Effect Of Grain Boundary Misorientation Research Articles

In-situ study of the effect of grain boundary misorientation on plastic deformation of Inconel 718 at high temperature

In-situ study of the effect of grain boundary misorientation on plastic deformation of Inconel 718 at high temperature

Migration of grain boundary triple junctions in nanocrystalline metals initiated by accumulated dislocations

The evolution of grain boundary (GB) networks in polycrystalline metals depends not only on the movement of the constituent GBs but also on the migration of GB triple junctions (TJs), the latter would have strong influence on the mechanical properties of polycrystals. We investigate the migration mechanism of TJs in face-centered cubic metals using molecular dynamics simulations. The TJ migration is revealed to follow a typical stick-slip behaviour that involves TJ grooving and coordinated GB migration. Upon external shear loading, the TJ experiences a quick migration process, resulting in a grooved TJ configuration with curved GBs at the TJ. The accumulation of dislocations at the TJ facilitates the TJ grooving process, since it compensates the angle gap caused by the inconsistent shear-migration coupling factors between the GBs. After reaching a critical grooving depth, the TJ migration is stagnated, while the connecting GBs begin to migrate via disconnections emitted from the TJ. Based on the mechanism of accumulated dislocation-initiated TJ migration, we propose an energetic model that considers the extra energy barrier for the accumulation of dislocations, to quantify the effect of GB misorientation on TJ migration. These findings deepen our understanding of the physical origin of TJ migration and point out the importance of dislocation accumulation on coordinated GB motion in nanocrystalline metals.

Simulation of Abnormal Grain Growth Using the Cellular Automaton Method.

The abnormal grain growth of steel, which is occurs during carburization, adversely affects properties such as heat treatment deformation and fatigue strength. This study aimed to control abnormal grain growth by controlling the materials and processes. Thus, it was necessary to investigate the effects of microstructure, precipitation, and heat treatment conditions on abnormal grain growth. We simulated abnormal grain growth using the cellular automaton (CA) method. The simulations focused on the grain boundary anisotropy and dispersion of precipitates. We considered the effect of grain boundary misorientation on boundary energy and mobility. The dispersion state of the precipitates and its pinning effect were considered, and grain growth simulations were performed. The results showed that the CA simulation reproduced abnormal grain growth by emphasizing the grain boundary mobility and the influence of the dispersion state of the precipitate on the occurrence of abnormal grain growth. The study findings show that the CA method is a potential technique for the prediction of abnormal grain growth.

Effect of grain boundary misorientation and carbide precipitation on damage initiation: A coupled crystal plasticity and phase field damage study

A coupled crystal plasticity phase field damage framework has been developed and applied to modelling damage initiation. A novel implementation of a grain misorientation angle dependent critical energy release rate has been used to determine a reduction in the local critical energy release rate resulting from the effects of intergranular carbide precipitates and grain boundary misorientation. When applied to a notched high temperature 316H austenitic stainless steel specimen, a good correlation between experimental results and void nucleation statistics for a misorientation dependent critical energy release rate was obtained. This has been evaluated through comparison with correlative electron microscopy experimental results, showing the potential of phase field models in the area of early damage formation. Additions to include plastic strain and creep deformation effects were made, and comparisons were drawn with experimental data to investigate the contributions of microstructural geometry properties such as the difference in and average values of Schmid factors across grain boundaries, as well as the loading direction stress and dislocation densities. The limitations to this approach and opportunities for further work in this area are discussed, with specific interest in the need for additional literature data characterising grain boundary carbide precipitation and cavity nucleation analysis.

Effects of Grain Boundary Misorientation Angle on the Mechanical Behavior of Al Bicrystals

This research article explores the effect of grain boundary (GB) misorientation on the mechanical behavior of aluminum (Al) bicrystals by means of molecular dynamics (MD) simulations. The effect of GB misorientation on the mechanical properties, fracture resistance, and crack propagation are evaluated under monotonic and cyclic load conditions. The J-integral and the crack tip opening displacement (CTOD) are assessed to establish the effect of the GB misorientation angle on the fracture resistance. The simulations reveal that the misorientation angle plays a significant role in the mechanical response of Al bicrystals. The results also evidence a gradual change in the mechanical behavior from brittle to ductile as the misorientation angle is increased.

Effect of misorientation of grain boundaries on the discontinuous precipitates of Cu–Ni–Si alloy

In this study, the effects of differences in grain size, grain boundary type and misorientation on the initiation of discontinuous precipitates are investigated for the Cu–5Ni–1.25Si alloy. Samples with different grain sizes are obtained by thermomechanical treatment, and the smaller the grain size, the higher the percentage of discontinuous precipitate at the same aging time, while there is no significant difference in the growth rate of the reaction front of discontinuous precipitates. EBSD is used to study the effect of grain boundary misorientation on discontinuous precipitates, the characteristics of discontinuous precipitates initiation on different grain boundaries are investigated by multivariate statistical analysis. It is found that discontinuous precipitates only initiate from random grain boundaries instead of coincident site lattice boundaries, where random grain boundaries with misorientation angles of 30–50° are more prone to discontinuous precipitates. And the initiation of DPs at different misorientation axes is different. There is a clear borderline for the axes such as <102>, <103>, and <203>. The grain boundaries below this borderline do not initiate DP, while all grain boundaries below this angle initiate DPs. The cases for axes such as <112>, <114>, and <214> are more complicated. On these misorientation axes, grain boundaries with lower or higher misorientation angles do not initiate DPs, while grain boundaries at intermediate range misorientation angles do initiate DPs.

Effect of Grain Boundary Misorientation on Spall Strength in Ta via Shock-Free Simulations with Relatively Few Atoms

A suite of 37 molecular dynamics simulations is conducted at two system sizes to systematically characterize the role of grain boundary (GB) misorientation on spall strength in pure BCC tantalum (Ta). The systems studied consist of bicrystals with a single [110] symmetric tilt grain boundary. Two loading conditions are compared: (i) homogeneous extension under uniaxial strain simulated in this study and (ii) piston/flyer impact of sample, which induces heterogeneous deformation via shockwave propagation along the length of the sample. The piston/flyer impact is taken from the literature and run on the same set of GB misorientation angles using LAMMPS. The major finding here is that both methods result in similar spall strength predictions, but the homogeneous extension method generally requires two to three orders of magnitude fewer atoms and similar reductions in computational costs. Spall strength results systematically overpredict using this method, by about 10% for the dataset three orders of magnitude smaller than piston/flyer simulations, and 5% for the dataset two orders of magnitude smaller. Lastly, the effect of system size and pre-compression magnitude on spall strength is systematically characterized.

The initial corrosion behavior of Al–Cu–Li bicrystals: effect of misorientation and precipitates

Al–Cu–Li bicrystals with different misorientation gradients were prepared to study the effect of misorientation and precipitates on the initial corrosion behavior of the Al–Cu–Li alloy by scanning electron microscopy (SEM), transmission electron microscopy (TEM), immersion testing, and electrochemical testing. The results show that the initial corrosions were generated on the matrix rather than on the grain boundary for the as-quenched alloy. The effect of grain boundary misorientation on the corrosion rates of the as-quenched alloy is very limited. On the other hand, for the aged alloy, the initial corrosions were generated on the grain boundaries, which suffer more severe corrosion than the matrix. Meanwhile, the corrosion of high angle grain boundary (HAGB) is more severe than that of low angle grain boundary (LAGB) for the aged alloy. These results are associated with the high number density of the continuously distributed precipitates on grain boundaries.

Precipitation on grain boundaries in Al-Mg-Si alloys: The role of grain boundary misorientation

The application of Al-Mg-Si alloys is growing in fracture critical applications such as transportation. The formation of grain boundary precipitates during cooling after solution treatment is an important factor in controlling low energy intergranular fracture modes. A systematic study was conducted on the effects of grain boundary misorientation and cooling rate (between 3 and 1000 °C/s) on the size and density of grain boundary precipitates before and after artificial ageing. It was found that precipitates on high angle grain boundaries were ≈3 times larger than those formed on low angle boundaries. The grain boundary precipitate size decreased by a factor of 3 when the cooling rate increased from 3 to 80 °C/s with no precipitates observed at 1000 °C/s. However, even for a cooling rate of 1000 °C/s, grain boundary precipitation was observed during subsequent artificial ageing. The effects of grain boundary misorientation and cooling rate were rationalized by enhanced diffusion at grain boundaries.

The Effects of Grain Boundary Misorientation on the Mechanical Properties and Mechanism of Plastic Deformation of Ni/Ni3Al: A Molecular Dynamics Study.

The effects of grain boundary misorientation angle (θ) on mechanical properties and the mechanism of plastic deformation of the Ni/Ni3Al interface under tensile loading were investigated using molecular dynamics simulations. The results show that the space lattice arrangement at the interface is dependent on grain boundary misorientations, while the interfacial energy is dependent on the arrangement. The interfacial energy varies in a W pattern as the grain boundary misorientation increases from 0° to 90°. Specifically, the interfacial energy first decreases and then increases in both segments of 0–60° and 60–90°. The yield strength, elastic modulus, and mean flow stress decrease as the interfacial energy increases. The mechanism of plastic deformation varies as the grain boundary misorientation angle (θ) increases from 0° to 90°. When θ = 0°, the microscopic plastic deformation mechanisms of the Ni and Ni3Al layers are both dominated by stacking faults induced by Shockley dislocations. When θ = 30°, 60°, and 80°, the mechanisms of plastic deformation of the Ni and Ni3Al layers are the decomposition of stacking faults into twin grain boundaries caused by extended dislocations and the proliferation of stacking faults, respectively. When θ = 90°, the mechanisms of plastic deformation of both the Ni and Ni3Al layers are dominated by twinning area growth resulting from extended dislocations.

Effect of grain boundary misorientation on the apparent diffusivity in nanocrystalline aluminum by atomistic simulation study

To obtain a fundamental understanding of the effect of structure and geometry of grain boundary on the diffusion kinetics in nanocrystalline materials, the influence of grain boundary misorientation on the effective diffusion coefficient (apparent diffusivity) in nanocrystalline aluminum was investigated using molecular dynamics simulations. Nine series of [001] symmetric tilt grain boundaries, including high and low symmetric boundary planes, were studied. The apparent diffusivity in the samples was calculated in the temperature range from 423 K to 823 K by monitoring the mean square displacement of atoms as a function of simulation time. A temperature dependence of the effective diffusion coefficient according to the Arrhenius law was obtained for all samples. It is found that the apparent diffusivity is anisotropic and it is a strong function of grain boundary misorientation at low and high temperatures. At all temperatures, Σ29 [001]/(520) symmetric tilt grain boundary with misorientation angle of 43.68° exhibits the highest effective diffusion coefficient among the investigated grain boundaries. The simulation results show that the activation energy and pre-exponential factor are affected significantly by the grain boundary misorientation angle. Moreover, the results indicated that the misorientation dependence of activation energy for diffusion exhibits two local maxima, which correspond to two symmetric tilt grain boundaries. Additional calculation of misorientation dependence of the pre-exponential factor shows two local minima at the same symmetric tilt grain boundaries. The misorientation dependence of the effective diffusion coefficient was explained on the basis of grain boundary energy and the crystallographic structure of grain boundary.

Uniaxial stress-driven grain boundary migration in Hexagonal Close-packed (HCP) metals: Theory and MD simulations

Stress-driven grain boundary (GB) migration is an important plastic deformation mechanism in nanopolycrystalline metals. Recently, Liu et al. (2014) and Zong et al. (2015) have shown a new mechanism for GB migration in a Hexagonal Close-packed (HCP) bicrystal with 90° GB misorientation under uniaxial stress loading conditions, which is different from shear-coupled GB migration. In real polycrystals, GBs have random orientations. To investigate the effects of GB misorientation and loading directions, we propose a mechanics model for uniaxial stress-driven GB migration in which the GB misorientation varies from 0° to 90°. Two types of imposed uniaxial loadings with loading directions parallel and perpendicular to the GB are considered. The model prediction is compared with that by molecular dynamics (MD) simulations. The results reveal that the uniaxial stress-driven GB migration can only occur in some range of GB misorientation under both loading conditions. The critical strain to trigger the GB migration varies with GB misorientation in a non-monotonic way. The proposed model provides new insights into the GB migration in HCP metals.

Effect of grain boundary misorientaton, deformation temperature and AlFeMnSi-phase on fatigue life of 6082 Al alloy

In the present work, the effect of grain boundary misorientation, deformation temperature and AlFeMnSi-phase on fatigue behavior of 6082 Al has been investigated. The 6082 Al alloy has been subjected to cryorolling followed by warm rolling at 100°C–250°C. The fatigue test of the processed alloy has been performed at stress ratio (R) of zero with different stress amplitude of 25, 50, 60, 70, 80, 90, and 110MPa. The deformed alloy and fracture surface was characterized by using XRD, DSC, SEM and EDS to elucidate the role of AlFeMnSi-phase and Mg2Si-precipitates on fatigue life of the alloy. Cryorolling followed by Warm rolling (CR+WR) 6082 Al alloy rolled at 100°C temperature showed higher dislocation density compared to other processed sample. The small size of AlFeMnSi-phase and Mg2Si-precipitates, high dislocation density and low angle grain boundaries had improved the fatigue life of 6082 Al alloy as observed in the present work.

Effects of Grain Boundary Misorientation and Chromium Segregation on Corrosion of CoCrMo Alloys

The influence of grain boundary interfacial energy on the structure of carbides and the local segregation of chromium were investigated at the nanoscale for coincident site lattice boundaries in a ...

In situ observations of silver-decoration evolution under hydrogen permeation: Effects of grain boundary misorientation on hydrogen flux in pure iron

An in situ silver decoration technique using a light microscope was developed. Hydrogen was introduced from the bottom surface of an annealed pure iron specimen, and its top surface was covered with a chemical solution containing silver ions. Samples were exposed for sufficient time for hydrogen permeation to occur from the back to the top surface. The resultant in situ silver decoration visualized that the hydrogen flux through low-angle grain boundaries is lower than that observed at high-angle grain boundaries. The in situ silver decoration technique can be used for kinetic analysis of solute hydrogen in ferritic iron and steels.

Characterization of heat affected zone liquation cracking in laser additive manufacturing of Inconel 718

The heat affected zone liquation cracking behavior was studied in laser additive manufactured Inconel 718. Liquation cracking was found initiating from the weak site near the fusion line in the pre-deposited layer, propagating along the interdendritic region with the further deposition proceeding layer by layer. Total cracking length calculation results showed that when controlling the heat input and height increment constant, liquation cracking susceptibility increased with the increase of laser scanning speed; and when controlling the laser scanning speed and height increment constant, liquation cracking susceptibility increased with the increase of heat input. The effect of grain boundary misorientation on susceptibility to liquation cracking was also investigated through electron backscatter diffraction (EBSD) measurement, and the results showed that liquation cracking tendency increased with the increase of grain boundary angle, which was considered to be attributed to the higher stability of liquation film at larger grain boundary during the last stage of solidification.

Hydrogen embrittlement in nickel, visited by first principles modeling, cohesive zone simulation and nanomechanical testing

Hydrogen (H) can have dramatic consequences on material properties, especially by reducing the fracture toughness. Degradation by H initiates through mechanisms at the nano-scale, and is normally not detectable prior to the final leakage or component fracture. Computational techniques are therefore being developed in order to provide both a wider understanding of the phenomenon and engineering tools for prediction of materials susceptibility toward hydrogen embrittlement. This work presents a preliminary study for the development of a novel computational approach in which density functional theory (DFT), nanoscale experiments and finite element (FE) modeling are combined and interrelated in order to improve the understanding of hydrogen induced intergranular cracking. Two low angle and low coincidence grain boundaries types have been considered: Σ3 and Σ5. Density Functional Theory has been applied to investigate the influence of an increasing number of H atoms on the cohesive strength of these grain boundaries in pure Nickel. This provided relations between the number of H atoms (coverage of the grain boundary) and the cohesive strength, which is further applied in cohesive zone FE modeling of a triangular nanometer sized fracture mechanics cantilever beam. For verification of the model such specimens will be tested experimentally both with and without in-situ electrochemical charging.The simulation results show that the model is suitable for describing the combined effect of grain boundary misorientation and the reduced cohesive energy due to hydrogen on the grain boundary propensity to cleave.

Effect of Grain Boundary Misorientation on Electromigration in Lead-Free Solder Joints

Reduction in microelectronic interconnect size gives rise to solder bumps consisting of few grains, approaching a single- or bicrystal grain morphology in C4 bumps. Single grain anisotropy, individual grain orientation, presence of easy diffusion paths along grain boundaries, and the increased current density in these small solder bumps aggravate electromigration. This reduces the reliability of the entire microelectronic system. This paper focuses on electromigration behavior in Pb-free solder, specifically the Sn-0.7 wt.%Cu alloy. We discuss the effects of texture, grain orientation, and grain boundary misorientation angle on electromigration (EM) and intermetallic compound formation in EM-tested C4 bumps. The detailed electron backscatter diffraction (EBSD) analysis used in this study reveals the greater influence of grain boundary misorientation on solder bump electromigration compared with the effect associated with individual grain orientation.

Effect of grain boundary orientation on radiation-induced segregation in a Fe–15.2 at.% Cr alloy

Ferritic chromium steels are important structural materials for future nuclear fission and fusion reactors due to their advantages over traditional austenitic steels, such as higher thermal fatigue resistance, lower thermal expansion coefficients and reduced swelling. However radiation-induced segregation or depletion (RIS/RID) of solute atoms at grain boundaries in these materials is a concern because these phenomena could adversely affect their mechanical properties. In an effort to develop a full mechanistic understanding of RIS/RID, a systematic approach combining orientation imaging, site-specific specimen preparation and three-dimensional atomic-scale analysis has been developed to characterize the behaviour of Cr and C at grain boundaries during irradiation. This methodology has been applied to a Fe–15.2at.% Cr alloy to investigate the effects of grain boundary misorientation, irradiation depth and impurities. Systematic differences in Cr segregation are reported as a function of grain boundary character and irradiation conditions. The similar properties demonstrated by grain boundaries of similar type means that it should be possible to apply relatively simple models to predict the long-term behaviour of these materials under irradiation conditions.

Effect of grain boundary misorientation on discontinuous precipitation in an AZ91 alloy

A scanning electron microscopy (SEM)-based electron backscatter diffraction (EBSD) analysis showed that the discontinuous precipitation (DP) reaction rate was dependent on the geometry of the grain boundary in Mg-9Al-1Zn (wt.%) alloys. DP converted a supersaturated solid solution,δ 0 (magnesium (Mg)-rich solid solution), into a two-phaseδ + γ aggregate, withδ being a precipitated Mg17Al12 (intermetallic phase) behind a migrating reaction front. The near-special grain boundary was rather inactive, whereas most of the random high-angle boundaries promoted the reaction. Prior deformation (hot rolling to achieve up to 80% thickness reduction) had no effect on the frequency of special-grain boundaries.