Alloy design for high-entropy alloys based on Pettifor map for binary compounds with 1:1 stoichiometry

Pettifor map for binary compounds with 1:1 stoichiometry was utilized as an alloy design for high-entropy alloys (HEAs) with exact or near equi-atomicity in multicomponent systems. Experiments started with selecting GuGd binary compound with CsCl structure from Pettifor map, followed by its extensions by selecting the binary compounds with the same CsCl structure to CuDyGdTbY equi-atomic quinary alloy and to Cu₄GdTbDyY and Ag₄GdTbDyY quinary alloys and Cu₂Ag₂GdTbDyY senary alloy in sequence. X-ray diffraction revealed that CuDyGdTbY alloy was formed into a HEA with mixture of bcc, fcc and hcp structures, whereas the Cu₂Ag₂GdTbDyY HEA was a single CsCl phase. The results suggest a potential of Pettifor map for the development of HEAs by utilizing its information of crystallographic structures. The further analysis was performed for composition diagrams of multicomponent systems corresponding to simplices in a high dimensional space. The present results revealed that a strategy of equi-mole of compounds instead of conventional equi-atomicity also works for the development of HEAs.     

An assessment on the future development of high-entropy alloys: Summary from a recent workshop

There is increasing interest in both relating the mechanical behavior of high-entropy alloys to their microstructural evolution and in their development for various applications. A special two-day international workshop on the above topic was held in Guiyang, China, in December 2014. The workshop gathered scientists and engineers to exchange information on recent progress in high-entropy alloys, to discuss the scientific issues and challenges to foster international collaborations, and to identify future directions. In this paper, a summary of this workshop is presented, including aspects of definition/terminology, phase formation, microstructure and phase stability, strengthening mechanisms, and high-temperature properties. Future research directions are also outlined.     

Dual HCP structures formed in senary ScYLaTiZrHf multi-principal-element alloy

A senary ScYLaTiZrHf alloy was investigated for its ability to form a solid solution with an hcp structure, also known as a high-entropy alloy (HEA). X-ray diffraction analysis of the ScYLaTiZrHf alloy produced by arc-melting method confirmed this hcp structure. The microstructure of the ScYLaTiZrHf was composed of dual phases that were enriched in (Y, La) or (Ti, Zr, Hf), with Sc distributed evenly across both phases. The positive mixing enthalpy of 11.4 kJ mol⁻¹ derived from Miedema's model, and the immiscible tendencies of its constituent binary phase diagrams help to explain the presence of dual phases in the ScYLaTiZrHf alloy. When formed into dual hcp solid solutions, the ScYLaTiZrHf alloy can be more accurately described as a multi-principal-element alloy (MPEA) rather than as a HEA, since the ScYLaTiZrHf alloy is a solid solution that is enthalpy-driven instead of entropy driven. The results also disclosed another procedure to stabilize solid solutions excepting for high-entropy scheme.     

Precipitation behavior and its effects on tensile properties of FeCoNiCr high-entropy alloys

In this work, we present a systematic study on the precipitation behavior and mechanical properties of a FeCoNiCr-based high-entropy alloy alloyed with dilute amounts of Ti and Al, (FeCoNiCr)_100-x-yTi_xAl_y (where x = 1–3, y = 4–9 at.%). It was found that, upon aging, nano-sized L1₂-Ni₃(Ti, Al) particles are formed within grains, whilst L2₁-(Ni, Co)₂TiAl Heusler particles are formed mainly along grain boundaries. The relative thermal stability of the two phases were studied at different aging temperatures (700–900 °C) with various durations of time (up to 48 h) and the results were directly compared with Thermo-calc calculations. Tensile tests were also conducted on alloys aged under different conditions. The measured properties, including strength and ductility, were correlated with the microstructure of aged (FeCoNiCr)_100-x-yTi_xAl_y alloys, with particular attention on the distribution and morphology of the two kinds of precipitate. Whereas both phases could contribute to the strengthening of the alloys via either Orowan bowing or particle shearing mechanism, the brittle (Ni, Co)₂TiAl Heusler phase was found to mainly affect the tensile plasticity. A simple composite model was proposed to describe the plastic strain of alloys. Based on observed microstructure and its corresponding mechanical performance, the alloy with the composition of (FeCoNiCr)₉₄Ti₂Al₄, when aged between 700 and 800 °C, gives the best balanced strength/ductility properties.     

Effects of electro-negativity on the stability of topologically close-packed phase in high entropy alloys

Topologically close-packed (TCP) phases with complex structures are often observed in high entropy alloys (HEAs). Currently, these TCP phases are garnering significant interest from both theoretical and experimental perspectives due to the ductility deterioration of the high strength HEAs. Alternatively, there are instances when TCP phases can actually benefit the mechanical performances of alloys, such as the wear resistance. Therefore, the stability of TCP phases should be taken into consideration in the alloy design. In this paper, the relationship between the TCP phase stability and the physicochemical/thermodynamic properties of alloying components in HEAs was systematically studied. The stability of TCP phases was found to correlate well with the electro-negativity difference (ΔX) for most of the reported HEAs. The stability of TCP phases is well delineated by the electro-negativity difference (ΔX): i.e., TCP phases are stable at ΔX > 0.133 except for some HEAs that contain a significant amount of aluminum.     

The general effect of atomic size misfit on glass formation in conventional and high-entropy alloys

It is a longstanding notion that atomic size misfit plays an important role with regard to glass formation in multi-component alloys. In the previous studies, this atomic size effect was commonly modeled as an “inclusion-in-matrix” problem and glass formation was usually linked to a threshold volume strain in “matrix” or solvent atoms. However, it becomes difficult to directly apply this approach to high entropy alloys, which are in lack of a clear distinction between solvent and solute atoms. With the simple geometric model we recently developed, here we show that glass formation in over two hundred glass-forming alloys, including conventional and high-entropy alloys, can be correlated with the excessive fluctuation in the intrinsic residual strains that result from the atomic size misfit. This interesting behavior suggests that, in most glass-forming multicomponent alloys hitherto reported, the atomic size effect acts with the chemistry effect to promote glass formation. Furthermore, our findings also imply that glass formation in multi-component alloys, regardless of their compositional complexity, may be rationalized with the Lindamann's criterion that was long established for the instability of crystalline lattices.     

Kinetics of phase separation in iron-based ternary alloys. II. Numerical simulation of phase separation in Fe–Cr–X(X=Mo, Cu) ternary alloys

Numerical simulations were performed using a model based on the Cahn–Hilliard equation in order to investigate asymptotic behavior of a minor element associated with phase decomposition of the major element in Fe–Cr–Mo and Fe–Cr–Cu ternary alloys. Bifurcation of peaks of Mo along peak tops of Cr concentration occurs in an Fe-40at.%Cr–5at.%Mo alloy at temperatures between 750 and 850 K. Bifurcation of peaks of Cr is also shown in an Fe–40at.%Mo–5at.%Cr alloy. The asymptotic behavior of Mo or Cr along a trajectory of a peak top of Cr or Mo concentration depends on the sign of the second derivative of the chemical free energy with respect to the concentrations of Cr and Mo. In an Fe–40at.%Cr–5at.%Cu alloy,the amplitude of Cu along peak tops of Cr concentration decreases with time, Simulated asymptotic behavior of Mo and Cr in Fe–Cr–Mo ternary alloys and Cu in an Fe–Cr–Cu alloy is in good agreement with that predicted by the theory proposed by the present authors.     

Effects of Nb additions on the microstructure and mechanical property of CoCrFeNi high-entropy alloys

A series of five-component CoCrFeNiNb_x high entropy alloys (HEAs) were synthesized to investigate alloying effects of the large atom Nb on the structure and tensile properties. Microstructures of these alloys were examined using scanning electron microscopy and the phase evolution was characterized and compared using the ΔH_mix–δ and ΔX criteria. It was found that the microstructure changes from the initial single face-centered cubic (FCC) to duplex FCC plus hexagonal close-packed (HCP) structure with additions of Nb. The current alloy system exhibits a hypoeutectic structure and the volume fraction of the Nb-enriched Laves phase with the HCP structure increases with increasing the Nb content, which is mainly responsible for the increment in the yield and fracture strength. Particularly, the Nb_0.155 alloy containing a 9.3% Nb-enriched Laves phase exhibits the most promising mechanical properties with the yield strength and plastic strain as high as 321 MPa and 21.3%, respectively. The ΔH_mix–δ criteria well describe the phase selection for the thermally treated alloys, while the physical parameter ΔX fails to predict the appearance of the Nb-enriched Laves phase in this alloy system.     

A geometrical parameter for the formation of disordered solid solutions in multi-component alloys

One or more disordered solid solutions (DSS) are entropically stabilized in high entropy alloys (HEA), in competition with possible intermetallic compounds or phase segregation. In spite of the supreme role of Gibbs free energy, various parameters have been used to understand the formation of DSS in multi-component alloys. These include, the δ-parameter (based on atomic size differences between the elements), the enthalpy of mixing (ΔH_mix) the Ω-parameter (T_mΔS_mix/|ΔH_mix|). These parameters have had different degrees of success in the context of understanding the formation of DSS in multi-component alloys. In the current work, we develop a purely geometrical parameter (Λ = ΔS_mix/δ²) to predict the formation of DSS. Ranges are prescribed for this parameter for the formation of: (a) DSS, (b) a mixture involving compounds and (c) (only) compound(s). Results from the literature are used to highlight the utility of the Λ-parameter, in the context of other standard approaches. The role of the value of the Λ-parameter in understanding the nature (complexity) and volume fraction of the compound formed is also highlighted.     

Effect of cryo-deformation on structure and properties of CoCrFeNiMn high-entropy alloy

Microstructure evolution in high-entropy alloy CoCrFeNiMn during plane-strain multipass rolling to a thickness strain of 80% at 293 and 77 K was studied. Deformation at both temperatures was found to be accompanied by twinning. At 77 K, twinning was more extensive in terms of the fraction of twinned grains and the length of the twinning stage thereby providing faster kinetics of the microstructure evolution. Micro-shear bands formed in the microstructure of the alloy at the late stages of rolling (at ε ≈ 80% at 293 K and ε ≈ 40% at 77 K). The ultimate tensile strength of specimens rolled at 77 K or 293 K was found to be 1500 or 1200 MPa, respectively while the strength in the initial homogenized condition was 440 MPa. The contribution of various mechanisms to the hardening of the alloy following rolling at 77 K and 293 K was analyzed quantitatively.     

Microstructure and mechanical performance of new Al0.5CrFe1.5MnNi0.5 high-entropy alloys improved by plasma nitriding

Effects of plasma nitriding at 525 °C on microstructure and mechanical performance of a brand-new Al_0.5CrFe_1.5MnNi_0.5 high-entropy alloy (HEA) were investigated. This alloy exhibits a large age hardening effect at temperatures from 600 to 800 °C and can be well-nitrided in the as-cast condition or the homogenized and furnace-cooled state. The nitrided layer has a thickness around 75 μm and a peak hardness level of Hv 1250 near the surface. The nitrided Al_0.5CrFe_1.5MnNi_0.5 alloys exhibit superior adhesive wear resistance to conventional nitrided steels by 25-54 times due to their much thicker highly-hardened layer and higher peak hardness than that of conventional steels.     

Effect of synthesis route on structure and properties of AlCoCrFeNi high-entropy alloy

A series of equatomic AlCoCrFeNi alloys prepared through various synthesis techniques is systematically investigated in the temperature range from 4 K to 1400 K. In this paper we consider their crystalline structure, microstructure, thermo-physical, electron-transport and magnetic properties. Phase transformations, Curie point and also the region for σ-phase existence are accurately determined in the system. A mixture of both body centered cubic (BCC) and face centered cubic (FCC) crystal structures is identified in all samples. Depending on synthesis conditions, BCC or FCC solid solution dominates that induces large variations in the properties of the alloys.     

Competition of XA and L21B ordering in Heusler alloys Mn2CoZ (Z = Al,Ga, Si,Ge and Sb) and its influence on electronic structure

Competition between the highly-ordered XA structure and disordered L2₁B structure in Heusler alloys Mn₂CoZ (Z = Al, Ga, Si, Ge, Sb) has been investigated. The relative stability of the two structures strongly depends on the main group element Z. When Z belongs to Al or Ga, the XA structure is stabler, but when Z belongs to Si, Ge or Sb, the L2₁B structure gains stability and is lower in energy. This is related to the different number of valence electrons in main group element Z, which influences the DOS structure near the Fermi level and changes N(E_F). The energy difference ΔE between the XA and L2₁B structures may be used to estimate the tendency to form L2₁B structure in different Heusler alloys qualitatively. A large negative ΔE is preferable to retard the A-C site disorder and retain the highly-ordered XA structure. That is just the case in Mn₂CoAl. A robust half-metallicity is observed in Mn₂CoSi, Mn₂CoGe and Mn₂CoSb, they are always half-metallic under either XA or L2₁B structure. But in Mn₂CoAl and Mn₂CoGa, their spin gapless semiconducting character will be destroyed and replaced by a half-metallic state if L2₁B disorder occurs. Finally, these results suggest that the L2₁B structure should be considered together with XA structure when discussing the electronic structure of “inverse” Heusler alloys.     

Diffusion kinetics,mechanical properties,and crystallographic characterization of intermetallic compounds in the Mg–Zn binary system

Pure Mg was diffusion bonded to pure Zn at 315 °C for 168 h to produce equilibrium intermetallic compounds of the Mg–Zn system. All equilibrium phases at 315 °C, Mg₂₁Zn₂₅, Mg₄Zn₇, MgZn₂, Mg₂Zn₁₁, were observed to develop. Concentration profiles by electron probe microanalysis, electron diffraction patterns by transmission electron microscopy, and load–displacement curves by nano-indentation were examined to characterize the phase constituents, crystal structure, diffusion kinetics, and mechanical properties. Mg₂₁Zn₂₅ with trigonal, Mg₄Zn₇ with monoclinic, and Mg₂Zn₁₁ with cubic structures were found and their lattice parameters were reported herein. Mg₄Zn₇ and Mg₂Zn₁₁ were observed to have a range of solubility of approximately 2.4 at% and 1.6 at%, respectively. Interdiffusion in MgZn₂ occurred most rapidly, was an order of magnitude slower in Mg₄Zn₇ and Mg₂Zn₁₁, and was the slowest in Mg₂₁Zn₂₅. Composition-dependence of interdiffusion within each intermetallic phase was negligible. The intermetallic phases exhibited insignificant creep, but evidence of discontinuous yielding was observed. The average hardness and reduced moduli were similar for Mg₂₁Zn₂₅, Mg₄Zn₇, and MgZn₂ phases, ∼5 GPa and ∼90 GPa, respectively. However, the Mg₂Zn₁₁ phase had lower hardness of 3.76 GPa and higher modulus of 108.9 GPa. The mechanical properties in the characterized intermetallic phases, exclusive of Mg₂₁Zn₂₅, were strongly concentration-dependent.     

Structural stability of some CsCl structure HfTM (TM = Co, Rh, Ru, Fe) compounds

In order to investigate Hf-TM (TM = Fe, Co, Rh, Ru) phase diagrams in the region of 50:50% atomic ratio, we performed ab initio Full-Potential Linearized Augmented Plane Waves calculations of the most stable Hf and TM elemental phases and HfTM compounds of the CsCl and CuAu structure types. The obtained electronic structures, cohesive energies and enthalpies of formation are discussed and compared to some of the existing models and available experimental data. The non-existing compound HfFe is found to be at least metastable, and the reason for its absence from the phase diagram is discussed.     

Thermodynamic modeling and composition design for the formation of Zr–Ti–Cu–Ni–Al high entropy bulk metallic glasses

This work explores the idea of predicting metallic glass forming composition in a multi component alloy for which an equilibrium phase diagram is yet to be deciphered. Deep eutectic regions in a quaternary alloy (Zr–Ti–Cu–Ni) have been extrapolated to the quinary Zr–Ti–Cu–Ni–Al system for designing a potential bulk glass forming composition. P_HSS parameter which is the product of mixing enthalpy, mismatch entropy and configurational entropy of an alloy, has been utilized for thermodynamic modeling. P_HSS values are computed through substitution of Al into the each of the fifteen quaternary eutectics that have been reported in the literature in the Zr–Ti–Cu–Ni system. A good correlation of P_HSS range between modeled alloys and established glass formers indicates the subtle efficacy of this method for high entropy amorphous alloy design through a rationale thermodynamic approach.     

Oxide morphology and adherence on dental alloys designed for porcelain bonding

The morphology and chemical makeup of oxides formed during degassing heat treatments on the surfaces of Ni-Cr and Co-Cr dental alloys designed for porcelain bonding were examined. A technique was developed for stripping intact oxides from these alloys for SEM examination of the oxide surface originally in contact with the metal. Information on the chemical composition of the films was obtained by energy dispersive X-ray spectroscopy. The poorly adherent oxides were found to be wrinkled, but the metal surfaces beneath these films were wrinkled correspondingly. The loss of oxide-metal contact appeared to be limited to the areas of localized void formation, predominantly in the Cr-rich phases. The undersurfaces of the strongly adherent oxides were found to be covered with minute protrusions of oxide. Large pegs of oxide were found to have extended into the NiBe intermetallic phase of these alloys. Beryllium appears to be the oxygen-active element responsible for peg formation.     

The use of diffusion multiples to examine the compositional dependence of phase stability and hardness of the Co-Cr-Fe-Mn-Ni high entropy alloy system

High entropy alloys (HEAs) or Multi-principal element alloys (MEAs) are a relatively new class of alloys. These alloys are defined as having at least five major alloying elements in atomic percent from 5% to 35%. There are hundreds of thousands of equiatomic compositions possible and only a fraction have been explored. This project examines diffusion multiples as a method to accelerate alloy development in these systems. Co-Cr-Fe-Mn-Ni quinary diffusion multiples were successfully created. Using these multiples, a quinary region of disordered of FCC was formed and examined using EDS and nanoindentation methods. From these techniques, maps of common HEA parameters (Ω, δ, ΔS_mix, ΔH_mix and Δχ) proposed in literature could be calculated and directly compared to observed phase stability. Similarly, hardness was examined as function of compositional complexity and atomic mismatch in the quinary disordered region in order to directly test the severe lattice distortion hypothesis. It was found that proposed HEA parameters were ineffective at single phase stability limits in the Co-Cr-Fe-Mn-Ni system. It was also observed that hardness did not correlate well to the maximum compositional complexity or to the maximum in atomic mismatch. This indicates the severe lattice distortion hypothesis is not the primary contributor to strengthening in the Co-Cr-Fe-Mn-Ni HEA system.