Strain-induced Boundary Migration Research Articles

In-situ observation of restoration of tungsten plate via high-temperature confocal laser scanning microscopy

The restoration of an yttria dispersion strengthened tungsten plate and the corresponding microstructural evolution are investigated through in situ annealing using high-temperature confocal laser scanning microscopy. The geometrically necessary dislocation density and subgrain/grain evolution at each annealing stage are confirmed using the electron backscatter diffraction technique. The microstructure's evolution is divided into distinct stages of recovery, recrystallization, and grain growth based on their characteristic features. During the recovery process, the fomation of subgrains occurs. The primary mechanism of recrystallization nucleation is dominated by the strain-induced boundary migration mechanism, and the subgrains preexisting at grain boundaries are the main nucleation sites.

Hot Deformation Behavior and Microstructural Evolution of a TiB2/Al-Zn-Mg-Cu-Zr Composite.

In the present work, the hot deformation behavior and microstructural evolution of a TiB2/Al-Zn-Mg-Cu-Zr composite were studied. Hot compression tests were conducted within a temperature range of 370 °C to 490 °C and a strain rate of 0.001 s-1 to 10 s-1. We established the Arrhenius constitutive equation with Zener-Hollomon parameters and processing maps and discussed the microstructural evolution during hot deformation. The results indicated that the safe processing parameter region falls within 370 °C-490 °C and 0.001 s-1-0.025 s-1. The influence of the strain rate on the safe processing range is more dominant than that of deformation temperature, which is primarily attributed to TiB2. Dynamic softening is primarily governed by dynamic recovery (DRV). Small particles (η, Al3Zr) can pin dislocations, promoting the rearrangement and annihilation of dislocations and facilitating DRV. Higher temperatures and lower strain rates facilitated dynamic recrystallization (DRX). Continuous dynamic recrystallization (CDRX) occurs near high-angle grain boundaries induced by strain-induced boundary migration (SIBM). TiB2 and large second-phase particles generate high-density geometrically necessary dislocations (GNBs) during hot deformation, which serve as nucleation sites for discontinuous dynamic recrystallization (DDRX). This enhances dynamic softening and improves formability.

Evolution of Grain Boundary Character Distribution in B10 Alloy from Friction Stir Processing to Annealing Treatment.

In this study, the grain boundary character distribution (GBCD) of a B10 alloy was optimized, employing thermomechanical processing consisting of friction stirring processing (FSP) and annealing treatment. Using electron backscatter diffraction, the effects of rotational speed of FSP and annealing time on the evolution of GBCD were systematically investigated. The GBCD evolution was analyzed concerning various parameters, such as the fraction of low-Σ coincidence site lattice (CSL) boundaries, the average number of grains per twin-related domain (TRD), the length of longest chain (LLC), and the triple junction distribution. The experimental results revealed that the processing of a 1400 rpm rotational speed of FSP followed by annealing at 750 °C for 60 min resulted in the optimum grain boundary engineering (GBE) microstructure with the highest fraction of low-Σ CSL boundaries being 82.50% and a significantly fragmented random boundary network, as corroborated by the highest average number of grains per TRD (14.73) with the maximum LLC (2.14) as well as the highest J2/(1 - J3) value (12.76%). As the rotational speed of FSP increased from 600 rpm to 1400 rpm, the fraction of low-Σ CSL boundaries monotonously increased. The fraction of low-Σ CSL boundaries first increased and then decreased with an increase in annealing time. The key to achieving GBE lies in inhibiting the recrystallization phenomenon while stimulating abundant multiple twinning events through strain-induced boundary migration.

Effects of KoBo-Processing and Subsequent Annealing Treatment on Grain Boundary Network and Texture Development in Laser Powder Bed Fusion (LPBF) AlSi10Mg Alloy

It is well known that the properties of polycrystalline metals are related to grain boundaries (GBs), which are fundamental structural elements where crystallographic orientations change abruptly and often exhibit some degree of symmetry. Grain boundaries often exhibit unique structural, chemical, and electronic properties that differ from bulk crystalline domains. Their effects on material properties, including mechanical strength, corrosion resistance, and electrical conductivity, make grain boundaries a focus of intense scientific investigation. In this study, the microstructural transformation of an AlSi10Mg alloy subjected to KoBo extrusion and subsequent annealing is investigated. A notable discovery is the effectiveness of a strain-annealing method for grain boundary engineering (GBE) of the LPBF AlSi10Mg alloy. In particular, this study shows a significant increase in the population of coincidence site lattice boundaries (CSL), which embody the symmetry of the crystal lattice structure. These boundaries, which are characterised by a high degree of symmetry, contribute to their special properties compared to random grain boundaries. The experimental results emphasise the crucial role of strain-induced boundary migration (SIBM) in the development of a brass texture in the microstructure of the alloy after annealing. In addition, the presented results demonstrate the feasibility of applying GBE to materials with high stacking fault energy (SFE), which opens up new possibilities for optimizing their properties.

Multi-scale investigation of microstructure and texture evolution during equal channel angular pressing of silver

A multi-scale investigation was carried out on pure silver subjected to equal channel angular pressing up to 3 passes. Microstructure and texture were examined using scanning and transmission electron microscopy as well as diffraction of neutron and high-energy synchrotron radiation. The evolution of the deformation substructure in the material is in accordance with its low stacking fault energy followed by restoration mechanisms leading to overall microstructural refinement. The restoration mechanism sets in as early as after 3 passes. Twinning involved in the deformation process supports grain refinement through mechanisms of dynamic recrystallization involving strain-induced boundary migration contributing to a steady-state grain size. The global texture, as measured by neutron diffraction, forms through a twin-assisted deformation mechanism. The texture is found heterogeneous through thickness.Graphical abstract

The Onset of Recrystallization in Polar Firn

AbstractConstraining the onset of dynamic recrystallization (DRX) and its effects on the mechanical properties of firn is crucial for firn densification modeling. To that end, samples from a depth of 13 m in a Summit, Greenland (72°35′N, 38°25′W) firn core were subjected to creep tests at −14°C and 0.21 MPa compressive stress to strains of 7%, 12%, 18%, and 29%. Microstructural analyses using thin‐section imaging and microcomputed x‐ray tomography (micro‐CT) revealed smaller grain sizes, reduced specific surface area and connectivity, and increased density in relation to reduced porosity as the strain increases. These results show that DRX occurs in firn under creep, with strain‐induced boundary migration (SIBM) and nucleation and growth starting at ∼7%. DRX leads to elongated grains, reduced grain size, and the development of a preferred crystallographic orientation, indicating that DRX occurs by both SIBM and nucleation and growth.

Texture evolution during multi-pass cold rolling and annealing of Ti-2Al-1.5Mn alloy

In this contribution, the underlying mechanisms of microstructure evolution and texture development during the cold rolling and annealing of a Ti-2Al-1.5Mn alloy were investigated and elucidated comprehensively. As a result of the heightened dislocation density and grain boundary, the hardness ascended as the successive reductions applied. However, the annealing treatment reinstated the structure through complete recrystallization. During cold working, the dominant basal <a> and prismatic <a> glide inclined the c-axis and <101̅0> axis towards the compressive direction and the rolling direction (RD), respectively. The primary rolled orientation consisted of <101̅0> fiber, the orientation of {1̅21̅3}<101̅0> with Euler angle {10°, 45°, 0°}and {1̅21̅7}<101̅0> with Euler angle {5°, 25°, 0 °} were predominant. Schmid Factor (SF) analysis and orientation rotation related to various textures demonstrated that basal <a> slipping resulted in a descendingϕangle in the Bunge system and <101̅0> pole parallel to the RD was the stable orientation. It was revealed that the recrystallized grains preponderantly exhibited textures inherited from the rolled matrix, but a new T-type texture also emerged. This implies that both the strain-induced boundary migration (SIBM) mechanism and selective grain growth, which nucleates at different sites, manipulate the annealing process. The study enhances our comprehension of texture evolution mechanisms involved in the plastic forming and helps ascertain the most effective operational parameters for producing high-performance titanium plates.

Nucleation of recrystallization

This paper reviews the mechanisms of nucleation during recrystallization of cold deformed, single phase polycrystalline metals including metals with large particles. The classic nucleation theories and conceptions are shortly summarized, while the main focus is on our results from recent 3D studies of nucleation. The novel results are related to the classic nucleation ideas and agreement/disagreement as well as new suggestions are discussed. More specifically the paper covers recovery leading to intragranular nucleation, bulge nucleation (which is often referred to as strain induced boundary migration), and particle stimulated nucleation. Also, effects of clustered nucleation, crystallographic orientation relationships and residual stress are considered. Finally future studies are suggested, which we consider key to advancing the understanding of nucleation during recrystallization.

Exceptional improvement in the yield strength of AZ61 magnesium alloy via cryo-stretching and its implications on the grain growth during annealing

This study explored the influence of pre-stretching temperature on the microstructural characteristics and deformation behavior of AZ61 magnesium alloy and its implications on grain growth during subsequent annealing. AZ61 alloy was stretched to 5% plastic strain along rolling (RD) and transverse direction (TD) at room (RT) and cryogenic temperature (−150 °C, CT) followed by annealing at 320 °C for 1 h to investigate the twinning and dislocation evolution and its consequent effect on the flow stress, plastic strain and strain hardening rate. Compared to RT-stretched samples, significant improvement in yield stress, strain hardening rate and moderate reduction in elongation to failure were witnessed for CT-stretched samples along RD and TD. The subsequent EBSD analysis revealed the increased fraction of fine {10−12} twins and nucleation of multiple {10−12} twin variants caused by higher local stress concentration at the grain boundaries in CT-stretched samples as manifested by the kernel average misorientation. This higher twin fraction and twin-twin interaction imposed the strengthening by restricting the mean free path of dislocations leading to higher flow stress and strain hardening rate. During annealing of the RT/CT-stretched samples, the residual strain energy and twin boundaries were decreased due to static recovery leading to a coarse-grained twin-free microstructure. Strain induced boundary migration (SIBM) was found to be the predominant mechanism governing the grain growth during annealing via movement of high angle grain boundaries.

Mechanisms of necklace recrystallization in a BCC Fe-Al-Ta alloy with strengthening Laves phase precipitates

A necklace structure composed of fine grains formed by dynamic recrystallization was uncommonly observed at the pre-existing grain boundaries during the hot compression of a BCC Fe-25Al-1.5Ta alloy containing C14 - (Fe, Al)2Ta Laves phase precipitates. Two possible mechanisms for necklace formation were proposed; particle-stimulated nucleation and grain boundary bulging, depending on whether the original grain boundaries are occupied by C14 particles, or they are free of them. Recrystallization was initiated preferentially around the clusters of large particles at the boundaries containing particles. In contrast, the bulging of the original grain boundaries by strain-induced boundary migration was observed as a preliminary stage for necklace formation at the particle-free boundaries. The necklace structure expanded into the deformed volume in such a way that low-angle subgrain boundaries decorating the necklace layers transformed into grains with increasing deformation strain.

Nucleation Sites in the Static Recrystallization of a Hot-Deformed Ni-30 Pct Fe Austenite Model Alloy

In the present study, the nucleation of static recrystallization (SRX) in austenite after hot deformation is experimentally analyzed using a Ni-30 pct Fe model alloy. In agreement with the predictions by current models, nucleation rate exhibits a strong peak, early during SRX. Whereas such an early peak is explained by current models by the saturation of nucleation sites, this condition is far from reached, even after the peak declines. In addition, triple-junction and grain-boundary sites are shown to make a quantitatively similar contribution to nucleation. However, for a given boundary between deformed grains, nucleation predominantly starts at one of the triple junctions. Triple-junction nucleation initiates by strain-induced boundary migration of the nucleus (bulging) along one of the boundaries at the junction. Annealing twin boundaries contribute negligibly to nucleation through their grain-boundary sites. By contrast, their junctions with the boundaries of the parent grains do play a relevant role. The earlier nucleation at the triple junctions is attributed to the higher dislocation density observed around them, and the energy of the boundary consumed by the bulge. Both the maximum and average number of nuclei formed per boundary between deformed grains increase with increasing boundary length.

Microstructure and corrosion resistance of Z3CN20.09M stainless steels after different thermo-mechanical processing

Thermo-mechanical process was used to control the microstructure of a nuclear grade cast duplex stainless steel Z3CN20.09M. It is revealed that heat treatment alone cannot change the casting microstructure of coarse columnar grains. After a low-strain deformation (7% tension) and annealing at 1180 °C, a high fraction of low-∑ CSL boundaries, about 76%, were formed due to the strain induced boundary migration. After higher deformation (>10%) and annealing, recrystallization microstructure with fine equiaxed grains could be formed. The electrochemical tests in lithium borate aqueous solution showed that a high fraction of low-∑ CSL boundaries and fine grains after the thermo-mechanical processing improved the corrosion resistance of the Z3CN20.09M.

Effect of pre-recovery annealing on recrystallisation behaviour of strip-cast non-oriented electrical steel

In order to improve the {100} recrystallisation texture of non-oriented electrical steel, a pre-recovery annealing process was proposed to regulate recrystallisation behaviour of {100} grains by altering the structure and stored energy of deformed microstructure. Pre-annealing was conducted at 500°C for 0–90 min. The microstructural and textural evolution was carried out by optical microscopy, electron backscattered diffraction and X-ray diffraction analysis. The results indicate that the pre-recovery annealing promotes the formation of large {100} recrystallised grains. With the prolongation of pre-annealing time, the {100} recrystallisation texture expands from Cube to the vicinity of {001}<130>-{001}<110>, and the volume fraction gradually increases. In addition, the changes of recrystallised microstructure are discussed on the basis of strain-induced boundary migration.

Microstructural evolution of pre-twinned Mg alloy with annealing temperature and underlying boundary migration mechanism

This study investigates the variations in the microstructural characteristics of a pre-twinned Mg alloy with the temperature of the subsequent annealing treatment. To this end, a rolled AZ31 alloy is compressed to 3% plastic strain along the rolling direction (RD) to activate {10-12} twinning and is subsequently annealed at 200, 250, 300, 350, and 400 °C. Numerous {10-12} twins are formed throughout the compressed material, leading to the formation of a RD-oriented texture. At an annealing temperature of 200 °C, no microstructural variations occur during annealing. As the annealing temperature increases from 250 to 400 °C, the residual strain energy and remaining twin boundaries of the annealed material decrease owing to the promoted static recovery and the increased area fraction of twin-free grown grains. Consequently, an increase in the annealing temperature results in a gradual microstructural transition from a fully twinned grain structure to a completely twin-free grain structure. The microstructural evolution during annealing is predominantly governed by the movement of high-angle grain boundaries via a strain-induced boundary migration mechanism, and a few twin boundaries migrate above 350 °C because of their lower boundary energy. The boundary migration behavior and resultant microstructural evolution are discussed in detail based on the variations in boundary mobility and driving force for boundary migration with annealing temperature.

Post-hot-deformation microstructure development in austenitic stainless steel

In the present work, austenitic stainless steel has been thermo-mechanically processed to study the post-deformation microstructure evolution at elevated temperatures. The samples were axially compressed (at a strain rate 0.1 s−1) and isothermally held at temperatures 900, 1000 and 1100 °C, for different time spans (2–1000 s). The electron back-scattered diffraction technique has been used to study the microstructure of the thermo-mechanically processed samples. The result shows for temperatures of 900 °C and 1000 °C, the resultant microstructures are comprised of deformed grains for samples held for 2 s. However, at 1100 °C sample isothermally held for 2 s shows a nearly complete recrystallization. The grain average misorientation approach has been used to estimate the softening fraction caused by recrystallization. The average grain size of the thermo-mechanically processed samples reveals grain refinement for the samples processed at 900 °C and 1000 °C. In addition to the grain refinement, the samples processed at 1000 °C shows a more consistent grain size. At all three temperatures, the increase in the holding time leads to a gradual decrease in the fraction of low angle boundaries and a simultaneous increase in high angle grain boundaries (HAGBs). The HAGBs are mainly contributed by Σ3 coincident site lattice boundaries. At the end, a discussion on post-deformation softening, considering strain-induced boundary migration, has been presented.

Fostering deep drawability through recrystallization texture strengthening in aluminum-rich interstitial free steel

The development of strong normal direction (ND)║〈111〉 fiber texture is crucial for deep drawability in interstitial free steels used for automotive applications. The present work focuses on comparative investigations on texture development, Lankford parameter measurement, microstructural evolution, recrystallization kinetics, and grain growth kinetics of titanium‑niobium stabilized interstitial free steels with different aluminum (Al) content. Industrially hot-rolled interstitial free steels with 0.06 wt% Al and 0.17 wt% Al were cold-rolled (∼90%), followed by isothermal annealing at 650 °C, 675 °C, and 700 °C for various time intervals. Texture studies using electron backscatter diffraction and X-ray diffraction showed the formation of a stronger ND fiber texture in 0.17 wt% Al steel compared to 0.06 wt% Al Steel after full recrystallization. A high r m value in 0.17 wt% Al steel than in 0.06 wt% Al Steel indicates the beneficial effects of Al addition in interstitial free steel. The study of recrystallization kinetics depicted faster recrystallization kinetics in interstitial free steel with 0.17 wt% Al. The grain growth kinetics of recrystallized grains illustrated similar behavior in both steels. At the onset of recrystallization in 0.06 wt% Al steel, a transient strengthening of the {001} 〈110〉 component (an effect of stress-induced boundary migration) was observed, which obstructed the nucleation of ND fiber grains. In contrast, the initial rapid nucleation of ND fiber grains in the 0.17 wt% Al steel was responsible for stronger ND fiber texture development. • Aluminum-interstitial-free steel developed stronger {111} <uvw> fiber texture. • Aluminum-interstitial-free steel showed high r m value. • Aluminum-interstitial-free steel possesses faster recrystallization kinetics. • Strain-induced boundary migration is dominant initially in normal interstitial free steel. • Rapid nucleation of {111} <uvw> grains occurs in Aluminum-interstitial free steel.

B2-precipitation induced optimization of grain boundary character distribution in an Al0.3CoCrFeNi high-entropy alloy

The grain boundary character distribution (GBCD) of Al0.3CoCrFeNi high entropy alloy (HEA) subjected to 5% cold rolling and subsequent annealing at 1000 °C from 1 h to 100 h was investigated. The results show that the microstructure of Al0.3CoCrFeNi HEA is composed of FCC structure and ordered BCC (B2) structure. With the increase of annealing time, the frequency of low coincident site lattice (low-ΣCSL (3 ≤ Σ ≤ 29)) is divided into three stages, i.e., increases slowly in stage I (1–10 h), increases sharply in stage II (10–50 h), and remains stable in stage III (>50 h). The HEA annealed for 100 h possesses the highest low-ΣCSL fraction of ~71.34% and the biggest special boundary cluster, indicating that GBCD is effectively optimized. To this end, GBCD optimization mechanisms are proposed to explain this evolution: strain-induced boundary migration (SIBM) mechanism in stage I, non-coherent Σ3 boundary migration reaction model in stage II, and maintain stability in stage III. In this process, B2-precipitation has a key influence on the evolution of mechanism. B2-precipitation pinning induces the SIBM mechanism in stage I to generate numerous non-coherent Σ3 boundaries, which provides a basis for the non-coherent Σ3 boundary migration reaction in stage II. Therefore, the introduction of precipitation in HEAs provides a new and effective pathway to optimize GBCD.

Microstructure Characteristic and Texture Evolution of TB18 Titanium Alloy during Hot Compression in the β Phase Zone

In the present study, microstructure characteristic and texture evolution of TB18 titanium alloy were studied by isothermal hot compression under different strain with strain rate of 0.001 s−1 and 0.1 s−1 in the β phase zone. Electron Back-Scattered Diffraction (EBSD) and XRD were used to characterize the microstructure and macrotexture. Microstructure characteristic demonstrated that coarse β grains flatten and elongated to the compression direction as strain increased. The dynamic recrystallization mechanism is continuous dynamic recrystallization (CDRX) at lower strain rate, while discontinuous dynamic recrystallization (DDRX) at higher strain rate. The deformation is strongly dependent on the dislocation density, geometrically necessary dislocations density increased with strain at different strain rate. The limited faction of dynamic recrystallization has little effect on texture evolution, and the texture evolution is ascribed to the strain induced boundary migration and activity of slip system. At lower strain rate, the lower activity of {110}〈111〉 slip system and higher activity of {123}〈111〉 enhanced the 〈110〉 texture and weakened the 〈111〉 texture; at higher strain rate, the higher activity of {123}〈111〉 slip system modified the 〈111〉 β texture intensity and enhances the 〈110〉 texture.

The Effect of Microstructural Evolution on the Brazeability of Two-Layer Al Sheets

In this study, the microstructural evolution and the interaction between the clad and the core alloys that occurs during the brazing process of two-layer Al sheets with equiaxed grains were examined. The study was carried out using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD) and glow discharge optical emission spectrometry (GDOES). The effects of microstructure on the brazing performances of two-layer sheets were clarified. Although the grains were fine and equiaxed before brazing, three typical microstructural evolutions happened during brazing, corresponding to three kinds of interactions between the clad and core alloys of the aluminum brazing sheets. In the alloys, which had either relatively uniform grain growth or no grain growth, the interaction between the clad alloy and the core alloy was weak; accordingly, they showed a smooth surface, an even microstructure, faint element mutual diffusion, and eventually good brazeability. Meanwhile, in the alloy with obvious abnormal grain growth (AGG), strain-induced liquid-film migration (SILFM) occurred when the energy was too low to cause strain-induced boundary migration (SIBM). This led to rough and uneven surface morphology, significant mutual diffusion, and surface segregation of elements; eventually, this produced the worst brazeability.

Correlation of microstructure and dynamic softening mechanism of UNS S32101 duplex stainless steel during elevated temperature tensile testing

The dynamic substructural development and softening mechanism of UNS S32101 duplex stainless steel were comprehensively investigated by employing hot-tensile tests at various strain rates of 0.1–10 s−1 at a fixed temperature of 1200 °C. Different flow behaviors were attributed to the microstructural evolution and restoration process under various hot-deformation conditions. The alternative restoration mechanisms of ferrite in the current alloy were closely associated with the evolution of the misorientation angle in the (sub)grains, depending on the applied strain rates. Therein, three distinct softening mechanisms were found in ferrite, i) subgrain coalescence (SC) at 0.1 s−1, ii) continuous dynamic recrystallization (CDRX) at 1 s−1 and iii) subgrain rotation-assisted discontinuous dynamic recrystallization (SR-assisted DDRX) at 10 s−1. During SR-assisted DDRX process, new DRX nuclei were preferentially formed at the high-angle grain boundaries/phase boundaries (HAGBs/PBs) through the growth of highly misoriented subgrains. In contrast to ferrite, the available dynamic softening behavior of austenite, unlike the classical DDRX mechanism characterized by strain-induced boundary migration (SIBM), is affected by a limited number of pre-existing HAGBs. At lower strain rates of 0.1 and 1 s−1, the nucleation process of DRX in austenite is analogous to the CDRX behavior, whereas the growth characteristics conform to DDRX, thus, it can be called dynamic recovery-assisted DDRX (DRV-assisted DDRX). At a high strain rate of 10 s−1, DRX nucleation mainly took place through the strain-induced twin boundaries (TBs) transformation into HAGBs, and then rapidly grew via SIBM, referred to as TB-assisted DDRX.