Single-phase High-entropy Alloys Research Articles

Combinatorial thin film screening to identify single-phase, non-equiatomic high entropy alloys in the MnFeCoNiCu system

Abstract

Prediction of Strength and Ductility in Partially Recrystallized CoCrFeNiTi0.2 High-Entropy Alloy.

The mechanical behavior of a partially recrystallized fcc-CoCrFeNiTi0.2 high entropy alloys (HEA) is investigated. Temporal evolutions of the morphology, size, and volume fraction of the nanoscaled L12-(Ni,Co)3Ti precipitates at 800 °C with various aging time were quantitatively evaluated. The ultimate tensile strength can be greatly improved to ~1200 MPa, accompanied with a tensile elongation of ~20% after precipitation. The temporal exponents for the average size and number density of precipitates reasonably conform the predictions by the PV model. A composite model was proposed to describe the plastic strain of the current HEA. As a consequence, the tensile strength and tensile elongation are well predicted, which is in accord with the experimental results. The present experiment provides a theoretical reference for the strengthening of partially recrystallized single-phase HEAs in the future.

Evaluating elastic properties of a body-centered cubic NbHfZrTi high-entropy alloy – A direct comparison between experiments and ab initio calculations

In this study, elastic constants of the equiatomic body-centered cubic NbHfZrTi high-entropy alloy (HEA) were experimentally evaluated using resonant ultrasound spectroscopy and compared directly with calculations based on density functional theory. Elastic properties, including Young's modulus, shear modulus, bulk modulus, Poisson's ratio, and Debye temperature, of polycrystalline aggregates were obtained from measurements and calculations, and excellent agreement was found between the experimental and theoretical data. We also made efforts to employ the rule-of-mixtures (ROM) to predict the elastic moduli of the current alloy, as well as other HEAs reported in the literature, and found that the lower-bound prediction provided a reasonable estimate of the elastic moduli of single-phase HEAs.

Ti2NiCoSnSb - a new half-Heusler type high-entropy alloy showing simultaneous increase in Seebeck coefficient and electrical conductivity for thermoelectric applications

A new single phase high entropy alloy, Ti2NiCoSnSb with half-Heusler (HH) structure is synthesized for the first time by vacuum arc melting (VAM) followed by ball-milling (BM). The BM step is necessary to obtain the single phase. Local electrode atom probe (LEAP) analysis showed that the elements are homogeneously and randomly distributed in the HH phase without any clustering tendency. When the BM was carried out for 1 hour on the VAM alloy, microcrystalline alloy is obtained with traces of Sn as secondary phase. When BM was carried out for 5 h, single HH phase formation is realized in nanocrystalline form. However, when the BM samples were subjected to Spark plasma sintering (SPS), secondary phases were formed by the decomposition of primary phase. Nanostructuring leads to simultaneous increase in S and σ with increasing temperature. The micro (1 h BM-SPS) and nanocrystalline (5 h BM-SPS) alloys exhibited a power factor (S2σ) of 0.57 and 1.02 mWm−1K−2, respectively, at 860 K. The microcrystalline sample had a total thermal conductivity similar to bulk TiNiSn sample. The nanocrystalline alloy exhibited a ZT of 0.047 at 860 K. The microcrystalline alloy showed a ZT to 0.144 at 860 K, in comparison to the nanocrystalline alloy.

Development of a Refractory Hexagonal Closed Packed High Entropy Alloy Based on Thin Film Screening

In order to identify candidate high entropy alloys (HEAs) that have the hexagonal closed packed crystal structure, gradient thin films in the OsRuWMoRe system were deposited by sputtering from multiple elemental targets onto Si substrates. In addition to having compositional gradients, the films exhibited regions with different phases, some of which were single-phase and non-equiatomic. Such alloys have the potential to exhibit properties superior to the primarily equiatomic HEAs that have been the focus of most work in this area. To screen the phases that exist across the thin film gradient samples, a range of characterization techniques were employed, including focused ion beam and scanning electron microscopy, X-ray energy dispersive spectroscopy, X-ray diffraction and electron backscattered diffraction analysis. The combinatorial method described in this study enabled the identification of a candidate single-phase HEA that was subsequently fabricated as a bulk alloy.

High-throughput solid solution strengthening characterization in high entropy alloys

While some high entropy alloys (HEAs) have been shown to display remarkable combinations of properties, exploration of the extensive multicomponent space by conventional methods is experimentally intractable. Thus, identifying and developing high-throughput screening methods is paramount to alloy design. Here, an experimental methodology is developed for rapid yield strength estimations of single-phase HEAs, which involves the production and testing of a compositionally-graded sample made by a diffusion-multiple approach. The sample is analyzed by a combination of nanoindentation and microstructural characterization, where the nanohardness results are analyzed by different conversion equations to determine yield strength. The values estimated by nanohardness agree with bulk tensile properties for a total of 8 compositions. Both are compared to a solid solution strengthening model, again yielding a good correlation. The experimental and simulation results indicate that, in this system, the strength is maximized when the atomic size mismatch is maximized. Furthermore, it is necessary to consider the strain hardening of these alloys to accurately estimate their strength by nanoindentation. A pathway is presented here. This work shows that high-throughput methodologies for predicting and measuring properties are promising for designing new HEAs with desirable combinations of properties.

Additively manufactured CoCrFeNiMn high-entropy alloy via pre-alloyed powder

An equiatomic CoCrFeNiMn high-entropy alloy (HEA) powder is produced by vacuum induction melting gas-atomization with argon gas. A detailed characterization reveals that gas atomized HEA powder is suitable for powder-bed-fusion additive manufacturing (PBFAM). Therefore, the powder with size ranging from 45 to 106μm is applied in one of PBFAM, electron beam melting (EBM). The final EBM part is strongly affected by the feedstock. The entrapped gas pores result in a high porosity of EBM-built parts, requesting extra efforts to eliminate the pores for processing parameters optimization. After the optimization, the highly dense EBM-built parts with a hierarchical microstructure and a strong 〈100〉 texture are obtained. The hierarchical microstructure displays long columnar grains containing intragranular cellular structure (dendrites) along build direction. Deformation mechanism of the EBM-built CoCrFeNiMn HEA is dominated by dislocation with limited contribution from mechanical twinning. Experimental results reveal that the cellular structure (dendrites) does not contribute the strength without the pre-existing surrounding dislocation network. As a result, EBM-built CoCrFeNiMn HEA parts exhibit comparable tensile properties to their conventional cast counterparts. It is suggested that the EBM process and gas atomized HEA powder can be employed for fabricating single-phase HEA parts with complex geometry and good mechanical properties.

High temperature, low neutron cross-section high-entropy alloys in the Nb-Ti-V-Zr system

High-entropy alloys (HEAs) with high melting points and low thermal neutron cross-section are promising new cladding materials for generation III+ and IV power reactors. In this study a recently developed high throughput computational screening tool Alloy Search and Predict (ASAP) has been used to identify the most likely candidate single-phase HEAs with low thermal neutron cross-section, from over a million four-element equimolar combinations. The selected NbTiVZr HEA was further studied by density functional theory (DFT) for moduli and lattice parameter, and by CALPHAD to predict phase formation with temperature. HEAs of NbTiVZrx (x = 0.5, 1, 2) were produced experimentally, with Zr varied as the dominant cross-section modifier. Contrary to previous experimental work, these HEAs were demonstrated to constitute a single-phase HEA system; a result obtained using a faster cooling rate following annealing at 1200 °C. However, the beta (BCC) matrix decomposed following aging at 700 °C, into a combination of nano-scale beta, alpha (HCP) and C15 Laves phases.

CALPHAD-aided development of quaternary multi-principal element refractory alloys based on NbTiZr

Equilibrium phase diagrams for ten MeX(NbTiZr)100-X alloy systems, where Me is Al, Cr, Fe, Hf, Mo, Re, Si, Ta, V or W and X ranges from 0 to 25 at% were calculated using the PanNb2018a database recently developed by CompuTherm, LLC. By the type of phases and sequence of formation, these alloy systems can be divided into two groups. The first group of quaternary alloys, containing Al, Hf, Mo, Ta, or V has a single-phase BCC region below the solidus line over the entire X range and can be considered as potential candidates for the development of single-phase high entropy alloys. The second group consists of the quaternary alloy systems in which the fourth element (Cr, Fe, Re, Si or W) has limited solubility in BCC NbTiZr, which leads to the formation of an additional phase below the solidus and above the solubility limit. These quaternary alloy systems can be used for the development of precipitation or dispersoid strengthened complex concentrated alloys. To verify CALPHAD calculations three representative alloys, Cr10Nb30Ti30Zr30, Ta10Nb30Ti30Zr30 and Re10Nb30Ti30Zr30, were prepared by arc melting. The alloy densities were 6.56, 7.81 and 7.85 g/cm3, respectively. The phase compositions of the produced alloys agreed satisfactorily with CALPHAD calculations. Mechanical properties were also studied and compared with those of NbTiZr. At room temperature (RT) all the alloys showed high hardness exceeding 350 Hv and high compression yield stress exceeding 1000 MPa. RT compression ductility of Re and Ta containing alloys was above 50%, but Cr-containing alloy showed low ductility of 5%. With an increase in temperature ≥800 °C, compression strength decreased more rapidly for the Cr-containing alloy, which at 1200 °C became softer than NbTiZr. The least temperature dependence of the strength was observed for the Ta-containing alloy, which became the strongest at 1200 °C.

Effect of Atomic Size Difference on the Microstructure and Mechanical Properties of High-Entropy Alloys.

The effects of atomic size difference on the microstructure and mechanical properties of single face-centered cubic (FCC) phase high-entropy alloys are studied. Single FCC phase high-entropy alloys, namely, CoCrFeMnNi, Al0.2CoCrFeMnNi, and Al0.3CoCrCu0.3FeNi, display good workability. The recrystallization and grain growth rates are compared during annealing. Adding Al with 0.2 molar ratio into CoCrFeMnNi retains the single FCC phase. Its atomic size difference increases from 1.18% to 2.77%, and the activation energy of grain growth becomes larger than that of CoCrFeMnNi. The as-homogenized state of Al0.3CoCrCu0.3FeNi high-entropy alloy becomes a single FCC structure. Its atomic size difference is 3.65%, and the grain growth activation energy is the largest among these three kinds of single-phase high-entropy alloys. At ambient temperature, the mechanical properties of Al0.3CoCrCu0.3FeNi are better than those of CoCrFeMnNi because of high lattice distortion and high solid solution hardening.

Thermal Stability of the HfNbTiVZr High-Entropy Alloy.

The multicomponent alloy HfNbTiVZr has been described as a single-phase high-entropy alloy (HEA) in the literature, although some authors have reported that additional phases can form during annealing. The thermal stability of this alloy has therefore been investigated with a combination of experimental annealing studies and thermodynamic calculations using the CALPHAD approach. The thermodynamic calculations show that a single-phase HEA is stable above about 830 °C. At lower temperatures, the most stable state is a phase mixture of bcc, hcp, and a cubic C15 Laves phase. Annealing experiments followed by quenching confirm the results from thermodynamic calculations with the exception of the Laves phase structure, which was identified as a hexagonal C14 type instead of the cubic C15 type. Limitations of the applied CALPHAD thermodynamic description of the system could be an explanation for this discrepancy. As-synthesized HfNbTiVZr alloys prepared by arc-melting form a single-phase bcc HEA at room temperature. In situ annealing studies of this alloy show that additional phases start to form above 600 °C. This indicates that the observed HEA is metastable at room temperature and stabilized by a slow kinetics during cooling. X-ray diffraction analyses using different cooling rates and annealing times show that the phase transformations in this HEA are slow and that completely different phase compositions can be obtained depending on the annealing procedure. In addition, it has been shown that the sample preparation method (mortar grinding, heat treatment, etc.) has a significant influence on the collected diffraction patterns and therefore on the phase identification and analysis.

Novel metastable engineering in single-phase high-entropy alloy

To improve the mechanical properties of high-entropy alloys (HEAs) and expand the application range of metastable engineering, the NbZrTiTa alloy was researched. The results show that this alloy exhibits uniform element distribution and a metastable single-phase body-centered cubic (BCC) structure. During loading, element diffusion occurs, and then the TiZr-rich and TaNb-rich regions form. The increased Ti and Zr content reduces the stability of the BCC structure and leads to in-situ structure transformation in the TiZr-rich region. Element diffusion and structure transformation improve ductility by absorbing the loading work and releasing internal stresses. Furthermore, interface strengthening caused by the formation of the dual-phase region and the coherent nano-precipitation due to the compositional fluctuations together enhance the strength. The co-contribution of various metastable-induced strengthening and toughening mechanisms distinguishes the strength and ductility of the single-phase NbZrTiTa HEA from those of all the reported refractory systems. More importantly, the successful utilization of the novel metastable engineering induced by element diffusion in single-phase HEA provides a useful guide to design HEAs and other structural materials.

Design of single-phase high-entropy alloys composed of low thermal neutron absorption cross-section elements for nuclear power plant application

Two quinary high-entropy alloys MoNbCrVTi and MoNbCrZrTi were designed and prepared for the two following goals. The first goal is to search for single-phase high-entropy alloys composed of low thermal neutron absorption cross-section elements, and the second goal is to verify the validity of the empirical parameters calculation and CALPHAD (acronym of CALculation of PHAse Diagrams) calculation in the initiatory selection process of desired alloys from numerous candidates. The density, hardness, microstructure, and compressive mechanical properties of the two alloys were preliminarily investigated, and the phase formation of these two alloys was also discussed. The MoNbCrVTi alloy consists of a single BCC phase with typical dendritic microstructure, while the MoNbCrZrTi alloy mainly consists of a BCC phase plus a C15 Laves phase. The densities of the as-cast MoNbCrVTi and MoNbCrZrTi alloys were determined to be 7.30 ± 0.01 and 7.33 ± 0.01 g/cm3, respectively. The MoNbCrVTi alloy exhibits a hardness of 494.4 ± 7.7 Hv, a high yield strength of 1281 MPa and a fracture strain of 9.4% at room temperature. By comparison, the MoNbCrZrTi alloy exhibits a hardness of 552.9 ± 8.6 Hv, a higher yield strength of 1454 MPa but a lower fracture strain of 2.7%. The proposed two goals are basically achieved based on the preliminary results.

Phase stability of single phase Al0.12CrNiFeCo high entropy alloy upon irradiation

Phase stability of NiCoFeCr and Al0.12NiCoFeCr single phase high entropy alloys (HEAs) was studied under 3 MeV Ni2+ irradiation at 500 °C to a fluence of 1 × 1017/cm2 reaching a peak dose of ~100 displacements per atom (dpa). Transmission electron microscopy (TEM) diffraction pattern and scanning transmission electron microscopy-energy dispersive X-Ray spectroscopy (STEM-EDS) were utilized to detect any second phases that formed in the irradiated regions of the specimens. While the single phase of NiCoFeCr alloy has remained stable under the irradiation, the phase in Al0.12NiCoFeCr alloy decomposed into FCC matrix phase, and Ni3Al type nanoprecipitates with L12 ordered structure. On the other hand, in Al0.12NiCoFeCr alloy, the non-irradiated region of the TEM specimen (annealed for 10 h at 500 °C) and the as-cast alloy annealed for 490 h at 500 °C exhibited no such precipitates similar to those observed in the irradiated region. Based upon the thermodynamic stability of these alloys, we propose that radiation-enhanced diffusion under the irradiation accelerated the formation of the Ni3Al type equilibrium phase. Our study clearly demonstrates that the phase stability of HEAs under irradiation should be considered as one of the deciding factors in the materials design and selection for nuclear applications.

Microstructural origins for a strong and ductile Al0.1CoCrFeNi high-entropy alloy with ultrafine grains

A single-phase Al0.1CoCrFeNi high-entropy alloy (HEA) with face-centered cubic structure was subjected to cryo-rolling at the liquid N2 temperature and subsequent annealing. Microstructural characterization of post-annealed samples by electron backscattered diffraction and transmission electron microscopy revealed that, the dominant grain boundaries (GBs) are of Σ3 type low energy twin boundaries owing to the low stacking fault energy of the HEA. The resulting ultrafine-grained single-phase HEA with abundant Σ3 twin boundaries shows a high strength above 1.0 GPa and a tensile strain larger than 20%. The quantitative analysis on the grain size dependence of strength suggests that a high lattice friction stress and a high GB strengthening via low energy Σ3 twin boundaries are two major contributions for the excellent strength-ductility balance of the ultra-fine grained HEA.

Nano-/Ultrafine Eutectic in CoCrFeNi(Nb/Ta) High-Entropy Alloys

The development of single-phase high-entropy alloys (HEAs) emerges as a new paradigm shift in material research society during the last decade. A strong rise in the demand for structural and functional applications leads to the design and fabrication of super-strong eutectic HEAs through solidification processing. We report on the evolution of eutectic microstructure in CoCrFeNiNb0.5 and CoCrFeNiTa0.4 HEAs synthesized by arc melting. The evolved microstructure consists of nanolamellae of FCC γ-Ni and hexagonal β (Fe2Nb or Co2Ta type) Laves phases of 150 nm lamellae thickness. These alloys exhibit high yield strength of 2 GPa, ultimate compressive strength up to 2.2 GPa and 20% plastic strain. The strain rate jump test and transmission electron microscopic studies of deformed specimens have been performed to explore the microscopic mechanism of deformation in these high-strength advanced eutectic alloys.

Entropy Determination of Single-Phase High Entropy Alloys with Different Crystal Structures over a Wide Temperature Range.

We determined the entropy of high entropy alloys by investigating single-crystalline nickel and five high entropy alloys: two fcc-alloys, two bcc-alloys and one hcp-alloy. Since the configurational entropy of these single-phase alloys differs from alloys using a base element, it is important to quantify the entropy. Using differential scanning calorimetry, cp-measurements are carried out from −170 °C to the materials’ solidus temperatures TS. From these experiments, we determined the thermal entropy and compared it to the configurational entropy for each of the studied alloys. We applied the rule of mixture to predict molar heat capacities of the alloys at room temperature, which were in good agreement with the Dulong-Petit law. The molar heat capacity of the studied alloys was about three times the universal gas constant, hence the thermal entropy was the major contribution to total entropy. The configurational entropy, due to the chemical composition and number of components, contributes less on the absolute scale. Thermal entropy has approximately equal values for all alloys tested by DSC, while the crystal structure shows a small effect in their order. Finally, the contributions of entropy and enthalpy to the Gibbs free energy was calculated and examined and it was found that the stabilization of the solid solution phase in high entropy alloys was mostly caused by increased configurational entropy.

Lattice distortion in a strong and ductile refractory high-entropy alloy

The maximization of the mixing entropy with the optimal range of enthalpy in high-entropy alloys (HEAs) can promote the formation of a stable single solid-solution phase with the absence of competing intermetallic compounds. The resultant effects, such as lattice distortion, can contribute to excellent mechanical properties, which has motivated numerous efforts to develop and design single-phase HEAs. However, challenges still remain, particularly on quantifying the lattice distortion and relating it to materials properties. In this study, we have developed a NbTaTiV refractory HEA with a single body-centered-cubic (BCC) structure using an integrated experimental and theoretical approach. The theoretical efforts include thermodynamic modeling, i.e., CALculation of PHAse Diagram (CALPHAD). The microstructural evolutions have been investigated by systematic heat-treatment processes. The typical dendrite microstructure was observed, which is caused by the elemental segregation during the solidification in the as-cast condition. The structural inhomogeneity and chemical segregation were completely eliminated by the proper homogenization treatment at 1200 °C for 3 days. The homogeneous elemental distribution was quantitatively verified by the Atom Probe Tomography (APT) technique. Importantly, results indicate that this HEA exhibits the high yield strength and ductility at both room and high temperatures (up to 900 °C). Furthermore, the effects of the high mixing entropy on the mechanical properties are discussed and quantified in terms of lattice distortions and interatomic interactions of the NbTaTiV HEA via first-principles calculations. It is found that the local severe lattice distortions are induced, due to the atomic interactions and atomic-size mismatch in the homogenization-treated NbTaTiV refractory HEA.

Entropy contributions to phase stability in binary random solid solutions

High entropy alloys contain multiple elements in large proportions that make them prone to phase separation. These alloys generally have shallow enthalpy of mixing which makes the entropy contributions of similar magnitude. As a result, the phase stability of these alloys is equally dependent on enthalpy and entropy of mixing and understanding the individual contribution of thermodynamic properties is critical. In the overall vision of designing high entropy alloys, in this work, using density functional theory calculations, we elucidate the contributions of various entropies, i.e., vibrational, electronic and configurational towards the phase stability of binary alloys. We show that the contribution of electronic entropy is very small compared to the vibrational and configurational entropies, and does not play a significant role in the phase stability of alloys. The configurational and vibrational entropies can either destabilize or can collectively contribute to stabilize the solid solutions. As a result, even those systems that have negative mixing enthalpy can show phase instability, revealed as a miscibility gap; conversely, systems with positive mixing enthalpy can be phase stable due to entropic contributions. We suggest that including entropic contributions are critical in the development of theoretical framework for the computational prediction of stable, single-phase high entropy alloys that have shallow mixing enthalpies, unlike ordered intermetallics.

Scandium-Based Hexagonal-Closed Packed Multi-Component Alloys

Since their early development, High-Entropy Alloys have fueled the investigation of exotic metal combinations. Here, we present a strategy for the rational design of a library for multi-component alloys based on six hcp-structured metals. Seven five- and six-component equimolar alloys based on Co, Gd, Y, Sc, Ti and Zr were prepared via induction melting and characterized by PXRD, SEM–EDX and Vickers hardness. They all present ternary hexagonal phases (ScTiZr or GdScY) co-existing with one or more cubic phases and intermetallic compounds. Both ScTiZr and GdScY appear promising as the starting point for new single-phase High-Entropy Alloys families.