Recrystallization Microstructure Research Articles

Study of the Stress Relaxation Resistance and Microstructural Evolution of a Cu-Ni-Si Alloy Strip

The stress relaxation resistance of a Cu-Ni-Si alloy strip after 48 h of initial loading stress of 80% σ0.2 at 125 °C was studied. The stress relaxation curve was fitted using a model. By employing EBSD, TEM, and other techniques, this study examined the microstructural characteristics of the Cu-Ni-Si alloy strip before and after stress relaxation. This study investigated the stress relaxation mechanism of the alloy and revealed that the stress relaxation rate of the Cu-Ni-Si alloy strip was 10.82% after 48 h at 125 °C. The stress relaxation process of the alloy strip was divided into two main stages. During the first stage, there was a significant rearrangement and movement of dislocations within the alloy strip, leading to a rapid decline in the remaining stress. In the second stage, the interaction among the Ni2Si phase, twins and dislocations led to a slow decrease in the remaining stress. After the stress relaxation of the alloy strip, the precipitated phase in the microstructure grew from 67 nm to 200 nm, the recrystallization microstructure increased from 57.91% to 62.42%, and the twin boundary content decreased from 22.1% to 18.5%. These observed changes in the microstructure are anticipated to further reduce the stress relaxation resistance of the alloy strip.

Continuous Dynamic Recrystallization and Deformation Behavior of an AA1050 Aluminum Alloy during High-Temperature Compression

Continuous dynamic recrystallization (CDRX) forms a new recrystallized microstructure through the progressive increase in low-angle boundary misorientations (LAGBs) during the hot forming of metallic materials with high stacking fault energy (SFE), such as aluminum alloys. The present work investigates the effect of deformation parameters on the evolution of the dynamic recrystallization microstructures of an AA1050 aluminum alloy during compression at elevated temperatures. The alloy microstructure is investigated at deformation temperatures and strain rates in the range of 300 °C to 500 °C and 0.001 to 0.8 s−1. A well-defined substructure and subsequent DRX grains provide indication that recrystallization can proceed with continued strain under high-temperature compression. At a strain rate of 0.1 s−1, the DRX fraction is observed to be 0.25 at a temperature of 300 °C. This fraction increases to 0.32 as the temperature rises to 400 °C. The recrystallization mechanism is identified by analyzing the flow stress, the evolution of the subgrain misorientation angle, and the distribution of recrystallized grains. The observations of discontinuous dynamic recrystallization (DDRX) and CDRX under various deformation parameters are discussed. Moreover, the main substructure evolution laws observed from the high-temperature compression of an AA1050 Al alloy are summarized.

Evolution and mechanism of recrystallization microstructure and texture in GH3536 superalloy foil and their influence on strength performance

To achieve isotropic strength in GH3536 foil, this study investigated the influence mechanism of initial recrystallization microstructure and texture characteristics of GH3536 superalloy foil with different cold-rolled reduction rates (referring to the reduction rate in thickness) on annealing recrystallization microstructure, including secondary recrystallization microstructure, and texture. This was studied by controlling the cold-rolled reduction rate and annealing temperature, in conjunction with microstructure and texture analysis. Meanwhile, this study clarified the dependency of strength performance on recrystallization microstructure and texture. The results of this study indicate that the Coincidence Site Lattice (CSL) grain boundaries and High Energy (HE) grain boundaries did not show significant superiorities, and were not the primary reasons for the formation of Brass ({011}<211>) and Goss ({011}<100>) textures, as well as secondary recrystallization. Instead, by controlling the cold-rolled reduction rate, the initial recrystallization stage formed a significant quantity and size superiority of α-fiber texture ({110}∥ND) in each grain size range. This led to preferential growth, and even abnormal growth of Brass and Goss grains in subsequent high-temperature annealing processes. The control of the α-deformation fiber texture was identified as the driving factor behind the formation of the initial recrystallization texture due to the cold rolling reduction rate. Cold-rolled processes with a deformation amount greater than 40 % yielded a quantity superiority of α-fiber texture shear bands and an appropriate stored energy advantage. This facilitated the nucleation and effective growth of more α grains, while suppressing the random growth of grains with γ-fiber texture ({111}∥ND). Ultimately, a quantity and size superiority was achieved in the initial recrystallization stage. The tensile strength of GH3536 foil was controlled by grain size and texture. By controlling the deformation amount to approximately 30 %, GH3536 foil was successfully produced with no pronounced orientation texture and isotropic strength.

Coordinated deformation mechanisms of high formability Al–Mg–Si–Cu–Zn–Fe alloys via coupling control of thermomechanical processes

Both the high formability and strengths are greatly important for the automotive application of Al–Mg–Si–Cu alloys. In the present work, four novel thermomechanical processing routes were designed to improve the comprehensive properties of Al–Mg–Si–Cu–Zn–Fe alloys. The results show that, the interspace between Fe-rich particles and Fe/Mn-containing dispersoids can be significantly reduced by an optimized thermomechanical processing route. This enhanced the interaction between them during subsequent cold rolling process, resulting in increased number density and uniform distribution level of multi-scale Fe-rich particles, which further substantially improve both the recrystallization microstructure and mechanical properties, i.e., higher bendability, average r value of 0.72 (0.59 for traditional sheet) and tensile strength of 302 MPa (283 MPa for traditional sheet). The microstructure evolution of alloy sheets was schematically illustrated, and in-depth analysis on the mechanism of high formability was proceeded from the aspects of grain boundary misorientations and recrystallization texture.

Study of the Dynamic Recrystallization Behavior of Mg-Gd-Y-Zn-Zr Alloy Based on Experiments and Cellular Automaton Simulation

The exploration of the relationship between process parameters and grain evolution during the thermal deformation of rare-earth magnesium alloys using simulation software has significant implications for enhancing research and development efficiency and advancing the large-scale engineering application of high-performance rare-earth magnesium alloys. Through single-pass hot compression experiments, this study obtained high-temperature flow stress curves for rare-earth magnesium alloys, analyzing the variation patterns of these curves and the softening mechanism of the materials. Drawing on physical metallurgical theories, such as the evolution of dislocation density during dynamic recrystallization, recrystallization nucleation, and grain growth, the authors of this paper establish a cellular automaton model to simulate the dynamic recrystallization process by tracking the sole internal variable—the evolution of dislocation density within cells. This model was developed through the secondary development of the DEFORM-3D finite element software. The results indicate that the model established in this study accurately simulates the evolution process of grain growth during heat treatment and the dynamic recrystallization microstructure during the thermal deformation of rare-earth magnesium alloys. The simulated results align well with relevant theories and metallographic experimental results, enabling the simulation of the dynamic recrystallization microstructure and grain size prediction during the deformation process of rare-earth magnesium alloys.

Microstructural response characteristics of SAC305/Cu column joint under friction embedded welding conditions

The friction welding method for macro-scale weldments was introduced into the manufacturing of electronic products. Without mold assistance, the positioning and connection of micro copper columns in column grid array packaging (CGA) devices were achieved. The average connection strength of 40.6 N and the intermetallic compound interface layer meant that effective connections were achieved. In the poorly identified stirring-mixing zone (SMZ) in scanning electron microscopy (SEM) images, electron backscatter diffraction (EBSD) results showed that this region was composed of recrystallized grains, twin grains, and Ag3Sn and Cu6Sn5 particles distributed inside or at the grain boundaries. The grain size was about 2 μm, and the Ag3Sn and Cu6Sn5 particle sizes were broken to 250 nm and 120 nm due to stirring. The SMZ at the bottom of the copper column only underwent deformation-induced dynamic recrystallization, while the SMZ on the side was discharged from the bottom of the copper column, so it underwent dynamic recrystallization through the process of cutting crushing-extruding and flow-bonding-deformation. Therefore, the hardness of SMZ was significantly reduced after welding. Unlike traditional friction welding, in the thermo-mechanical affected zone (TMAZ), the solder not only underwent deformation but also formed 47.8% recrystallization microstructure and 14.6% recovery microstructure. Therefore, the effect of deformation strengthening was weakened and recrystallization led to a decrease in hardness. Recovery was the main response behavior in the micro-deformation zone (MDZ). Combined with the numerical simulation results, the temperature and stress driving conditions of the microstructure of each micro-region in instantaneous friction embedded welding (FEW) were also discussed.

The dynamic recrystallization microstructure characteristics and the effects on static recrystallization and mechanical properties of Al–Mg–Si alloy

It is essential to clarify the dynamic recrystallization microstructure characteristics of age-hardened aluminum alloys and the effects on the static recrystallization microstructure and aging mechanical properties, with the aim of selecting the deformation conditions reasonably. This work investigated the evolution of the hot deformation and solid solution microstructures and aging mechanical properties of Al–Mg–Si alloy with deformation temperatures of 400–550 °C, strain rates of 0.01–10 s−1, and true strains of 0.2–0.8. Research has shown that it is advantageous for the DDRX and CDRX mechanisms to annihilate and rearrange dislocations simultaneously for forming deformation microstructure with uniform grain size at higher deformation temperatures and lower strain rates (such as 500 °C, 0.01 s−1). Additionally, the lower dislocation density in deformation microstructure facilitated the inhibition of static recrystallization during subsequent solid solution treatment, whereas lower deformation temperature and higher strain rate conditions should result in a greater degree of static recrystallization and reduce the uniformity of the solid solution microstructure. Hardness test results demonstrated that the deformation temperature of 480–520 °C and strain rate of 0.03–0.2 s−1 were the appropriate conditions for Al–Mg–Si alloy to attain finer and uniform grains and superior properties after solid solution treatment. The extruded samples within the above conditions (500 °C, 0.032 s−1) exhibited higher UTS and YS after solid solution and aging treatment, reaching 444 ± 2 MPa and 426 ± 4 MPa, respectively.

Evolution of static recrystallization microstructure caused by residual strain of an Al-richen Ni-based single crystal superalloy

Recrystallization has a devastating effect on the service process of single crystal superalloys (SX). This study mainly investigated the static recrystallization (SRX) behavior of an as-cast high Al content nickel-based SX under local or global deformation at room temperature, as well as heat treatment at various temperatures. The microstructural evolution was observed under different conditions using electron back scatter diffraction (EBSD), which was employed to determine the SRX threshold. The results indicated that the critical SRX temperature exceeds 1260 °C, and the critical plastic strain is greater than 5 %. In particular, the characteristic of the higher SRX tendency in the interdendritic region (IDR) of the as-cast SX was discovered, which was attributed to the phenomenon that the IDR with lower deformation resistance was prone to significant plastic accumulation and the bulk γ' could be the nucleation substrate of the SRX. Furthermore, with the increase in heat treatment temperature, there are certain changes in the morphology characteristics of re-precipitated γ' in the vicinity of the SRX grain boundary with a higher migration rate. This is mainly attributed to the decrease in the internal subcooling of the bulk γ' phase and the increase in the concentration of γ' forming elements at the grain boundary front.

Genetic implications of vein texture and mineralogy at the Au-bearing El Morado mining district, Sierras Pampeanas, San Juan, Argentina

El Morado mining district is located in Sierra de La Huerta, in the Sierras Pampeanas of San Juan province, Argentina. It comprises numerous Au-bearing quartz veins hosted by intensely deformed igneous-metamorphic units of lower Paleozoic age. In the El Morado mining district, Maira mine is the only deposit producing gold at the present time. Quartz veins in this mine and surroundings, occur in concordance with their host rocks foliation and show numerous evidences of brittle-ductile deformation, mainly represented by a pinch-and-swell structure and extended development of breccia bodies.The application of scanning electron microscopy cathodoluminiscence in quartz samples lead to the recognition of homogeneous texture, which is evidence of recrystallization, commonly observed in orogenic gold quartz veins. Intracrystalline deformation is ubiquitous and represented by undulose and patchy extinction in relicts of large old grains where bulging and sub-grain rotation recrystallization microstructures are well represented. These dynamic quartz deformation processes indicate temperatures going up to ∼350 °C. Fluid inclusion post-entrapment modifications were recognized in quartz veins as numerous collapsed, small-sized fluid inclusions.Ore mineralization at Maira mine occurred after quartz veins emplacement and deformation in the upper crustal brittle-ductile transition zone. Galena, chalcopyrite and lesser pyrite and bornite, together with gold and tetrahedrite were identified infilling fractures in quartz veins. Gold grains were also identified closely related to malachite, cuprite, goethite and lepidocrocite in oxidizing horizons.

The role of primary recrystallization microstructure in secondary recrystallization of Goss grain in industrial low-temperature grain-oriented silicon steel

Low-temperature grain-oriented silicon steel has very excellent magnetic properties along the rolling direction. During industrial mass production, slight process fluctuations can directly lead to variations in the primary recrystallization microstructure, which in turn can have a dramatic effect on the magnetic properties of the final product. However, it is unclear how different primary recrystallization microstructures influence the magnetic properties of the industrial final products, making it difficult to further optimize the production process of industrial low-temperature grain-oriented silicon steel. Therefore, it is very important to clarify the transformation behavior of different primary recrystallization microstructures and their influences on the transformation behavior of inhibitors during the high-temperature annealing process. Here, we analyzed the differences between two primary recrystallization microstructures in terms of microstructure homogeneity, Goss grain characteristics and texture characteristics and investigated the secondary recrystallization behavior of Goss grains and the dissolution behavior of inhibitors in these two primary recrystallization microstructures during high-temperature annealing. Based on the results of this work, it was found that a finer primary recrystallization microstructure could be more favorable to the formation of a sharp Goss texture, which in turn could significantly improve the magnetic properties, and in addition, new insights into the transformation behavior and the grain boundary pinning behavior of inhibitors during high-temperature annealing process were established. The results of this work could help to further improve the understanding of the actual role of inhibitors during high-temperature annealing and could also provide new ideas for further improving the industrial production process.

Evolution of recrystallization texture in medium to low stacking fault energy alloys: Experiments and simulations

The present work aims to investigate the evolution of static recrystallization microstructure and texture in medium to low stacking fault energy (SFE) alloys. In these categories of materials, the process of recrystallization becomes complex because of the presence of extensive deformation heterogeneity. Ni-xCo (40 and 60wt.% Co) alloy has been chosen for this purpose, where Ni-40Co belongs to medium SFE, and Ni-60Co belongs to low SFE regimes. The effect of solid solution strengthening and deformed microstructural features on the mechanism of recrystallization are explored. Both the alloys were subjected to 50% cold-rolling reduction followed by isothermal annealing at 600°C. Recrystallization texture of Ni-40Co shows a non-uniform α-fiber having a peak intensity at the Goss component, whereas Ni-60Co shows uniform α-fiber texture. Both the alloys also show rotated cube (Rt C) and rotated Cu (Rt Cu) components after recrystallization. Apart from that Ni-60Co exhibits brass recrystallization (BR) texture component. The differences in the recrystallization texture in both the Ni-Co alloys are attributed to the role of different heterogeneous deformation features in the microstructure and transition from Cu-type to Bs-type as-deformed texture. The recrystallization microstructure is dominated by the formation of substantial annealing twin (Σ3) boundaries, which also produces many new orientations; thereby weaken the recrystallization texture. These experimental findings and the proposed mechanism were used as input to simulate the recrystallization microstructure and texture in the alloy using the parallelized cellular automata (CA) technique. Parallelized CA model has successfully predicted the evolution of recrystallization texture and microstructure through simulation except parallel annealing twin feature.

Effects of Al additions on precipitation, recrystallization and mechanical properties of hot-rolled ultra-super ferritic stainless steels

In this study, 1.0 wt%Al element was added into the traditional 26.8Cr-3.7Mo–2Ni super-ferritic stainless steel to tailor the brittle phase precipitation, recrystallization microstructure and mechanical property via solution annealing at temperatures of 950–1150 °C. The experimental results demonstrated that hot-rolled microstructure for 1.0Al sample consisted of surface fine grain region and core elongated grains together with TiN, NbC and Laves phase, while none of these fine grains appeared in 0Al samples. After annealing below 1050 °C, both σ-phase and Laves phase were formed in Al-free samples, while only abundant Laves phase precipitated in 1.0Al sample indicating the inhibition effect of Al on the σ-phase and the promotion effect on Laves phase. When annealing above 1050 °C, recrystallization was completed and neither σ-phase nor Laves phase were observed. Relatively stable mean grain size was produced due to the Laves phase pinning effect during annealing at 1050 °C, while the Nb/Al solute drag effect bore responsibility for the situation at 1050–1150 °C. Sudden grain coarsening phenomenon appeared after annealing above 1150 °C. Al additions gave a solution strengthening and intensive Laves phase precipitation strengthening for the designed alloys, and good comprehensive mechanical properties were obtained demonstrating the probability of developing of the Al-modified ultra-super stainless steels.

Characterization of Hot Deformation Behavior and Processing Maps Based on Murty Criterion of SAE8620RH Gear Steel

The hot deformation behavior and microstructure evolution of the SAE8620RH gear steel were investigated through a single-pass hot compression test at deformation temperatures between 850 and 1100 °C and strain rates between 0.02 and 8.0 s−1 by 60% reduction. A novel strain compensation constitutive model was developed, and the 2D processing maps were established by Murty’s criterion. Results showed that the relationship between material-related parameters and strain can be mathematically expressed by a highly reliable 8th-order polynomial. The constructed strain compensation constitutive model demonstrated remarkable predictive precision, as evidenced by the correlation coefficient (R) and the absolute values of average relative error (AARE) of 0.978 and 4%, respectively. The flow instability domains considerably expanded towards the high deformation temperature region as the strain increased. Microstructure analysis confirmed the accuracy of the processing map constructed by Murty’s criterion. The most noticeable optimum processing windows for SAE8620RH gear steel at a strain of 0.7 occurred within the temperature range of 1000–1100 °C and the strain rate range of 0.3–1.0 s−1, due to high η values exceeding 0.3 and equiaxial dynamic recrystallization microstructure.

Effect of heat treatment on abrasive wear behaviour of Ni-WC coatings

In this study, an experimental investigation of the abrasive wear behaviour of HVOF-sprayed Ni-10WC metal coatings was conducted. Different heat treatment temperatures, such as 400 °C, 600 °C, and 800 °C, were used to modify the coating. In order to explore microstructure, element analysis, and phase identification were measured by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and x-ray diffraction. The porosity and hardness of coatings were performed using a Vickers microhardness tester and image-J software. Abrasive wear tests were performed using a pin on the disc tester against silicon carbide of 100 µm grit as a counter material mounted on a 165 mm circular disc. The outcome demonstrates that coatings' mechanical and microstructure properties are greatly improved by heat treatment. The optimum temperature was determined to be 600 °C, which exhibited maximum hardness as 750 Hv0.3, minimal porosity as 1.8 %, and maximum wear resistance. Heat treatment lowers the development of the W2C amorphous phase, increasing the microstructure's recrystallization and density.

A multi-scale modelling by coupling cellular automata with finite element method and its application on cross-wedge rolling

Cross-wedge rolling is an advanced plastic forming process for manufacturing stepped-shafts. During the cross-wedge rolling, the workpiece undergoes dynamic recrystallization, which has a significant effect on the grain refinement. A multiscale modelling method is required to predict and simulate the microstructure evolution. This work proposes a novel multi-scale modelling approach by coupling the cellular automata model and finite element method, which can visually simulate and predict the evolution and overall distribution of dynamic recrystallization microstructure. This novel multi-scale modelling method is verified by comparing the experimental results and simulation results of the microstructure of the uniaxial thermal compression test. The developed multi-scale modelling approach is applied to simulate the microstructure evolution and distribution of the high-speed railway axle formed by cross-wedge rolling. The simulation results show that cross-wedge rolling is a typical local forming process, and the microstructure of different regions has significant differences.

Magnetic Domains and Their Power Spectral Densities in Non-Oriented Electrical Steel after Thermal Compression at Different Rates.

The magnetic domains of non-oriented electrical steel bearing cumulative thermal compressions made by a Gleeble 3500 Thermal System were observed using an atomic force microscope (AFM). The component forces, comprising the magnetic forces between the AFM probe and magnetic domains of the samples, along the freedom direction of the probe, were measured, and they formed the value fluctuation of the magnetic domains. The fluctuations of the magnetic domains were analyzed by examining the power spectral density (PSD) curves. The hysteresis curves of the samples were measured using a highly sensitive magnetic measurement system. An analysis of the magnetic force microscope (MFM) maps suggested that some magnetic domains were compressed into crushed and fragmented shapes, similar to the microstructure of deformed grains. Meanwhile, some were reconstructed within the thermal compressions, like dynamic recrystallization microstructures. Meaningfully, the MFM probe moved and deformed the proximal magnetic domains of tested samples within the region of its weak magnetic field. The peak positions of the magnetic domains with a high deformation rate were shifted and moved during the measuring processes by the weakly polarized probe. Both windward and leeward sides simultaneously expressed a slope towards each co-adjacent valley in the MFM maps and induced a statistical throbbing within a narrow band in the PSD curves. Thus, the MFM scanning mode was also analyzed and improved to obtain accurate MFM maps with low disturbances from the weak magnetic field of the probe. Swapping the order positions of the middle processes in the MFM scanning and adding a gliding step between them could offset the peak skewing of magnetic domains caused by the weakly polarized probe during MFM measurement process without incurring excessive replacement costs. Accumulative compression at a high rate (10 s-1) would crush magnetic domains into irregularly decreasing sizes with messy boundaries. This investigation provides an example of the complete relationships among deformations, magnetic domains, and magnetic properties.

Evolution of recrystallization texture in nickel-iron alloys: experiments and simulations

ABSTRACT The present work is aimed at examining the effect of solid solution on the development of recrystallization microstructure and texture in FCC materials where SFE remains unchanged on alloying addition. To elucidate the mechanisms of texture formation during recrystallization, pure Ni and Ni-Fe (20 and 40 wt.% Fe) alloys were investigated. After recrystallization, pure Ni showed a cube and a non-uniform α-fibre texture, whereas the Ni-Fe alloys showed a texture characterised by the rotated cube component, brass recrystallization (BR) orientation, and a non-uniform α-fibre. Addition of Fe to pure Ni has led to some fine differences in the recrystallization texture that have been attributed to the role of highly heterogeneous deformed microstructure in the Ni-Fe system because of alloying. However, in all the cases Cu-oriented grains are prone to early recrystallization due to relatively more heterogeneously deformed regions, whereas deformed grains having orientation <110> || ND have shown slow recrystallization. In all cases, the entire stage of recrystallization is dominated by the formation of annealing twin (Σ3) boundary. The mobility of these twin boundaries plays an important role in the evolution of the recrystallization texture, which in turn, depends on its coherency, i.e. grain boundary plane (K1) of twin boundaries. The mechanism of evolution of recrystallization texture and the role of different deformation features during recrystallization is investigated. The cellular automata simulation technique was used to simulate the recrystallization behaviour of the alloys. The simulation results were used to discuss experimental observations.

Effect of Cold Rolling Reduction Ratio on Microstructure and Magnetic Properties of Secondary Cold‐Rolled High‐Grade Nonoriented Silicon Steel

Trial production of 0.25 mm‐thick thin‐gauge high‐grade nonoriented silicon steel with a Si content of 3.5% by the secondary cold rolling process is studied. The recrystallization microstructure and magnetic properties are systematically studied by the microstructure, texture evolution, and the effect of two‐stage cold rolling reduction ratios (37.5–70.5%) during the whole process. The results show that the magnetic induction intensity B5000 first increases and then decreases; the iron loss first decreases and then increases at the frequencies of 50 and 400 Hz with the reduction of the cold rolling reduction rate in the second stage. The cubic and Goss texture of the finished annealed sheet has the highest strength when the reduction ratio of the second stage cold rolling is 58.3%. The highest magnetic induction intensity, B5000 = 1.671 T. Lowest iron loss, P1.0/50 = 0.83 W kg−1, P1.5/50 = 1.98 W kg−1, and P1.0/400 = 11.94 W kg−1, respectively.

Extreme ductile thinning of Cambrian marbles in the Northern Snake Range metamorphic core complex, Nevada, USA: Implications for extension magnitude and structural evolution

The detailed stratigraphy of rocks within the Northern Snake Range metamorphic core complex provides an exceptional opportunity to investigate the geometry and magnitude of ductile strain during high-magnitude continental extension. In the northern part of the range, Middle-Late Cambrian marbles in the footwall of the Northern Snake Range dècollement (NSRD), which have a stratigraphic thickness of 1107 ± 107 m in adjacent ranges, were thinned during Late Eocene-Late Oligocene ductile extension. From west to east across the range, these rocks have been thinned from 869-935-m-thick (15–21% structural thinning) to 54-88-m-thick (92–95% structural thinning) across a 12 km lineation-parallel distance. Ductile extensional strain was accompanied by the development of pervasive linear-planar fabrics and produced megaboudins of calcareous schist units that are 100-500-m-long, 15-25-m-thick, and separated by as much as 1 km. The magnitude of lineation-parallel ductile extension increases eastward across the range from 24 ± 21% to 1226 ± 256%, and total ductile extension across the range is 12.1 ± 2.2 km (167 ± 31%). Quartz recrystallization microstructures and published calcite-dolomite thermometry indicate deformation temperatures of ∼400–550 °C during initial Late Eocene-Late Oligocene ductile extensional shearing. West to east across the range, rocks in the NSRD footwall experienced a progressively longer ductile extensional strain history and a prolonged residence time at higher temperatures, which was aided by the eastward migration of denudation-related cooling and was likely enhanced by strain heating and/or a pre-extensional eastward dip of footwall rocks. These factors promoted the development of the extreme strain gradient.

Elucidation of interface joining mechanism of aluminum alloy/dual-phase steel friction stir riveting (FSR) joint

The mixed use of aluminum (Al) alloy and high strength steel (HSS) is an effective way to reduce the weight of vehicles, which meanwhile brings great challenges to the traditional spot joining technologies. Three-stage friction stir riveting (FSR) process using a precisely designed semi-hollow rivet was developed for Al-steel joining. By coordination control the rotation and feed movement of the rivet, a “tube-sheet” annular solid-phase welding zone is formed between the semi-hollow rivet and the lower steel sheet, realizing the reliable joining of Al-steel dissimilar sheets combination. The rivet structure was designed to accommodate the trapped Al alloy in the rivet cavity during the friction welding stage, obtaining an inclusion-free friction welding interface which was not only mechanically mixed but also fully interatomic binding. The optimal FSR joint is composed of ferrite softening zone (FSZ), carbide precipitation zone (CPZ), ultrafine martensite zone (UMZ), and fine dual-phase zone (FDPZ). Moreover, due to the circumferential flow of materials during friction welding stage, the dynamic recrystallization (DRX) microstructures of the rivet and steel sheet both have shear texture consistent with {112}<111> and {110}<001> cube texture, which brings the uniformity of microstructure and performance. With the combined contribution of inclusion-free joining interface and the microstructure with uniform cube texture, the optimal FSR joints have the maximum average tensile-sheer strength of 281.1 MPa (90.7%·σ6061-T6) and peak load of 5.29 kN, presenting pull-out fracture mode with good fracture energy absorption.