Search Results (70)

A (NiCoCr)_99.25C_0.75 medium entropy alloy (MEA) was developed via laser powder bed fusion (LPBF) using pre-alloyed powder feedstock containing 0.75 at%C, followed by a precipitation heat treatment. The as-built alloy exhibited high density (>99.9%), columnar grains, fine substructures, and strong <111> texture. Heat treatment at 700 °C for 1 h promoted the precipitation of Cr-rich carbides (Cr₂₃C₆) along grain and substructure boundaries, which stabilized the microstructure through Zener pinning and the consumption of carbon from the matrix. The heat-treated alloy achieved excellent cryogenic tensile properties at 77 K, with a yield strength of 1230 MPa and an ultimate tensile strength of 1.6 GPa. Compared to previously reported LPBF-built NiCoCr-based MEAs, this alloy exhibited superior strength at both room and cryogenic temperatures, indicating its potential for structural applications in extreme environments. Deformation mechanisms at cryogenic temperature revealed abundant deformation twinning, stacking faults, and strong dislocation–precipitate interactions. These features contributed to dislocation locking, resulting in a work hardening rate higher than that observed at room temperature. This study demonstrates that carbon addition and heat treatment can effectively tune the stacking fault energy and stabilize substructures, leading to enhanced cryogenic mechanical performance of LPBF-built NiCoCr MEAs. Full article

In this work, the W element, with a larger atomic radius compared to Co, Fe, and Ni, was added to modify the microstructure and enhance the yield strength of CoFeNi medium entropy alloy (MEA). A detailed study was conducted to clarify the effects of W additions and annealing temperatures on the microstructure evolution and mechanical properties of CoFeNiW_x (x = 0, 0.1, and 0.3) MEAs. CoFeNiW_0.1 retained a single FCC structure without the formation of precipitates in the FCC phase, indicating that W, with a larger atomic radius, can completely dissolve in CoFeNiW_0.1. For CoFeNiW_0.3 MEA, coarse particles with an average diameter of ~2 μm appeared after homogenizing. Nevertheless, when the alloy was annealed at 800 °C and 900 °C, fine particles formed, with the average diameters of approximately 144 nm and 225 nm, respectively. After annealing at 800 °C, the CoFeNiW_0.3 with a partially recrystallized microstructure exhibited better comprehensive mechanical properties. Full article

Medium-entropy alloys (MEAs) have attracted significant attention due to their unique structure–property relationships. In this study, we examine the effects of lattice distortion on the mechanical properties of Nb-Ti-V-Zr MEAs, focusing on two alloy series: Nb(Ti_1.5V)_xZr and Nb(TiV)_xZr (x = 1, 2, 3, 4 and 5). Experimental results show that the Nb(TiV)_xZr r alloys exhibit greater atomic size mismatches and increased lattice distortion compared to the Nb(Ti_1.5V)_xZr alloys, leading to higher yield strengths via enhanced solid-solution strengthening. However, excessive lattice distortion does not ensure an optimal strength–ductility balance, as the alloys with the highest distortion demonstrate limited plasticity. Thus, moderate reduction in lattice distortion proves beneficial in achieving an excellent compromise between strength and ductility. These findings offer valuable guidance for leveraging lattice distortion in the design of high-strength, high-ductility, body-centered cubic (BCC) MEAs for extreme environments. Full article

In this study, 20 wt.% of Nb was incorporated into a CrFeNi medium-entropy alloy (MEA) powder system to prepare CrFeNi-Nb composite coatings on a Q235B mild steel substrate by laser cladding technology. The effects of Nb addition on the phase composition, microstructure, and wear resistance of CrFeNi coatings were systematically investigated. Microstructural characterization revealed that the CrFeNi coating exhibited a single face-centered cubic (FCC) phase structure, while the CrFeNi-Nb composite coating demonstrated a dual-phase structure comprising FCC phase and Laves phase. The Laves phase significantly enhanced the microhardness and wear resistance of the coating. The average microhardness of the CrFeNi-Nb coating increased by 259.62% compared to the substrate and 190.58% compared to the Nb-free CrFeNi coating. The average coefficient of friction (COF) of the coating decreased from 0.74 to 0.68; the wear rate reduced from 5.77 × 10⁻⁴ mm³ N⁻¹ m⁻¹ to 2.26 × 10⁻⁴ mm³ N⁻¹ m⁻¹; and the weight loss decreased from 10.77 mg to 4.3 mg. The experimental results demonstrated that the addition of Nb promoted the formation of the Laves phase in the CrFeNi MEA, which effectively improved the wear resistance of the coating. Full article

This study investigates the interfacial reactions of FeCoNiCr and FeCoNiMn medium-entropy alloys (MEAs) with Sn and Sn-3Ag-0.5Cu (SAC305) solders at 250 °C. The evolution of interfacial microstructures is analyzed over various aging periods. For comparison, the FeCoNiCrMn high-entropy alloy (HEA) is also examined. In the Sn/FeCoNiCr system, a faceted (Fe,Cr,Co)Sn₂ layer initially forms at the interface. Upon aging, the significant spalling of large (Fe,Cr,Co)Sn₂ particulates into the solder matrix occurs. Additionally, an extremely large, plate-like (Co,Ni)Sn₄ phase forms at a later stage. In contrast, the Sn/FeCoNiMn reaction produces a finer-grained (Fe,Co,Mn)Sn₂ phase dispersed in the solder, accompanied by the formation of the large (Co,Ni)Sn₄ phase. This observation suggests that Mn promotes the formation of finer intermetallic compounds (IMCs), while Cr facilitates the spalling of larger IMC particulates. The Sn/FeCoNiCrMn system exhibits stable interfacial behavior, with the (Fe,Cr,Co)Sn₂ layer showing no significant changes over time. The interfacial behavior and microstructure are primarily governed by the dissolution of the constituent elements and composition ratio of the HEAs, as well as their interactions with Sn. Similar trends are observed in the SAC305 solder reactions, where a larger amount of fine (Fe,Co,Cu)Sn₂ particles spall from the interface. This behavior is likely attributed to Cu doping, which enhances nucleation and stabilizes the IMC phases, promoting the formation of finer particles. The wettability of SAC305 solder on MEA/HEA substrates was further evaluated by contact angle measurements. These findings suggest that the presence of Mn in the substrate enhances the wettability of the solder. Full article

Because of their low density and excellent material properties, lightweight Ti-rich medium-entropy alloys (MEAs) have great potential for application in the aerospace and automotive industries. This study investigated the effects of B doping on the microstructure and mechanical properties of a (Ti₆₅(AlCrNbV)₃₅)_100−xB_x alloy series. The mechanical properties of the alloys were then enhanced through thermomechanical treatment, and the strengthening mechanism was explored by characterizing the alloys’ microstructure and mechanical properties. X-ray diffraction revealed that the (Ti₆₅(AlCrNbV)₃₅)_100−xB_x alloys retained their body-centered cubic structure. However, the addition of B resulted in a rightward shift in the diffraction peaks due to B having a smaller atomic radius compared with the other constituent elements. Weak diffraction peaks corresponding to TiB were discovered in the diffraction patterns for the alloys with 0.4 or 0.6% B content (named B0.4 and B0.6, respectively). The hardness of the homogenized alloys was increased from 321 Hv for the base alloy (B0) to 378 Hv for B0.6. In tensile testing, the homogenized alloy with 0.2% B content (B0.2) exhibited a yield strength of 1054 MPa and 21% elongation, which represented 17% greater strength compared with B0. Conversely, the mechanical properties of B0.4 and B0.6 were poorer due to precipitation at grain boundaries. After thermomechanical treatment, the alloys’ strength and hardness increased with increasing B content despite various heat treatment conditions. The recrystallization behavior of the alloys tended to be delayed by B doping, resulting in an increase in the recrystallization temperature. After recrystallization at 900 °C, the elongation of B0, B0.1, and B0.2 exceeded 20%. Of the (Ti₆₅(AlCrNbV)₃₅)_100−xB_x alloys in the series, B0.2 presents the optimal combination of favorable yield strength and ductility (1275 MPa and 10%, respectively). Full article

Aerospace and marine engineering impose higher requirements on mechanical properties and lightweight design of materials. In this work, combining the high mechanical properties of FeCrNi medium entropy alloy (MEA) and the lightweight advantages of lattice structure, four types of high-performance FeCrNi MEA lattice structures (BCC, BCCZ, FCC, and FCCZ) were prepared by selective laser melting (SLM) technology, and their dynamic mechanical properties were systematically characterized via split Hopkinson pressure bar (SHPB) method. The results demonstrate that the FCCZ FeCrNi MEA lattice structure exhibits superior comprehensive performance among the four lattice structures, achieving the highest specific compressive strength of 59.1 MPa·g⁻¹·cm⁻³ and specific energy absorption of 26.3 J/g, significantly outperforming conventional lattice materials including 316L and AlSi10Mg alloys. Furthermore, the finite element simulation and Johnson-Cook (J-C) constitutive model of the dynamic compression process can effectively predict the microstructural evolution and mechanical response of lattice structure, providing critical theoretical guidance for optimizing the design of high-performance lattice structure materials. Full article

Medium-entropy alloys (MEAs) exhibit properties comparable or even superior to high-entropy alloys (HEAs). Due to their very good resistance in thermomechanical conditions and corrosive environments and unique electrical and magnetic properties, medium-entropy alloys are good candidates for coating applications. One of the most economically effective methods of producing metallic coatings is electrodeposition. In this work, the structure of an electrodeposited CoFeNi medium-entropy alloy coating on a copper substrate from a metal chlorides solution (FeCl₂ ∙ 4H₂O + CoCl₂ ∙ 6H₂O + NiCl₂ ∙ 6H₂O) with the addition of boric acid (H₃BO₃) was investigated. The coating was characterized by a nanocrystalline structure identified by transmission electron microscopy examination and X-ray diffraction methods. Based on XRD and TEM, the face-centered cubic (FCC) phase of the CoFeNi MEA coating was identified. The high corrosion resistance of the MEA coating in a 3.5% NaCl environment at 25 °C was confirmed by electrochemical tests. Full article

Medium-entropy alloys (MEAs) allow the formation of different phases, generally in a solid-solution state, and compounds that favor obtaining alloys with properties superior to those of conventional alloys. In this study, medium-entropy CuNiSiCrCoTiNbx alloys were fabricated via melting in a vacuum induction furnace. The influence of the Nb addition (X = 0, 0.5 and 1 wt%) alloying elements on the microstructure, hardness, and wear resistance of the CuNiSiCrCoTiNb₀ (M1), CuNiSiCrCoTiNb_0.5 (M2), and CuNiCoCrSiTiNb₁ (M3) alloys were explored using X-ray diffraction (XRD), scanning electron microscopy (SEM), and a ball-on-disc tribometer, respectively. In general, the results indicated that the incorporation of Nb alloying element promoted the evolution of the microstructure, increased the hardness, and improvement of the wear resistance. The XRD and SEM findings demonstrate that higher Nb addition and aging heat treatment (AT) modification mainly favored the formation of dendritic regions and the precipitation of the Co₂Nb, Cr₃Si, and Ni₂Si phases, which promoted the refinement and strengthening of the microstructure. Significant increases in hardness were recorded: 11.95% increased, promoted by the addition of Nb before (E1) and after (E2, E3, and E4) the heat treatments. The maximum hardness values recorded were 92 ± 0.11 (AC) and 103 ± 0.5 HRB (AT-60 min) for the M3 alloy. The increase in hardness caused by Nb addition and aging heat treatments contributed to the dry sliding wear resistance response, decreasing material loss by 20%. This was related to the high concentration of precipitated phases rich in CoNb, CrSi, and NiSi with high hardness. Finally, the M3 alloy aged for 60 min exhibited the best specific wear rate behavior, with a material loss of 1.29 ${\mathrm{m}\mathrm{m}}^3$. The commercial Cu-Be C17510 alloy experienced a maximum hardness of 83.47 Hardness Rockwell B, HRB, and a high wear rate of 3.34 mm³. Full article

CoCrNi medium-entropy alloy (MEA) coatings prepared using laser cladding (LC) with unique properties have aroused great interest in recent years and have been widely studied. However, limited studies have been conducted on the effect of adding Al/Fe on the wear properties of CoCrNi MEA coatings prepared on the surface of stainless steel. In this study, AlCoCrFeNi, CoCrFeNi, and CoCrNi MEA LC coatings were prepared on a stainless steel substrate. The grain structures and microscopic morphologies of coatings were characterized, and the wear mechanisms were analyzed using the nano-indentation and wear tests. The hardness-strengthening mechanism was theoretically investigated using phase diagrams and molecular dynamics (MD). The phase diagram results show that the addition of Al lowered the nucleation initiation temperature, thereby increasing the nucleation rate and forming more grains. Moreover, according to the Voronoi volumes and mean–square atomic displacements (MASD) results using MD, the addition of Al makes the appearance of severe localized lattice distortions, while the addition of Fe tends to form short-range ordered structures. In summary, fine-grain strengthening and the hardness strengthening caused by local lattice distortion were the main strengthening mechanisms of AlCoCrFeNi. These findings are highly significant for expanding the application potential and provide profound insights into the wear properties of the CoCrNi MEA coatings. Full article

Alloying provides an effective approach to designing metallic materials with unique microstructures and enhanced performance. In this work, we developed a series of (CoCrNi)_100−xZr_x (where x = 1, 2, 3, 4, and 5) medium entropy alloys (MEAs) by vacuum arc-melting method. The effects of Zr addition on the microstructures and mechanical properties of (CoCrNi)_100−xZr_x MEAs were systematically investigated. Due to the negative mixing enthalpy of Zr with Co, Cr, and Ni, lamellar C15 Laves-phase precipitates formed within the ductile FCC matrix. As the Zr content increases, the alloys exhibit higher strength but become more brittle at room temperature. Among the (CoCrNi)_100−xZr_x MEAs series, the CoCrNiZr₃ MEA shows an excellent balance between strength and ductility, achieving a compressive yield strength of 610 MPa and a hardness of 249 HV, respectively, while maintaining a good ductility beyond 45%. Microstructural analysis using scanning electron microscope and transmission electron microscope suggests that this outstanding strength-ductility balance of CoCrNiZr₃ MEA arises from the synergistic effect of precipitation strengthening and solid solution strengthening. These findings not only provide deeper insight into the interaction between different strengthening mechanisms but also offer valuable guidance for designing high-performance multi-component alloys through strategic alloying. Full article

Refractory high-entropy or medium-entropy alloys (RHEAs, RMEAs) exhibit outstanding strength and hold significant promise for high-temperature applications. However, their pronounced brittleness at room temperature restricts their industrial application. Recently, the introduction of interstitial oxygen has proven effective in refining the microstructure and improving the mechanical properties of RMEAs. In this study, we investigated the effect of interstitial oxygen content ranging from 0.5 to 6 at.% on the microstructures and mechanical properties of TiZrNb MEA. The alloys display a single BCC structure, showing a dendritic crystal morphology. At an oxygen content of 4 at.%, the alloy shows a room-temperature compressive yield strength of 1300 MPa and compressive strain of over 50%, achieving a balanced strength and ductility combination. Moreover, it shows excellent high-temperature mechanical properties, with yield strength exceeding 500 MPa at 800 °C. The Toda-Caraballo and Labusch theoretical models were used in the study to clarify the strengthening mechanism of the alloys, and the theoretical yield strengths obtained by calculation coincided with the experimental yield strengths. This validation not only confirms that the primary strengthening mechanism is solid solution strengthening, but also proves the reliability of the model in predicting the mechanical properties of MEAs and provides a theoretical basis for the use of interstitial atoms to strengthen MEAs. Full article

The corrosion characteristics and passive behavior of as-cast Ni₄₀Fe₃₀Co₂₀Al₁₀ medium-entropy alloy (MEA) fabricated by the vacuum arc melting technique were investigated in 3.5 wt.% NaCl, 0.5 M HCl, and 0.5 M H₂SO₄ solutions. Although the impact of different solutions on the corrosion current density was not pronounced, the corrosion potential values of MEAs in H₂SO₄, HCl and NaCl solutions were −0.37, −0.58 and −1.16 V, respectively, indicating that the resistance to general corrosion in acidic solutions becomes strengthened. Through electrochemical passive region tests, surface morphology analysis and ICP testing, it was found that, due to the high-entropy effect and uniform single-phase structure, an optimized and stable passive film formed specifically in the Cl⁻-containing solution. The ion concentrations in the passive region of MEA in NaCl solution were an order of magnitude lower than those of other two samples, suggesting that its passive film formed exhibits a more prominent capacity to inhibit metal dissolution. Compared with electrochemical reactions in H₂SO₄ and HCl solutions, MEA shows enhanced pitting resistance in NaCl solution, which could be attributed to the presence of abundant unoxidized metal atoms (51.9 at.%). Al is identified as the primary component in the formation of the passive film, which plays a protective role for the Co-rich interior of the MEA. Although MEA has a relatively high passivation current in the H₂SO₄ solution, it has the widest passivation zone (1.87 V), indicating the optimized stability of the formed passive film. Moreover, it displays a high level of resistance to pitting corrosion in the solution containing only H⁺- and free of Cl⁻. Both the MEAs show significant grain-boundary corrosion in H₂SO₄ and HCl solutions. Among them, the MEA in HCl experiences more severe intragranular corrosion. Notably, MEA withstands the erosion of a single Cl⁻- or H⁺-containing solution, but it is unable to resist the synergistic effect of a solution containing both H⁺ and Cl⁻. Full article

Materials utilized in extreme environments, such as those necessitating protection and impact resistance at cryogenic temperatures, must exhibit high strength, ductility, and structural stability. However, most alloys fail to maintain adequate toughness at cryogenic temperatures, thereby compromising their safety during cryogenic temperature service. This study investigates the quasi-static mechanical properties of a CoCr₀.₄NiSi₀.₃ medium-entropy alloy (MEA) at room temperature, −75 °C, and −150 °C. The deformation behavior and mechanisms responsible for strengthening and toughening at reduced cryogenic temperatures are analyzed, revealing that decreasing cryogenic temperature enhances the strength of the as-cast MEA. Specifically, both the yield strength (YS) and ultimate tensile strength (UTS) of the MEA increase significantly with decreasing temperature during cryogenic tensile testing. Under tensile testing at −150 °C, the YS reaches 617.5 MPa, the UTS is 1055.0 MPa, and the elongation to fracture remains approximately 21.0% at both −150 °C and −75 °C. After cryogenic temperature tensile deformation, the matrix exhibits a dispersed distribution of nanoscaled tetragonal and orthorhombic phases, a coherent hexagonal close-packed phase, L1₂ phase and layered long-period stacking ordered (LPSO) structures, which are rarely observed in the cryogenic deformation of metals and alloys. The metastable phase evolution path of this MEA at cryogenic temperatures is closely associated with the decomposition of perfect dislocations into a/6<112> Shockley partial dislocations and their subsequent evolution at reduced cryogenic temperatures. At −75 °C, the a/6<112> Shockley partial dislocation interacts with the L1₂ phase to form antiphase boundaries (APBs) approximately 3 nm thick. At −150 °C, two phase transition paths from stacking faults (SFs) to nanotwins and LPSO occur, leading to the formation of layered LPSO structures and deformation-induced nanotwins. The dispersion of these coherent nanophases and nanotwins induced by the reduced stacking fault energy under cryogenic temperatures is the key factor contributing to the excellent balance of strength and plasticity in the as-cast MEA, providing an important basis for research on the cryogenic mechanical properties of CoCrNi-based MEAs. Full article

The temperature dependence of the mechanical properties of the CoCrFeNi medium-entropy alloy (MEA) manufactured by laser-directed energy deposition (L-DED) and additionally annealed at 1200 °C for 24 h was studied. The microstructure of the as-deposited alloy was represented by a single-phase face-centered cubic structure with coarse columnar grains and a high density of dislocation. Annealing resulted in the development of recrystallization and a reduction in dislocation density. The CoCrFeNi alloy produced by L-DED demonstrated mechanical properties comparable with those of the fine-grained equiatomic CoCrFeMnNi alloy, produced by casting followed by thermomechanical processing. Namely, as-deposited CoCrFeNi had a yield strength (YS) and ultimate tensile strength (UTS) of YS = 370 MPa and UTS = 610 MPa at room temperature, and YS = 565 MPa and UTS = 965 MPa at cryogenic temperature, along with a ductility of ~60%. Annealing resulted in a decrease in strength to YS = 180/350 MPa at 293/77 K. A quantitative analysis of various strengthening mechanisms showed that some strength increment of the as-deposited alloy was ensured by the high dislocation density formed during L-DED. Full article