Search

Directed energy deposition (DED), as an additive manufacturing technology, has shown unique advantages in multi-material additive manufacturing and remanufacturing. In this study, two types in-situ alloyed CrCoNi medium entropy alloys that have thin-walled structures with different thicknesses (T1 and T2) were manufactured by the DED process, and the mechanisms of differences in relative density, microstructure, and mechanical properties at different heights were systematically analyzed. In terms of microstructure, the T1 and T2 samples along the building direction exhibit significant differences in crystallographic orientation, grain size, and dislocation density, which are related to the local temperature gradient differences caused by the scanning path and heat accumulation. In terms of mechanical properties at different heights of the two types of thin-walled structures, the yield strength is higher but the elongation is lower at the bottom position of sample, while the yield strength is lower but the elongation is higher at the middle and top positions. The differences of mechanical properties at different heights of the T1 and T2 samples are related to the microstructure and relative density. This finding provides new insights for the design and performance analysis of complex thin-walled structures formed by additive manufacturing. [ABSTRACT FROM AUTHOR]

Cold metal transfer (CMT) welding is an advanced arc welding technology, which is promising for joining dissimilar materials such as aluminum, magnesium, titanium alloys and steels for automotive and aerospace industries, as their high demands on reliable lightweight structures. Meanwhile, medium entropy alloys (MEAs) are a kind of highly promising new materials because of the excellent functional and mechanical performances. In this work, AA5052 Al/galvanized steel dissimilar joints were obtained by CMT-CS (cycle step) lap welding technology with CoCrNi MEA powder interlayer. When the MEA interlayer was applied, the contact angle reduced from 53.6° to 31.5°, and brazed width increased from 6.1 to 6.9 mm, which improved the wettability and spreadability of the molten filler on the steel substrate. In terms of interfacial microsture, the addition of the MEA layer with Co, Cr, Ni elements promoted the geneation of a relatively ductile Fe-rich Fe-Al IMCs, i.e., FeAl and Fe3Al, and thus enhanced the mechanical propeties of the laser joints. The maximum tensile load of the joints with interlayer reached 274 N/mm, which was 27.4% higher than that of the joints without interlayer. The introduction of MEA interlayer also improved the corrosion resistance and reduced the corrosion rate of the dissimilar joints. [ABSTRACT FROM AUTHOR]

The effective atomic radii of the constituent elements and their temperature dependence in quaternary and ternary subset alloys derived from the CrMnFeCoNi high-entropy alloy have been experimentally determined by a combination of lattice parameter measurements at room temperature and thermal expansion measurements down to the liquid helium temperature. The determined relative order of the effective atomic radii at 0 K is Co << Cr < Ni < Fe << Mn. The temperature dependence of the effective atomic radii differs from each other, but the difference is not large enough to change the relative order of the effective atomic radii at 0K and room temperature. Absolute values and relative order among the elements do not agree with those calculated using the ab-initio calculations. [ABSTRACT FROM AUTHOR]

FeCrNi medium entropy alloy (MEA) was prepared by selective laser melting (SLM) method, and the microstructure and mechanical properties of the FeCrNi MEAs under different laser energy density were investigated. The results show that the as-SLMed samples have a single-phase FCC structure and show a hierarchical microstructure of micro-scale matrix grains and submicron-scale cellular grains. The density of the as-SLMed samples is greatly affected by the energy density, which will increase rapidly and then decrease slowly with the increasing energy density. Energy densities between 45 and 80 J/mm3 are considered to be the optimum process window for preparing the FeCrNi MEA, which show good combination of strength (σ0.2 > 720 MPa, σUTS > 950 MPa) and ductility (εf = 20 ~ 40%). The high-density boundary networks from coarse matrix grains and fine cellular grains are the main reason for the high strength and the outstanding plasticity is due to the highly activated deformation twinning behavior resulting from the FCC matrix. [ABSTRACT FROM AUTHOR]

An ultra-high strain rate (104 s−1) dynamic plastic deformation treatment at liquid nitrogen temperature (LNT-DPD) followed by annealing is carried out to obtain dual heterogeneity of grain size and dislocation density in an equiatomic CrCoNi medium entropy alloy (MEA). Such extreme loading conditions resulted in extensive phase transformation in this MEA. Subsequent annealing at 650 °C for 1 h further induced reverse phase transformation and partial recrystallization, forming a complex heterogeneous microstructure characterized by nested trimodal grain sizes and partitioned dislocation density. A superior yield strength of ∼800 MPa and a good ductility of ∼40% were simultaneously achieved in this heterogeneous alloy. In order to reveal the effects of grain size and dislocation density distributions on the mechanical property improvements, the underlying deformation mechanisms were systematically discussed. High density of geometrically necessary dislocations (GNDs) would be induced in complex heterogeneous structures during tensile deformation due to strain gradients or partitioning between different regions, which would lead to additional strengthening and work hardening. These results provide a novel approach to overcome the strength-ductility trade-off dilemma for FCC-structured MEAs.

Heterogeneous grain structures with coherent L12 nanoprecipitates were constructed in a medium entropy alloy with chemical composition of Co34.5Cr32Ni27.5Al3Ti3 (at. %). The volume fraction and size of L12 nanoprecipitates are observed to become higher and larger, and the interspacing is found to become smaller with increasing aging time, resulting in a severer heterogeneity. The domain of tensile properties was expanded by varying the size and volume fraction of coherent L12 nanoprecipitates after aging treatment. The samples after longer-time aging treatment show stronger strain hardening as compared to those after shorter-time aging treatment. The hetero-deformation-induced (HDI) hardening plays a more vital role in the longer-time aged samples than the shorter-time aged samples, especially at the elasto-plastic transition stage. The dominant precipitation hardening mechanism is observed to transit from dislocation-cutting to Orowan dislocation-looping with increasing precipitation size, as predicted by a theoretical model and validated by experimental results. Multiple deformation defects, such as deformation twins, stacking faults and Lomer-Cottrell locks, are the dominant deformation mechanisms for all samples, while interactions between these defects and L12 nanoprecipitates were observed to be more intensive in the longer-time aged samples than in the shorter-time aged samples, resulting in stronger precipitation and HDI hardening.

A Cantor’s alloy with modified contents in manganese and chromium, elaborated by classical foundry, was enriched either in tantalum and carbon or in hafnium and carbon, to promote the formation of MC carbides to try improving the poor creep resistance of the original quinary alloy for possible use at 1000°C. Similarly to what was observed in recent investigations, metallography allowed checking that interdendritic/intergranular MC carbides were obtained here too. The two monocarbides–containing alloys were first subjected to differential thermal analysis to specify notably their melting start temperatures, to validate the chosen temperature for the creep tests. Three-point flexural creep tests were then performed at 1000°C, which indisputably demonstrated the outstanding strengthening effect of these MC carbides for the modified Cantor’s alloy.

Purposes A novel type of Ni2CoCr0.5Si0.3Al0.1Ti0.1 medium entropy alloy is designed in this work. Methods Trace amounts of Si, Al, and Ti elements are added to the non-equiatomic ratio NiCoCr-based medium entropy alloy, and dispersed coherent L12 phase precipitation is introduced through aging heat treatment at different temperatures. Findings The results show that the Ni2CoCr0.5Si0.3Al0.1Ti0.1 medium entropy alloy aged at 700 ℃ has the best mechanical properties, with a yield strength of 783 MPa, an ultimate tensile of 1 252 MPa, and an elongation of 41.6%. Compared with the annealed samples, the alloy after aging treatment is increased in yield strength by 310 MPa. Further analysis of the initial microstructure of the alloy aged at 700 ℃ reveals the presence of coherent L12 phase structure and trace amount of intermetallic compound Ni16Ti6Si7 phase. Meanwhile, because of the severe deformation through cold rolling and the short time of annealing treatment, trace stacking faults, nano stacking fault networks, Lomer-Cottrell locks, and dislocation pinning phenomena are found in the initial structure. The geometrically necessary dislocation exhibits a uniform distribution during the stretching process, and the main deformation mode is dominated by dislocation slip with a stacking fault structure.

An extended crystal plasticity model and an optimized neural network are established to investigate the mechanical behavior of CoCrNiSi0.3C0.048 Medium entropy alloy (MEA) with a three-level hierarchical precipitation structure under dynamic compressive deformation at a variety of ranges of strain rates. The interaction between the matrix and first-level precipitates is described by a presented hybrid representative volume element RVE model, involving the effects of volume fraction (Vf), radius and geometric distribution on heterogeneous deformation. Effects of second- and third-level precipitates are realized through the extended crystal plasticity constitutive model, reflecting the motion mechanism of dislocations near the precipitates in the form of initial slip resistance. Meanwhile, this study investigates the influence of the presence of primary precipitates on the initiation of slip systems within special grains with Goss or S orientations in the matrix. Additionally, it provides a more in-depth explanation of the underlying principles behind the suppression of Goss texture formation by the precipitates from the perspective of slip systems analysis. The changing of volume fractions affects flow stress evolution. Avoiding suffering from expensive computational consumption by the crystal plasticity finite element method, an artificial neural network model optimized through a genetic algorithm, is presented to predict stress-strain responses and texture evolution results under varying compression strain rates. A reliable dataset derived from crystal plasticity finite element models and experimental results is utilized to train, test, and validate the genetic algorithm-based artificial neural network model. This model demonstrates good prediction capability in dynamic mechanical behavior and texture evolution, showcasing the feasibility and efficacy of the proposed neural network model. Compared to the CPFEM method, neural network models can greatly improve computational efficiency.

In this work, a series of Fe-free AlCoaCrbNic (AlCo1.31Cr0.5Ni2.4, AlCo1.71Cr0.5Ni2.0, AlCo1.8Cr0.6Ni1.8, and AlCo1.16CrNi2.0 denoted as E1, E2, E3, and E4, respectively) medium-entropy alloys (MEAs) were designed though the mathematical model. Among the four MEAs, E2 and E3 exhibit uniform and typical eutectic structural characteristics, consisting of face-centered cubic (FCC) and ordered body-centered cubic (B2) phases, with ordered FCC (L12) nanoprecipitates uniformly distributed in the FCC matrix. The E4 alloy is composed of primary B2 phase and eutectic FCC + B2 phases, and the high-density Co- and Cr-rich BCC-structured spherical precipitates are distributed in the B2 phase. Importantly, the E1 alloy is characterized by a phase structure comprising both FCC and body-centered tetragonal (BCT) phases, a combination that is notably infrequent among high-entropy alloys. There are high-density twinning and stacking faults (SFs) in the BCT phase. All four alloys exhibit excellent compressive properties, with fracture strength surpassing 2000 MPa and compression ratio exceeding 30 %. The potentiodynamic polarization curves implied that the E4 MEA presented excellent corrosion resistance with the lowest corrosion current density of 4.05 × 10−7 A/cm2 and the lowest passive current density of 7.01 × 10−6 A/cm2 in 3.5 wt% NaCl solution. Moreover, the electrochemical impedance spectroscopy (EIS) test results also indicating that the E4 MEA has the best corrosion resistance among the four alloys because it has the highest electrode resistance (Rp = 171.76 × 103 Ω cm2). The ordered FCC/BCT phase structure in E1 alloy provides effective exploration for the development of new structured MEAs.

In this work, the (10 wt%, 30 wt%, 50 wt%) TiC reinforced AlFeCrCo medium entropy alloy (MEA) lightweight composite coatings with the extremely low porosity were successfully fabricated on Mg alloy substrate by resistance seam processing. The results showed that the microstructure of composite coating was consisting of a BCC-based MEA matrix and TiC particles, and a good metallurgical bonding with a semi-coherent relationship was formed between the coating and Mg alloy. Furthermore, the lightweight composite coatings (5.6–6.5 g cm−3) showed improved corrosion resistance over Mg alloy substrate. Specifically, the composite coatings exhibited optimized wear performance in the dry, deionized water and 3.5 wt% NaCl solution conditions with the increasing TiC content, surpassing related high/medium entropy alloy coatings, which is attributed to the formation of a coherent interface between the MEA matrix and the TiC particles. The first-principles calculations were performed to elucidate the nature for the higher bonding strength of TiC/MEA interface. In these wear conditions, the main wear mechanisms of composite coatings were discussed in terms of adhesive wear, oxidative wear and/or corrosive wear, in connection with their microstructure features and electrochemical behaviors. Based on the elevated anti-corrosion ability, this work has provided a strategy to fabricate advanced coatings on Mg alloys endowing with lower density, interfacial metallurgical bonding and optimized wear resistance, substantially contributing to the development of high-performance composite coatings.

Multi-principal element, medium, and high entropy alloys are recently known to exhibit a wide spectrum of intriguing mechanical, physical, electrocatalytic, and anticorrosive properties that are superior to traditional alloys. In this regard, the NiCoCr and NiCoV systems, which are among the most promising medium entropy alloys for a wide range of applications, show strong corrosion resistance in alkaline or acidic environments, in addition to their attractive potential for electrochemical water splitting. The electrochemical characteristics of these alloys are primarily linked to their versatility in adjusting compositions and constituent elements. This adaptability gives rise to distinctive properties, including a metal-hydrogen bond strength on par with platinum, resulting in significant catalytic activity. In this work, we demonstrate that the laser structured NiCoCr(V) system exhibits electrochemical performance in water splitting comparable to commercial electrodes with overall water splitting potential of 1.61V@10 mA/cm2 which is very close to 1.56 V of Pt/C||Ir/C electrodes, thanks to its richness in oxidation states. Femtosecond laser surface structuring has been used to further enhance the oxidation states through tailored laser-matter interactions and increase the electrochemically active surface area by creating laser-induced periodic structures on the surface. A combination of X-ray photoelectron spectroscopy, Raman spectroscopy, X-ray diffraction, and high-resolution electron microscopy have been used to evaluate the impact of the laser processing on the surface. The electrochemical performances of the studied alloys were characterized for both Hydrogen evolution and Oxygen evolution reactions. In both cases, critical electrochemical parameters such as overpotentials and Tafel slopes monotonically improved by around 10–15 % compared to the unprocessed electrodes. [Display omitted] • Impact of femtosecond laser on NiCoCr and NiCoV alloys for water splitting were studied. • The fs laser treatment increased surface area and induced functional oxides for HER/OER. • There was notable reduction in bi-functional voltage of laser processed samples. • LS-NiCo(Cr/V) exhibited outstanding bi-functional activities in alkaline electrolysis. [ABSTRACT FROM AUTHOR]

Metal lattice structural materials are widely used in aerospace, automotive industry, and other fields due to their advantages of lightweight, high specific strength, energy absorption, and porosity. High strength and toughness FeCrNi medium entropy alloy (MEA) was taken as the research object, and selective laser melting (SLM) was used to prepare FeCrNi medium entropy alloy lattice structure materials with four simulated lattice structures: BCC, BCCZ, FCC and FCC. The microstructure, mechanical properties, and deformation behavior of these materials were systematically studied.The results indicate that the FeCrNi medium entropy alloy lattice structure prepared by the skip scanning strategy has high node overlap quality, dense interlaced stacking of molten pools, and uniform and fine grains. When the relative density is similar, the specific strength and specific energy absorption values of BCC, FCC, BCCZ, and FCCZ lattice structures increase sequentially.The specific energy absorption of FeCrNi medium entropy alloy material with FCCZ lattice structure reaches 49.8J·g-1, significantly higher than that of Ti6Al4V and 316L stainless steel lattice materials.The finite element simulation analysis shows that the presence of Z-shaped pillars increases the apparent strength and stiffness of the lattice material, and leads to a transition in deformation behavior from bending dominated to tensile dominated, which is the main reason for the strength improvement of the FCCZ lattice structure.

Directed energy deposition (DED), as an additive manufacturing technology, has shown unique advantages in multi-material additive manufacturing and remanufacturing. In this study, two types in-situ alloyed CrCoNi medium entropy alloys that have thin-walled structures with different thicknesses (T1 and T2) were manufactured by the DED process, and the mechanisms of differences in relative density, microstructure, and mechanical properties at different heights were systematically analyzed. In terms of microstructure, the T1 and T2 samples along the building direction exhibit significant differences in crystallographic orientation, grain size, and dislocation density, which are related to the local temperature gradient differences caused by the scanning path and heat accumulation. In terms of mechanical properties at different heights of the two types of thin-walled structures, the yield strength is higher but the elongation is lower at the bottom position of sample, while the yield strength is lower but the elongation is higher at the middle and top positions. The differences of mechanical properties at different heights of the T1 and T2 samples are related to the microstructure and relative density. This finding provides new insights for the design and performance analysis of complex thin-walled structures formed by additive manufacturing.

The development of high-performance, robust, and economical bifunctional electrode materials for efficient H 2 and O 2 evolution reactions (HER and OER) in water splitting is one of the fundamental aspects of the ever-growing H 2 research. In this regard, high and medium entropy alloy systems exhibit great potential in cost-to-performance trade-off as bifunctional electrodes. In particular, the richness in mixing various elements with different hydrogen bonding abilities brings the possibility to tune the catalytic properties that aim for high HER/OER efficiency. In this regard, CoNiCr and CoNiV systems are highly promising model systems with only 3 elements and low hydrogen diffusivity. Also, excellent corrosion resistance in acidic/alkaline environments and high conductivity, and simple crystal structure make these materials very auspicious candidates for HER/OER studies. Here, we have evaluated the electrocatalytic performance of the medium entropy alloy system NiCo(Cr/V) for water splitting. For our investigations, thin films of three compositions namely, NiCoCr, NiCoV, and NiCo(Cr/V) were fabricated using magnetron sputtering. In particular, the NiCo(CrV) (thickness: 1 μm) demonstrates excellent performance with overpotentials of 87 and 320 mV at 10 mA/cm2 and Tafel slopes of 96 and 69 mV/dec towards HER and OER, respectively in 1 M KOH solution. Our findings suggest that the higher HER catalytic activity originates from the synergistic effects of the multicomponent alloy system in single-phase, while the higher OER activity comes from the multicomponent functional oxides of NiCo(CrV) generated on the surface during CV activation. An alkaline electrolysis cell with NiCo(CrV) as bifunctional electrode requires only 1.58 V to maintain 10 mA/cm2, with outstanding electrochemical stability with no compositional reorganization after long-term durability tests. [Display omitted] • Thin films of NiCoCr, NiCoV and NiCo(CrV) were studied in order to evaluate their catalytic performance in water splitting. • The NiCo(CrV)/CC MEA showed superior HER/OER activity in alkaline media. • This NiCo(CrV)/CC exhibits excellent stability as a bifunctional electrode. • NiCo(CrV) exhibited an over potential value of 87 mV at 10 mA/cm2. [ABSTRACT FROM AUTHOR]

The sluggish de/hydrogenation kinetics and stable thermodynamics of magnesium hydride (MgH 2) are unfavorable for its large-scale application. Herein, the medium-entropy alloy CrCoNi nanosheets were synthesized and remarkably enhanced the low-temperature hydrogen storage performance of MgH 2. Surprisingly, the initial dehydrogenation temperature of 9 wt% CrCoNi modified MgH 2 was greatly reduced from 325 °C to 195 °C, a drop of 130 °C compared to non-additive MgH 2. Also, the MgH 2 –CrCoNi composite released 4.84 wt% hydrogen at 300 °C even in 5 min and absorbed 3.19 wt% hydrogen at 100 °C within 30 min (3.2 MPa). The calculated de/rehydrogenation activation energy were reduced by 45 and 55 kJ mol−1, respectively. Further cyclic kinetics investigation indicates that the 9 wt%-CrCoNi doped MgH 2 still presented good stability after 20 cycles, losing only 0.36 wt% hydrogen capacity. The XRD pattern validated that CrCoNi remained stable during the cyclic reaction process. Besides, the uniformly distributed CrCoNi nanosheets were in tight contact with the MgH 2 surface, providing abundant catalytic active sites and low-energy barrier diffusion channels. Under synergistic catalysis, the H atoms are rapidly absorbed and released across the Mg/MgH 2 interface, resulting in excellent kinetic properties. Briefly, this paper provides new references and inspirations to design efficient polymetallic catalysts for hydrogen storage materials. • CrCoNi nanosheets were prepared via a facile wet chemical ball milling method. • The de/hydrogenation performance of 9 wt% CrCoNi modified MgH 2 was greatly enhanced. • The synergistic effect of Cr, Co and Ni elements played a key role in the catalysis on MgH 2. [ABSTRACT FROM AUTHOR]

In this work, a novel multiple-phase CrFeNiAl0.2Ti0.1 medium entropy alloy was designed by introducing large atomic radius elements Al and Ti with the molar ratio of 2:1, which was systematically investigated from microstructure to mechanical properties. The results show that the alloy formed a typical dendritic structure with (Cr, Fe)-rich BCC dendrites and Ni-rich FCC interdendrites through uniform transformation. In the interdendrites, a small number of microscale strips and nanoscale particles composed of Ni3(Al, Ti)-type L12 phase precipitated via a mechanism of precipitation decomposition. Compared to the CrFeNi base alloy with a yield strength of 160 ± 5 MPa, the as-cast CrFeNiAl0.2Ti0.1 exhibited a superior strength-plasticity combination with a high yield strength of 903 ± 10 MPa and a great plastic strain of more than 50%. The significant improvement of the yield strength was mainly attributed to solid solution strengthening from the large lattice-distortion and modulus-distortion of Al and Ti atoms. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Materials Engineering / Cailiao Gongcheng is the property of Journal of Materials Engineering Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Joining of barium titanate (BaTiO 3 , BT) ceramics to metals plays an important role in their wide applications. However, conventional joining methods, including solid state diffusion joining and liquid state joining, have several drawbacks. In this paper, BT ceramic was flash joined to NiCrFe medium-entropy alloy (MEA) in seconds with an electric field (E-field) using current density of 10–80 mA/mm2 at 700–1000 °C. Shear strength of the joint first increased with applied current density and dwell time of the E-field and then decreased. It exceeded 40 MPa when the temperature was above 900 °C with current density of 60 mA/mm2 for 60 s. Both the real part (ε') and imaginary part (ε ") of complex permittivity of the joined sample were larger than those of the as-sintered BT ceramic at high frequency range. Microstructure of the joint suggests that a layer of chromium oxide was formed at faying surfaces, achieving effective bonding of BT and MEA. This research provides an efficient method for the manufacturing of BT based devices. [ABSTRACT FROM AUTHOR]

Medium entropy alloys (MEAs) have a high potential in industrial applications due to their excellent mechanical properties. Here we report a strategy to improve the ductility of the Ni40Co30Cr20Al5Ti5 MEAs by rapid solidification (RS). The as-cast alloy is composed of dendritic structure and L12 nano-precipitates. After RS, the lattice misfit of precipitate and matrix is reduced from 0.251 % to 0.167 %, and the size of the precipitate decreases from 85 nm to 28 nm. Meanwhile, RS also reduces the grain size of matrix. Gliding dislocation can cut through precipitates with low lattice misfit and smaller size, which can avoid strain accumulation and prevent crack initiation at the precipitate-matrix interfaces. Compared with the as-cast alloy, the ductility of the RS alloy is increased by 17 %–42 % without a decrease in strength. Our results can provide a guidance for other experiments using the RS technology (laser cladding, magnetron sputtering, etc.) to achieve the strength-ductility balance in the MEAs.

This study investigates the hot deformation behavior and microstructural evolution of a novel medium entropy alloy subjected to low strain rates and temperatures ranging from 650 °C to 850 °C. The alloy was subjected to hot compression testing under various conditions to observe its response. At all deformation condition, discontinous dynamic recrystallization was the main restoratory mechanism. After full recrystallization, grain sizes ranged from 1 to 5 μm, accompanied by an average hardness of 250 HV. The flow curves demonstrated a typical three-stage work hardening behavior across all deformation conditions, indicative of dynamic recrystallization. It is found that the deformation activation energy (Q) exhibited a negative slope with increasing strain, with an average value of Q = 267.7 kJ/mol. Zener-Hollomon parameter was determined from the experimental results and effectively used to simulate deformation, demonstrating good agreement with experimental findings. Remarkably, following the initial cycle of recrystallization, the deformation process shifted towards grain boundary sliding instead of secondary recrystallization, displaying characteristics reminiscent of superplastic behavior. Further analysis revealed a high strain rate sensitivity (m > 0.3) at 750 °C and 850 °C, with grain sizes lower than anticipated values. In summary, this study provides a comprehensive understanding of the hot deformation behavior and microstructural evolution in the Fe50-(CoCrMnNi)50 alloy. The findings shed light on dynamic recrystallization, deformation activation energy, and the unexpected shift to grain boundary sliding. These insights are valuable for advancing the knowledge of medium entropy alloys and their potential applications in various industries.

Experiments have shown that planar defects play an important role in strengthening and toughening medium- and high-entropy alloys (M/HEAs). Understanding the underlying defect dependent deformation mechanisms is helpful in designing strong and ductile M/HEAs. In this work, the compressive mechanical responses of columnar-grained CrCoNi MEAs are investigated via molecular dynamics simulations, considering the interspacing effect and deformation difference of pre-existing nanotwins (NTs) and stacking faults (SFs). It is found that the strength of MEAs is enhanced by the introduction of planar defects through impeding dislocation motion. “Smaller and stronger” is found between the NT/SF interspacing and strength, which is attributed to the stronger obstruction of dislocation motion for the higher fraction of planar defects at smaller interspacing. During compression, as compared with NTs, dislocations are easier to penetrate the SF planes, and partial SFs are annihilated due to the reaction between partial dislocations and SFs. Thus, at an identical NT/SF interspacing, the stacking faulted specimen manifests lower strength than the nanotwinned counterpart. The strain localization and shear deformation are observed in the specimen with a small NT/ST interspacing but dislocation motion dominates plastic deformation. Only at sufficiently small NT interspacing, less than 1 nm, the dominant deformation of the nanotwinned specimen is transformed into strain localization deformation, leading to softening. Our finding provides further atomic insights for understanding the strengthening mechanisms of M/HEAs with pre-existing planar defects.

The mechanical properties and serviceability of the alloy fabricated by laser powder bed fusion (L-PBF) additive manufacturing (AM) strongly depend on the phase composition and microstructure, which are affected by the thermal condition of the process, such us cooling rate. In this study, AlCrFe2Ni2 medium entropy alloy (MEA) was fabricated by L-PBF under varying cooling rate via changing process parameters. The phase transformation, microstructure and mechanical properties were experimentally investigated using X-ray diffraction (XRD), scanning transmission electron microscopy (TEM), scanning electron microscope (SEM) and electron backscatter diffraction (EBSD). The cooling rate was numerically calculated using a well-tested multi-physic thermal fluid model. The results showed that cooling rate had a significant effect on phase composition, microstructure and mechanical properties. The as-built MEA with lower cooling rate showed a nano-size structure composed of BCC and ordered B2 phases and a higher yield strength of 2055 MPa. While for the higher cooling rate, the as-built MEA showed a homogeneous structure consisted of single ordered B2 phase and a lower yield strength of 1544 MPa. The threshold value of cooling rate for phase transformation was between 4.9 × 106 K/s and 6.0 × 106 K/s. Through the strengthening mechanism analysis, it was crucial to achieve the nano-size dual-phase microstructure by manipulating cooling for obtaininng desirable mechanical properties in L-PBF AMed AlCrFe2Ni2 MEA.

Fe40Cr25Ni25Al5Ti5 (atom fraction/%) medium entropy alloys (MEAs) were prepared by a vacuum arc-melting furnace with a water-cooled copper mold, and the microstructure, mechanical properties and strengthening deformation mechanism of the alloy in solid solution and annealed states were studied by scanning electron microscopy (SEM), energy spectrometry(EDS), X-ray diffractometer (XRD), transmission electron microscopy(TEM) and tensile testing machine.The results show that the Fe40Cr25Ni25Al5Ti5 medium entropy alloy has FCC+BCC1+BCC2 triple-phase organization of solid solution state, with yield strength of 520 MPa, fracture strength of 852 MPa and elongation of 13%. After annealing at 600 ℃ for 2 h, the phase composition of the alloy has no change, the size of granular BCC2 phase increases, and the volume fraction of FCC zone and BCC zone has no change significantly. The yield strength is 668 MPa, the fracture strength is 1029 MPa, and the elongation is reduced to 9%. The strength of Fe40Cr25Ni25Al5Ti5 alloy originates from the synergistic effect of coherent strengthening, solid solution strengthening and phase boundary strengthening. Dislocation slips are the main deformation mechanism of alloy.

In order to explore the mechanism of mechanical properties of TiNbTaZr after non‐Ti elements are replaced by Hf from the perspective of electronic structure. Analyzing the structural characteristics of TiNbTaZr, TiHfNbZr, TiNbHfZr, and TiNbTaHf medium entropy alloys by solid solution characteristic parameters. Establishing the corresponding structural models and dealing with the first‐principles calculations. By analyzing the binding energy and formation enthalpy, it is determined that these four alloys are metastable β‐Ti alloys of disordered solid solution, and the thermodynamic stability of the structure is confirmed by phonon spectrum. The elastic constants, Poisson's ratio, and hardness of these four alloys show that the strength of Hf‐containing alloys is improved and the ductility is slightly reduced compared with TiNbTaZr. From the analysis of population and density of states, it is concluded that the reason for the above phenomenon is that Hf will cause the improvement of bonding stability and covalency of alloy materials, thus promoting the improvement of theoretical strength of alloys. TiNbHfZr exhibits excellent properties of high strength and low elastic modulus and has good ductility. [ABSTRACT FROM AUTHOR]

Titanium matrix composites (TMCs) reinforced with 2–8.3 wt.% NiCoCr medium-entropy alloy particles (NiCoCrp/Ti) were synthesized through an in-situ reaction between NiCoCr and Ti powders during a spark plasma sintering process at 850 °C and 1000 °C. At 850 °C, there are obvious interdiffusion layers with Ti3(Ni, Co) intermetallic around NiCoCr particles. At 1000 °C, the sufficient diffusion of Ni, Co and Cr away from the prior NiCoCr particles results in clusters of granular intermetallic (Ti3(Ni, Co) and Ti2(Ni, Co)) in the Ti matrix. Besides, the addition of NiCoCr particles leads to a significant improvement in tensile strength and hardness. Ti-2wt.%NiCoCr sintered at 1000 °C has a combination of high tensile strength (788 MPa) and good ductility (23.4%). Furthermore, the mechanical properties are mainly correlated to the amount and distribution of the intermetallic compounds in TMCs.

As-cast CrCoNi Medium Entropy Alloy (as-casted MEA) was obtained by vacuum arc melting. The as-casted MEA was rolled 3 times at the temperature of liquid nitrogen (77 K), the sum of the total deformation was 50%. Then, the rolled CrCoNi Medium Entropy Alloy (as-rolled MEA) were annealed with different parameters, the annealing parameters were 923 K/30min, 973 K/30min and 1073 K/30min respectively. The experimental samples were characterized by uniaxial tensile test at room temperature, electron back scatter diffraction (EBSD) and transmission electron microscopy (TEM). The tensile test results showed the CrCoNi MEA was deformed by Liquid Nitrogen rolling, the tensile strength increased from 421 MPa to 1312 MPa, but the elongation was only 7.4%. The as-rolled MEA annealed at moderate temperature and short time, the ductility of the CrCoNi was improved significantly. When the as-rolled MEA annealed at 1073 K/30 min, the elongation of CrCoNi increased to 65.3%, while the tensile strength maintained 800 MPa. The EBSD and TEM results showed that the CrCoNi MEA was deformed by Liquid Nitrogen rolling, there were a lot of high density dislocation regions in the alloy. At the same time, a large number of bunches of stacking faults (SFs), intersected SFs and deformation twins (DT) were found in the rolled MEA. The EBSD and TEM results also showed that when the CrCoNi MEA was annealed, a large number of type B and type C annealing twins and 9 R structures were found in the alloy. The growth of annealing twins were achieved by the mechanism of grain boundary migration caused by stacking faults annexation. After annealing, the transformation of FCC→9 R structure was occurred, and the 9 R structure caused more boundary nucleation of annealing twins. The annealing twins and 9 R structure played a key role in improving the ductility of the annealed CrCoNi MEA. Adopt the technology of moderate temperature combining short time annealing process, can effectively control the properties of CrCoNi MEA.

Medium entropy alloys (MEAs) are new emerging engineering alloys comprising four principal elements characterised by a low enthalpy of mixing and entropies of formation between 1 and 1.5 molar gas constant. They have high strength, wear, and thermal properties. MEAs have generated interest as an alternative material in the last two decades in nuclear, aerospace, and high strength engineering applications. In this research a medium entropy alloy Fe20Cr20Mn20Ni20Ti10Co5V5 with principal elements Fe, Cr, Mn, and Ni was fabricated using spark plasma sintering. Elemental powder mixture was sintered at temperatures of 870oC, 900oC and 950oC under 35 bar pressure under an inert argon atmosphere for 45 minutes using an FCT Systeme GmbH spark plasma sintering machine. After sand blasting the densities of the samples were measured before grinding, polishing, and etching for characterisation. Microstructure analysis was carried using scanning electron and optical microscopy. Microhardness was measured using Falcon 507 hardness tester, modulus using by Anton-Paar Nanoindenter and wear using Anton-Paar TRB3 Tribometer. The elements form a solid solution with presence of a hard μ phase and soft γ-phase were observed. The hardness of the alloys sintered at 870oC and 900oC were 397 and 424 Vickers respectively. The alloy sintered at 950oC showed a hardness of 674 Vickers. After annealing the hardness increased to 736 Vickers. The modulus of elasticity and creep resistance increased after heat treatment at 700oC.The other alloys showed a decrease in hardness and other properties after annealing. Unlike in steels, where annealing reduces, in this alloy annealing increased hardness at an appropriate temperature. Fe20Cr20Mn20Ni20Ti10Co5V5 MEA exhibited good thermal stability. Further work on the alloy will involve its crystallography, and feasibility for use elevated temperature energy applications such as fuel cells, turbines and wear resistant machine components.

To avoid brittle intermetallic compounds during direct connection of the Ti-6Al-4V alloy and 316L stainless steel, the FeCrCuV medium entropy alloy (MEA) is designed as the transition layer, and the 316L/FeCrCuV/Ti-6Al-4V dissimilar materials are fabricated by laser metal deposition (LMD). The heterogeneous interfaces of 316L/FeCrCuV and FeCrCuV/Ti-6Al-4V are investigated by a scanning electron microscope (SEM), an energy dispersive spectroscope, and electron backscatter diffraction. The results indicate that common brittle intermetallic compounds TiFe and TiFe2 at Ti-6Al-4V/316L heterogeneous interfaces disappear, and the BCC/FCC dual-phase solid solution structure is obtained due to the solid solution effect of the FeCrCuV transition layer. Refined grains appear at heterogeneous interfaces of 316L/FeCrCuV and FeCrCuV/Ti-6Al-4V for the rapid cooling rate during LMD, which results in fine grain strengthening. The microhardness near heterogeneous interfaces increases the solution strength and fine grain strengthening. Furthermore, the design of the FeCrCuV transition layer with a dual-phase structure improves the coordinated deformation ability of 316L/FeCrCuV/Ti-6Al-4V and results in higher tensile strength. [ABSTRACT FROM AUTHOR]

Due to the presence of multi-principal elements, the multi-principal element alloys (MPEAs) are supposed to undergo competing oxidation reactions during micro-arc oxidation (MAO). In this work, Ti–Nb–Zr medium entropy alloy (MEA, MAO-Free) was MAO-treated at various applied voltage. For Ti–Nb–Zr MEA treated at 250 V (MAO-250 V), the coating microstructure was characterized by amorphous oxides of Ti, Nb and Zr from the substrate and amorphous compounds of CaO, Ca(OH) 2 , SiO 2 and CaSiO 3 from the electrolyte, which distributed homogeneously in coating matrix. Different from the microstructure of MAO-250 V group, tetragonal-ZrO 2 nano-crystals dispersed in amorphous coating matrix were noticed for that treated at 300 V (MAO-300 V). For specimen treated at 350 V (MAO-350 V), the coating matrix exhibited higher crystallinity by composing of major tetragonal-ZrO 2 and minor rutile-TiO 2 , with the presence of nano-sized amorphous regions. The pore size, porosity, surface roughness and hydrophilicity of coatings increased with the increase of applied voltage. An order of MAO-300 V > MAO-250 V > MAO-Free > MAO-350 V group was identified for the wear resistance. Moreover, the MAOed coatings exhibited comparable corrosion resistance and cyto-compatibility to MAO-Free group. Together, the results indicate that the MAO-300 V group with favorable wear resistance, corrosion resistance and cyto-compatibility is promising for biomedical applications. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Materials Engineering / Cailiao Gongcheng is the property of Journal of Materials Engineering Editorial Office and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The present study investigated the cryogenic mechanical properties and microstructure of CrCoNi medium entropy alloy (MEA) fabricated by selective laser melting (SLM) with hot isostatic pressing (HIP). The SLM processing parameters were optimized with respect to the relative density and micro-hardness of as-built CrCoNi specimens. The cryogenic (150 K) deformation mechanisms (compared to room (298 K) temperature) of SLM CrCoNi MEA were characterized by uniaxial tensile test, electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). At 150 K, it reveals superior mechanical properties with the noticeable increase in both strength and ductility by 6.5% and 10.0% (as-built), 22.7% and 6.6% (HIP-treated), respectively. It is found that the high cooling rate of 3.2 × 106 K/s obtained from the simulation results in the fine microstructure and high dislocation density, accordingly the high yield strength of SLM CrCoNi MEA. The steady work hardening, which postpones the onset of necking instability, is attributed to the pronounced dislocation activity and deformation twinning. In addition, a new phase with hexagonal closed-packed (HCP) substructure is apparently observed. Thus, a synergistic effect of dislocations, deformation twinning and HCP substructures contributes to the simultaneous enhancement of strength and ductility. These findings reveal that the CrCoNi MEA fabricated by SLM exhibits an excellent strength and ductility at cryogenic temperature. Furthermore, the SLM CrCoNi MEA with HIP shows significant improvement in the ductility with exceptional strength at 150 K. It demonstrates that a combination of SLM and HIP promotes the CrCoNi MEA to be promising for the cryogenic applications.

The present work investigates the corrosion properties and passivation layer of VCrFeCoNi medium-entropy alloy (V-MEA). The V-MEA exhibits the corrosion current density of ∼0.101 μA and corrosion potential of ∼ -0.336 V with a broad passivation region of ∼0.763 V from the potentiodynamic polarization measurement. The V-MEA shows the multilayer passivation with vanadium oxides-rich outer layer and chromium oxide-rich inner layer. The outer layer as a cation-selective layer defeats Cl− ions and the inner layer as an anion-selective species captures the Cl− ions. Resultantly, fewer Cl− ions can go through the passivation layer, leading to the broad passivation region of V-MEA.

We investigated the effects of interstitial N and C on the stacking fault energy (SFE) of an equiatomic CoCrNi medium entropy alloy. Results of computer modeling were compared to tensile deformation and electron microscopy data. Both N and C in solid solution increase the SFE of the face-centered cubic (FCC) alloy matrix at room temperature, with the former having a more significant effect by 240% for 0.5 at % N. Total energy calculations based on density functional theory (DFT) as well as thermodynamic modeling of the Gibbs free energy with the CALPHAD (CALculation of PHAse Diagrams) method reveal a stabilizing effect of N and C interstitials on the FCC lattice with respect to the hexagonal close-packed (HCP) CoCrNi-X (X: N, C) lattice. Scanning transmission electron microscopy (STEM) measurements of the width of dissociated ½ dislocations suggest that the SFE of CoCrNi increases from 22 to 42–44 mJ·m−2 after doping the alloy with 0.5 at. % interstitial N. The higher SFE reduces the nucleation rates of twins, leading to an increase in the critical stress required to trigger deformation twinning, an effect which can be used to design load-dependent strain hardening response.

The recycled copper alloy powders prepared by physical method were further alloyed, and therefore four medium entropy alloys (MEAs), (Fe40Ni40Mn20)50Cu50, (Fe38Ni38Mn19Al5)50Cu50, (Fe36Ni36Mn18Al10)50Cu50 and (Fe32Ni32Mn16Al20)50Cu50were successfully prepared via mechanical alloying (MA) and spark plasma sintering (SPS). The influence of Al content on the microstructure and mechanical properties of the MEAs were systematically studied. Following 60 h of MA, the mechanical alloyed powders of the four MEAs consist of a primary supersaturated FCC solid solution along with a trace amount of WC contaminants. Following SPS, the (Fe40Ni40Mn20)50Cu50, (Fe38Ni38Mn19Al5)50Cu50 and (Fe36Ni36Mn18Al10)50Cu50 show a dual-phase structure consisting of a Cu-rich phase (FCC1) and a Fe-Ni-rich phase (FCC2), displaying a multiscale grain structure of ultrafine grains and micron grains. However, the (Fe32Ni32Mn16Al20)50Cu50 alloy shows a primary Cu-rich phase (FCC1) with a small amount of Fe-Mn rich phase (FCC2) and Ni-Al rich B2 phase. The plasticity of the four MEAs is gradually decreased, while the strength and hardness are gradually increased with the increase of Al content. The compressive yield strength, compressive strength and Vickers hardness of (Fe40Ni40Mn20)50Cu50 MEAs are 878 MPa, 1257 MPa and 248.5HV, respectively. Compared with (Fe40Ni40Mn20)50Cu50, the compressive yield strength and hardness of (Fe32Ni32Mn16Al20)50Cu50 are increased by 50.1% and 50.4%, respectively, whereas the fracture strain is decreased from 19.55% to 8.31%.

The hydrogen embrittlement micromechanisms and the effect of carbon interstitial on hydrogen distribution were characterized in Fe40Mn40Ni10Cr10 and Fe38Mn41Ni10Cr10C1 medium entropy alloy. Ex-situ microstructural observations revealed that the segregation of carbon on grain boundaries suppresses hydrogen from being trapped in the grain boundaries for carbon-doped alloy before deformation. However, the distribution of hydrogen was similar for both alloys after plastic strain so that the grain boundaries trapped a large fraction of hydrogen during deformation. The fully intergranular fracture mode in the hydrogen affected area of both alloys was explained by the synergy of grain boundary–dislocation reactions and hydrogen-enhanced grain boundary decohesion effects.

The FCC duel-phase Fe-based medium entropy alloys were fabricated by the water-cooled copper crucible magnetic levitation melting and negative pressure copper mold suction casting. The effects of Ag addition on the microstructure and corrosion resistance in different mediums were investigated. The results show that the binary mixing enthalpy of the main group elements in the alloy with the added Ag elements is positive, thus the FCC2 phase will be enriched in the dendrites due to phase separation. With the addition of Ag, the corrosion potential of the alloys moves forward, the corrosion current density decreases, the radius of the capacitive arc increases, and the corrosion resistance of the alloy is significantly improved. Among them, the corrosion current density of alloy Fe60.8Mn13.4Si8.7Cr9.4C3.7Ag4 in PBS solution can reach 2.139×10-8A/cm-2 with the best corrosion resistance. [ABSTRACT FROM AUTHOR]

FeCrNi medium entropy alloy (MEA) has been widely regarded for its excellent mechanical properties and corrosion resistance. However, insufficient strength limits its industrial application. Intermetallic particle dispersion strengthening is considered to be an effective method to improve strength, which is expected to solve this problem. In this work, microstructural evolution and mechanical behavior of FeCrNi MEA with different Si content were investigated. We found that the precipitation of fine σ particles can be formed in situ by thermomechanical treatment of Si doping FeCrNi MEAs. The FeCrNiSi0.15 MEA exhibits a good combination of strength and ductility, with yield strength and tensile elongation of 1050 MPa and 7.84%, respectively. The yield strength is almost five times that of the as-cast FeCrNi MEA. The strength enhancement is mainly attributed to the grain-boundary strengthening and precipitation strengthening caused by fine σ particles. [ABSTRACT FROM AUTHOR]

In our study, molecular dynamics (MD) simulations of laser powder bed fusion (LPBF) have been conducted on equimolar FeNiCr medium entropy alloy (MEA) powders. With the development of newer LPBF technologies capable of printing at the microscale, an even deeper understanding of the underlying atomistic effects of the process parameters on the microstructural and mechanical properties of the manufactured FeNiCr MEA products is required. In accordance with previous literature, the parameters of the LPBF process have been systematically varied, including layer resolution from 1 to 6, laser power from 100 µW to 220 µW, bed temperature from 300 to 600 K, and laser scan speed from 0.5 Å/ps to 0.0625 Å/ps. Consistent with prior macroscopic experimental findings, the atomistic results suggest that additive manufacturing using thinner layers imparts higher ultimate tensile strength (UTS) than fabricating with thicker layers. The latter, however, requires a shorter process time but induces keyhole defect formation if the laser-induced temperature is not sufficiently high enough. Increasing the temperature proves useful in mitigating this problem. Enhancement of UTS for the multi-rowed powders has been observed by raising the substrate temperature to 600 K, slowing down the laser or by raising the power to 160 µW during production. Beyond certain critical power, however, the UTS of the product diminishes due to the emergence of multiple vacancies. These results will help researchers to find a good balance between the production speed and strength of additive manufactured products at the nanoscale. [ABSTRACT FROM AUTHOR]

The Laser Powder Bed Fusion (LPBF) in-situ alloying is a novel technology for preparing novel alloys efficiently. However, the narrow process window limit its application. A micro-scale integrated simulation model is designed and constructed based on the Discrete Element Method (DEM), the Finite Element Method (FEM), and Calculation of phase diagram (CALPHAD) method in the work. The equation of the methods are introduced. By defining the parameters such as composition inhomogeneity ω, the forming performance of LPBF in-situ alloying FeCoCrNi medium entropy alloys are predicted. The calculation results show that the hysteresis diffusion effect makes the mixing powders more challenging to be in-situ alloyed than conventional main component alloys. For the mixed powder beds consisting of 15–53 μm particles with equal mass ratio, the composition deviation on the micro-scale is within 2.5%, and the composition homogeneity reaches 98.9 %. Under a laser power of 150 W and a scanning speed of 500 mm/s, several laser remelting process are indispensable to eliminate unmelted elements. Experiments are carried out to verify the accuracy of the integrated simulation model, and the high-efficiency preparation experiment of non-equiatomic ratio medium entropy alloy was carried out by using the technology.

The effect of grain size variation (11 μm, 34 μm) on the hydrogen-induced tensile properties degradation of a Co-free cost-effective Fe 40 Mn 40 Ni 10 Cr 10 austenitic medium entropy alloy was investigated using a slow strain rate test. Despite improving both strength and ductility with decreasing the grain size in non-charged conditions, the fine-grain alloy showed a higher relative elongation loss after electro-chemical hydrogen charging. The larger ductility loss in the fine-grain alloy was ascribed to the fast propagation rate of major intergranular cracks and the drastic strain hardening rate drop of the alloy under hydrogen charging. The Schmid factor analysis showed that the enhanced dislocation activity in the fine-grain alloy compared with coarse-grain one was responsible for rapid hydrogen transfer to the grain boundaries, fast dislocation pile-up behind the grain boundaries, and, consequently, more severe hydrogen embrittlement. The significant stress concentration near the grain boundaries and fast intergranular crack propagation were recognized to be the main reason for premature fracture in fine-grain alloy. • The elongation loss under hydrogen charging was higher in fine-grained Fe 40 Mn 40 Ni 10 Cr 10 alloy than the coarse-grained one. • The propagation route of hydrogen-induced cracks was different in fine-grained and coarse-grained specimens. • More drastic elongation loss in the fine-grained alloy was related to developing major cracks and dislocations activity. [ABSTRACT FROM AUTHOR]