Strain-induced Boundary Migration Research Articles

Modeling length scale effects on strain induced grain boundary migration via bridging phase field and crystal plasticity methods

A coupled crystal plasticity and phase field modeling framework is employed to simulate polycrystalline grain growth via static strain induced grain boundary migration. Specifically, the effect of the average grain size as a microstructural length scale on the development of microstructure and texture in columnar–grained aluminum polycrystal is examined. During annealing, subsequent to plastic tensile straining, a nearly normal grain growth regime is observed for comparatively small microstructural length scales. However, for comparatively large microstructural length scales, abnormal grain growth and development of a specific texture emerge. Hence, grain growth is found to transition from normal to abnormal by enlarging the microstructural length scale. These observations are attributed to the fact that the competing driving forces for grain boundary migration scale differently with the microstructural length scale. In addition, for the case of large microstructural length scale, a transition from abnormal to normal grain growth at later annealing times is observed. This observation is attributed to the weakening of the strain induced boundary migration effects due to the initial rapid reduction of dislocation content.

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).

Grain size effect on twinning and annealing behaviors of rolled magnesium alloy with bimodal structure

Many wrought Mg alloys, which are manufactured by hot forming processes, have an inhomogeneous grain structure with locally different grain sizes. In the present study, variations in twinning and annealing behaviors caused by the difference in grain size within a material were investigated by subjecting a rolled Mg alloy with a bimodal grain structure to precompression and subsequent annealing. Activation of {10-12} twinning during precompression was found to be more pronounced in the coarse-grain region (CGR) than in the fine-grain region (FGR) owing to a lower twinning stress in the former. As a result, the twin structure formed in the FGR was distinctly different from that in the CGR; the CGR had a larger twinned area, stronger twin texture, fewer twin boundaries, and weaker grain refinement effect than the FGR. During the subsequent annealing, the microstructure of the FGR was greatly altered through grain growth driven by strain-induced boundary migration on account of its high internal strain energy, whereas that of the CGR remained nearly unchanged owing to its relatively lower internal strain energy. Consequently, the microstructural differences between the FGR and CGR induced by the precompression almost disappeared due to the subsequent annealing treatment, which resulted in a homogeneous microstructure.

Evaluation of dynamic development of grain structure during friction stir welding of pure copper using a quasi in situ method

By employing a quasi in situ method, we investigated the dynamic evolution of the grain structure considering the material flow, strain, and strain rate in the friction stir welding of pure copper. The tool ‘stop action’ and rapid cooling were employed and a brass foil was used as a marker to show the material flow path. The grain structure along the material flow path was characterised using electron backscatter diffraction. Static recrystallization occurs for the work-hardened base material in the preheating stage in front of the tool. In the acceleration flow stage, grains are significantly refined by plastic deformation, discontinuous dynamic recrystallization, annealing twinning during the strain-induced boundary migration and slight continuous dynamic recrystallization. In the deceleration flow stage, due to a strain reversal, the grain first coarsens, and is thereafter refined again. Finally, the hot-deformed material in the shoulder-affected zone is ‘frozen’ directly whereas that in the probe-affected zone undergoes significant annealing; thus, the recrystallized microstructure and 45°-rotated cube texture are obtained in the probe-affected zone.

Tailoring the crack-tip microstructure: a novel approach

ABSTRACTThis investigation demonstrates a way to innovatively modify the ferritic microstructure at a local scale, particularly at the failure prone area such as Charpy V-notch (CVN) root. Tensile pre-strain (PS) up to 6% and 12% were employed before annealing (An) the samples at 650°C for 15 minutes. Ferrite grain size increased sharply and gradually (along the distance ahead of the notch root) within the microstructurally modified region in 6–12% pre-strained and annealed samples, respectively. The critical strain which promotes strain-induced boundary migration (SIBM), was found to be 0.1 which resulted in abnormally coarse ferrite grains.

Influence of delta phase precipitation on static recrystallization of cold-rolled Inconel 718 alloy in solid solution treatment

Due to the improvement in fatigue strength of cold-rolled blades, the static recrystallization behavior of cold-rolled Inconel 718 alloy in the solid solution treatment was investigated with the methods of optical microscopy and electron backscattered diffraction. The effect of δ phase precipitation on the static recrystallization process was also analyzed. The results show that δ phase preferentially precipitates comparing to static recrystallization. Affecting by the δ phase preferential precipitation process, sub-grain migration is the main nucleation mechanism in the static recrystallization of cold-rolled Inconel 718 alloy at 980 °C. Meanwhile, strain-induced boundary migration also exists as an aided nucleation mechanism when the rolling reduction is less than 30%.

Evolution of Grain Boundary Character Distribution in Pure Copper during Low-Strain Thermomechanical Processing

In order to get optimal grain boundary character distribution (GBCD) and grain boundary properties, thermomechanical processing (TMP) is usually adopted in grain boundary engineering. However, the mechanism behind the TMP treatments and GBCD optimization is still unclear. The present study has conducted a series experiments involving low-strain TMPs to study the relationship between TMP parameters and the behind microstructural evolution. The experimental results indicate that in the scope of low-strain TMP, strain induced boundary migration (SIBM) is the most effective process for GBCD optimization. Besides, SIBM and grain growth would gradually transfer to recrystallization with the increase of pre-deformation level and annealing temperature. Further quasi in-situ EBSD results infer that SBIM is activated locally in some region with high stored energy, and further gradual initiation of SIBM from one region to another contributes to the gradual increase of special boundaries with annealing time.

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.

Understanding the effects of recrystallization and strain induced boundary migration on ∑3 twin boundary formation in Ni-base superalloys during iterative sub-solvus annealing

The formation of ∑3 twin boundaries in an experimental low stacking fault Ni-based superalloy containing 24 wt% Co was investigated. Normalized microstructures were hot deformed to strain limits of 0.10 and 0.50 at a strain rate of 0.1/s and a temperature of 1020 °C. The hot-deformed microstructures were characterized using electron backscatter diffraction, and were subsequently subjected to a series of iterative sub-solvus anneal heat treatments at 1100 °C for one to two minutes. By characterizing the changes in the microstructure and grain boundary character distribution as a function of annealing time, this investigation sought to quantify and detail how twin boundary generation and growth occurs during annealing. This behavior was investigated in the same alloy, but with two distinct as-deformed starting microstructures associated with the different strain limits. The starting microstructures possessed varying levels of intragranular misorientation networks, which were found to have greatly affected the resultant formation of ∑3 annealing twin boundaries. Strain induced boundary migration (SIBM) was observed to have been restricted in the iteratively annealed sample deformed to a strain of 0.10. Due to the sub-solvus heat treat temperature, grain boundary precipitates were present in the microstructure which limited the mobility of the grain boundaries and suppressed grain growth. As a result, the expansion in the length fraction of ∑3 twin boundaries was limited as a function of annealing time. The sample deformed to 0.50 strain, however, was shown to have recrystallized both upon deformation and subsequent annealing. Although recrystallization was found to largely remove much of the pre-existing twin boundaries, the mobility of the newly formed boundaries during growth of the recrystallized grains resulted in a dramatic increase in both the density and length fraction of twin boundaries.

Effect of temper rolling and subsequent annealing on texture development and magnetic permeability of semi-processed electrical steel with 2.3 wt.% Si

Abstract The effect of temper rolling and subsequent annealing on texture development and magnetic properties of semi-processed non grain oriented electrical steel has been investigated. The result shows that 5% temper rolling and final annealing at 850°C resulted in strong α-fiber component. The etch-pit technique shows that strain induced boundary migration is responsible for new texture component augmentation. Vibrating sample magnetometery reveals that maximum and minimum magnetic permeability are related to rolling and transverse directions, respectively. Average magnetic permeability is improved to some extent as a result of temper rolling and subsequent annealing.

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.

Microstructural Evolution in High-Strain-Rate Deformation of Ti-5Al-5Mo-5V-1Cr-1Fe Alloy.

To study the microstructural evolution in high-strain-rate shear deformation of Ti-5Al-5Mo-5V-1Cr-1Fe (Ti-55511) alloy, a series of forced shear tests of hat-shaped specimens have been conducted using a split Hopkinson pressure bar combined with the “strain-frozen” technique. A localized shear band is induced in Ti-55511 alloy in these tests. The experimental results demonstrate that the flow stress in hat-shaped specimens remains constant (about 600 MPa) and is independent of punching depth. The width of the adiabatic shear band increases with increasing punching depth and tends to saturate at 30 μm, and the estimation of the adiabatic shear band (ASB) width in hat-shaped (HS) specimens has been modified. Relying on the experimental results, thermal softening has a minor effect on the onset of the adiabatic shear band and dynamic recrystallization formation, and the nucleation mechanism for dynamic recrystallization is strain-induced boundary migration and subgrain rotation and coalescence. In addition, we suggest the concept of adhesive fracture as the dynamic failure mechanism for Ti-55511 alloy.

Development of microstructure and texture during single and multiple pass friction stir processing of a strain hardenable aluminium alloy

In the present study, microstructure and texture development during single and multiple pass friction stir processing (FSP) of a strain hardenable wrought AlMg alloy (AA5086) was investigated. Subtle differences were observed while comparing with heat treatable alloys in the nucleation mechanism of the recrystallized microstructure observed in the nugget zone. Strain induced boundary migration was the dominant mechanism of microstructure evolution in the alloy, which influenced the crystallographic texture development by weakening it. Micro-texture measurements reveal variations in the crystallographic texture along the thickness of the sample. Recrystallization texture components were observed in the nugget zone indicative of a pronounced static recrystallization in the alloy as compared to the heat treatable alloys. Bulk texture measurements within the nugget zone of the optimally processed sample reveal a relatively dominant C component of shear texture. Average grain size in the nugget zone remained the same and the bulk crystallographic texture components were retained during multiple-pass FSP. The lower strain energies involved and the enhanced recovery processes due to the high temperature materials processing of the alloy during FSP resulted in a stable microstructure and texture. In summary, FSP could be promoted as a competent and suitable secondary processing technique for the bulk production of ultra-fine-grained materials in strain hardenable aluminium alloys.

Cube texture evolution and magnetic properties of 6.5 wt% Si electrical steel fabricated by surface energy and three-stage rolling method

0.1 mm-thick 6.5 wt% Si electrical steel sheets with strong cube texture were successfully produced by surface energy effect and three-stage rolling method. The texture evolution and magnetic properties during the processing were briefly investigated, special attention was paid on the development of cube texture. The study shows that cube texture component was originated from the deformed cube grains existed in center layer of hot rolled sheet. During three-stage rolling and intermediate annealing, the cube texture component was enhanced. Attributed to the frequencies as well as size advantages of cube recrystallized grains during nucleation, a homogeneous microstructure associated with cube texture was produced after primary recrystallization. The mechanism responsible for the microstructure and texture of primary recrystallization was explained by strain induced boundary migration. After high temperature annealing for a long time, complete secondary recrystallization microstructure consisting of entirely large cube grains was produced by means of the surface energy effect. Both the strong cube texture and proper grain size enable final annealed sheets have excellent magnetic properties. The present study provides a new efficient way to optimize the recrystallization texture and improve the magnetic properties of double grain-oriented 6.5 wt% Si electrical steel.

Utilization of hot deformation to trigger strain induced boundary migration (SIBM) in Ni-base superalloys

The effect of strain on the resultant microstructure of an experimental low stacking fault energy Nickel based superalloy containing 24 wt. pct. Co was investigated. Billets subjected to a preliminary heat treatment at 1110 °C were compressed to strain limits of 0.15 and 0.5 at strain rates ranging from 0.1/s to 0.01/s and temperatures at 1020 °C and 1060 °C. The as-deformed microstructures were assessed and characterized using electron backscatter diffraction, as were microstructures corresponding to a super-solvus anneal heat treatment at 1160 °C for one hour. This study sought to identify a critical strain limit at which conditions indicative of Strain induced boundary migration (SIBM) could be effectively triggered for the experimental Ni-based superalloy over a set range of thermal-mechanical parameters. Microstructures corresponding to SIBM were then compared to more extensively deformed billets which contained notable fractions of dynamically recrystallized grains to quantify differences in the length fraction and density of ∑3 twin boundaries of the respective microstructures. Though billet samples deformed to both 0.15 and 0.5 contained notable magnitudes of stored strain energy, microstructures deformed to 0.15 were noted as having maintained larger length fractions of ∑3 twins due to a predominant absence of dynamic recrystallization. Annealed samples originally deformed to 0.15 yielded annealing twin length fractions as high as 59% when compared a sample deformed to the 0.5 strain limit under equivalent thermal-mechanical conditions that resulted in a twin length fraction of 50%. Although samples deformed to the lower strain limit exhibited higher length fractions of annealing twins, samples deformed to the higher strain limit of 0.5 were noted to yield ∑3 densities as high as 0.65 μm−1, whereas the annealed sample deformed under equivalent thermal-mechanical parameters to the 0.15 strain limit produced ∑3 densities as low as 0.32 μm−1.

The effect of thermomechanical controlled processing on recrystallisation and subsequent deformation-induced ferrite transformation textures in microalloyed steels

The evolution of texture components for two experimental 0.06 wt% C steels, one containing 0.03 wt% Nb (Nb steel) and the second containing both 0.03 wt% Nb and 0.02 wt% Ti (Nb–Ti steel), was investigated following a new thermomechanical controlled process route, comprising first deformation, rapid reheat to 1200 °C and final deformation to various strains. Typical deformation textures were observed after first deformation for both steels. Following subsequent reheating to 1200 °C for various times, the recrystallisation textures consisted primarily of the α- leftlangle {011} rightrangle //RD texture fibre with a weak γ-{111}//ND texture fibre, similar to deformation textures, indicative of the dominance of a strain-induced boundary migration mechanism. The texture components after finish deformation were different from the rough deformation textures, with a strong α- leftlangle {011} rightrangle //RD texture fibre at the beginning, and then the strong peaks move to (111) leftlangle {1bar{2}1} rightrangle and (111) leftlangle {bar{1}bar{1}2} rightrangle textures due to the deformation-induced ferrite (DIF) transformation. The effect of Ti on the recrystallisation textures and deformation textures has also been analysed in this study. The results illustrate that Ti significantly influences the γ-{111}//ND texture fibre. Finally, the textures after deformation and recrystallisation in the austenite were calculated based on the K–S orientation relationship between the austenite and ferrite. This allowed the understanding of the mechanism of recrystallisation between first and final deformation and the DIF textures during phase transformation.

Microstructure and Texture Development in Alpha-Brass Using Repetitive Thermomechanical Processing

Repetitive thermomechanical processing (TMP) has been applied to evaluate the effect of compression strain and temperature on microstructure and texture development in an alpha-brass alloy. Two TMP schemes were employed using four cycles of low-strain compression (ε = 0.15) and annealing, and two cycles of medium-strain compression (ε = 0.3) and annealing. Compression tests were conducted at 25, 250, and −100 °C, while annealing was made at 670 °C for 10 min. Examination by electron backscattered diffraction (EBSD) indicated that the low-strain scheme was capable to increase the fraction of Σ3n boundaries (n = 1, 2, and 3) with increasing cycles, producing maximum fraction of 68%. For medium-strain scheme, a drop in the fraction of Σ3n boundaries occurred in cycle 2. Reducing compression temperature lowered the fraction of Σ3n boundaries for low-strain scheme, while it enhanced the formation of Σ3n boundaries for medium-strain scheme. Annealing textures showed that 〈101〉 compression fiber was strongly retained for samples processed by small-strain scheme, while weakening of 〈101〉 fiber accompanied by the formation of 〈111〉 recrystallization fiber occurred for the medium-strain scheme. The results indicate that the increase in strain energy stored during compression, via increasing strain and/or decreasing deformation temperature, is responsible to favor recrystallization twinning over strain-induced grain boundary migration (SIBM). Both mechanisms are important for the formation of Σ3n boundaries. Yet, SIBM is thought to strongly promote regeneration of Σ3n boundaries at higher TMP cycles. This is consistent with the development of microstructure and texture using small-strain scheme.

A critical evaluation on efficacy of recrystallization vs. strain induced boundary migration in achieving grain boundary engineered microstructure in a Ni-base superalloy

A critical evaluation of recrystallization vs. strain induced boundary migration (SIBM) as the mechanism responsible behind the grain boundary engineering (GBE) in alloy 617 is made. Towards this hot deformation processing (in the temperature range of 1173–1473 K and strain rates of 0.001–10 s−1) as well as GBE-type iterative processing is performed on solution-annealed specimen. GBE-quantifying parameters such as Σ3n fraction, triple junction distribution, twin-related grain size ratio, twin-related domain (TRD) parameters and fractal dimension have been utilized to quantify the extent of GBE in the processed microstructures. Occurrence of dynamic recrystallization (DRX) during hot deformation processing does not induce notable multiple twinning leading to a microstructure analogous to that of solution-annealed condition even after complete DRX. Following GBE-type iterative processing, GBE microstructure has only been achieved when prolific multiple twinning occurred due to activation of SIBM leading to a high fraction of Σ3n boundaries (∼79%) with most of the twins being part of the grain boundary network. Consequently, an increasing proportion of J2 and J3 triple junctions with a concomitant increase in both the average number of grains per TRD and twin-related grain size ratio are achieved. However, occurrence of static recrystallization during GBE-type processing has led to smaller TRD size and copious random boundaries connectivity as confirmed by fractal analysis thus clearly suggesting that SIBM, not recrystallization catalyzes the profusion of Σ3n boundaries. As GBE relies on the multiple twinning mechanisms promoted via SIBM, recrystallization needs to be circumvented for successful attainment of GBE microstructure.

Modeling the viscoplastic behavior and grain size in a hot worked Nb-bearing high-Mn steel

In this study, hot compression tests were performed on a Nb-containing high-Mn steel at temperatures in range of 850–1150°C and strain rates of 0.001–1s−1. The stacking fault energy of the material at different conditions, i.e. without Nb, with Nb and C in solution and with Nb and C as NbC, was calculated by JMat Pro software as 10.9, 10.15 and 10mJ/m2. The software predicted that the stacking fault energy reaches 200–250mJ/m2 as temperature rises to the deformation range. A new model with interactive effects of temperature, strain and strain rate was constructed on the basis of Johnson-Cook equation to predict the experimental flow curves satisfactorily. Optical microscopy and SEM-EBSD observations showed that strain-induced boundary migration (SIBM) is the major mechanism of dynamic recrystallization giving rise to a necklace structure at the early stages of recrystallization. It was found that very fine layers of lath martensite form within the large austenitic grains during post-deformation quenching. However, the martensite layers were less discernible in the small DRX grains. The dynamically recrystallized grain size decreased with increase in strain rate or decrease in temperature. The model developed based on Bate and Hutchinton approach showed that the recrystallized grain size has an indirect relationship with the flow stress level.

Direct observation of recrystallization mechanisms during annealing of Cu in low and high strain conditions

Recrystallization and subsequent twin formation in >99.99% pure copper is investigated by in situ annealing in a scanning electron microscope. Distinct microstructural evolution mechanisms are observed for specimens deformed using 5% and 25% cold-rolled reductions. Upon annealing the former, growth and twinning of existing grains via strain-induced boundary migration is observed while the latter exhibits nucleation and growth. Serial sectioning indicates that recrystallization in the low strain condition is surface dominant while nucleation occurs through thickness in the high strain condition. Geometrically necessary dislocation densities are calculated and mapped to identify the localized driving pressures for each recrystallization mechanism.