Search Results (401)

Dynamic recrystallization processes are known to significantly affect both the mechanical properties and the microstructure of materials. In this paper, we investigate the influence of discontinuous dynamic recrystallization (dDRX) during deformation at high strain rates (from 10⁴ to 10⁵ s⁻¹) and elevated temperatures in pure aluminum and copper (in the range of 700–800 K for aluminum and 800–1100 K for copper). For this purpose, we propose a theoretical model in which the material is described within the framework of continuum mechanics, plastic deformations are modeled using a dislocation plasticity approach, the equation of state is represented by a neural network, and the microstructure evolution is simulated using the cellular automata method. The model is applied to uniaxial compression and tension of copper and aluminum polycrystals with an initial average grain size of 14 μm. It is shown that grain refinement occurs in all systems. The average grain size decreases from 14 μm to 4–5 μm. The distribution of plastic and total strains in the polycrystals is presented. In all considered systems, deformation localization is observed, and the localization pattern changes due to the nucleation of new grains and grain boundary surfaces during dynamic recrystallization. Full article

Ductility dip cracking (DDC) remains a persistent challenge in multipass welds of high-chromium nickel-based alloys used in the nuclear power generation industry. While dynamic recrystallization (DRX) has been observed to arrest DDC crack growth and has been associated with weld regions that experience less DDC, there exists no quantitative relationship between the extent of recrystallization in a microstructure and DDC susceptibility. This research examines the influence of intragranular carbides on DRX behavior and establishes an experimental relationship between DDC susceptibility and extent of recrystallization in high-chromium nickel-based weld metals, novel contributions for this alloy system. In this work, the DRX behavior of the weld metal of high-chromium nickel-based filler metals (FM-52, FM-52M, FM-52i, and FM-52xl) was investigated under controlled thermo-mechanical conditions, and its effect on DDC susceptibility was established. Weld metal specimens were subjected to uniaxial deformation at 1100 °C to a true strain of 2% at strain rates of 10⁻³/s and 10⁻⁴/s using a Gleeble 3800^TM. Recrystallization was quantified using electron backscatter diffraction (EBSD) via grain orientation spread (GOS) analysis and dislocation–precipitate interactions were examined using transmission electron microscopy (TEM). Strain-to-fracture (STF) testing at 950 °C was employed to assess DDC susceptibility as a function of the extent of recrystallization and grain surface area. All tested weld metals exhibited increased recrystallization and grain refinement, as the strain rate decreased from 10⁻³/s to 10⁻⁴ s. The FM-52i weld metal specimens exhibited the highest grain refinement under high temperature deformation, followed by the FM-52xl, FM-52, and FM-52M weld metals with a percent reduction in average grain surface area of 51.22%, 41.66%, 35.48%, and 24.40%, respectively. The FM-52i weld metal specimens also exhibited the highest recrystallization response, followed by FM-52M, FM-52xl, and FM-52 weld metals at 75%, 40%, 39% and 21% recrystallized, respectively. Weld metals containing strong carbide formers experienced higher recrystallization responses than those without due to precipitate–carbide interactions. All tested weld metals experienced drastic reductions in DDC response with increasing extent of recrystallization and decreasing average grain surface areas. DRX in STF specimens was observed to facilitate uniform plastic strain accumulation, lowering overall DDC susceptibility compared to non-recrystallized specimens. Full article

Friction stir welding (FSW) of dissimilar aluminium alloys often results in non-uniform microstructure and hardness distributions due to asymmetric temperature fields and material flow. The objective of this study is to establish a quantitative relationship between thermal history, microstructural evolution, and hardness distribution in dissimilar AA5052-H32/AA6061-T6 FSW joints by combining experimental characterisation with validated thermal modelling. AA5052-H32 and AA6061-T6 plates were welded under five different parameter sets. A thermal finite element model was developed in COMSOL Multiphysics to simulate temperature evolution during welding and was validated using embedded thermocouple measurements, with predicted peak temperatures ranging from 455 °C to 641 °C. Optical microscopy, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) were employed to characterise grain structure and dynamic recrystallisation (DRX) behaviour, while Vickers microhardness mapping was used to evaluate the local mechanical response. The results show that DRX occurred in the nugget zone (NZ), leading to significant grain refinement, with a minimum grain diameter of 6.07 µm, representing an approximately eightfold reduction compared with the base material AA5052-H32. In contrast, the thermo-mechanically affected zone (TMAZ) experienced limited recrystallisation due to insufficient plastic deformation and temperature. The lowest hardness was observed in the TMAZ on the AA5052-H32 side, with the hardness reduction of 22% primarily caused by work hardening loss. Hardness was also reduced by 34% on the AA6061-T6 side due to decreased precipitation strengthening caused by high temperatures. This combined experimental–numerical study provides a systematic thermal–microstructure–hardness framework for understanding and predicting local property variations in dissimilar FSW joints. Full article

Friction stir welding (FSW) is a key technology for manufacturing T-shaped thin-walled structures and avoiding fusion welding defects. However, the quantitative relationship between its process parameters and the microstructure properties of the joint remains unclear. To address this, this study established regression models via response surface methodology (RSM) relating rotational speed (w), welding speed (v), and plunge depth (h) to the mechanical properties of T-joints. The optimal process parameters (400 rpm, 60 mm/min, 0.21 mm) were determined, under which the ultimate tensile strength (UTS) and weld nugget hardness (WNH) of the joint reached 74.1% (377 MPa) and 94.4% (153 Hv) of the base materials (BM) respectively, with v showing the most significant influence on joint mechanical properties. Microstructural observations revealed that from the BM to the stirring zone (SZ), the grains underwent a continuous evolution from coarsening, partial recrystallization to complete dynamic recrystallization (DRX). In the SZ, due to severe plastic deformation and high heat input, the continuous dynamic recrystallization (CDRX) was the dominant mechanism, and the grain was significantly refined. The heat input in the thermomechanical affected zone (TMAZ) is relatively low, mainly geometric dynamic recrystallization (GDRX). DRX-driven grain refinement was the primary strengthening factor in the joint, with hardness closely related to grain size. However, thermal cycling induced softening in the heat-affected zone (HAZ) and promoted the precipitation of brittle compounds such as Al₃Mg₂ and MgZn₂, which caused crack initiation exhibiting intergranular brittle fracture. Subsequently, under stress drive, it extends to SZ, mainly characterized by ductile fracture. Full article

To give more insight into the microstructural evolution and deformation mechanisms governing the long-term service performance of additively manufactured TiAl-based composites at elevated temperatures, this study investigated the high-temperature compressive creep behavior of a laser powder bed-fused LaB₆ reinforced high-Nb TiAl-based composite after hot isostatically pressing (HIP), with emphasis on the creep response and dynamic recrystallization (DRX) mechanisms under different applied stress levels. The results showed that, as the applied stress increased from 200 MPa to 450 MPa, the steady-state creep rate rose from 2.88 × 10⁻⁸ s⁻¹ to 3.85 × 10⁻⁷ s⁻¹. Stress exponent analysis indicated that creep deformation was predominantly controlled by dislocation climb, and no tertiary creep stage was observed within the investigated stress range. At 200 MPa and 300 MPa, a certain fraction of recrystallized grains formed during prolonged creep exposure. When the stress increased to 400 MPa, the recrystallization process was restricted due to the limited creep duration. In contrast, at 450 MPa, the accelerated accumulation of strain energy significantly promoted recrystallization. Both continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) were identified, jointly governing the microstructural evolution. Superior creep resistance can be attributed to multiple synergistic strengthening mechanisms, including the refined α₂/γ lamellar structure induced by HIP treatment, the strong pinning effect of dispersed La₂O₃ nanoparticles on dislocation motion, and the suppression of diffusion-controlled dislocation climb by Nb addition. These combined effects enhance the high-temperature creep performance of the TiAl composite and provide important insights for the application of LPBF-fabricated TiAl-based composites under elevated-temperature service conditions. Full article

To study the influence of freckle defects on the hot deformation behavior and the inheritance of dynamic recrystallization (DRX) structure in GH4706 alloy, the microstructures of specimens with and without freckles and the evolution laws of hot-processing parameters were compared. Hot compression experiments were conducted on a thermal simulation testing machine at 950–1150 °C, strain rates of 0.001–1 s⁻¹, and 55% deformation. Freckle-containing specimens were tested under DRX critical conditions. The flow stresses of both specimens increase with strain rate or with decreasing temperature. The power dissipation coefficient (η) and instability value (ξ) follow complex laws. Electron back-scattering diffraction (EBSD) was used to analyze DRX microstructures and nucleation mechanisms. The DRX degree of freckle-containing specimens is lower, with a larger average grain size. The DRX mechanism initiates preferentially in freckle-containing specimens, and its volume fraction changes in a complex manner. Grain coarsening occurs in freckle-containing specimens at high temperatures and low strain rates. Freckle defects lead to significant differences in the DRX mechanism of GH4706 alloy. Freckle-containing specimens exhibit both discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX), whereas freckle-free specimens primarily display DDRX and second-phase particle-stimulated nucleation (PSN). The presence of MC carbides and Laves phases within freckle defects provides nucleation sites, further supporting a typical second-phase particle-stimulated nucleation mechanism. Full article

The development of sustainable pigments from natural sources is gaining interest due to environmental concerns and the need for bio-based alternatives to synthetic dyes. This study investigates the synthesis of hybrid pigments by adsorbing anthocyanins—extracted from pomegranate agro-waste—onto halloysite (HA) nanotubes. A full factorial design was applied to evaluate the influence of pH and surfactant type (cetylpyridinium bromide and sodium dodecyl sulfate) on pigment colour and the thermal and structural stability of the hybrids. Adsorption was carried out in 400 mL dispersion baths containing 10 g of HA and 5% w/w anthocyanins. Surfactants (2% w/w) were added before the pigment, followed by 200 µL of silane. Dispersions were stirred at high speed for 1 h and then at 500 rpm for 23 h to ensure adsorption without premature desorption. Characterisation (TGA, XRD, FTIR, UV-Vis/NIR, SEM, EDX, BET) confirmed the preservation of HA structure and minimal changes in thermal behaviour. Pigment colour varied with synthesis conditions, especially pH: a higher pH increased brightness and yielded yellowish tones, while a lower pH resulted in reddish-blue hues with greater variability. The results confirm halloysite’s potential as a stable carrier for natural dyes and demonstrate that pH effectively tunes hybrid pigment colour. Full article

This study investigates the hot deformation behavior of electroslag remelted (ESR) 25Cr2Ni2MoV steel, focusing on the effects of deformation temperature and strain rate on flow stress, microstructure evolution, and dynamic recrystallization (DRX) mechanisms. Hot compression tests were performed at temperatures ranging from 1120 °C to 1210 °C and strain rates from 0.01 s⁻¹ to 10 s⁻¹ to generate true stress–strain curves. The friction and adiabatic temperature effects were corrected to ensure accurate results. The data reveal that the material exhibits a single-peak true stress–strain curve, characteristic of dynamic recrystallization softening. The flow stress is negatively sensitive to temperature and positively sensitive to strain rate. An Arrhenius-type constitutive model was developed, and the activation energy for hot deformation was determined to be 371.3 kJ/mol. EBSD analysis show that the recrystallized grain size is highly dependent on strain rate, with finer grains formed at lower strain rates (0.01–0.1 s⁻¹). A processing map constructed at a true strain of 0.5 identified an optimal hot-working window at deformation temperatures of 1160–1200 °C with strain rates below 0.37 s⁻¹, providing guidance for the forging process of large 25Cr2Ni2MoV steel. Full article

By means of characterization techniques such as XRD, TEM, and in situ EBSD, the texture evolution, recrystallization behavior, and their modulation by the Al₃Zr phase in hot-rolled Al-Zn-Mg-Cu-Zr alloys with varied homogenization treatments were investigated. Results show that both the single-stage homogenized (SH) alloy and the double-stage homogenized (DH) alloy acquired a typical β-fiber texture after hot rolling, including brass, S, and copper orientations. The DH alloy experienced suppressed recrystallization (a recrystallization fraction of 6.05%) owing to its higher density of Al₃Zr precipitates. In contrast, the SH alloy exhibited more significant dissolution and agglomeration of Al₃Zr, leading to extensive recrystallization peaking at 78.1%. The primary recrystallization mode was identified as continuous recrystallization, characterized by the growth and coarsening of subgrains. Although dynamically recrystallized (DRx) grains formed during hot rolling could act as potential recrystallization nuclei, most of them exhibited weak growth capability, except the cube-oriented grains. During recrystallization, deformed grains with S orientation tended to transform into cube-oriented grains, while those with brass orientation prefer to convert into Goss-oriented grains. This can be attributed to the presence of highly mobile grain boundaries between these specific orientation pairs. In the DH alloy, subgrain growth and DRx grain consumption during annealing reduced orientation dispersion in deformed grains, promoting marked brass texture strengthening, with its volume fraction reaching 57.7%. Full article

To achieve an optimal balance of mechanical properties in low-cost alloy systems, this study tailored a heterogeneous bimodal structure in dilute Mg-0.4Al-0.3Ca-0.2Mn-xSn alloys (x = 0, 0.1 wt.%) and systematically investigated the critical role of trace Sn microalloying during hot extrusion. Mg-0.4Al-0.3Ca-0.2Mn-xSn alloys were fabricated via melting, homogenization, and subsequent hot extrusion at 320 °C. Trace Sn addition induced the formation of uniformly distributed CaMgSn phases within the homogenized matrix, facilitating a synergistic enhancement of strength and ductility. Specifically, the extruded alloys exhibited a characteristic bimodal grain structure consisting of coarse un-dynamic recrystallized (unDRXed) grains and fine dynamic recrystallized (DRXed) grains. Sn microalloying effectively refined the DRXed grains from 2.66 μm to 2.11 μm and significantly boosted the elongation (EL) from 12.9% to 26.3% while maintaining an Ultimate Tensile Strength (UTS) of 274 MPa. The Sn-containing secondary phases served as potent sites for particle-stimulated nucleation (PSN), thereby promoting the DRX process and reducing the texture intensity from 20.89 to 9.99. Overall, the superior strength-ductility synergy is primarily governed by the formation of the heterogeneous bimodal structure, where trace Sn facilitates grain refinement and texture weakening through PSN mechanisms, providing a robust strategy for the design of high-performance dilute magnesium alloys. Full article

Additive manufacturing (AM) of aluminium by solid-state routes offers a promising pathway to overcome the limitations of fusion-based processes, such as porosity and hot cracking. This study investigates the potential of wire-based friction stir additive manufacturing (W-FSAM) as an innovative solid-state process. A test specimen made of EN AW-1050 was fabricated and characterised using mechanical testing as well as optical and electron microscopy. Microstructural characterisation revealed a fully consolidated, pore-free build with fine equiaxed grains and partial dynamic recrystallisation (DRX). The average grain size decreased from 13.4 µm near the substrate to 9.7 µm at the top, reflecting the variation in cumulative thermal exposure along the build height. A homogeneous hardness distribution (21.2 HV) and smooth interlayer interfaces were observed. Tensile tests in the travel direction yielded an ultimate tensile strength of approximately 85 MPa and an elongation exceeding 60%, while high-cycle fatigue tests demonstrated a fatigue strength of about 30 MPa at $2\times {10}^6$ cycles ($R=0.1$) with ductile fracture features. The results confirm that W-FSAM enables the production of fine-grained, defect-free CP-Al structures whose mechanical properties, in terms of strength and ductility, exceed those of the reference material. Thus, W-FSAM represents a promising solid-state additive manufacturing route for the production of high-performance CP-Al components. Full article

Differential speed rolling (DSR) has been recognized as a unique processing technique in recent years, which has been used to plastically deform Mg-based alloys and to investigate the role of dynamic recrystallization (DRX) and its influence on both microstructure and mechanical properties. In this study, Mg–2Al–0.5Ca–0.5Mn (AXM20504) was solution-heat-treated (T4 condition) and subjected to single-pass DSR at both 20 and 40% thickness reductions, followed by post-annealing at temperatures of 350, 400, and 450 °C for the durations of 20, 40, and 60 min to evaluate the onset and development of static recrystallization (SRX) and its overall effect on the formability of Mg-based alloys. The results demonstrate how post-annealing yields nearly complete SRX at 400 °C for 60 min and 450 °C for 40 min with a significant improvement in ductility, increasing from 5% to 12% while maintaining an average tensile strength above 200 MPa. Thus, the improvement in mechanical properties demonstrates that post-annealing can deliver significant potential in terms of the enhanced formability of Mg alloys used in sheet metal forming applications. Full article

A hot extrusion deformation test of sintered Cu-10wt%Mo composite was carried out under deformation conditions, with deformation temperatures ranging from 800 °C to 950 °C, and extrusion ratios ranging from 2.9 to 10.5. The hot extrusion process eliminated the original interfaces between copper powder particles in sintered Cu-10wt%Mo composite. While the copper phase experienced dynamic recrystallization, the molybdenum particles effectively pinned the boundaries and inhibited subsequent grain growth. As the extrusion ratio increased, the composite material’s tensile strength, elongation, and thermal conductivity first increased and then decreased. With the rise in hot extrusion deformation temperature, the composite material’s tensile strength, elongation, and thermal conductivity gradually increased, but stabilized after reaching 900 °C. Deformation during hot extrusion is confined to the copper phase, which undergoes dynamic recrystallization (DRX), with no significant deformation occurring in the molybdenum phase. The molybdenum phase promotes an increased local strain rate in the copper phase, resulting in the formation of a certain number of twin grains. Full article

Dry heat inactivates pathogens on personal protective equipment without chemical residues, but its effects on material integrity and performance across multiple reprocessing cycles have not been comprehensively assessed. We evaluated five filtering facepiece respirator (FFR) models and three surgical mask (SM) models after one, two, and three cycles of dry heat (80 °C, 90 min). We measured fabric and strap tensile properties as indicators of mechanical durability [Young’s modulus (E), yield strength (σ_y), ultimate tensile strength (σ_UTS), and strain at failure (ε_f)]. We also assessed particle filtration efficiency (PFE) and airflow resistance (breathability). Under the methods applied herein, all untreated SMs and FFRs performed within the range anticipated for their type. Tensile properties exhibited heterogeneous, model-specific responses to thermal stress. FFR fabrics ranged from progressive stiffening (Dräger DR-X1720C; +120% E) to marked softening (3M-8210; −82% E), while SM fabrics exhibited softening, consistent with thermal relaxation. Straps made of thermoplastic elastomer (3M-8210 and 3M-9320A+) weakened (15–31% σ_UTS decrease), whereas braided polyisoprene straps (3M-1860S and 3M-1870+) maintained their original strength. Despite these changes, all treated FFR replicates met filtration requirements across all cycles (45/45). For SMs, 24/27 treated replicates met the required PFE threshold (≥98%), but 3 treated RH-S919B replicates fell below this threshold (PFE 94.9% and 97.7% after one cycle, and PFE 97.3% after three cycles), identifying a potential model-specific vulnerability to the treatment. Breathability remained within control ranges for most models; however, the Level 2 ZA-S001B showed decreased breathability (higher airflow resistance) after two (+11.1 Pa) and three (+13.3 Pa) dry-heat cycles, whereas the Level 3 RH-S920TFG showed modest improvements in breathability (lower airflow resistance, up to −10.1 Pa). Under these laboratory conditions, up to three cycles of dry heat at 80 °C for 90 min preserved PFE and breathability in all treated FFR replicates and in most treated SM replicates. Nonetheless, there were measurable, component-specific mechanical changes (especially in some straps) that could compromise fit and durability with repeated use. These findings support dry heat at 80 °C for 90 min as a potential component of emergency PPE processing strategies, provided that model-specific quantitative fit testing and extended-wear studies confirm safe real-world reuse, regulatory approvals are met, and end-user acceptability is considered. Full article

The present study systematically investigated the hot deformation behavior of GH2787 superalloy within the temperature range of 1060–1120 °C and strain rates of 0.1–10 s⁻¹. An Arrhenius-type constitutive equation was developed that accurately predicts the flow behavior, and the calculated thermal deformation activation energy Q is 364,401.19 J/mol. The hot working map was constructed based on the dynamic material model, which identified two preferred processing regions with power dissipation efficiency exceeding 0.3, and no flow instability was observed across the entire parameter range. Microstructural analysis reveals that the extent of dynamic recrystallization significantly increases with rising temperature and strain rate. Discontinuous dynamic recrystallization (via grain boundary bulging nucleation) serves as the dominant recrystallization mechanism in GH2787 superalloy during hot deformation, while continuous dynamic recrystallization (via subgrain rotation and coalescence) acts as a synergistic auxiliary mechanism, jointly driving microstructural evolution. This study provides important theoretical foundations for optimizing the hot working processes of GH2787 superalloy. Full article