Strain-induced Boundary Migration Research Articles

Implication of initial grain size on DRX mechanism and grain refinement in super-304H SS in a wide range of strain rates during large-strain hot deformation

In this work, we studied the effect of initial grain size on the hot compression characteristics of super-304H austenitic stainless steel in a range of strain rates (0.001–1 s−1) at a fixed temperature of 1223 K. Analysis of the flow curves reveals that the flow stress is inversely proportional to the average sub-grain diameter in both coarse and fine-grained specimens at higher strain (≥0.5) levels. Further, the fine-grained specimen following deformation at a low strain rate (0.001 s−1) reveals the occurrence of both continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) mechanisms. In this condition, the CDRX is characterized by a progressive increase of boundary misorientation, while the DDRX is characterized by bulging and strain-induced boundary migration. In contrast, the CDRX characterized by the formation of microbands is majorly responsible for the grain refinement in the fine-grained specimen at higher strain rates (∼1 s−1) and in the coarse-grained specimen at all strain rates (0.001–1 s−1). The superposition of different DRX mechanisms leads to significant variations in grain refinement kinetics with strain rates and initial grain sizes. The findings of this investigation provide a foundation for the accurate control of the microstructures in the studied alloy with different initial grain sizes during the hot working at various strain rates.

The evolution of abnormally coarse grain structures in beta-annealed Ti-6Al%-4V% rolled plates, observed by in-situ investigation

The factors controlling the evolution of abnormally coarse grain (ACG) structures during standard β annealing of titanium-6Al%-4V% (Ti64) hot-rolled plates, have been investigated in more detail than previously, using both ex-situ through-thickness large area, and in-situ EBSD micro-texture observation. Starting with a typical, through thickness, α+β deformation texture distribution that gives rise to ACGs at the plate mid-section, each stage of the annealing process has been sequentially characterised and phenomenologically linked to the spatial texture evolution that occurs during transient heating through the α→β phase transformation and the subsequent isothermal hold at the β-annealing temperature. It was found that during the ramp heating phase, the rotated cube texture component greatly expands from just below the β transus by broad front strain induced boundary migration (BF-SIBM), driven by a disparity in stored energy with neighbouring texture bands from the α and γ fibres. This subsequently sets up the necessary conditions for unstable coarsening of surviving grain clusters with predominately near α fibre orientations during the super-transus hold, in a process with similar characteristics to discontinuous recrystallisation. Humphreys’ mean field model of the stability of cellular microstructures, effectively explains the process by which these grain clusters, which have a lower misorientation range relative to the rotated cube component matrix, have a higher probability of entering a discontinuous growth regime, compared to other components from the α and γ fibres. Furthermore, the conditions that give rise to the development of ACG structures could be linked to the high strain rate and temperature experienced at the central mid-thickness of rolled plates.

In Situ Observations and Microstructure Evolution Behavior of Static Recrystallization of Microalloyed Continuous Casting Slabs in a Solidification End Reduction Process

Solidification end reduction is a novel technique that can effectively refine the microstructure and improve the homogeneity in continuously cast steel. Herein, the static recrystallization (SRX) behaviors and microstructure evolution of microalloyed continuous casting slabs during the heavy reduction (HR) process are simulated and analyzed by means of high‐temperature laser scanning confocal microscopy (HT‐LSCM). According to the statistical results of microstructure, the insulation temperature has a more significant effect on the SRX fraction and recrystallization grain size compared with the strain rate, initial austenite grain size, and prestrain, and a modified SRX kinetic model is proposed. Furthermore, a deforming microstructure is observed at different strains and holding times with electron backscatter diffraction (EBSD), and it is found that the reduction of high‐angle boundaries (HAGBs, θ > 15°) of 25°–35° is related to strain‐induced boundary migration (SIBM). The stored deformation energy of HAGBs is higher than that of very low‐angle boundaries (VLAGBs, θ < 5°) by the detailed misinterpretation profile analysis of the block and lath boundaries. Finally, the combination of in situ observation and EBSD analysis shows that the stored deformation energy provides the driving force for the SRX process, which, in turn, consumes dislocations to stabilize the microstructure.

Micrograin structure evolution associated with grain boundary characteristics in grade 91 steel during long-term creep exposure

Micrograin structure evolution in ASME Grade 91 steel after 233,823 h of creep exposure at 600 °C was characterized after grain boundary classification in terms of misorientation angle (MOA), boundary energy, and lath martensite substructure. Under long-term creep with an applied stress of 5.9 MPa, micrograin coarsening occurs by annihilation of lath boundaries, consisting of hexagonal dislocation networks (HDN), thanks to the “knitting-out” of dislocations from the boundary, and the absorption of interior mobile dislocations into the boundary. This annihilation is expectedly faster when the original lath boundary shows a small MOA, and is decorated by little precipitation (MX carbonitrides and boundary M23C6 carbides). Under long-term creep with an applied stress of 86.7 MPa, micrograin coarsening occurs not only by lath boundary annihilation, but also by the migration of all other boundary types found in the lath martensite microstructure. This migration is described as a strain-induced boundary migration (SIBM), and mainly occurs when boundaries are relatively precipitation-free, and a gradient of stored internal energy is present between adjacent grains. Even within the same class of boundary, the progress of SIBM differs due to the local evolution of precipitates and the actual boundary energy.

Monte Carlo modelling of recrystallization in alpha Zirconium

A Monte Carlo based modelling framework combined with experimental EBSD microstructures was used in order to simulate partial recrystallization taking place during short annealing of moderately deformed hexagonal Zirconium and discuss the role played by Strain Induced Boundary Migration (SIBM). The experimental part started from low alloyed Zr702α which was channel-die-compressed along two different compression directions (Normal Direction - ND and Transverse Direction – TD of initial sample coordinate system) until 17% of strain was reached. As a result, two different microstructure and texture evolutions were witnessed and characterized using EBSD measurements. In particular mechanism of accumulation of local misorientations as well as formation of grain boundaries is analysed. Textures of grain boundary areas also provides important information about the way how new recrystallization grains are formed. Based on EBSD results, a SIBM model of recrystallization is proposed and compared with classical nucleation using Monte Carlo simulations. It is concluded that only simulations performed using the SIBM model provide results which are in a very good agreement with experimental data in terms of texture as well as microstructural evolution.

Mechanism of microstructure evolution and grain growth in friction stir welding of AA6061-T6 and AISI304 in air and water media

Grain size has a significant effect on the tensile strength and ductility of the friction stir welded (FSWed) joints. The size and morphology of grains, developed during friction stir welding (FSW) process, depends on amount of frictional heat generated and rate of deformation involved. In addition, grain size also depends on rate of cooling of the weld region. In the present work, evolution of microstructure in FSW of AA6061-T6 and AISI304 has been investigated by conducting FSW experiments in two different media i.e. air and water. In order to get a detailed insight of the microstructure evolution, stop-action technique was used where the experiments were interrupted by stopping the machine, followed by water quenching of the samples. Detailed study about the microstructure evolution, in different weld regions, has been performed by using Electron Back Scattered Diffraction (EBSD) and Transmission Electron Microscopy (TEM) techniques. The microstructures were characterized by analyzing the recrystallized grains, and grain growth. Particle stimulated nucleation and strain induced boundary migration were observed to be the recrystallization mechanisms. FSW in the water medium yields finer microstructure in the weld regions than air medium. Grain growth was observed after coalescences of sub-grains. Results have shown higher grain growth for FSW in air medium. Results have also shown transformations of Goss {110}<100> and B{110}<112> textured grains of base aluminum into Copper {112}<111>, E{111}<110> and F{111}<112> dominated textured grains at the stir zone (SZ). Water medium produced relatively highly shear textured grains than air medium at SZ.

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.

Control of deformation and annealing process to produce incoherent Σ3 boundaries in Hastelloy C-276 alloy

Hastelloy C-276 samples were cold-rolled with their thickness reduction of 7.5% and 15%, and then annealed at 1100 °C for 5–60 min in solid solution. The grain boundary character distributions (GBCDs) were characterized by a technique of electron back scatter diffraction (EBSD). Results indicated that, more convoluted Σ3 boundaries (i.e. incoherent Ʃ3 boundaries) are generated by reasonable annealing time in low strain (7.5%) sample during strain-induced boundary migration (SIBM) processing. They can induce higher Ʃ3n fraction and activate more interactions between special boundaries for GBE. Instead, more coherent Ʃ3 boundaries were produced by moderate strain (15%) and annealing treatment with the occurrence of static recrystallization (SRX). The recrystallized fine grains possess fewer incoherent Ʃ3 boundaries cannot promote to get more multiple twinning for optimization of GBCD. Therefore, reasonable deformation and annealing treatment parameters lead to more incoherent Ʃ3 boundaries would play an important role in the optimization of GBE.

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.

Grain Boundary Engineering of Strain-Annealed Hastelloy-X

The present study investigates the occurrence and effectiveness of the dissociation mechanism of Σ3 CSL boundaries into its variants such as Σ9 and Σ27a-b during strain-annealed grain boundary engineering (GBE) of Hastelloy-X. Multiple cold-rolling strain levels and annealing conditions are studied and it is observed that the density of ∑3 boundaries decreases proportionally to the amount of strain induced boundary migration (SIBM) during the GBE process. The dissociation mechanism of Σ3 annealing twins is activated at the onset of SIBM, causing an increase in the density of the Σ3n variants. It is shown that at high annealing times or temperatures, the rate of generation of CSL boundaries through dissociation mechanism is lower than their annihilation rate. It is further suggested that the dissociation mechanism of ∑3 boundaries during GB migration is more efficient when the amount of applied strain prior to annealing is kept low, thus promoting disruption of the random GB network.

Suppression of Intergranular Fracture in 7000 Series Aluminum Alloys

The 7000 series aluminum alloys suffer from intergranular fracture (IGF) that limits the use of the alloys, although they have highest strength among aluminum alloys. The types of IGF can be classified into two categories: (i) with smooth fracture surface showing practically no plastic deformation that takes place in hydrogen embrittlement and stress corrosion cracking, and (ii) with shallow and fine dimples on the fracture surface showing localized plastic deformation inside precipitate free zones. In this study, attempts have been made to suppress the IGF of both types by (a) controlling precipitate microstructure on grain boundaries by quench control and (b) controlling grain boundary morphology by strain induced boundary migration. The IGF of type (i) (hydrogen embrittlement) was successfully suppressed both by the two controlling processes.

Thermomechanical Processing Map in Retaining {100}//ND texture via Strain-Induced Boundary Migration Recrystallization Mechanism

The feasibility of establishing thermomechanical conditions to promote {100}//ND fiber texture via strain-induced boundary migration (SIBM) recrystallization mechanism in a non-grain oriented (NGO) electrical steel was investigated. Single-hit uniaxial compression at various temperatures and strains has been applied on Fe-6 wt pct Si to establish the relationship between stored energy and the softening mechanisms. Recovery only and recrystallization by SIBM or by subgrain growth (SGG) have been observed depending on the stored energy level. A strong {100}//ND fiber recrystallization texture, i.e., 45 pct area fraction, was seen in the sample which was deformed to 0.2 strain at 650 °C and then annealed at 1000 °C for 15 minutes, whereas only 13 pct {100}//ND fiber component was observed after 0.4 strain at 500 °C followed by the same annealing treatment. By examining the same microstructural region before and after annealing via EBSD, it has been shown that {100}//ND textured recrystallized grains were formed adjacent to the {100}// ND textured deformed matrix. Low stored energy has been shown to favor the formation of {100}//ND texture recrystallized grains via SIBM recrystallization mechanism attributed to its slow recrystallization nucleation rate. The results from the deformation studies have been used to suggest a processing window map concept to define the recovery, SIBM, and SGG regions for the starting as-cast columnar microstructure.

Refinement of the Ti-17 microstructure after hot deformation: Coupled mesoscale model

The thermo-mechanical processing of Ti-alloys comprises several steps where complex deformation and temperature cycles are achieved. In this work, the static recrystallization behaviour of a Ti-17 alloy is investigated using ex-situ characterization and in-situ synchrotron radiation experiments aiming to understand the operating mechanisms and to establish the recrystallization kinetics. Hot compression in the β- field for different strain rates is applied to provide different initial microstructures before isothermal heat treatments and continuous cooling. Strain induced boundary migration is the main operating nucleation mechanism during static recrystallization. A simple mesoscale model is proposed to couple the evolution of the microstructure during hot deformation followed by annealing considering the heterogeneity of deformation within the β-grains, for the nucleation and growth of grains and the formation of the substructure by static recovery. Electron backscattered diffraction measurements are used after isothermal annealing and continuous cooling treatments to validate the model. A strong influence of the localization of deformation in the vicinity of the prior β-high angle grain boundaries is observed and empirically implemented in the mesoscale model. The strong influence of the temperature is attributed to the difference in high angle grain boundary mobility during static recrystallization. Grain refinement is not successfully achieved up to the investigated strain due to the insufficient nucleation rate with respect to the growth rate. However, a homogenous recrystallized microstructure is observed. The model can predict the microstructure for any starting microstructure, even beyond the experimental validation.

Optimization of the grain boundary character distribution of pure copper by low-strain thermomechanical processing

Abstract Although thermomechanical processing has been widely used in grain boundary engineering, the relationship between thermomechanical parameters and grain boundary character distribution is still not well understood. In the present study, electron backscatter diffraction was used to study the grain boundary character distribution in pure copper after lowstrain thermomechanical treatments. It was found that the low-R coincidence site lattice frequency and grain size decreases in the following sequence, 10% compression &gt; 15% compression &gt; 5% compression, except for 700 0C. During thermomechanical treatments of copper, strain-induced grain boundary migration and grain growth may occur during annealing of 5% compressed copper, and recrystallization dominates during annealing of 15% compressed copper, while the annealing mechanism of 10% compressed copper changes from strain-induced grain boundary migration and grain growth to recrystallization when the annealing temperature exceeds 600 0C. The results indicated that during single-step low-strain thermomechanical treatments, straininduced grain boundary migration and grain growth would gradually change to recrystallization with the increase of pre-deformation level and annealing temperature. Among the three mechanisms, strain-induced grain boundary migration seems to be more effective than recrystallization and grain growth in the optimization of grain boundary character distribution, and it is suggested that this is due to the high boundary migration rate of strain-induced grain boundary migration.

High‐Temperature Deformation Behavior of a Novel Hot‐Rolled Transformation‐Induced Plasticity Alloy: Experiments, Constitutive Modeling, and Processing Maps

Hot deformation behavior of a novel hot‐rolled transformation‐induced plasticity (TRIP) alloy is studied by compression test in the temperature range of 1123–1323 K and strain rate range of 0.1–10 s−1, and the relation between flow stress and Zener–Hollomon parameter is systematically analyzed. The study indicates that the flow stress of experimental alloy increases with the decrease in deformation temperature and increase in strain rate. The value of stress exponent (n) and activation energy (Q) is 7.4 and 323.5 kJ mol−1. Processing maps developed using the dynamic material model at strains of 0.25, 0.40, and 0.55 predict optimum hot deformation temperature of 1273 K and strain rate of 0.8 s−1. The study of dynamic recrystallization (DRX) behavior under different deformation conditions by transmission electron microscope (TEM) suggests that two DRX mechanisms are operative—continuous dynamic recrystallization (CDRX) mechanism, which depends on high density of dislocations at the grain boundaries, and the second mechanism involves strain‐induced boundary migration (SIBM), governed by grain boundary bulging.

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.

Assessing the potential of sparsely nucleated recrystallized grains to lead grain boundary engineering during extending annealing in Alloy 600H

Alloy 600H is subjected to a lower extent of deformation (7.5% and 10%) followed by annealing at a higher temperature (1273 K and 1373 K) for different time duration (5 min to 600 min) to explore whether sparsely nucleated recrystallized grains can promote grain boundary engineering (GBE). Since strain-induced boundary migration (SIBM) may also occur in the aforementioned deformation-annealing conditions, this study is expected to explicate the relative role of recrystallization vs. SIBM on GBE. It is observed that the sparsely nucleated grains formed during annealing (60 min at 1273 K) of the specimen subjected to critical strain (i.e., 10%) for recrystallization does not lead to a GBE microstructure following extended annealing treatment even up to 600 min. This is due to the dominant nucleation of new strain-free grains during the extended annealing treatment, rather than the growth of initially nucleated recrystallized grains. As a result, the product microstructure post extended annealing treatment yields lower fraction of Σ3n boundaries and lesser numbers of grains per twin-related domains (TRDs) with concomitant increase in random high angle boundaries (HAGBs) connectivity, as corroborated by its higher fractal dimension (Df) and lower J2/1-J3 value. In contrast, significant SIBM has happened in the specimens subjected to 7.5% deformation (i.e., lower than the critical strain for recrystallization) when annealed at 1373 K. During such SIBM, the existing TRDs in the as-received specimen have grown allowing the occurrence of multiple twinning behind the migrating grain boundaries. The evolution of a higher fraction of Σ3n boundaries, promoted via SIBM, and complete suppression of recrystallization in these specimens lead to the significant disruption in random HAGBs connectivity as substantiated by their lower Df and a higher J2/1-J3 value.

Dynamic Recrystallization in Cu-Cr-Zr-Ti Alloy Under Large Plane Strain Conditions

The Cu-Cr-Zr-Ti alloy was subjected to single-hit plane strain compression tests in the temperature range of 923 K to 1073 K (650 °C to 800 °C) and at two different strain rates of 0.1 and 1 s−1. These tests were performed to ascertain the kinetics of microstructural evolution resulting by dynamic recrystallization (DRX) in the alloy under hot deformation conditions. The differences in the extent of DRX under various deformation conditions were established by plotting fractional softening as a function of strain and time. Further, the Avrami exponent was found to decrease with increase in deformation temperature. This was attributed to early onset of recrystallization leading to subsequent work hardening of the recrystallized grains and grain growth at higher temperatures which results in the loss of nucleation sites due to reduced grain boundary area. Microstructural observations made using optical microscopy, scanning electron microscopy (SEM), and electron backscattered diffraction (EBSD) revealed nucleation to occur in the following steps: elongation of grains perpendicular to the compression direction, bulging of elongated grain boundaries due to strain-induced boundary migration (SIBM), and subsequent subgrain formation behind the bulged boundary due to strain-induced low-angle boundaries. Finally, the equation describing the DRX kinetics under plane strain conditions for the alloy was established as $$ X = 1 - \exp \left[ { - 0.455 \times \left( {{{\left( {\varepsilon - \varepsilon_{\text{c}} } \right)} \mathord{\left/ {\vphantom {{\left( {\varepsilon - \varepsilon_{\text{c}} } \right)} {\varepsilon_{\text{p}} }}} \right. \kern-0pt} {\varepsilon_{\text{p}} }}} \right)^{2.84} } \right] $$ .