Site Lattice Grain Research Articles

Evolution and mechanism of recrystallization microstructure and texture in GH3536 superalloy foil and their influence on strength performance

To achieve isotropic strength in GH3536 foil, this study investigated the influence mechanism of initial recrystallization microstructure and texture characteristics of GH3536 superalloy foil with different cold-rolled reduction rates (referring to the reduction rate in thickness) on annealing recrystallization microstructure, including secondary recrystallization microstructure, and texture. This was studied by controlling the cold-rolled reduction rate and annealing temperature, in conjunction with microstructure and texture analysis. Meanwhile, this study clarified the dependency of strength performance on recrystallization microstructure and texture. The results of this study indicate that the Coincidence Site Lattice (CSL) grain boundaries and High Energy (HE) grain boundaries did not show significant superiorities, and were not the primary reasons for the formation of Brass ({011}<211>) and Goss ({011}<100>) textures, as well as secondary recrystallization. Instead, by controlling the cold-rolled reduction rate, the initial recrystallization stage formed a significant quantity and size superiority of α-fiber texture ({110}∥ND) in each grain size range. This led to preferential growth, and even abnormal growth of Brass and Goss grains in subsequent high-temperature annealing processes. The control of the α-deformation fiber texture was identified as the driving factor behind the formation of the initial recrystallization texture due to the cold rolling reduction rate. Cold-rolled processes with a deformation amount greater than 40 % yielded a quantity superiority of α-fiber texture shear bands and an appropriate stored energy advantage. This facilitated the nucleation and effective growth of more α grains, while suppressing the random growth of grains with γ-fiber texture ({111}∥ND). Ultimately, a quantity and size superiority was achieved in the initial recrystallization stage. The tensile strength of GH3536 foil was controlled by grain size and texture. By controlling the deformation amount to approximately 30 %, GH3536 foil was successfully produced with no pronounced orientation texture and isotropic strength.

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.

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.

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.

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.

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.

Effects of Coincident Site Lattice Grain Boundaries and Ordered Structures on Mechanical Properties of High Silicon Steel

The coincident site lattice (CSL) grain boundaries and ordered structures of high silicon steel are investigated through different warm rolling reductions. The results reveal that the plastic deformation ability of Fe–6.9 wt%Si warm‐rolled sheet mainly depends on the total content of special ∑CSL boundaries and antiphase domain of ordered structures. With the increase of rolling reductions from 65% to 86%, the superdislocation glide behavior of warm‐rolled sheets is changed into the single dislocation glide, as well as the fragmentation behavior of ordered structures at different rolling reductions causes a decrease in the sizes of antiphase domains. While the sharp and homogeneous distribution of γ‐orientation texture is beneficial to obtain a large fraction of ∑CSL boundaries that have great contribute to high resistance to the initiation and propagation of cracks. In the three‐point bending tests, the values of fracture deflection improves from 3.5 mm to 12.1 mm, and the mixed fracture mode of inter‐and trans‐granular is transformed into quasi‐cleavage fracture mode, which indicates the high rolling reduction is conducive to the improvement of plastic deformation ability.

Secondary recrystallisation behaviours of grain-orientated silicon steel produced by TSCR process

ABSTRACTGrain-oriented electrical steel is mainly used as the core material in electrical transformers, and its magnetic properties are closely related to the sharpness of Goss texture formed by secondary recrystallisation during high-temperature annealing. However, the mechanisms of abnormal Goss grain growth are still disputed in the literature. In this paper, grain-oriented silicon steel strips with AlN as main inhibitor were produced by thin slab casting and rolling process, and the evolution of microstructure and texture during high-temperature annealing was investigated. Moreover, the mechanisms of secondary recrystallisation were further discussed. The results show that secondary recrystallisation is actually a multi-stage process, and different mechanisms are at work at different stages. Coincident site lattice grain boundaries play an important role at the early stage of secondary recrystallisation, however, the role of high-energy grain boundaries and solid-state wetting should be more important at the later stage of secondary recrystallisation.

Geometrically necessary dislocation density measurements at a grain boundary due to wedge indentation into an aluminum bicrystal

An aluminum bicrystal with a symmetric tilt Σ43 (3 3 5)[1 1 0] coincident site lattice grain boundary was deformed plastically via wedge indentation under conditions that led to a plane strain deformation state. Plastic deformation is induced into both crystals and the initially straight grain boundary developed a significant curvature. The resulting lattice rotation field was measured via Electron Backscatter Diffraction (EBSD). The Nye dislocation density tensor and the associated Geometrically Necessary Dislocation (GND) densities introduced by the plastic deformation were calculated. The grain boundary served as an impediment to plastic deformation as quantified through a smaller lattice rotation magnitude and smaller GND density magnitudes in one of the crystals. There is evidence that the lattice rotations in one grain brought a slip system in that grain into alignment with a slip system in the other grain, upon which the impediment to dislocation transmission across the grain boundary was reduced. This allowed the two slip systems to rotate together in tandem at later stages of the deformation. Finite element crystal plasticity simulations using classical constitutive hardening relationship capture the general features observed in the experiments.

A Predictive Framework for Dislocation-Density Pile-Ups in Crystalline Systems With Coincident Site Lattice and Random Grain Boundaries

Evolving dislocation-density pile-ups at grain-boundaries (GBs) spanning a wide range of coincident site lattice (CSL) and random GB misorientations in face-centered cubic (fcc) bicrystals and polycrystalline aggregates has been investigated. A dislocation-density GB interaction scheme coupled to a dislocation-density-based crystalline plasticity formulation was used in a nonlinear finite element (FE) framework to understand how different GB orientations and GB-dislocation-density interactions affect local and overall behavior. An effective Burger's vector of residual dislocations was obtained for fcc bicrystals and compared with molecular dynamics (MDs) predictions of static GB energy, as well as dislocation-density transmission at GB interfaces. Dislocation-density pile-ups and accumulations of residual dislocations at GBs and triple junctions (TJs) were analyzed for a polycrystalline copper aggregate with Σ1, Σ3, Σ7, Σ13, and Σ21 CSLs and random high-angle GBs to understand and predict the effects of GB misorientation on pile-up formation and evolution. The predictions indicate that dislocation-density pile-ups occur at GBs with significantly misoriented slip systems and large residual Burger's vectors, such as Σ7, Σ13, and Σ21 CSLs and random high-angle GBs, and this resulted in heterogeneous inelastic deformations across the GB and local stress accumulations. GBs with low misorientations of slip systems had high transmission, no dislocation-density pile-ups, and lower stresses than the high-angle GBs. This investigation provides a fundamental understanding of how different representative GB orientations affect GB behavior, slip transmission, and dislocation-density pile-ups at a relevant microstructural scale.

Characterization of commercially cold sprayed copper coatings and determination of the effects of impacting copper powder velocities

Copper coated steel containers are being developed for the disposal of high level nuclear waste using processes such as cold spray and electrodeposition. Electron Back-Scatter Diffraction has been used to determine the microstructural properties and the quality of the steel-copper coating interface. The influence of the nature of the cold-spray carrier gas as well as its temperature and pressure (velocity) on the coating's plastic strain and recrystallization behaviour have been investigated, and one commercially-produced electrodeposited coating characterized. The quality of the coatings was assessed using the coincident site lattice model to analyse the properties of the grain boundaries. For cold spray coatings the grain size and number of coincident site lattice grain boundaries increased, and plastic strain decreased, with carrier gas velocity. In all cases annealing improved the quality of the coatings by increasing texture and coincidence site-lattices, but also increased the number of physical voids, especially when a low temperature cold spray carrier gas was used. Comparatively, the average grain size and number of coincident site-lattices was considerably larger for the strongly textured electrodeposited coating. Tensile testing showed the electrodeposited coating was much more strongly adherent to the steel substrate.

Atomistic investigation of the structure and transport properties of tilt grain boundaries of UO2

We apply atomistic simulation techniques to address whether oxygen shows higher diffusivity at the grain boundary region compared to that in bulk UO2, and whether the relative diffusivity is affected by the choice of the grain boundary. We consider coincident site lattice grain boundaries, Σ3, Σ5, Σ9, Σ11 and Σ19, expressing the {nn1}, {n11}, and {n10} surfaces, and evaluate the extent that the grain boundary structures affect the diffusion of oxygen. We found that oxygen diffusion is enhanced at all boundaries and in the adjacent regions, with strong dependence on the temperature and local structure.

Boundary plane distribution for Σ13 grain boundaries in magnesium

Grain boundary plane distribution is analyzed for the Σ13 coincident site lattice grain boundaries in magnesium. These boundaries are relatively frequent in materials with hexagonal structure. It was shown that the most frequent boundary plane is (0001) basal plane. The experimental data obtained by stereological approach are in the agreement with the grain boundary energies obtained by atomistic calculations.