Boundary Migration Mechanism Research Articles

A strategy to introduce gradient equiaxed grains into Zr sheet by combining laser surface treatment, rolling and annealing

A novel strategy combining laser surface treatment, rolling and annealing (LST-RA) to produce gradient equiaxed grains is proposed and verified after applying to pure Zr sheet. Dense high angle boundaries (HABs) are first produced by the laser-induced rapid β→α transformation in the surface layer of pure Zr while a considerable amount of stored energy is then introduced by 50% rolling. During subsequent annealing, the preexisting denser HABs facilitate generating more recrystallization nuclei in the surface layer than in the matrix through the strain induced boundary migration mechanism. This eventually results in a gradient equiaxed grain structure from surface to interior of the pure Zr sheet. Furthermore, the fine-grained surface layer is found to have a weakened texture compared to the coarse-grained matrix.

Evolution mechanisms of recrystallized grains and twins during isothermal compression and subsequent solution treatment of GH4586 superalloy

In present study, isothermal compression, solution treatment, electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM) observations were carried out to correlate the microstructure and deformation-solution parameters for GH4586 superalloy. And, the recrystallized mechanisms, twin evolution, γ′ precipitation and their interaction were clearly illustrated based on the analysis of the recrystallized fraction, size of γ grains, grain boundary misorientation and length fraction of twin boundaries. The results show that the process of dynamic recrystallization is controlled by the mechanisms of sub-grain rotation and strain-induced boundary migration while the process of static recrystallization is controlled by the mechanism of subgrain boundary migration. The evolution of twin boundaries during deformation is affected by two aspects: (i) the losing of twin’s identity due to higher strain; and (ii) the nucleation and growth of new twins within the DRX grains. During deformation of GH4586 superalloy, fine γ′ precipitates have a hindered effect on the recrystallization nucleated from pre-existing subgrains and twin growth. Moreover, the recrystallization process has a noticeable influence on the formation of new twins. Conversely, the existing of these twins is also beneficial for the recrystallization.

Evolution of rotated Brass texture by cross rolling: implications on formability

In the present work, the possibilities of tailoring crystallographic texture via cross rolling are presented. It is shown that a strong rotated Brass texture develops upon cross rolling in aluminium alloys which also remains intact during the subsequent recrystallisation annealing treatment. The governing mechanisms behind the evolution of deformation and recrystallisation texture are discussed in terms of effect of strain path on stability of deformation texture components and strain-induced boundary migration mechanism, respectively. In addition, the likelihood of rotated Brass texture having a positive effect on formability is discussed in terms of sluggish cross-slip criteria as the rotated Brass texture presents a unique scenario where cross-slip is inhibited along all the three principal directions.

Effect of Dynamic Recrystallization on the Transformed Ferrite Microstructures in HSLA Steel

The flow stress behavior of high-strength low-alloy (HSLA) steel at different true strains was studied using a hot compression test. The effect of dynamic recrystallization (DRX) on the transformed ferrite microstructures was investigated with electron backscatter diffraction (EBSD). The EBSD analysis indicated that the fraction of high-angle grain boundaries (HAGBs) and DRX increased with increasing true strain. The low-angle grain boundaries (LAGBs) were gradually transformed into HAGBs with increasing DRX degree. When the true strain was increased to 0.916, the fraction of HAGBs increased to 85% and the fraction of DRX increased to 80.3%. The relatively high fraction of HAGBs was related to the complete DRX. The dislocations and substructures in the tested steel at different true strains were characterized by transmission electron microscopy (TEM). TEM observation shows that the nucleation of the dynamically recrystallized grains occurred by the bulging of the original grain boundaries. The DRX nucleation mechanism of the HSLA steel is the strain-induced grain boundary migration mechanism.

Atomistic mechanism for vacancy-enhanced grain boundary migration

Mechanical behavior of polycrystalline materials is intimately connected to migration of grain boundaries, which in turn is dramatically impacted by the presence of defects. In this paper, we present atomistic simulations to elucidate the elementary mechanism that dictates the role of vacancies in enhancing grain boundary migration via shear-coupled normal motion. The minimum energy pathway and the associated energy barriers are calculated using the nudged elastic band method. Fully three-dimensional atomistic simulations provide excellent verification of the three-dimensional disconnection model and furnish quantitative evidence that vacancies facilitate grain boundary migration by weakening the line tension of a disconnection loop. It is also revealed that vacancies serve as energetically favorable sites for the nucleation of grain boundary disconnections, thereby inducing shear-coupled grain boundary migration.

Atomistic study of [formula omitted]-oriented interfacial crack behaviors in Mg bicrystal

In this study, 12¯10-oriented interfacial crack behaviors and mechanical properties of Mg bicrystal subjected to uniaxial tensile deformation are systematically studied by molecular dynamic (MD) simulation. According to the simulation results, the behaviors of 12¯10-oriented crack associated with {101¯1} twin boundary (TB), {101¯2} TB and basal/prismatic (BP) interface show a strong dependence on the crystalline orientation of Mg. For the bicrystals with TB, the brittle and ductile fracture rely on the competition of dislocation emission and cleavage at the crack-tip under low temperature. The brittle-ductile transition can be observed with the increased temperature. Interestingly, the BP transformation dominates the initial plastic deformation for Mg bicrystal with BP interface which accompanies the boundary migration under tension. The boundary migration mechanism is revealed to influence on the interfacial cracking behavior and the correlative mechanical property of materials.

Hot Deformation Behavior and Softening Mechanism of As-Cast S32750 Super Duplex Stainless Steel at Low and High Strain Rates

In the present study, the hot deformation behavior and softening mechanism of as-cast S32750 super duplex stainless steel were investigated at low and high strain rates by SEM, EBSD and TEM techniques. Hot compression tests were carried out at the temperature range of 950-1200 °C and strain rates of 0.1, 25 s−1 up to true strain of 1.0 using Gleeble-3800 thermomechanical simulator. The coupling mechanism of the two phases under different deformation conditions determined two types of flow curve characteristics: “yield-point-like” characteristic and typical dynamic recrystallization characteristic. It was found that dynamic recovery and dynamic recrystallization (DRX) were the dominant restoration mechanisms at low and high temperatures, respectively, in austenite phase. The DRX grains were mainly formed through strain-induced boundary migration mechanism located near the austenite/ferrite interface regions. A strong and positive correlation was observed between DRX fraction and the formation of Σ3 twin boundaries. There was an abnormal relationship between DRX fraction and strain rate. The DRX was promoted by increasing strain rate from 0.1 to 25 s−1 at 1200 °C due to weakened strain partitioning and increment in dislocation density, while the strain rate had a minimal effect on DRX fraction at 950 and 1050 °C. A high degree of DRX, approximately 60%, was found within the ferrite phase under all deformation conditions. The softening mechanism within ferrite could be explained by continuous DRX.

Molecular dynamics simulation of migration behavior of FCC-BCC atomic terrace-step phase boundary in iron-based alloy

The martensitic transformation between the high-temperature face-centered cubic (FCC) phase and the low-temperature body-centered cubic (BCC) phase in iron-based alloys has been studied for years, which plays a critical role in controlling microstructures and hence properties of the alloys. Generally, the BCC structure martensitic phase forms from the FCC parent phase, involving a collective motions of atoms over a distance less than the interatomic distance in the vicinity of the interphase boundary. Thus the structure of interphase boundary separating the FCC and BCC phases is the key characteristics to quantitatively understanding the mechanism and kinetics of martensitic transformation. Due to the difficulty in observing the atomic motions taking place at a velocity as high as the speed of sound, the experimental investigation on the migration of FCC/BCC interphase boundary during the transformation is as yet limited. Noteworthily, molecular dynamics (MD) simulation has been applied to studying the martensitic transformation, in particular for investigating the mobility of the FCC/BCC interphase boundary in iron. However, in most of the MD studies the atomistically planar interfaces of {111}<sub>FCC</sub> // {110}<sub>BCC</sub> are considered as the initial configuration of the interphase boundary between FCC and BCC phases, which is in contradiction to the high-resolution TEM observations. In fact, the FCC/BCC interphase boundary, which is known as the macroscopic habit plane, is a semi-coherent interface consisting of several steps and terrace planes on an atomic scale. In the present work, the atomic configuration of a terrace-step FCC/BCC interphase boundary of iron is built in terms of the topological model. The MD simulation is conducted to clarify the mechanism of interphase boundary migration in the FCC-to-BCC transformation. The results show that the FCC/BCC boundary migrates along its normal at the expense of FCC phase as a result of the lateral motions of the transformation dislocations. Meanwhile, the interphase boundary maintains the stable terrace-step structure during the transformation. Further examinations reveal that the transformation dislocations move steadily at a velocity as high as (2.8 ± 0.2) × 10<sup>3</sup> m/s, affecting the migration of the interphase boundary with a constant velocity of about (4.4 ± 0.3) × 10<sup>2</sup> m/s. The effective migration velocity of FCC/BCC interface exhibits dynamic properties consistent with the characteristic features commonly observed in a displacive martensitic transformation. Additionally, the motion of transformation dislocations gives rise to the macroscopic shape strain composed of a shear component <inline-formula><tex-math id="M3">\begin{document}$ {\varGamma _{{\rm{yz}}}} = 0.349$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20191903_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20191903_M3.png"/></alternatives></inline-formula> parallel to the boundary and a dilatation <inline-formula><tex-math id="M4">\begin{document}$ {\varGamma _{{\rm{zz}}}} = 0.053$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20191903_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20191903_M4.png"/></alternatives></inline-formula> normal to the boundary in the MD simulation, which is close to the crystallographic calculations by the topological model.

Grain growth in Nio–MgO and its dependence on faceting and the equilibrium crystal shape

The impact of faceting on grain growth was approached by model experiments in NiO–MgO. Grain growth rates were found to be 10 times higher in NiO compared to MgO. As the self-diffusion acoefficients differ by a factor of 250, grain growth in NiO is unexpectedly slow compared to MgO. Recently, the movement of steps was identified as the atomic mechanism of grain boundary migration. According to the equilibrium crystal shape, grain boundaries in NiO are more faceted. The faceted grain boundaries of NiO have fewer steps at the grain boundaries resulting in a relatively lower mobility.

In-situ observations and modeling of metadynamic recrystallization in 300M steel

The metadynamic recrystallization (MDRX) behaviors and microstructure evolution of 300 M steel are studied by hot compression experiments and high temperature laser scanning confocal microscopy (HTLSCM) experiments. The effects of insulation temperature, pre-strain rate and holding time on the MDRX behaviors are discussed in detail. Based on the statistical results of the microstructure evolution, the MDRX kinetics model and recrystallization (RX) grain size model of 300 M steel are established, respectively. It can be confirmed from the in-situ observations that strain-induced boundary migration mechanism and curvature-driven grain boundary migration mechanism are the main MDRX mechanisms of 300 M steel. The MDRX develops rapidly under the high temperature or low pre-strain rate. The insulation temperature has a more important effect on the MDRX rate. The RX grain size increases with the increased insulation temperature, pre-strain rate and holding time. Compared to the pre-strain rate, the effects of insulation temperature on the maximum MDRX fraction and RX grain size are negligible. Comparisons between the experimental and predicted results verify that the proposed MDRX kinetics models and RX grain size model can quantificationally reveal the MDRX behaviors of 300 M steel well.

Retaining {100} fibre texture in electrical steel via strain-induced boundary migration recrystallisation.

In this paper, the feasibility of retaining the preferred {100} fibre texture for non-grain oriented (NGO) electrical steel from the as-cast columnar {100} grains by encouraging recrystallisation via the strain induced boundary migration (SIBM) mechanism was investigated. Rolling with intermediate annealing stages before the final anneal was used to reduce the stored energy through recovery before the final recrystallisation step. A strong {100} fibre recrystallisation texture, i.e. 32% area fraction, was seen in the sample which was warm deformed to 1.6 strain with 5 intermediate annealing stages then annealed at 950 °C for 5 minutes (to observe the early stages of recrystallisation) or 1 hour. Whereas only 12 - 14% {100} fibre component was observed after 1.2 strain with a single or with no intermediate annealing stage. It was found that the {100} fibre recrystallisation texture was formed in regions adjacent to parental deformed grains with {100} fibre texture due to the SIBM recrystallisation mechanism. EBSD imaging analysis was used to follow the recrystallisation process in the same microstructural region using interrupted heat treatments and it was seen that the {100} texture grains carried very low stored energy after deformation, which meant the subgrains in these grains bulged into the surrounding grains with higher stored energy. Due to its slow recrystallisation nucleation rate, a low overall stored energy is preferred to encourage {100} texture recrystallised grains to form via the SIBM recrystallisation mechanism.

Improving the Magnetic Properties of Non-Oriented Electrical Steels by Secondary Recrystallization Using Dynamic Heating Conditions.

In the present work, we have used unconventional short-term secondary recrystallization heat treatment employing extraordinary high heating rate to develop coarse-grained microstructure with enhanced intensity of rotating cube texture {100}<011> in semi-finish vacuum degassed non-oriented electrical steels. The soft magnetic properties were improved through the increase of grains size with favourable cube crystallographic orientation. The appropriate final textural state of the treated experimental steels was achieved by strain-induced grain boundary migration mechanism, activated by gradient of accumulated stored deformation energy between neighbouring grains after the application of soft cold work, combined with steep temperature gradient during subsequent heat treatment under dynamic heating conditions. The materials in our experimentally prepared material states were mounted on the stator and rotor segments of electrical motors and examined for their efficiency in real operational conditions. Moreover, conventionally long-term heat treated materials, prepared in industrial conditions, were also tested for reference. The results show that the electrical motor containing the segments treated by our innovative approach, exhibits more than 1.2% higher efficiency, compared to the motor containing conventionally heat treated materials. The obtained efficiency enhancement can be directly related to the improved microstructural and textural characteristics of our unconventionally heat treated materials, specifically the homogenous coarse grained microstructure and the high intensity of cube and Goss crystallographic texture.

Atomistic simulations of grain boundary migration under recrystallisation conditions

We present details of an approach to simulating the migration of grain boundaries using classical molecular dynamics (MD) in a regime in which the boundary motion is driven by the presence of dislocation defects resulting from plastic deformation. By simulating the shrinkage of spherical grains of deformed material embedded in a perfect crystal, we create conditions in which grain boundary motion takes place on time scales accessible to MD simulations, in which the defect free energy acts as a driving force additional to the capillary force due to excess grain boundary energy. This approach is particularly flexible in allowing arbitrary choices of misorientation axis and angle and providing an interface in which all grain boundary plane orientations are represented. This flexibility is at the cost of additional complexity in the system under study and we establish an approach suitable for analysing the excess grain boundary energy and the components of the force for boundary motion on a local basis over the surface of the shrinking grain. We demonstrate that this approach is able to resolve variation in the grain boundary energy related to the intrinsic grain boundary dislocation network in lower angle boundaries and to detect local maxima in the driving force due to the dislocation structure within the deformed grain. We show that the variation in measured grain boundary velocities is accounted for by the excess energy of the dislocations. We further show that the presence of dislocations in the shrinking grain has a tendency to reduce the anisotropy in grain boundary behaviour. Indicative evidence is presented that the dislocation network may act as a focal point for mechanisms of grain boundary migration.

An EBSD investigation on the columnar grain growth in non-oriented electrical steels assisted by strain induced boundary migration

Recent studies regarding the development of high efficiency grain non-oriented (GNO) electrical steels demonstrate that development of columnar-grained microstructures prior to cold rolling is an efficient method to improve the magnetic behavior of the final product. However, the columnar morphology obtained by annealing of the hot-rolled bands was not developed after further processing by cold rolling and final annealing. Understanding the effects of plastic deformation on the development of such microstructures will help to promote them during the final annealing and improve even more the magnetic behavior of these materials. The present research reports the effects of small plastic deformations (0–25%) and annealing time (10–180 min) on the development of columnar microstructures in GNO electrical steels. The columnar grain growth is explained in terms of the strain induced grain boundary migration (SIBM) mechanism, based on the changes in the grain average misorientation (GAM) maps obtained by electron backscattering diffraction (EBSD).

Formation and disintegration investigation of fivefold annealing twins in copper nanoparticles

Fivefold annealing twins (FAT) have been assumed to form by grain boundary migration according to the Ashby model. However, the mechanism of boundary migration lacks experimental support. Moreover, the FAT disintegration process has never been reported. In our experiment, the complete formation and disintegration process of FAT in copper nanoparticles was investigated by in-situ transmission electron microscopy with high temporal and spatial resolution. The results show that FAT formation and disintegration arises from the layer-by-layer migration of twin boundaries, induced by partial dislocation emission. This experimental observation is significant to the basic understanding of twinning mechanisms.

Mechanisms of grain boundary migration and growth in nanocrystalline metals under irradiation

Atomistic simulations of radiation damage uncover how grain boundaries (GBs) migrate and coalesce under irradiation in bicrystalline Cu. Planar GB migration biased by defect cluster-mediated attraction first leads to slow and steady motion. Subsequently, adjoining GBs coalesce into curved surfaces, where curvature-driven migration with a velocity three orders of magnitude higher than that of a planar boundary dominates motion, triggering rapid grain growth. This study reveals the atomistic mechanisms of radiation-induced grain growth, and has practical implications towards engineering radiation-tolerant nanostructures.

Microstructural Evidence for Grain Boundary Migration and Dynamic Recrystallization in Experimentally Deformed Forsterite Aggregates

Plastic deformation of peridotites in the mantle involves large strains. Orthorhombic olivine does not have enough slip systems to satisfy the von Mises criterion, leading to strong hardening when polycrystals are deformed at rather low temperatures (i.e., below 1200 °C). In this study, we focused on the recovery mechanisms involving grain boundaries and recrystallization. We investigated forsterite samples deformed at large strains at 1100 °C. The deformed microstructures were characterized by transmission electron microscopy using orientation mapping techniques (ACOM-TEM). With this technique, we increased the spatial resolution of characterization compared to standard electron backscatter diffraction (EBSD) maps to further decipher the microstructures at nanoscale. After a plastic strain of 25%, we found pervasive evidence for serrated grain and subgrain boundaries. We interpreted these microstructural features as evidence of occurrences of grain boundary migration mechanisms. Evaluating the driving forces for grain/subgrain boundary motion, we found that the surface tension driving forces were often greater than the strain energy driving force. At larger strains (40%), we found pervasive evidence for discontinuous dynamic recrystallization (dDRX), with nucleation of new grains at grain boundaries. The observations reveal that subgrain migration and grain boundary bulging contribute to the nucleation of new grains. These mechanisms are probably critical to allow peridotitic rocks to achieve large strains under a steady-state regime in the lithospheric mantle.

Recrystallisation behaviour of a fully austenitic Nb-stabilised stainless steel.

We have performed an in-depth characterisation of the microstructure evolution of 20Cr-25Ni Nb-stabilised austenitic stainless steel during 1 h isochronal annealing up to 1100°C using scanning electron microscopy. This steel grade is used as cladding material in Advanced Gas-cooled fission reactors, due to its resistance to thermal creep and oxidation. The initial deformed microstructure undergoes recrystallisation via a strain-induced boundary migration mechanism, attaining a fully recrystallised microstructure at 850°C. A number of twins are observed in the vicinity of deformation bands prior to the start of recrystallisation. New Nb(C, N) particles form gradually in the microstructure, and the particle dispersion presents a maximum volume fraction of 2.7% at 930°C. At higher temperatures, the smaller particles become unstable and gradually dissolve in the matrix. Consequently, the Zener pinning pressure exerted on the grain boundaries is progressively released, triggering the growth of the austenite grains up to an average size of ∼47 μm at 1100°C. The observed temperature window for recrystallisation and grain growth can be predicted by a unified model based primarily on the migration of high- and low-angle grain boundaries. LAY DESCRIPTION: Austenitic stainless steel containing high percentage of chromium and nickel is currently used as fuel cladding material in the British Advanced Gas-cooled Reactors (AGR). This material has been chosen because of its high resistance to thermal creep and corrosion, both enhanced by the presence of a fine dispersion of carbonitrides precipitated during the cladding thermomechanical processing. During the time spent in the reactor core, few fuel cladding elements can become susceptible to local chromium depletion at grain boundaries, which is ascribed to the time evolution of the microstructural damage caused by the neutron bombardment in the reactor core. This depletion might increase the susceptibility of this steel to intergranular corrosion attacks during medium-to-long term storage of spent fuel elements in water ponds. The severity of the local chromium depletion depends not only on the irradiation conditions, but also on the grain boundary geometry. We have investigated the recovery, recrystallisation and grain growth of AGR stainless steel during 1 h annealing at selected temperatures relevant for the thermomechanical processing of the steel claddings, focusing on the formation and evolution of grain boundaries and second phases. These two features play a key role in the progression of the neutron damage and the subsequent development of local chromium depletion during reactor service operations. A deep understanding of the mechanisms and conditions behind their formation during the thermomechanical processing of the cladding material and their interaction with each other constitutes the foundation to evaluate, and potentially mitigate, the effect of irradiation on the cladding material.

Microstructure and mechanical performance tailoring of Ti-13Nb-13Zr alloy fabricated by selective laser melting after post heat treatment

The poor ductility of the selective laser melting (SLM)-fabricated parts may lead to the limitation of its applications. Therefore, in the present work, four types of post heat treatments of SLM-proceeded Ti-13Nb-13Zr were performed to tailor the microstructure and mechanical performances. In the super-transus heat treatment, the morphologies of grains are transformed from columnar to equiaxed because of the strain-induced grain boundary migration mechanism (SIBM), while in the sub-transus process, the columnar β grains are remained. In the water quenching process, high resolution transmission electron microscopy analysis revealed that the collapse of the partial layers in the (110) planes and [111] direction results in the original stacking sequence from …ABCABC… to a new stacking sequence …AB’AB’… and an w structure is formed, resulting in the highest ultimate (1155 ± 30 MPa), yield strength (874 ± 30 MPa) and unfortunately the lowest ductility (2.3 ± 0.2%). In the air cooling process, as the heat temperature declines from 750 °C to 650 °C, the electron back-scattered diffraction results show that the coarse and irregular α grains with strong {0001} and {211¯0} orientations are gradually transformed into fine acicular α crystals with the dominated {0001} orientation. In this condition, the residual stresses were released and the fine microstructure was largely retained, thus the high yield strength is maintained (799 ± 10 MPa) and the ductility is improved (7.2 ± 0.3%).

Isothermal annealing behaviour of nuclear grade 20Cr-25Ni austenitic stainless steel

We have performed an in-depth characterisation of the microstructure evolution of 20Cr-25Ni Nb-stabilised austenitic stainless steel during isothermal annealing at 930 °C using scanning and transmission electron microscopy. This steel grade is used as cladding material in advanced gas-cooled fission reactors, due to its resistance to thermal creep and water corrosion. The initial deformed microstructure undergoes recrystallisation via a strain-induced boundary migration mechanism, attaining a fully recrystallised microstructure after 120 s of annealing. The transition from low-to-high grain boundaries has already occurred after 15 s, together with an increase in the cube grain orientation at the expense of the S texture component. After 120 s, the grain boundary migration induces the formation of new fine Nb(C,N) particles, whereas the pre-existing particles become enriched in Ni and Si. The resulting particle population limits the grain growth in the austenitic matrix, based on the Zener pinning model, resulting in relatively small recrystallised austenite grains and a high density of high-angle and special coincidence-lattice-site grain boundaries, together with a large number of particle/matrix interfaces.