in the material that leads in general to a grain size refinement, a precipitate dispersion and a redistribution of dislocations depending on experiment conditions. The objective of the present investigation is to understand how the ECAP can influence the surface and the bulk mechanical properties of double aluminium alloy specimens. Each specimen was composed of a tubular part of the series AA6026 as well as of a cylindrical part of the series AA6012 assembled together before ECAP. A negligible bonding effect was observed after ECAP and after uniaxial compression tests performed at constant temperatures varying between 200 and 300°C with different press ram velocities. The characterization of each ECAP condition was initially represented in terms of the stress versus deformation flow curves. The load versus stroke curves were preferred due to the friction acting at the forming tool- specimen interface. It was obtained a decrease in the load versus stroke levels with increasing the number of ECAP passes under the experiment conditions of the present investigation. The increase in the press ram velocity determined an increase in the load for a given stroke.

Relaxation processes during high-frequency mechanical loading of metals deformed at low temperatures

Experimental data on the influence of high-frequency (∼20 kHz) mechanical loading (exposure to ultrasound) on hcp and bcc metals that have been pre-deformed under cryogenic conditions are generalized and analyzed. It is shown that under exposure to ultrasound, the energy of the mechanical oscillations is predominantly dissipated at the boundaries of twins and that this determines the processes for redistribution of dislocations in accommodation zones of twinning layers and in the matrix, while ensuring relaxation of internal stresses at these sites. The efficiency of twin boundaries as stress concentrators is balanced and this reduces the cold brittleness of metals and enhances the contribution of polygonization processes during subsequent annealing, thereby delaying recrystallization. These results substantiate ultrasonic irradiation as an analog of local thermal activation and as a method for regulating the degree of conjugation of phase-matrix interfaces.

Unexpected travel of Lomer-type dislocations in Ge/GexSi1-x/Si(001) heterostructures

High-resolution electron microscopy at the atomic scale has been applied to study the edge dislocation redistribution between interfaces in Ge/Ge0.5Si0.5/Si(001) heterostructures. It is demonstrated that such heterostructures relax in two stages: a network of edge dislocations is formed on the Ge/GeSi interface during heterostructure growth; after that, during additional annealing, some of these edge dislocations move through the volume of initially elastically strained GeSi to the GeSi/Si interface. Our results provide a direct explanation that plastic relaxation of the GeSi buffer layer proceeds owing to motion of Lomer-type dislocation complexes consisting of a pair of complementary 60° dislocations with the ends of the {111} extra planes being located at a distance of ~2-12 interplanar spacings from each other.

Experimental observation of the dislocation walls in heterostructures with two interfaces: Ge/Ge0.5Si0.5 10 nm/Si(001) as an example

High-resolution electron microscopy (HREM) at the atomic scale has been applied to study the edge dislocation redistribution between interfaces in Ge/Ge0.5Si0.5/Si(0 0 1) heterostructures. Our results provide a direct explanation that plastic relaxation of the GeSi buffer layer proceeds owing to motion of Lomer-type dislocation complexes consisting of a pair of complementary 60° dislocations with the ends of the {1 1 1} extra planes being located at a distance of ~2–12 interplanar spacings from each other. It is demonstrated that edge dislocations belonging to the upper and lower interfaces become arranged one under the other and dislocation walls are formed. The distributions of tension and compression in the [0 0 1] direction between two edge dislocations, obtained by processing the HREM image, testify to superposition of strain fields.

Recrystallization and Grain Growth in Accumulative Roll-Bonded Metal Composites

We examine recrystallization and grain growth during processing of accumulative roll-bonded (ARB) Cu-Nb and Zr-Nb composites. Throughout the ARB process, from initial millimeter thick layers down to nanometer thick layers, the mechanism for recrystallization and grain growth is the motion of high-angle grain boundaries (HAGBs). However, the driving forces for these phenomena change as the densities of different types of defects evolve during the process. The creation and redistribution of dislocations, grain boundaries, and phase boundaries has significant effects on recrystallization and grain growth and, thus, on microstructural evolution. Both Cu-Nb and Zr-Nb exhibit a distinct transition in recrystallization and growth behavior at around 500-nm average layer thicknesses. For the thicker layered materials, the microstructure evolution during recrystallization and growth is determined by the density and distribution of dislocations and HAGBs. For layers less than 500 nm, the layers are largely one-grain thick and the grains are nearly dislocation free; coarsening of grains within layers at the nanoscale is due to reduction in phase boundary energy.

Dislocation density distribution around an indent in single-crystalline nickel: Comparing nonlocal crystal plasticity finite-element predictions with experiments

We present a physics-based constitutive model of dislocation glide in metals that explicitly accounts for the redistribution of dislocations due to their motion. The model parameterizes the complex microstructure by dislocation densities of edge and screw character, which either occur with monopolar properties, i.e. a single dislocation with positive or negative line sense, or with dipolar properties, i.e. two dislocations of opposite line sense combined. The advantage of the model lies in the description of the dislocation density evolution, which comprises the usual rate equations for dislocation multiplication and annihilation, and formation and dissociation of dislocation dipoles. Additionally, the spatial redistribution of dislocations by slip is explicitly accounted for. This is achieved by introducing an advection term for the dislocation density that turns the evolution equations for the dislocation density from ordinary into partial differential equations. The associated spatial gradients of the dislocation slip render the model nonlocal. The model is applied to wedge indentation in single-crystalline nickel. The simulation results are compared to published experiments (Kysar et al., 2010) in terms of the spatial distribution of lattice rotations and geometrically necessary dislocations. In agreement with experiment, the predicted dislocation fluxes lead to accumulation of geometrically necessary dislocations around a vertical geometrical border with a high orientation gradient below the indenter that is decisive for the overall plastic response. A local model variant without dislocation transport is not able to predict the influence of this geometrical transition zone correctly and is shown to behave markedly softer.

Graininess variation research of steel 20 welding joints at deformation-thermal treatment

It is known that the metal grains’ size affect on the metal physical-and-mechanical properties. At the fusion welding the metal grains irregular growth takes place, graininess is increasing. It affects negatively on physical-and-mechanical properties on the welding joint metal. For decreasing graininess variation thermal and thermoplastic treatment of welding joints (annealing, thermoplastic treatment) is used. In was carried out the comparative analysis of the results of steel 20 welding joints structure research after deformation at the temperature-rate regime of super plastic deformation and after annealing and after thermo cyclical treatment. At superplastic deformation during hot deformation, became possible processes of dislocation redistribution, including vacancy creeping, forming recrystallization nuclei and their growth, migration of the large angular boundaries. In this research a thermo mechanical treatment at super plasticity regime deformation is studied. In this case the effect of super plasticity deformation is not revealed in full, because for that there must present fine grain structure. The results of the research showed the positive effect of the thermoplastic treatment at super plasticity regime deformation. Slow rate of deformation in combination with definite temperature affects on the thermal activated processes of thermal loss of strength. Microstructure research was carried out on Neophot-30 microscope. For studying the graininess structure variation changing the parameters of grains dimensions distribution were calculated by using the industry system of images analysis SIAMS 600. It was determined that in the zone of the welding joint thermal effect the increasing of graininess variation in metal took place. After annealing of the welding joint the graininess variation in the zone of thermal effect almost had not changed. After isothermal rolling with heated rollers at the regime of super plastic deformation and thermo cyclical treatment the graininess variation in the zone of thermal effect decreased and approached to the value of the basic metal graininess variation.

Dislocation interaction of layers in the Ge/Ge-seed/GexSi1−x/Si(0 0 1) (x ∼ 0.3–0.5) system: Trapping of misfit dislocations on the Ge-seed/GeSi interface

High-resolution electron microscopy has been applied to study the dislocation redistribution between Ge and GeSi layers at the atomic scale. Ge0.3Si0.7 (30nm in thickness) and Ge0.5Si0.5 (10nm) buffer layers buried between the Si(001) substrate and the plastically relaxed Ge layer 0.5μm thick remain in a metastable (stressed) state during the growth of Ge/Ge-seed/GexSi1−x/Si(001) (x∼0.3–0.5) heterostructures, though the buffer layer thickness is several times greater than the critical value for insertion of misfit dislocations (MDs). An ordered grid of edge MDs is observed only on the Ge/GeSi interface; the mean distance between the MDs is ∼10nm (which is close to the equilibrium value for the non-stressed Ge/Si system). After 30min of annealing at 700°С, the Ge0.3Si0.7 buffer layer still remains in a metastable state, and the edge MDs are located only on the Ge/GeSi interface with the same dislocation spacing of ∼10nm. At the same time, approximately one-half of MDs in the structure with the Ge0.5Si0.5 buffer layer passes through the Ge/GeSi interface to the GeSi/Si(001) interface, and the buffer layer plastically relaxes by almost 100%. An assumption is put forward that there exists a barrier for the MD transition from the Ge layer to the GeSi layer, which results in MD trapping on this interface. The magnitude of this barrier depends on the difference in the compositions of the main Ge (x=1) film and the GexSi1−x buffer layer, and increases with increasing this difference.

Mechanism of Residual Stress Reduction in Low Alloy Steel by a Low Frequency Alternating Magnetic Treatment

Residual stress reduction in low alloy steel by a low frequency alternating magnetic treatment and its mechanism were investigated. Experimental results revealed that average stress reductions of 20%-24% were obtained in the welded samples. Moreover, compared with the zones with lower initial stress levels, more remarkable stress reductions were obtained in the stress concentration zones. The microstructures and magnetic domains were observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Based on the analysis of the microstructure and magnetic domain changes, the mechanism of stress reduction by the magnetic treatment has been concluded: (1) the magneto-plastic deformations mainly due to the more uniform redistribution of dislocations are the fundamental cause of stress relaxation; and (2) surface topography is also proved to affect the magnetic treatment results to some degree by influencing magnetic domains.