Boundary Migration Mechanism Research Articles

Influence of Chromium Atoms on the Shear-Coupled Motion of [110] Symmetric Tilt Grain Boundary in α-Iron: Atomic Simulation

Shear-coupled grain boundary motion (SCGBM) is an important mechanism of plastic deformation, especially in the cases of ultrafine-grained or nanocrystalline materials at low temperatures. Much research work has been focused on the geometric rules of coupling, the grain boundary migration mechanisms, or the temperature effect of SCGBM, but the effect of the alloy atoms is seldom involved. In this work, molecular dynamics (MD) simulations were carried out to examine the SCGBM of the Σ17[110](223) and Σ9[110](221) grain boundaries (GBs) in iron-chromium alloys containing from 1 at.% to 9 at.% Cr. A constant shear velocity corresponding to 10 m/s parallel to the boundary plane was applied to the models. Our simulation results indicate that the critical stress of GB migration reduces due to the addition of Cr atoms for the Σ17(223) GB. As for the Σ9(221) GB, sliding occurs simultaneously with coupling in the shear process when the atomic amount of Cr reaches 3%. This phenomenon was also observed in the Σ9(221) GB in pure Fe when the temperature was elevated to 300 K, which was studied in our previous simulation work. The existence of new structural units was demonstrated to be responsible for the sliding of the grain boundary.

Homogenization and short-range chemical ordering of Co–Pt alloys driven by the grain boundary migration mechanism

Binary magnetic alloys like Co–Pt are relevant for applications as components of magnetic exchange coupled composites. Numerous approaches exist to tune the coercive field of Co–Pt alloys primarily relying on high-temperature processing aiming to realize chemically long-range ordered phases. The peculiarity of Co–Pt is that large coercive field and magnetic anisotropy can be achieved even in chemically disordered alloys relying on short-range order. Here, we study alloying of Co–Pt from bilayers of Pt(14 nm)/Co(13 nm) at temperatures up to 550 °С, where bulk diffusion processes are suppressed and the dominant diffusion mechanism is grain boundary migration. We demonstrate that grain boundary diffusion mechanism can lead to the realization of a homogeneous yet chemically disordered Co56Pt44 alloy at temperatures of 500 °С and higher. A pronounced increase of the coercive field for samples processed at temperatures higher than 400 °С is attributed to short-range ordering. With this work, we pinpoint the grain boundary diffusion as the mechanism responsible not only for the homogenization of binary alloy films but also as a driving force for the realization of short-range order in Co–Pt. Our results motivate further research on grain boundary diffusion as a mechanism to realize chemically long-range ordered phases in Co–Pt alloys.

Effects of hot rolling and coiling temperatures on microstructure, texture and magnetic properties of 1.6 wt% Si non-oriented silicon steel

Bigger grains prior to cold rolling are favorable to the magnetic properties of non-oriented electrical steels. However, it is hard to coarsen grains in hot rolled sheets of low silicon steels due to γ→α transformation. In the work, the influences of different hot rolling temperatures (820 °C, 840 °C and 875 °C) and coiling temperatures (680 °C and 750 °C) on the microstructural and textural evolution of 1.6 wt% Si steel were comparatively investigated in detail. The mechanism of grain growth during coiling was investigated using a quasi in-situ electron backscattered diffraction (EBSD) technique. The main finding was that pancake grains with an aspect ratio of 1.5–3.5 formed after hot rolling at 840 °C and further significantly grew after coiling at higher temperature 750 °C through a strain induced boundary migration mechanism. However, the significant grain growth did not occur after coiling at low temperature of 680 °C. By contrast, elongated deformed grains and equiaxed grains were produced after finish rolling at 820 °C and 875 °C, respectively, in which pronounced grain growth was not observed even after coiling at 750 °C. The significant grain growth promoted the formation of shear bands during cold rolling and led to weakened γ-fiber (<111 >//ND) texture, enhanced λ-fiber (<100 >//ND) and big grains in the annealed sheet. As a result, both magnetic induction and iron loss were significantly improved. This work provides a promising method to coarsen grains before cold rolling by design hot rolling process for low silicon non-oriented silicon steels.

An investigation on microstructure and mechanical properties of 316 stainless steel: a comparison between ultrasonic treatment and thermal annealing

ABSTRACT The effect of ultrasonic treatment (UST) and thermal annealing (THA) post-processes on the mechanical properties and the related microstructural mechanisms of the tensile pre-strained 316 stainless steel was investigated. It was shown that both processes reduce the microhardness and the yield point as well as increasing the elongation of the pre-deformed alloy. A 10% reduction of the yield point and 28% increase in the elongation was observed after the higher power UST (500 W), while an enhanced ductility of 56% and 41% reduction of the yield point was measured for the high-temperature THA (800°C) treated steel. The increased ductility was related to de-twinning and dislocation annihilation mechanisms, which increase the mean free path distance of dislocations. The de-twinning mechanism was proposed as the boundary migration mechanism and reverse gliding of the partial dislocations by cyclic shear stress for the THA and UST processes, respectively. Unlike the UST process, the high-temperature thermal annealing was associated with the formation of M23C6 precipitates, which causes depletion of alloying elements from the vicinity of grain boundaries and makes the alloy more prone to intergranular corrosion. Compared with THA, the advantages of the UST process are as follows: a rapid and straightforward process, low energy consumption, enhanced ductility without significant reduction in strength, and inhibition of grain boundary precipitation.

Effect of heat treatment temperature on the microstructural evolution of CM247LC superalloy by laser powder bed fusion

To attain the desired mechanical properties of an additively manufactured component, robust post-processing in term of thermal treatment is highly required to reduce the crystallographic anisotropy. However, the microstructure appearance with respect to the post-treatment temperature is not well understood mechanistically. In this study, the microstructural evolution of grains of a laser-powder bed fused (L-PBF) nickel-base superalloy, CM247LC, during post-processing heat treatment is investigated systematically. Recrystallization barely happens below the γ' solvus temperature leading to a remaining unhomogenized dendritic(cellular) structure. However, recrystallization is introduced above the γ' solvus temperature. By considering the grain boundary (GB) migration mechanisms and supported by experimental observations, the sluggish recrystallization behavior of this γ'-strengthened nickel-based superalloy has been understood. Owing to lack of the difference in stored energy between adjacent grains, this primary driving force is constrained. The GB migration is majorly driven by capillarity force (1–10 MPa) before the recrystallization occurrence, which is evident by the evolution of GB curvatures. On the other hand, the Zener pinning force generated from GB precipitates including carbides and γ' precipitates provides the dragging force in the comparable scale (1–10 MPa) against the GB migration.

Revealing the size effect mechanism of reversible grain boundary migration in nanocrystalline coppers: Molecular dynamics simulations and a refined disconnection model

Reversible grain boundary (GB) migration is of great significance for promoting cyclic deformability and has been observed in nanocrystalline metals. However, GBs as dislocation sources generally emit more dislocations under deformation with increasing grain sizes. The size effect mechanism of reversible GB migration is thus crucial but remains unknown. We demonstrate that the reversibility of the representative Σ11(113) GB migration in copper (Cu) bicrystals under cyclic shear strongly depends on their grain sizes using molecular dynamics (MD) simulations. Atomic observation of the GB plane reveals the transition mechanism from the fully reversible disconnection-mediated GB migration to the disconnection pair grooving (DPG) induced irreversible structural damage with increasing grain size (d), where the disconnections at the groove are proved to be screw disconnections with Burgers vectors of 1/22[332¯] using Burgers circuit and lattice analysis. A refined disconnection model considering grain size is developed to predict disconnection-mediated GB migration behavior and interpret the forming mechanism of DPG. When satisfying d > 1.6651Gb/πτ (shear modulus G, Burgers vector b, and shear stress τ), the DPG mode is triggered, while the single disconnection sliding provokes the reversible GB migration when d ≤ 1.6651Gb/πτ. This study holds implications for nanocrystalline metals towards endurable cyclic deformability by controllable grain sizes.

Faceting of twin interfaces in rolled pure magnesium

ABSTRACT Recent studies proved that the faceting of twin boundaries is a common feature in hcp materials. It was demonstrated that the asymmetrical facets in the most frequent and twin interfaces are the consequence of the twin boundary migration mechanism. We demonstrate that a similar mechanism is also applicable to less common twins, observed in pure magnesium rolled at room temperature. The migration of twin boundary is mediated by disconnections, which have Burgers vector and step height equal to four interplanar distances. The mechanism of asymmetrical facets nucleation is also demonstrated by using atomistic simulations. The simulations are supported by transmission electron microscopy (TEM) of asymmetrical basal-pyramidal interfaces of twins.

Correlation between the mechanical properties and the 〈110〉 texture in a hot-rolled near β titanium alloy

Correlation between mechanical properties and texture characteristics was investigated in a hot-rolled Ti-7333 alloy. After rolling deformation, two strong fiber-textures with 〈100〉 parallel to rolling direction (RD) and 〈110〉 parallel to RD were determined. The mechanism of stress-induced boundary migration mainly controls the nucleation of recrystallized grains holding the similar orientation with the adjacent deformed grains on the bulged grain boundaries. It was revealed that the 〈110〉 fiber-texture with higher Schmid factor (SF) possesses the high deformation ability along RD.

Fabrication of high-performance low silicon non-oriented electrical steels by a new method: Low-finishing-temperature hot rolling combined with batch annealing

The grain size prior to cold rolling plays an important role in the microstructural, textural evolution and magnetic properties of non-oriented silicon steel sheets. In this work, we proposed a new way to obtain large grain size before cold rolling and applied it to a 1.0 wt% Si-0.2 wt% Al non-oriented silicon steel, i.e., appropriate low-finishing-temperature hot rolling combined with batch annealing. The grain growth process during batch annealing was investigated using a quasi in-situ electron backscattered diffraction (EBSD) technique. It was demonstrated that significant grain growth took place through a strain-induced boundary migration mechanism. Then a large number of in-grain shear bands were formed in the cold rolled microstructure and could provide nucleation sites for Goss ({110} 〈001〉 ) and λ-fiber (〈001〉//ND) recrystallized grains, thus resulting in a coarse-grained and homogeneous recrystallization microstructure with weakened γ-fiber (〈111〉//ND) texture, strengthened Goss and λ-fiber texture. Consequently, superior magnetic properties (B50 = 1.763 T, P1.5/50 = 4.30 W/kg) were obtained. For comparison, the conventional processing route without batch annealing was also investigated. A small-grained and inhomogeneous recrystallization microstructure with pronounced γ-fiber texture was produced in the finally annealed sheet, which could significantly deteriorate both the magnetic induction and iron loss. This work provided a new way to enlarge the grains in hot rolled sheets by low temperature batch annealing instead of the conventional high temperature normalization. Thus a new processing route to fabricate low silicon non-oriented electrical steels with high permeability and low iron loss was established.

Effect of welding speed on microstructure and mechanical properties of Al−Mg−Mn−Zr−Tialloy sheet during friction stir welding

Effects of welding speed on the microstructure evolution in the stir zone (SZ) and mechanical properties of the friction stir welding (FSW) joints were studied by OM, XRD, SEM, TEM, EBSD and tensile testing. Compared with the base metal (BM), an obviously fine dynamic recrystallization (DRX) microstructure occurs in the SZ and the DRX grain size decreases from 5.6 to 4.4 μm with the increasing of welding speed. Fine DRX microstructure is mainly achieved by continuous dynamic recrystallization (CDRX) mechanism, strain induced boundary migration (SIBM) mechanism and particle stimulated nucleation (PSN) mechanism. Meanwhile, the geometric coalescence and the Burke−Turnbull mechanism are the main DRX grain growth mechanisms. Among all the welding speeds, the joint welded at rotation speed of 1500 r/min and welding speed of 75 mm/min has the greatest tensile properties, i.e. ultimate tensile strength (UTS) of (509±2) MPa, yield strength (YS) of (282±4) MPa, elongation (El) of (23±1)%, and the joint efficiency of 73%.

A taxonomy of grain boundary migration mechanisms via displacement texture characterization

Atomistic simulations provide the most detailed picture of grain boundary (GB) migration currently available. Nevertheless, extracting unit mechanisms from atomistic simulation data is difficult because of the zoo of competing, geometrically complex 3D atomic rearrangement processes. In this work, we introduce the displacement texture characterization framework for analyzing atomic rearrangement events during GB migration, combining ideas from slip vector analysis, bicrystallography and optimal transportation. Two types of decompositions of displacement data are described: the shear-shuffle and min-shuffle decomposition. The former is used to extract shuffling patterns from shear coupled migration trajectories and the latter is used to analyze temperature dependent shuffling mechanisms. As an application of the displacement texture framework, we characterize the GB geometry dependence of shuffling mechanisms for a crystallographically diverse set of mobile GBs in FCC Ni bicrystals. Two scientific contributions from this analysis include 1) an explanation of the boundary plane dependence of shuffling patterns via metastable GB geometry and 2) a taxonomy of multimodal constrained GB migration mechanisms which may include multiple competing shuffling patterns, period doubling effects, distinct sliding and shear coupling events, and GB self diffusion.

Twin boundary migration mechanisms in quasi-statically compressed and plate-impacted Mg single crystals.

Twinning is a prominent deformation mode that accommodates plasticity in many materials. This study elucidates the role of deformation rate on the atomic-scale mechanisms that govern twin boundary migration. Examination of Mg single crystals deformed under quasi-static compression was compared with crystals deformed via plate impact. Evidence of two mechanisms was uncovered. Atomic-level observations using high-resolution transmission electron microscopy revealed that twin boundaries in the <a>-axis quasi-statically compressed single crystals are relatively smooth. At these modest stresses and rates, the twin boundaries were found to migrate predominantly via shear (i.e., disconnection nucleation and propagation). By contrast, in the plate-impacted crystals, which are subjected to higher stresses and rates, twin boundary migration was facilitated by local atomic shuffling and rearrangement, resulting in rumpled twin boundaries. This rate dependency also leads to marked variations in twin variant, size, and number density in Mg. Analogous effects are anticipated in other hexagonal closed-packed crystals.

On the effect of rapid annealing on the microstructure and mechanical behavior of additively manufactured stainless steel by Laser Powder Bed Fusion

This work presents an investigation on the effect of rapid annealing on the microstructure evolution and mechanical performance of stainless steel 316L (SS316L) fabricated by Laser Powder Bed Fusion. In this process, it is well documented that the intensive thermal gradient in the heat flux direction leaves remarkable residual stress and promotes strong texture along the building direction (BD). In the current research, to swiftly reduce residual stresses, a short-term heat treatment at 1300 °C for different holding times of 10–120 s was designed and studied. Microstructural observation through the optical microscopy and scanning electron microscopy, as well as the X-ray diffraction analysis, revealed that rapid annealing longer than 30 s resulted in the gradual annihilation of the cell walls which are indeed the sub-grain boundaries (SGBs). In addition, electron backscatter diffraction outputs demonstrated that rapid annealing significantly reduced the low angle grain boundaries and attenuated the texture developed along the BD. Moreover, the analysis of grains size and shape illustrated that annealing up to 30 s recovered the structure. In contrast, longer annealing resulted in increasing the aspect ratio of grains along the direction with maximum residual strain (i.e. BD) due to activation of the strain induced grain boundary migration (SIGBM) mechanism. After the rapid annealing, the mechanical performance of the samples also indicated that annealing up to 30 s would not reduce the compressive and tensile strength more than 6%. In comparison, longer annealing resulted in 24% reduction in mechanical properties along with coarsening of dimples in the fracture surface of the samples.

Correction to: Grain boundary migration and deformation mechanism influenced by heterogeneous precipitate

Correction to: Grain boundary migration and deformation mechanism influenced by heterogeneous precipitate

Impurity effect on recrystallization and grain growth in severe plastically deformed copper

The impurity effect on commercial purity copper (4 N and 3 N) was investigated by comparing their annealing induced recrystallization and grain growth behavior after severe plastic deformation. Equal channel angular pressing (ECAP) with additional cryogenic rolling was applied to produce equiaxed ultra-fine grained (UFG) and nano-laminated (NL) coppers with high stored excess energy. The stored excess energy increased with decreasing purity from 4 N to 3 N, whereas the grain boundary velocity and recrystallized grain size decreased for both UFG and NL Cu. In particular, the NL structures were found to be more sensitive to impurities than equiaxed ultra-fine grained ones, and consequently the abnormal grain growth at the early stages of recrystallization in NL 4 N Cu was effectively suppressed in NL 3 N Cu. The analysis of the grain growth rate during recrystallization, using the microstructural path methodology (MPM), demonstrates that the correlation between the abnormal grain growth and the significantly enhanced grain boundary mobility cannot be solely explained by the high stored excess energy. Under all conditions, the impurity drag effect poses a kinetic constraint on the rate-controlling mechanism for grain boundary migration.

Multiphase field modeling of grain boundary migration mediated by emergent disconnections

Knowledge about grain boundary migration is a prerequisite for understanding and ultimately modulating the properties of polycrystalline materials. Evidence from experiments and molecular dynamics (MD) simulations suggests that the formation and motion of disconnections is a mechanism for grain boundary migration. Here, grain boundary migration is modeled using a multiphase field model based on the principle of minimum dissipation potential with nonconvex boundary energy, along with a stochastic model for thermal nucleation of disconnection pairs. In this model, disconnections arise spontaneously in the presence of an elastic driving force, and that their motion mediates boundary migration. The effect is due to the fact that the formation of the disconnections pairs results in a stress concentration, causing the elastic driving force to exceed the threshold value and driving the propagation of the disconnection along the interface. The model is applied to study the propagation/annihilation of single disconnection pairs, the relaxation of a perturbed interface, and shear coupling at various temperatures. The results are consistent with the current understanding of disconnections, and capture the effect of thermal softening.

Microstructure and Texture Development in Thermomechanically Processed Leaded Brass

Leaded brass alloys with high zinc content are classified as dual-phase (α-β′) brass alloys. For these alloys, α-β′ phase transformation induced by thermomechanical processing (TMP) is expected to play a significant role not only in the formation of α and β′ phases and uniformity of lead particles but also in the development of Σ3 boundaries and their variants in the α phase. To assess such impact, the current study has employed two TMP schemes, with each scheme consisting of four cycles of compression and annealing. Samples of the first scheme (TMP-1) were annealed at 670 °C to promote phase transformation from α to β′, as compared to the second scheme (TMP-2) samples annealed at 525 °C. Microstructure examination indicated that TMP-1 scheme fostered a remarkable growth in the area fraction of β′ phase, accompanied by grain coarsening of α and β′ phases. This resulted in the isolation of α grains and less interaction of Σ3 boundaries at trip junctions, and therefore, a reduction in the fraction of Σ3 boundaries occurred in the α phase. On the other hand, the TMP-2 scheme successfully enhanced cluster formation of α grains, enabling further generation of Σ3 boundaries via strain-induced boundary migration mechanism and ultimately leading to breakup of the random boundary network. In addition, the two schemes resulted in limited coarsening of lead particles. Most lead particles were located at the α-βʹ interfaces, which suppressed grain boundary migration via the particle-dragging effect. For both TMP schemes, the initial texture of tension fibers was largely preserved after four cycles for both α and βʹ phases. Nevertheless, the TMP-2 scheme produced texture randomization of the α phase due to increasing the fraction of Σ3 boundaries.

Texture evolution associated with the preferential recrystallization during annealing process in a hot-rolled near β titanium alloy

Texture evolution associated with the recrystallization behavior during solution treatment was investigated in a hot-rolled Ti-7333 by virtue of Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD) and Electron Backscatter Diffraction (EBSD). The results show that the microstructure of the rolled specimens consists of a large number of elongated β grains and a small amount of equiaxed β grains. The dominant texture components are confirmed to be {111}<110> and {100}<001>. With the progressing of the solution treatment, the recrystallization occurs and the orientations of {111}<110> and {001}<100> hold the preferred orientations to act as the nuclei for the recrystallized grains. It is found that the recrystallized grains are mainly located on the bulged boundaries or within the deformed grains, and the mechanism of the strain-induced boundary migration (SIBM) mainly accounts for the formation of the recrystallized grains nucleated on the bulged boundaries, which eventually leads to the texture of the solution-treated specimens inheriting the previous orientation of <110> parallel to the rolling direction. The results of the present work provide a valuable reference for understanding the relationship between the texture evolution and the recrystallization behavior during the annealing process in near β titanium alloys.

Understanding the interaction of extension twinning and basal-plate precipitates in Mg-9Al using precession electron diffraction

Precession electron diffraction is used to characterize the interaction between {101¯2} tensile twins and basal plate-like precipitates in a post-deformed, precipitate-dispersed Mg-9Al micropillar. We observed a heterogeneous distribution of precipitates in the micropillar sample, which enabled the study of the different stages involved in twin-precipitate interactions. We show that twin nucleation was promoted, taking place on the surface, as well as from the interior of the micropillar. Twin tip propagation and twin growth were hindered by the precipitates. Twin tips either were arrested by precipitates and new twins formed on the other side of the precipitate, or continued to grow around the precipitate without the re-nucleation events. During twin thickening, the precipitates did not significantly alter the shear-dominant twin boundary migration mechanism, as evidenced by the relatively flat twin boundaries around the partially embedded precipitates. However, the twin boundary migration was retarded by the precipitates, especially in regions confined by closely-spaced precipitates.

Grain boundary migration and deformation mechanism influenced by heterogeneous precipitate

AbstractUnderstanding the interaction between heterogeneous precipitates and grain boundaries (GBs) is of great significance for tailoring the stability and mechanical properties of nanograined materials. In this work, the nanoscale interaction between the cylindrical precipitate and the migrating GB is investigated by atomic simulation. The results show that the resistance for GB migration can be increased by decreasing the direction angle $$\alpha$$ α (the angle between the axis of the precipitate and the tilt axis of GB). For the larger precipitate, the influence of direction angle is more pronounced. With the increase in shear strain, the interaction between the specific precipitate and GB changes the material deformation mechanism from “GB migration” to “GB migration accompanied with activated dislocations or GB deformation,” which contributes to the softening of the material. By simultaneously tuning the direction angle and size of heterogeneous precipitates, the GB deformation can be strongly inhibited and the stability of GBs can be significantly improved.