Grain Boundary Junctions Research Articles

Deformation behavior of high-entropy alloy system Al – Co – Cr – Fe – Ni achieved by wire-arc additive manufacturing

A non-equiatomic high-entropy alloy (HEA) of the Al – Co – Cr – Fe – Ni system was obtained using wire-arc additive manufacturing technology in the atmosphere of pure argon. The initial wire had 3 conductors with different chemical composition: pure aluminum wire (Al ≈ 99.95 %), chromium-nickel wire (Cr ≈ 20 %, Ni ≈ 80 %), and cobalt alloy wire (Co ≈ 17 %, Fe ≈ 54 %, Ni ≈ 29 %). The resulting sample of high-entropy alloy was a parallelepiped consisting of 20 deposited layers in height and 4 layers in thickness. The alloy had the following elemental composition, detected by energy-dispersive X-ray spectroscopy: aluminum (35.67 ± 1.34 at. %), nickel (33.79 ± 0.46 at. %), iron (17.28 ± 1.83 at. %), chromium (8.28 ± 0.15 at. %) and cobalt (4.99 ± 0.09 at. %). Scanning electron microscopy revealed that the source material has a dendritic structure and contains particles of the second phase at grain boundaries. Element distribution maps obtained by mapping methods have shown that grain volumes are enriched in aluminum and nickel, while grain boundaries contain chromium and iron. Cobalt is distributed in the crystal lattice of the resulting HEA quasi-uniformly. It is shown that during tensile tests, the material was destroyed by the mechanism of intra-grain cleavage. The formation of brittle cracks along the boundaries and at the junctions of grain boundaries, i.e., in places containing inclusions of the second phases, is revealed. It was suggested that one of the reasons for the increased fragility of HEA, produced by wire-arc additive manufacturing, is revealed uneven distribution of elements in microstructure of the alloy and also the presence in material volume of discontinuities of various shapes and sizes.

Vacancy release upon heating of an ultrafine grain Al-Zr alloy: In-situ observations and theoretical modeling

The present study demonstrates the direct observation of pore formation by rapid release of vacancies in ultrafine grain (UFG) Al-0.4at%Zr alloy during in-situ annealing in a scanning transmission electron microscope. The ultrafine grain structure was preliminary obtained by high pressure torsion processing at a hydrostatic pressure of 6 GPa up to 10 revolutions. In-situ annealing reveals the rapid formation of pores in triple junctions (TJs) of grain boundaries (GBs) during the first few minutes followed by their subsequent slow resorption with the complete disappearance of some of them. During annealing, no noticeable displacement of GBs is observed. By considering the evolution of non-equilibrium GBs inherited by the severe plastic deformation, a theoretical description is suggested which describes: (i) the pore formation at disclinated TJs as a thermodynamically driven process of free volume dissolution through generation of vacancies, which then migrate to the TJs and coagulate at them with growth of the pores diminishing the strain energy of the TJ disclinations, and (ii) further decrease of the TJ disclination strain energy through the climb of extrinsic GB dislocations towards the disclinated TJs, accompanied with dissolution of the TJ pores by emission of vacancies which provide the dislocation climb. The rapid release of excess vacancies during the early stage of heat treatment is consequently identified as a phenomenon responsible for accelerated atomic mobility. This work hence provides a new perspective for understanding the accelerated precipitation kinetics observed in severely deformed alloys.

Деформация и разрушение высокоэнтропийного сплава AlCoCrFeNi

Using wire-arc additive manufacturing (WAAM)technology in an atmosphere of argon gas a non - equatomic high entropy alloy (HEA) of AlCoCrFeNi system is obtained: Al (35.67±1.34 at%), Ni (33.79±0.46 at%), Fe (17.28±1.83 at%), Cr (8.28±0.15 at%), Co (4.99±0.09 at%). Scanning electron microscopy method revealed that HEA is a polycrystal material having the grain size (4-15) µm with the particles of second phase located along the grain boundaries. Mapping methods showed that grain volumes are enriched in aluminum and nickel, while grain boundaries contain chromium and iron. Cobalt is distributed in the crystal lattice of the resulting HEA quasi-uniformly. It is shown that during tensile tests, the material was destroyed by the mechanism of intra-grain cleavage. The formation of brittle cracks along the boundaries and at the junctions of grain boundaries, i.e., in places containing inclusions of the second phases, is revealed. It was suggested that one of the reasons for the increased brittleness of HEA, is revealed uneven distribution of elements in the microstructure of the alloy and also the presence in the volume of material discontinuities of various shapes and sizes.

ANALYSIS OF LOCAL FIELDS OF ELASTIC STRESS GENERATED BY ROTATIONAL-SHEAR MESODEFECTS NEAR THE JUNCTIONS OF GRAINS

The work is devoted to the study of the structure of the elastic stress field in the area of junctions of grain boundaries containing strain-induced rotational-shear mesodefects. The jumps in plastic distortion of grains when passing through grain boundaries create additional misorientations on them, the mismatch of which at the junctions of grains leads to the appearance of linear mesodefects of the rotational type – junction disclinations. Planar mesodefects of the shear type appear on the flat sections of the boundaries, which are plastic shears uniformly distributed along the boundaries. These mesodefects create spatially inhomogeneous elastic stress fields near the junctions and ledges of the grains. They increase during plastic deformation and, at sufficiently large value of strain, initiate the formation of a fragmented material structure. Rotational-shear mesodefects are also the cause of the nucleation and accumulation of microcracks at the stage of viscous fracture of polycrystalline solids. In this work, asymptotic formulas are obtained that make it possible to analyze the distribution and anisotropy of the internal stress field in the vicinity of the grain junction from rotational-shear mesodefects. It was found that the components of the stress field weakly depend (logarithmically) on the length of the grain boundaries formatting junction of the grains. It is shown that the screening disclination of the dipole leads to the appearance of an angular dependence of the diagonal components of the elastic stress tensor in the vicinity of the junctions and its contribution to the elastic field of the disclinations dipole is about 10–15%. The obtained asymptotic expressions can be used to study the kinetics of a dislocation ensemble and to analyze the conditions for the nucleation of microcracks in the vicinity of joints and ledges of grains.

Grain Boundary α-Phase Precipitation and Coarsening: Comparing Selective Laser Melted and Conventional Manufactured Ti6Al4V

Investigation into the precipitation and coarsening behavior of grain boundary α-phase (GB-α) in Ti6Al4V (Ti-64) manufactured by selective laser melting (SLM) were carried out and compared with that in as-cast Ti64 in this study. It was found that in SLM Ti64, the GB-α tends to precipitate first at the triple junctions (TJs) of the β-phase grain boundaries during the further annealing heat treatments in the α+β phase regime. With 750 ℃ annealings, the coarsening process of GB-α was controlled by the bulk diffusion mechanism with a small coarsening coefficient, different from as-cast Ti64. The interface reaction which has a large coarsening coefficient was dominant for the GB-α coarsening at 850℃ and 950 ℃ annealings. Such a difference in coarsening behavior at different annealing temperatures in SLM Ti64 was found to be related to the different curvatures of GB-α caused by the different nucleation mechanisms of α lamellae.

Partial melting of ultrahigh-pressure eclogite by omphacite-breakdown facilitates exhumation of deeply-subducted crust

Results from numerical modelling and experimental petrology have led to the hypothesis that partial melting was important in facilitating exhumation of ultrahigh-pressure (UHP) metamorphic rocks from mantle depths. However, the melting reactions responsible are rarely well-documented from natural examples. Here we report microstructural features and compositional data that indicate in situ partial melting dominated by breakdown of omphacite in UHP eclogite from the Sulu belt, China. Diagnostic microstructures include: (i) the presence of in situ leucosome pockets composed of plagioclase, euhedral amphibole, minor K-feldspar and epidote within host zoisite- and phengite-bearing eclogite; (ii) skeletal omphacite within the leucosome pockets that has a lower jadeite content (25–45 mol.%) than rock-forming omphacite (39–54 mol.%); and, (iii) seams of Na-rich plagioclase that extend along grain boundaries separating phengite, quartz and zoisite, and which commonly exhibit low dihedral angles where they terminate at triple grain-boundary junctions. Major oxide proportions of 57 leucosome pockets, calculated using mineral modes and compositions, yield leucodiorite bulk compositions characterized by intermediate SiO2, high Al2O3 and Na2O, and low K2O contents. In primitive mantle-normalised trace element diagrams, the leucosome pockets show enrichment in large ion lithophile elements, U, Pb, Zr, Hf and Ti, but depletion in Th and Ta, patterns that are similar to those of rock-forming omphacite. Rather than forming predominantly by breakdown of phengite and/or zoisite, as widely proposed in the literature, the leucosome pockets have petrographic characteristics and major oxide and trace element compositions that are consistent with partial melting dominated by omphacite breakdown. Based on conventional thermobarometry, the eclogite was exhumed from pressure–temperature (P–T) conditions of 3.6–3.1 GPa and 900–840°C. Partial melting led to the formation of the leucosome pockets, which equilibrated with the rims of surrounding rock-forming garnet and pyroxene during crystallisation. Conventional thermobarometry using rim compositions yields P–T conditions of 1.6–1.2 GPa and 780–690°C, broadly consistent with calculated phase equilibria and Ti-in-zircon temperatures from zircon overgrowths. Weighted mean ages of ca 217–214 Ma from thin overgrowths on zircon are interpreted to record melt crystallisation. This study provides insight into an overlooked mechanism by which eclogites partially melt during exhumation from UHP conditions, and permits a better understanding of the processes that assist deeply-subducted continental crust to return to shallower depths.

Mechanism of Implementation of High Ductility in Ultrafine-Grained Aluminum after Annealing and Subsequent Deformation

A theoretical model that describes the implementation of high ductility in high-pressure torsion (HPT) processed ultrafine-grained aluminum subjected to annealing and additional HPT deformation, is suggested. In the framework of the model, the plastic deformation is realized through the successive emission of lattice dislocations from triple junctions of grain boundaries due to splitting of head dislocations in pile-ups of grain boundary dislocations, which are pressed by an external shear stress against these triple junctions, and their subsequent glide in grain interior and climb along surrounding grain boundaries. The energy characteristics and the critical stresses of dislocation emission have been determined for ultrafine-grained aluminum subjected to annealing with small additional HPT deformation. The theoretical stress–strain curves demonstrate good agreement with experimental data available in the literature.

Generation of dislocation clusters at triple junctions of random angle grain boundaries during cast growth of silicon ingots

Three-dimensional distribution of grain boundaries (GBs) and generation sources of dislocation clusters is examined in a cast-grown high-performance multicrystalline silicon ingot for commercial solar cells. A significant number of dislocations are generated nearby some triple junctions of random angle GBs, although it is believed that such non-coherent GBs would not induce large strain during the cast growth. This explosive generation of dislocations would take place when the triple junctions are interacted with multiple Σ3{111} GBs. A segment of the random angle GB connected with a pair of Σ3{111} GBs nearby the triple junction would act as a dislocation source.

Surface oxidation phenomenon of Mg–10 wt% Sn–1.5 wt% Al alloy during homogenization

Homogenization heat treatment of Mg–10 wt% Sn–1.5 wt% Al alloy is studied. The results show that aliquid phase begins to form in the alloy at the temperature (Tm) of 550.2 °C, so the homogenization is performed at 460 °C–510 °C, based on the empirical formula T= (0.9–0.95) Tm. But the surface oxidation phenomenon takes place when treating at 460 °C, which makes the progress of the process difficult to accomplish. Metallograph depicts that wetting phase transformation emerges during the process. For further researching, vacuum quartz tube experiment is conducted, which indicates that the phenomenon is contact with the wetting phase transformation of grain boundaries (GBs) and grain-boundary triple junctions (GB TJs).

The significant size effect on nucleation and propagation of crack during tensile deformation of copper foil: Free surface roughening and crystallography study

Tensile tests were conducted on copper foils with 80 μm in thickness to elucidate the effect of grain size on nucleation and growth of crack. Experimental results indicated that samples with larger grain size had smaller fracture stress and strain when the ratio of sample thickness to grain size (T/D) was below 10. This is attributed to strain localization during the early stage of tensile deformation. Confocal laser scanning microscopy (CLSM) indicated an increase in free surface roughening with increase in grain size during tensile straining. The local depression trough and thinning of sample caused by surface roughening were precursors to crack propagation. (EBSD) studies of crack in sample with T/D value of 1.8, it was observed that in the vicinity of the crack tip, a high density of dense and deep slip traces were present. The slip process gradually changed from single slip to multiple slip and then to cross slip. Both grain boundary and annealed twin boundary acted as obstacles to the movement of dislocations and led to strain localization. Furthermore, during tensile deformation, the crack propagation direction was not always perpendicular to the loading direction. A change in the direction of crack propagation occurred at the junction of grain boundaries and entered into the adjacent grain. Large angle grain boundaries provided greater resistance to the propagation of crack.

Effect of symmetrical tilt grain boundary on the compatibility between copper and liquid lithium: Atomistic simulations

Abstract The compatibility between Cu and liquid Li directly affects the safety of fusion devices. In this work, we investigated the compatibility between Cu Ʃ3 (111), Ʃ3 (112) and Ʃ5 (310) symmetrical tilt grain boundaries (STGB) and liquid Li using molecular dynamics (MD) simulation. The Li atoms rapidly penetrate the intergranular area along Ʃ3 (112) and Ʃ5 (310) grain boundaries (GBs), resulting in the formation of liquid grooves at the junctions of GBs and solid-liquid interfaces. However, the Ʃ3 (111) GB shows its resistance to Li penetration. In addition, the interfacial alloying and dissolution of Cu atoms are accelerated by Ʃ5 (310) GB via promoting the escape of Cu atoms. Then we calculated the potential energy distribution of Cu atoms which indicates the liquid metal embrittlement (LME) effect exists between Cu bicrystal and liquid Li. And we demonstrate this embrittlement has a strong correlation with the vacancy formation energy and the segregation energy of Li atoms. Comparing with the experimental results, our simulation results explain the corrosion morphology of polycrystalline Cu in liquid Li, and point out that the LME is the root cause of Cu specimen fragmentation.

The Effect of Magnetic Flux Focusing on the Current–Voltage Characteristics of YBa2Cu3O7- δ Grain Boundary Josephson Junctions

In order to obtain experimental evidence for the influence of flux focusing effect on the characteristics of planar grain boundary (GB) Josephson junctions, we measured the current–voltage characteristics and magnetic-field-dependent current–voltage steps of integrated YBa2Cu3O7- δ bicrystal GB Junctions with different junction widths. Under the self-magnetic field, the critical current of the GB junction is proportional to the square root of the junction width, which is different from the traditional linear relationship between the critical current and the junction width. When a vertical magnetic field is applied, the flux flow current–voltage step could be observed, and the step voltage increases monotonously with an increase in the junction width. When a magnetic field rotating in the bc plane is applied, the voltage value of the flux flow step decreases slowly as the magnetic field inclined angle increases when the field inclined angle is relatively large. Based on these results, we demonstrate the presence of the magnetic flux focusing effect on YBa2Cu3O7- δ GB Josephson junctions. The size of flux focusing in the GB is closely associated with the width of the junctions and the orientation of the magnetic field.

Texture Evolution and Dynamic Recrystallization Behavior of Hybrid-Reinforced Titanium Matrix Composites: Enhanced Strength and Ductility

TiB, TiC and La2O3 hybrid-reinforced Ti-6Al-4V matrix composites were fabricated by common casting and processed by the following forging and isothermal extrusion. The texture evolution, dynamic recrystallization (DRX) behavior and mechanical properties of the as-extruded composites were investigated. The results show that DRXed regions and deformed regions formed during extrusion, strong 〈0001〉α, $$ \left\langle {10\bar{1}0} \right\rangle_{\alpha } $$ and weak $$ \left\langle { 1 0 {\bar{\text{1}}\text{1}}} \right\rangle_{\alpha } $$ fiber textures parallel to extrusion direction were formed due to the selection of slip system in α phase and inheritance from phase transformation according to Burgers orientation relationship. Additionally, the orientation of α grain in DRXed regions became randomized in comparison with the deformed regions. Micro-sized TiB short fibers not only acted as source to generated dislocations, but hindered dislocation slip to cause pile-ups around TiB, which is beneficial to stimulate dynamic recrystallization. Submicro-sized La2O3 particles distributed at the triple junction of grain boundaries could pin grain boundaries and hinder grain growth. Tensile test results show that the ultimate tensile strength got a 9.8 pct enhancement, from 834.3 ± 10 to 916.0 ± 10.6 MPa, accompanied by simultaneous improvement of elongation from 5.4 ± 0.6 to 12.2 ± 0.7 pct at room temperature. The simultaneous increase of strength and ductility was principally attributed to TiB fracture modes and grain refinement after extrusion.

Influence of Carbon and Oxygen Impurities on Migration Velocity of Grain-boundary Triple Junctions in FCC Metals

Using molecular dynamics simulation, this paper studies the influence of impurity atoms of carbon and oxygen on the migration velocity of triple junctions of grain boundaries created by misorientations about axis in Ni, Ag, Al metals with the face-centered cubic crystal system. It is shown that the addition of impurity atoms of light elements substantially constrains the triple junctions from migration. Carbon atoms tend to create clusters which become effective locks when fixed at the grain boundaries, thereby preventing their migration. Oxygen atoms do not form aggregates, but due to high values of the binding energy between atoms and grain boundaries, they effectively inhibit their migration. Binding energies between carbon and oxygen atoms and grain boundary dislocations are calculated in metals at issue. The obtained results correlate with the dependencies between the migration velocity of the triple junctions and the impurity concentration.

Constitutive analysis, twinning, recrystallization, and crack in fine-grained ZK61 Mg alloy during high strain rate compression over a wide range of temperatures

This study aimed to explain the uniaxial compression of T5 heat-treated extruded ZK61 magnesium alloy at the temperatures of 298−673 K and the strain rates of 0.001−10 s−1. Flow stress data were analyzed to establish a constitutive equation by means of numerical simulation and activation energy was estimated to be 141 ± 1 kJ/mol. Microstructure analysis revealed a low fraction of primary and secondary twinning at temperature 623 K in the early stages of deformation and dense twinning at low temperatures ≤ 473 K. Pole figure analysis suggested virgin fiber texture where c−axis of HCP is tilted at angle ~25° with respect to extrusion direction. During early stages of deformation at a strain 0.1 and at high-temperature c-axis of the crystal reorients and changes to ~40° along transverse direction, while at strain 0.45 highly aligned typical extruded texture was developed, which proposed that {0002} basal planes are perfectly parallel to the extrusion direction. Moreover, the results indicated that at high temperature> 548 K the dominant discontinuous dynamic recrystallization was changed into continuous dynamic recrystallization with the increase in strain; this phenomenon is attributed to initial processing history; fine grain size and activation energy 141 ± 1 kJ/mol. The twin dynamic recrystallization was dominant at temperatures ≤548 K. Thus, hot deformation in the strain rate range of 0.001−1 s−1 at temperature 548 K or in the strain rate range of 0.001−10 s−1 and in the temperature range of 573−623 K is optimum for the ZK61 Mg alloy (T5). Fracture behavior revealed that principal and secondary cracks were nucleated and propagated owing to the low fraction of dynamic recrystallization, twin−twin, twin−dislocation interaction and voids at triple junctions of grain boundaries.

Mechanism of Hardening of Ultrafine-Grained Aluminum after Annealing

A theoretical model is proposed that describes the hardening mechanism of ultrafine-grained aluminum obtained by high pressure torsion after low-temperature annealing. Within this model, the hardening is due to the successive transformation of the grain-boundary dislocation structure. In particular, plastic deformation is occurs through the emission of lattice dislocations from triple junctions of grain boundaries containing pile-ups of grain-boundary dislocations, the subsequent sliding of lattice dislocations in the grain body, and the formation of walls of climbing grain-boundary dislocations along opposite grain boundaries. The energy characteristics and critical stresses for emission of lattice dislocations are calculated. Theoretical dependences of the flow stress on the plastic strain, which demonstrate good qualitative and quantitative agreement with experimental data, are constructed.

Determination of the Radius of Triple Junctions of Tilt Boundaries: A Molecular Dynamics Simulation

Using the method of molecular dynamics, an investigation of the free volume and energy distribution is performed in the region of triple junctions of high-angle tilt boundaries with the and misorientation axes in nickel formed as a result of recrystallization. Crystallization is simulated from three crystalline nuclei for the varied free volume value. It is shown that during recrystallization the free volume is mainly concentrated in the region of grain boundaries and its fraction increases on approaching the grain-boundary junction. From the effective energy distribution in the region of triple junctions, their effective radius is determined. In the tilt boundaries and their junctions, the free volume during recrystallization is dispersed more effectively that it is in the junctions formed by the boundaries. For this reason, the energy radius of the triple junction of the boundaries hardly varies with an introduction of a free volume: from 5 to 7 A, while that of the junction of the boundaries upon introduction of 2% vacancies into the initial computational cell becomes as large 14 A.

Fracture behavior of twin induced ultra-fine grained ZK61 magnesium alloy under high strain rate compression

In this study ultra-fine grained single pass extruded ZK61 magnesium alloy sheet is processed at temperature 350 °C with area reduction ratio ˜30. The complex dynamic mechanical behavior is studied experimentally with Split Hopkinson pressure bar over wide ranges of strain rates (1000–4000 s−1) along the normal direction. Electron backscattered diffraction, scanning electron microscopy and optical microscopy analysis are employed to reveal the changes in texture and fracture analysis. Positive strain rate sensitivity is observed up to strain rate 3000 s−1 that is signature of basal slip and extension twinning although the grain size was very small. This finding is contradiction to experimental and theoretical predictions suggesting the elimination of twinning induced deformation in magnesium alloys in very fine small grains <3 μm. Flow stress and a fraction of extension twinning are apparently decreased as the applied strain rate increased from 3000 to 4000 s−1; that is attributed to adiabatic rise in temperature, early reorientation of crystal. Extensive grain reorientation causes the nucleation of extension twinning that changes fiber texture to strong double peak basal texture under high strain rate compression. Nucleation of principle crack was attributed to twin–dislocation interaction that provides dynamic recrystallized grains followed by the adiabatic shear band at the boundary of coarse grains. Contrary, secondary cracks are nucleated and propagated due to high local stresses at the triple junction of grain boundaries. Besides, scanning electron microscopy revealed ductile fracture under high strain rate compression.

Механизм упрочнения ультрамелкозернистого алюминия после отжига

A theoretical model is proposed that describes the mechanism of hardening of ultrafine-grained aluminum, obtained by severe plastic torsion deformation, after low-temperature annealing. In the framework of the model, hardening is realized due to the sequential transformation of the grain-boundary dislocation structure. In particular, plastic deformation occurs through the emission of lattice dislocations from triple junctions of grain boundaries containing pile-ups of grain-boundary dislocations, the subsequent sliding of lattice dislocations in the bulk of the grain, and the formation of walls of grain-boundary dislocations climbing along opposite grain boundaries. The energy characteristics and critical stresses for the emission of lattice dislocations are calculated. The theoretical dependences of the flow stress on the plastic deformation are plotted, which show good qualitative and quantitative agreement with experimental data.

Kinetics of Grain Boundary Networks Controlled by Triple Junction and Grain Boundary Mobility

The kinetics of a triple junction of grain boundaries with distinct specific energies and mobilities and a finite mobility of the triple junction is investigated. The microstructure is approximated by different 2D settings consisting of typical structural elements. First, the migration of the triple point together with the adjacent grain boundaries, is simulated, assuming that the grains are infinitely large. Secondly, growth or shrinkage of finite n-sided grains is simulated by altering the boundary conditions and the results are compared to the already published analytical solution. The numerical results coincide with the corrected analytical solution. This solution can be derived either by applying the principle of maximum dissipation, or by applying the force balance at the triple junction within the framework of linear irreversible thermodynamics. The change of the area of infinite and finite grains is investigated analytically and numerically. By comparing the results of both approaches, the influence of the initial topology of the structural elements on the kinetics of grain growth can be estimated. Furthermore, the kinetics of grain growth of different idealized grain boundary networks is investigated. It is shown that square shaped grains surrounded by hexagons and dodecagons result in a more realistic grain growth scenarios than squares surrounded by octagons. A deviation from idealized grain boundary arrangements is e.g., observed, due to different triple junction mobilities, and the initially n-sided regular grain deforms in a complex manner.