Coincidence Site Lattice Grain Research Articles

Optimization of the grain boundary network and tensile properties of powder metallurgy-hot isostatic pressed Ni-based alloy Inconel 625

Powder metallurgy-hot isostatic pressing (PM-HIPing) is an advanced manufacturing technology for large-sized and complex-shaped components for nuclear applications. In the present study, a novel PM-HIPing strategy was proposed to optimize the grain boundary network and tensile properties of Ni-based alloy Inconel 625, and it was verified by a series of experiments. Samples with 69 % and 74 % of coincidence site lattice (CSL) grain boundaries were obtained when the narrow-size ranged powder was consolidated at 1140 °C and 1180 °C, respectively. The fraction of CSL grain boundaries was further increased to ∼78.5 % after a high temperature solution treatment. Room temperature yield strength and ultimate tensile strength of PM-HIPed alloy 625 were comparable to that of the wrought specimens, elongation of the specimens was ∼56–60 %, the samples all featured with dimple ductile dominant fracture mode.

Structural transformation behavior of oxide scale during coiling of 0.9 wt% Cr-containing high-strength steel

The phase transformation mechanism of oxide scale on the surfaces of chromium (Cr)-free carbon steel and 0.9 wt% Cr-containing steel at different coiling temperatures and cooling rates was studied using thermal simulation and electron backscattering diffraction experiments. The results showed that the eutectoid transformation of both the steel followed the “C” curve relationship. The Cr-containing steel had a larger eutectoid transformation area fraction than the Cr-free carbon steel. The nose tip temperature of the eutectoid transformation of the oxide scale on the surfaces of Cr-free carbon steel and 0.9 wt% Cr-containing steel were 420∼510 °C and 420–480 °C, respectively. For the 0.9 wt% Cr-containing steel, the proportion of Σ3 and Σ13b grain boundaries in cubic Fe3O4 increased, followed by a decrease, when the coiling temperature decreased from 610 to 410 °C. The Σ3 and Σ13b grain boundaries of the Fe3O4 low-dimensional coincident site lattice had the highest proportion for 510 °C coiling temperature. The low-dimensional coincident site lattice grain boundaries could be used to enhance crack resistance and improve the oxide scale on the steel plate surface.

Grain boundary assembly in a 316 L steel produced by selective laser melting and annealing

The effect of recrystallization annealing on the grain boundary assembly of a 316 L-type austenitic stainless steel produced by selective laser melting was studied. The primary recrystallization readily developed upon heating to 1373–1473 K. Using the orientation imaging microscopy, the recrystallization development could be evaluated by means of the grain orientation spread and/or the kernel average misorientation. The recrystallization kinetics was mainly controlled by the grain nucleation rate rather than the grain growth rate. The recrystallized microstructures were characterized by large fractions of special Σ3n CSL (coincident site lattice) grain boundaries above 0.7 that was associated with a high number density of annealing twins. The change in the fraction of Σ3n SCL boundaries could be expressed by unique linear function of the recrystallized fraction irrespective of annealing temperature. The development of Σ3n CSL boundaries interrupted the network of ordinary grain boundaries, when the number of triple junctions with 3 ordinary boundaries decreased and that with 1 ordinary and 2 Σ3n CSL boundaries increased. An increase in the fraction of triple junctions with 1 ordinary and 2 Σ3n SCL boundaries clearly correlated with the recrystallized fraction.

Role of inhibitor behavior in abnormal growth of Goss grain in grain-oriented silicon steel: experiments and modeling

The role of inhibitor behavior in abnormal growth of Goss grains during secondary recrystallization was studied in combination with experimental observation and phase-field model simulation in grain-oriented silicon steel. The frequency advantages of both HE (high energy) and CSL (coincidence site lattice) grain boundaries around Goss grains over the matrix were identified, with frequency advantages of ∼15% and ∼2% respectively. The HE grain boundary was supposed to play a more important role in abnormal grain growth over CSL grain boundary in this study. The boundary character distribution was necessary but not sufficient for the abnormal growth of Goss grains. Appropriate inhibitor behavior was a prerequisite for secondary recrystallization. The initial content and stability of inhibitor played a key role in stabilizing the matrix, which guaranteed beneficial fine grains around Goss grains. While if the inhibitor force rapidly declined, matrix grains might grow obviously, resulting in coarsened grain colonies with Cube, {112}<110>, {111}<112>, {411}<148>and {210}<001> orientation, which further hindered the abnormal growth of Goss grains. The candidate Goss grains with different misorientations competed with each other during secondary recrystallization and the exact Goss grain exhibited an obvious growth advantage. Only with an appropriate pinning force and grain boundary character, perfect secondary recrystallization can be achieved. The present findings can provide guidance to understand and precise control of abnormal grain growth phenomenon.

Atomistic investigation of the impact of phosphorus impurities on the tungsten grain boundary decohesion

In the present work, we have generated a new second-nearest neighbour modified embedded atom method potential (2NN-MEAM) for the W–P system to investigate the impact of P impurity segregation on the strength of symmetric 〈110〉 tilt coincident site lattice grain boundaries (GBs) in tungsten. By incorporating the impurity-induced reduction of the work of separation in the fitting strategy, we have produced a potential that predicts decohesion behaviour as found by ab initio density functional theory (DFT) modelling. Analysis of the GB work of separation and generalized stacking fault energy data derived from DFT and the 2NN-MEAM potential show that P-impurities reduce the resistance to both cleavage and slip. Mode I tensile simulations reveal that the most dominant mode of GB failure is cleavage and that pristine GBs, which are initially ductile, on most accounts change to brittle upon introduction of impurities. Such tendencies are in line with experimentally observed correlations between P-impurity content and reduced ductility.

Особенности микроструктуры и микротекстуры среднеуглеродистой стали, подвергнутой сероводородному растрескиванию под напряжением

Increasing the resistance of steel products to sulfide stress cracking (SSC) is one of the topical issues of the oil and gas industry. Among various factors determining the SSC resistance of a material is the structure-phase state of the material itself and the crystallographic texture associated with it. This paper analyzes these features using the scanning electron microscopy (SEM), transmission electron microscopy (TEM), and microroentgen electron backscattered diffraction (EBSD) techniques. As the research material, a production string (PS) coupling made of medium-carbon steel was selected, which collapsed by the mechanism of hydrogen embrittlement and subsequent SSC. For the first time, by the SEM method, using the location and mutual orientation of cementite (Fe3C) particles, at high magnifications, the authors demonstrated the possibilities of identifying the components of upper bainite, lower bainite, and tempered martensite in steels. The presence of the detected structural components of steel was confirmed by transmission electron microscopy (TEM). Using the EBSD method, the detailed studies of microtexture were conducted to identify the type and nature of the microcrack propagation. It is established that the processes of hydrogen embrittlement and subsequent SSC lead to the formation of {101} &lt;0¯10&gt;, {100} &lt;001&gt;, {122} &lt;2¯10&gt;, {013} &lt;211&gt;, {111} &lt;¯100&gt;, {133} &lt;1 ̅2 ̅1&gt;, {32 ̅6 ̅} &lt;201&gt; grain orientations. It is shown that the strengthening of orientations of {001} &lt;110&gt;, {100} &lt;001&gt;, {112} &lt;111&gt;, and {133} &lt;1 ̅2 ̅1&gt; types worsens the SSC resistance of the material. Using the EBSD analysis method, the influence of coincident site lattice (CSL) grain boundaries on the nature of microcrack propagation is estimated. It is found that the Σ 3 CSL grain boundaries between the {122} &lt;2¯10&gt; and {111} &lt;¯100&gt;, {012} &lt;1 ̅1 ̅0&gt;, {100} &lt;001&gt; plates of the upper bainite inhibit the microcrack development, and the Σ 13b, Σ 29a, and Σ 39a CSL grain boundaries, contribute to the accelerated propagation of microcracks. For comparative analysis, similar studies were carried out in an unbroken (original) coupling before operation.

Uniaxial ratcheting behavior and molecular dynamics simulation evaluation of 316LN stainless steel

In this study, a series of uniaxial ratcheting experiments under different mean stresses with a constant stress amplitude were performed on a 316LN stainless steel at room temperature. Meanwhile, molecular dynamics (MD) simulation was carried out to reveal the microstructure evolution mechanism at the initial stage of ratcheting deformation at atomic scale. The results showed that the initial stage of ratcheting deformation could be divided into three stages: the stage of beginning (ratcheting strain growth rate (RSGR, dεr/dN) >0.01%), the stage of decreasing ratcheting rate (RSGR at 0.00001–0.01) and the stage of elastic/plastic shakedown (RSGR <0.00001) based on the changes of ratcheting strain rate. The shakedown ratcheting strain was linearly increased with increase of the mean stress in both MD simulation and experiment. And the ratchet deformation at different stages strongly depended on dislocation evolution. At the stage of beginning, the dislocation originated from coincident site lattice (CSL) boundaries and formed pile-up structures at random angle grain boundaries (RAGBs); while at the stage of decreasing ratcheting rate, the dislocation tangled at CSL grain boundaries or inside the grain, the dislocation density gradually increased and tended to be stable. When the given stress was insufficient to produce more plastic strain, the dislocation density balanced dynamically and the ratchet deformation entered the stage of elastic/plastic shakedown.

The Effects of Microstructure Refinement and Coincidence Site Lattice Boundaries on the Plasticity of FeCrAlY Alloys

FeCrAlY alloys are widely used as thin‐gauge catalyst carriers for automotive exhaust gas purification. However, they exhibit low ductility due to coarse Fe–Y phases in the alloys, which decreases the carrier yield. To improve the plasticity of the alloys, a novel method for fabricating FeCrAlY alloys using twin‐roll strip casting (TRSC) is proposed. It is found that the plasticity of the FeCrAlY alloy is significantly improved by TRSC compared to conventional ingot casting. Then, to understand the mechanism of plasticity improvement, the microstructures of the alloys are investigated. The Fe–Y phases and grains are significantly refined by TRSC due to its characteristics of subrapid solidification. In addition, the TRSC samples produce several low‐Σ coincidence site lattice (CSL) boundaries, and some of these CSL grain boundaries are inherited by the cold‐rolled sheets. The impact characteristic of steel liquid in the TRSC is the major reason for the formation of CSL grain boundaries. The finer microstructure and greater number of low‐Σ CSL boundaries in the TRSC samples result in strong resistance to fracture, which improves the plasticity of the FeCrAl alloys.

Influence of Welding Speeds on the Morphology, Mechanical Properties, and Microstructure of 2205 DSS Welded Joint by K-TIG Welding.

In this paper, 8.0 mm thickness 2205 duplex stainless steel (DSS) workpieces were welded with a keyhole tungsten inert gas (K-TIG) welding system under different welding speeds. After welding, the morphologies of the welds under different welding speed conditions were compared and analyzed. The microstructure, two-phase ratio of austenite/ferrite, and grain boundary characteristics of the welded joints were studied, and the microhardness and tensile properties of the welded joints were tested. The results show that the welding speed has a significant effect on the weld morphology, the two-phase ratio, grain boundary misorientation angle (GBMA), and mechanical properties of the welded joint. When the welding speed increased from 280 mm/min to 340 mm/min, the austenite content and the two-phase ratio in the weld metal zone (WMZ) decreased. However, the ferrite content in the WMZ increased. The proportion of the Σ3 coincident site lattice grain boundary (CSLGB) decreased as the welding speed increased, which has no significant effect on the tensile strength of welded joints. The microhardness of the WMZ and the tensile strength of the welded joint gradually increased when the welding speed was 280–340 mm/min. The 2205 DSS K-TIG welded joints have good plasticity.

A single grain boundary parameter to characterize normal stress fluctuations in materials with elastic cubic grains

A finite element analysis of intergranular normal stresses is performed in order to identify a possible statistical correlation between the intergranular normal stresses and the corresponding grain boundary type within a polycrystalline aggregate. Elastic continuum grains of cubic lattice symmetry are assumed in the analysis. Meaningful results are obtained by analyzing the first two statistical moments of grain boundary normal stresses obtained on several grain boundary types.Among the five macroscopic parameters (5D) describing a grain boundary, the orientation of the grain boundary plane relative to the two adjacent crystal lattices (4D) is found to be the most important property influencing the normal stresses. To account for it, a single new (1D) parameter E12 is introduced, which combines the geometrical aspect of grain boundary with its material properties and measures the average stiffness of grain boundary neighborhood along the grain boundary normal direction. It is demonstrated that E12, in combination with Zener elastic anisotropy index A, is able to accurately predict normal stress fluctuations on any grain boundary type in a material with cubic lattice symmetry. It is argued that largest normal stresses most likely form on grain boundaries whose normals are oriented along the stiffest direction in both adjacent grains (〈111〉 for crystals with A>1 or 〈001〉 for crystals with A<1). Moreover, it is shown that classification of grain boundaries according to their propensity to exhibit large normal stresses can be trivially reduced to the (analytical) calculation of the corresponding effective stiffness parameter E12.A few practical implications are discussed relevant to intergranular stress-corrosion cracking of Coincidence Site Lattice grain boundaries. For example, it is highlighted that in face-centered cubic materials the coherent Σ3 twin grain boundaries, which are known for their very high cracking resistance, nevertheless exhibit largest intergranular normal stresses, indicating that cracking resistance is associated with high grain boundary strength.

Research on the stress relaxation behaviour of Cu–0.23Cr–0.08Ag alloy

The stress relaxation behaviour of Cu–0.23Cr–0.08Ag alloy was studied at different deformations (10%, 20%, 30%). The high percentage of low ΣCSL (Coincidence Site Lattice) grain boundary could improve the stress relaxation resistance of the alloy. There are two main factors affecting the stress relaxation resistance: one is the pinning effect of Cr to movable dislocations; the other is the obstacle of twins to the movement of movable dislocations.

Improving mechanical anisotropy and corrosion resistance of extruded AA7075 alloy by warm cross rolling and annealing

The anisotropy of mechanical properties and poor corrosion resistance are important problems for high strength aluminum plate. In this study, the combined process of hot extrusion and warm cross rolling was conducted on AA7075 alloy, and both the extruded and rolled plates were annealed at various temperatures. The effects of warm cross rolling and annealing on the microstructure, mechanical properties, and corrosion behavior were studied. The results indicated that both cross rolling and annealing weakened the intensity of main textures of brass, Dillamore, and S. Compared to the extruded plate, the cross rolled ones exhibited higher hardness, higher conductivity, and lower anisotropy of tensile strength. Both cross rolling and annealing made the tensile strength of a plate along different directions more uniform. Moreover, annealing greatly enhanced the corrosion resistance by consuming the distortion energy, reducing the number of crystalline defects and increasing the fraction of low-Σ coincident site lattice grain boundaries. After annealing, the rolled AA7075 plates exhibited better corrosion resistance than the extruded ones, since they possessed lower fractions of brass texture, finer grains, and higher fraction of low-Σ coincident site lattice grain boundaries.

Influence of Laser-Welding on Microstructure and Corrosion Properties of Twinning-Induced Plasticity (TWIP) Steel.

The effect of welding speed on microstructure, mechanical properties, and corrosion properties of laser-assisted welded joints of a twinning-induced plasticity (TWIP) steel was investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) analysis, electrochemical test, and micro-area scanning Kelvin probe test (SKP). The results reveal that the welded joints, with a fully austenitic structure, are obtained by laser welding. In addition, the preferred orientation of grains in fusion zone (FZ) increased with the increase of welding speed. Additionally, the coincidence site lattice (CSL) grain boundaries of FZ decreased with increasing welding speed. However, potentiodynamic polarization and SKP results demonstrated that the welding speed of 1.5 m/min renders superior corrosion resistance. It can also be inferred that the corrosion properties of the welded joints are related to the grain size and frequency of CSL grain boundary in FZ.

Optimization of the grain boundary character distribution of pure copper by low-strain thermomechanical processing

Abstract Although thermomechanical processing has been widely used in grain boundary engineering, the relationship between thermomechanical parameters and grain boundary character distribution is still not well understood. In the present study, electron backscatter diffraction was used to study the grain boundary character distribution in pure copper after lowstrain thermomechanical treatments. It was found that the low-R coincidence site lattice frequency and grain size decreases in the following sequence, 10% compression &gt; 15% compression &gt; 5% compression, except for 700 0C. During thermomechanical treatments of copper, strain-induced grain boundary migration and grain growth may occur during annealing of 5% compressed copper, and recrystallization dominates during annealing of 15% compressed copper, while the annealing mechanism of 10% compressed copper changes from strain-induced grain boundary migration and grain growth to recrystallization when the annealing temperature exceeds 600 0C. The results indicated that during single-step low-strain thermomechanical treatments, straininduced grain boundary migration and grain growth would gradually change to recrystallization with the increase of pre-deformation level and annealing temperature. Among the three mechanisms, strain-induced grain boundary migration seems to be more effective than recrystallization and grain growth in the optimization of grain boundary character distribution, and it is suggested that this is due to the high boundary migration rate of strain-induced grain boundary migration.

Optimization of the grain boundary character distribution of pure copper by low-strain thermomechanical processing

Abstract Although thermomechanical processing has been widely used in grain boundary engineering, the relationship between thermomechanical parameters and grain boundary character distribution is still not well understood. In the present study, electron backscatter diffraction was used to study the grain boundary character distribution in pure copper after lowstrain thermomechanical treatments. It was found that the low-R coincidence site lattice frequency and grain size decreases in the following sequence, 10% compression &gt; 15% compression &gt; 5% compression, except for 700 0C. During thermomechanical treatments of copper, strain-induced grain boundary migration and grain growth may occur during annealing of 5% compressed copper, and recrystallization dominates during annealing of 15% compressed copper, while the annealing mechanism of 10% compressed copper changes from strain-induced grain boundary migration and grain growth to recrystallization when the annealing temperature exceeds 600 0C. The results indicated that during single-step low-strain thermomechanical treatments, straininduced grain boundary migration and grain growth would gradually change to recrystallization with the increase of pre-deformation level and annealing temperature. Among the three mechanisms, strain-induced grain boundary migration seems to be more effective than recrystallization and grain growth in the optimization of grain boundary character distribution, and it is suggested that this is due to the high boundary migration rate of strain-induced grain boundary migration.

Role of Random and Coincidence Site Lattice Grain Boundaries in Liquid Metal Embrittlement of Iron (FCC)-Zn Couple

Liquid metal embrittlement (LME) has frequently been reported in various systems: Cu-Bi, Ni-Bi, Al-Ga, and Fe-Zn for more than 60 years. Although advances have been made in understanding the phenomenon, the role of grain boundary (GB) type and characteristics in LME has remained unclear. The present work shows that liquid metal penetration only occurs in random GBs, where its propagation path is a function of misorientation angle (θ) and stress component perpendicular to GB plane. In contrast, low-Σ coincidence site lattice (CSL) boundaries: Σ3 (θ = 60 deg) and Σ5 to 9 (~ θ = 40 deg) resist LME, even at the maximum stress component. Triple junctions of CSL and random GB block liquid metal penetration by modifying of the random GB misorientation angle. These findings provide insights to employ grain boundary engineering techniques to increase population of CSLs over random GBs and, hence, mitigate LME.

Grain boundary Li-ion conductivity in (Li0.33La0.56)TiO3 polycrystal

Lithium lanthanum titanate (LLTO) is one of the promising solid-state Li-ion electrolytes for an all-solid-state Li-ion battery system. Although LLTO shows a significantly high Li-ion conductivity of 2.2 × 10−3 S cm−1 in the bulk, the Li-ion conductivity at the grain boundary is largely reduced to 4.2 × 10−5 S cm−1, which prevents the practical application of solid-state Li-ion electrolytes. To solve this problem, the origin of such a low Li-ion conductivity at the grain boundary should be clarified. In this study, we investigated the relationship between the Li-ion conductivity and the geometric structure of the grain boundary in the (Li0.33La0.56)TiO3 polycrystal by using electrochemical strain microscopy in atomic force microscopy combined with electron backscatter diffraction in scanning electron microscopy. The experimental data suggest that the Li-ion conductivity is significantly reduced at the random grain boundaries but not at the coincidence-site-lattice (CSL) grain boundaries. Such a small reduction of the Li-ion conductivity at the CSL grain boundaries may originate from the smaller increment of the activation energy, owing to less composition deviations and less structural distortions at the CSL grain boundaries. These results suggest that it is effective to control the geometries of the grain boundaries for further improvement of the Li-ion conductivity in LLTO.

Effects of Magnetic Field on the Residual Stress and Structural Defects of Ti-6Al-4V

In this work, the influences of a magnetic field of 2.4 T on the macro residual stress and the status of structural defects, including grain boundaries, dislocations and the Fe-rich clusters of Ti-6Al-4V were investigated by X-ray Diffraction (XRD), Electron Backscatter Diffraction (EBSD) and magnetic measurement. The XRD test results show that the applied magnetic field can cause the relaxation and homogenization of macro residual stress. The maps of Kernel Average Misorientation (KAM) values obtained by EBSD tests present a significant dislocation multiplication caused by a magnetic field, and the rise of dislocation density was estimated to be about 32% by XRD tests. The EBSD test results also show an increase in the fraction of Coincidence Site Lattice (CSL) grain boundaries and a decrease in the fraction of low-angle grain boundaries. The results of magnetic measurement show that Ti-6Al-4V has mixed magnetism consisting of paramagnetism and weak ferromagnetism, and that the ferromagnetic saturation magnetization decreased after exposing the alloy to the magnetic field, which suggests the dissolution of the Fe-rich clusters in the alloy. These magnetically-induced changes are related to magnetoplastic effects, a kind of phenomena on which there have been some research, and the possible mechanism of them is discussed in this paper.

Atomic Structure and Dynamics of Epitaxial Platinum Bilayers on Graphene.

Platinum atomic layers grown on graphene were investigated by atomic resolution transmission electron microscopy (TEM). These TEM images reveal the epitaxial relationship between the atomically thin platinum layers and graphene, with two optimal epitaxies observed. The energetics of these epitaxies influences the grain structure of the platinum film, facilitating grain growth via in-plane rotation and assimilation of neighbor grains, rather than grain coarsening from the movement of grain boundaries. This growth process was enabled due to the availability of several possible low-energy intermediate states for the rotating grains, the Pt-Gr epitaxies, which are minima in surface energy, and coincident site lattice grain boundaries, which are minima in grain boundary energy. Density functional theory calculations reveal a complex interplay of considerations for minimizing the platinum grain energy, with free platinum edges also having an effect on the relative energetics. We thus find that the platinum atomic layer grains undergo significant reorientation to minimize interface energy (via epitaxy), grain boundary energy (via low-energy orientations), and free edge energy. These results will be important for the design of two-dimensional graphene-supported platinum catalysts and obtaining large-area uniform platinum atomic layer films and also provide fundamental experimental insight into the growth of heteroepitaxial thin films.

First-Principles Study of the Impact of Grain Boundary Formation in the Cathode Material LiFePO4

Motivated by the need to understand the role of internal interfaces in Li migration occurring in lithium-ion batteries, a first-principles study of a coincident site lattice grain boundary in LiFePO4 cathode material and in its delithiated counterpart FPO4 is performed. The structure of the investigated grain boundary is obtained, and the corresponding interface energy is calculated. Other properties, such as ionic charges, magnetic moments, excess free volume, and the lifetime of positrons trapped at the interfaces are determined and discussed. The results show that while the grain boundary in LiFePO4 has desired structural and bonding characteristics, the analogous boundary in FePO4 needs to be yet optimized to allow for an efficient Li diffusion study.