Grain Boundary Misorientation Research Articles

Controllable Pseudoelasticity in Metallic Nanocrystals by Grain Boundary Engineering.

Materials with pseudoelasticity can recover from large strains exceeding their elastic limits during unloading, making them promising damage-tolerant building blocks for advanced nanodevices. Nevertheless, a practical approach to realize controllable pseudoelastic behavior at nanoscale remains challenging. Here, we proposed a grain boundary (GB) engineering approach to endow metallic nanocrystals with a controllable pseudoelasticity. Both in situ nanomechanical testing and atomistic simulations demonstrate that such controllable pseudoelasticity is governed by the extension and contraction of an inherent stacking fault array at the GB. By precisely tuning GB misorientation and inclination, our simulation results reveal that metallic nanocrystals can exhibit tailored pseudoelastic performance across a broad spectrum of GBs in different face-centered cubic metals. These findings enrich our understanding of the intrinsic pseudoelasticity of GBs and provide a GB engineering approach toward metallic materials with reversible deformability.

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.

Atomic simulation study of the factors affecting nucleation in deformation-induced martensitic transformation in grains and at grain boundaries in pure iron

Molecular dynamics simulations were performed to investigate the dominant factors affecting the nucleation in deformation-induced martensitic transformation in grains and at grain boundaries in pure iron, and the effectiveness of microstructure designs incorporating the obtained dominant factors was verified. First, the local nucleation stress required for grain-boundary nucleation was lower than that for in-grain nucleation, indicating that the grain boundary is an effective nucleation site for martensitic transformation. Next, we investigated the dominant factors affecting the in-grain nucleation and the grain-boundary nucleation, and found that the dominant factor of the in-grain nucleation is the magnitude of the loading-direction component of the transformation strain induced by lattice deformation (Bain distortion), while the dominant factors of grain-boundary nucleation are the grain-boundary misorientation angle and grain-boundary free volume. Finally, the possibility of controlling the nucleation timing by designing the microstructures based on the obtained nucleation dominant factors was demonstrated by comparing two polycrystalline models with the same texture but different distributions of grain-boundary misorientation angles.

Temperature effect on the transition between grain boundary migration and sliding

Grain boundary (GB) dynamics plays an important role in the mechanical and physical properties of nanocrystalline metals. In this study, we investigate the temperature effect on GB deformation transition using molecular dynamics simulations of [100] symmetric tilt GBs in Au bicrystals. Different deformation behaviours were revealed in GBs with the same structures at varied temperatures. GB sliding occurs as the predominant deformation mechanism at elevated temperatures, while GB migration dominates at low temperatures. The temperature-dependent critical stresses for GB migration and GB sliding were compared, revealing that temperature alters the critical GB misorientation at which GB migration transitions to GB sliding. Our study provides new insight into GB-mediated plastic deformation in nanocrystalline materials.

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.

Room temperature control of grain orientation via directionally modulated current pulses

Traditional approaches to control the microstructure of materials, such as annealing, require high temperature treatment for long periods of time. In this study, we present a room temperature microstructure manipulation method by using the mechanical momentum of electrical current pulses. In particular, a short burst of high-density current pulses with low duty cycle is applied to an annealed FeCrAl alloy, and the corresponding response of microstructure is captured by using Electron Backscattered Diffraction (EBSD) analysis. We show evidence of controllable changes in grain orientation at specimen temperature around 28 °C. To demonstrate such microstructural control, we apply the current pulses in two perpendicular directions and observe the corresponding grain rotation. Up to 18° of grain rotation was observed, which could be reversed by varying the electropulsing direction. Detailed analysis at the grain level reveals that electropulsing in a specific direction induces clockwise rotation from their pristine state, while subsequent cross-perpendicular electropulsing results in an anticlockwise rotation. In addition, our proposed room temperature processing yields notable grain refinement, while the average misorientation and density of low-angle grain boundaries (LAGBs) remain unaltered. The findings of this study highlight the potentials of ‘convective diffusion’ in electrical current based materials processing science towards microstructural control at room temperature.

Comparison of the Thermal Stability in Equal‐Channel‐Angular‐Pressed and High‐Pressure‐Torsion‐Processed Fe–21Cr–5Al Alloy

Nanostructured steels are expected to have enhanced irradiation tolerance and improved strength. However, they suffer from poor microstructural stability at elevated temperatures. In this study, Fe–21Cr–5Al–0.026C (wt%) Kanthal D (KD) alloy belonging to a class of (FeCrAl) alloys considered for accident‐tolerant fuel cladding in light‐water reactors is nanostructured using two severe plastic deformation techniques of equal‐channel angular pressing (ECAP) and high‐pressure torsion (HPT), and their thermal stability between 500–700 °C is studied and compared. ECAP KD is found to be thermally stable up to 500 °C, whereas HPT KD is unstable at 500 °C. Microstructural characterization reveals that ECAP KD undergoes recovery at 550 °C and recrystallization above 600 °C, while HPT KD shows continuous grain growth after annealing above 500 °C. Enhanced thermal stability of ECAP KD is from significant fraction (>50%) of low‐angle grain boundaries (GBs) (misorientation angle 2–15°) stabilizing the microstructure due to their low mobility. Small grain sizes, a high fraction (>80%) of high‐angle GBs (misorientation angle >15°) and accordingly a large amount of stored GB energy, serve as the driving force for HPT KD to undergo grain growth instead of recrystallization driven by excess stored strain energy.

Mesoscale quantification of grain boundary character and local plasticity in hydrogen-assisted intergranular and intergranular-like cracking paths in tempered martensitic steel

The effects of the misorientation of prior austenite grain boundary (PAGB) segments on the local plasticity evolution in intergranular (IG) and IG-like fractures were investigated using a tempered martensitic steel. When a diffusible hydrogen content of 6.3 mass ppm was introduced, IG fractures with smooth fracture surfaces occurred in the elastic regime of the tensile test. According to the grain orientation spread (GOS) distribution, local plasticity did not contribute to the mesoscopic (block size) scale, irrespective of the PAGB segment misorientation. With a relatively low hydrogen content (4.0 mass ppm), an IG-like feature appeared on the fracture surface, which included plasticity-related traces such as tear ridges or nanovoids. The GOS values near the IG-like cracks were higher than those near the IG cracks and exhibited significant misorientation dependence. Specifically, at the low-angle PAGB segments, the GOS values of the grains neighboring the IG-like cracks increased with a decreasing misorientation. On the other hand, most of the cracked high-angle PAGB segments exhibited nearly constant GOS values. Remarkably, a Σ3 PAGB segment exhibited an abnormally high GOS value. These facts and corresponding microstructural observation results indicate that the low angle and Σ3 PAGB segments allow crack-tip blunting before crack growth.

Exploring the evolutionary behavior of grain boundaries in titanium induced by temperature fluctuations using a combination of molecular dynamics simulations and experiments

Temperature fluctuation in low-temperature range is common environmental condition experienced by precision instruments. However, the effect of temperature fluctuation on grain boundary, which correlated with the mechanical and thermodynamics of materials, has not been elucidated. In this study, we used molecular dynamics to simulate the evolutionary behavior of grain boundary during temperature fluctuations. Unexpectedly, we found irreversible structural transformation in grain boundaries that experienced temperature fluctuations of low temperature range (<100 °C) and then returned to the initial temperature. Subsequently, we explored its influence on the macroscopic properties of material by experiments. The results show that structural transformation of grain boundary leads to a decrease in misorientation of grain boundary and macro size of material. Moreover, our experiments also show that structural transformation of grain boundary induced by temperature fluctuations is accomplished through the migration of atoms around grain boundary. It provides insight to investigate temperature dependence of grain boundary.

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.

Convection-Induced Compositional Patterning at Grain Boundaries in Irradiated Alloys.

We consider the stability of precipitates formed at grain boundaries (GBs) by radiation-induced segregation in dilute alloys subjected to irradiation. The effects of grain size and misorientation of symmetric-tilt GBs are quantified using phase field modeling. A novel regime is identified where, at long times, GBs are decorated by precipitate patterns that resist coarsening. Maps of the chemical Péclet number indicate that arrested coarsening takes place when solute advection dominates over thermal diffusion right up to the precipitate-matrix interface, preventing interfacial local equilibrium and overriding capillary effects. This contrasts with liquid-solid mixtures where convection always accelerates coarsening.

Microscale Imaging of Thermal Conductivity Suppression at Grain Boundaries.

Grain-boundary engineering is an effective strategy to tune the thermal conductivity of materials, leading to improved performance in thermoelectric, thermal-barrier coatings and thermal management applications. Despite the central importance to thermal transport, a clear understanding of how grain boundaries modulate the microscale heat flow is missing, owing to the scarcity of local investigations. Here, thermal imaging of individual grain boundaries is demonstrated in thermoelectric SnTe via spatially resolved frequency-domain thermoreflectance. Measurements with microscale resolution reveal local suppressions in thermal conductivity at grain boundaries. Also, the grain-boundary thermal resistance - extracted employing a Gibbs excess approach - is found correlating with the grain-boundary misorientation angle. Extracting thermal properties, including thermal boundary resistances, from microscale imaging can provide comprehensive understanding of how microstructure affects heat transport, crucially impacting the materials design of high-performance thermal-management and energy-conversion devices. This article is protected by copyright. All rights reserved.

Comparative assessment of gas and water atomized powders for additive manufacturing of 316 L stainless steel: Microstructure, mechanical properties, and corrosion resistance

In this study, the microstructure, and mechanical properties of 316 L stainless steel (SS316L) produced by laser powder bed fusion (LPBF) using water-atomized (WA) and gas-atomized (GA) powders were compared. The results showed that the use of WA powder, with a finer average particle size and better spreadability, led to significantly higher values of tensile strength (UTS), yield strength (YS), elongation (El%), and toughness in the WA samples (728 MPa, 580 MPa, 31.8%, and 215 J/m3, respectively) compared to the GA sample (602 MPa, 503 MPa, 25.2%, 145 J/m3, respectively). The WA samples also exhibited a non-uniform hardness distribution and superior work-hardening rate due to the presence of multiple inclusions that tightly bound to the matrix and created stress fields, increasing the required energy for dislocation motion. The higher solidification rate of melt pools in the WA sample left more intensive residual stress with distorted grains, exhibiting a higher grain orientation spread (GOS). Additionally, a multitude of geometrically necessary dislocations (GNDs) formed around the boundaries of elongated grains with tilted boundaries to maintain lattice continuity, resulting in a higher kernel average misorientation (KAM) and congestion of low-angle grain boundaries (LAGBs), particularly in the WA sample. XRD patterns confirmed the higher lattice distortion in the WA sample, and the smaller cellular structures observed in SEM images were consistent with the higher dislocation density observed in the WA specimens. Finally, the WA sample exhibited lower surface roughness and rather higher resistance to corrosive media containing chlorides.

Precipitation of δ phase in Inconel 718 superalloy: The role of grain boundary and plastic deformation

The control of δ precipitations is important during the thermal processing of Inconel 718 superalloy (IN718) because it can control the grain size and affect the mechanical properties of the material. The main purpose of this study was the effects of plastic deformation and Grain Boundary Misorientation (GBMO) on δ phase precipitation at grain boundaries (GBs). The content of the δ phase and the number of precipitated orientations at the GB were statistically analyzed. It was found that the δ phase grew into grains along the {111} plane of γ-matrix after nucleation at the GB. Plastic deformation can change the morphology and precipitate location of δ phase, with the increasing of plastic strain, the content of the δ phase and the number of precipitated orientations in GBs increased, which was related to the piling up of dislocations caused by deformation and the slip of {111} plane. For the undeformed samples, the δ phase precipitation at the GB was affected by the GBMO, which was consistent with the change of GBs energy with the GBMO. The influence of the structure and characteristics of the interfaces (GBs and twin boundaries) on the precipitation behavior of the δ phase is also analyzed in this study.

Misorientation and Temperature Dependence of Small Angle Twist Grain Boundaries in Silicon: Atomistic Simulation of Directional Growth

In nano-crystalline and multi-crystalline silicon, grain boundaries (GBs) and their properties may dominate the overall material performance. With a hybrid Monte Carlo and molecular dynamics (MC/MD) approach capable of reproducing the natural formation process of silicon (001) small angle twist GBs, the misorientation and temperature dependence of GB properties were examined at the atomic level. The GB structures and energies show various transition characteristics around three critical misorientation angles. Structure–property correlations are established by converting the three critical misorientation angles to the corresponding dislocation spacings, which are equal to 6-, 2-, and 1-times dislocation core radius. Stress fields and elastic strain energies agree well with the Continuum theory, and their effects on the dislocation structures and defect sink are discussed. This work also reports the variations of GB structures and energies are governed by a critical temperature at around 800 K, where the GB energies reach the minimum and the dislocation dissociations are suppressed.

Overcoming the strength-ductility trade-off in additive manufactured AlSi10Mg plates by friction stir welding at a low rotational speed

The main problem in post-treatment of laser powder bed fusion (L-PBF) AlSi10Mg is the sharp reduction in strength when the ductility enhances, i.e. the problem of strength-ductility trade-off. Friction stir welding (FSW) at a low rotational speed of 200 rpm was employed to join AlSi10Mg plate produced by laser-powder bed fusion in conjunction with overcoming the strength-ductility trade-off. The microstructure, texture, and mechanical properties of the as-built and friction stir welded samples were examined. FSW resulted in the elimination of porosities, formation of ultrafine-grains with an average grain size of 0.7 μm, increment in dislocation density and Taylor factor, and fragmentation of Si particles. In addition, after FSW, B/B¯ and C shear texture components were mainly formed in the center of the stir zone, while at the shoulder-dominated zone the A1∗(111)[1¯1¯2] and A2∗(111)[112¯] components might be formed. The main grain structure formation mechanism was continuous dynamic recrystallization occurring by a gradual increase in misorientation of low angle grain boundaries and geometrically dynamic recrystallization. Consequently, tensile elongation was enhanced by ≈173%, while a negligible decrease in strength (≈9%) was observed. During tensile loading, the joint was fractured from the stir zone by the initiation of cracks from the surface of the sample, resulting in a bimodal dimple structure.

High-temperature fatigue strength of grain boundaries with different misorientations in nickel-based superalloy bicrystals

Nickel-based single-crystal superalloys are widely used in the manufacture of aeroengine turbine vanes for their excellent high-temperature performance. Low-angle grain boundaries (LAGBs) will be generated inevitably during their manufacture, which are often characterized by grain boundary misorientation (GBM) and will weaken the mechanical properties of superalloys. However, the relationship between GBM and the fatigue properties of superalloys at elevated temperatures has seldom been investigated due to the difficulty in the sample preparation and experiment process. Based on six kinds of bicrystals with different tilt LAGBs made by a second-generation single-crystal superalloy, the effects of misorientation on the grain boundary microstructure and fatigue properties (980 °C) of superalloys were studied systematically in this work. It is found that, with the increase of GBM, the GB precipitates combined with the cast micropores increase monotonically, accordingly both the fatigue life and fatigue strength decrease successively. Fatigue fracture observations show that the cracks of all the bicrystals initiated from the cast micropores at GBs, and then propagated along the GBs. Therefore, the coupling effect of cast micropores and GBM on the fatigue damage mechanisms of the bicrystals are evaluated according to their hindering degrees on the piled-up dislocations. Combining with a hysteresis energy model, a quantitative fatigue strength prediction model of superalloys is established and is well verified by abundant experimental data. This study could provide guidance for fatigue performance prediction and structural design of superalloys.

Precipitation characteristics of γ' precipitates of the GH4742 nickel-based superalloy at a slow cooling rate

The double cone samples of the predeformed GH4742 superalloy were treated by subsolid solution heat treatment (Sub-SHT) at 1080 ℃/8 h and then cooled at a cooling rate of 22 ℃/min. The evolution of γ' precipitates by coupling hot deformation and heat treatment was studied by in situ electron backscatter diffraction + scanning electron microscopy. This research explains the morphological variations between the recrystallized and deformed grains of the γ' precipitates, as well as the relationship between the characteristics of the grain boundaries (GBs) and γ' precipitates at the GBs. The results show that when the predeformed samples were subjected to an 8 h Sub-SHT and then cooled at 22 ℃/min, the γ' precipitates exhibit a tri-modal size distribution, and the primary γ' precipitates are dendritic. The morphology of γ' precipitates within deformed grains is related to the accumulation of dislocations and other defects. The maximum size of columnar γ' precipitates is obtained at the high-angle grain boundary, where the misorientation ranges from 30° to 40°. The length of columnar γ' precipitates results from the comprehensive effect of the GB energy determined by the GB misorientation and the GB mobility determined by the stored energy difference. Owing to the low energy and low mobility of the Σ3 twin boundary, the γ' precipitates at the Σ3 twin boundaries remain stable, and the morphology and size are no different from those within the grains. These findings may be helpful for the design and optimization of Ni-base superalloys.

Dependency of grain boundary dislocation network configuration on generalized stacking fault energy surface in FCC metals

We use a microscopic phase-field (MPF) model, incorporating the generalized stacking fault energy (GSFE) surfaces (i.e., the γ-surface) as inputs, to investigate systematically the evolution of dislocation network configurations at low-angle pure twist grain boundaries (GBs) in six face-centered cubic (FCC) metals (Ag, Cu, Rh, Ir, Pd and Pt) that have different GSFE surfaces. The equilibrium configurations of GB dislocation networks are obtained via the interplay between the crystalline energy and the elastic strain energy during the energy minimization process. It was found that for {111} twist GBs, though the geometrically necessary dislocations (GNDs) of the screw type are observed, they can be either full or partial ones depending on the GSFE surfaces of the above metal systems. The areas of the staking faults regions formed between partial dislocations are quantitatively characterized as a function of the magnitude of the stacking fault energy, the elastic modulus and the GB misorientation angle. A fast-acting model for predicting the geometric characteristics of the GB dislocation networks is developed based directly on the material properties and GB misorientation, which is validated by its applications to another two metal systems (Au and Ni) with the same GB.

Statistical characterization of twin transmission across grain boundaries in magnesium

A detailed statistical analysis of twin transmission (TT) across grain boundaries is performed in rolled, commercial purity magnesium compressed along the rolling direction. EBSD images are acquired from two different cuts: a section containing the rolling (RD) and normal (ND) directions; and a section at 35° to the ND which contains transverse direction (TD). An automated twinning analysis software, METIS, is used to obtain the statistical correlations between deformation twins and other microstructural features by analyzing EBSD microstructures comprising thousands of grains and twins. This detailed statistical analysis reveals that the TT propensity is sensitive to the grain boundary (GB) misorientation angle but not to the GB misorientation axis. Specifically, TT propensity decreases with increasing GB misorientation angle; however, the decreasing trend is not monotonic. Further, the detailed analysis of TT events combining macroscopic Schmid factor and the geometric measures (m’: accounts for the alignment of both twin plane normal and shear directions; m”: accounts for the alignment of only twin shear directions; and m”’: accounts for the alignment of only twin plane normal) helps in understanding and identifying the process of twin-pair formation, i.e., co-nucleation versus transmission, also to identify the role of local stresses at GBs induced by inter-grain interaction versus twinning shear transformation processes. In addition, Bayesian inference is used to draw statistically meaningful conclusions as to the likelihood of transmission given misorientation angles. Lastly, twin chain frequency as a function of chain length is predicted using the probability distribution obtained from the Bayesian inference and compared with the actual data from the EBSD microstructures.