Dual-phase High-entropy Alloys Research Articles

Impact response of metastable dual-phase high-entropy alloy Cr10Mn30Fe50Co10

Plate impact experiments are conducted on a metastable dual-phase high-entropy alloy (HEA) Cr10Mn30Fe50Co10 with different matrix grain sizes and phase fractions. Free surface velocity histories are obtained along with microstructure characterizations. Postmortem samples are characterized with transmission electron microscopy and electron backscatter diffraction. Multiple deformation mechanisms are observed, including dislocation slip and stacking fault in the face-center cubic (FCC) phase, dislocation slip and deformation twinning in the hexagonal close-packed (HCP) phase, and the FCC to HCP transformation. For spallation, damage is ductile in nature. Despite different microstructures, micro-voids prefer to nucleate at high-angle grain boundaries within the FCC phase, leading to similar spall strength. Upon shock loading, a dynamic plastic deformation partitioning behavior among the two phases is observed; at high shock stress, denser dislocations in the HCP phase result in increased fractions of voids around the FCC/HCP phase interface and within the HCP phase during spallation damage.

Atomistic insights on the deformation mechanisms of amorphous/crystalline dual-phase high entropy alloys under nanoindentation

The amorphous/crystalline (A/C) dual-phase nanostructured model is an effective method to improve the mechanical properties of high entropy alloys (HEAs). However, the deformation mechanism of the dual-phase HEAs during nanoindentation is still unclear. Here, the effect of the amorphous layer position and thickness on the deformation behavior and mechanical properties of the A/C dual-phase CoCrFeMnNi HEAs under nanoindentation is investigated by molecular dynamics simulation. The results show that the amorphous phase has a significant impact on the mechanical behavior of the HEAs, and the amorphous layer plays a role in absorbing dislocations and hindering their continued movement. The embedding position of the amorphous layer has almost no effect on the elastic deformation of the HEAs. However, during the plastic deformation stage, as the insertion position of the amorphous layer in the HEAs moves downwards, the nanoindentation force of the HEAs shows an obvious increasing trend, which promotes deformation strengthening. The results also indicate that the amorphous layer thickness affects its ability to absorb dislocations, and the dislocation lengths in the HEA with smaller amorphous phase thickness are significantly higher than those in the models with larger amorphous phase thickness.

High strain rate deformation behavior of Al0.65CoCrFe2Ni dual-phase high entropy alloy

An industrially-cast high entropy alloy with a nominal composition of Al0.65CoCrFe2Ni, having FCC and BCC phases, was compressed at high strain rates using a split Hopkinson pressure bar under a range of test temperatures. The strain rate sensitivity and strain hardening behaviour of the alloy were analyzed. The alloy exhibited high strain hardening and excellent strain rate sensitivity owing to its large solid solution strengthening and low athermal strength contributors. The alloy also displayed outstanding dynamic strain rate sensitivity compared to similar dual-phase high entropy alloys of the AlCoCrFeNi alloy system. The alloy exhibited excellent resistance to thermal softening at high strain rate loading due to higher phonon drag associated with an increase in the operating temperature. The substructures formed in the alloy during high strain rate deformation at room temperature comprised of Lomer-Cottrell (L-C) locks, deformation twins, planar deformation bands, and high dislocation pile-ups at interphase boundaries and subgrain boundaries. The high strain rate deformation at high temperatures resulted in the formation of dislocation cells, kinks and jogs, enabling the alloy to retain strain hardening behavior. The Johnson-Cook (J-C) model accurately predicted the plasticity behavior of the alloy under high strain rate loading.

Tuning microstructure via cold deformation and annealing for superb mechanical properties in Al0.5CoFeCrNiSi0.25 dual-phase high-entropy alloys

In the field of metal materials, deformation and annealing play an irreplaceable role in improving the microstructure and optimizing the properties. In this study, we prepared Al0.5CoFeCrNiSi0.25 dual-phase high-entropy alloys (DHEAs) by vacuum arc melting and investigated their microstructure evolution and mechanical properties at different rolling and annealing temperatures. The results showed that the volume ratio of the FCC phase remained largely unchanged with increasing annealing temperature, with only small recovery for 10% reduction alloy. In contrast, the volume fraction and recrystallization ratio of the FCC phase in a 40% reduction alloy increased, and its recrystallization rate was higher than that of the BCC phase. Annealing the alloys at 900 °C formed the FCC, BCC, σ, L12, and B2 phases. As the annealing temperature increased to 1100 °C, the lamellar structure was changed, and the L12 and σ phases dissolved, leading to the gradual increase in the spacing size and volume fraction of the FCC phase. Increasing the annealing temperature reduced the yield strength but enhanced the ductility of DHEAs. Annealing for 1 h at 900 °C after 40% cold rolling enhanced their strength to 1360.61 MPa due to the high dislocation density and presence of σ phases but led to poor ductility. Annealing for 1 h at 1100 °C after 40% cold rolling produced a good combination of tensile strength (∼1267.8 MPa) and ductility (uniform elongation of ∼34.4%). Such remarkable strength and ductility may be attributed to the increased volume fraction of the FCC phase and the dual-phase heterogeneous deformation induction strain hardening effect.

Microstructure and properties analysis of FeCoNiAlCu dual-phase high-entropy alloy coating by laser cladding

In this work, a new dual-phase high-entropy alloy (HEA) coating is successfully synthesized by laser cladding. XRD, SEM, TEM, EBSD, and TG-DSC are employed to investigate the microstructures and high-temperature stabilities. Simultaneously, under high-temperature friction and universal mechanical tester, the wear properties at 25 °C, 400 °C, and 800 °C and the mechanical properties at the compression speeds of 1 %, 2 %, and 3 % are explored. As indicated by the results, the FeCoNiAlCu HEA coating is composed of FCC phase with small angle grain boundaries and small size grains and BCC phase with HAGBs and large size grains. In terms of mechanical properties, FeCoNiAlCu HEA coating has good strength and toughness. When considering the fracture mechanism, it transfers gradually from a ductile fracture to a brittle fracture with increased compression speed. Besides, FeCoNiAlCu HEA coating maintains good stability at a high temperature of 764.2 °C. Meanwhile, the wear resistance and friction reduction of HEA coating are improved obviously at 800 °C. With the rise of temperature, the friction coefficient decreases gradually, the wear rate reduces, and the antifriction and wear resistance are enhanced.

Surface Integrity Analysis in Grinding of Dual-Phase High-Entropy Alloy

Abstract High-entropy alloys (HEAs) are highly anticipated because of their superb properties in strength, hardness, wear resistance, etc. However, compared with numerous studies on the design and properties of HEAs, the machinability research of HEAs is extremely rare, which limits the application of HEAs. In this work, grinding experiments of (FeCoNi)86Al7Ti7 dual-phase HEA workpieces were carried out, and the results are analyzed from a general machinability perspective (the machining parameters’ effect on grinding force and surface roughness) to a more in-depth perspective, including grinding-induced changes in morphology and microstructure on the ground surface and subsurface. With scanning electron microscope (SEM) and electron backscatter diffraction (EBSD) information of subsurface, the deformation mechanisms have been studied, including the role of the second-phase (Ni2AlTi) in the grinding process, the material removal modes of the different phases, and the morphology of the nanoprecipitates in the matrix, based on the completely opposite properties of different phases in HEA. It is noticed that the hard and brittle property of the second phase brings support to the material, reduces the plastic deformation, and also makes its own removal brittle, while the plastic matrix experiences shear deformation in grinding, which makes the nanoprecipitates in it assume different morphologies. These detailed findings could be of help to understand the effect of grinding on material properties so as to improve the machining quality of this material.

Hot deformation mechanisms of dual phase high entropy alloys

The microstructure of high entropy alloys can finally be designed via thermomechanical treatments to tune the mechanical properties. This work investigates the modification of the microstructure after treatments at 1100 °C for three hypo-eutectic high entropy alloys. Two phases were indexed according to the BCC and FCC crystal structures using electron backscattered diffraction. Their microstructure is investigated for three hot deformation tests: at a constant strain rate of 0.001s−1, at a strain rate jumps from 0.001s−1 to 1s−1 and from 1s−1 to 0.001s−1. The BCC size and fraction strongly influence the deformation of the FCC matrix. Due to its typical semi-interconnected hypo-eutectic structure, the BCC phase carries the load at the beginning of the deformation. Progressively, the FCC phase deforms to accommodate the plastic strain due to the bending and fragmentation of the BCC phase. Fine particles of the BCC phase are formed within the FCC matrix at high temperatures, and they pin the high-angle grain boundaries formed by continuous dynamic recrystallisation. The fragmentation of the BCC phase occurs faster for thinner eutectic BCC particles, and it is a consequence of I) the formation of boundaries during plastic deformation via dynamic recovery followed by continuous dynamic recrystallisation; II) the movement of phase boundaries consuming the formed boundaries within the BCC phase, fragmenting them. A fine substructure with a high density of high-angle grain boundaries is formed at 1100 °C for the alloy with initial fine BCC eutectic particles.

Probing plastic mechanisms in gradient dual-phase high-entropy alloys under nanoindentation

The plastic deformation mechanisms of gradient nanostructured high entropy alloys (HEAs) subjected to nanoindentation tests have been examined. First, we have constructed gradient glass/crystal (GGC) dual-phase HEAs by embedding crystalline CoCrFeNi HEA grains into CoCrFeNiAlx (x = 0.5, 1, and 2) metallic glass (MG) matrix in a gradient manner, leading to a gradient change in both the grain size and MG matrix shell thickness. Then, nanoindentation tests on the specimens under proper boundary and loading conditions are simulated by molecular dynamics software. It is found that the gradient-distributed MG shell effectively reduces the degree of strain localization within the region around the indenter, which results in a considerable hardening effect, thanks to the coexistence of both dislocation motion in the grain phase and plastic flow activated simultaneously in the MG phase. The results also show that the damage tolerance of GGC specimens is higher than that of the polycrystalline specimen because gradient-distributed MG phases can effectively prevent FCC to HCP transformation. Besides, the Al addition in the gradient-distributed MG shell can reduce its deformation propagation capability, weakening the hardening effect. The proposed method of constructing GGC can be promisingly helpful, for example, in the surface engineering of high-entropy alloys.

Development of wear-resistant dual-phase high-entropy alloys enhanced by C15 Laves phase

Development of wear-resistant dual-phase high-entropy alloys enhanced by C15 Laves phase

Solid-particle erosion of a dual-phase AlCoFeNi2 high-entropy alloy

Material removal due to solid-particle erosion results in mechanical failure of equipment and tremendous financial impact. To decrease the removal, materials with a good combination of high hardness and ductility are highly desirable. Dual-phase high-entropy alloys (HEAs) have the potential for impact resistance and have thus attracted attention from researchers. However, there is a lack of knowledge on phase-specific microstructural evolution under repeated particle impacts. In this study, an FCC (face-centred cubic) + B2 dual-phase AlCoFeNi2 HEA was fabricated, and its erosion resistance was tested under a particle velocity range of 97–230 m/s and incident angle range of 20 °-90 ° using nominally 50 μ m angular aluminum oxide particles. 316 stainless steel (SS) was tested under the same conditions as a benchmark. Characterization tools including an electron backscatter diffraction technique were applied to assess the microstructural evolution of the eroded surfaces and subsurface damage. The dual-phase HEA exhibited an approximately 10% lower erosion rate at oblique incidence (<40 °) than 316SS. The enhanced erosion resistance of the HEA was associated with the hard B2 phase that strengthened the AlCoFeNi2 and protected the matrix from cutting erosion while the soft FCC phase served as a plastic deformation carrier. Dynamic recrystallization with refined grains beneath the eroded surface was observed in the FCC phase after the erosion test. This study not only revealed the phase-specific microstructural evolution during erosion but also indicates that dual-phase HEAs are promising for use in erosive conditions.

Effect of solution and aging treatments on the microstructure and mechanical properties of dual-phase high-entropy alloy prepared by laser-powder bed fusion using AlSi10Mg and FeCoCrNi powders

Effect of solution and aging treatments on the microstructure and mechanical properties of dual-phase high-entropy alloy prepared by laser-powder bed fusion using AlSi10Mg and FeCoCrNi powders

Development of low-temperature impact-damage tolerant high entropy alloy with sequential multi-deformation mechanisms

Metals often lose their ductility at cryogenic temperatures owing to the decreased mobility of dislocations. TRansformation-induced plasticity (TRIP), a toughening mechanism at room temperature, can increase damage susceptibility at low temperatures, as the resultant martensite phases can become more brittle than the parent phases. Herein, we develop a high-entropy alloy (HEA) with an improved low-temperature impact-damage tolerance through a sequential plasticity mechanism. We design a trip-assisted dual-phase HEA (TADP HEA) and investigate the effects of Al addition on its mechanical properties upon deformation at different temperatures, depending on stacking fault energy (SFE). Our analysis shows that a senary (Cr20Mn6Fe34Co34Ni6)98Al2 HEA exhibits superior mechanical properties, including a 641 MPa yield strength (σy), 964 MPa ultimate tensile strength (σUTS), and 40% uniform elongation (ɛUTS) at ambient temperature (25 °C), and a 1 GPa σy, 1.5 GPa σUTS, and 36% ɛUTS at −100 °C. Notably, despite the presence of hexagonal-close packed martensite, the HEA exhibits a higher Charpy impact energy (406 J) than Cantor HEA (344 J) at −100 °C. We attribute this improvement to the sequential deformation mechanism of mechanical twinning and martensitic transformation in the HEA at −100 °C, which results in sustainable steady strain-hardening during deformation. We suggest that optimizing the sequential deformation mechanism by manipulating SFE in multi-component alloys can be an effective route for improving the damage tolerance of metals at cryogenic temperatures.

Realizing excellent tribological properties of BCC/FCC gradient high -entropy alloy coating via an in-situ interface reaction

The simultaneous improvement of strength and toughness has been a long-standing problem in the pursuit of high-performance materials. Using a gradient-structure design that exploits the migration and recrystallization processes of elements in the molten pool, this study generates an FeCoCrNiAl intermediate interfacial layer from FeCoCrNi and FeCoCrAl powders in situ. Further, a strong and tough integrated composite high–entropy alloy (HEA) coating with an outer layer of high-strength body centered–cubic (BCC) HEA, an interface of BCC and face centered–cubic (FCC) dual-phase HEA, and an inner layer of high-toughness FCC HEA is innovatively designed and prepared. The Vickers hardness of the composite exhibited a stepped gradient of 594.4Hv0.5, 311.8Hv0.5, and 182.9Hv0.5 from top to bottom. Subsequently, the surface and cross-sectional tribological behaviors of the gradient coating are studied in various air environments. The composite HEA coating overcomes the brittle fracture problem of single FeCoCrAl coatings and the adhesion problem of single FeCoCrNi coatings. The composite HEA coating exhibits a low friction coefficient, a small width/depth, and a low wear rate (0.374 × 10−5 mm3/(N·m)). The cross-section wear is a dynamic friction process coupled with multiple wear mechanisms, and the wear marks are characterized by ladders, width changes, and debris accumulation. This new approach can guide the future development and design of HEA wear-resistant gradient coatings and HEA composite coatings.

Effect of Manufacturing Conditions and Al Addition on Inclusion Characteristics in Co-Based Dual-Phase High Entropy Alloy

Three Co-based dual-phase high-entropy alloys (HEAs) were produced by different manufacturing conditions: arc-melting with Ar protection (Ar-HEA), vacuum induction melting in Al2O3 crucible (Cr-HEA) and vacuum induction melting with 0.5 at. pct Al (Al-HEA), which resulted in different levels of impurity elements and inclusion characteristics. The inclusions that precipitated in different HEA samples were investigated through an electrolytic extraction process and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) characterization. The results showed that Mn(S,Se) inclusions were presented in all three alloys. MnCr2O4 inclusions were presented only in Ar-HEA, pure Al2O3 inclusions were presented in Cr-HEA and Al-HEA, and Mn–Cr–Al–O inclusions were also found in Al-HEA. Thermodynamic calculation software FactSage and Thermo-calc were used to predict the inclusion formations of the HEAs, which showed a good agreement with the experimental findings. The stable inclusions can transform from MnCr2O4 to Mn(Cr,Al)2O4 and then to pure Al2O3 with the increase of Al content. The inclusions in Al-containing HEA are spinel or Al2O3 depending on the content levels of Al and O. It is proposed that the formation of spinel and Al2O3 inclusions can be avoided in liquid HEA when the O content is controlled to be very low, which can result in smaller-sized inclusions. Moreover, the calculated coagulation coefficient of spinel inclusions is close but lower than that of Al2O3 inclusions. The collision growth of inclusions was affected by a combination of physical parameters of HEA and inclusions as well as the inclusion size and amount.Graphical

A hypoeutectic high-entropy alloy with hierarchical microstructure for high-temperature application

Dual-phase high entropy alloys have great potential for high-temperature applications. In this work, a hypoeutectic Ni41.8Co19Cr10Fe10Al15Ti2Mo2Hf0.1B0.1 high-entropy superalloy consisting of FCC and B2 matrix was developed with superior high-temperature mechanical properties. After aging, high-density L12 particles precipitated in the FCC matrix, while lath-shaped FCC and Cr-rich σ phases precipitated in the B2 matrix. The hierarchical microstructure contributed to the high yield strength of ∼1.1 GPa MPa at 25 °C and ∼614 MPa at 800 °C. In addition, the B2 matrix with a serrated phase boundary retarded crack nucleation and propagation, leading to an excellent tensile elongation of ∼26.1% at 800 °C. These results are significant to the development of high-performance hypoeutectic alloys for application over a wide temperature range.

The microstructural evolution of dual phase high entropy alloy thin films by annealing

The thermal stability of the dual phase AlCrFeCoNiCu0.5 high entropy alloy thin films at up to 600 °C under vacuum was studied through detailed characterization. The evolution of microstructure and chemical composition was revealed by X-ray diffraction, transmission Kikuchi diffraction and transmission electron microscopy. Also, the mechanical properties were measured by nanoindentation. Although the crystalline phase of all the HEA thin films was BCC-dominated, the FCC phase fraction increased from 16.6% (as-deposited) to 28.1% (600 °C-annealed). The as-deposited films exhibited a columnar morphology with an average grain size of 106.1 nm for length and 31.3 nm for width and remained the columnar shape until 600 °C. The columnar grains showed an increasing tendency of size at 200 °C (221.3 nm and 45.6 nm for length and width) and 400 °C (283.9 nm and 56.2 nm). At 600 °C, the columnar grain size decreased to 211.8 nm in length and 59.3 nm in width. Concurrently, the occurrence of new equiaxed grains was observed with an average diameter of 42.4 nm. The reduction of columnar grain size and the formation of equiaxed grains at 600 °C were explained based on the energy dispersive spectroscopy. Although no preferential evaporation was found, the elemental distribution revealed the formation of two secondary phases, i.e., Cu-rich FCC phase and Cr-rich BCC phase. The nanoindentation results indicated that the mechanical properties increased as the increase of annealing temperature. The hardness and elastic modulus of 600 °C-annealed sample were 8.5 ± 0.3 GPa and 205.4 ± 3.5 GPa, respectively, which were 10.4% and 41.5% higher than those of the as-deposited thin film. These findings are critical for a fundamental understanding of the microstructure and properties change of the dual-phase HEA thin films in high working temperatures.

Additive manufacturing of a metastable high entropy alloy: Metastability engineered microstructural control via process variable driven elemental segregation

Compositional paradigm shift in high entropy alloys (HEAs) provided new opportunities for microstructural engineering, whereas process control during laser powder bed fusion (LPBF) additive manufacturing (AM) enable fine microstructure tailoring. Metastability engineering in transformation induced plasticity (TRIP) HEAs by addition of minor alloying elements is an attractive strategy for fine microstructural tuning. This study explored in detail the microstructural evolution during LPBF AM of a metastable Fe40Mn20Co20Cr15Si5 (at.%) dual phase HEA (CS-HEA). LPBF processing window for CS-HEA was established based on quantitative analysis and experiments. Based on melt pool overlap lack of fusion pores were observed at lower energy densities (J) of J ≤ 31.25 J/mm3 and key-hole formation by melt pool destabilization in case of J ≥ 75 J/mm3. The microstructure of CS-HEA consists of metastable FCC-γ and HCP-ε phases; LPBF process parameters governed the final phase fraction in the alloy which has been correlated to metastability alteration of the high temperature γ phase. Final microstructural engineering was devised by LPBF process control which enabled cooling rate manipulation to guide Mn and Si segregation at the cell boundaries, thereby controlling the matrix metastability and final phase fraction. Additionally, high resolution transmission electron microscopy (TEM) revealed disparity in stacking fault morphology in CS-HEA with LPBF process variable alterations associated with local variations in chemical composition and stacking fault energy. The phase evolution with process parameters also affected the nanomechanical behavior of the alloy.

High Temperature Deformation Behavior and Processing Maps of FeCoNiCrAlTi Dual Phase High Entropy Alloy

High Temperature Deformation Behavior and Processing Maps of FeCoNiCrAlTi Dual Phase High Entropy Alloy

Understanding the microstructure refinement and mechanical strengthening of dual-phase high entropy alloy during ultrasonic shot peening

The ultrastrong gradient nanostructured dual-phase high entropy alloy (DP-HEA) Fe50Mn30Co10Cr10 has been fabricated by an efficient surface nanocrystallization method, e.g. ultrasonic shot peeing (USP) at room temperature. We investigated the effect of the USP duration on the microstructure of the DP-HEA Fe50Mn30Co10Cr10 by the advanced materials characterization methods, e.g. Electron Backscatter Diffraction (EBSD), X-ray Diffraction (XRD), and Transmission Electron Microscopy (TEM). We found that the microstructure of the USPed DP-HEA Fe50Mn30Co10Cr10 can be refined into several nanometers. Additionally, the FCC γ phase is transferred into the HCP ε phase during the USP process of the DP-HEA Fe50Mn30Co10Cr10 and the portion of the HCP ε phase increases gradually with the increase of the USP duration. Nanoindentation tests indicate that the nanohardness of the nanostructured DP-HEA Fe50Mn30Co10Cr10 can reach as high as 7.49 GPa, suggesting a yield strength of 2.49 GPa, which is 10 times higher than that of the coarse-grained materials. Additionally, nanocrystallization of the DP-HEA Fe50Mn30Co10Cr10 can be triggered with a short peening duration and the transformation-induced plasticity (TRIP) effect can benefit the formation of a thick gradient nano-grained layer during USP. Micro-tensile tests revealed that the gradient nano-grained layer can significantly improve the yield strength of the DP-HEA Fe50Mn30Co10Cr10 with sacrificing ductility.

Microstructural stability and properties of Al1.2CrCuFeNi2 dual-phase high entropy alloy

The microstructural stability and mechanical properties of novel Al1.2CrCuFeNi2 dual-phase high entropy alloy (HEA) were investigated. The as-cast alloy was composed of a uniform near-eutectic structure consisting of Ni-Al rich B2 matrix and Cr-Fe rich BCC spherical phases (mean grain size: ∼100 nm). Annealing at 1000 °C results in the precipitation of ultrafine particles and transformation from spherical phase to needle shaped structure. The as-cast HEA has an excellent combination of super yield strength (∼1326 MPa), fracture strength (∼1762 MPa) and plastic strain (∼24 %). After annealing, the compressive yield strength and fracture strength increase by ∼ 18 % and ∼ 13 %, respectively. This is mainly ascribed to the precipitation strengthening of ultrafine particles and the significant crystal lattice distortion strengthening in HEA. Besides, Vickers hardness increased from 548 HV to 612 HV implying that the Al1.2CrCuFeNi2 HEA projects excellent tempering softening resistant.