Strain-induced Grain Boundary Migration Research Articles

Preparation of ultra-high ductility and high strength Mg-Sn-Zn-Zr alloy by differential thermal ECAP (DT-ECAP) induced heterogeneous structure

Developing high-ductility magnesium (Mg) alloys has become an imminent issue for their wide application. In this work, a new Mg-Sn-Zn-Zr alloy with ultra-high ductility (elongation, El. over 40 %) and high ultimate tensile strength (UTS, ∼309–354 MPa) was prepared by a novel differential thermal equal-channel angular pressing (DT-ECAP). Heterogeneous structures, including bimodal grain structures and inhomogeneous distribution of second phases composed of banded structure and particle free zone (PFZ), were induced by DT-ECAP process. Based on the results of electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and selected area electron diffraction (SAED), the bimodal grain structure originated from incomplete dynamic recrystallization (DRX) dominated by Zener pinning, strain-induced grain boundary migration (SIBM) and the limitation of polycrystallization due to lower dislocation density. Meanwhile, the bimodal distribution of second phases was highly associated with the defect density and initial structure. More importantly, the enhanced strength of DT-ECAPed alloys can be primarily attributed to hetero-deformation induced (HDI) strengthening, grain boundary strengthening, and precipitation strengthening. Moreover, HDI hardening, texture weakening or randomizing activation of non-basal slip, high density of dislocations in sub-structures, and twining induced superior work-hardening effect, which was highly responsible for the ultra-high ductility in sixth pass (6P) alloy. The current work provides a novel DT-ECAP process for inducing heterogeneous structure and offers beneficial insight into the development of ultra-high ductility and high strength for rare-earth-free Mg alloys via a combination of HDI strengthening and hardening and other vital mechanisms.

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

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

Modeling discontinuous dynamic recrystallization containing second phase particles in magnesium alloys utilizing phase field method

Pre-existing second phase particles can induce particle stimulated nucleation (PSN) and pinning effect on grain boundaries during DRX process of magnesium alloys. The interaction among pinning effect, PSN mechanism and strain-induced grain boundary migration (SIBM) nucleation mechanism imposes challenge in quantitative study of microstructure evolution. A phase field model that describes discontinuous dynamic recrystallization process considering second phase particles (PF-DDRX-SP) which mainly distribute along grain boundaries is developed. The second phase order parameter with diffusion interface is introduced to the free energy function. With considering the pinning effect of second phase particles on grain boundaries, the grain boundary energy couples the interaction between second phase and grains. The effect of second phase on dislocation density evolution is considered in the model, and the PSN and SIBM mechanism are both executed by limiting the nucleation position to the grain boundary and the second phase interface. For the validation, the initial topology consistent with initial microstructure is applied to the simulation of DRX kinetics and flow behavior of AZ80 magnesium alloy, and great reliability and accuracy of developed model is indicated. Furthermore, through the model, the microstructure evolution with different particle size and with different volume fraction of round particle during DRX process were simulated. The results show that particles with higher dispersion reduce the number of DRX nuclei, which indicate that second phase particles suppress the nucleation at grain boundaries because of the pinning effect. More importantly, the proposed model could also quantitatively predict the relationships between the parameters of second phase particles and DRX behaviors, and enable to optimize the initial second phase structure in a uniform grain structure during thermomechanical process.

Kinking facilitates grain nucleation and modifies crystallographic preferred orientations during high-stress ice deformation

Kinking is an important strain-accommodating process during crystal plastic deformation under relatively large stresses and may influence the mechanical properties of the Earth's lithosphere and planetary cryosphere. To better understand the origins, mechanisms, and microstructural effects of kinking, we present detailed microstructural analyses of coarse-grained (∼1300 μm) ice samples deformed under uniaxial compression at -30°C. Deformed samples show elongated (aspect ratio ≥ 4) kink domains developing within or at the tips of remnant original grains (≥300 μm, aspect ratio < 4). Small, equiaxed subgrains also develop along the margins of remnant grains. Moreover, many remnant grains are surrounded by mantles of small, recrystallized grains (<300 μm, aspect ratio < 4). Together, these observations indicate that grain nucleation is facilitated by both kinking and dynamic recrystallization. Low- (<10°) and high-angle (mostly >10°, many >20°) kink bands within remnant grains have misorientation axes that lie predominantly within the basal plane. The c-axes of most kink domains are oriented sub-perpendicular to the sample compression axis, indicating that kinking may produce or modify a crystallographic preferred orientation. Kink band densities are highest within remnant grains that have basal planes sub-parallel to the compression axis—these data are inconsistent with models suggesting that (if kinking is the only strain-accommodating process) there should be higher kink band densities within grains that have basal planes oblique to the compression axis. One way to rationalize this inconsistency between kink models and experimental observations is that kinking and dynamic recrystallization are both active during deformation, but their relative activities depend on the crystallographic orientations of individual grains. For grains with basal planes sub-parallel to the compression axis, strain-induced grain boundary migration (GBM) is inhibited, and large strain incompatibilities can be relaxed via kinking when other processes such as subgrain rotation (SGR) recrystallization are insufficient. For grains with basal planes oblique to the compression axis, strain-induced GBM might be efficient enough to relax the strain incompatibility via selective growth of these grains, and kinking is therefore less important. For grains with basal planes sub-perpendicular to the compression axis, kink bands are seldom observed—for these grains, the minimum shear stress required for kinking exceeds the applied compressive stress, such that kinks cannot nucleate.

Using grain boundary irregularity to quantify dynamic recrystallization in ice

Dynamic recrystallization is an important mechanical weakening mechanism during the deformation of ice, yet we currently lack robust quantitative tools for identifying recrystallized grains in the “migration” recrystallization regime that dominates ice deformation at temperatures close to the ice melting point. Here, we propose grain boundary irregularity as a quantitative means for discriminating between recrystallized (high sphericity, low irregularity) and remnant (low sphericity, high irregularity) grains. To this end, we analysed cryogenic electron backscatter diffraction (cryo-EBSD) data of deformed polycrystalline ice, to quantify dynamic recrystallization using grain boundary irregularity statistics. Grain boundary irregularity has an inverse relationship with a sphericity parameter, Ψ, defined as the ratio of grain area and grain perimeter, divided by grain radius in 2-D so that the measurement is grain size independent. Sphericity (Ψ) typically decreases with increasing grain size, up to a threshold grain size, above which Ψ either plateaus (at temperature, T< -10 °C) or increases much more gradually (at T≥ -10 °C). There is no apparent relationship between grain boundary sphericity and grain c-axis orientation even at very high temperatures (-4 and -5 °C), where GBM dominants, suggesting little crystallographic control on the activity of grain boundary migration (GBM). Decreasing sphericity up to the threshold grain size can be explained by newly-formed, small, spherical recrystallized grains growing via strain-induced GBM and thereby developing increasingly irregular grain boundaries. We suggest that the plateau (or gradual decrease) in sphericity at larger sizes represents a population of original grains (i.e., remnant grains) that becomes increasingly irregular (at similar rates) due to the balance between GBM and nucleation. In this interpretation, the threshold grain size represents the largest grain size reached by a growing recrystallized grain by the end of an experiment. Thus, grain boundary irregularity provides a means for discriminating between recrystallized and remnant grains—a capability that is potentially useful for evaluating dynamic recrystallization processes in ice deformed at temperatures close to the melting point. The threshold grain size and experiment duration can be used to calculate the rates of recrystallization and grain size evolution associated with GBM. Grain size evolution rates are similar at high and low temperatures, suggesting similar GBM rates. Previous studies show that grain boundary mobility decreases with decreasing temperature. The driving force of GBM, on the other hand, has a positive correlation with stress, which increases with a decreasing temperature if strain rate remains unchanged. The balance between boundary mobility and driving force is likely the cause of similar GBM rates between high and low temperatures.

Using Grain Boundary Irregularity to Quantify Dynamic Recrystallization in Ice

Dynamic recrystallization is an important mechanical weakening mechanism during the deformation of ice, yet we currently lack robust quantitative tools for identifying recrystallized grains in the “migration” recrystallization regime that dominates ice deformation at temperatures close to the ice melting point. Here, we propose grain boundary irregularity as a quantitative means for discriminating between recrystallized (high sphericity, low irregularity) and remnant (low sphericity, high irregularity) grains. To this end, we analyzed cryogenic electron backscatter diffraction (cryo-EBSD) data of deformed polycrystalline ice, to quantify dynamic recrystallization using grain boundary irregularity statistics. Grain boundary irregularity has an inverse relationship with a sphericity parameter, ψ, defined as the ratio of grain area and grain perimeter, divided by grain radius in 2-D so that the measurement is grain size independent. Sphericity (ψ) typically decreases with increasing grain size, up to a threshold grain size, above which ψ either plateaus (at temperature, T < -10 °C) or increases much more gradually (at T ≥ -10 °C). There is no apparent relationship between grain sphericity and grain c -axis orientation, suggesting little crystallographic control on the activity of grain boundary migration (GBM) that governs grain boundary irregularity. Decreasing sphericity up to the threshold grain size can be explained by newly-formed, small, spherical recrystallized grains growing via strain-induced GBM and thereby developing increasingly irregular grain boundaries. We suggest that the plateau (or gradual decrease) in sphericity at larger sizes represents a population of original grains (i.e., remnant grains) that becomes increasingly irregular (at similar rates) due to GBM. In this interpretation, the threshold grain size represents the largest grain size reached by a recrystallized grain at the end of each experiment. Thus, grain boundary irregularity provides a means for discriminating between recrystallized and remnant grains—a capability that is potentially useful for evaluating dynamic recrystallization processes in ice deformed at temperatures close to the melting point. The threshold grain size and experiment duration can be used to calculate the rates of recrystallization and grain size evolution associated with GBM. Grain size evolution rates are similar at high and low temperatures, suggesting similar GBM rates. Previous studies show that grain boundary mobility decreases with decreasing temperature. The driving force of GBM, on the other hand, has a positive correlation with stress, which increases with a decreasing temperature if strain rate remains unchanged. The balance between boundary mobility and driving force is likely the cause of similar GBM rates between high and low temperatures.

New perspectives on twinning events during strain-induced grain boundary migration (SIBM) in iteratively processed 316L stainless steel

Characterization of microstructure and grain boundary character distribution (GBCD) during iterative thermo-mechanical processing (TMP) of 316L stainless steel were done using electron backscatter diffraction (EBSD). Results from EBSD scans were analyzed in terms of the fraction of special boundaries, their deviation from ideal misorientation, connectivity of random high-angle grain boundaries, and grain size. In order to understand the efficacy of the iterative processing route leading to a well-connected twin boundary network, additional analysis was done in terms of twin-related domain statistics and triple junction distribution. Results from these analyses show that initial iteration results in a microstructure with insufficient twin boundary density, and a poor twin boundary network. Although the next iteration leads to a small increase in twin statistics, it also does not significantly improve the statistics of favorable grain boundaries and their network. However, the fourth iteration of the processing results in a large improvement in both the population and distribution of favorable grain boundaries. All the subsequent iterations were found to result in microstructural deterioration in terms of both population and network of these grain boundaries. Based on the analysis, role of underlying mechanisms such as strain-induced grain boundary migration and static recrystallization in the effective optimization of GBCD was analyzed. The present research gives important insights into these mechanisms and their control during TMP in order to achieve an engineered microstructure.

Effects of Combined Additions of Mn and Zr on Dispersoid Formation and Recrystallization Behavior in Al-Zn-Mg Alloys

The effects of Mn or Zr additions on the dispersoid formation and recrystallization behavior in Al-Zn-Mg alloys were investigated. The combined additions of Mn and Zr are expected to increase the uniformity of dispersoids and increase the sizes of Al3Zr and Al6Mn dispersoids but decrease their volume fraction, which reduces the pinning force of dispersoids to grain boundaries. Al6Mn dispersoids are incoherent with the Al matrix and result in lower pinning force compared to that of Al3Zr dispersoids. Consequently, the inhibiting effect of combined additions of Mn and Zr on recrystallization is not as strong as that due to Zr addition alone. After hot rolling, the long axis of most ellipsoidal Al6Mn dispersoids tends to be parallel to the rolling direction (RD), resulting in a larger pinning force in the normal direction (ND) than in the RD; therefore, the aspect ratios of recrystallized grains are larger in Mn-containing alloys than in Mn-free alloys. Further, the addition of Mn in Al-Zn-Mg alloy can lead to coarse α-AlFeMnSi constituent particles and, therefore, a higher area fraction of second phase in Mn-containing alloys; as a result, particle-stimulated nucleation (PSN) is the dominant recrystallization mechanism in Mn-containing alloys, while both PSN and strain-induced grain boundary migration (SIBM) exist in Zr-containing alloys.

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).

Plastic strain-induced grain boundary migration (SIBM) in pure aluminum: SEM in-situ and AFM examinations

Plastic strain induced grain boundary migration (SIBM) is investigated by means of in-situ SEM experiments and AFM surface observations in the case of 4N pure aluminum. The study focuses on two polycrystalline samples obtained through different thermo-mechanical treatments that provide different initial (grain size and orientation) microstructures with different evolutions during heating. A total of 77 grain boundaries (GBs) were characterized from both samples. Evaluation of GB displacements was allowed by determining fixed points on initial and final EBSD maps and marks from thermal grooving along GB contours. Some sub-surface final examinations ensured the surface ones being pretty well representative of the bulk behaviour. The GB displacements were related to their geometry and to their initially available migration driving force P, the two main contributions of which were estimated. The boundary curvature contribution Pc is estimated from SEM observations and the so-called "stored energy" contribution PΔρ (that results from the differential of dislocation densities Δρ across the boundary) is estimated using a crystal plasticity modeling within a homogenization scheme for aggregates validated on slip trace identifications from AFM observations. The resulting driving force P, related to the GB velocity V through the widely used law V=MP is compared with the observed displacement during a finite annealing time. Additional effects as thermal grooving and triple junction pinning or pulling are also discussed from complementary SEM and AFM observations of some typical GBs. Some evidences of GB out-of-plane displacements possibly contributing to the migration process are also commented. The quite extensive set of data regarding grain orientation, GB misorientation and curvature, intracrystalline slip activity and evolution during heating constitutes references for future comparisons with mesoscale simulations.

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.

Constrained groove pressing, cold-rolling, and post-deformation isothermal annealing: Consequences of their synergy on material behavior

Stress-relieved Al-Mn-Si specimens were constrained groove pressed (CGPed) and in the following, cold-rolled under different strains of 0.47, 0.8 and 1.27. Dual strained sheets were isothermally heat treated at 150, 250, and 350 °C. Microstructure survey revealed that generated shear-bands by CGP acted as talent sites for further strain-induced grain boundary migration (SIGBM) during annealing. SEM micrographs pointed out that coarse particles (1 μm <) had not preferential positions within the aluminum matrix and often comminuted into fine dispersoids (0.5 μm >) under heavy strains. Assessment of the softening fraction (Rrec) depicted that greater accumulated strains along with higher post-annealing temperatures accelerated the thermal recovery and hence, reduced the Rrec. Tensile testing was performed repeatedly while fluctuation range for obtained stress-strain curves for each specimen as a criterion of structural uniformity was considered. It was found that post-deformation treatment of severely deformed sheets at 250 °C supplied maximum structural uniformity due to perfect restoration and evolution of inhomogeneity agents such as shear-bands, and precipitates within the matrix. In this regard, processing route contained cold-rolling by 0.8 strain of two-pass CGPed specimen and subsequent annealing at 350 °C ascertained to be an adequate combination of deformation and heat treatment with respect to achievement of the maximum uniform elongation (UE) of 22.96% and contribution of uniform deformation energy (UD) in total mechanically absorbed energy equal to 84.18% that provided the optimal formability for the applied alloy. Fracture analysis was performed by fracture angle measurement and SEM fractography. Thermal annealing of CGPed sheet at 150 and 250 °C declined its fracture angle from 55° to 50° and 46.5°, respectively. However, annealing at 350 °C changed the dominant fracture mode from shear to necking by incidence of fibrous morphology and enhanced the angle to 61°. Likewise, it was revealed that in spite of fracture angle increment by rolling reduction, post-rolling prevented the necking fracture in CGPed sheet treated at 350 °C and this might be shifted to higher temperatures.

Recrystallization behavior of multi-directionally forged over-aged and solution treated Al-Cu-Mg alloy during non-isothermal annealing

The recrystallization behavior of an Al-Cu-Mg alloy is investigated in multi-directionally forged over-aged and solution treated alloys, during non-isothermal annealing. Deformation and non-isothermal annealing are performed with a Gleeble 3800 thermo-mechanical simulator. The hardness measurements show that there is a thermal stability in mechanical properties during non-isothermal annealing up to 250°C with heating rate of 10K·min−1. Differential scanning calorimetry curves of deformed over-aged and solution treated alloys describe the related precipitation phenomena. EBSD maps demonstrate that partially recrystallized and fully recrystallized microstructures appear in deformed over-aged and solution treated alloys, respectively. During over ageing, small S′/S particles are formed and increase the critical particle diameter for particle stimulated nucleation (PSN). Evidence is also reported on grain boundary bulging in the over-aged alloy, related to strain induced grain boundary migration (SIBM). In the solution treated case, since there is not sufficient time for S′/S phase growth during non-isothermal annealing, Zener pinning pressure is reduced and PSN sites can be activated. The association of recrystallized microstructure and randomly oriented grains confirms that PSN is the governing nucleation mechanism in deformed solution treated alloys. In the deformed over-aged alloy, a strong γ-fiber is developed by consuming other texture components like {001}〈110〉.

Constitutive Equation and Dynamic Softening Behavior of 7A55 Aluminum Alloy during Compression at Elevated Temperatures

The hot deformation behaviors of 7A55 aluminum alloy were investigated by compression tests at temperatures ranging from 270°C to 450°C and strain rate ranging from 0.1s−1 to 25s−1. Tha rResults show that the flow stress increased with increasing strain rate and decreasing temperature. A two-stage constitutive equation was established and the hot deformation activation energy was 140 kJ/mol. EBSD observations show that the fine and equiaxed grains with the misorientation angle above 15° nucleated at the initial grain boundaries under high temperature and low strain rate conditions. It is concluded that the softening mechanism of 7A55 aluminum alloy is dynamic recovery (DRV), together with a partial dynamic recrystallization (DRX). The nucleation mechanism of DRX could be explained by the strain induced grain boundary migration (SIBM). The DRX softening model was established based on the dislocation density theory finally.

Constrained groove pressing and subsequent annealing of Al-Mn-Si alloy: Microstructure evolutions, crystallographic transformations, mechanical properties, electrical conductivity and corrosion resistance

Al-Mn-Si specimens were severe plastic deformed (SPDed) through constrained groove pressing (CGP) by εeff=1.16, 232, and 3.48. CGPed sheets were subsequently annealed at 150, 250 and 350°C to investigate complementary treatment route on macro- and microscale properties of heavily strained alloy. Microstructure evolutions in deformed and post-annealed states along with their associated mechanisms such as recovery, recrystallization and strain induced grain boundary migration (SIGBM) were studied and analyzed. SIGBM as an indication for inhomogeneous grain growth was traced by transformations in grains' aspect ratio. Microanalysis of crystallographic characteristics by means of X-ray diffraction (XRD) patterns revealed that (111) planes were the main crystallographic index in CGPed and annealed alloys since preservation up to 350°C had amplified the (200) and deteriorated the intensity of (311) planes. Dislocation density measurements implied the dynamic recovery occurrence in CGP (εeff=2.32) which had affected mechanical characteristics, electrical conductivity and corrosion resistance of the utilized alloy. Mechanical properties through tension and hardness tests had been examined since the maximum YS, UTS, and hardness of 118MPa and 141MPa, 52Hv obtained for CGPed specimen (εeff=3.48) compared with annealed alloy with the values of 85MPa and 112MPa, and 29Hv, respectively.

Influence of strain induced grain boundary migration on grain growth of 300M during static and meta-dynamic recrystallization

Abstract The microstructural evolution of 300M steel during the isothermal treatment between deformations is studied. Firstly, interrupt compressions of 300M steel at temperature of 1273K and strain rate of 0.01s -1 are conducted to figure out the kinetics of static recrystallization (SRX) and meta-dynamic recrystallization (MDRX), subsequently an in situ heat treatment experiment is designed to measure the amount of strain induced grain boundary migration (SIGBM) and compared with interrupt compressions. The results show that MDRX is verified to have a lower nucleation rate than SRX in 300M steel. In addition, SIGBMs mainly appear at the triple junctions, and the amount of SIGBMs during SRX and MDRX both reach the maximum value between the steady state time and the time of 50% fraction softening. Finally, the grain growth driven by SIGBMs dominates the subsequent isothermal treatment after the new round of nucleation induced by MDRX and SRX.

Recrystallization of quartz after low-temperature plasticity – The record of stress relaxation below the seismogenic zone

Quartz microfabrics in rocks from the Silvretta basal thrust and the Defereggen-Antholz-Vals (DAV) shear zone in the Eastern Alps, are analysed by polarized light and electron microscopy. The microfabrics from both shear zones record a switch from low-temperature plasticity at transient high stress to recrystallization at relaxing stresses at greenschist facies conditions. The development of new grains is dominantly by subgrain rotation and subsequent strain-induced grain-boundary migration in areas of localized high strain developed during initial low-temperature plasticity. The findings suggest that new grains develop at almost random crystallographic orientations at fast rates of stress relaxation (i.e. at low stress), as indicated by recrystallized quartz zones in the Silvretta fault rocks. In contrast, at slow rates of stress relaxation, new grains develop at moderately high stresses with crystallographic preferred orientation characterized by high Schmid factor for basal <a> glide, as indicated by vein quartz samples from the DAV shear zone. Both recorded histories with transient peak stresses and different rates of stress relaxation are interpreted to be related to seismic activity of the fault systems. This study demonstrates that characteristic microfabrics provide important information about the deformation history of natural shear zones developed in different tectonic regimes.

Recrystallization of coherent nanolamellar structures in Ti48Al2Cr2Nb intermetallic alloy

This study is about the recrystallization of a nanolamellar structure in Ti48Al2Cr2Nb intermetallic alloy. This is a specific case of recrystallization, the driving force comes only from the elastic coherency strain and the interfacial energy. Using an estimation of the stored energy, it is shown that the driving force is sufficient for recrystallization to occur without any prior plastic deformation. Two kinds of nucleation sites are identified: at the border between nanolamellar colonies and inside the nanolamellar colonies. For the former, it is proposed that a nucleus preexists. It is formed by local plastic relaxation of strong internal stresses at the tip of the lamellae and by subsequent recovery. For the latter, it is asserted that a nucleus is generated with a new orientation, by formation of a new interface and its migration. These two different mechanisms of nucleation are discussed based on microscopy observations and driving force estimation. In both cases growth proceeds by strain induced grain boundary migration (SIGBM) assisted by successive (111) twinning.

Strain induced grain boundary migration effects on grain growth of an austenitic stainless steel during static and metadynamic recrystallization

Static and metadynamic recrystallization of an AISI 304L austenitic stainless steel was investigated at 1100°C and 10−2s−1 strain rate. The kinetics of recrystallization was determined through double hit compression tests. Two strain levels were selected for the first compression hit: εf=0.15 for static recrystallization (SRX) and 0.25 for metadynamic recrystallization (MDRX). Both the as-deformed and the recrystallized microstructures were investigated through optical microscopy and electron back-scattered diffraction (EBSD) technique. During deformation, strain induced grain boundary migration appeared to be significant, producing a square-like grain boundary structure aligned along the directions of the maximum shear stresses in compression. EBSD analysis revealed to be as a fundamental technique that the dislocation density was distributed heterogeneously in the deformed grains. Grain growth driven by surface energy reduction was also investigated, finding that it was too slow to explain the experimental data. Based on microstructural results, it was concluded that saturation of the nucleation sites occurred in the first stages of recrystallization, while grain growth driven by strain induced grain boundary migration (SIGBM) dominated the subsequent stages.

Effects of Different Modes of Hot Cross-Rolling in 7010 Aluminum Alloy: Part I. Evolution of Microstructure and Texture

The current study describes the evolution of microstructure and texture in an Al-Zn-Mg-Cu-Zr-based 7010 aluminum alloy during different modes of hot cross-rolling. Processing of materials involves three different types of cross-rolling. The development of texture in the one-step cross-rolled specimen can be described by a typical β-fiber having the maximum intensity near Copper (Cu) component. However, for the multi-step cross-rolled specimens, the as-rolled texture is mainly characterized by a strong rotated-Brass (Bs) component and a very weak rotated-cube component. Subsequent heat treatment leads to sharpening of the major texture component (i.e., rotated-Bs). Furthermore, the main texture components in all the specimens appear to be significantly rotated in a complex manner away from their ideal positions because of non-symmetric deformations in the two rolling directions. Detailed microstructural study indicates that dynamic recovery is the dominant restoration mechanism operating during the hot rolling. During subsequent heat treatment, static recovery dominates, while a combination of particle-stimulated nucleation (PSN) and strain-induced grain boundary migration (SIBM) causes partial recrystallization of the grain structure. The aforementioned restoration mechanisms play an important role in the development of texture components. The textural development in the current study could be attributed to the combined effects of (a) cross-rolling and inter-pass annealing that reduce the intensity of Cu component after each successive pass, (b) recrystallization resistance of Bs-oriented grains, (c) stability of Bs texture under cross-rolling, and (d) Zener pinning by Al3Zr dispersoids.