Mixed Ductile-brittle Fracture Research Articles

Effect of electromigration on microstructure and properties of CeO2 nanopartical-reinforced Sn58Bi/Cu solder joints

To mitigate the decrease in mechanical performance of Sn58Bi/Cu solder joints resulting from electromigration-induced damage. The CeO2 nanoparticles were incorporated into Sn58Bi solder by a melt-casting method, and their effects on the microstructure and properties of Sn58Bi/Cu solder joints under electromigration were investigated. The study results demonstrate that the addition of 0.125 ~ 0.5 wt% CeO2 nanoparticles refines the eutectic microstructure of Sn58Bi solder alloy. At an addition amount of 0.5 wt%, the composite solder alloy exhibits the maximum tensile strength of 68.9 MPa, which is 37% higher than that of the base solder. CeO2 nanoparticle-reinforced Sn58Bi solder can achieve excellent solderbility with Cu substrates and the joints can significantly inhibit the growth of the anodic Bi-rich layer, which is responsible for electromigration. With the extension of current stressing time, Bi-rich and Sn-rich layer are respectively formed on the anode and cathode in the joints. The intermetallic compound (IMC) layer grows asymmetrically, transitioning from a fan-shaped morphology to a flattened structure at the anode and to a thickened mountain-like morphology at the cathode. Adding the CeO2 nanoparticles helps to mitigate the decrease in mechanical performance caused by electromigration damage during current application to some extent. Over the current stressing period of 288 ~ 480 h, the fracture position shifts from the anodic IMC/Bi-rich interface to the cathodic Sn-rich/IMC interface. The fracture mechanism transitions from a brittle fracture characterized by plate-like cleavage to a ductile–brittle mixed fracture with fine dimples and cleavage.

Influence of microstructure evolution on hot ductility behavior of a Cr and Mo alloyed Fe-C-Mn steel during hot deformation

Cr and Mo alloyed Fe-C-Mn steel was conducted by hot tensile experiments under different strain rates and deformation temperatures. The influence of microstructure evolution on fracture mechanism was studied. The principal failure mechanism was attributed to intergranular crack propagation and microvoid aggregation when deformed at 800–1100 ℃, changing to a mixed ductile-brittle intergranular fracture when deformed at 1200 ℃. After hot deformation, the microstructure transformed from a mixed phase of pearlite and ferrite to a lath martensite structure. Moreover, the lath martensite structure transitioned from disorder to order as the deformation temperature increases. The study revealed that in case of lower temperature when dynamic recovery (DRV) was dominated, the growth of voids was impeded by fine dynamic recrystallization (DRX) grains, resulting in an irregular void morphology. In case of higher temperature, the degree of DRX was high, voids were surrounded and impeded by newly formed DRX grains, which were gradually elongate with deformation. On the other hand, when there were DRX grains present between voids, void coalescence was less likely to occur, thereby inhibiting damage propagation.

Multi-objective optimization of friction stir additively manufactured Al-7075 multi-layered laminates

Friction Stir Additive Manufacturing (FSAM) is an advanced technique for non-fusion aluminum alloys, enabling the production of large, irregular-shaped parts without the typical solidification defects associated with melting. FSAM relies on a critical interplay of parameters that significantly influence the microstructure and mechanical properties of the final fabricated structure. This study focuses on developing seven-layered Al-7075 laminates following the Taguchi L9 matrix array under forced cooling. Three input parameters, namely tool stirring speed (TSS), tool welding speed (TWS), and the number of tool passes (NPs) were selected for multi-objective optimization using Taguchi Grey Relational Analysis (GRA) for higher microhardness, UTS, and % elongation. Microstructural analysis revealed fine equiaxed grains within the stir zone, indicating a substantial 95.5% reduction in grain size compared to the base metal, along with a mixed ductile-brittle fracture in samples with higher UTS. Taguchi single-response optimization identified optimal settings in sample L6 (800 rpm, 90 mm/min, and 1 NP) for increased hardness and reduced % elongation. Conversely, sample L7 (1000 rpm, 30 mm/min, and 3 NPs) exhibited the lowest hardness and UTS, along with the highest % elongation due to the high density of precipitate dissolution. Notably, sample L9 (1000 rpm, 90 mm/min, and 2 NPs) achieved the highest UTS of 415 MPa as a result of an even distribution of precipitates with low dissolution density, representing 74.3% of the base metal's strength. TWS emerged as the predominant input parameter, contributing 56.94%, 52.33%, and 47.33% to hardness, UTS, and % elongation, respectively. Moreover, Taguchi-GRA identified the optimal parameter set as TSS of 1000 rpm, TWS of 60 mm/min, and 2 NPs for higher hardness, UTS, and % elongation. ANOVA analysis of GRA demonstrated robust predictive capabilities with an R-squared value of 88.09%.

Improving bonding strength of medium-thick Al-Cu dissimilar joint by a novel splat cooling assisted double side friction stir welding

To obtain a high-strength medium-thick Al-Cu dissimilar joint, a novel splat cooling assisted double side friction stir welding (SDS-FSW) is proposed. The microstructure and tensile properties of conventional double-side friction stir welding (DS-FSW) and the novel SDS-FSW joints under optimal welding parameters are systematically compared, it shows that the utilization of in-process splat cooling significantly reduces the welding peak temperature and the high-temperature dwell time. The in-process splat cooling causes significant grain refinement in the welding nugget zone (WNZ) of the Al-Cu dissimilar SDS-FSWed joint, it exists the lower recrystallization fraction and the higher KAM value. Although both DS-FSW and SDS-FSW joints fracture at the Al-Cu interface, the DS-FSW joint represents a brittle fracture characteristic, while the SDS-FSW joint represents a mixed ductile-brittle fracture mode. Under the drive of the mechanical stirring of the tool and the welding thermal cycle, the aluminium atoms and copper atoms diffuse to the copper base material and aluminium base material, respectively. Since copper diffuses in aluminium at a higher diffusion rate than aluminium diffuses in copper, and the solubility of copper in aluminium is much smaller than that of aluminium in copper, thus the Al2Cu becomes the first phase with the nucleation and growth. As the welding temperature and time continue to increase, Al2Cu reaches the supersaturation state, and the Al4Cu9 phase is produced between the Cu (Al) and the Al2Cu phase. In the end, the interface forms the stable double-layer IMCs structure (Al2Cu/Al4Cu9). The optimal tensile strength of the SDS-FSWed joint is 159 MPa, which is increased by 42% compared to the DS-FSWed joint at the same welding parameter. This was mainly because in-process splat cooling contributed to forming a thinner IMCs layer at the A-Al-Cu interface. It reveals that the application of in-process splat cooling is a versatile method to thin the IMCs layer at the Al-Cu medium-thick DS-FSW process.

Effect of Aging Time on Microstructure and Properties of Cold-Rolled Ni-W-Co-Ta Medium–Heavy Alloy

A systematical exploration of the effect of aging time on the microstructure and mechanical properties of cold-rolled Ni-W-Co-Ta medium–heavy alloy with 90% thickness reduction at the aging temperature of 700 °C was performed. The results demonstrate that the volume fraction of the precipitation (Ni4W), which persists under various aging times, increases from 13.7% (2 h) to 28.7% (32 h) with the extension of aging time. Meanwhile, the microstructure after aging treatment is still dominated by dislocation entanglement and dislocation walls, although the degree of lattice distortion and dislocation density attributed to heavy deformation decreases. The maximum tensile strength, yield strength, and microhardness (2286 MPa, 1989 MPa, 766 HV) of the cold-rolled Ni-W-Co-Ta medium–heavy alloy under the 16 h aging treatment at 700 °C are reached, respectively. The ductile–brittle mixed fracture morphology is maintained in the fracture morphology of the medium–heavy alloy before and after aging treatment.

Effect of Vanadium Addition on Solidification Microstructure and Mechanical Properties of Al-4Ni Alloy.

The effects of vanadium addition on the solidification microstructure and mechanical properties of Al-4Ni alloy were investigated via thermodynamic computation, thermal analysis, microstructural observations, and mechanical properties testing. The results show that the nucleation temperature of primary α-Al increased with increased vanadium addition. A transition from columnar to equiaxed growth took place when adding vanadium to Al-4Ni alloys, and the average grain size of primary α-Al was reduced from 1105 μm to 252 μm. When the vanadium addition was 0.2 wt%, the eutectic nucleation temperature increased from 636.2 °C for the Al-4Ni alloy to 640.5 °C, and the eutectic solidification time decreased from 310 s to 282 s. The average diameter of the eutectic Al3Ni phases in the Al-4Ni-0.2V alloy reduced to 0.14 μm from 0.26 μm for the Al-4Ni alloy. As the vanadium additions exceeded 0.2 wt%, the eutectic nucleation temperature had no obvious change and the eutectic solidification time increased. The eutectic Al3Ni phases began to coarsen, and the number of lamellar eutectic boundaries increased. The mechanical properties of Al-4Ni alloys gradually increased with vanadium addition (0-0.4 wt%). The Al-4Ni-0.4V alloy obtained the maximum tensile strength and elongation values, which were 136.4 MPa and 23.5%, respectively. As the vanadium addition exceeded 0.4 wt%, the strength and elongation decreased, while the hardness continued to increase. Fracture in the Al-4Ni-0.4V alloy exhibited ductile fracture, while fracture in the Al-4Ni-0.6V alloy was composed of dimples, tear edges, and cleavage planes, demonstrating mixed ductile-brittle fracture. The cleavage planes were caused by the primary Al10V and coarse Al3Ni phases at the boundary of eutectic cells.

Effect of annealing on the microstructure and mechanical anisotropy of Mg/Al composite plate

In this paper, the effects of recrystallization, grain growth and texture evolution on the formability and anisotropy of Mg/Al composite plates were studied by different annealing processes. The results show that the rolled Mg/Al composite plate exhibits obvious anisotropy and poor formability in the rolling directions (RD) and transverse directions (TD), which is the result of the synergistic effect of matrix texture and grain characteristics. Annealing-activated recrystallization can eliminate local shear bands, hardening and other defects formed by rolling, thereby softening the structure and improving the formability of the sheet. The occurrence of recrystallization behavior also makes the bimodal texture parallel to the TD distribution in the Mg matrix transform into the RD-TD surface dispersion distribution, which weakens the strong deformation texture. The Schmidt factor (SF) of each slip system is calculated to evaluate the slip system start-up probability (deformation difficulty) of the material in a certain direction. The results show that the average SF values of matrix slip in RD and TD directions increase after annealing treatment, which makes the slip system easier to start. The tensile test results show that the formability of the composite plate is the best when the annealing temperature is 300 °C. At this time, the ultimate tensile strength (UTS) and yield strength (YS) are small, the elongation (EL) is large and the anisotropy of mechanical properties is the weakest. In addition, the annealing of the composite plate leads to the transformation of the fracture mechanism from brittle fracture to ductile-brittle mixed fracture. In summary, annealing treatment can effectively improve the formability and weaken the anisotropy of Mg/Al composite plates.

Comparison of electron-beam-welded joints manufactured by rolled and selective laser melted reduced activation ferritic/martensitic steel

As the core component of China Fusion Engineering Test Reactor (CFETR), the forming and manufacturing of the first wall faces severe challenges due to the constraints of materials, service environment and geometric structure. The microstructure and mechanical properties of welded joints obtained by rolled and laser selective melted (SLMed) reduced activation ferritic/martensitic (RAFM) steel were investigated in depth. The microstructure analysis results showed that the weld metal (WM) was composed of coarse lath martensite and δ-ferrite, while the microstructure were mainly coarse grains in heat affected zone (HAZ) which was easy to break. The average grains size of SLMed RAFM steel (S-RAFMs) was smaller than that of rolled RAFM steel (R-RAFMs), since the rapid melting and solidification of laser in the additive manufacturing process prevented grains to grow. The microstructure of R-RAFMs joint was relatively more uniform and there was no obvious texture and preferred orientation, while the WM of S-RAFMs joint had a weak cubic texture. The results of mechanical properties showed that the S-RAFMs joints exhibited higher strength (UTS = 697.46 MPa) and the R-RAFMs joints exhibited better plasticity (EL = 13.49 %) under the same tensile conditions. The tensile and impact fracture morphologies showed that the fracture mechanism of two joints was ductile-brittle mixed fracture, and the dimples of R-RAFMs joints were finer and denser. It was clearly observed that the hardness of WM (∼390–430 HV0.3) was higher than that of the base metal (BM) (∼190–230 HV0.3) from the hardness distributions along the width directions. The average hardness of BM in R-RAFMs joint (227 HV0.3) was lower than S-RAFMs joint (197.3 HV0.3), which was mainly related to the fine grain strengthening effect during SLM forming. The above results may provide theoretical guidance for the high-performance manufacturing of the first wall and promote the application of SLM and EBW technology in the field of nuclear fusion.

The microstructure and mechanical properties of aluminum/steel welded joint with Ni foil interlayers by FSLW

Friction stir lap welding experiments between 6061 aluminum alloy plate and DP590 dual-phase steel plate were carried out. The effects of welding parameters and addition of Ni foil interlayers with different thickness on the microstructures and mechanical properties of the lap joints were investigated. The shear strength of the joint reached the maximum value of 6597 N and showed a mixed ductile-brittle fracture mode when the welding speed, the rotating speed and the thickness of the Ni foil intermediate layer were set to 100 mm/min, 900 rpm and 0.3 mm, respectively. A certain amount of beneficial plastic AlNi and FeNi3 intermetallic compounds are formed through the reaction of Al, Fe and Ni at the aluminum alloy/steel interface.

Vacuum brazing of SiCp/Al composites and TC4 titanium alloy: Microstructure evolution and mechanical properties

In this paper, a dissimilar material consisting of 55% SiCp/ZL102 composites and TC4 titanium alloy was brazed using Al-8.0 Mg-17.0Cu-1.0Ti foil-like brazing alloy manufactured by rapid solidification technology. Brazing was completed in a vacuum of 1.0 × 10−4 Pa. Si element from Al composites accumulated at the interface of TC4 titanium alloy to form Ti7Al5Si12 intermetallic compound layer. The intermetallic compound α2(TiAl3) grew perpendicular to the brazing seam below 590 °C. The α2(TiAl3) intermetallic compound grown into long strips at 590 °C. The strip structure had a bonding interface in the form of pinning at a certain angle. The brazing interface was continuous and dense, without defects and exhibited good bonding performance. The shear strength of the brazed joint reached 98.01 MPa at 590 °C for 30 min, and the gas tightness was 1.0 × 10−10 Pa m3/s. The brazed joint failed in filler metal due to ductile-brittle mixed fracture mechanism, which resulted in shear lips, parabolic dimples, and furrows in the fracture surface.

Study on the properties of low silver Sn1.0Ag0.5Cu composite solder reinforced with nickel-plated zinc oxide and its soldering joint

Based on the preparation of nickel-plated zinc oxide (Ni-ZnO) by pyrolysis-reduction method, low silver Sn1.0Ag0.5Cu lead-free composite solders reinforced with Ni-ZnO were prepared by the powder metallurgy method, and the microstructure and properties of the soldering joints were investigated accordingly. The results showed that the particle size of ZnO was reduced after ball milling. The Ni-ZnO reinforcement phase was successfully prepared by pyrolysis-reduction method, in which Ni particles with an average particle size of 9.7 nm were uniformly and densely deposited on the surface of ZnO particles, and the interface between ZnO (002) and Ni (111) showed a semi-coherent relationship with a mismatch of 0.203. The moderate addition of Ni-ZnO had little effect on the electrical conductivity of the Sn1.0Ag0.5Cu composite solders reinforced with Ni-ZnO. But it refined the primary β-Sn grains in the composite solders and improved the wettability and tensile strength of the composite solders. At an addition of 1.0 wt% Ni-ZnO reinforcement phase, the tensile strength and shear strength of the composite solder and its soldering joint reached a maximum value of 41.8 MPa and 28.9 MPa, with an increase of 39.3 % and 41.7 % respectively compared to the base solder material. The fracture position of composite solder/Cu soldering joint occurred in the transition zone of the soldering seam and IMC (Intermetallic Compound) layers. With the increase of Ni-ZnO reinforcement phase addition, the shear fracture location of the soldering joint moved from the near copper substrate side to the near soldering seam side in the transition zone, and the fracture mode gradually changed from a mixed ductile-brittle fracture to a typical ductile fracture dominated by parabolic toughness dimples.

Characterization and constitutive analysis-based crystal plasticity of warm flow and fracture behaviours of 2060 Al–Cu–Li alloy

AA2060-T8 is a relatively new Al–Cu–Li alloy developed in the last few years as a lightweight alternative to conventional Al alloys. Thus, it is essential to characterize the warm flow, fracture, and anisotropic behaviours of this alloy and propose constitutive analysis-based crystal plasticity (CP) modelling to link its mechanical behaviour with the microstructural state. Therefore, in this study, isothermal warm tensile tests were conducted using a Gleeble-3800 thermomechanical simulator and different sample orientations at temperatures and strain rates varying from 373 to 573 K and 0.001–0.1 s−1, respectively. The tensile behaviour of the 2060 Al–Cu–Li alloy sheet depends on the sample orientations, which results in significant anisotropy. The anisotropic degree of the tensile properties of this alloy was significantly reduced by increasing the forming temperature above 473 K. In addition, the fracture behaviours of 2060 Al–Cu–Li sheets were transformed from mixed ductile-brittle fracture mode to ductile fracture mode by decreasing strain rates and increasing the temperatures, respectively. Afterward, a novel crystal plasticity model was proposed to describe the grain behaviour and the deformation mechanism of 2060 Al–Cu–Li alloy under warm forming conditions. The developed constitutive model was implemented using the monolithic time-integration algorithm-based backward Euler method. The results acquired from the proposed constitutive analysis agree well with those obtained from experimentation in all testing conditions. This indicates its ability to accurately predict the warm flow behaviour of 2060 Al–Cu–Li alloy under different strain rates and loading directions.

Microstructural evolution and interfacial properties of explosively welded Nb/steel composite plate during post-heat treatment

In this work, Nb/steel composite plates were fabricated by explosive welding technique. Then post-heat treatments were conducted for large temperature range of 723 K–1273 K. Microstructural evolution and interfacial properties in welded interface were revealed by SEM, EBSD, XRD, microhardness, nanohardness and shear strength examinations. The effects of annealed temperature on interfacial microstructures and properties of Nb/steel composites were systematically investigated. The results demonstrated that microstructures and microhardness of Nb-side almost presented no changes with annealed temperature; Carbon elements from steel-side diffused easily into Nb-side forming a thin Nb carbide layer in which content of NbC increased as increasing annealing temperature; At 1273 K, formation of Nb2C was also found in carbide layers. In the steel-side, during low temperature (723 K–923 K) annealing, the deformed pearlite near interface recrystallized to form ferrite of which grains grew up with increasing annealed temperature; During high temperature (1073 K–1273 K) annealing, a large number of voids appeared at 1073 K, and typical normalizing structure generated at 1173 K–1273 K. Meanwhile, it was also found that with increasing of annealed temperature, shear strength firstly increased to maximum value of 326 MPa at 923 K and then decreased to 267 MPa at 1273 K. Failure mechanism of shear tests exhibited mixed ductile-brittle fracture at low temperature annealing; Then it converted into brittle fracture at high temperature annealing. Microhardness near interface of steel side firstly decreased to the minimum value of 106HV at 923 K and then increased to value of 220HV at 1273 K.

Microstructure and low-temperature impact fracture behavior of QT400-18AL containing Ni

The effects of Ni content on the microstructures and low-temperature impact fracture behaviors of QT400-18AL ferritic ductile cast iron were systematically investigated by XRD, SEM, EBSD, and low-temperature impact test technique. The results demonstrate that the average diameter of graphite spheres and HAGBs increase with the increase of Ni content, whereas the nodule count, the crystal lattice constant and average grain size of the ferrite phase decrease. Additionally, the UTS, YS, and hardness of the QT400-18AL at room temperature increase with the increasing Ni content. The Ni-0.75 cast iron has the best room temperature mechanical properties with UTS of 391 MPa, YS of 253 MPa, and hardness of 140 HB. The average impact energy of the QT400-18AL decreases with the decrease in test temperatures and increase with the increase in Ni content at the same test temperatures. The fracture morphologies show that the ductile fracture zones increase with the increase in Ni content at the same impact temperature. Moreover, they decrease with the decreasing impact temperatures at the same Ni content, and the fracture types change from ductile fracture to ductile-brittle mixed fracture. The Ni-0.75 cast iron exerted satisfactory impact performance at −40 °C owing to the optimized combination of GBCD and dislocation density (KAM value), and its low-temperature impact energy was 12.64 J.

Effect of process induction on densification behavior, microstructure evolution and mechanical properties of Ti-5Al-2.5Sn ELI fabricated by laser powder bed melting

In this paper, Ti-5Al-2.5Sn ELI α titanium alloy was prepared by laser powder bed fusion. The effects of heat treatment and HIP process on the densification behavior, microstructure evolution, low-temperature mechanical properties, and fracture behavior of Ti-5Al-2.5Sn ELI titanium alloy were analyzed. The experimental results show that the density of Ti-5Al-2.5Sn ELI titanium alloy is improved after HIP treatment. Complete recrystallization occurred in the deposited microstructure, and the recrystallized α grains coarsened with the increase in holding time. After heat treatment, the tensile strength decreases gradually, while the elongation increases with the increase of holding time. Compared with 300 K, the ultimate tensile strength and yield strength of the sample at 77 K were improved, but the elongation decreased. Ti-5Al-2.5Sn ELI titanium alloy annealed at 850 °C/4 h exhibits mixed ductile-brittle fracture morphology at 77 K and obtains ultrahigh strength and ductility. At the same time, Ti-5Al-2.5Sn ELI titanium alloy exhibits unit slip deformation behavior at 300 K, while at 77 K, it mainly exhibits the coupling of dislocation slip and twin deformation behavior. These findings lay a foundation for the rapid preparation of high-strength α-titanium alloy products with good ductility.

The microstructure and mechanical properties of aluminum/steel welded joint by FSLW

Friction stir lap welding experiments between 6061 aluminum alloy and DP590 dual-phase steel plates were carried out. The effects of welding parameters and the addition of Cu foil interlayers with 0–0.3 mm thickness on the microstructures and mechanical properties of the lap joints were investigated. The shear strength of the joint reached the maximum value of 5337 N and showed a mixed ductile-brittle fracture mode when the welding speed, the rotating speed, and the thickness of the Cu foil intermediate layer were set to 80 mm/min, 900 rpm, and 0.1 mm, respectively. The plastic Al2Cu and AlCu intermetallic compounds formed through Al and Cu reaction at the aluminium alloy/steel interface play a beneficial role in the Cu-added joint strength.

Chip formation mechanism in cryogenic machining of high temperature alloy-Inconel 718 and Ti-47.5Al-2.5V-1.0Cr

Inconel 718 and TiAl alloy are typical difficult-to-cut materials for their high cutting temperature, and cryogenic machining is an effective method to solve this problem. In order to reveal the material removal mechanism of these two materials in cryogenic machining, turning tests with different cooling media were designed and carried out. Cryogenic cooling could increase the brittleness of the material, making the material removal mechanism change from ductile removal to brittle removal. Due to the different plasticity of the material and their different sensitivity to temperature, the cooling conditions have slightly different effects on in the change of the removal mechanism of these two materials. By observing the geometric characteristics of chip, the deformation mechanism of the material near the chip free surface area was studied. Besides, the variation of chip root morphology was discussed to reveal the deformation mechanism of the primary deformation zone near the cutting edge. The experimental results showed that cryogenic cooling could change the material removal mechanism, but the sensitivity of different materials to temperature also have a significant influence, and the more sensitive to temperature, the more the material removal mechanism changes. The formation mechanism of serrated chips of Inconel 718 in cryogenic cutting was a combined action of ductile-brittle mixed fracture near the chip free surface and adiabatic shearing near the cutting edge. And the formation mechanism of Ti-47.5Al-2.5V-1.0Cr serrated chips is a combined action of periodic brittle fracture near the chip free surface and ductile-brittle fracture near the cutting edge.

Research on the Microstructure and Properties of a Flux-Cored Wire Gas-Shielded Welded Joint of A710 Low-Alloy High-Strength Steel

In this study, a 16 mm thick A710 low-alloy high-strength steel was welded by using flux-cored wire gas-shielded welding with an E81T1-Ni1M flux-cored wire. The microstructure characteristics and mechanical properties of the joints were systematically studied. The results showed that the joint was well formed without obvious welding defects. The center of the weld was mainly needle-like ferrite, the coarse grain area was mainly slat-like and granular bainite, and the fine grain area was mainly ferrite and pearlite. The lowest hardness in the weld area was the weakest area of the joint. The average tensile strength of the joint was 650 MPa, reaching 95% of the base metal; the samples were all fractured in the weld area, and the fracture morphology showed typical plastic fracture characteristics. The low-temperature (−40 °C) impact energy of the joint weld area and the heat-affected zone were 71 J and 253 J; the fracture morphology was characterized by a ductile–brittle mixed fracture, and the ductile area of the specimen fracture in the heat-affected zone was larger. The bending performance was good. Under the specified life of 2 × 106 cycles; the median fatigue limit and the safety fatigue limit were 520 MPa and 492 MPa, and the fatigue cracks germinated on the surface of the priming weld.

Exploring strengthening mechanism of FeCoNiAl high-entropy alloy by non-metallic silicon addition produced via powder metallurgy

Multi-component high-entropy alloys (HEAs) have recently attracted interest in balancing the bottleneck of high hardness and high fracture toughness. This study demonstrates the influence of Si addition to FeCoNiAl HEA fabricated by advanced powder metallurgy. The FeCoNiAlSix (x = 0, 0.2, 0.4, 0.6, and 0.8 molar ratio) HEA was fabricated by mechanical alloying for 45 h and subsequent powder compact densification by spark plasma sintering (SPS). FeCoNiAlSix HEA morphology and phase evolution were investigated to understand the influence of Si content on the microstructure, compressive strength, and fracture mechanisms of the FeCoNiAl alloy. The results indicate that FeCoNiAlSix powders HEAs have single-phase body-centered cubic (BCC) structures. However, densification of powders HEAs facilitates dual-phase face-centered cubic (FCC) and BCC formation in FeCoNiAl, whereas the FCC phase transforms completely to BCC or BCC/B2 with Si addition. The Vickers microhardness of sintered FeCoNiAlSix HEAs varies from 524 HV (for FeCoNiAl HEA) to 798 HV (for FeCoNiAlSi0.8 HEA). The maximum compressive strength of HEAs varies from 1325 MPa (for FeCoNiAl HEA) to 2031 MPa (for FeCoNiAlSi0.8 HEA), respectively. The absorption energies of FeCoNiAl and FeCoNiAlSi0.2 were 198 and 203 MJ/m3, respectively. In contrast, the absorption energies of FeCoNiAlSi0.6 and FeCoNiAlSi0.8 HEAs reduced drastically (115 and 96 MJ/m3, respectively). The failure mechanism of the FeCoNiAlSix HEAs was a mixed ductile-brittle fracture in FeCoNiAl. Brittle fracture dominates in FeCoNiAlSi0.2 and FeCoNiAlSi0.4 HEAs. A complete brittle fracture was noticed with river-like patterns in FeCoNiAlSi0.8 HEAs. The results of this investigation suggest that HEAs with different Si molar ratios and tuned phase structures can be used to design high-strength structural alloys for automotive applications.

Effects of Rotational Speed on the Microstructure and Mechanical Properties of 2198-T8 Al-Li Alloy Processed by Friction Spot Welding

The friction spot welding (FSpW) method was used to weld 2198-T8 Al-Li alloy at rotational speeds of 500 rpm, 1000 rpm, and 1800 rpm. It was shown that the grains in the FSpW joints were transformed from "pancake" grains to fine equiaxed grains by the heat input of welding, and the reinforcing phases of S' and θ were all redissolved into the Al matrix. This leads to a decrease in the tensile strength of the FsPW joint compared to the base material and a change in the fracture mechanism from mixed ductile-brittle fracture to ductile fracture. Finally, the tensile properties of the welded joint depend on the size and morphology of the grains and their dislocation density. At the rotational speed setting of 1000 rpm in this paper, the mechanical properties of welded joints consisting of fine and uniformly distributed equiaxed grains are best. Therefore, a reasonable set of the rotational speed of FSpW can improve the mechanical properties of the welded joints of 2198-T8 Al-Li alloy.