Ordered B2 Phase Research Articles

Atomic mechanisms for the fracture of AlMo0.5NbTa0.5TiZr refractory high entropy superalloy

Refractory high entropy superalloys (RHESs), known for their excellent high temperature performance, exhibit promising characteristics but are challenged by significant brittleness. Efforts to enhance plasticity through microstructure regulation have achieved only limited success, largely due to the unclear underlying fracture mechanisms of the superstructure. In this study, we systematically investigate the fracture mechanisms of the AlMo0.5NbTa0.5TiZr RHES from microscopic to electronic scales. Interestingly, both experimental and simulation results reveal that the ordered B2 phase demonstrates non-negligible plastic deformation capabilities during fracture, including deformation twinning and amorphization. Despite this, the fracture resistance of the B2 phase is lower compared to the A2/B2 interface and disordered A2 phase, even though the A2 phase shows less twinning and amorphization. Ab initio molecular dynamics simulations, combined with electronic behavior analysis, indicate that bonds involving Al and Zr in the B2 phase often exist in an anti-bonding state, making them more prone to breaking under load. This study provides deeper insights into the fracture mechanisms of the A2/B2 superstructure and its constituent phases at both atomic and electronic levels, offering a systematic approach to improving the fracture properties of such RHESs.

Directionally solidified NiCoCrFeAlW eutectic high-entropy alloy: Microstructure and mechanical properties

The NiCoCrFeAlW eutectic high-entropy alloy exhibits a balanced combination of ductility and strength. This study examines the microstructure evolution and mechanical properties of the Ni30Co30Cr10Fe10Al18W2 alloy at varying growth velocities (10, 20, 50, 100, and 200 μm/s), which in its as-cast state, features a microstructure that includes both coarse dendrites and lamellar eutectic. The structure of the coarse dendrites is characterized by an ordered body-centered cubic (B2) phase, while the lamellar eutectic, features a combination of face-centered cubic (FCC) and ordered B2 phases. The microstructure, characterized by lamellar and dendritic structures that are parallel to the growth direction after directional solidification, undergoes a morphological transition at the solid-liquid interface from a planar to a dendritic form as growth velocity increases. The tensile fracture strength initially increases with the growth velocity, reaching a peak of 1286 MPa at a velocity of 50 μm/s, and then subsequently decreases as the growth velocity continues to increase. Directional solidification results in an increase in sample elongation with increasing growth velocity, achieving an elongation of 22.5% at 200 μm/s. An examination of the deformation mechanism reveals that the lamellar structure, composed of the more ductile FCC phase and the harder B2 phase, effectively harmonizes the plasticity and strength of the alloy. Therefore, directional solidification significantly adjusts the eutectic microstructure's orientation, thereby enhancing the mechanical properties along the direction of solidification.

Microstructure, corrosion resistance and high temperature oxidation properties of AlCrFeNiCu high-entropy alloy coating by high-speed laser cladding

AlCrFeNiCu high-entropy alloy (HEA) coating was prepared on Zirconium (Zr) rod by high-speed laser cladding (HSLC) technique. The effect of different laser power and scanning speed on the forming quality of the coating was studied. When the specific energy density was in the range of 3.0–3.2 J/mm2, the AlCrFeNiCu HEA coating with good adhesion to the substrate, low dilution rate (small heat affected zone) and no cracks were obtained. The phase structure, microstructure, microhardness, corrosion resistance and high temperature oxidation resistance of the coating were tested and analyzed. The results show that the AlCrFeNiCu HEA coating is composed of 76.9 % BCC phase and 23.1 % FCC phase, in which the BCC phase is mainly composed of disordered BCC phase rich in Cr and Fe elements and ordered B2 phase rich in Al and Ni elements, and the FCC phase is rich in Cu elements. Due to the rapid cooling of HSLC, the coating structure gradually changes from columnar crystal to equiaxial crystal from bottom to top. The hardness of the coating is as high as 805 HV, which may be caused by the solution strengthening effect, lattice distortion effect and slow diffusion effect of HEA. In 3.5 % NaCl solution and 0.1 mol/L KOH solution, the self-corrosion current density of the coating is 4.209 × 10−8 A/cm2 and 4.331 × 10−8 A/cm2, respectively. The passivation film on the surface of the coating is composed of a variety of oxides, and the oxygen vacancy density of the passivation film in the two solutions is 2.552 × 1020 cm−3 and 5.794 × 1020 cm−3, with fewer pitting pits on the surface. The structural integrity of the coating can be maintained after 60 min oxidation at 1200 °C. The oxidation process follows the growth kinetics curve, and the surface oxides mainly comprise FeAl2O4, Al2O3 and Cr2O3. AlCrFeNiCu HEA coating improves the corrosion resistance and high temperature oxidation resistance of the Zr-4 alloy.

Compositional effect on microstructure and mechanical properties of AlNbTiZr-based refractory high entropy alloys

This study aims to investigate the phase composition, microstructure, and mechanical properties of three refractory high-entropy alloys (RHEAs): Al7(NbTiZr)93, Al7(NbTiZrMo)93, and Al7(NbTiZrTa)93. The as-cast Al7(NbTiZr)93 alloy exhibits a single-phase BCC structure, with a yield strength (YS) of 1050 MPa at room temperature. The addition of Mo and Ta leads to the formation of Al7(NbTiZrMo)93 and Al7(NbTiZrTa)93 alloys with BCC-B2 and B2-FCC dual-phase structures, whose YS values reach 1741 MPa and 2350 MPa, respectively. At 800 °C, trace amounts of ordered B2 phase are generated in the Al7(NbTiZr)93 matrix, and the YS decreases from 1050 MPa at room temperature to 219 MPa at 800 °C. In the Al7(NbTiZrMo)93 alloy, the 10 nm B2 precipitates grow to several hundred nanometers, and the YS decreases to 431 MPa after compression at 800 °C. In the Al7(NbTiZrTa)93 alloy, the grid B2-FCC structure at room temperature transforms into a disordered BCC structure at 800 °C, yet possessing the YS of 908 MPa and demonstrating acceptable ductility. Moreover, Ta exhibits the better structural and performance matching than Mo in AlNbTiZr-based RHEAs. Therefore, it opens up new prospects for regulating the structure and high-temperature performance of AlNbTiZr-based RHEAs.

Revealing intermetallic strengthening in bainitic steel designed for nickel aluminide formation

BainNiAlCu steel was designed to achieve a cementite-free bainitic structure and susceptibility to the formation of intermetallic phases at elevated temperatures. Based on Transmission Electron Microscopy (TEM) observations, the intermetallic strengthening occurring in the bainitic ferrite matrix was revealed. The B2-ordered phase are rich in Ni, Al, and Cu, which indicates the growth of NiAl/Cu co-precipitates. The intermetallic phase was characterized by a spherical morphology and nanometer size. The micro-segregation of Ni, Al and Cu was observed at the boundaries of cementite particles and the bainitic matrix. The redistribution of atoms during the growth of carbides forms a heterogeneous nickel aluminide nucleation site. Intermetallic strengthening is a promising strategy for controlling the thermal stability of bainitic structures. The results of this research constitute fundamental knowledge regarding the design of ultra-fine, cementite-free bainitic steels subjected to intermetallic strengthening.

Understanding the thermal stability and friction behavior of ultrasonic shot peening-introduced gradient nanostructured M50 bearing steel

The M50 steel is a common material for main shaft bearings in aerospace applications. The thermal stability of gradient ultrafine grains/nanograins steels is of critical importance for engineering applications. Here, we fabricated gradient nanostructured M50 steel using ultrasonic shot peening and subsequent annealing. This study investigated the effect of annealing treatment on the microstructure evolution of gradient nanostructured M50 steel and the thermal stability of the USPed sample post-annealing. Furthermore, the friction behavior of gradient nanostructured M50 steel was also examined. After annealing at 350 °C for 30 min, ferrite phase at the depth span of 0–100 μm has better thermal stability, and formed a high-density dislocation network and cells at the top surface of the gradient layer. When annealed samples were continued annealed at 350 °C for 100 h, the average ferrite phase size at the depth span of 0–100 μm reduced to a fixed value (∼434 nm). High-density dislocation cells with smaller and more dispersed ordered B2 phases have a stronger pinning effect of dislocation. Hetero-deformation induced hardening by a dislocation network of grain interior and dislocation wall (∼200 nm thick) and precipitation hardening of dislocation cells with more dispersed ordered B2 phase is the main reason for the high microhardness of the samples annealing at 350 and 450 °C for 30 min. The USPed sample within the 100–200 μm depth span exhibits better friction properties attributed to the strengthened martensitic matrix, partial decomposition of spherical carbides, and the presence of high-density dislocations, which reduce the cracking of carbides and matrix, and carbide spalling. Although heat treatment results in a slight decrease in the friction properties of the USPed sample, they still surpass those of the tempered sample. This reduction in friction properties post-annealing primarily arises from the annihilation of high-density dislocations.

Dual wire arc additive manufacturing of compositionally graded Alx-Co-Cr-Fe-Ni high-entropy alloy

Graded materials are promising alloys due to their improved strength and ductility. In this study, a functionally graded Alx-Co-Cr-Fe-Ni high entropy alloy with a variation of Al concentration along the building direction was produced in-situ by dual wire arc additive manufacturing for the first time, and the microstructure and mechanical properties of the deposited alloy were investigated. The results showed that the FCC + BCC dual-phase gradually transformed to a single BCC phase with increasing Al content, the precipitation temperature of the ordered B2 phase became high, and the precipitation temperature range of the disordered BCC phase became wider, which promoted the formation of the disordered BCC phase. With the change in phase ratio, the hardness gradually increased from 342 HV to 397 HV, and the bottom deposited alloy had an optimal combination with a compressive strength of 827.4 MPa and elongation of 42.3%. Compared to the bottom region, the top region shows much higher sliding wear resistance. The high-density dislocations and grain refinement result in better toughness and strength in the bottom region, while the segregation of Cr elements at the grain boundaries changes the properties of the grain boundaries and makes them more brittle in the top region. This investigation realizes a novel method to fabricate GHEAs, which can produce a more convenient and cost-effective gradient material with a complex composition.

Structure and Properties of Melt-Quenched Al<sub>4</sub>CoCrCuFeNi High-Entropy Alloy

The structure and mechanical properties of a multicomponent high-entropy Al4CoCrCuFeNi alloy in the as-cast and melt-quenched states were investigated. The alloy composition was analyzed based on the literature criteria for predicting the phase formation in high-entropy alloys, which considered the entropy and enthalpy of mixing, valence electron concentration as well as the atomic size difference of the components. The alloy films were synthesized by quenching from the melt using a splat-quenching technique. The cooling rate of the films was estimated to be ~ 106 K/s based on the film thickness. The X-ray diffraction analysis revealed that both as-cast and melt-quenched Al4CoCrCuFeNi alloy samples had an ordered B2 phase in their structure. The microhardness of the as-cast alloy was 6500 MPa, while the microhardness of the melt-quenched film was significantly higher and reached 9400 MPa.

Initial Oxidation Behavior of AlCoCrFeNi High-Entropy Coating Produced by Atmospheric Plasma Spraying in the Range of 650 °C to 1000 °C.

As protective coatings for the thermal parts of aero-engines, AlCoCrFeNi coatings have good application prospects. In this study, atmospheric plasma spraying (APS) was used to prepare AlCoCrFeNi high-entropy coatings (HECs), which were oxidized from 650 °C to 1000 °C. The mechanism of the oxide layer formation and the internal phase transition were systematically investigated. The results show that a mixed oxide scale with a laminated structure was formed at the initial stage of oxidation. The redistribution of elements and phase transition occurred in the HECs' matrix; the BCC/B2 structure transformed to Al-Ni ordered B2 phase and Fe-Cr disordered A2 phase.

An order-disorder phase transition in alloy 783 bolts after long-term service

Ni-Co-Fe based alloy 783 is commonly employed as bolt material in power plants. Its initial state possesses a γ matrix strengthened through B2-type β phase and L12-type γ' phase. Over prolonged service, the alloy displays an observed increase in strength along with certain embrittlement. After 100 and 123 months of service at 600 °C, a pine-like phase enriched with Co and Fe occurs in the matrix, which is confirmed to be a BCC phase with a lattice parameter of 2.87 Å. These BCC precipitates also manifest within the coarse β particles and are coherent with the B2 phase, showing a cuboidal shape. Thermodynamic calculations suggest the transformation from the ordered B2 phase to the disordered BCC phase at temperatures below 670 °C. The identification of the order-disorder phase transition deepens the understanding of phase transition after long-term service in alloy 783 and the related mechanical properties evolution in addition to previous works.

Analysis of phase stability and chemical segregation in the Mo-V alloys using a generalized embedded atom method potential

A new interatomic potential for the Mo-V system is introduced to facilitate the study of phase stability and mechanical properties at lower temperatures. This potential is based on a generalization of the embedded atom method and includes contributions from embedding energy, explicit two- and three-body interactions and nonlocal many-body interaction terms. The parameters of the potential are optimized by using data from ab initio density functional theory (DFT) calculations. The potential is rigorously validated across a range of physical properties, such as elastic constants, equation of states, phonon dispersion curves, point defect properties and melting temperatures for different compositions. Even though our potential is trained on a small dataset, its accuracy is comparable to available machine learning potentials for Mo and V. Our results show that an ordered B2 phase is stable at low temperatures in alloys containing 50% V, but the solid solution phase is stable above 800 K. However, such long-range ordering is not observed in V-rich or Mo-rich alloys. In addition, our results show that V segregates to dislocation cores and grain boundaries.

The influence of cooling rate on the structure and mechanical properties of Al4CoCrCuFeNi multicomponent alloy

The present study focused on examining the composition, structure, and mechanical properties of an Al4CoCrCuFeNi multicomponent high-entropy alloy. Two states of the alloy, namely as-cast and melt-quenched, were investigated. The composition of the alloy was determined based on established criteria found in literature, which consider factors such as entropy, enthalpy of mixing, valence electron concentration, and atomic size difference of the components. To synthesize the alloy films, a splat-quenching technique was employed, where the films were rapidly cooled from the molten state. The estimated cooling rate of the films was approximately 106 K/s, calculated based on the film thickness. X-ray diffraction analysis was conducted on both the as-cast and melt-quenched Al4CoCrCuFeNi alloy samples, revealing that they exhibited an ordered B2 phase in their structure. The microhardness measurementswere carried out to evaluate the mechanical properties of the alloy. The as-cast alloy displayed a microhardness of 6500 MPa, while the melt-quenched film exhibited a significantly higher microhardness, reaching 9400 MPa.

Stress-assisted atomic diffusion in NiTiHf shape memory alloys

Stress-assisted atomic diffusion behavior in NiTiHf shape memory alloys was investigated by measuring the internal friction at temperature range of 300–1070 K. A thermally activated internal friction peak was recorded in the B2-ordered phase in the temperature range of about 850–950 K (at 1 Hz, depending on alloy composition), with activation energy of 1.79 ± 0.05 and 2.00 ± 0.10 eV and relaxation time of 2.7−2+1∙10−12 and 1.7−5+1∙10−13 s for Ni50Ti35Hf15 and Ni50Ti30Hf20 alloys, respectively. It was proposed that the short-range diffusion of Ni-atoms under the stress is the main relaxation mechanism in the studied alloys. The diffusion coefficients of Ni in the B2-ordered phase were estimated to be 3.72·10−8 and 5.90·10−7 m2s−1 for Ni50Ti35Hf15 and Ni50Ti30Hf20 alloys, correspondingly.

Introducing a new heterogeneous nanocomposite thin film with superior mechanical properties and thermal stability

High entropy alloy (HEA) films offer excellent mechanical properties due to their random solid solution structure. However, their large grain boundary volume fraction can cause thermal instability, resulting in phase decomposition that affects their high-temperature performance. Nevertheless, it remains an interesting question whether phase decomposition can be used as a processing tool to create new HEA materials. In this study, AlCrFeCoNiCu0.5 HEA thin films were fabricated and annealed at 500 °C for up to 72 h. The 72-hour annealed thin film exhibited a 30 % increase in mechanical properties, exceeding most other HEA thin films. Characterization using X-ray and electron microscopy revealed a decomposition-induced phase transformation, which produced four new phases, including Cu-rich FCC phase, Cr-rich BCC phase, and ordered B2 phase of AlNi and FeCo. The enhanced mechanical properties were due to back stress strengthening and BCC + B2 phase strengthening. The 72-hour annealed thin film also showed excellent thermal stability through a new round of annealing, exhibiting a low variation in microstructure and chemical composition after being annealed at 500 °C for 100 h.

The cobalt-free Fe35Mn15Cr15Ni25Al10 high-entropy alloy with multiscale particles for excellent strength-ductility synergy

The evolution of microstructure and tensile properties of a newly designed cobalt-free high entropy alloy, Fe35Mn15Cr15Ni25Al10 (at.%), has been investigated during the annealing and aging processes. A dual-phase alloy with a face-centered cubic (FCC) and body-centered cubic (BCC) or ordered B2 structure, along with multiscale heterogeneous precipitates, has been formed. The precipitated phases consist of bulk primary B2–NiAl and secondary BCC/B2 particles that range in size from the nanometer to micrometer scale. By prolonging the aging time, the yield strength is strongly enhanced, increasing from 250 MPa to 860 MPa at the peak aging stage. The primary sources of strengthening are precipitation hardening and hetero-deformation-induced strengthening from the BCC or ordered B2 phase. Furthermore, the incorporation of various strengthening mechanisms and high-strain hardening capability at cryogenic temperatures (77 K) results in improved strength and ductility simultaneously. A highly cost-effective high-entropy alloy, doped with a high concentration of aluminum, has been developed.

Phase prediction, densification, and microstructure of AlCrFeNi(TiO2)x high entropy alloy composite fabricated by spark plasma sintering

This study incorporates the knowledge of empirical calculation and computational concepts using Thermo-Calc as a design guide to develop AlCrFeNi high entropy alloy (HEA). Subsequently, a novel AlCrFeNi HEA composite consisting of varying weight percentages (wt%) of TiO2 reinforcements is synthesized using mechanical alloying (MA) and spark plasma sintering (SPS) techniques. Our work investigated the role of TiO2 reinforcement on the densification behavior, phase changes, microstructure, and properties of AlCrFeNi HEA. The phase diagram was constructed using Thermo-Calc software. Powder densification during SPS was analyzed using obtained sintering data. The samples were characterized using Xray Diffraction (XRD) and Scanning Electron Microscopy (SEM) techniques. The density and micro-hardness were evaluated. The result shows that the densification during SPS is dependent on kinetic mechanisms such as melt diffusion, surface diffusion, and plastic flow. Phases of the disordered BCC phase (rich in FeCr) and ordered B2 phase (rich in NiAl) were found in the AlCrFeNi HEA. The phases were consistent with the prediction using thermo-calc software. The BCC phases in the AlCrFeNi are retained regardless of the addition of TiO2 particles but with the diffusion of Ti with O atoms fitting into the interstitial sites in the BCC and B2 lattice. The microstructure verified the homogeneous distribution of the TiO2-rich phase within the BCC and B2 phases of the AlCrFeNi HEAs. The density decreases with the adding TiO2. The hardness of reinforced AlCrFeNi is enhanced from 537.24 Hv to 752.74 Hv with an increment of TiO2 reinforcement from 0 to 8 wt%. TiO2 particles acted as impediments to the mobility of dislocation, thereby increasing the resistance to deformation of HEA composites during micro indentation.

Development of a novel CoCrNi-based eutectic high entropy alloy for a wide temperature range

This short communication elucidates the developed as-cast CoCrNi-based eutectic high entropy alloy by prefixing valence electron concentration to balance strength and ductility. The developed EHEA includes a face-centered cubic (FCC) phase, ordered B2 phase, and precipitates. The dislocation pile-up at the FCC/B2 interface balances the strength and ductility in a wide temperature range.

Improvement of thermal shock resistance and hot mechanical properties by FCC/BCC/B2 multiphase strengthened microstructure in laser cladded high-entropy alloy coatings

NiCoFeCrSiAlxCu0.5TiMoB0.4 high-entropy alloy (HEA) coatings were prepared by laser cladding, and the microstructure, phase structure and thermal properties were studied. The results showed that the solid solution phase of the coating matrix changed from FCC + BCC to BCC as the main phases with increasing Al content. Moreover, the enrichment of Ti resulted in the significant formation of a stacking fault structure in the interdendritic region of the dual-phase coatings. The ordered B2 phase precipitated from the BCC structure exhibited high hardness and crack damage resistance. The wear rates of the HEA-Al0.5 and HEA-Al1.5 coatings at 600 °C were 3.78 × 10−5 and 4.35 × 10−5 mm3/(N·m)−1, respectively. The wear mechanisms of HEA coatings were mainly abrasive wear and oxidation wear. Because of the high microhardness of the BCC phase, cracks initiated and propagated on the worn surface, which aggravated the oxidation wear deterioration of the HEA coating. Overall, this study may provide a theoretical basis for the microstructure-property relationships of HEA coatings and contribute to the development of high-performance coatings.

The CrFeNbTiMox refractory high-entropy alloy coatings prepared on the 40Cr by laser cladding

In order to improve the performance of 40Cr coal mining cutter teeth, the CrFeNbTiMox (x = 0.2, 0.4, 0.6, 0.8, 1) refractory high entropy alloy (RHEAs) coatings were prepared on the 40Cr by laser melting. The effects of the Mo on the phase, microstructure, hardness, wear and corrosion resistance of the coatings were investigated. The results show that the coatings consist of BCC (Im3m), C14-Laves (P63/MMC), C15-Laves (Fd3m) and ordered B2 phase (Pm3m). The BCC phase increased and the C14-Laves phase decreased as the Mo element content increased. The central part of the coating is mainly composed of dendrites and some swelling crystals, the dendritic areas increase and the swelling crystals decrease with the increase of Mo elements. The Mo and Nb elements are mainly concentrated in the dendritic areas, and the Cr and Fe are enriched in the interdendritic areas. With the increase of the Mo element, the hardness of the coating increased from 676 HV to 841 HV, which is about 3.7 times higher than that of the substrate. Dry sliding friction and wear tests show that the wear resistance of the coating improves with the increase in Mo. The Mo1 coating has the lowest wear loss weight, wear volume and wear rate. In the 3.5 % NaCl solution, the polarization curves and the EIS fitting results show that the corrosion resistance of the coating improves with the addition of Mo elements. The hardness, wear and corrosion resistance of the 40Cr substrate is significantly improved due to the refractory high entropy alloy coating.

Phase evolution and mechanical properties of AlxCoCrFeNi high-entropy alloys by directed energy deposition

Directed energy deposition (DED), as an important branch of additive manufacturing (AM), has been widely used in the design and fabrication of high-entropy alloys (HEAs). Al is a common alloying element, it is critical to understand the effect of Al content on the phase evolution and mechanical properties of AlxCoCrFeNi HEAs. Here, a series of AlxCoCrFeNi (x = 0.2, 0.6 and 1.0) HEAs were manufactured by DED to investigate the alloying effect of Al on the phase evolution and mechanical properties. The phase transformation process was explored in conjunction with thermodynamic calculations. The results showed that the AlxCoCrFeNi HEAs transformed from single FCC phase (Al0.2) to FCC + BCC duplex phases (Al0.6), and then to single BCC phase (Al1.0) with the increase of Al content. Moreover, BCC phase further decomposed into the nanoscale two-phase structure containing ordered B2 phase and disordered A2 phase, which is beneficial to improve microhardness, yield strength and tensile strength. This work provides useful guidelines for the preparation and research of HEA systems with desirable properties.