Evolution Of Grain Boundaries Research Articles

In-situ EBSD tensile revealing the evolution mechanism of high angle grain boundaries in Al–Zn–Mg alloy profile with heterogeneous structures

The evolution of grain boundary (GB) of an Al–Zn–Mg alloy profile with heterogeneous structures is studied based on in-situ EBSD tensile. This evolution is not only related to its grain boundary misorientation angle (GBMA) and slip transfer parameter (m'/(Δb/b)) but also to the behavior of the geometrically necessary dislocation (GND) tensor in the deformation. The effect of GBMA and m'/(Δb/b) for three types of high angle grain boundary (HAGB) on GND density and back stress has been revealed. The increase of GND density is not obvious near the HAGBs with 45°–55° GBMA of neighboring coarse grains or neighboring fibrous grains. The optimal grain boundary for forming back stress is the HAGBs between coarse grain and fibrous grain, which has 35°–45° GBMA and a low m'/(Δb/b). The back stress is hard to form near these HAGBs with 25°–35° GBMA and a low m'/(Δb/b). Combined with the neighboring grain type, slip transfer and GBMA, the new criterion is proposed to predict GND behavior near HAGBs.

Microstructure characterization and dynamic recrystallization behavior of Ni–Cu alloy during hot deformation

Flow curve and microstructure of Ni–30Cu alloy were studied after hot compression at 1150 °C and strain rate of 0.01 s−1. The stress-strain and work hardening rate curves showed a weak peak at strain of 0.32, followed by a slight dynamic softening. The dominant microstructural mechanisms in low (0.1–0.23) and medium strains (0.23–0.32) were dynamic recovery and dynamic recrystallization, respectively. Microstructural characterizations by electron back scattered diffraction (EBSD) showed that continuous dynamic recrystallization brings about after the peak strain (ε = 0.32) and leads to grain refinement. It was found that the twin boundaries help to activate a grain dissociation mechanism which works as a new variant of continuous dynamic recrystallization. The grain boundary maps developed by EBSD showed that new twin boundaries form through the evolution of low angle grain boundaries. This step, named as “recovery twinning”, was identified as the origin for the observed twinning-assisted continuous dynamic recrystallization (TCDRX). The Kocks-Mecking dislocation evolution model was modified in order to take the dynamic softening by TCDRX into account and describe the variation in the frequency of high angle grain boundaries with strain. Model predictions were validated and confirmed by the experimental results.

Evolution of microstructure and grain boundaries during annealing of high-purity tantalum materials

The microstructure and texture evolution of tantalum (Ta) during annealing were studied in this work, in addition to simulating and analyzing of the grain boundary motions and their interactions using molecular dynamics simulations (MD). The results showed that the evolution of the recrystallized texture in the surface region of Ta is different from that in the core region. Coincident site lattice (CSL) is significantly related to the grain orientation and thus directly affects the recrystallization texture. MD results showed that the grain boundary mobility rate is higher at low angle grain boundaries (LAGB), which implied that the mobility rate is influenced by the tilt angle. Lower mobility rate of high angle grain boundary (HAGB) possibly caused by a large number of stabilized Σ3 grain boundaries (GBs) between them. It is shown that the inhomogeneous distribution of the recrystallization texture during the annealing process can be possibly attributed to the presence of a large number of Σ3 GBs.

He bubble-induced phase transformation of W grain boundaries revealed by accelerated molecular dynamics

The growth of He bubbles and the resulting impact on the microstructural evolution of W are of paramount importance to the plasma-facing materials community due to the application of W in Tokamak fusion reactors. Using accelerated molecular dynamics (AMD) techniques, we outline the structural evolution of grain boundaries (GBs) caused by growing He bubbles. It is discovered that when an alternative, low energy, high density GB structure or phase is available, He bubbles can induce a progressive phase transformation of the GB to the higher density phase by the continual nucleation of W Frenkel pairs. We find that the resulting W self-interstitials migrate to sites at the GB which are structurally related to the higher density phase, leading to the transformation. We discuss the implications of this progressive microstructural evolution on the growing He bubble and consider in general how He bubbles will impact the structural evolution of an arbitrary W GB. These findings of GB phase transformation are predicted to impact other damage events in W such as recrystallization, GB migration and defect segregation which must take these findings into account in order to accurately simulate a realistic W microstructure and hence extract experimentally meaningful data.

Understanding the formation and evolution of low-angle grain boundaries in Nickel-Based single crystal superalloys

Grain boundaries have long been of utmost importance in the research and application of single-crystal superalloys. However, there are still many unsolved mysteries regarding the understanding of grain boundaries throughout the solidification process. The work focuses on investigating the formation, the evolution, and the disappearance of low-angle grain boundaries, including subgrain boundaries. The findings show that the low-angle grain boundaries and subgrain boundaries in various dendritic arrays exhibit morphological variances and different development processes. The subgrain boundaries in the aligned array appear to be straight, where irregularly fine-grained γ' precipitates have formed. In contrast, low-angle grain boundaries show a curved and sawtooth-like structure in misaligned arrays. The γ' precipitates locating in the region are regularly long strips. The generation of grain boundaries is a result of the adjustment in dendritic spacing, where the influence of solute leads to dendrite deflection. The misorientation of grain boundary is constantly changing in response to dynamic changes in dendrite spacing. The low-angle grain boundaries eventually dissolve as the dendritic array evolves. This study enriches understanding of the interplay between dendrite arrays and grain boundaries, offering theoretical insights into the evolutionary patterns of grain boundaries within single-crystal superalloys.

Evolution of Grain Boundaries Promoted Hydrogen Production for Industrial-Grade Current Density.

The development of efficient and durable high-current-density hydrogen production electrocatalysts is crucial for the large-scale production of green hydrogen and the early realization of hydrogen economic blueprint. Herein, the evolution of grain boundaries through Cu-mediated NiMo bimetallic oxides (MCu-BNiMo), which leading to the high efficiency of electrocatalyst for hydrogen evolution process (HER) in industrial-grade current density, is successfully driven. The optimal MCu0.10-BNiMo demonstrates ultrahigh current density (>2 A cm-2) at a smaller overpotential in 1m KOH (572mV), than that of BNiMo, which does not have lattice strain. Experimental and theoretical calculations reveal that MCu0.10-BNiMo with optimal lattice strain generated more electrophilic Mo sites with partial oxidation owing to accelerated charge transfer from Cu to Mo, which lowers the energy barriers for H* adsorption. These synergistic effects lead to the enhanced HER performance of MCu0.10-BNiMo. More importantly, industrial application of MCu0.10-BNiMo operated in alkaline electrolytic cell is also determined, with its current density reached 0.5 A cm-2 at 2.12V and 0.1 A cm-2 at 1.79V, which is nearly five-fold that of the state-of-the-art HER electrocatalyst Pt/C. The strategy provides valuable insights for achieving industrial-scale hydrogen production through a highly efficient HER electrocatalyst.

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.

Disconnection flow-mediated grain rotation.

Grain rotation is commonly observed during the evolution of microstructures in polycrystalline materials of different kinds, including metals, ceramics, and colloidal crystals. It is widely accepted that interface migration in these systems is mediated by the motion of line defects with step and dislocation character, i.e., disconnections. We propose a crystallography-respecting continuum model for arbitrarily curved grain boundaries or heterophase interfaces, accounting for the disconnections' role in grain rotation. Numerical simulations demonstrate that changes in grain orientations, as well as interface morphology and internal stress field, are associated with disconnection flow. Our predictions agree with molecular dynamics simulation results for pure capillarity-driven evolution of grain boundaries and are interpreted through an extended Cahn-Taylor model.

Grain boundary migration behavior and microstructure evolution during multilayer additive hot-compression bonding

Multilayer additive hot-compression bonding (MAHCB) was a new technology to prepare large-size homogeneous materials, the strain hardening at the bonding interface was an urgent issue for its application. In this paper, the effect of grain boundary migration on strain hardening was studied by the MAHCB process of pure copper. The results show that prolonging the holding time can significantly promote both interfacial grain boundary migration (IGBM) and matrix grain boundary migration (MGBM). The bonding interface became tortuous due to the promotion of IGBM, accompanied by the reduction in dislocation density at the interface and the gradual elimination of interfacial defects. It occurred mainly by the evolution of the triple junction grain boundary from T-type to equal type, which was driven by the decrease in the interface energy. On the other hand, the growth of dynamic recrystallization (DRX) grains was improved with MGBM, and enlarged the low dislocation density region. With the combined effect of both, strain hardening was completely eliminated, when the strain was 0.15 and the holding time was 3 h. Then the optimal bonding properties were obtained, the average tensile-shear strength and elongation were 191.2 MPa and 212 %, and the recovery rates were 99.7 % and 98.9 %, respectively, compared to the as-cast pure Cu samples.

Microstructural evolution and mechanical property of friction stir welded joints in AlSi10Mg alloys fabricated by laser powder bed fusion

Friction stir welding (FSW) was introduced to join laser powder bed fusion (LPBF) AlSi10Mg alloys in this study. Microstructure and texture evolution, microhardness distribution and tensile properties of the FSW joints were investigated. The evolution of grain size, grain boundary and misorientation angles distribution, dynamic recrystallization and crystallographic texture in various regions of the FSW joint concerning base metal (BM), thermo-mechanically affected zone (TMAZ), upper part of nugget zone (UPNZ) and low part of nugget zone (LPNZ) was characterized by using electron back-scattered diffraction (EBSD). The analysis reveals a strong texture evolution of shear components on the TMAZ, UPNZ and LPNZ as well as a fully recrystallized grain structure on the UPNZ and LPNZ. Additionally, a gradual evolution in Si-rich eutectic from continuous cellular networks in the BM to totally globularized particles in the UPNZ and LPNZ occurs in the FSW joint. The microhardness rapidly decreases to the TMAZ from the BM through heat affected zone (HAZ) reaching the lowest value of ∼65.0 HV at the TMAZ/NZ boundary. The FSW joint in LPBF AlSi10Mg alloys exhibits an ultimate tensile strength (UTS) of 247 MPa and an elongation-to-failure (EI) of 8.8%. The UTS of FSW joints is reduced compared to the BM, but the EI has been increased by 160%, implying an improvement in tensile ductility.

Microstructure evolution and EBSD analysis of multi-principal element alloy coatings after high-temperature oxidation

Multi-principal element alloy coatings (MPEACs) have great application potential for high-temperature diffusion barrier coatings. The sluggish diffusion and lattice distortion of this new coating material due to its own atomic size effect give the coating excellent resistance to high temperature oxidation and grain coarsening. In this study, the high-temperature oxidation properties, and microstructure evolution of quaternary, quinary, senary, and denary alloy coatings (whose lattice distortion magnitude is linearly increased) at 1100 °C were studied. The evolution of grain orientation, size, and grain boundaries of alloy coatings after high temperature exposure were analyzed by EBSD. The results showed that from quaternary to denary alloy coatings, with the rise in lattice distortion magnitude of the alloy system, the sluggish diffusion ability of the coating at high temperatures was more obvious. Specifically, the oxidative activation energy of quaternary to denary alloy coatings is 355.41, 480.96, 478.76, and 654.03 kJ/mol, respectively. A high distortion magnitude is beneficial to increase the activation energy of the alloy coating, as a result, grain growth of alloy coatings with high activation energy is significantly inhibited, and under the combined action of storage strain energy and solute dragging, the grain boundary migration rate of the coatings also declines. Finally, the denary alloy coating shows good resistance to high temperature softening. This study provides a positive reference for promoting the application of multi-principal alloys in diffusion barrier coatings.

The effects of strain path change on the microstructure, texture, and mechanical properties of accumulative roll-bonded (ARBed) AA1050 strips

In this study, the effect of unidirectional accumulative roll bonding (UARB) and rotational ARB (RARB) on the microstructure, texture, and mechanical properties of AA1050 sheets was investigated. The X-ray diffraction technique was used to study the microstructural and textural evolutions in the processed samples. It was found that the successive sample rotation suppressed the evolution of low-angle grain boundaries (LAGBs) to high-angle ones in the RARBed sheets. However, the grain size was dramatically reduced from 35.7 μm to 169 nm in the starting Al and the processed sample by eight RARB cycles, respectively. Akin to the UARB path, the RARB route led to a drastic drop in Cube {001} <100> texture intensity. Compared to the UARBed strips, additional Goss {011} <100>, A {011} <111>, P {011} <122> and BR {236} <385> components were observed in the RARB-processed sheets. Interestingly, the rotation of Goss-oriented grains towards the Brass {011} <211> component was frustrated by consecutive sample rotation around its normal direction (ND) between RARB passes. In addition to the microhardness test, the uniaxial tensile test was performed at room temperature to evaluate the mechanical properties of the processed aluminum. The results showed that a sharp increase in hardness reached a plateau at higher cycles. In terms of tensile properties, the ultimate tensile strength (UTS) increased to its peak of 332 MPa after eight cycles of the RARB method. Although, their lower interlayer bond strength gave them low elongation compared to their UARBed counterparts.

Phase-field modeling of interaction between void and grain boundaries and its swelling effect

The interaction of irradiated voids and grain boundaries in zirconium is systematically studied by using a composite multiphase-field model that couples the evolution of irradiated voids and grain boundaries. In phase-field modeling, the polycrystalline evolution phase-field governing equation and void evolution phase-field governing equation are coupled together for modeling. In the numerical solution, the aforementioned two sets of equations are coupled to solve the absorption of vacancies and interstitial atoms by grain boundaries. First, we studied the influence of the existence of voids on the evolution of polycrystalline grains when the voids did not evolve. Thereafter, we examined the influence of the grain boundaries on the evolution of the voids when the grain boundaries did not evolve. Finally, we analyzed the interaction process between the grain boundaries and voids at the same time. The preceding simulation research can reproduce the pinning effect of the void on the grain boundary and the phenomenon that the grain boundary acts on the void evolution process to form a “void-denuded zone” near the grain boundary. The interaction mechanism between grain boundaries and voids is revealed through systematic simulation research. On this basis, the influence of the interaction process between the grain boundary and the void on the swelling behavior is further discussed.

Short‐time existence for the network flow

AbstractThis paper contains a new proof of the short‐time existence for the flow by curvature of a network of curves in the plane. Appearing initially in metallurgy and as a model for the evolution of grain boundaries, this flow was later treated by Brakke using varifold methods. There is good reason to treat this problem by a direct PDE approach, but doing so requires one to deal with the singular nature of the PDE at the vertices of the network. This was handled in cases of increasing generality by Bronsard‐Reitich, Mantegazza‐Novaga‐Tortorelli and eventually, in the most general case of irregular networks by Ilmanen‐Neves‐Schulze. Although the present paper proves a result similar to the one in Ilmanen et al., the method here provides substantially more detailed information about how an irregular network “resolves” into a regular one. Either approach relies on the existence of self‐similar expanding solutions found in Mazzeo and Saez. As a precursor to the main theorem, we also prove an unexpected regularity result for the mixed Cauchy‐Dirichlet boundary problem for the linear heat equation on a manifold with boundary.

Microstructural Evolution and Distributions of Grain Boundary in SPD Processed Al–3 mass%Mg Alloy

SPD-processed Al alloys have an ultra-fine-grained microstructure and extremely high strength. The strength of SPD-processed Al alloy exceeds the estimated it by the grain size using Hall-Petch relationship. SPD-processed Al has high dislocation density because the excess introduced strain remains in dynamically recrystallized grains. The excess dislocation may form the low angle grain boundary during dynamic recrystallization. Al-Mg alloy represents especially large strengthening by dislocation hardening after SPD not only smaller grain size than pure Al. This is because the stacking fault energy (SFE) of the alloy is lowered by Mg solute atom. The dislocation movement become suppressed during DRX in low SFE alloys. As a result, SPD-processed Al-Mg alloys have extremely fine-grained (~0.25 µm) microstructure. SFE of the alloy may also affect the formation of low angle grain boundary and dislocation accumulation in the grain interior. The dislocation distributions could result the degree of extra-hardening. The distributions of dislocation and low angle grain boundary in ECAE processed Al-3 mass%Mg alloy were investigated by EBSD and XRD techniques. Annealing after ECAE for recovery the strain provides the relaxation of the strain hardening, in spite of the LAGB fraction remains static.

Effects of temperatures on microstructure evolution and deformation behavior of Fe–32Ni by in-situ EBSD

Fe–32Ni alloy has attracted the attention of researchers owing to its excellent thermal expansion properties and mechanical properties. In this study, the macroscopic mechanical properties, surface morphology, crystal orientation, evolution of grain boundary and kernel average misorientation (KAM) and deformation mechanism of Fe–32Ni alloy were systematically investigated by in-situ loading equipment from 25 °C to 800 °C combined with electron backscatter diffraction (EBSD). The results showed that the grains with (111) and (101) orientations formed a special ring texture in {100} and {110} poles of the polar diagram after grain deformation and rotation. The rotational ability of the grains with (111) and (101) orientations were significantly promoted as the temperature increased. The number and proportion of low-angle grain boundaries (LAGBs) increased with increasing strain and temperature. By analyzing the microstructure evolution during in-situ EBSD testing, it was found that the rotation of the crystals significantly improved the deformability of the material. The effect of selective passage for dislocations by LAGBs both enhanced the plasticity and caused hardening of the material. Dislocations tended to accumulate at grain boundaries causing strain concentration. Cracks sprouted at grain boundaries and expand along the grain boundaries. This study can provide profound insights on the deformation mechanism of Fe–32Ni alloy for the possible design and process optimization.

Transformation of borides in directionally solidified nickel base superalloy and its mechanical response

Directionally solidified (DS) GTD444 is a boron-modified nickel base superalloy which is used in later stage tubine blades. Boron is added as a grain boundary strengthener and it modifies the grain boundary microchemistry. During service, the blades undergo thermal exposure that could lead to further changes in the grain boundary chemistry. This is known to influence the grain boundary strength and affects the overall mechanical response of the superalloys. In the present study high resolution characterization has been carried out to investigate the evolution of grain boundary precipitates in DS GTD444 alloy under thermal aging at 900 ºC. In addition, in situ picoindentation and high-temperature tensile tests were performed to determine the effect of changes in microstructure on the mechanical properties. Atom probe tomography, in conjunction with electron microscopy, shows that nano-sized borides present along grain boundaries in the as-received DS blades completely transform into carbides after aging. These carbides are of two types: M23C6, and M6C. These carbides are present discretely along the grain boundary. The evolved microstructure after 320 h of aging shows higher transverse ductility as compared to the as-received DS blades, and the failure in all tested samples was transgranular. This suggests that the presence of discrete precipitates (borides or carbides) along the DS boundaries strengthens the grain boundaries.

Effect of welding speed on the microstructure and texture development in the individual weld zone of friction stir welded DP780 steel

In this study, microstructural and texture development in the individual weld zone during friction stir welding (FSW) of DP780 steel at similar tool rotation speed (200 rpm) different welding speeds (200 mm/min and 400 mm/min) was investigated. Phase transformations and competition between recovery and recrystallization mechanisms in the distinct weld zones were explored by electron back scattered diffraction (EBSD). More pronounced grain refinement was observed in stir zone (SZ) of sample welded at higher speed (0.62 μm) as compared to lower welding speed (0.51 μm) which is attributed to the enhanced Zener pinning effect at higher welding speeds. Continuous dynamic recrystallization (cDRX) was found to be the dominating recrystallization mechanism in SZ of both welded samples. Orientation distribution function (ODF) based texture investigations of SZ revealed the development of D1 {1¯1¯2}<111> and D2 {112¯}<111> shear components along with the cube {001}<100> texture. The lower texture intensities witnessed in SZ of higher welding speed sample was due to the accelerated recrystallization kinetics at higher welding speeds. The microstructural investigations of thermo-mechanically affected zone (TMAZ) of both welded samples revealed that cDRX was predominant recrystallization mechanism while microshear bands, grain boundary bulging and geometric DRX (gDRX) also assisted the grain refinement. Texture analysis showed the development of D2 {112¯}<111>, J{110}<11¯2> and J¯{1¯1¯0}<1¯12¯> shear components in the TMAZ lower welding speed sample and D1 {1¯1¯2}<111>, D2 {112¯}<111>, J{110}<11¯2> and J¯{1¯1¯0}<1¯12¯> shear components in the TMAZ of higher welding speed sample. Moreover, heat affected zone (HAZ) of both samples showed higher evolution of low angle grain boundaries (LAGBs) indicating that the grains are in the dynamic recovery stage due to lower temperatures and strain.

Evolution of dislocations and grain boundaries during multi-axial forging of tantalum

Presented here is the evolution of microstructure of Ta during multi-axial forging, with a specific understanding of how grain boundary movement and their interaction occurs during deformation. Coarse grained (CG) tantalum was multi-axially forged till 3, 6, and 9 passes in a closed channel die to refine grain size and increase its strength. The grain size was refined from 64 μm to 760 nm, the dislocation density increased by an order of magnitude, the Vickers hardness increased from 92 to 223 HV, and the yield strength at room temperature increased from 178 to 653 MPa. It was found that the contribution of GBs to strengthening was higher than that of dislocations. Mechanisms for GB structure evolution during deformation were studied using molecular dynamics (MD) simulations. GB mobility was high for low angle GBs (LAGBs) and reduced with increasing tilt angle, after which it took on low values for high angle GBs (HAGBs). The mobility of specific HAGBs were due to the formation of ledges/disconnections and dissociation into other GB types. MD results showed that the decrease in fraction of HAGBs during initial deformation can be attributed to the dissociation of HAGBs with low mobility into GBs with higher mobility.

Recent Advances in the Equal Channel Angular Pressing of Metallic Materials

Applications of a metallic material highly depend on its mechanical properties, which greatly depend on the material’s grain sizes. Reducing grain sizes by severe plastic deformation is one of the efficient approaches to enhance the mechanical properties of a metallic material. In this paper, severe plastic deformation of equal channel angular pressing (ECAP) will be reviewed to illustrate its effects on the grain refinement of some common metallic materials such as titanium alloys, aluminum alloys, and magnesium alloys. In the ECAP process, the materials can be processed severely and repeatedly in a designed ECAP mold to accumulate a large amount of plastic strain. Ultrafine grains with diameters of submicron meters or even nanometers can be achieved through severe plastic deformation of the ECAP. In detail, this paper will give state-of-the-art details about the influences of ECAP processing parameters such as passes, temperature, and routes on the evolution of the microstructure of metallic materials. The evolution of grain sizes, grain boundaries, and phases of different metallic materials during the ECAP process are also analyzed. Besides, the plastic deformation mechanism during the ECAP process is discussed from the perspectives of dislocation slipping and twinning.