Grain Boundaries Research Articles

An understanding of the segregation and migration mechanism of point defects in tungsten grain boundaries: An atomic scale simulation

In this study, molecular dynamics (MD) simulations are employed to investigate the interactions between point defects and grain boundaries (GBs) in tungsten. Firstly, the segregation energy of vacancies (Vs) and self-interstitial atoms (SIAs) to GBs is examined. The results indicate that both Vs and SIAs tend to segregate towards GBs. Notably, the different types of GBs exhibit varying attraction to Vs and SIAs. The interaction radius is applied to investigate the degree of segregation, which is a significant index of GBs’ attraction to Vs and SIAs. The segregation radius of SIAs is typically larger than that of Vs. High-angle grain boundaries (HAGBs) show strong segregation for SIAs, facilitating the aggregation at GBs. Additionally, elastic theory is applied to qualitatively discuss the factors influencing the segregation energy distribution. The differences in atomic free volume caused by hydrostatic stress and the lattice distortion induced by point defects lead to Vs and SIAs occupying different sites. Finally, the nudged elastic band (NEB) method is employed to study the migration of Vs and SIAs near GBs, revealing that migration energy barriers for Vs and SIAs in GBs are much lower than in bulk. GBs facilitate the migration of point defects. Low-angle grain boundaries (LAGBs) present a higher energy barrier for V migration. Vs tend to occupy the compressive region, while SIAs tend to occupy the tensile region. Particularly, in comparison to Vs, SIAs are more likely to segregate to GBs due to higher binding energy, wider interaction radius, and extremely lower diffusion energy barrier. This provides some insights into the segregation and migration of point defects in GBs.

Effect of grain boundary precipitation on corrosion behaviour of nonisothermally aged Al-Zn-Mg-Cu alloys

The corrosion properties of five nonisothermally aged (NIA) Al-Zn-Mg-Cu alloys with different heating rates are studied and compared with those of the T6-treated alloys by electrochemical experiments and weight loss test. The results show that reducing the heating rate results in an increase in the corrosion resistance of the alloy. The aging treatment at a heating rate of 10℃/h results in an alloy with an excellent corrosion resistance and a low corrosion current density. Additionally, the hardness of the alloy is similar to that of the alloy treated with T6. Coarse precipitates are obtained by 10℃/h aging treatment, with the grain boundaries (GBs) transferring to intermittent phases with narrow precipitation-free zones (PFZs). Therefore, this approach effectively prevents anodic dissolution and improves the corrosion resistance of the alloy. These findings suggest that NIA can enhance the mechanical properties and corrosion resistance of an alloy. Additionally, the 10°C/h aging process reduces the processing time by approximately 50%, compared to that of the T6 treatment, offering a cost-effective alternative. These results provide valuable insights into the processing of large aluminium components.

In-situ EBSD-DIC simulation of microstructure evolution of aluminum alloy welds

A comprehensive understanding of the dynamic evolution of weld microstructure under external loads can provide key insights for high-performance laser welding. A novel in-situ EBSD-DIC simulation method is introduced to study the microstructure evolution of laser welded aluminum alloys under uniaxial tensile action. An advanced crystal plastic finite element model (CPFEM) is developed, which combines the real microstructure, grain orientation and grain size effects. The results show that the dislocation density in columnar grain zone is higher than that in equiaxed grain zone. The continuous accumulation of dislocations in the columnar region results in a high work-hardening rate. This high work hardening rate enhances the plastic deformation capacity of the columnar crystal region, resulting in local strain concentration. Columnar zones are more prone to fracture because the high-strain region is a potential fracture site. In addition, low Angle grain boundary (LAGBs) is one of the reasons that the dislocation density of the columnar grain zone is higher than that of equiaxed grain zone during tensile process, which is conducive to dislocation slip of columnar grains. This study is a fundamental innovation in simulating the microstructure evolution of laser welding. This marks a major breakthrough in simulating the evolution of crystallographic features such as grain orientation, microstress and strain and dislocation density under external loads. This work can further provide practical guidance for “microstructure characteristics - mechanical property regulation”.

Recrystallization and spheroidization mechanisms in Ti6246 alloy under thermomechanical treatment

This paper provides a detailed analysis of the dynamic microstructure evolution of Ti-6Al-2Sn-4Zr-6Mo alloy (Ti6246) through a 4-pass hot rolling process. Additionally, it examines the effect of heat treatment on microstructure evolution during the post-annealing of the hot-rolled alloy. During the rolling process, dynamic recrystallization (DRX) predominantly takes place in the α-phase, leading to grain refinement and spheroidization as a consequence of DRX and lamellae spheroidization. Spheroidization of the lamellar structure is more pronounced in the 2-pass rolling process, while the level of DRX is greater in the 4-pass rolling process. The dislocations present within the α lamellae accumulate and entangle at the sub-grain boundary, leading to the formation of a high-angle grain boundary (HAGB). Meanwhile, the β phase infiltrates the grain boundaries of the α phase, leading to the formation of spheroidized equiaxed α grains. Schmid factors associated with the pyramidal slip systems are relatively high, facilitating the activation of slip systems during hot rolling processes. The evolution of the α-phase morphology during heat treatment processes in various rolling passes is primarily driven by static recrystallization (SRX) and grain boundary terminal migration mechanisms. The deformed microstructure volume fraction diminishes and transforms into substructures and recrystallized grains through post-annealing. Post-annealing facilitates the dissipation of stored deformation energy with increasing temperature, leading to a decrease in dislocation density and an enhancement in the extent of recrystallization. Accumulated deformation during rolling and the temperature of the heat treatment both play a role in affecting the static spheroidization process. Greater accumulated deformation and elevated post-annealing temperatures are advantageous for achieving grain spheroidization. The grain boundary splitting process is more pronounced, and the deformation energy is elevated during the 3,4-pass rolling.

Stabilizing Wide‐Bandgap Perovskite with Nanoscale Inorganic Halide Barriers for Next‐Generation Tandem Technology

AbstractWide‐bandgap (WBG) perovskite solar cells (PSCs) play a crucial role in advancing perovskite‐based tandem solar cells. In WBG perovskite films, grain boundary (GB) defects are the main contributors to open‐circuit voltage (VOC) deficits and performance degradation. This report presents an effective strategy for passivating GBs by incorporating an inorganic protective layer and reducing the density of GBs in perovskite films. This is achieved by integrating potassium thiocyanate (KSCN) into I‐Br mixed halide WBG perovskites. It is reported for the first time that the incorporation of KSCN creates band‐shaped barriers along the GBs. In addition, KSCN enlarges the grains of perovskite film. Elemental and structural analyses reveal that these barriers are composed of potassium lead halide. Incorporating KSCN significantly enhances the fill factor and VOC of WBG single‐junction PSCs by reducing trap density. This results in high power conversion efficiencies of 19.22% (bandgap of 1.82 eV), 20.45% (1.78 eV), and 21.54% (1.70 eV) with a C60/bathocuproine electron transport layer, and 18.51% (1.82 eV) with a C60/SnO2. Furthermore, both operational and shelf stabilities are significantly improved due to reduced light‐induced halide segregation. By using inorganic‐halide‐passivated WBG sub‐cells, a monolithic all‐perovskite tandem solar cell with an efficiency of 27.04% is demonstrated.

Machine learning interatomic potential with DFT accuracy for general grain boundaries in α-Fe

AbstractTo advance the development of high-strength polycrystalline metallic materials towards achieving carbon neutrality, it is essential to design materials in which the atomic level control of general grain boundaries (GGBs), which govern the material properties, is achieved. However, owing to the complex and diverse structures of GGBs, there have been no reports on interatomic potentials capable of reproducing them. This accuracy is essential for conducting molecular dynamics analyses to derive material design guidelines. In this study, we constructed a machine learning interatomic potential (MLIP) with density functional theory (DFT) accuracy to model the energy, atomic structure, and dynamics of arbitrary grain boundaries (GBs), including GGBs, in α-Fe. Specifically, we employed a training dataset comprising diverse atomic structures generated based on crystal space groups. The GGB accuracy was evaluated by directly comparing with DFT calculations performed on cells cut near GBs from nano-polycrystals, and extrapolation grades of the local atomic environment based on active learning methods for the entire nano-polycrystal. Furthermore, we analyzed the GB energy and atomic structure in α-Fe polycrystals through large-scale molecular dynamics analysis using the constructed MLIP. The average GB energy of α-Fe polycrystals calculated by the constructed MLIP is 1.57 J/m2, exhibiting good agreement with experimental predictions. Our findings demonstrate the methodology for constructing an MLIP capable of representing GGBs with high accuracy, thereby paving the way for materials design based on computational materials science for polycrystalline materials.

The Effect of Solute Elements Co-Segregation on Grain Boundary Energy and the Mechanical Properties of Aluminum by First-Principles Calculation

The segregation of solute atoms at grain boundary (GB) has an important effect on the GB characteristics and the properties of materials. The study of multielement co-segregation in GBs is still in progress and deserves further research at the atomic scale. In this work, first-principles calculations were carried out to investigate the effect of Mg and Cu co-segregation on the energetic and mechanical properties of the Al Σ5(210) GB. The segregation tendency of Mg at the GB in the presence of Cu is characterized, indicating a preference for substitutional segregation far away from Cu atoms. Cu segregation can facilitate the segregation of Mg due to their mutual attractive energy. The GB energy results show that Mg and Cu co-segregation significantly decreases GB energy and thus enhances the stability of the Al Σ5(210) GB. First-principles tensile test calculations indicate that Cu effectively counteracts the weakening effect of Mg segregation in the GB, particularly with the high concentration of Cu segregation. The phenomenon of Cu compensating the strength of the GB is attributed to an increase of charge density and the formation of newly formed Cu-Al bonds. Conversely, Mg segregation weakens the strengthening effect of Cu on the GB, but it can increase the strength of the GB when high concentrations of Cu atoms are present in the GB. The ICOHP and Bader charge analysis exhibits that the strengthening effect of Mg is attributed to charge transfer with surrounding Al and Cu, which enhances the Cu-Al and Al-Al bonds. The results provide a further understanding of the interplay between co-segregated elements and its influence on the energetic and mechanical properties of grain boundary.

Effect of grain size gradient on the mechanical behavior of gradient nanograined pure iron: an atomic study

Abstract Using molecular dynamics (MD) simulation, the deformation mechanisms of gradient nanograined (GNG) pure iron (Fe) were investigated. Simulations of uniaxial tensile experiments were conducted on samples exhibiting different grain size gradients (GSGs). The simulation results reveal the presence of a critical GNG parameter (g), at which point the GNG-Fe attains its highest strength. The deformation mechanisms of three representative samples, namely GNG-2 with the g value at the threshold, GNG-1 with a g value smaller than the critical threshold and GNG-4 with a g value exceeding it, were thoroughly investigated. Within the coarse-grained (CG) region of GNG-1, the primary deformation mechanism is predominantly characterized by planar defects, rather than being dominated by dislocations. The mechanisms of both “strain hardening” and “softening” were observed and discussed in this region. The deformation of the coarse grains occurs in a coordinated manner, and the magnitude of the back-stress is insufficient to trigger grain boundary (GB) motion in the fine-grained (FG) region. In contrast, the deformation of the CG region in the GNG-4 primarily depends on dislocation. The “hardening” and “softening” effects of the dislocations were discussed. In the FG region of GNG-4, the grains undergo deformation primarily through GB motion, a phenomenon attributed to the significant back-stress generated by the uncoordinated deformation exhibited by the coarse grains. In the CG area of sample 2 with the g value at threshold, both dislocation- and planar defects-controlled mechanisms are observed. In the FG of this sample, neither GB migration and grain rotation are found. Only the GB width becomes larger, indicating that the back-stress transferred from the CG area makes the GB more active, but not large enough to induce the GB migration or grain rotation. The results of this work may provide some theoretical supports for the deformation mechanism of the GNG materials.

Multi-pass weld influence on metallurgical, tensile, impact toughness and fatigue crack growth behavior of gas tungsten arc welded Ti-6Al-4V joints

A comparative study on the influence of changing the number of weld passes on the metallurgical and mechanical properties of Ti-6Al-4 V alloy was carried out. Two welded joints involving 3-pass and 4-pass welding procedure were fabricated for 7 mm thick Ti-6Al-4 V plates using gas tungsten arc welding (GTAW) process. A compatible solid filler of Ti-6Al-4 V was used to weld these plates. A significant variation in the microstructure and the microhardness of the upper part of the fusion zones of these welds was observed. Equiaxed microstructure possessed by the base metal resulted into relatively better transverse tensile strength, ductility, and impact toughness than the 3-pass and the 4-pass weld. Widmanstätten and martensite-α′ microstructure associated with the fusion zone of the 3-pass weld resulted into a larger fraction of high angle grain boundaries (HAGBs) which accounted for higher resistance to fatigue crack propagation in this weld. Fusion zone of the 4-pass weld possessed lamellar and basketweave microstructure which had a lower fraction of HAGBs and, hence exhibited relatively lower resistance to fatigue crack propagation as compared to the 3-pass weld. However, equiaxed microstructure of the base metal could not offer much resistance to fatigue crack propagation, which resulted into relatively poor fatigue performance of the base metal. These fatigue studies indicate that both the weld joints exhibited better fatigue crack resistance than the base metal. This study established that 7 mm thick Ti-6Al-4 V joint welded using 3-pass welding procedure resulted into superior mechanical properties than those obtained by using 4-pass welding procedure.

Stress-driven triple junction reconstruction facilitates cooperative grain boundary deformation

Triple junctions (TJs), essential components linking neighboring grain boundaries (GBs), are of great significance for the deformation of entire GB networks in polycrystalline materials. However, kinetic behaviors of TJs and their coupling with GB plasticity remain largely unexplored, especially at atomic scale. Using atomistic in situ nanomechanical testing, we reveal a regime of dynamic TJ reconstruction for accommodating the coordinated deformation of GB network in gold and platinum polycrystals, proceeding through different modes of structural transformations, including disordered atomic arrangement, subgrain, dense stacking faults, and nanotwins. Such TJ reconstruction preferentially nucleates at TJs predicted with strong dragging effect, which serves as an effective route to facilitate the cooperative motion of neighboring GBs, in contrast to the widely-believed TJ deformation in steady state. This reconstruction-coordinated TJ kinetics provides novel insights into complicated GB network evolution and calls for a revisit of TJ roles in polycrystalline materials.

Enhanced CO2-to-CH4 conversion via grain boundary oxidation effect in CuAg systems

The authentic active sites of oxide-derived copper (OD-Cu), namely grain boundaries (GBs) and oxidized Cuδ+ species, is still debatable, and their role in governing CH4 conversion remains unclear. Herein, this study answers these questions using bimetallic catalysts by novel electro-shock strategy with controllable GBs for the oxidization of Cuδ+ species by modulating Ag loading. The Ag enrichment at the GBs facilitates the bonding of oxygen with the uncoordinated Cu atoms, resulting in GB oxidation effect. The obtained CH4 selectivity is twice that of GBs or nanoalloy effect. The enhanced performance is attributed to the stable Cuδ+ species and unique electron transfer mechanism from GB oxidation structure. Operando attenuated-total-reflection Fourier-transform-infrared-spectroscopy unveils the reaction pathway of CO2-to-CH4 and the sluggish reversible quenching processes of intermediates. Theoretical calculations indicate that the weak *CO adsorption on GB oxidation structure facilitates *CO hydrogenation, promoting CO2-to-CH4 conversion.

Non-Arrhenius grain growth in SrTiO3: Impact on grain boundary conductivity and segregation

In this study, a correlation between the conductivity, space charge layers, solute drag and non-Arrhenius grain growth in SrTiO3 was investigated. Strontium titante is known for its non-Arrhenius grain growth where grain growth rates decrease by orders of magnitude with increasing temperatures between 1350°C and 1425°C. Here, undoped SrTiO3 was sintered and annealed at temperatures below and above the grain growth transition. The influence of the annealing temperature and time on the conductivity and space charge layers at grain boundaries (GBs) in SrTiO3 was investigated by electrochemical impedance spectroscopy (EIS). STEM-EDS analysis indicates the presence of GBs with qualitative different cationic segregation in SrTiO3. A distortion of the GB semi-circle in the impedance plots was found which was attributed to the presence of multiple GB types with different electric properties and space charge layers. The presence of two GB types as indicated by impedance analysis corresponds well with previous results, where solute drag and segregation was supposed to cause the non-Arrhenius grain growth behavior of SrTiO3.

A method for determining efficiency parameters based on efficiency-quality graph in incremental sheet forming

This study reveals the mechanisms by which efficiency parameters affect surface quality, mechanical properties, and configuration offset in incremental sheet forming (ISF), focusing on surface morphology and microstructural evolution. By constructing an efficiency-quality graph, a method is developed to determine efficiency parameters that balance quality and efficiency synergistically. The results show that increasing Z and decreasing v intensify plowing and adhesion effects at the interface, degrading surface quality and enriching the presence of {001}<100> Cube and {112}<111> Copper textures, while the cross-sectional crystal orientation remains unaffected. Additionally, increasing Z enlarges grain size and reduces high-angle grain boundaries (HAGBs), amplifying micro-stress and defects, which deteriorates mechanical properties and increases configuration offset. Conversely, increasing v has minimal impact on mechanical properties but significantly increases configuration offset due to the rise in HAGBs, micro-stress, and defects. The efficiency-quality graph elucidates the complex synergistic relationship between forming efficiency and comprehensive quality of the formed parts, and the optimal efficiency parameters are identified by considering the priority order of different quality dimensions. Specifically, when prioritizing surface quality and hardness, the optimal parameters are Z=0.1mm and v=5000mm/min; when prioritizing configuration offset, the optimal parameters are Z=0.1mm and v=1000mm/min. Therefore, this study provides a comprehensive method for evaluating efficiency parameters in incremental sheet forming, emphasizing the importance of balancing efficiency with quality.

The evolution of grain boundary structure mediated by disclinations in magnesium alloys under superplastic deformation

Superplastic deformation in metals and alloys, characterized by ultrahigh ductility (exceeding 300%) without cracking at elevated temperatures, is a critical process for manufacturing complex-shaped components. While a few grain-boundary (GB)-mediated deformation mechanisms have been identified as essential contributors to superplasticity in fine-grained polycrystals (grain size is typically less than 10 μm), it is still a challenge to maintain a steady fine-grained microstructure and sustainable plastic flow at high temperatures. Partially due to the lack of a quantitative description of dislocation-GB reactions, it has not been well recognized how grain coarsening can be suppressed by the external loading during superplastic deformation. In this work, we address this challenge by formulating a disclination-dislocation coupling equation within the Lie-algebra framework, providing a quantitative understanding of the interactions between disclinations, dislocations, and GBs. Using quasi-in-situ electron backscattered diffraction (EBSD) analysis in Mg alloys, we systematically investigate the multiscale interactions of the defects and their impact on grain structure evolution. Three key mechanisms that suppress conventional grain coarsening have been identified, i.e., disclination-assisted GB accommodation, disclination-GB pinning, and disclination-induced sub-GB crossing, all of which are captured by the proposed equation. This study contributes to the broader field of plasticity by linking macroscopic deformation behavior with microscopic mechanisms, offering new insights into the theory of superplastic deformation in metals and alloys.

Evolution of microstructure and mechanical properties in TiBw/Ti65 composites during vacuum solid-phase diffusion bonding

This paper examines the solid-phase diffusion bonding (DB) of TiBw/Ti65 composites, focusing on the effects of processing parameters such as temperature, time, pressure, and interlayer thickness. The findings show that lower processing conditions resulted in slower atomic diffusion and more defects, whereas higher conditions reduced pores but led to grain growth. Optimal bonding was achieved at 950°C, 10MPa, and 60minutes, yielding a shear strength of 686MPa, which is 81.86% of the base material's strength. The introduction of pure titanium interlayer results in a concentration gradient, which serves as an additional driving force for atomic diffusion, thereby enhancing the quality of diffusion bonding. Shear strength varied with interlayer thickness, peaking at 668MPa for a 5 μm interlayer. Microplastic deformation, dynamic recrystallization (DRX), and grain boundary (GB) migration affected the microstructure and mechanical properties of the bonding interface. TiB whiskers (TiBw) near the bonding interface significantly influenced DRX and GB migration during the DB process.

Influence of post-weld rolling and artificial aging on microstructure and mechanical properties of friction stir welded 2195-T4 Al-Li alloy joints

Fiction stir welding (FSW) of a naturally aged (T4) 2195 Al-Li alloy was conducted to study the effect of post-weld artificial aging (AA) and rolling+artificial aging (R+AA) on microstructure and mechanical properties of the FSW joints. Grain features were analyzed by electron back scattered diffraction (EBSD) technology, and precipitates were characterized using transmission electron microscopy (TEM) along [011]Al and [001]Al zone axes. Mechanical properties of the joints were evaluated through micro-hardness and tensile testing. The results indicate that grain development, including grain orientation, grain boundary components and dislocation density, varies within different regions of the As-welded joints due to complicated thermal-mechanical history. The alloying elements mostly exist as Guinier-Preston zones in the base material (BM) with a few formations of δ'/β', and slightly precipitate into T1 in the heat affected zone (HAZ), form σ in the thermal-mechanical affected zone (TMAZ), as well as develop T1 and S' in the nugget zone (NZ) after FSW. The lowest hardness zone (LHZ) exhibits elevated precipitation levels, generating coarse T1 and grain boundary precipitates with evident σ and precipitate free zone. The overall precipitation level of the As-welded joints remains minor, resulting in a slight decrease in hardness at the LHZ. Post-weld AA promotes many T1 formations, enhancing the joint strength and hardness. As a result, the ultimate tensile strength (UTS) of the joints increases from 440 MPa to 506.5 MPa, while the elongation decreases from 13.5% to 3.4%. Furthermore, hardness profiles transform from a low-amplitude wave curve to a high-amplitude W-shaped curve, generating a significant LHZ due to the minimal precipitation of T1. Pre-rolling deformation can enhance dislocation density and low angle grain boundaries (LAGBs), promoting the nucleation of high-density, fine T1 during artificial aging, and especially improves precipitation ability of the LHZ, leading to an elevated hardness increment in the region after AA. Compared to the Joint-AA, both strength and elongation of the Joint-R+AA increase, obtaining 530.5 MPa and 3.9%, respectively.

Investigation of the recrystallization behavior of cold-rolled A1050 thin sheets based on misorientation measurements and grain boundary information

The recrystallization behaviors of severely cold-rolled A1050 thin sheets (100μm thick) with a reduction area of 98.3% were examined in detail. The thin sheets consisted of elongated and banded grain structures with typical β-fiber (Brass-S-Copper) texture components. Based on electron backscatter diffraction (EBSD), microstructural features such as misorientation measures of the kernel average misorientation (KAM) and grain boundary information of the lattice misorientation angle and axis were characterized. The elongated grains with representative orientations near Copper and S components possessed slightly greater KAM values than those near Brass. During annealing, newly recrystallized grains mainly contained the representative orientations near Copper and S, as opposed to Brass. Low angle grain boundaries (LAGB) observed between the recrystallized and deformed β-fiber grains revealed the occurrence of subgrain growth. The typical recrystallization Cube and Goss textures were also found during annealing. An equi-axed Cube grain growing inside deformed grains with the representative orientation near Copper reveals widespread grain boundary misorientation. An ellipsoidal Goss texture grows up along the two deformed bands with equivalent near-S orientations. One band is mainly consumed by Goss, and the other band prevents it from growing inward by means of grain boundaries with 60∘〈111〉. Various grain boundary misorientations reveal the dynamic evolution of the newly recrystallized grains.

Effect of the flow behavior and dynamic recrystallization of EH420 marine steel on the morphology evolution of martensite substructures

The hot deformation behavior of EH420 marine steel was investigated by isothermal compression tests in the temperature range of 850~1050°C and the strain rate range of 0.01~10s−1. A high-temperature constitutive model and a hot processing map were developed. The results indicated that the microstructure of EH420 marine steel after hot deformation was mainly bainite and lath martensite. The martensite blocks gradually coarsened with the increase in temperature within the range of 950~1050°C/0.01s-1, and the average width increased from about 0.7 μm to 2.3 μm. At high strain rates, martensite blocks became coarse and disordered due to flow instability. The flow stress curve showed a single peak when the strain rate was low. There were more dynamic recrystallization (DRX) structures, resulting in fine and ordered martensite blocks. Under the deformation conditions of 0.1~10s-1/1000°C, the proportion of high-angle grain boundaries (HAGBs) decreased from 53.5% to 40.7% as the strain rate increased. Meanwhile, the hot deformation mechanism shifted from discontinuous dynamic recrystallization (DDRX) to the combined effect of DDRX and dynamic recovery (DRV) and finally transformed into DRV. The flow stress derived from the constitutive model was consistent with the experimental results. The activation energy of EH420 marine steel was 3.16×105J/mol. The optimal hot processing parameters were 950~1050°C and 0.01~0.1s-1.

A novel cobweb-like sub-grain structured Al-Cu-Mg alloy with high strength-plasticity synergy

Al-Cu-Mg alloys, as the most widely used lightweight structural materials, have been recognized as promising candidates in the transportation field for a low-carbon economy. However, the tensile strength and plasticity of alloys cannot be simultaneously improved to satisfy the requirements of continuously upgraded transportation vehicles. In this work, inspired by high-tensile strength and high plasticity of cobweb structure, a novel cobweb-like sub-grain structure was developed in Al-Cu-Mg alloys by a successive solution, high-strain-rate rolling (4.4 s-1), cryogenic treatment (–196°C) and aging process (SRCA). Notably, the tensile strength and plasticity of this alloy were superior to those reported in the current study. An ultrahigh Vickers hardness and tensile strength value of 206.2 Hv and 619.6 MPa, which were 39.8% and 31.8% higher than those of traditional T6 heat-treated Al-Cu-Mg alloys, were obtained after SRCA. Meanwhile, an increase in the elongation of this alloy from 4.31% to 8.23% (increase of 90.9%) was also achieved. More importantly, the high strength-plasticity (“double high”) Al-Cu-Mg alloy was attributed to a cobweb-like sub-grain structure, which was proposed for the first time by utilizing reverse thinking to enhance plasticity through elevating dislocations, due to the formation of high-density dislocations from high-strain-rate rolling and rearrangement effect of dislocations from cryogenic treatment. Furthermore, the strength-plasticity mechanism was verified using in-situ tensile electron back scatter diffraction (EBSD), molecular dynamics (MD) simulations, and crystal plasticity (CP) models. The results indicated that the cobweb-like sub-grain structure, resembling countless walls, formed barriers that hindered dislocation migration towards high-angle grain boundaries (HAGBs) and absorbed them, thereby reducing the occurrence of stress concentration zones, i.e., the dislocation absorption and stress-strain sharing mechanisms. Additionally, the strengthening mechanism was associated with synergistic strengthening by multiscale microstructures, including micron-sized grains, micron-sized high-density dislocation lattices, and nanosized Al2CuMg phases, which were activated by successive deformation processes. Consequently, the concept of biomimetic structure design, which may serve as an effective method for achieving structural materials with high strength-plasticity synergy, can be extended to transportation fields, such as railway tracks and body structure design.

Design and control the selection of martensitic variant to simultaneously improve strength and toughness of low-carbon martensitic steel

In this paper, the microstructure evolution and mechanical properties of low carbon steels during direct quenched and rolling followed by water-cooled processes were studied. Two experimental steels, which were directly quenched at 900 °C (Q900) and 1000 °C (Q1000), were compared with steels that were water-cooled after rolling at the same temperatures (R900 and R1000). Microstructural analyses using EBSD and TEM revealed that rolling reduced the size of prior austenite grains (PAGs), resulting in an average width of 3.6 μm, which influenced grain boundary distributions and variant selection. The best combination of strength, ductility and toughness was obtained in R900 steel, including tensile (with the yield strength of 1304 MPa, the total elongation of 22.95 %), Charpy impact (with the impact energy at 20 °C is 182 J), and fracture toughness evaluations (with the J1c is 326.28 KJ/m2), this demonstrates that R900 steel exhibited significantly enhanced strength and ductility compared to Q900 steel. Moreover, EBSD analysis of crack propagation paths highlighted the role of high-angle grain boundaries (HAGBs) in enhancing fracture toughness by deflecting cracks. These findings underscore the critical role of PAGs size in tailoring microstructures to achieve superior mechanical properties in low carbon martensitic steels, offering insights for advanced material design and application in demanding structural and industrial contexts.Keyworks.low-carbon martensitic steel; strength and toughness; martensitic transformation; ductile-to-brittle transition phenomenon; selection of martensitic variants.