Grain Boundary Migration Research Articles

Study on hot deformation behavior and dynamic recrystallization mechanism of recycled Al–Zn–Mg–Cu alloy

This study utilized Al–Zn–Mg–Cu alloy chips as raw materials to prepare recycled billets through a solid-state recycling process. Isothermal hot compression tests were conducted using a thermal simulation machine to establish an Arrhenius-type constitutive equation and the hot processing map based on the dynamic materials model. The microstructure of the recycled Al–Zn–Mg–Cu alloy after hot compression was characterized and analyzed using SEM, EBSD, and TEM, exploring the microstructural evolution mechanisms of the recycled alloy at different temperatures and strain rates. The results indicate that the optimal hot deformation parameters for the recycled Al–Zn–Mg–Cu alloy are 300–360 °C at a strain rate of 0.01–0.05 s⁻1 and 400–450 °C at a strain rate of 0.05–0.3 s⁻1. At the same strain rate, as the deformation temperature increases, low-angle grain boundaries gradually disappear, and the grain structure becomes more stable. Dynamic recrystallization replaces dynamic recovery as the dominant mechanism. At the same deformation temperature, low strain rates allow more time for the migration of low-angle grain boundaries, thereby promoting the growth of dynamically recrystallized grains and ultimately forming new, undistorted grains.

Atomic-scale insights into the effect of layer thickness ratio on the tensile properties of heterostructured nano-aluminum

Gradient-structured alloys with lamellar microstructures show extraordinary potential in breaking the strength-ductility trade-off dilemma. When partial grain size exceeds the Hall-Petch limit, the deformation mechanism is not fully understood. In this study, the competing relationship between grain boundary (GB) softening and hetero-deformation-induced (HDI) strengthening in laminated nano-grained aluminum was investigated under uniaxial tensile loading. In the soft domains, the high activity of grain boundaries results in pronounced GB softening. However, when the volume fraction of the soft layer exceeds 50 %, the flow stress surpasses the predicted values of the rule of mixtures (ROM), demonstrating a significant HDI strengthening. A detailed analysis of microstructural evolution is conducted, including strain gradients, GB migration, and dislocation distribution. The effects of mechanical incompatibility between soft and hard layers on the distribution of plastic strain and overall material strength are elucidated in gradient structures. The study provides critical theoretical insights into the design and development of high-performance gradient nanocrystalline aluminum alloys, particularly in applications where a balance between strength and ductility is of paramount importance.

Flow behavior and microstructural evolution of Al-1.2Mg-2.4Si-1.2Cu-0.6Mn alloy during hot compression

Thermal compression tests were conducted on Al-1.2Mg-2.4Si-1.2Cu-0.6Mn alloys with process parameters including deformation temperatures ranging from 400°C to 550°C and strain rates ranging from 0.05 s⁻¹ to 1 s⁻¹. The hot working properties of aluminum alloys were evaluated with intrinsic equations and machining diagrams. The results show that the processing safety zones of the alloy are 425–450℃ and 0.05–0.1 s−1, 450–475℃ and 0.1 s−1-0.5 s−1, and 475–500℃ and 0.5 s−1. The microstructure of the aluminum alloy following hot pressing was investigated through the use of optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and backscattered electron diffraction (EBSD). When the deformation temperature of the alloy is below 450°C, the alloy organization exhibits a greater prevalence of inhomogeneous defects, which manifest as coarser granular regions in the micro-morphology. Upon reaching a deformation temperature of above 525°C, the deformed alloy organization exhibits an evident thermal cracking phenomenon. At the same temperature, an increase in strain rate is accompanied by a decrease in the Mg2Si phase and an increase in the precipitation of the Al(Fe,Mn)Si eutectic phase. Furthermore, the precipitated phase tends to become coarser. The presence of certain Mn-containing nanoparticle dispersions can impede the migration of subgrain grain boundaries, while simultaneously pinning dislocations and accelerating the generation of dislocation entanglement. This also can serve to inhibit the coarsening of the grains to some extent.

Revealing the Effect of Dynamic Structural Evolution on Triple Junction Deformation Mechanism: Molecular Dynamics Simulations and an Energetic Model

Abstract Triple junction (TJ) migration is a plastic deformation mechanism in polycrystalline metals mediated by the interplay between neighboring grain boundaries (GBs). Compared with GB migration, the dynamic change of TJ structure during migration and its impact on subsequent TJ behavior is often overlooked. In this study, we explore the effect of dynamic TJ structure evolution on TJ deformation mechanism in a face-centered cubic metal Au model using molecular dynamics simulations. Our analysis reveals that the dislocation accumulation at TJ due to inconsistent disconnection nucleation from neighboring GBs can influence the stick-slip behavior of TJ migration. With the dynamic change of TJ structure, the velocity of TJ migration is gradually reduced, leading to a transition from TJ migration to dislocation emission from TJ. We have developed an energetic model to evaluate the critical stresses required for TJ migration and dislocation emission, providing an explanation of the competition between TJ migration and dislocation emission. Our findings deepen the understanding of TJ-governed plastic deformation mechanisms in polycrystalline materials.

Study on the Plastic Deformation Mechanism of Void‐Containing Twin‐Crystal Magnesium with Symmetric Tilt Grain Boundary Angles

AbstractA twin‐crystal magnesium (Mg) model with 9 different symmetric tilt grain boundary (STGB) angles (20°–80°) with preset nanohole defects is established by atomsk and lammps software. The mechanical behavior of grain boundary (GB) angles on twin‐crystal Mg with cavity defects and its cavity evolution are simulated by the molecular dynamics method with embedded atomic potential, and the plastic deformation mechanism is revealed. The results show that the yield stress decreases with the increase of the STGB Angle during the stretching process. When the GB Angle increases from 20° to 80°, the yield stress decreases from 2.28 to 1.42 Gpa. This is because the larger the STGB Angle is, the larger the Schmidt factor is, and the easier it is to start dislocation slip during the stretching process. On the other hand, the larger the Angle of STGB, the more the number of atomic voids at the interface, and the more the number of dislocation nucleation points. The larger the Angle of STGB, the lower the strength of twinning Mg but the better the plasticity to avoid fracture. The plastic deformation mechanism mainly includes the nucleation of Shockley incomplete dislocation at STGBs, dislocation slip generates stacking faults (SFs), GB migration, and base plane dislocations.

Orientation-related and temperature-dependent continuous grain boundary migration in multi-principal element alloys

Grain boundaries (GBs) significantly affect the mechanical properties of metals and alloys. In this study, we investigated using molecular dynamic simulations the migration behavior of Σ25 (710), Σ5 (310), and Σ37 (750) [001] symmetric tilt GBs in CoCrCuFeNi multi-principal element alloy (MPEA) and Cu samples subjected to shear deformation. In Cu, the migration of the GBs exhibits a coupled migration pattern, consistent with the Cahn model; while in MPEA, the migration pattern varies with GB angle and temperature. Both Σ25 (710) and Σ37 (750) GBs, along with higher temperatures, induce GB roughening and continuous migration in the MPEA samples. Further investigation to the effects of GB angle and temperature was conducted through microstructure evolution tracing and quantitative analysis. A model was developed to describe the temperature-dependent continuous GB migration and average flow stress in MPEA samples with Σ25 (710) or Σ37 (750) GBs. This work can help understand the mechanical behavior of GB in MPEA and provide valuable insights for the development of high-performance materials.

Structural and phase transformations in Fe-Pd-Ag layered thin films by grain boundary diffusion

The influence of the stacking order and of an additional Ag layer on the formation of ordered phases in thin (< 50 nm) layered Fe/Pd and Fe/Ag/Pd films was investigated at 460 °C. The samples were prepared by magnetron sputtering at room temperature on Si/SiO2 substrate and were post-annealed in vacuum. Composition depth profiling and x-ray diffraction were used to characterize the processes. It has been shown, that the stacking order strongly influences the formation of ordered phases both in Fe/Pd bilayered and Fe/Ag/Pd trilayered films. In bilayered Fe/Pd films for both stacking orders the FePd3 phase appeared and it also showed L12 ordering for one stacking order. Addition of Ag layer between the Fe and Pd layers found to promote the formation of FePd phase which showed L10 ordering or A1 disordered structure depending on stacking order. Based on the analysis of the chemical depth profiles and XRD patterns the transformations were interpreted by grainboundary diffusion mechanisms including grainboundary diffusion induced grainboundary migration and solid state reaction.

Mechanical response and grain boundary behavior of (HfNbTaTiZr)C high-entropy carbide ceramics and its constituent binary carbides

The structure and mechanical response of grain boundaries (GBs) are essential for predicting the mechanical properties of polycrystalline materials. Understanding the properties of GBs in carbide ceramics is essential for the development of applications of high entropy carbide ceramics (HECCs) in extreme environments. In this work, we investigated the intrinsic properties and mechanical responses of {210}/[001] GBs in (HfNbTaTiZr)C HECC subjected to shear deformation and its constituent binary carbides using first-principles calculations. The results showed that the GBs in Group VB carbides undergo migration, whereas the formation of CC bonds within the GBs in Group IVB carbides can inhibit GB migration and result in the failure of supercells due to the breakage of metal-C bonds. Therefore, tailoring the GBs in (HfNbTaTiZr)C based on metal atoms from different groups can induce a "pinning" effect, potentially enhancing plasticity without sacrificing strength. Furthermore, HECC exhibits higher GB migration stress compared to conventional binary carbides. These results can provide in-depth insights into the mechanical behavior of GB structure of high-entropy carbide ceramics.

Interface migration and grain growth in NbC-Ni cemented carbides with secondary carbide addition

Cemented carbides are widely used for cutting and drilling tools. They usually combine a WC hard carbide phase and a Co-based ductile binder. NbC-Ni materials are considered as a possible alternative, especially for wear applications. The advantageous economic situation for raw materials sourcing, their interesting mechanical properties and low density have raised a new interest for these materials. However, mechanical properties can be limited by the rapid grain growth during liquid phase sintering, as compared to WC-Co. Grain growth can be controlled by the addition of secondary carbides. In this paper, a quantitative EBSD analysis of grain growth is performed for NbC-12 vol%Ni materials sintered at 1450 °C with controlled addition of Mo2C and WC. The average grain size decreases continuously with Mo2C addition. The results are discussed based on a more detailed interface characterization and on a previous model for the cooperative migration of phase boundaries and grain boundaries.

Comparative study of intergranular acicular nano-precipitate evolution and their effects on creep failure behavior for BM and HAZ in a novel Fe-Ni-based alloy joint

Intergranular acicular nano-precipitate evolution and their effects on creep failure behavior for the base metal (BM) and heat affected zone (HAZ) in a novel cost-effective Fe-Ni-based alloy joint were investigated comparatively. It was found that the BM tended to initiate acicular nano-precipitates perpendicular to grain boundaries, while their formation was suppressed at the HAZ. After creep at 650 °C under 300 MPa, nano-precipitates composed of γ´-Ni3(Al, Ti), M23C6 and σ phase at BM would expand along the grain boundaries (GBs) with significant GB migration. Creep cavities were more prone to initiate at the precipitate-free zone and the acicular nano-precipitate roots due to the precipitation strengthening reduction and local strength mismatch. Furthermore, grain boundary embrittlement induced by acicular precipitates and the presence of recrystallized grains at the crack tip also promoted the crack propagation. In contrast, the welding thermal cycle contributed to the M23C6 redistribution at the GBs, which decreased element diffusion rate and dragging grain boundary movement, thereby exhibiting better creep crack resistance at the HAZ. The finding provided profound implications for the comprehensive understanding of acicular nano-precipitate evolution in the newly designed Fe-Ni-based alloy joint under service condition.

Disruptive atomic jumps induce grain boundary stagnation

Grain growth in polycrystalline materials can be impeded by grain boundary (GB) stagnation. Using atomistic simulations, we observed that during GB migration, the disruptive jumps of a few GB atoms can disturb the original ordered collective movement of GB atoms, leading to the stagnation of the entire GB. These disruptive atomic jumps can be activated by both high driving forces and high temperatures, with even jumps of a few atoms capable of causing the stagnation of an entire GB. This mechanism also explains the non-Arrhenius behavior observed in some GBs. Additionally, a large model size could increase the rate of disruptive atomic jumps, and a clear transition in thermal behavior is observed with the increase of the GB size in GBs exhibiting clear thermally activated stagnation. Our further investigation shows that the disruptive atoms involved in these jumps do not differ from other GB atoms in terms of atomic energy, volume, density, local entropy, or Voronoi tessellation, and no “jam transition” was observed in the energy barrier spectra. This fact makes those disruptive jumps challenging to detect. To address this issue, we propose a displacement vector analysis method that effectively identifies these subtle disruptive jumps.

The influence of zirconium addition on microstructure evolution, precipitation behavior, and properties of Cu–Ni–Ti alloy

This study systematically investigated the effects of a small amount of Zr addition on the microstructure evolution, mechanical, and electrical properties of Cu–Ni–Ti alloy, specifically focusing on Cu–3Ni–1Ti - (0.3Zr). The results indicate that adding an appropriate amount of Zr significantly enhances the mechanical properties and over-aging resistance of the Cu–Ni–Ti alloy, with only a slight decrease in conductivity. After aging at 500 °C for 2 h, Cu–3Ni–1Ti-0.3Zr exhibits an ultimate tensile strength of 673 MPa and a conductivity of 53 % IACS. Even after extending the aging time to 10 h, the tensile strength and conductivity remain high at 646 MPa and 61 % IACS, respectively. Microscopic analysis reveals that the addition of Zr promotes the formation of fine Ni3Ti phase during solidification and solution treatment, improving the deformation resistance and enhances its yield strength of the alloy in the cold-rolled state. Additionally, it hinders the migration of grain boundaries，suppressing the recovery and recrystallization processes, thus maintaining high mechanical properties and over-aging resistance. Furthermore, the precipitation of coherent or semi-coherent nano Ni3Ti phases during the aging process positively affects the alloy's yield strength and conductivity. The Oworan and dislocation strengthening mechanisms resulting from nano Ni3Ti phases are the primary mechanisms enhancing the yield strength of Cu–3Ni–1Ti-0.3Zr alloy, contributing approximately 87 %.

High temperature deformation behaviour and recrystallization mechanism of TiC/Ti6Al4V gradient material fabricated by laser directed energy deposition

Metal-ceramic gradient materials are of considerable interest due to their unique capability to integrate the ductility of metals with the mechanical strength and high-temperature resistance of ceramics, thereby reducing the risk of cracking induced by thermal stress. In this article, TiC/Ti6Al4V gradient materials were prepared through the laser directed energy deposition (LDED) process. Subsequently, high-temperature compression tests were performed at various temperatures to investigate the deformation behaviour and recrystallization mechanism of the gradient materials. The Zener-Holomon constitutive equation was then used to calculate the gradient material at 750–900 °C and 0.001–0.1 s−1 to obtain a deformation activation energy of 362.36 KJ·mol−1. The EBSD was used to analyze the microstructural evolution of the gradient material. The results indicate a significant reduction in the average grain size of the specimens after high-temperature compression, and a more random crystal orientation due to the occurrence of dynamic recrystallisation (DRX). The primary softening mechanism of TiC/Ti6Al4V gradient materials is discontinuous dynamic recrystallisation, with TiC primarily affecting the nucleation of discontinuous dynamic recrystallisation (DDRX) and the subsequent grain growth. The nucleation becomes easier with increasing TiC content. However, the TiC hinders the migration of grain boundaries, resulting in a smaller DDRX grain size. After high-temperature compression, the main slip system of the crystal is converted from Pri to Bas. Pyr is difficult to activate due to its high critical resolved shear stress.

Research on hot deformation behavior and microstructure evolution mechanism of GH4169 superalloy

The hot deformation behavior and microstructure evolution of GH4169 superalloy were investigated through hot compression experiments with a temperature range of 900–1100 °C and strain rates ranging from 0.01 to 5 s−1. Concurrently, the flow stress curve of the alloy was obtained and constitutive equations were established. Based on the dynamic material model, a hot working diagram was established and the hot working window was optimized. The findings demonstrate that the high-temperature softening mechanism of this alloy primarily involves dynamic recrystallization (DRX), while both discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) exist. According to the hot processing map, the process parameters corresponding to the unstable areas are low temperatures and medium-high strain rates (900–985 °C, 0.15–5 s−1), and high temperatures and high strain rates (1025–1100 °C, 2.5–5 s−1). The reason is that the appearance of deformation bands and flow localization in the microstructure. The optimal hot working parameters for GH4169 are 1000 °C/0.01 s−1 and 1050–1100 °C/0.01–0.1 s−1, where the microstructure consists of uniformly distributed DRX grains. The volume fraction of DRX in GH4169 alloy rises with higher temperatures or lower strain rates. In addition, the reduction of intergranular dislocations promotes the migration of grain boundaries from low angle grain boundaries (LAGBs) to high angle grain boundaries (HAGBs).

The influence of coupling effect between stress and high temperature on the degradation of microstructure and hardness in P91 steel

Interrupted aging and creep tests were performed in the condition of 650 °C/70 MPa for P91 steel. The mechanisms of hardness and microstructure evolution during aging and creep were studied through hardness test, SEM/EBSD and TEM/EDS. By comparing the difference between aging and creep, the coupling effect of temperature and stress in P91 steel was elucidated. Results show that creep stress could accelerate the entire degradation process in P91 steel. Firstly, the dislocation density increased rapidly at the beginning of creep, and then decreased also at a high rate after reaching its peak value. On the other hand, the dislocation density decreased slowly in aging condition. Additionally, with the migration of grain boundaries under creep stress, some intergranular M23C6 entered inside grains and afterwards dissolved into matrix. On the other hand, other M23C6 which remained intergranular continuously ripened and congregated with time, and finally turned into large ones at triple junctions of grain boundaries. Furthermore, stress had the greatest impact on dislocation density, whereas temperature affected volume fraction of M23C6 the most. In creep condition, the degradation of mechanical properties of P91 steel was mainly caused by the reduction of dislocation strengthening which was followed by grain boundary strengthening.

Shear response of 〈110〉 asymmetric tilt grain boundaries in BCC-Fe

The present study investigated the effect of shear load on the 110 asymmetric tilt grain boundaries of body-centered cubic iron with molecular dynamics simulations. Various mechanisms like grain boundary (GB) sliding, shear-coupled migration, GB migration with rotation, GB decomposition, and GB deformation with dislocations emission were observed. Low-angle tilt GBs migrated with the help of two dislocation mechanisms, gliding and climbing. Two types of twin nucleation mechanisms were noticed in high-angle tilt GBs with varying tilt angles (η). For 16° ≤ η ≤ 112° and η ≥ 130° cases, dynamic self-adjustment of the atoms at the GBs caused GBs decomposition with the formation of twin boundaries (TBs). Newly nucleated TBs migrated under further applied shear load. For 112° < η < 130° case, TBs nucleated from the GB on either side of the misfit dislocations due to their localized core structure on the GB plane.

Solute segregation in a moving grain boundary: a phase-field approach

We present a phase-field approach for investigating monolayer and multilayer type solute segregation in a moving Grain boundary (GB). In this model, we introduce an expression for the GB solute interaction potential which allows for easy modification of the shape of the solute segregation profile at the GB. As a consequence, our phase-field simulations capture various segregation profiles in both stationary and migrating GB that agree with Cahn’s solute drag theory. Furthermore, we explore how different segregation profiles evolve at varying GB velocities owing to the inequality of the atomic flux of solute between the front and back faces of the moving GB. At a low-velocity regime, we observe that multilayer segregation results in significantly increased drag force compared to monolayer segregation. At a high-velocity regime, the opposite holds. Our simulation results also provide valuable insights for predicting grain growth in polycrystalline materials in the presence of solute segregation.

Effect of the atomic structure of complexions on the active disconnection mode during shear-coupled grain boundary motion

The migration of grain boundaries leads to grain growth in polycrystals and is one mechanism of grain-boundary-mediated plasticity, especially in nanocrystalline metals. This migration is due to the movement of dislocationlike defects, called disconnections, which couple to externally applied shear stresses. While this has been studied in detail in recent years, the active disconnection mode was typically associated with specific macroscopic grain boundary parameters. We know, however, that varying microscopic degrees of freedom can lead to different atomic structures without changing the macroscopic parameters. These structures can transition into each other and are called complexions. Here, we investigate [111¯] symmetric tilt boundaries in fcc metals, where two complexions—dubbed domino and pearl—were observed before. We compare these two complexions for two different misorientations: In Σ19b[111¯](178) boundaries, both complexions exhibit the same disconnection mode. The critical stress for nucleation and propagation of disconnections is nevertheless different for domino and pearl. At low temperatures, the Peierls-like barrier for disconnection propagation dominates, while at higher temperatures the nucleation is the limiting factor. For Σ7[111¯](145) boundaries, we observed a larger difference. The domino and pearl complexions migrate in different directions under the same boundary conditions. While both migration directions are possible crystallographically, an analysis of the complexions' structural motifs and the disconnection core structures reveals that the choice of disconnection mode and therefore migration direction is directly due to the atomic structure of the grain boundary. Published by the American Physical Society 2024

Importance of grain boundary processes for plasticity in the quartz-dominated crust: Implications for flow laws

When H2O is present along grain boundaries, the deformation processes responsible for plasticity in silicate mineral aggregates can deviate from what may be conventionally expected. Although a necessary component of understanding crustal deformation processes, there is no theoretical framework that incorporates grain boundary processes into polycrystalline quartz rheology. To address this issue, we carried out high-pressure and high-temperature deformation experiments on fine-grained quartz aggregates. Our study illustrates that grain boundary migration (GBM) through dissolution-precipitation (in the presence of an aqueous fluid) and grain boundary sliding (GBS) may act as accommodation mechanisms to prevent hardening from dislocation glide. GBM and GBS can relax incompatibilities resulting from an inadequate number of independent slip systems, plastic anisotropy between neighbouring grains, and non-planar grain boundaries together with grain boundary junctions. As demonstrated earlier in the literature, GBM may act as a recrystallization mechanism counteracting hardening, but also is a potential mechanism that allow H2O to enter in the quartz crystal (hydrolization) at the experimental time-scale. The above serial processes occur over a range of more than two orders of magnitude in grain size (∼3 to 200 μm) and explain a grain-size-insensitive stress exponent (n = 2) and low activation energy (Q = 110 kJ/mol). In the absence of a switch to grain size sensitive deformation mechanisms induced by grain size reduction, our results imply that only a modest weakening (∼5 times the strength of the protolith) is needed (or possible) to localize shear zones in the Earth's crust.

Microstructural evolution and mechanical response of micro-deformation diffusion bonding Ti-6Al-4V with designed interface morphology

This study explicates the microstructural evolution and mechanical response of the Ti-6Al-4 V joints diffusion-bonded with a micro-deformation less than 1%, assisted by pre-designed interface morphology. The interfaces composed of interfacial voids and bonded zones were developed, exhibiting varying bond rates. The interfacial voids evolved from valleys of bonding surfaces, and the bonded zones were derived from comparatively deformed ridges mainly depending on dynamic recrystallization, strain-induced migration of grain boundaries, and diffusion mechanism. The recrystallized α grains was generated due to the nucleation and rotation of sub-grains driven by pile-up of dislocations. Additionally, the diffusion of elements was significantly activated at the deformed zones (herein, referring to the bonded zones), promoting the metallurgical bonding of α / β phase interfaces. The formation of bonded zones supported for sound joining equivalent to the substrate in mechanical properties, confirmed by the void-free joint. Nevertheless, the mechanical properties of the joints were decreased when interfacial voids led to premature failure, and deteriorated more severely with narrowing void spacing. The concentration of stress and strain at void tips induced untimely initiation of cracks, and propagation guided by the trajectory of high stress. The interaction of stress field surrounding adjacent voids was enhanced under narrow spacing (less than 4 times of void length), leading to acceleration of crack initiation and propagation.