Ultrafine Grains Research Articles

Effect of ultra-fine grain zone on mechanical properties of CuCrZr-Hastelloy X bimetallic structure manufactured by laser-directed energy deposition

In this study, CuCrZr-Hastelloy X bimetallic structure featuring an ultra-fine grain zone (UFGZ) near the interface were fabricated by laser-directed energy deposition (L-DED). Samples with UFGZ were obtained with 300 °C preheating, whereas samples without preheating did not contain UFGZ. The microstructures and mechanical properties of samples with and without UFGZ were compared. The formation of UFGZ was attributed to the presence of a spherical phase near the interface, which was identified as Ni (Fe, Cr, Mo). Due to the higher melting point of Hastelloy X compared to CuCrZr, Ni elements mixed into CuCrZr with the flow of the molten pool, solidified first, and served as substrates for the heterogeneous nucleation, ultimately promoting the formation of UFGZ. With 300 °C preheating, the hardness of CuCrZr near the interface increased from 115.07 HV to 148.51 HV due to the presence of UFGZ. And the hardness gap near the interface decreased from 172.58 HV to 147.19 HV, which improved the uniformity of mechanical properties. Moreover, the nanoindentation tests results that UFGZ increased the hardness of the zone near the interface from 1.42 GPa to 1.72 GPa. Tensile test results indicated that the UFGZ altered the fracture mode from brittle to ductile. Samples with UFGZ exhibited ductile fracture, while those without UFGZ exhibited brittle fracture. At room temperature, the tensile strength of samples with UFGZ increased from 298.44 MPa to 347.05 MPa. For tests conducted at 400 °C, the tensile strength increased from 165.12 MPa to 229.53 MPa. This enhancement indicated that UFGZ could improve the strength and toughness of the interface, thereby enhancing the interfacial bonding strength. This study is of great significance for improving the interfacial bonding strength of CuCrZr-Hastelloy X bimetallic structures.

Achieving balanced mechanical properties in Fe–0.16C plain low-carbon steel by trimodal microstructure and fine cementite

In the present study, for the first time, a trimodal microstructure (including coarse, fine, and ultrafine ferrite grains) was created in plain low-carbon steel by simple thermomechanical processing including intercritical annealing (at 770 °C, 800 °C, and 830 °C), 75% cold rolling, and finally heat treatment at 550 °C. The results showed that martensite with two different morphologies consisting of lath (low-carbon) and plate (high-carbon) were created in the microstructures of 800-75% and 830-75% sheets due to the non-homogeneous distribution of carbon atoms as a result of not using the homogenization treatment. After the final heat treatment, the samples exhibited trimodal grain size distribution of ferrite including coarse grains (larger than 5 micrometers), fine grains (between 1 and 5 micrometers), and ultrafine grains (UFGs) (smaller than 1 micrometer). The formation of ultrafine grains was ascribed to the presence of martensite with lath morphology in the 800-75% and 830-75% samples. The size of cementite particles in the 770-75%-550 sample was larger than the other two samples (800-75%-550 and 830-75%-550) owing to the presence of martensite phase only with plate morphology in 770-75%-550 steel. Unlike the 800-75%-550 sheet, the entire microstructure of the 770-75%-550 and 830-75%-550 samples had not undergone complete recrystallization and several deformed ferrite grains were still visible in the microstructures. The strength and hardness of the heat-treated sheets was larger than that of the as-received steel due to the presence of fine spherical cementite particles and fine ferrite grains in the microstructure of the heat-treated samples. Based on the obtained results, the 800-75%-550 sample revealed the best combination of strength-ductility-toughness due to trimodal microstructure along with the formation of fine spherical cementite particles. The failure mode of all heat-treated samples was ductile and there were both fine and coarse dimples in the fracture surfaces.

Grain size dependent deformation microstructure evolution and work-hardening in CoCrNi medium entropy alloy

This study clarified the grain size dependence of the deformation microstructure evolution and work-hardening behavior in CoCrNi medium entropy alloy. We fabricated fully recrystallized specimens with coarse-grained (CG) and ultrafine-grained (UFG) specimens by severe plastic deformation and subsequent annealing processes. Tensile deformation was applied to the specimens at room temperature. The UFG specimen exhibited both high strength and high ductility compared to conventional UFG metals due to the high work-hardening ability. In the CG specimen, three distinct types of deformation microstructures consisting of dislocations and deformation twins developed depending on grain orientations, similar to the single-crystalline specimens. In the UFG specimen, widely extended stacking faults and randomly-tangled dislocations were found to coexist in most grains. Deformation twins were found to nucleate without evidence of dislocation reactions regardless of grain orientations, implying abnormal nucleation mechanisms of deformation twins in the UFG specimen. Dislocation densities quantified by in-situ synchrotron XRD measurements during tensile deformation were higher in the UFG specimen than those in the CG specimen and conventional UFG metals. Our analysis showed that the work-hardening behavior of the specimens was primarily controlled by increases in dislocation density as well as the introduction of planar defects during deformation. Through comparisons with the CG specimen and conventional UFG metals, we concluded that the excellent work-hardening ability of the UFG specimen was mainly due to the evolution of unique deformation microstructures and rapid increase in dislocation density, which could be due to inhibited dynamic recovery in the MEA.

Nano gradient structuring at Ti-6Al-4V surface induced by ultrashort-pulse laser peening

We have successfully fabricated gradient micro/nano structuring on the surface top layers of Ti-6Al-4V alloy using a shock-peening technique facilitated by femtosecond and picosecond laser pulses, without the need for coatings or confinement. These micro/nano structures encompass ultrafine grains, extensive subgrain boundaries, hierarchical nanotwins, and complex dislocation morphologies. The ultrafine grains, ranging in size from hundreds of nanometers to a few micrometers, were predominantly located in the grain-refined regions, whereas the micro/nano dislocation structures were predominantly found in regions of severe plastic deformation. These observations suggest a promising avenue for achieving high-precision gradient structuring in the field of metallic surface engineering.

Modeling complex polycrystalline alloys using a Generative Adversarial Network enabled computational platform

Creating statistically equivalent virtual microstructures (SEVM) for polycrystalline materials with complex microstructures that encompass multi-modal morphological and crystallographic distributions is a challenging enterprise. Cold spray-formed (CSF) AA7050 alloy containing coarse-grained prior particles and ultra-fine grains (UFG) and additively manufactured (AM) Ti64 alloys with alpha laths in beta substrates. The paper introduces an approach strategically integrating a Generative Adversarial Network (GAN) for multi-modal microstructures with a synthetic microstructure builder DREAM.3D for packing grains conforming to statistics in electron backscatter diffraction (EBSD) maps for generating SEVMs of CSF and AM alloy microstructures. A robust multiscale model is subsequently developed for self-consistent coupling of crystal plasticity finite element model (CPFEM) for coarse-grained crystals with an upscaled constitutive model for UFGs. Sub-volume elements are simulated for efficient computations and their responses are averaged for overall stress-strain response. The methods developed are important for image-based micromechanical modeling that is necessary for microstructure-property relations.

Cryogenic strength-ductility mechanism of heterostructured CoCrFeMnNi high-entropy alloy

A typical heterogeneous structure with ultrafine grains embedded in coarse grains is obtained by partial recrystallization annealing, which exhibits excellent cryogenic mechanical properties. Researches reveal that the hard domains, high-density GND pile-ups, and multi-axial stress states of heterogeneous structures provide nucleation positions and critical stresses for triggering twinning, meanwhile, nanotwins can refine the hard domain grains to improve the heterogeneity of heterogeneous structures during cryogenic plastic deformation. Therefore, the heterostructure-twinning interaction is identified as the key factor in realizing the excellent cryogenic strength-ductility synergy of the heterostructured CoCrFeMnNi high-entropy alloy. Besides, the nucleation and propagation of generous microcracks at single-order twin boundaries induced by the excellent twinning activity result in the rapid fracture of the heterostructured samples, whereas the microcracks can hardly be observed nearby high-order nanotwins, which indicates that generating more high-order nanotwins is a promising guiding scheme to prepare the heterostructured materials with superior cryogenic mechanical properties.

Ultra-strong cold-drawn 2507 stainless steel wire with heterogenous microstructure of dual-phase and grain size

Obtaining ultrafine grains through severe plastic deformation is a highly effective strategy for improving the strength of metallic materials. Due to the different deformation mechanisms between ferrite and austenite, the heavily drawn 2507 stainless steel wire possesses a significant heterogeneous structure at drawing strain ε = 6.65.In this study, a 2507 duplex stainless steel (DSS) with a strength of 2.7 GPa was prepared by cold drawing process. The deformation production of the ferrite in the wire is simply dislocation, which subsequently transforms to subgrain boundary and high angle grains boundary. While the production of the austenite is dislocation and twinning. In addition, based on the boundary strengthening, solid solution hardening and lattice friction, the contribution of the deformed ferrite and deformed austenite to the yield stress is 2260 MPa and 2800 MPa, respectively.

Achieving strength-ductility synergy of ODS-FeCrAl alloys via heterostructured strategy

Oxide dispersion strengthened (ODS)-FeCrAl alloys are candidate structural materials for advanced nuclear reactor applications, which have excellent strength and radiation tolerance but low ductility. In this study, a novel ODS-FeCrAl heterostructured composite, reinforced with FCC-structured ODS-CoCrFeNiMn high-entropy particles, was prepared by using mechanical alloying and spark plasma sintering to achieve the strength-ductility synergy. The effect of different reinforcement contents (0, 10, 15 and 20 wt.%, designated as RC-0, RC-10, RC-15, and RC-20, respectively) on the microstructure and mechanical properties of the composites was investigated. The results showed that the unreinforced reference alloy, RC-0, consists of BCC-structured ODS-FeCrAl matrix and high-density oxides. In the RC-10, RC-15, and RC-20 composites, in addition to the ODS-FeCrAl matrix and nanoscale oxides, FCC-structured ODS-CoCrFeNiMn high-entropy reinforcement with ultrafine grain size and FCC transition layer with gradient grain size are observed. As the reinforcement content increases, the thickness of transition region increases. The composites show an increase in ultimate compressive strength and compressive strain with increasing reinforcement content in the following order: 2951 MPa/33.6%, 3169 MPa/40.0%, 3290 MPa/43.2%, and 3489 MPa/48.8%. And the compressive yield strength exhibits an initial increase and subsequent decrease, i.e. 1523 MPa (RC-0), 1680 MPa (RC-10), 1635 MPa (RC-15), and 1355 MPa (RC-20), respectively. The increase in yield strength is mainly attributed to hetero-deformation induced (HDI) strengthening and reinforcement hardening. The improvement in ductility is mainly attributed to the HDI work hardening and the suppression of microcrack formation and propagation at prior powder boundaries.

Ultra-fine nano-crystalline optimize electrostatic energy storage

Grain size pays a crucial role in the properties of ferroelectrics and dielectrics. Reducing the grain size is considered to be an effective mean for enhancing dielectric energy storage. In this work, high recovered energy storage density and efficiency were achieved in three-layered Aurivillius thin films by ultra-fine grain nano-crystalline engineering. The ultra-low remanent polarization can be attributed to the emergence of polar nano-regions due to the disruption of macroscopic continuity of ferroelectric domains by ultra-fine nano-grains. At the same time, all thin films have high dielectric breakdown strength due to the presence of extremely high grain boundary density. Thus, a high recovered energy storage density of 71 J/cm3 and an efficiency of 76% were achieved, and the thin film capacitors show good fatigue endurance and temperature stability. The results suggest that ultra-fine nano-crystalline engineering can expand the application of traditional ferroelectric thin films in energy storage devices.

Strain-gradient and damage failure behavior in particle reinforced heterogeneous matrix composites

A strain gradient plasticity model with strengthening effect and damage effect (SGSED) is developed under the continuum medium framework for particle-reinforced heterogeneous matrix composites (PRHMCs). Boron carbide (B4C) particle-reinforced ultrafine-grained (UFG)/fine-grained (FG) heterogeneous matrix composites (B4Cp/(UFG/FG)) are fabricated, in which the UFG region consisted of carbon nanotubes (CNTs)/6061 aluminum (Al) flakes with grains in the ultrafine range and the FG region is processed via 6061Al. The SGSED model is written into the user subroutines using commercial finite element (FE) calculation software, and the three-dimensional (3D) FE representative volume element (RVE) for B4Cp/(UFG/FG) composites is established, from which the distribution of the interface-affected-zone (IAZ) formed of the strain gradient caused by the uncoordinated deformation of the UFG-FG heterogeneous matrix and reinforced phase-matrix is calculated. The evolution of the strain gradient in the deformation process of composites and the influence of the strain gradient on the progressive damage and crack evolution of composites are analyzed, and the strain gradient strengthening-toughening mechanism of composites is revealed. It is found that the IAZ has a considerable strengthening-toughening effect on the composites, which can reduce stress concentration at the interface between the reinforced phase and the matrix, and slow down the crack propagation of the matrix.

Optimized energy storage performances via high-entropy design in KNN-based relaxor ferroelectric ceramics

Dielectric capacitors play an irreplaceable role in complex and integrated electronic systems. However, attaining ultrahigh recoverable energy storage density (Wrec) alongside energy storage efficiency (η) poses a formidable challenge, impeding the advance towards the miniaturization and integration of cutting-edge energy storage devices. In this work, a high-entropy strategy with a viscous polymer process is adopted, bringing about ultrafine grains and polar nanoregions (PNRs) accompanied by enhanced electrical homogeneity and polarization relaxation characteristics. As a result, an ultrahigh Wrec ∼ 7.51 J/cm3 is achieved in a high-entropy KNN-based energy storage ceramic with a high η ∼ 88.4% at 750 kV/cm. Moreover, the ceramic capacitor exhibits a colossal power density reaching approximately 420 MW/cm3 and a discharge energy density of approximately 4.85 J/cm3 at 160 ℃. Impressively, it maintains low variation rates of less than 4% across a broad temperature range from 20 ℃ to 160 ℃. The new approach opens avenues for the design and development of superior dielectric materials used in next-generation high-power pulse devices.

A bimodal and heterogeneous laminate structure with alternately distributed copper-base and nickel-base alloys via multi-material laser powder bed fusion (MM-LPBF) additive manufacturing

A micro-laminated bimodal heterostructure with alternately distributed copper-base and nickel-base alloys was additively manufactured via the multi-material laser powder bed fusion (MM-LPBF) technique. The high melting point elements such as nickel act as the heterogeneous nucleation sites for the copper melt, promoting the generation of ultrafine grains and forming a multi-layered structure with alternating coarse and fine grains. The differentiated grain size, shape and orientation in the bimodal heterostructure lead to the diversity of deformation mechanisms in different localized areas, which helps to achieve the internal stress gradient distribution and plastic deformation harmonization, thereby overcoming the strength-ductility trade-off for the overall structure. This work provides a novel fabrication path and experimental basis for understanding bimetallic laminated heterostructures.

Mitigating adverse effects of Cu-containing intrauterine devices using a highly biocompatible Cu[sbnd]5Fe alloy

Copper-containing intrauterine devices (Cu-IUD) are adopted by worldwide women for contraception with the advantages of long-term effectiveness, reversibility and affordability. However, adverse effects occur in the initial implantation stage of Cu-IUD in uterine because of the burst release of Cu2+. To minimize the burst release, in this study, we designed a series of Cu–Fe alloys with 0.5 wt%, 1 wt% and 5 wt% Fe and also further produced ultrafine grained (UFG) structure for these alloys via equal-channel angular pressing. The microstructures and properties of the coarse grained (CG) Cu, CG Cu–Fe alloys and UFG Cu–Fe alloys were systematically investigated, including grain structure and phase compositions, metallic ions release behavior, electrochemical corrosion performance, and in vitro cytotoxicity. With careful comparison and selection, we chose the CG Cu–5Fe and UFG Cu–5Fe for in vivo tests using rat model, including tissue biocompatibility, in vivo corrosion behavior, and contraceptive effectiveness. Moreover, the corrosion mechanism of the Cu–5Fe alloy and its improved biocompatibility was discussed. Both CG and UFG Cu–5Fe alloys exhibited dramatic suppression of Cu2+ release in simulated uterine fluid for the long-term immersion process. The in vivo tissue compatibility was significantly improved with both CG and UFG Cu–5Fe alloys implanted in the rats’ uterine while the high contraceptive efficacy was well maintained. Due to the superior biocompatibility, the CG and UFG Cu–5Fe alloys can be the promising candidate material for Cu-IUD. Statement of significanceA highly biocompatible Cu–Fe alloy was designed and fabricated for Cu-containing intrauterine devices (Cu-IUD). With 5 wt% Fe, the burst release of Cu2+ is inhibited due to the formed galvanic cell of Cu and Fe, resulting in earlier release of Fe3+. As Fe is the most abundant essential trace element of human body, it can mitigate the toxic effects of Cu2+, thus significantly improving both in vitro cell compatibility and in vivo tissue compatibility. More importantly, the Cu–5Fe alloy exhibits 100 % contraceptive efficiency as the CG Cu, but with greatly reduced adverse effects to the uterus tissues. An advanced Cu-IUD can be developed using Cu–Fe alloys.

Pursuing ultrahigh strength–ductility CoCrNi-based medium-entropy alloy by low-temperature pre-aging

Developing high-performance alloys with gigapascal strength and excellent ductility is crucial for modern engineering applications. The concept of multi-component high/medium entropy alloys (H/MEAs) provides an innovative approach to designing such alloys. In this work, we developed the Co1.5CrNi1.5Al0.2Ti0.2 MEA, which exhibits outstanding mechanical properties at room temperature through low-temperature pre-aging followed by annealing treatment. Tensile testing reveals that the MEA possesses an ultrahigh yield strength of 20±0785 MPa, an ultimate tensile strength of 2365 ± 70 MPa, and exceptional ductility of 15.8% ±1.7%. The superior tensile properties are attributed to the formation of fully recrystallized heterogeneous structures (HGS) composed of ultrafine grain (UFG) and fine grain (FG) regions, along with discontinuous precipitation of coherent nano-size lamellar L12 precipitates. The mechanical incompatibility between the UFG region and the FG regions during deformation induces the accumulation of a large number of geometrically necessary dislocations at the interface, resulting in strain distribution and hetero-deformation-induced (HDI) stress accumulation, contributing significantly to HDI strengthening. HDI strengthening, precipitation strengthening, and grain boundary strengthening are the primary mechanisms responsible for the ultra-high yield strength of the MEA. During deformation, the dominant deformation mechanisms include dislocation slip, deformation-induced stacking faults, and Lomer–Cottrell locks, with minor deformation twinning. The synergistic interaction of these multiple deformation modes provides the MEA with excellent work hardening capability, delaying plastic instability and achieving an excellent combination of strength and ductility. This study provides an effective strategy for synergistically strengthening MEAs by combining HDI strengthening with traditional strengthening mechanisms. These findings pave the way for the development of advanced structural materials with high performance tailored for demanding applications in engineering.

Design of tri-phase lamellar architectures for enhanced ductility in ultra-strong steel

Achieving exceptional ductility in ultra-high strength steels has long been a formidable challenge, particularly when the yield strength reaches 2.0 GPa. In this study, we have designed an innovative tri-phase lamellar microstructure in medium Mn steel that successfully achieves both an ultrahigh yield strength of approximately 2.0 GPa and an impressive uniform elongation ranging from 22.5 % to 24.5 %. The ultra-high yield strength of this steel is primarily due to the ultrafine lamellar grain with high-density dislocations and HDI strengthening resulting from meticulously designed tri-phase lamellae. The remarkable ductility is attributed to the synergistic action of multiple plasticity mechanisms. The inherently excellent plasticity of the lamellar ferrite, in coordination with the lamellar martensite, generates significant HDI hardening, thereby suppressing the onset of necking in the early stages of deformation. During the mid-stage of deformation, high strain activates the transformation-induced plasticity effect in the highly stable austenite, which remains stable due to substantial HDI hardening and effectively mitigates strain localization. In the later stages of deformation, delamination cracking relieves stress accumulation at the interfaces, delaying material failure. These mechanisms elevate yield strength and uniform elongation product to 47.3 GPa·%, showcasing the tri-phase lamellar structure’s potential for ultrahigh-strength, high-ductility steels.

High-strength and high-plasticity ZK60 magnesium alloy prepared by rapid solidification and high-speed extrusion

The extrusion speed of ZK60 magnesium alloy is usually lower than 5 m/min. The rapidly solidified (RS) ZK60 magnesium alloy extruded at 350 ℃ with die-exit speeds of 10 m/min, 20 m/min and 30 m/min was investigated. The results reveal that after the RS ZK60 alloy was extruded at different die-exit speeds, the average size of dynamically recrystallized grains is 1.19–1.40 μm, the percent of high-angle grain boundaries (HAGBs) become larger with the average orientation degree of 57.83 ° to 54.81 °, the intensity of texture at (0001)<1̅100> is below 3.4, and the Schmid factors of the prismatic and pyramidal planes reach the high value of no lower than 0.42 overall, while no coarse MgZn phase was observed. The yield tensile strength, ultimate tensile strength and elongation of the RS as-extruded ZK60 magnesium alloy change slowly from 321 to 299 MPa, -355–348 MPa, and -23–27 % when the die-exit speed increase from 10 to 30 m/min, respectively. The ultra-fine grains and disappearance of the coarse low melting-point MgZn phase are the main reasons for the high-speed extrusion, high strength, high plasticity and low texture of the RS as-extruded ZK60 magnesium alloy.

Mechanical Behavior of Oxide Dispersion Strengthened Steel Directly Consolidated by Rotary Swaging.

Among the main benefits of powder-based materials is the possibility of combining different constituents to achieve enhanced properties of the fabricated bulk material. The presented study characterizes the micro- and sub-structures and related mechanical properties of ferritic steel strengthened with a fine dispersion of nano-sized Y2O3 oxide particles. Unlike the typical method of preparation via rolling, the material presented herein was fabricated by direct consolidation from a mixture of powders using the versatile method of hot rotary swaging. The mechanical properties were evaluated at room temperature and also at 1300 °C to document the suitability of the prepared steel for high-temperature applications. The results showed that the imposed shear strain, i.e., swaging ratio, is a crucial parameter influencing the microstructure and, thus, material behavior. The workpiece subjected to the swaging ratio of 1.4 already exhibited a sufficiently consolidated structure with ultra-fine grains and featured high room-temperature microhardness values (up to 690 HV0.5), as well as a relatively high maximum flow stress (~88 MPa) when deformed at the temperature of 1300 °C with the strain rate of 0.5 s-1. However, the dispersion of oxides within this sample exhibited local inhomogeneities. Increasing the swaging ratio to 2.5 substantially contributed to the homogenization of the distribution of the Y2O3 oxide particles, which resulted in increased homogeneity of mechanical properties (lower deviations from the average values), but their lower absolute values due to the occurrence of nucleating nano-sized recrystallized grains.

Microstructure, crystallographic texture and mechanical properties of the Zn–1%Mg–1%Fe alloy subjected to severe plastic deformation

The paper covers the production, analysis of the microstructure, crystallographic texture and deformation mechanisms of the ultrafine-grained (UFG) Zn–1%Mg–1%Fe zinc alloy demonstrating unique physical and mechanical properties compared to its coarse-crystalline analogs. The zinc alloy with improved mechanical properties was developed in two stages. At the first stage, based on the analysis of literature data, an alloy with the following chemical composition was cast: Zn–1%Mg–1%Fe. Then, the alloy was subjected to high-pressure torsion (HPT) to improve mechanical properties due to grain structure refinement and implementation of dynamic strain aging. The conducted mechanical tensile tests of the samples and assessment of the alloy hardness showed that HPT treatment leads to an increase in its tensile strength to 415MPa, an increase in hardness to 144HV, and an increase in ductility to 82%. The obtained mechanical characteristics demonstrate the suitability of using the developed alloy in medicine as some implants (stents) requiring high applied loads. To explain the reasons for the improvement of the mechanical properties of this alloy, the authors carried out comprehensive tests using microscopy and X-ray diffraction analysis. The microstructure analysis showed that during the formation of the ultrafine-grained structure, a phase transition is implemented according to the following scheme: Zneutectic + Mg2Zn11eutectic + FeZn13 → Znphase + Mg2Zn11phase + MgZn2particles + Znparticles. It was found that as a result of high pressure torsion in the main phases (Zn, Mg2Zn11), the grain structure is refined, the density of introduced defects increases, and a developed crystallographic texture consisting of basic, pyramidal, prismatic, and twin texture components is formed. The study showed that the resistance of pyramidal, prismatic and twin texture components at the initial stages of high-pressure torsion determines the level and anisotropy of the strength properties of this alloy. The relationship between the discovered structural features of the produced alloy and its unique mechanical properties is discussed.

Achieving superior strength and ductility synergy in bulk ultrafine grained Al-Mg-Sc-Zr alloy via powder pre-aging

Introducing high density of nano-precipitates to recrystallized ultrafine grains is helpful to realize strength-ductility synergy but is a challenging task, because recrystallization and precipitate growth/coarsening usually concur. Here we develop a pre-aging powder metallurgy processing route to achieve such microstructure in Al-Mg-Sc-Zr alloy. During the pre-aging stage, atomic clusters including short-range order are formed within the grains, which provide new sites for the nucleation and enable the formation of fine Al3(Sc, Zr) precipitates. Subsequent high-temperature sintering and hot extrusion lead to grain recrystallization. The nano-precipitates not only further strengthen the ultrafine-grained alloy by Orowan mechanism, but also greatly enhance the strain-hardening rate by dislocation-precipitate interaction, resulting in excellent strength-ductility synergy. The utilization of digital image correlation (DIC) analysis allows for the observation of dynamic strain aging during the tensile process, whereby the strain demonstrates a distinctive step-like transition coinciding with the passage of the Portevin-Le Chatelier (PLC) band. This work provides a new path for improving the mechanical properties of the same type of metallic materials.

Achieving ultra-fine grains in Ti-6Al-4V alloy welds through pre-weld friction stir processing

Titanium Ti-6Al-4V alloy, recognized for its exceptional strength, is extensively employed in the aerospace, biomedical, and automotive industries. Friction Stir Processing (FSP) has been found to enhance the overall performance, while friction stir welding (FSW) is recognized as the most effective technique for joining the Ti-6Al-4V alloy. This study focusses on the implementation of friction stir welding on a Ti-6Al-4V alloy that had previously undergone friction stir processing. The objective was to analyse the microstructure and mechanical characteristics. The examination using Electron Backscatter Diffraction revealed notable alterations in the microstructure, such as variations in grain size, misorientation angle, and grain boundaries. The proportion of high angle grain boundaries (HAGBs) on the advancing side and stir zone of the friction stir treated Ti-6Al-4V were 59 % and 66 %, respectively. Signifying grain refinement, grains measuring sizes between 0.83 μm and 1.05 μm were achieved as result of processing. Subsequent, friction stir welding resulted in about 50 % further decrease in grain size compared to base metal, with HAGBs comprising 71 % and 52 % at the advancing side and stir zone, respectively. As a result the Vickers micro hardness values increased to 397 ± 13Hv upon friction stir processing to 444 ±7Hv upon subsequent friction stir welding respectively.