Ultrafine Grains Research Articles

Achieving unexpected strength and ductility synergies in heterogeneous metastable lamellar steels

High-strength steel with excellent ductility is pivotal for the formability and safety of critical structural components. Here, a heterogeneous metastable lamellar steel, composed of alternating lamellar ferrite and austenite aligned with the rolling direction, was developed through an innovative combination of warm rolling and immediate annealing processes. This novel design overcomes the strength-ductility trade-off, achieving high ultimate tensile strength (∼1.2 GPa) and excellent uniform elongation (∼78%), pushing the product of ultimate tensile strength and uniform elongation to an ultra-high level (> 90 GPa %). The high tensile strength is attributed to ultrafine lamellar grains and significant work hardening induced by the hetero-deformation and transformation-induced plasticity (TRIP) effect. The exceptional ductility is a result of the synergy of multiple plasticity mechanisms, including (i) the inherent plastic deformation ability of lamellar microstructure and the hetero-deformation-induced hardening in the early deformation period, (ii) the persistent TRIP effect induced by the lamellar austenite with high mechanical stability and the elimination of strain localization caused by prolonged strain hardening due to the coordinated deformation of lamellar austenite and ferrite in the middle deformation period, and (iii) delamination cracking in the late deformation period. This approach adopted in current work offers a straightforward and economically feasible pathway for fabricating advanced high-strength steel with superior performance.

Simultaneously improving the strength and ductility of an oxide dispersion-strengthened high-entropy alloy by employing innovative precursors for oxide formation

The engineering of a composite microstructure, achieved by coupling heterogeneous grain distribution with oxide dispersion, has been demonstrated as an effective strategy for enhancing the strength-ductility synergy of alloys at both room and elevated temperatures. The effectiveness of this approach is strongly correlated with the micro-features of dispersed oxides. In this study, we selected the Ni38Co20Cr20Fe18Ti4 high-entropy alloy (HEA) as the base material, which could achieve the desired composite structure through preparation using mechanical alloying and spark plasma sintering methods. Our primary objective was to investigate the effects of incorporating Y2O3 or utilizing hydrides (TiH2 and YH3) as alternative precursors on the modification of oxide precipitates, evolution of a bimodal grain microstructure, and alteration in mechanical properties for the oxide dispersion strengthened (ODS) HEA. The results show that the incorporation of Y2O3 promotes the formation of ultrafine and semi-coherent Y2Ti2O7 ternary oxide particles, in addition to the pre-existing coarse binary TiO in the HEA matrix. Moreover, this leads to a significant increase in the fraction and a decrease in the average size of ultrafine grains (UFGs) within the bimodal microstructure. Notably, utilizing Ti- and Y-hydrides instead of Y2O3 and Ti as oxide-forming precursors within an equivalent composition remarkably amplifies the precipitation proportion of Y2Ti2O7 among dispersoids while further refining UFGs. The ODS-HEA synthesized using hydrides exhibits a simultaneous enhancement of 12 % in yield strength and 13 % in elongation to fracture, compared to that prepared from Ti and Y2O3. This intriguing phenomenon primarily arises from the heightened strengthening and toughening contributions, facilitated by the increased precipitation of advantageous Y2Ti2O7 nanoparticles.

Optimizing the mechanical and corrosion properties of an ultrafine-grained Al-Cu-Mg alloy through cyclic deformation: Clusters and lattice defects

To optimize its corrosion and mechanical properties, an Al-Cu-Mg alloy is cyclically processed by high-pressure torsion (HPT), involving a 1/2 turn clockwise and a 1/2 turn counter-clockwise per cycle. The dislocation density increases to 3.04×1014 m−2 on 1 cycle HPT and decreases gradually to 2.23×1014 m−2 for higher cyclic rotations (i.e. 5 cycle HPT). Due to the rapid elimination of the geometrically necessary dislocations, the refinement of grain size in cyclic HPT is more efficient than the monotonic HPT (i.e. it decreases from 2.48 μm to 112 nm on 1 cycle HPT). The HPT samples are electrochemically corroded in a simulated seawater environment. 1-cycle HPT process causes a strong increase in hardness (to 232 HV) and an increase in corrosion potential (to −0.517 V). The trade-off between hardness/strength and corrosion resistance is broken on cyclic HPT processing. Aided by excess vacancies induced by HPT, Cu-Mg clusters form on and near grain boundaries through a ‘vacancy-pump’ mechanism, causing solute depleted zones (SDZ) adjacent to GBs. The combined effects of ultrafine grains, solute cluster segregation and SDZs enhance the strength and corrosion resistance of the Al-Cu-Mg alloys. This study provides a potential strategy to design the microstructures of an Al alloy, and to achieve a higher hardness and a better corrosion performance of that material.

Effect of multi-pass overlapping friction stir processing on fatigue behavior of Al-Cu alloy

More than half of mechanical breakdowns stem from fatigue failure, often occurring suddenly and without warning. Friction stir processing (FSP) enhances the material's toughness and resilience to fatigue by refining its grain structure. This study investigates how multi-pass overlapping technique impacts the fatigue crack growth rate of FSPed Al-Cu alloy. Microstructural analysis revealed that the stir region exhibited uniformly dispersed and fragmented precipitates and finely recrystallized ultrafine grains. The hardness and strength were reduced, and ductility was enhanced after FSP due to high thermal cycling. Fatigue testing demonstrated a significant increase in fatigue life and reduced fatigue crack growth rate attributable to the combined effects of precipitation and grain refinement during cooling-assisted FSP. SEM examination of fatigue fracture surfaces revealed dimples indicative of ductile failure in the rapid crack propagation zone, while the steady-state propagation region displayed striation markings and secondary fractures.

Friction weldability of ultrafine-grained titanium grade 2

The paper presents the results of an experimental study of the friction weldability of Titanium Grade 2 with ultrafine-grained (UFG) microstructure obtained by hydrostatic extrusion and rotary swaging (HE+RS). High-speed friction welding with different rotational speed (n) values was used as a joining method. The best properties were obtained for n=7000 rpm. Metallographic observations confirmed the formation of a continuous friction joint with a changed microstructure of the base material in the thermal-mechanical affected zone (TMAZ) and a friction weld (FW). The results of the mechanical properties showed the greatest decrease in hardness in the TMAZ. The friction welded joint had an 18 % lower tensile strength value than initial state UFG Titanium Grade 2 after the HE+RS process (with UTS=1050 MPa).

Microstructure and mechanical properties of a severely cold-rolled and annealed dual-phase compositionally complex alloy (CCA) with an exceptionally deformable Laves phase

A novel CCA was designed by substituting Nb in (FCC + C14 Laves) CoCrFeNi2.1(Nb)0.2 CCA by (Hf + Nb + Ta). The (HfNbTa)0.2 CCA was homogenized, heavily cold-rolled, and isothermally annealed at 800 °C and 1000 °C for different time intervals. The (HfNbTa)0.2 alloy revealed the presence of a Hf and Ni enriched cubic C15 Laves phase. The considerations of site occupancy behavior, formation energy, and highly off-stoichiometric composition stabilized the (Hf, Ni) rich cubic C15 Laves phase. In contrast to the brittle hexagonal C14 Laves phase in (Nb)0.2 CCA, the C15 Laves phase in (HfNbTa)0.2 CCA showed exceptional deformability owing to the high propensity for nano-twin formation. Meanwhile, the FCC matrix developed a deformation-induced nano-lamellar structure with a spacing of ∼45 nm. Annealing resulted in ultrafine recrystallized FCC matrix and precipitation of DO19 structured ε nano-precipitates. The isothermal grain growth kinetics revealed a high grain growth exponent (n) ∼7, which confirmed a Zener-drag mediated process due to the ε nano-precipitates. The Hall-Petch analysis of the hardness data showed relatively high friction stress originating from the dissolution of Hf, Nb, and Ta in the FCC matrix. A high Hall-Petch coefficient indicated increased shear stress for plastic flow across the boundaries, resulting from the elongated Laves phase at the boundaries. The highly deformable Laves phase, ultrafine grain size, and ε nano-precipitates resulted in high yield strength (∼975 MPa) and superior ductility (∼16 %) in the (HfNbTa)0.2 CCA, even surpassing the (Nb)0.2 CCA. It was envisaged that strong yet deformable Laves phases could pave the pathway for developing Laves phase-based CCAs for advanced structural applications.

Grain refinement of Fe4N compound layer in nitrided steel

This study elucidated the microstructure of the Fe4N (γ’) compound layer in specimens subjected to the nitriding to the specimens with various finished surfaces. Gyrofinishing produces a specimen surface with an ultra-fine-grained (UFGed) structure and a rough surface, whereas the buff-finished specimen exhibited a coarse grain but smooth surface. The γ’ grains formed on the gyrofinished specimens after nitriding were considerably finer and more equiaxial compared with those on the buff-finished specimen. The γ’ nucleated from the grain boundary in the matrix, indicating that the grain boundaries act as a nucleation site. Therefore, the UFGed structure promotes the nucleation of γ’ during the nitriding, resulting in fine grains. Numerous pores were formed in the γ’ layer of the gyrofinished specimen owing to the rough surface. The UFGed structure and surface roughness before nitriding are key factors for controlling the microstructure in the γ’ compound layer.

Pre-deformation assisted fabrication of bulk Nd2Fe14B/α-Fe nanocomposites with high energy density

For Nd2Fe14B/α-Fe nanocomposite magnets, which exhibit great superiority in theoretical energy products, it is a great challenge to obtain controlled nano-scale microstructural features while eliminating the harmful metastable phases due to the general metastable fabrication methods. Here, we report a strategy to control the nano-scale features and simultaneously eliminate metastable phases for Nd2Fe14B/α-Fe nanocomposites through low-temperature pre-deformation of amorphous alloys combined with subsequent thermal annealing. The resultant bulk Nd2Fe14B/α-Fe nanostructure exhibits desired ultrafine grain sizes, ∼20 nm for soft-magnetic α-Fe phase with a high fraction of Vα-Fe ≈ 30 wt% and ∼33 nm for hard-magnetic Nd2Fe14B phase. The desired microstructure results in a larger coercivity (Hci = 4.1 kOe) and higher saturation magnetization (Ms = 1.51 T) compared to those (Hci = 3.3 kOe and Ms = 1.35 T) of the counterpart without pre-deformation, contributing to a high energy density of 26.3 MGOe. This energy density is 200% larger than that of the counterpart without pre-deformation and far beyond that (around 20 MGOe) of previously reported bulk Nd2Fe14B/α-Fe nanocomposites with Vα-Fe ≥ 20 wt%. The superior property stems from the purposely introduced low-temperature pre-deformation that changed the structure and local chemistry of the amorphous alloy, facilitating the decomposition of harmful metastable phases and the formation of Nd2Fe14B phase during annealing processes, which lowers the temperature, from 750 to 700 °C for producing the Nd2Fe14B/α-Fe nanostructure and thus yields ultrafine nanograins. These results demonstrate an effective means to prepare high-performance bulk Nd2Fe14B/α-Fe nanocomposite magnets.

Enhancing strength-ductility synergy in low-Mn lightweight steel by large-deformed warm rolling and flash annealing

The presence of δ-ferrite makes it challenging to achieve higher strength-ductility in low-Mn lightweight steels. In this study, a large deformation warm rolling process was used to construct a precursor enriched with Mn, nano-precipitations, ultra-fine grains, and high-density dislocations. The subsequent flash annealing process not only promotes the formation of multi-heterogeneous structures but also refines and reduces δ-ferrite while increasing the ultrafine-grained hard phase martensite. Additionally, this process enhances the retention of retained austenite, whose ultrafine grains contribute to improved mechanical stability. The synergistic effect of multiple strengthening mechanisms enables low-Mn lightweight steel to achieve outstanding comprehensive mechanical properties, with a tensile strength of 1423 MPa and a total elongation of 23.0 %.

Influence of ultrafine-grained structure of Ti-42Nb-7Zr alloy on energy dissipation and storage under quasi-static tension

The ultrafine-grained (UFG) structure and plastic deformation of the Ti-42Nb-7Zr alloy have been investigated using infrared thermography. The initial stage occurs at a constant temperature when the total energy of plastic strain absorbed by the alloy is 1.75 times higher for the UFG alloy, than for the alloy with a coarse-grained structure. This fact is due to substructural strengthening caused by severe plastic deformation and dispersion hardening with ω-phase nanoparticles. The amount of heat released before the UFG alloy fracture is 2 times larger than the amount of stored energy, and this results in localized softening of the UFG alloy prior to its fracture.

Fabricating a pure titanium foil with excellent strength-ductility combination via bimodal grain structure coupling with deformation twins

Preparation of pure titanium foil by cold rolling deformation often confronts the strength-ductility trade-off dilemma, how to obtain an excellent strength-ductility combination remains a great challenge. In the present work, we have prepared a pure titanium foil with a bimodal grain (BG) structure consisting of coarse grain (CG) and ultrafine grain (UFG), coupling with high-density deformation twins (DT) via a cryogenic-temperature rolling (CTR) and partial recrystallization annealing (PRA) process. Notably, a high strength-ductility synergy with YS of ∼483 MPa, UTS of ∼517 MPa and elongation of ∼21 % is achieved. The high strength is mainly caused by pronounced fine grain strengthening from the UFG region and twin boundary strengthening from strong interactions between DT and dislocations activated in the CG region. The GND caused by the hetero-deformation between CG and UFG, along with the activation of pyramidal <c+a> dislocation via DT in the CG region endows the pure titanium foil with excellent ductility.

Autoclave grid-to-rod fretting corrosion behaviors of the Zr alloy fuel cladding with and without Cr coating through advanced characterization

The fretting corrosion behaviors and the associated mechanisms of Zr alloy claddings with and without Cr coating in the primary water (310 °C/15.5 MPa) of the simulated pressurized water reactor were studied and compared. The results demonstrated that the fretting corrosion resistance of Zr alloy cladding was improved significantly by the introduction of Cr coating. The increased fretting corrosion resistance of the Cr-coated Zr alloy cladding was mainly attributed for two reasons. Firstly, the metallic Cr had much greater hardness than the Zr alloy and the formation of the ultra-fine grains layer under the worn scar further increased its hardness. Secondly, the metallic Cr had much better plasticity than the ZrO2 layer, resulting in better resistance to fatigue failure.

The Influence of Microstructure Evolution on the Mechanical and Electrochemical Properties of Dissimilar Welds from Aluminum Alloys Manufactured Via Friction Stir Welding

The present study investigated a new configuration of friction stir welded joints from two aluminum alloys. Dissimilar welds AA6082/AA1350 were examined, whereas, for AA1350, two states were investigated—coarse-grained (CG) and ultrafine-grained (UFG). Changes in the mechanical and electrochemical properties regarding the microstructure evolution across the welds were discussed. The average grain size in the stir zone (SZ) for all materials equaled 4 to 5 µm with a fraction of high-angle grain boundaries of about 77 pct, indicating the occurrence of continuous dynamic recrystallization. Changes in the microhardness across the welds were connected with differences in grain size (AA1350) and dissolution of β″ precipitates in the SZ of AA6082. As a result, the tensile strength of the welds decreased compared to base materials AA6082 and AA1350 UFG; however, there was an increase when compared to the base material AA1350 CG. Electrochemical experiments revealed that pitting corrosion occurred for AA1350, while for AA6082, it was a combination of pitting and intergranular corrosion. The depth of corrosion attack was higher for AA1350, with a maximum value of ~ 70 µm for base materials, while in the SZ, a depth decreased to 50 µm. For the AA6082, the maximum depth was measured in the SZ and did not exceed 30 µm.

Investigating the microstructure and mechanical properties of Al–Ag-Sc ultra-fine grain alloy processed by accumulative rolling bonding method

The aim of the present study is to investigate the effect of accumulative roll bonding (ARB) and precipitation hardening on the microstructure development and there of mechanical properties of ultrafine grained Al–Ag-Sc alloy. For this purpose, the samples were heated to a temperature of 600 °C for 24 h, and then they were quenched in water. In order to carry out the two-stage precipitation hardening process, the samples were placed in the furnace at a temperature of 350 °C for 5000 s in the first stage, which was followed by quenching in water. In the second stage, they were heated to 250 °C for 5000 s following by water quenching too. Following, the samples were subjected to the ARB for up to 8 cycles. Scanning Electron Microscope (SEM) equipped with Electron Backscatter Diffraction (EBSD) and energy dispersive spectroscopy (EDS) techniques, and X-Ray Diffraction (XRD) was used to verify the microstructure and phase analysis of the prepared samples. Also, tensile and microhardness tests were conducted to confirm the mechanical properties, and pin-on-disk tests were done to check wear behavior. Microstructural parameters such as crystallite size, microstrain, and density of dislocations in rolled sheets were estimated using the Rietveld method. The Results have shown that applying various cycles of the ARB process leads to the development of ultra/nano grain structure. In that sense, performing 8-cycle of the ARB transformed the low angle grain boundaries (LAGBs) of the 4-cycle ARB condition to high angle grain boundaries (HAGBs). Results showed that there is proper welding between different layers, except for the layers created in the last cycle. Similarly, the obtained size of the crystallites demonstrates that the samples become finer when ARB cycles are increased. The results of the mechanical tests indicated that after 8 cycles, there is approximately 3.5-fold and 2.5-fold increase in the tensile strength and hardness compared to the original (annealed) sample, respectively. While the elongation decreased slowly and reached 1.5% in the 8th cycle. A fractography of the fracture surface of the original sample included deep and coaxial dimples, indicating a ductile fracture. Analysis of wear surface degradation showed that scratch wear occurred for all samples.

Investigations on diamond rotary rolling treatment induced gradient microstructure and its effects on mechanical properties

Surface and subsurface gradient microstructures play a significant role in mechanical properties. To obtain gradient microstructures, a surface strengthening method, namely diamond rotary rolling treatment (DRRT), was proposed in this study. Varied gradient microstructures in 316 L stainless steel were obtained through different DRRT processing parameters. The micro-hardness distribution along the gradient microstructure was tested. The resulting gradient micro/nanostructures in terms of grain size, dislocation density and phase transformation were characterized and evaluated with electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and X-ray diffraction (XRD) analyses. The corresponding mechanical properties were investigated by slow strain rate test. The results showed that the DRRT could introduce a gradient hardening layer on the surface, accompanied by a gradient distribution of nano-grains, ultrafine grains, fine grains and coarse grains extending from the surface to the material interior. Besides, the gradient microstructure was associated with martensitic phase transformation, slip and dislocation. Compared to milled and untreated specimens, the DRRT-treated specimens exhibited a significant increase in yield strength and reduced ductility. Particularly, a well-designed DRRT process can enhance the yield limit while preserving higher ductility. Additionally, a quantitative relationship between the overall yield strength and the hardness distribution of the gradient microstructure was established.

Intergranular Shielding of Ultrafine-Grained Ni-Rich Layered Cathode for Advanced Lithium-Ion Batteries

To achieve carbon neutrality on a global scale, fuel-powered vehicles are being steadily phased out over environmentally recommended electrified vehicles (EVs) with low greenhouse gas emissions. In a battery, the cathode is among the foremost components, as it predominantly determines the LIB’s performance and price. However, since current fleet of layered oxide NCM and NCA cathodes have a relatively lower Ni content, they can extract only a limited amount of reversible capacity (210 mAh∙g-1), restricting the driving range of EVs. To achieve higher energy densities, many studies have focused on increasing the average fraction of Ni in NCM or NCA cathodes to more than 90%. However, the extraction of large amounts of lithium ions is seriously compromised by the rapid deterioration of both cycle life and thermal safety. Recent studies on X-doped cathodes (X = B, Nb, Ta, Sb, W, and Mo) have offered promising results by engineering their geometric microstructural configurations to suppress intergranular cracking.1,2 This approach has proven its potential to improve the electrochemical cycling performance of Ni-rich NCA, NCM(A), and NM cathodes. However, prolonged exposure of the Ni-enriched (Ni ≥ 95%) cathode to the electrolyte generally results in continuous oxidative decomposition of electrolytes and subsequent cathode structural transformation, rendering the cathode unsuitable for applications requiring long battery life.3 Herein, we propose a highly stabilized Ni-enriched (Ni ≥ 95%) layered cathode by adopting a multi-stage engineering strategy, i.e., the construction of an ultrafine-scaled microstructure and subsequent intergranular shielding of the refined grains. The formation of an F induced protective layer at the intergranular boundaries of the ultrafine grains in Li[Ni0.95Co0.04Mo0.01]O2 (denoted as Mo1-NC96) noticeably improved both the structural and chemical stability of the cathode, with a limited degree of structural degradation and gas evolution. Reference s : [1] U.-H. Kim, G.-T. Park, B.-K. Son, G.W. Nam, J. Liu, L.-Y. Kuo, P. Kaghazchi, C.S. Yoon, Y.-K. Sun, Nat. Energy, 5 (2020) 860-869.[2] G.-T. Park, B. Namkoong, S.-B. Kim, J. Liu, C.S. Yoon, Y.-K. Sun, Nat. Energy, 7 (2022) 946-954.[3] F. Kong, C. Liang, L. Wang, Y. Zheng, S. Perananthan, R.C. Longo, J.P. Ferraris, M. Kim, K. Cho, Adv. Energy Mater., 9 (2019) 1802586.

Formation of Symmetric Gradient Microstructure in Carbon Steel Bars

In recent years, severe plastic deformation has attracted the most attention as a way to improve the mechanical properties of steel bars. Obtaining ultrafine grains and nanostructures in such bars leads to a strong increase in strength properties but strongly reduces their plastic properties. This study shows that the formation of a gradient microstructure allows simultaneous improvement in the strength and plastic properties of carbon steel bars, taking into account the symmetry of the microstructure distribution from the center of machining. A new combined technology is proposed to obtain such a microstructure. This technology consists of drawing bars from medium carbon steel on a radial-displacement rolling mill and carrying out subsequent drawing. Steel bars with a diameter of 30 mm were strained in three passes to a diameter of 16 mm at room temperature. The results show that the average value of microhardness in the center, neutral, and surface areas for the three straining cycles were 1890 MPa, 2335 MPa, and 2920 MPa, respectively. This symmetrical distribution of microhardness confirms the gradient microstructure. Strength characteristics also increased almost twofold: the yield strength increased from 330 to 735 MPa, and the ultimate strength increased from 600 MPa to 1025 MPa. Relative elongation decreased from 18 to 14 MPa, and relative reduction decreased from 40 to 31%, but remained at a fairly good level for AISI 1045 steel. The validity of all results was confirmed through numerous experiments using a set of traditional and modern research methods, which included optical, scanning, and transmission microscopy. EBSD analysis allowed precise positioning of the field of vision for studying microstructural changes across the entire bar cross-section. All of these methods used together, including tensile testing of the mechanical properties and the fractographic method, allow us to assess changes in microhardness and the reproduction of results.

Harmony of Self‐Similarity: Overcoming the Strength–Ductility Trade‐Off Through Harmonic Structure

Within the new generation of structural materials having heterogeneous microarchitectures, one of the top performers is harmonic structure (HS). Herewith, the exciting discovery of self‐similarity in their stress–strain behavior is reported. HS materials consist of the 3D skeleton of ultrafine grains (UFG) encompassing coarse‐grained (CG) counterparts. Our discovery simplifies the mathematical description of such a system and helps unraveling hidden patterns. Specifically, it is demonstrated that overcoming the strength–ductility trade‐off in HS occurs since its components have a characteristic strengthening mechanism, which realizes the synergy of three factors: 1) hetero‐deformation‐induced (HDI) strengthening of the CG phase, the deformation of which increases with acceleration with respect to the deformation of the entire sample; 2) the transition of the CG phase to the strengthened to saturation UFG phase; and 3) the work of UFG phase deformation, which increases with acceleration when the sample is strained due to the rapidly growing value of and the factor (2).

Mechanical and tribological behaviours of friction stir welding using various strengthening techniques

The present study investigates the research conducted on friction stir welding of aluminium alloys AA6061. The experiments were conducted under optimal parametric settings, and the specimens were subsequently subjected to post-processing techniques such as heat treatment and shot peening. The enhancement of weldment strength is contingent upon secondary processing, eliminating internal stress and improving surface qualities. Subsequently, the weldment will undergo mechanical testing and tribometer testing. The conventional testing protocol was implemented to assess the hardness, yield, and tensile strength. The wear and friction test examined the effects of various process factors. The load conditions were set at 10, 30, 50, and 70 N, while the sliding distance was set at 700 m and 1400 m. The experiments were carried out in dry sliding conditions at room temperature and the resulting data were recorded as average values. The welded AA6061 samples exhibited superior hardness and ultimate tensile strength, with the highest average values observed after heat treatment and subsequent shot peening. Attributed to the strengthening effect and achieving an ultra-fine grain size. The results of this inquiry indicate that the yield strength and ultimate tensile strength of the welded sample were significantly increased by heat treatment and shot peening. In particular, when compared to the as-welded AA6061 aluminium alloy sample, the sample's yield and ultimate tensile strength were 39.49 % and 38.7 % greater, respectively. The tribometer test revealed that a rise in load and sliding distance results in an increase in wear rate and a drop in the coefficient of friction. Under a load of 70 N and a sliding distance of 1400 m. s, the heat-treated and shot-peened AA6061 specimen showed a 27.3 % lower wear rate and a 29.6 % lower coefficient of friction compared to the as-welded AA6061 specimen.

Trimodal Grain Structured Aluminum Matrix Composites Regulated by Transitional Hetero-Domains

Aluminum matrix composites (AMCs) with hetero-grains exhibit high strength with good ductility. A trimodal grain structure composed of ultrafine grains (UFGs), fine grains (FGs) and coarse grains (CGs) prevents the pre-mature cracking of hetero-zone boundaries in conventional bimodal grain structures; thus, it is favored by AMCs. However, the design of the size and distribution of hetero-domains in trimodal AMCs is tough, with complicated multi-scale deformation mechanisms. This study tunes the distribution of FG domains elaborately via altering the volume fraction of FG from 10 vol.% to 60 vol.% and investigates the distribution effect of FG domains on strength–ductility synergy. The optimized 2024 Al matrix composites with 30 vol.% FG exhibited a tensile strength of over 700 MPa and an elongation of 7.5%, respectively, realizing a good combination of high strength and ductility. This work enlightens the heterostructure design with a balance between heterogeneous deformation induced (HDI) strain hardening and high-content soft phase induced strain homogenization.