High Mixing Entropy Research Articles

Formation and Disruption of W-Phase in High-Entropy Alloys

High-entropy alloys (HEAs) are single-phase systems prepared from equimolar or near-equimolar concentrations of at least five principal elements. The combination of high mixing entropy, severe lattice distortion, sluggish diffusion and cocktail effect favours the formation of simple phases—usually a bcc or fcc matrix with minor inclusions of ordered binary intermetallics. HEAs have been proposed for applications in which high temperature stability (including mechanical and chemical stability under high temperature and high mechanical impact) is required. On the other hand, the major challenge to overcome for HEAs to become commercially attractive is the achievement of lightweight alloys of extreme hardness and low brittleness. The multicomponent AlCrCuScTi alloy was prepared and characterized using powder X-ray diffraction (PXRD), scanning-electron microscope (SEM) and atomic-force microscope equipped with scanning Kelvin probe (AFM/SKP) techniques. Results show that the formation of complex multicomponent ternary intermetallic compounds upon heating plays a key role in phase evolution. The formation and degradation of W-phase, Al2Cu3Sc, in the AlCrCuScTi alloy plays a crucial role in its properties and stability. Analysis of as-melted and annealed alloy suggests that the W-phase is favoured kinetically, but thermodynamically unstable. The disruption of the W-phase in the alloy matrix has a positive effect on hardness (890 HV), density (4.83 g·cm−3) and crack propagation. The hardness/density ratio obtained for this alloy shows a record value in comparison with ordinary heavy refractory HEAs.

不锈钢表面FeCoCrAlCuNiMo x 激光高熵合金化层的相演变

High-entropy alloys (HEAs), defined as solid solution alloys which have at least 5 principal elements but no more than 13 elements, with concentrations of each principal element ranging from 5% to 35% in atomic fraction, are emerging as one of the hot research frontiers in the metallic materials field. The significance of HEAs originates from their various combinations of high strength, good thermal stability and excellent resistance to corrosion, wear and oxidation. HEAs exhibit simple solid solutions with bcc and/or fcc structure(s) due to the effect of high mixing entropy in the solid solution state of HEAs, which may make the HEAs with improved mechanical and physical properties. However, a small quantity of intermetallic compounds can also form in certain HEAs, indicating that the formation of simple solid solutions cannot solely depend on the high mixing entropy. Then, the theory of HEAs based on the concept of entropy-enthalpy competition to judge whether or not simple phases will form was proposed. However, even if an alloy meets these criterions, it can still contain intermetallic phases. Why *国家自然科学基金项目51271126和沈阳市科技计划项目F16-032-0-00资助 收到初稿日期: 2016-01-02,收到修改稿日期: 2016-02-20 作者简介:吴臣亮,男, 1989年生,博士生 DOI: 10.11900/0412.1961.2016.00004

Microstructure and strengthening mechanisms in an FCC structured single-phase nanocrystalline Co25Ni25Fe25Al7.5Cu17.5 high-entropy alloy

We report on a study of the design, phase formation, microstructure, mechanical behavior and strengthening mechanisms of a novel single-phase Co25Ni25Fe25Al7.5Cu17.5 (at.%) high-entropy alloy (HEA). In this investigation, a bulk nanocrystalline (nc) Co25Ni25Fe25Al7.5Cu17.5 HEA with the face-centered cubic (FCC) crystal structure was fabricated by mechanical alloying (MA) followed by consolidation via spark plasma sintering (SPS). The X-ray diffraction (XRD) and transmission electron microscopy (TEM) results revealed that a single FCC solid-solution phase with an average grain diameter of 24 nm was produced following MA. Following SPS, bulk samples exhibiting a bimodal microstructure with both nanoscale grains and ultra-fine grains (UFGs) and with an average grain diameter of 95 nm were obtained, possessing a single FCC solid-solution phase identical to that in the milled powders. The single-phase feature of the Co25Ni25Fe25Al7.5Cu17.5 HEA principally resulted from remarkably high mutual solubility in most binary atom-pairs of the constituent elements, which appears to correspond to a high entropy of mixing. Approximately 5 vol.% of nanoscale twins were observed in the bulk nc samples. The bulk nc Co25Ni25Fe25Al7.5Cu17.5 HEA exhibits a compressive yield strength of 1795 MPa with a hardness of 454 Hv, which is dramatically higher than the yield strength of most previously reported FCC structured HEAs (∼130–700 MPa). Compared to those of the bulk coarse-grained (CG) Co25Ni25Fe25Al7.5Cu17.5 HEA fabricated by arc-melting, the yield strength and Vickers hardness values of the bulk nc samples increased by 834.9% and 251.9%, respectively. Quantitative calculations of the respective contributions from each strengthening mechanism demonstrate that grain boundary strengthening and dislocation strengthening are principally responsible for the measured ultra-high strength of the bulk nc Co25Ni25Fe25Al7.5Cu17.5 HEA.

Onset of Cooperative Dynamics in an Equilibrium Glass-Forming Metallic Liquid.

Onset of cooperative dynamics has been observed in many molecular liquids, colloids, and granular materials in the metastable regime on approaching their respective glass or jamming transition points, and is considered to play a significant role in the emergence of the slow dynamics. However, the nature of such dynamical cooperativity remains elusive in multicomponent metallic liquids characterized by complex many-body interactions and high mixing entropy. Herein, we report evidence of onset of cooperative dynamics in an equilibrium glass-forming metallic liquid (LM601: Zr51Cu36Ni4Al9). This is revealed by deviation of the mean effective diffusion coefficient from its high-temperature Arrhenius behavior below TA ≈ 1300 K, i.e., a crossover from uncorrelated dynamics above TA to landscape-influenced correlated dynamics below TA. Furthermore, the onset/crossover temperature TA in such a multicomponent bulk metallic glass-forming liquid is observed at approximately twice of its calorimetric glass transition temperature (Tg ≈ 697 K) and in its stable liquid phase, unlike many molecular liquids.

A hexagonal close-packed high-entropy alloy: The effect of entropy

The formation of disordered solid solution in the hexagonal close-packed (hcp) structure in the GdHoLaTbY alloy and its mechanical properties were investigated in this study. The single hcp phase of the alloy in the as-cast state was confirmed by X-ray diffraction and scanning electron microscopy analyses. The compressive yield strength, fracture strength, and plastic strain of the alloy are 108MPa, 880MPa, and 21.8%, respectively, and the Vickers hardness is 96HV. The results show that the yield strength, fracture strength, and hardness of the alloy obey the rule of mixture, which indicates that there is no hardening effect from entropy. Although the high entropy of mixing stabilizes the solid solution against intermetallic compounds, lack of severe lattice distortion from elastic strain or electronic interactions between principal elements impacts little on the mechanical properties.

A Brief Review of High Entropy Alloys and Serration Behavior and Flow Units

Multicomponent alloys with high entropy of mixing, e.g., high entropy alloys (HEAs) and/or multiprincipal-element alloys (MEAs), are attracting increasing attentions, because the materials with novel properties are being developed, based on the design strategy of the equiatomic ratio, multicomponent, and high entropy of mixing in their liquid or random solution state. Recently, HEAs with the ultrahigh strength and fracture toughness, excellent magnetic properties, high fatigue, wear and corrosion resistance, great phase stability/high resistance to heat-softening behavior, sluggish diffusion effects, and potential superconductivity, etc., were developed. The HEAs can even have very high irradiation resistance and may have some self-healing effects, and can potentially be used as the first wall and nuclear fuel cladding materials. Serration behaviors and flow units are powerful methods to understand the plastic deformation or fracture of materials. The methods have been successfully used to study the plasticity of amorphous alloys (also bulk metallic glasses, BMGs). The flow units are proposed as: free volumes, shear transition zones (STZs), tension-transition zones (TTZs), liquid-like regions, soft regions or soft spots, etc. The flow units in the crystalline alloys are usually dislocations, which may interact with the solute atoms, interstitial types, or substitution types. Moreover, the flow units often change with the testing temperatures and loading strain rates, e.g., at the low temperature and high strain rate, plastic deformation will be carried out by the flow unit of twinning, and at high temperatures, the grain boundary will be the weak area, and play as the flow unit. The serration shapes are related to the types of flow units, and the serration behavior can be analyzed using the power law and modified power law.

SURFACE LAYER HIGH-ENTROPY STRUCTURE AND ANTI-CORROSION PERFORMANCE OF AERO-ALUMINUM ALLOY INDUCED BY LASER SHOCK PROCESSING

7075 aluminum alloy is an ultra-high strength alloy containing Al, Zn, Mg, Cu and Cr elements,and is widely used in the aviation industry, but it has severe intergranular corrosion characteristics. The high-entropy alloys are composed of more than five major metallic elements and possess excellent corrosion resistance.When laser shock, featuring ultra high energy as well as the thermodynamic and kinetic loading characteristics farfrom-equilibrium states, acts on the surface of alloys with multiple elements, high-entropy alloy surface layer with specific properties may be obtained. In this work, surface modification of 7075-T76 aluminum alloy by laser shock was investigated. The microstructure, formation cause of the amorphous/nano- crystalline composite high- entropy alloy surface layer obtained by laser shock, hardness and corrosion resistance of the laser were analyzed by means of SEM and TEM. The results show that the adiabatic shear thermal effect induced by super high energy, ultra-fast process of laser shock causes surface alloy system to occur entropy increase effect and partitioning. The high mixing entropy contributes to the randomization increase of the alloy system. Thus, the elements in the system spontaneously self- organize in accordance with the law of Boltzmann. The dynamical formation of the nano- crystalline grains coordinates the thermodynamic equilibrium during the process. The strain- hardened layer is composed of amorphous microstructure and nanocrystalline grains, and the total depth of it reaches up to about 100 μm. After 1time laser shock,the depth of the surface high entropy layer is about 20 μm, of which the diameter of the nanocrystalline grains is 6~8 nm. After 3 times laser shock, the thickness of the layer can increase to more than 40 μm, and the diameter of the nanocrystalline grains is 2~3 nm. Meanwhile, the intense ultra high strain-rate induced by the laser shock makes precipitates deform, producing parallelly distribution of deformation twins in order to balance the laser energy. After repeated laser shocks, the hardness of the amorphous/nanocrystalline layer gradually closes to that of the matrix of the alloy because of the disappearing of the support of grain boundaries to the strength, the dislocation strengthening effect in nano-crystalline grains, and the coherent relationship between precipitates and matrix. Due to that the amorphous microstructure can prevent galvanic effect around precipitates, and nano-crystalline has good chemical stability, the nano-crystalline/amorphous composite high-entropy layer on surface of 7075-T76 aluminum alloy induced by laser shock can significantly improve the corrosion resistance, and effectively block the intergranular corrosion of the alloy.

The microstructure and mechanical behavior of modern high temperature alloys

Article history: Received September 6, 2014 Accepted 2 January 2015 Available online 5 January 2015 Over the past decades, high entropy alloys (HEAs) have attracted continuously increasing research efforts because of their technological promise for structural applications and their scientific interest as a multi-component alloy exhibiting an overall random solid solution structure with high mixing entropy at high temperature. In this summary, we briefly review the recent studies focused on the structure and mechanical behavior of HEAs, covering the important issues from phase stability to elastic modulus, mechanical strength, hardness and fatigue resistance. Finally, we highlight a few key findings recently reported for HEAs and discuss the outstanding issues yet to be resolved. © 2015 Growing Science Ltd. All rights reserved.

A centimeter-size Zr40Hf10Ti4Y1Al10Cu25Ni7Co2Fe1 bulk metallic glass with high mixing entropy designed by multi-substitution

In this paper, a Zr40Hf10Ti4Y1Al10Cu25Ni7Co2Fe1 (at.%) bulk metallic glass (BMG) with high mixing entropy was synthesized. Designed by (Hf, Ti, Y) multi-substitution for Zr and (Ni, Co, Fe) for Cu in Zr55Al10Cu35 BMG with low mixing entropy, this new BMG exhibits significant improvement in glass forming ability (GFA) and good plasticity among TiZrCuNi-containing BMGs with high mixing entropy. In addition, thermal properties, crystallization behaviors and mechanical properties of Zr40Hf10Ti4Y1Al10Cu25Ni7Co2Fe1 BMG are discussed in detail. Successful formation of Zr40Hf10Ti4Y1Al10Cu25Ni7Co2Fe1 BMG with high mixing entropy provides a novel approach in search for new high entropy alloys with non-equal atomic ratios.

Discovery of a superconducting high-entropy alloy.

High-entropy alloys (HEAs) are multicomponent mixtures of elements in similar concentrations, where the high entropy of mixing can stabilize disordered solid-solution phases with simple structures like a body-centered cubic or a face-centered cubic, in competition with ordered crystalline intermetallic phases. We have synthesized an HEA with the composition Ta34Nb33Hf8Zr14Ti11 (in at. %), which possesses an average body-centered cubic structure of lattice parameter a=3.36 Å. The measurements of the electrical resistivity, the magnetization and magnetic susceptibility, and the specific heat revealed that the Ta34Nb33Hf8Zr14Ti11 HEA is a type II superconductor with a transition temperature Tc≈7.3 K, an upper critical field μ0H_c2≈8.2 T, a lower critical field μ0Hc1≈32 mT, and an energy gap in the electronic density of states (DOS) at the Fermi level of 2Δ≈2.2 meV. The investigated HEA is close to a BCS-type phonon-mediated superconductor in the weak electron-phonon coupling limit, classifying it as a "dirty" superconductor. We show that the lattice degrees of freedom obey Vegard's rule of mixtures, indicating completely random mixing of the elements on the HEA lattice, whereas the electronic degrees of freedom do not obey this rule even approximately so that the electronic properties of a HEA are not a "cocktail" of properties of the constituent elements. The formation of a superconducting gap contributes to the electronic stabilization of the HEA state at low temperatures, where the entropic stabilization is ineffective, but the electronic energy gain due to the superconducting transition is too small for the global stabilization of the disordered state, which remains metastable.

High-entropy alloys in light transition metal systems

High-entropy alloys (HEAs) are a new class of alloys designed with the approach of maximization of configurational mixing entropy by increasing the number of constituents [1,2]. Alloys produced in such a way are reported for a variety of promising properties (high hardness and strength, wear resistance, magnetism etc.) [3]. However, origin of these properties (microstructure, phase content, element composition, thermal history) is not always clear. High mixing entropy in HEAs favours the formation of single-phase substitutional solid solutions at elevated temperatures with approximately equiatomic compositions and simple average crystal structures of either the cF4-Cu (fcc) or the cI2-W (bcc). Nevertheless, only a few element combinations produce truly single-phase materials. In order to search for new HEAs compositions samples in the systems Cr-Fe-Co-Ni-Al and Cr-Fe-Co-Ni-Mn were synthesized by arc melting and homogenized in tantalum ampoules at 1100 and 1300 °C for 2 weeks. DTA, X-ray diffraction and electron microscopy measurements were performed. Only samples with small Al content (~ 5 at.%) showed the single-phase microstructure. Their local atomic structure is under investigation.

Influence of Al and Cu elements on the microstructure and properties of (FeCrNiCo)AlxCuy high-entropy alloys

(FeCrNiCo)AlxCuy high-entropy alloys were designed using the strategy of equiatomic ratio, high entropy of mixing and different mixing enthalpies of atom-pairs. The effects of entropy and enthalpy on phase forming process of the alloys were clearly studied and the influences of Al and Cu elements on the microstructure and mechanical properties were investigated. As long as Al element level increased from 0.5 to 1, the microstructure of the alloy system changed from fcc structure to duplex fcc plus bcc structure and then a single bcc structure. Increase of Al element greatly enhanced the Young’s modulus, hardness and yield strength of these alloys. (FeCrNiCo)Al0.75Cu0.5 alloy got the most excellent comprehensive mechanical properties; its fracture strength and plastic strain were as high as 2270MPa, and 42.70%, respectively. Cu-rich phase formed in the alloys when Cu element was in high levels. Increase of Cu element greatly decreased fracture strength of the high-entropy alloys when Al element was in the high level of x=1.

High-Entropy Metallic Glasses

The high-entropy alloys are defined as solid-solution alloys containing five or more than five principal elements in equal or near-equal atomic percent. The concept of high mixing entropy introduces a new way for developing advanced metallic materials with unique physical and mechanical properties that cannot be achieved by the conventional microalloying approach based on only a single base element. The metallic glass (MG) is the metallic alloy rapidly quenched from the liquid state, and at room temperature it still shows an amorphous liquid-like structure. Bulk MGs represent a particular class of amorphous alloys usually with three or more than three components but based on a single principal element such as Zr, Cu, Ce, and Fe. These materials are very attractive for applications because of their excellent mechanical properties such as ultrahigh (near theoretical) strength, wear resistance, and hardness, and physical properties such as soft magnetic properties. In this article, we review the formation and properties of a series of high-mixing-entropy bulk MGs based on multiple major elements. It is found that the strategy and route for development of the high-entropy alloys can be applied to the development of the MGs with excellent glass-forming ability. The high-mixing-entropy bulk MGs are then loosely defined as metallic glassy alloys containing five or more than five elements in equal or near-equal atomic percent, which have relatively high mixing entropy compared with the conventional MGs based on a single principal element. The formation mechanism, especially the role of the mixing entropy in the formation of the high-entropy MGs, is discussed. The unique physical, mechanical, chemical, and biomedical properties of the high-entropy MGs in comparison with the conventional metallic alloys are introduced. We show that the high-mixing-entropy MGs, along the formation idea and strategy of the high-entropy alloys and based on multiple major elements, might provide a novel approach in search for new MG-forming systems with significances in scientific studies and potential applications.

Alloy Design Strategies and Future Trends in High-Entropy Alloys

High-entropy alloys (HEAs) are newly emerging advanced materials. In contrast to conventional alloys, HEAs contain multiple principal elements, often five or more in equimolar or near-equimolar ratios. The basic principle behind HEAs is that solid-solution phases are relatively stabilized by their significantly high entropy of mixing compared to intermetallic compounds, especially at high temperatures. This makes them feasibly synthesized, processed, analyzed, and manipulated, and as well provides many opportunities for us. There are huge numbers of possible compositions and combinations of properties in the HEA field. Wise alloy design strategies for suitable compositions and processes to fit the requirements for either academic studies or industrial applications thus become especially important. In this article, four core effects were emphasized, several misconceptions on HEAs were clarified, and several routes for future HEA research and development were suggested.

Microstructure and oxidation behavior of new refractory high entropy alloys

High-entropy alloys (HEAs) are defined as the alloys composed of at least five principal elements in equimolar or near equimolar ratios, which can facilitate the formation of simple solid solutions during solidification. Recent studies suggested that the refractory HEAs exhibited great promise for high temperature structural materials. However, their oxidation behavior had received little attention. In the present study, Cr, Al and Si elements were added to improve the oxidation resistance, four types of new refractory HEAs were designed and synthesized, including NbCrMoTiAl0.5 (H-Ti), NbCrMoVAl0.5 (H-V), NbCrMoTiVAl0.5 (H-TiV) and NbCrMoTiVAl0.5Si0.3 (H-TiVSi0.3). Their microstructures and oxidation behavior were studied. As expected, these refractory HEAs mainly consist of a simple body-centred cubic (BCC) refractory metal solid solution (RMss) due to the high mixing entropy effect. Solidification process and thermodynamic analysis were investigated to explain the formation mechanism of their microstructures. For all the refractory HEAs, the oxidation kinetics at 1300 °C follows a linear behavior. The oxidation resistance of the HEAs is significantly improved with Ti and Si addition, but reduced with V addition.

More than entropy in high-entropy alloys: Forming solid solutions or amorphous phase

Metastable solid solutions can form preferably over intermetallic compounds, in cast high-entropy alloys or multi-component alloys with equi- or nearly equi-atomic compositions, due to the entropy contribution at elevated temperatures. Meanwhile, the high mixing entropy also favors the amorphous phase formation. The phase selection between solid solutions and the amorphous phase upon alloying in high-entropy alloys is intriguing. A two-parameter physical scheme, utilizing the atomic size polydispersity and mixing enthalpy, is found to be capable of capturing this phase selection mechanism.

High mixing entropy bulk metallic glasses

Bulk metallic glasses (BMGs) are usually based on a single principal element such as Zr, Cu, Mg and Fe. In this work, we report the formation of a series of high mixing entropy BMGs based on multiple major elements, which have unique characteristics of excellent glass-forming ability and mechanical properties compared with conventional BMGs. The high mixing entropy BMGs based on multiple major elements might be of significance in scientific studies, potential applications, and providing a novel approach in search for new metallic glass-forming systems.

Microstructure and Mechanical Properties of MgMnAlZnCu High Entropy Alloy Cooling in Three Conditions

In this paper, alloy with composition of MgMnAlZnCu was designed by using the strategy of equiatomic ratio and high entropy of mixing. The microstructure and mechanical parameters of the new high entropy alloy cooling in three conditions were studied. The alloy was prepared by induction melting and then it was cast in copper molds in air, water and salt water respectively. The microstructure and properties of alloy samples were examined by microhardness tester, XRD, TEM, SEM and testing machine for material strength. The results showed that the alloy was composed mainly of h.c.p and Al-Mn quasicrystal phases. The alloy exhibit high hardness (431HV-467HV) and high compression strength (428MPa-450MPa) at room temperature. The alloy was fragile and the strains were from 3.29% to 5.53%.

Study to Microstructure and Mechanical Properties of Mg Containing High Entropy Alloys

In this paper, alloys with compositions of Mgx(MnAlZnCu)100-x (x: atomic percentage; x=20, 33, 43, 45.6 and 50 respectively) were designed by using the strategy of equiatomic ratio and high entropy of mixing. Microstructure and mechanical properties of the new high entropy alloy were studied. The alloys were prepared by induction melting and then were cast in a copper mold in air. The alloy samples were examined by microhardness tester, XRD, SEM, thermal analyzer and testing machine for material strength. Alloys were composed mainly of h.c.p phase and Al-Mn icosahedral quasicrystal phases. The alloys exhibited moderate densities which were from 4.29g•cm-3 to 2.20g•cm-3, high hardness (429HV-178HV) and high compression strength (500MPa-400MPa) at room temperature. The extensibility was increased with Mg from 20at% (atomic percentage) to 50at%.

Formation of ordered/disordered nanoparticles in FCC high entropy alloys

Abstract This study comprises the microstructure characterization of four as-cast CoCrFeNiAl0.3, CoCrFeNiTi0.3, CoCrFeNiMo0.3 and CoCrFeNiAl0.3Mo0.1 FCC high entropy alloys. Massive nanoparticles are observed in the CoCrFeNiAl0.3, CoCrFeNiTi0.3 and CoCrFeNiAl0.3Mo0.1 alloys, whereas no nanoparticle is found in the CoCrFeNiMo0.3 alloy. These nanoparticles are multi-elemental solid solutions instead of inter-metallic compounds, and the crystal structure possesses both the ordered L12 and the disordered FCC structures. It is suggested that the formation of nanoparticles is dominated by the effects of the high mixing entropy, the sluggish cooperative diffusion of atoms, and the relatively negative mixing enthalpies of the atomic pairs between Al (or Ti) and other compositional elements. Furthermore, the formation of L12 ordered nanoparticles should fulfill the criteria of alloy composition having both more than 25 at.% of Ni (Ni could be partially substituted by Co) and at least 6.25 at.% of Al or Ti.