Formation Of High-entropy Alloys Research Articles

The phase selection via machine learning in high entropy alloys

The phase formation of high entropy alloys (HEAs) is diversity due to the breadth of the composition space. Most of the studies about phase formation are based on features of solid solution. However, the establishment of the link between compositions and phase formation plays an important role in accelerating novel HEAs development. To achieve this aim, we employed the support vector machine (SVM) to build phase selection models with both the composition dataset and the thermodynamic parameters dataset. The accuracies of both models are above 85%. A dataset with more than a thousand data has been established, which covers 18 elements and most of the HEAs families. The feature dataset was transferred by calculation of thermodynamic parameters. The similar accuracies of the models with compositions dataset and with thermodynamic parameters proved that the prediction of phase formation in HEAs can be carried out directly from compositions. Our models provide a universal approach of phase formation prediction in HEAs.

Design of high-entropy alloys with a single solid-solution phase: Average properties vs. their variances

The empirical thermo-physical parameters for designing of high-entropy alloys (HEAs) with a single solid-solution phase (SSP) are based on the average properties of constituent elements, significant derivation from which might make them out of work. In the current work, both the important roles of average properties and their variances were studied. After an introduction of the variance of dimensionless enthalpy of mixing to characterize the chemical bond mismatch and a comparative study between two typical collections of HEAs to show the validity of previous designing rules, it was found that all the previous parameters to some extent fail to predict the formation of HEAs with a single SSP whereas the variance of dimensionless enthalpy of mixing is able to separate the regions of HEAs with a single SSP, with multi-phases and with an amorphous phase. Application to three typical HEA systems proved further that the lower value of the variance of dimensionless enthalpy of mixing corresponds to the higher possibility to form HEAs with a single SSP. The current work provides some necessary conditions for designing of HEAs, based on which preparation of potential HEAs with a single SSP could be accelerated.

Effects of Mutual Interaction between Constituent Elements on Phase Formation of High Entropy Alloys

High entropy alloys (HEAs) are alloys that contains multiple elements, often five or more principal elements in equiatomic or near equiatomic ratio with or without minor elements. As felt from the name, high mixing entropy is the sole parameter to decide the solid solution alloy formation in HEAs but, the mutual interaction between elements are key parameters to predict the phase formation in HEAs as proposed by some researchers.

Big-data analysis of phase-formation rules in high-entropy alloys

Big-data analysis of phase-formation rules of high-entropy alloys (HEAs) was conducted and a phase formation rule from a dynamic view was deduced. It was indicated in literatures that cooling rate has a strong influence on the phase formation of HEAs. Higher cooling rate may promote the generation of amorphous phase, and accordingly suppress the formation of intermetallics. Meanwhile, it was also shown that cooling rate had little impact on the formation of solid-solution phase. To demonstrate this rule, a series of FeCoNi(AlSiB)x HEAs ribbons were fabricated by a melt-spinning technique, and the microstructure, mechanical, and magnetic properties were also investigated. The results show that all ribbons exhibit disordered solid-solution structure. The addition of boron changes the alloy from ductility to brittleness, but without evident change of magnetic properties. The alloy in the nominal composition of FeCoNi(AlSi)0.2 has the best combination of mechanical and magnetic properties. A distinct feature of HEAs in magnetization was noticed and explained.

MnFeNiCuPt and MnFeNiCuCo high-entropy alloys designed based on L10 structure in Pettifor map for binary compounds

Quinary exact equi-atomic MnFeNiCuPt and MnFeNiCuCo alloys were investigated to examine their formation of high-entropy alloys (HEAs) by focusing on an L10 structure from Pettifor map for binary compounds with 1:1 stoichiometry. The MnFeNiCuPt alloy was practically selected through computer-assisted alloy design under conditions of ≤ 20 at% noble metals, and the condition that the L10 structure appears as frequently as possible in the constituent binary equi-atomic compositions comprised of 78 elements. MnFeNiCuCo was selected by substituting Pt with Co from the MnFeNiCuPt alloy as the second candidate. X-ray diffraction and observations by scanning electron microscopy (by energy dispersive spectroscopy for composition analysis) revealed that as-prepared MnFeNiCuPt and MnFeNiCuCo alloys were formed into HEAs with dual fcc structures containing dendrites of ∼10 μm in width. The MnFeNiCuPt and MnFeNiCuCo alloys annealed at 1373 K for 43.2 ks and subsequently quenched in water formed single fcc phases and dual fcc phases, respectively. The annealed MnFeNiCuPt and MnFeNiCuCo alloys were subsequently cooled in a furnace and formed single L12 ordered phases and dual fcc phases, respectively. These phases, experimentally observed in the annealed samples, could be partially explained by thermodynamic calculations using Thermo-Calc with SSOL4 and SSOL5 databases for solid solutions. The MnFeNiCuPt and MnFeNiCuCo alloys exhibit soft magnetism with saturation magnetization of 0.23 and 0.43 T, respectively, with coercivity values of ∼1 kA m−1. An alloy design for HEAs based on digitalized crystallographic data of these samples could lead to the discovery of new HEAs.

A criterion for the formation of high entropy alloys based on lattice distortion

Lattice distortion in high entropy alloys (HEAs) is one of their main crystallographic features. Its description is possible by means of unit cell parameter and bulk modulus variations of their constituent elements. The balance of forces acting on the lattice atoms under such distortion is related to the formation of a solid solution of a given crystal structure. This leads to the definition of a novel criterion for selecting HEA compositions. The main existing families of HEAs have been classified under this approach, in addition to an extensive list of multicomponent alloys including intermetallics and bulk metallic glasses. Criteria reported in the literature have been revised with the multicomponent alloy database used in this work which, together with the proposed approach, can be used to improve our understanding of HEAs formation.

Senary Refractory High-Entropy Alloy HfNbTaTiVZr

Discovery of new single-phase high-entropy alloys (HEAs) is important to understand HEA formation mechanisms. The present study reports computational design and experimental validation of a senary HEA, HfNbTaTiVZr, in a body-centered cubic structure. The phase diagrams and thermodynamic properties of this senary system were modeled using the CALPHAD method. Its atomic structure and diffusion constants were studied using ab initio molecular dynamics simulations. The microstructure of the as-cast HfNbTaTiVZr alloy was studied using X-ray diffraction and scanning electron microscopy, and the microsegregation in the as-cast state was found to qualitatively agree with the solidification predictions from CALPHAD. Supported by both simulation and experimental results, the HEA formation rules are discussed.

Prediction and validation of quaternary high entropy alloys using statistical approaches

Prediction of the crystalline structure formation of high entropy alloys is addressed in a novel way by applying principal components analysis to their thermodynamic and electronic parameters. In the simplest form, it shows an excellent discrimination between both face and body centred cubic structures when taking into account the valence electron concentration and enthalpy of mixing. Our analysis indicates that there is a stronger correlation between the formation of multiprincipal components alloys and these parameters than with entropy. The successful prediction of a multiphase structure in TiMnFeNi and the discovery of two novel four component HEAs, MnFeCoNi and TiVMnNb, lends credence to this approach.

Relative effect of electronegativity on formation of high entropy alloys

From the view point of chemical compatibility of elements, the electronegativity difference Δe was investigated as a characterisation parameter to evaluate the formation possibility of solid solution phases. The relationship between the formation of high entropy alloys (HEAs) and Δe indicates that a lower value of Δe is favourable to form solid solution phases. Based on the rule of topological stability and chemical compatibility, a δ − Δe plot was built, and four multicomponent alloys were designed. X-ray diffraction results show that three of the four designed alloys were fabricated as HEAs by arc melting and suction casting. The new HEA FeCoCuNiAgGe with the lowest value of Δe has typical dendrite and interdendrite microstructures, and the phase separation phenomenon is a common characteristic of the designed alloys containing Ag. Therefore, the application of Δe can explore the composition range for the development of new HEAs.

On the formation of disordered solid solutions in multi-component alloys

Abstract In high entropy alloys (HEA) a disordered solid solution (DSS) is entropically stabilized in competition with possible intermetallic compounds or phase separation. Ranges for many parameters (ΔRmax, ‘δ’, ΔHmix, ΔSmix, Ω) have been prescribed in literature for the formation of HEA. In the current investigation, we show that not withstanding the centrality of entropy in the stabilization of a DSS, other factors (parameters) like valence electron concentration, atomic size differences between the elements and enthalpy of mixing, play a significant role in the formation of phases. To illustrate the issues involved in the formation of DSS and to understand the phase formation, selected ternary (CoFeNi, CrFeNi), quaternary (CoCrFeNi, CuCoFeNi, AlCrFeNi) and penternary alloys (AlCuCoFeNi) are chosen for the experimental investigations and for the calculation of different parameters. It is shown that the ranges for the relevant parameters prescribed in literature are not satisfactory. It is possible that a (single) DSS can form, even if the entropy of mixing is small, provided all the relevant parameters are within favorable ranges. Contrastingly enough, violation of any of the parameters results in the formation of multiple phases or compounds. Strict and reduced definitions of HEA are invoked to understand the results.

Alloy Design and Properties Optimization of High-Entropy Alloys

This article reviews the recent work on the high-entropy alloys (HEAs) in our group and others. HEAs usually contain five or more elements, and thus, the phase diagram of HEAs is often not available to be used to design the alloys. We have proposed that the parameters of δ and Ω can be used to predict the phase formation of HEAs, namely Ω ≥ 1.1 and δ ≤ 6.6%, which are required to form solid-solution phases. To test this criterion, alloys of TiZrNbMoVx and CoCrFeNiAlNbx were prepared. Their microstructures mainly consist of simple body-centered cubic solid solutions at low Nb contents. TiZrNbMoVx alloys possess excellent mechanical properties. Bridgman solidification was also used to control the microstructure of the CoCrFeNiAl alloy, and its plasticity was improved to be about 30%. To our surprise, the CoCrFeNiAl HEAs exhibit no apparent ductile-to-brittle transition even when the temperatures are lowered from 298 K to 77 K.

Formation and Stability of Equiatomic and Nonequiatomic Nanocrystalline CuNiCoZnAlTi High-Entropy Alloys by Mechanical Alloying

Nanocrystalline equiatomic high-entropy alloys (HEAs) have been synthesized by mechanical alloying in the Cu-Ni-Co-Zn-Al-Ti system from the binary CuNi alloy to the hexanary CuNiCoZnAlTi alloy. An attempt also has been made to find the influence of nonequiatomic compositions on the HEA formation by varying the Cu content up to 50 at. pct (Cu x NiCoZnAlTi; x = 0, 8.33, 33.33, 49.98 at. pct). The phase formation and stability of mechanically alloyed powder at an elevated temperature (1073 K [800 °C] for 1 hour) were studied. The nanocrystalline equiatomic Cu-Ni-Co-Zn-Al-Ti alloys have a face-centered cubic (fcc) structure up to quinary compositions and have a body-centered cubic (bcc) structure in a hexanary alloy. In nonequiatomic alloys, bcc is the dominating phase in the alloys containing 0 and 8.33 at. pct of Cu, and the fcc phase was observed in alloys with 33.33 and 49.98 at. pct of Cu. The Vicker's bulk hardness and compressive strength of the equiatomic nanocrystalline hexanary CuNiCoZnAlTi HEA after hot isostatic pressing is 8.79 GPa, and the compressive strength is 2.76 GPa. The hardness of these HEAs is higher than most commercial hard facing alloys (e.g., Stellite, which is 4.94 GPa).