This study investigates the microstructural evolution and mechanical property enhancement of secondary-hardening ultra-high-strength steel AF1410 subjected to electropulsing (EP) treatment. To decouple the thermal and athermal effects of the pulsed current, a reference condition with identical thermal exposure (390 °C for 352 seconds) was introduced for direct comparison. Results show that EP treatment enhances mechanical properties compared to the as-forged baseline: ultimate tensile strength increases from 1629 MPa to 1785 MPa (+ 9.6 pct), Vickers hardness rises from 472 HV to 538 HV (+ 14.0 pct), while elongation remains essentially unchanged. EP yields additional improvements compared to the thermally matched reference (1675 MPa, 496 HV), providing unambiguous evidence for an athermal contribution beyond pure Joule heating. Microstructural analyses reveal that both EP and thermal exposure induce substantial dislocation density increases (8–9 times) and grain refinement, suggesting these evolutions are primarily thermally driven. However, high-resolution transmission electron microscopy discloses distinct carbide evolution pathways: the thermally treated reference develops an M2C-MC(I)-MC(II) carbide system, whereas EP treatment promotes the formation of a M2C-M23C6-MC(II) carbide system. The athermal effects of the electric pulse lower the effective precipitation temperature and accelerate carbide transformation kinetics. Notably, the EP process achieves these enhancements within a duration of only 352 s, offering an ultra-rapid, energy-efficient pathway for microstructural refinement and precipitation control.
Precise control of crystallographic texture is an effective strategy for enhancing the ductility of magnesium alloys. This work establishes a direct correlation between non-basal dislocation evolution and texture modification, and elucidates the mechanisms responsible for the improved ductility of texture-modified alloys. Mg-Y alloys extruded at 350 °C, 380 °C, and 420 °C, exhibiting gradual changes in recrystallization fraction and texture components, were used as experimental materials. The results demonstrate that, with increasing extrusion temperature, both the densities and the ratio of pyramidal II ⟨c + a⟩ to prismatic ⟨ a ⟩ dislocations increase, driving recrystallized nuclei to deviate progressively from their parent orientations and promoting the formation of ⟨112̅1⟩ -oriented and ⟨11̅01⟩ -oriented nuclei, whose orientations are parallel to the extrusion direction. The enhanced ductility of the texture-modified alloys is attributed to the combined activation of multiple non-basal slip systems and improved intergranular deformation compatibility. In particular, compared with a single ⟨112̅1⟩ texture, the dual texture of ⟨112̅1⟩ + ⟨11̅01⟩ exhibits superior intergranular compatibility, primarily due to more effective basal-to-basal slip transfer.
High-temperature shape memory alloys (HTSMAs) are promising functional materials; however, their brittleness limits the fabrication of components with complex geometries. In this study, a direct laser fabrication process was developed for Ni–Pt–Ir–Ti HTSMAs using Ni–Ti and Pt–Ir wires. This approach preferentially removed Ni while suppressing the loss of other elements from the melt pool through selective evaporation. As a result, a quasi-equiatomic composition of M (Ni, Pt, and Ir) and Ti with a high Pt concentration, (Ni22.1Pt25.2Ir2.6)Ti50.1, was achieved after 30 laser irradiations at a peak power of 4.0 kW and a pulse duration of 1.0 ms. The laser-processed region consisted predominantly of B19 martensite containing (111) Type I twins without brittle intermetallic compounds. The evaporation behavior of the alloying elements was analyzed based on the Hertz–Knudsen–Langmuir model in combination with thermodynamic calculations. The calculated compositional changes in the Ni–Pt–Ir–Ti alloys at high temperatures aligned with the experimental results, suggesting that the compositional changes during laser processing were primarily governed by selective evaporation. The methodology proposed in this work sheds light on the flexible manufacturing of HTSMAs and provides new insights into laser alloying in multi-component material systems.
This paper investigates the mechanical behavior of AA2024-T4 under various tension-dominated loading conditions using dog-bone, butterfly, plane strain, and notched specimens with three different notch radii. The results indicate that the total deformation of each specimen is non-uniform, and the location of strain concentration varies with the specimen type. It is also shown that the stress and strain state can be effectively controlled by altering the specimen size and geometry. Furthermore, by employing finite element simulations to extract the stress state at fracture initiation for different specimen types, it is revealed that both stress triaxiality and the Lode parameter jointly influence the equivalent fracture strain of the material. Based on the DF2012 fracture criterion, a modified failure criterion is proposed by introducing a hyperbolic functional relationship for the effects of stress triaxiality and the Lode parameter, thereby achieving accurate prediction of the equivalent fracture strain of AA2024-T4.
High strength line pipe steels obtain their properties from chemistry design and thermomechanically controlled processing (TMCP). Maintaining sufficient fracture toughness of the coarse grain heat affected zone (CGHAZ) is critical during welding of these steels. Here, austenite grain size is an important parameter. Austenite grain growth was measured in 37 line pipe steels during thermal cycles relevant to the CGHAZ, using a laser ultrasonics for metallurgy (LUMet) system attached to a Gleeble thermomechanical simulator. LUMet is a non-contact high throughput technique that allows in-situ measurements of austenite grain growth. A range of chemical compositions were selected to quantify the effect of alloying elements on austenite grain growth. Nb and C are found to retard austenite grain growth and an empirical equation is proposed for their effect on the austenite grain boundary mobility. The limiting grain size is found to depend on the fraction and average size of Ti-rich carbo-nitrides. The particle size prediction based on the LUMet measurements has been validated using Transmission Electron Microscopy (TEM) and Scanning Transmission Electron Microscopy (STEM). The grain growth model has been successfully applied for different thermal cycles and can effectively predict which chemistries will experience substantial austenite growth in the CGHAZ. As a result, it is a useful tool that can be used both by steel manufacturers for chemistry design and by pipeline constructors to predict the behaviour of the employed steels.
High-resolution digital image correlation (HRDIC) has been used to compare strain localization in conventional AZ31 and dilute rare-earth containing commercial ZEK100 magnesium alloys. The materials were processed to obtain a similar grain structure but have a different texture. It is demonstrated that whilst both alloys show strong concentration of strain in grain boundary regions, this is much more intense in AZ31 than ZEK100. Basal slip is observed to dominate in both AZ31 and ZEK100, although non-basal modes are also more active in the later case. Crystal plasticity simulations have been used to help explain the results. The predictions suggest that the higher local strains observed in AZ31 compared to ZEK100 can be reproduced through the texture difference between the materials alone, at least at the low plastic strains studied here. Other differences between the alloys, such as the ease of activating non-basal slip or grain boundary segregation, are expected but are therefore not essential to produce the difference in strain localization behaviour. These results are consistent with the hypothesis that the improved formability of ZEK100 compared to AZ31 is predominantly a texture effect.
This study investigates the strain-induced martensitic transformation (SIMT) and springback behavior of 304 austenitic stainless steel during multi-pass roll forming. By utilizing cyclic tensile tests, quantitative X-ray diffraction (XRD), and electron backscatter diffraction (EBSD), we established a critical correlation between microstructural evolution and macroscopic mechanical response. The martensite volume fraction increased monotonically with the cumulative forming angle. A significant acceleration in transformation kinetics was observed at a 40 deg forming angle, coinciding with a rapid decrease in twin boundary density and widespread slip band activation, and the extensive nucleation of martensite in 304 stainless steel triggers the transformation-induced plasticity (TRIP) effect, effectively suppressing springback. Mechanistic analysis revealed that α′-martensite nucleation sites evolved from high-energy twin boundaries to slip bands within soft-oriented grains as deformation progressed, while strictly adhering to the Kurdjumov–Sachs orientation relationship. Furthermore, the biaxial stress state characteristic of roll forming promoted a higher transformation rate compared to uniaxial tension. These findings provide a critical microstructural basis for accurate springback prediction in metastable stainless steels.
Wire arc additive manufacturing (WAAM) and casting are both viable for fabricating large, complex duplex stainless steel (DSS) components. However, their distinct processing characteristics yield significant differences in microstructure and properties. To address the current lack of direct comparisons between these methods, this study systematically contrasts WAAM and cast DSS in terms of forming processes, microstructures, mechanical properties, and tribological performance. Microstructural analysis (OM/EBSD) reveals that WAAM produces a more balanced α/γ phase ratio (1:2.28 vs 1:4.41 in cast DSS), finer grains (8.93 vs 22.26 μm), and a weakened 110<001> Cube texture in the α-phase. Mechanically, WAAM-fabricated DSS exhibits superior tensile strength (735.4 vs 619.7 MPa) with 21.81 pct elongation and inclusion-free fractures, whereas cast DSS shows higher elongation (25.07 pct) but contains MnCrO4 inclusions. The WAAMed DSS also exhibits superior wear resistance, characterized by a lower friction coefficient, reduced wear rate/volume, and a primary oxidative wear mechanism with secondary micro-abrasive/adhesive wear. Cast DSS primarily experiences abrasive/adhesive wear, with oxidative wear as a secondary contributor.
To satisfy the requirements for high-temperature and long-term service of carbide-strengthened 25Cr35NiNb alloys in the petrochemical industry, the effects of C addition (0.2 0.6 wt pct) on microstructural evolution and creep behavior were systematically investigated at 1173 K (900 °C) and 1323 K (1050 °C). Increasing C content significantly promoted the formation of Cr-rich carbides rather than Nb-rich carbides, while simultaneously suppressing the precipitation of the G phase. Multistep tensile creep tests revealed a pronounced enhancement in creep resistance with increasing C addition. The high apparent stress exponent and activation energy associated with power-law creep were rationalized by introducing a threshold stress, which originated from multiscale precipitation strengthening involving nanosized MC and sub-micron M23C6 carbides. The increase in threshold stress with C addition was mainly attributed to secondary M23C6 precipitates, which facilitated general dislocation climb and introduced additional back stress through sub-grain boundary impediment. Owing to the analogous creep-controlling mechanisms, the steady-state creep behavior can be reliably predicted over a wide range of C contents, applied stresses, and temperatures. This study provides fundamental insight into composition- and temperature-dependent microstructural evolution and offers guidance for the design of heat-resistant alloys operating above 1273 K (1000 °C).
To address the limitations of conventional fusion welding, the development of high-speed and high-strength solid-state joining technologies for dissimilar materials is essential. In this study, cold spot forge welding was applied to the lap joining of 1‑mm‑thick SUS304 stainless steel and A6061‑T6 aluminum alloy sheets with a spot diameter of 8 mm. The effects of the bonding temperature and the reduction ratio R on the joint strength and reaction layer (RL) formation were investigated at the bonding temperatures of 538 K to 693 K (265 °C to 420 °C) and the reduction ratios of 1.02 to 3.98. Tensile‑shear testing was conducted, and the interfacial reaction behavior was examined using electron probe microanalysis, electron backscatter diffraction, transmission electron microscopy, and related techniques. The diffusion-barrier effect of the oxide film decreased with increasing R, enabling the control of the RL thickness through the combined influence of R and the bonding temperature. The optimal processing conditions were identified as 663 K (390 °C) and R = 2.6, under which the RL was 10 to 30 nm thick and was effectively free of intermetallic compounds (IMCs). Future prospects for establishing predictive formulas for joint quality based on the relationship between the RL thickness and tensile strength are also presented. These findings advance solid-state joining technology by demonstrating a high-throughput method capable of overcoming IMC-related brittleness.
The evolution of size and distribution of second-phase reinforcements will cause significant local stress gradient in metal matrix composites fabricated by additive manufacturing and post heat treatment. However, current studies focused on macroscopic mechanical performance, which is unable to quantitatively unravel the local structural stability and nanomechanical responses of composites and its correlation with the evolution of melt pool regions. Here, the local deformation behavior and nanomechanical response of TiBw-reinforced Ti6Al4V composites fabricated by electron beam powder bed fusion and post-annealing treatment have been investigated via instrumented nanoindentation. Distinct microstructural evolution of melt pool boundary (MPB) and melt pool center (MPC), such as the width of melting layer and the variation of TiBw in size and distribution, was observed in the composites as annealing temperature increased from 800 °C to 1100 °C. Relative to the narrow dispersion of nanohardness and elastic modulus for MPB regions, the load-displacement data obtained from MPC region showed a clear transition from low to high dispersion at the critical temperature of 950 °C, which is consistent with macroscopic tensile results. This transition implied a microstructural change of recrystallization and precipitation of nano-TiBw, and the migration of boron atoms from MPB into MPC regions. In addition, the characteristic parameters of nanohardness and elastic modulus were statistically analyzed by Weibull distribution model. The local stress–strain relationship and strain hardening exponent was also determined by indentation inverse algorithm and dimensional analysis. Our findings provide a microscopic perspective on the understanding of macroscopic strain hardening and structural stability of metallic composites upon deformation.
Achieving high-strength diffusion bonding between titanium alloys and steels remains challenging because brittle Ti–Fe reaction products can form at the interface and provide preferential paths for brittle fracture. In this study, TC4/17-4PH joints with an Nb/Cu multilayer interlayer were investigated to clarify the temperature-dependent evolution of interfacial microstructure, mechanical performance, and fracture behavior at 850 °C to 950 °C. Microstructural and chemical analyses were combined with tensile and lap-shear testing and fractographic characterization. At 850 °C and 900 °C, interfacial diffusion remained moderate and did not lead to the formation of a continuous brittle reaction layer. The joints were mainly bonded through diffusion-assisted solid-solution-type interfacial interactions. Increasing the bonding temperature from 850 °C to 900 °C improved the joint strength, and the best overall performance was obtained at 900 °C, with ultimate tensile and lap-shear strengths of 849.13 ± 20.24 and 410.89 ± 24.19 MPa, respectively. At 950 °C, the Cu interlayer was markedly consumed, accompanied by formation of Ti–Fe reaction products and a more continuous Nb-rich reaction region, leading to a sharp decrease in the strength. Fractographic analysis further showed a temperature-dependent transition of the fracture path. At 850 °C to 900 °C, both tensile and shear fractures occurred within or near the Cu interlayer region and were accompanied by plastic deformation and tearing. By contrast, fracture at 950 °C shifted toward regions containing Ti–Fe intermetallics and Nb-rich constituents, resulting in predominantly brittle failure. These results indicate that the effective bonding window of the Nb/Cu-interlayered TC4/17-4PH joints is governed by the continuity of brittle interfacial products and the resulting fracture-mode change.
In the present study, a novel Ni matrix composite reinforced with in situ duplex TiC and Ni3(Al, Ti) phases is successfully fabricated by reactive hot-press sintering Cu@Ti3AlC2 and metallic Ni powders. Through high-energy ball milling, Ti3AlC2 particles are predominantly coated with Cu, which greatly prompted particle dispersion and interfacial compatibility with the Ni matrix. Microstructural characterization demonstrates that Cu completely diffused into the Ni matrix. The in situ L12-ordered Ni3(Al, Ti) phase is ultrafine (about 92 nm) in the T30C20N50 composite, whereas it grows to 1 to 2 μm in the T40C20N40 composite. In situ submicron TiC particulates are penetrated by Ni(Cu) phase, indicating the superior wettability. The T40C20N40 composite exhibit optimal mechanical performance, including a Vickers hardness of 5.82 GPa, compressive strength of 1519 MPa, flexural strength of 1047 MPa, and apparent fracture toughness of 28.13 MPa·m1/2. The enhancement of mechanical properties is attributed to the synergistic effect of austenitic solid solution strengthening, dispersion strengthening, and dislocation strengthening mechanisms.
Bi–Sb–Te alloys are known as the current dominant materials for thermoelectric cooling applications used at near room temperatures. In this work, the available literature data on thermodynamic properties and phase equilibria of the Bi–Sb, Bi–Te, Sb–Te, and Bi–Sb–Te systems are critically reviewed and evaluated. Thermodynamic assessments of the Bi–Sb, Bi–Te, Sb–Te, and Bi–Sb–Te systems are then performed based on the reliable experimental thermochemical and phase equilibrium data. The entire liquid solution phase is modeled using the modified quasi-chemical model in pair approximation. The compound energy formalism and the regular solution model are used for the Gibbs energies of the solid phases. The homologous series of Bi–Sb-rich phases, [(Bi, Sb)2]n[(Bi, Sb)2Te3]m, are modeled as one single solid solution phase. One set of self-consistent thermodynamic model parameters is optimized for the Bi–Sb–Te ternary system, which can reasonably describe the reliable thermodynamic and phase equilibrium data.
Phase equilibria and solidification behavior of the Sm–Co–Cu ternary system were studied by using scanning electron microscope with energy dispersive spectroscopy (SEM–EDS) and X-ray diffraction (XRD). The results of phase equilibria reveal that ten binary intermetallic compounds including SmCo2, SmCo3, Sm2Co7, Sm5Co19, SmCo5, Sm2Co17, SmCu2, SmCu4, SmCu5, and SmCu6 were observed, and the ternary intermetallic compounds were not detected. The continuous solid solution phase Sm(Co, Cu)5 is formed from the respective SmCo5 and SmCu5. The solubility of Cu in SmCo2, SmCo3, Sm2Co7, and Sm2Co17 and that of Co in SmCu2 and SmCu4 were determined. Three isothermal sections at 873 K, 1073 K, and 1273 K were established. Meanwhile, solidification microstructures of Sm10.5Co89.5−xCux as-cast alloys were examined. Furthermore, on the basis of the present and earlier experimental results, thermodynamic calculation of the Sm–Co–Cu ternary system was carried out using the CALPHAD method. The calculated isothermal sections and vertical sections in this ternary system are in good agreement with the experimental results. Finally, solidification processes of several Sm–Co–Cu as-cast alloys were simulated using the Scheil–Gulliver module with thermodynamic parameters. The simulated results are satisfactorily consistent with the experimental solidification microstructure. It indicates that thermodynamic parameters of this ternary system obtained in this work are self-consistent and reasonable. The present parameters would provide the proper foundation to develop a thermodynamic database of multi-component Sm–Co-based magnetic alloys, which is valuable information for exploring high-performance and low-cost Sm–Co–Cu-based permanent magnets.
Anomalous elimination refers to a dendrite competition outcome that deviates from the predictions of the classical Walton–Chalmers model during directional solidification. This phenomenon may contribute to the expansion of defects, thereby degrading the mechanical properties of single crystal alloys. However, the underlying mechanism of anomalous elimination remains poorly understood. In this work, the phase-field model, which comprehensively considers the temperature gradient, multi-components segregation, and melt convection, is developed to simulate the dendrite growth and competition process. The simulation result indicated that the melt convection is able to change dendrite morphology and thus may affect the dendrite competition through affecting the distribution of solutes around dendrites. At converging grain boundaries, when the melt flows from the favorably oriented dendrite side toward the unfavorably oriented dendrite side, convection benefits the growth of the unfavorably oriented dendrites, which may induce anomalous elimination. At diverging grain boundaries, the favorably oriented dendrites have a spatial advantage for developing tertiary arms. Melt convection at typical velocities (below 10 mm/s) is insufficient to reverse the competitive outcome at diverging boundaries.
The structural integrity of welded joints is critical to the safety and long-term durability of aluminum alloy components in automotive and aerospace applications, where fatigue failure frequently initiates at localized stress concentrations. This study investigates the fatigue performance of representative welding configurations in 6061 aluminum alloy. The fracture mechanisms and underlying microstructural evolution, particularly dislocation behavior, in the welded joints were characterized to elucidate the failure origins. Additionally, this study proposes a high-precision fatigue life prediction method for welded components, leveraging finite element numerical simulations, empirical fatigue performance data of 6061 aluminum alloy base material, and key correction parameters from the Forschungskuratorium Maschinenbau guideline. The method establishes fatigue life predictions based on stress characteristics at critical locations—specifically, those corresponding to characteristic points on the fatigue S-N curve. While validated for aluminum alloy welded joints, the approach is broadly applicable to engineering components, offering a novel and robust framework for fatigue performance assessment.
Coarse prior austenite grain (PAG) structures can sometimes be observed in high-strength steels, e.g., in parts produced by wire-arc additive manufacturing (WAAM), and are problematic as they are often associated with poor mechanical performance. In the case of WAAM, cyclic austenitisation can refine coarse PAG structures either during or after the building process. However, in this study we show that there can be a strong effect of heating rate during austenitisation on the resulting grain refinement. The heating rate effect was systematically studied in two steels—a low-alloy steel (300 M) and a maraging steel (Custom 465®)—from an as-WAAM’d state where both comprised coarse columnar PAGs. On heating, for all heating rates investigated and in both steels, the austenite ‘memory effect’ was first observed, which retained the columnar structure. This was then sometimes followed by autogenous recrystallisation (without applied external deformation), which greatly refined the PAGs. High-temperature in-situ electron backscatter diffraction was used to directly observe austenitisation in 300 M, revealing that the recrystallisation shifted from a continuous to a discontinuous mechanism with decreasing heating rate, and that a further decrease in rate suppressed recrystallisation altogether, leaving only a memory effect. Conversely, in Custom 465®, recrystallisation produced significant PAG refinement across all heating rates investigated. However, the recrystallised PAGs retained some crystallographic texture inherited from the columnar PAGs and twins in the as-built material. These findings provide mechanistic insight into grain refinement strategies for steels with coarse PAGs and highlight the importance of controlling heat rates during industrial processing.
Employing high-damping steels is advantageous for applications that require vibration and noise reduction. However, the intrinsic tradeoff between strength and damping limits their use in load-bearing applications. Previous studies established processing pathways to overcome this tradeoff, e.g., through partial recrystallization, but these approaches are unsuitable to be scaled up for thick-section plates. To address this challenge, we employed the CALPHAD-based genomic design framework to develop a precipitation-strengthened, high-strength, high-damping steel compatible with thick-section manufacturing processes. Experimental findings on a fully homogenized and solution-treated Fe–Mn–Nb–C design prototype confirmed that the employed precipitation-strengthening strategy enhances the strength of the alloy by 60 pct, while preserving its damping performance at low oscillation strain amplitudes (< 0.03 pct). Further, we applied the Richman-Bolling technique to conduct a thermomechanical assessment of a high-damping Fe–Mn prototype alloy, confirming the employed strengthening approach. The presented findings, beyond providing other insights, establish genomic design as a promising pathway for developing high-damping steels deployable for thick-section load-bearing applications.
This study introduces a novel methodology for quantifying strain-induced martensite in austenitic TRIP steels using magnetic flux density measurements from a Teslameter as the primary physical quantity. Incremental tensile tests combined with XRD and microstructural analysis established the correlation between martensite fraction and magnetic flux density. To model this nonlinear relationship, three formulations were developed and optimized: a proposed empirical model, the physically based Olson–Cohen (O-C) model, and a modified Avrami (M-A) model. A multi-algorithm machine learning approach was employed for calibration, comparing deterministic Levenberg-Marquardt with metaheuristics including genetic algorithm, particle swarm optimization, and grey wolf optimizer in Python. The empirical model achieved the highest fitting accuracy (MSE: 0.5919, R2: 0.9978), while the O-C model demonstrated superior algorithmic stability with all optimizers converging identically. Results reveal a fundamental trade-off between empirical accuracy and physical consistency of transformation kinetics. This work enables reliable, data-driven quantification of martensitic evolution through non-destructive magnetic evaluation, facilitating real-time structural condition monitoring of TRIP steel components.
