L10 Phase Research Articles

Microstructure and corrosion resistance properties of (FeCoNi)86Al7Ti7 high-entropy alloy via different aging time

The effects of heat treatment processes with different aging times on the corrosion behavior and passive film properties of the (FeCoNi)86Al7Ti7 high-entropy alloy were investigated. The study shows that aging time has a significant effect on the grain characteristics and L21 phase distribution. A suitable aging time optimizes the compositional segregation between the face-centered cubic and L21 phases. Samples aged for 1 h exhibit the best corrosion resistance, which can be attributed to the homogeneity of the composition and the quality of the passive film. Ti oxides play a major role in the passive film.

Quantitative magnetization measurements of magnetic particles with FePt standard samples

Micrometer-sized magnetic particles have been widely used in magnetic force microscopy, magnetic resonance force microscopy, and bio-sensing. To quantitatively interpret the data obtained with magnetic particles, it is important to know the magnetic properties of the particles. However, the magnetic moment of individual particle is usually too small to be measured by common instruments for samples with large volume. Here, we present a method to characterize magnetic microspheres using patterned FePt thin films as standard samples. The FePt thin film in the L10 phase has perpendicular magnetic anisotropy, and the patterned features can be magnetized to near single-domain magnets, which make them suitable standards for magnetic sphere calibration with magnetic force microscopy. Multiple linear regression is used to analyze the frequency shift images and obtain the effective dipole moment of the spheres. The position of the dipole moment is obtained by minimizing the residuals in multiple linear regression with a gradient descent algorithm. Three NdFeB spheres of different diameters were measured. It was found that the magnetization increases with the increase in the diameter of the sphere, possibly due to the weakening of ferromagnetism on the surface.

Effect of Heat Treatment on the Microstructure and Mechanical Properties of the Al0.6CoCrFeNi High-Entropy Alloy.

The effect of heat treatment on the microstructure and tensile properties of an as-cast Al0.6CoCrFeNi high-entropy alloy (HEA) was investigated in this paper. The results show that the as-cast Al0.6CoCrFeNi HEA presents a typical FCC dendrite morphology with the interdendritic region consisting of BCC/B2 structure and heat treatment can strongly affect the microstructure and mechanical properties of HEA. Microstructure analysis revealed the precipitation of a nano-sized L12 phase in the FCC dendrite and the formation of the FCC and σ phases in the interdendritic region after annealing at 700 °C. The coarse B2 phase was directly precipitated from the FCC dendrite in the 900 °C-annealed sample, with the coexistence of the B2, FCC, and σ phases in the interdendritic region. Then, the interdendritic region converted to a B2 and FCC dual-phase structure caused by the re-decomposition of the σ phase after annealing at 1100 °C. The tensile test results show that the 700 °C-annealed HEA presents the most significant strengthening effect, with increments of corresponding yield strength being about 107%, which can be attributed to the numerous nano-sized L12 precipitates in the FCC dendrite. The mechanical properties of 1100 °C-annealed alloy revert to a level close to that of the as-cast alloy, which can be attributed to the coarsening mechanism of B2 precipitates and the formation of a soft FCC phase in the interdendritic region. The observed variation in mechanical properties during heat treatment follows the traditional trade-off relationship between strength and plasticity.

Double heterogeneous structures induced excellent strength–ductility synergy in Ni40Co30Cr20Al5Ti5 medium-entropy alloy

The application of single-phase face-centered cubic (FCC) medium entropy alloys (MEAs) in the engineering industry is often hindered by the challenge of insufficient strength. In this study, a novel non-equiatomic ratio Ni40Co30Cr20Al5Ti5 MEA was successfully fabricated. Through the well-designed mechanical heat treatment processing, we introduced a heterogeneous grain structure comprising 67.4% fine grain and 32.6% coarse grain. Additionally, heterogeneous size L12 phases consisting of 18.7% submicron precipitates and 11.7% nano-sized precipitates, were incorporated into the alloy. Tensile tests conducted at room temperature revealed that the double heterogeneous structure alloy demonstrated remarkable strength–ductility synergy. It exhibited a yield strength of 1200 MPa, an ultimate tensile strength of 1560 MPa and a total elongation of 33.6%. The exceptional strength of the alloy can be primarily attributed to heterogeneous deformation induced strengthening, grain boundary strengthening and precipitation strengthening. The excellent ductility is mainly attributed to the high-density stacking faults and Lomer–Cottrell locks. This study not only contributes to the clarification of the strengthening and deformation mechanism of double heterogeneous structure alloys but also provides an effective strategy for the development of high-performance alloys with high strength and ductility.

Influence of Various Alloying Element Additions on Microstructure and Magnetic and Mechanical Properties and Corrosion Behavior of Cast Fe–Ga–Z Shape Memory Alloys

Fe–Ga alloys are attractive materials where high mechanical strength, toughness, ductility, and large low-field magnetostriction combine to give unique properties. Adding alloying elements is an effective method to further enhance these properties. In order to integrate these alloys into the operating environments, e.g., micro-robots and magnetic actuators, the corrosion behavior should be addressed. This work analyzed the microstructure, magnetization, hardness, and corrosion properties of Fe81Ga19−xZx (X = 5 at.% of Ni, Mn, or Ti, and 2 at.% Al; separately) alloys. X-ray diffraction (XRD), scanning electron microscope-electron (SEM), vibrating sample magnetometer (VSM), Vickers hardness (HV), and a potentiostat were used for characterization. XRD revealed that the prominent peak belongs to the bcc disorder A2 phase and a small peak for the cubic order L12 phase. Fe–Ga–Al alloy got the maximum Ms value, while Fe–Ga–Mn alloy gained the lowest one. However, the Mr and Hc properties for Fe–Ga alloy were distinctly improved by adding Al but slightly affected by doping Mn. Addition of Ti achieved the highest hardness, followed by Ni, Mn, and Al. The microstructure of the different alloys significantly influenced their corrosion behavior. Fe–Ga–Mn alloy with the fine globular grain structure showed the lowest corrosion rate (C R = 0.03 mm/year), whereas Fe–Ga–Al alloy with the coarse longitudinal grains exhibited the highest corrosion rate (C R = 0.19 mm/year).

Abnormal annealing-induced strengthening in Ni39.3Al15.7Fe45 eutectic medium entropy alloy

Cold-working combined with annealing is frequently used to enhance strength of traditional alloy materials. However, this thermal-mechanical processing is not adapted to the preparation for some near net shape parts. In this paper, a dual-phase Ni39.3Al15.7Fe45 eutectic medium entropy alloy (EMEA) was prepared by vacuum arc melting and abnormal annealing-induced strengthening is demonstrated and analyzed via XRD, SEM, EDS, TEM and room temperature tensile testing. It is found that the Ni39.3Al15.7Fe45 EMEA is composed of FCC solid solution phase and BCC phase. After annealing, ordered L12 phase precipitated from FCC solid solution, which induces a significant increment of tensile strength and a weak decrease of tensile plasticity. The tensile strength and elongation are 991 MPa and 24.1 % at as-cast state, the tensile strength and elongation are 1279 MPa and 22.4 % after annealing at 700 °C for 1 h.

Impact of Temperature on the Tensile Properties of Hypereutectic High-Entropy Alloys

Eutectic high-entropy alloys (EHEAs) can achieve a balance of high strength and ductility. It has been found that the mechanical properties of hypoeutectic high-entropy alloys are superior to those of EHEAs. In this work, hypereutectic Al1.1CoCrFeNi2.1 alloy was prepared, and the mechanical properties in a wide temperature range were studied. The presence of both soft ordered L12 and hard BCC (B2) phases results in a combination of ductile and brittle fracture modes. The Al1.1CoCrFeNi2.1 hypereutectic high-entropy alloy contains more primary soft L12 phases, which ensure excellent ductility. Moreover, the Orowan by-passing mechanism caused by the B2 precipitates increases in the strength of the alloy for low-temperature tensile tests (−100 °C and 23 ± 2 °C). The −100 °C test exhibits a dimple morphology and demonstrates the highest ultimate tensile strength of 1231 MPa, along with an excellent elongation of 44%. At high tensile temperatures (650 °C, 750 °C, and 850 °C), the dislocation cutting mechanism and dynamic recrystallization increase the plasticity. However, the presence of a large number of cracks near the spherical primary L12 phase significantly reduces the ductility and strength. The results show that the hypereutectic Al1.1CoCrFeNi2.1 exhibits superior plasticity and strength properties at low temperatures. The findings of the article provide a new approach to enhancing the comprehensive mechanical properties of hypereutectic alloys.

Enhanced properties of a novel medium-entropy alloy base diamond composite by introducing coherent L12 phase at interface

In this study, a medium-entropy alloy (MEA) was used as a new binder to fabricate diamond composite. The MEA base diamond composite (MEA-Comp) shows a much higher transverse rupture strength (TRS, 1453.5 MPa) and a stronger bonding interface than the referenced Co base diamond composite (TRS, 744.9 MPa). It is for the first time found that a (Ni, Co)3Al L12-type phase was in-situ formed between diamond and the MEA matrix. The L12 phase is fully coherent with the diamond in orientation relationship of [001]L12//[001]Diamond and (010)L12//(010)Diamond. Thus, the metallurgical bonding between diamond and metal matrix can be greatly improved, resulting in an enhanced wearing performance. Additionally, chromium carbides (majority Cr7C3 and minor Cr3C2) were also in-situ formed in the interlayer between diamond and matrix. The interlayer composed of L12 phase and chromium carbides can also block the direct contact between diamond and metal matrix, which might slow down the graphitization of diamond under service condition.

Remarkable cryogenic properties by deformation twinning in a coherent precipitation-containing high entropy alloy

Developing structural alloys for critical cryogenic applications is a challenging yet essential task, primarily due to the ductile-brittle transition phenomenon. To address this issue, we developed a twinning-aided coherent precipitation-containing Co45Cr15Ni30Al5Ti5 high entropy alloy (HEA) with remarkable cryogenic properties. The activation threshold for twinning was successfully reduced via the disordering behavior of the L12 phase. In the present work, distinct temperature-dependent deformation behavior was observed. The dominant room-temperature strain accommodation mechanism was found to be the dislocation slip coupled with architectured stacking faults (SFs) and immobile Lomer-Cottrell (L-C) locks. Whereas, deformation twins were readily formed under cryogenic tensile loading, leading to significantly increased yield strength and ultimate tensile strength, approaching 1.5 GPa and 1.9 GPa, respectively. The remarkable cryogenic work hardening behavior contributes to persistent elongation of up to 31 % and the super-high ultimate tensile strength. The presence of SFs, deformation twins, and their interactions provide significant strain-hardening capability.

Investigation of multi-phase strengthened Ni36Co30Cr11Fe11Al8Ta4 high entropy alloy prepared by directional solidification

The directional solidification Ni36Co30Cr11Fe11Al8Ta4 high entropy alloys were prepared by changing the withdraw rates to reveal the phase formation and reinforcing mechanism. The different withdraw rates changes microstructure of the alloy. The primary dendrite spacing was reduced from 250 μm to 100 μm. L12 phases were observed in inter-dendritic region. The phase analysis result indicates that the Laves phases are nucleated first. When the withdraw rate increases from 5 μm s−1 to 100 μm s−1, the yield strength increases from 556.21 MPa to 664.31 MPa. The directional solidification alloy shows the higher strain than the as-cast alloy. The dislocation loops can be observed due to the high misfit of L12 phases. The main strength mechanisms are Orowan bypassing mechanism and interface strengthening mechanism.

Unraveling the two-stage precipitation mechanism in a hierarchical-structured fcc/L21 high-entropy alloy: Experiments and analytical modeling

Understanding the phase stability and precipitation mechanisms is crucial for engineering multiphase nanostructured alloys with optimal mechanical properties. In this work, we studied the formation and temporal evolution of nanoprecipitates and their effect on mechanical properties of an fcc/L21 eutectic high-entropy alloy through a combination of experiments and analytical modeling. Aging the alloy at 1023 K results in the precipitation of coherent L12 nanoparticles in the fcc phase and coherent bcc nanoparticles in the L21 phase, leading to the formation of an fcc/L12 + L21/bcc hierarchical structure. Notably, the scanning transmission electron microscopy (STEM) results reveal that the precipitation in both the fcc and L21 phases is not through a one-step nucleation, but a two-stage transformation consisting of an initial chemical separation via spinodal decomposition and subsequent structural ordering/disordering. The Gibbs free energy diagrams of the fcc and L21 phases were modeled through numerical techniques, and the spinodal decomposition regions of the two systems at different temperatures were calculated. Based on the modeling results, we discussed the phase stability and thermodynamics of spinodal decomposition of the two phases. In addition, the formation of hierarchical structure substantially enhances the strength of the alloy. Modeling of the strengthening mechanisms reveals that the order strengthening of L12 nanoparticles plays a major role in enhancing the yield strength of the alloy, whereas the contribution from the bcc nanoparticles can be negligible. Our findings provide insights into the phase stability, precipitation and strengthening mechanisms of hierarchical-structured alloys.

A eutectic high-entropy alloy coating of novel hard-surface and soft-core structure with improved tribological properties

Structures with a hard surface and soft core have always been the goal of surface-coating technology. In this study, this goal was achieved in a eutectic high-entropy alloy (EHEA) surface by using aging treatment at elevated temperature. The EHEA surface structure comprised L21 and B2 phases, and a substantial amount of nanoprecipitates were observed inside the B2 and L21 phases. Interestingly, the structure near the interface resembled a “flowerpot seed”, whereas the structure away from the interface resembled a “flower-like” one. After aging treatment, Al diffused to the area away from the interface, resulting in increased volume fraction of L21. Cr and Fe diffused toward the interface, resulting in increased B2 phase. The hardness gradually decreased from the surface to the interior, creating a structure with a hard surface and soft core. The tribological properties of the surface also significantly improved, with decreased coefficient of friction from 0.514 to 0.471. Therefore, this EHEA is a promising candidate as a coating material for long-term applications at elevated temperatures.

Roles of solidification rate and Co substitution for Cr on single phase ordered Fe2CrSi alloy formation

A comprehensive experimental and theoretical study on the structural, electronic, and magnetic properties of bulk Co-substituted Fe2CrSi Heusler alloys has been carried out. The impact of rapid solidification on the formation of phase pure Fe2CrSi alloy has also been explored. Bulk Fe2Cr1-xCoxSi (where x = 0, 0.1, 0.2, 0.3, and 0.5) alloy samples were prepared using arc melting, and melt-spinning was employed to prepare Fe2CrSi ribbons at two different wheel velocities (20 and 30 ms−1). Our investigation revealed that the Fe2CrSi Heusler alloys with a Co substitution in the range of 0.2 ≤ x ≤ 0.5 exhibited a single L21 phase structure. However, for x = 0 and 0.1, there were 27 % and 11 % A15 impurity phases, respectively, coexisting with the L21 phase. On the other hand, the sample melt-spun at 20 ms−1 contained only 7 % impurity phase, while the one melt-spun at 30 ms−1 was free of any impurity phase. First principles calculations using the GGA + U approach showed that the single phase ribbon sample exhibited 12.5 % DO3 type disorder, indicating a change in site preference resulting from a high solidification rate. All the experimental and calculated magnetic moments of the alloys were in agreement with the Slater-Pauling rule. Our results suggest that the Fe2Cr0.75Co0.25Si alloy is a promising candidate for spintronic device applications due to its half-metallic nature.

Hot deformation behavior and dynamic recrystallization of dual-phase (CrCoNi)94Al3Ta3 medium entropy alloy

The microstructural evolution and dynamic recrystallization (DRX) behavior of a dual-phase (CrCoNi)94Al3Ta3 medium entropy alloy (MEA) was studied using thermal simulated test machine at temperature ranging from 1000 ℃ to 1150 ℃ under strain rate of 0.001–1 s−1. The initial dual-phase microstructures were equiaxed grains with face-centered cubic structure (γ) and L12 phase (γ') precipitated homogeneously in the grain, besides a certain amount pinned at boundary. The stress-strain curves exhibited a typical DRX softening behavior, in which the flow stress was sensitive to deformation temperature and strain rate. The deformation activation energy was calculated to be 612.04 kJ/mol (γ + γ' dual-phase region) and 391.02 kJ/mol (γ single-phase region) respectively. The appropriate hot deformation parameters of (CrCoNi)94Al3Ta3 MEA were 1100 ℃/0.001 s−1 and 1150 ℃/0.01 s−1 based on the hot processing map and deformation microstructure. The discontinuous DRX transformed to continuous DRX depended on deformation temperature, this was because that the interaction between precipitates and dislocation was weakened as the deformation temperature beyond the dissolution temperature of L12 phase.

Facile and fast synthesis of highly ordered L10-FeNi nanoparticles

The chemically ordered L10-FeNi alloy is a promising candidate for next generation rare-earth-free permanent magnets, which can revolutionize the high-performance magnets market currently dominated by Nd-Fe-B. Despite many efforts, the experimental results fall short of theoretical predictions, and current approaches are not suitable for industrial implementation. In this work, we propose an innovative and efficient synthesis method that exploits the natural order of a crystalline Ni/Fe complex, which closely mimics the atomic organization in the L10 structure, to drive the formation of the ordered phase. By low-temperature reduction of the complex salt, carbon coated aggregates of FeNi alloy nanoparticles (20 – 120 nm) with a >55% of L10 phase, high coercivity (up to 65 mT) and large saturation magnetization (∼ 140 Am2/kg) were obtained. The results pave the way for the development of a novel and sustainable route to produce high-anisotropy FeNi nanoparticles of potential interest for next generation critical-element-free permanent magnets.

Microstructure evolution and strengthening mechanism of directional solidification Ni40.5Co30Cr11Fe11Al6Ta1.5 high entropy alloy

Directional solidification Ni40.5Co30Cr11Fe11Al6Ta1.5 high entropy alloys (HEAs) with different withdraw rates (5 μm/s-100 μm/s) were prepared to reveal the microstructure evolution, the deformation mechanism and the strengthening mechanism. The results show that the solid-liquid (S-L) interface of the alloy changes from convex to planar with the increase of withdraw rates. The primary and second dendritic arm spacing decreases from 248 μm and 53 μm (20 μm/s sample) to 177 μm and 37 μm (100 μm/s sample), respectively. The alloy consists of FCC, L12 and topologically close packed (TCP) phases, which is in good agreement with the analysis of phase volume fraction. The L12 phases are formed in IR due to the lower mixing enthalpy. The ultimate tensile strength of the alloy increases from 412.36 MPa and 25.75% to 547.89 MPa and 48.1%, respectively. The strength of the alloy is mainly attributed to precipitation strengthening.

Mechanisms underlying the influence of Co and Ti on the microstructure, mechanical and wear properties of A2/B2 typed AlCrFeNi alloy

AlCrFeCoNi is one of widely studied high-entropy alloys, in which Co is considered to be beneficial. Here we demonstrate in detail that Co does not play a positive role. Instead, Co-free AlCrFeNi possesses higher ductility than AlCrFeCoNi while maintaining a similar mechanical strength. It is of interest to observe that removing Co results in a two-fold increase in the wear resistance. The improvements are related to the formation of a soft-hard alternating lamellar structure which is absent in the Co-containing alloy. We further tailor the alloy by replacing Co with Ti, leading to further enhanced wear resistance. AlCrFeCoxNi (x = 0.5 and 1) and Co-free AlCrFeNiTix (x = 0, 0.2, 0.5, 0.7 and 1) all contain a Fe-Cr-rich disordered BCC phase (A2) and a NiAl (Ti)-rich ordered BCC phase (B2). For x = 0.5, a small amount of L21 phase with embedded nanoscale A2 phase is observed. For x ≥ 0.7, an additional CrFeTi Laves phase forms mainly at grain boundaries. The increase in Ti content alters the lamellar eutectic microstructure of AlCrFeNi, forming a cellular eutectic microstructure. Ti considerably increases hardness and compressive strength at the expense of ductility. AlCrFeNiTi0.7 appears to have a proper balance between hardness and ductility with a superior wear resistance comparable to that of a well-known industrial wear-resistant high-Cr cast iron (Cr30C2.85). This HEA has demonstrated itself a promising candidate for the mechanical and anti-wear applications.

Tunable magnetic properties of CoPt nanoparticles: Impacts of phase coexistence and thermal annealing

Nanostructured materials such as CoPt nanoparticles with tunable hard magnetic properties are desirable for a wide range of technological applications. In CoPt nanoparticle systems, a structurally disordered fcc phase may be transformed into an ordered fct L10 phase via annealing. However, the effects of this thermally induced disorder to order transition on the magnetic properties and phase coexistence still need to be understood. Here, we have studied these effects in Co50Pt50 nanoparticles, which were prepared by chemical reduction combined with ultrasound. By varying the annealing temperatures from 450 °C to 700 °C for 1 h, we observe that the magnetic properties are heavily dependent on the annealing temperature. The nanoparticles annealed at 500 °C exhibit the highest coercivity, while the 600 °C annealed nanoparticles possess the largest saturation magnetization. We demonstrate the variation in coercivity with annealing temperature follows that of the order parameter. This study establishes, for the first time, a correlation between the order parameter and the coercivity for annealed CoPt nanoparticles and paves a pathway for the development of nanostructured magnets with tunable magnetic properties.

Reliable identification of the complex or superlattice nature of intrinsic and extrinsic stacking faults in the L12 phase by high-resolution imaging

Planar defect-based deformation mechanisms are becoming increasingly important for understanding and tuning the mechanical properties of superalloys. Typically, to elucidate such deformation mechanisms, the type of planar defect is determined by conventional transmission electron microscopy or by high-resolution imaging along 〈110〉 crystallographic directions. However, an unambiguous identification of the complex or non-complex nature of intrinsic and extrinsic stacking faults in the L12 phase may not be possible by analyzing only one projection. To overcome this limitation, we present a straightforward approach to identify the superlattice or complex character of intrinsic and extrinsic faults in the L12 structure in high-resolution scanning transmission electron microscopy by tilting to a different zone axis and examining the shift of superlattice planes. Using this approach, two key aspects of the Kolbe mechanism for the formation of superlattice extrinsic stacking faults are verified for the first time: the presence of a complex intrinsic fault segment at the leading end and the occurrence of re-ordering.

Thermal Stability and Hot Corrosion Performance of the AlCoCrFeNi2.1 High-Entropy Alloy Coating by Laser Cladding.

Both crack-free AlCoCrFeNi2.1 eutectic high-entropy alloy (EHEA) and Y and Hf co-doping AlCoCrFeNi2.1 EHEA (YHf-EHEA) coatings were prepared by laser cladding. The solidification microstructure, thermal stability, and hot corrosion performance of the coatings at 900 °C under 75% Na2SO4 + 25% NaCl molten salts were investigated. The experimental results showed that the structure of the as-deposited coatings consisted of FCC and BCC/B2 phases. After heat treatment, an Al-rich L12 phase was precipitated in the FCC phase of all coatings. The grain sizes of the EHEA and YHf-EHEA coatings after heat treatment at 900 °C for 10 h increased by 27.5% and 15.7%, respectively, compared to the as-deposited coatings. Meanwhile, after hot corrosion, the spallation areas of the YHf-EHEA and EHEA coatings accounted for 14.98% and 5.67% of the total surface area, respectively. In this study, the Y and Hf co-doping did not change the microstructure morphology and phase structure of the coatings but did improve the thermal stability and resistance of the hot corrosion oxide scale spallation, providing a certain amount of data and theoretical support for the application of EHEA coatings as high-temperature protective coatings.