Symposium MB3 : High-Entropy Alloys

Multiple single-crystal micropillars for uniaxial compression testing were obtained from a recrystallized, coarse grained polycrystalline Fe₂₀Mn₂₀Ni₂₀Co₂₀Cr₂₀ high entropy alloy; the loading axis was selected in individual specimens to lie along the [123], [001], [111] or [114] directions to evaluate single and multiple slip response. Compression stress-strain curves were obtained from which CRSS and early stage work-hardening behavior were documented; a pillar size effect on CRSS was observed using micropillars of different diameter oriented for single slip. Micropillars deformed by single slip exhibited a low work hardening rate and in some cases stage I hardening was recognized; in contrast those that deformed by multiple slip showed high early stage work-hardening, with the work hardening rate increasing with the number of active slip systems. Post-deformation microstructure analysis was performed using the scanning electron microscope to document slip traces resulting from deformation of these micropillars. Focused ion beam milling was used to lift-out transmission electron microscope specimens from the deformed micro-pillars to develop an appreciation for the dislocation structures in the deformed micropillars. Results from these experiments will be presented and discussed.

Damage tolerance is a desired yet often elusive characteristic of structural materials, particularly single-phase materials, as it requires a combination of high strength and high ductility, properties that are generally mutually exclusive. Certain newly developed alloys are of special interest in this regard. Medium- to high-entropy alloys are an intriguing new class of materials in which three, four, five, or more elements are present in equiatomic or near-equiatomic concentrations, with the striking characteristic that they can crystallize as single-phase solid solutions with simple crystal structures, despite containing high concentrations of multiple elements with very different crystal structures. Here we examine the equiatomic face-centered-cubic high-entropy alloy CrMnFeCoNi, which exhibits a remarkable combination of strength above 1 GPa, tensile ductility of more than 50% and fracture toughness exceeding 200 MPa.m^1/2 at crack initiation and more than 300 MPa.m1/2 (J > 500 kJ/m²) during stable crack growth, properties which actually improve from ambient to cryogenic temperatures. This appears to result from continuous steady strain hardening, which acts to suppress instability, consistent with planar dislocation slip at ambient temperatures which transitions into deformation-induced nano-twinning at lower temperatures. We will show our work on specifically designed ‘hourglass-shaped’ samples containing gradients in cross-section to determine the stresses required for the onset of twinning and its evolution with increasing deformation in this alloy in order to shed light on the micro-scale mechanisms underlying the excellent mechanical properties of these materials. Furthermore, we will report results of the material’s fatigue-crack propagation behavior at room temperature and below.

High throughput, combinatorial approaches dramatically accelerate the discovery and development of functional materials, where properties depend primarily on composition. In structural materials, many properties depend sensitively on sample dimensions and microstructural length scales, complicating sample miniaturization that is critical to high throughput techniques. As a result, high throughput tests to rapidly evaluate structural materials are not currently available to meet the challenges offered by the extreme numbers of alloy compositions and microstructures. Here we develop a strategy to accelerate the exploration of conventional and multi-principle element structural alloys, and describe new tests for the rapid evaluation of structural alloys.

Equiatomic multi-component alloys, referred to variously as medium-/high-entropy alloys, multi-component alloys, or compositionally complex alloys in the literature, are an intriguing new class of materials that have received significant attention in the materials science community as of late. Some of these alloys can crystallize as single-phase solid solutions with simple crystal structures despite containing high concentrations of elements with very different crystal structures which makes them interesting from a fundamental scientific viewpoint. Additionally, they can display a good combination of mechanical properties making them attractive for a wide range of applications. Here we examine equiatomic medium- and high-entropy, face-centered-cubic CrCoNi-based alloys, which exhibit exceptional combinations of strength, ductility and fracture toughness at ambient to cryogenic temperatures, consistent with their high lattice friction and low stacking fault energy characteristics. We further use in situ transmission electron microscopy to identify in real time a synergy of deformation mechanisms including planar dislocation slip, rapid motion of partial dislocations, near-tip crack bridging and deformation-induced nano-twinning.

High Entropy Alloys (HEAs) is a new class of alloys with promising mechanical behavior such as higher specific yield strength. HEAs are composed of 5 or more elements with equal or near equal atomic weight. In this study a Continuum Dislocation Dynamics (CDD) framework is used to study the deformation behavior of High Entropy Alloys. In this framework, the dislocations are divided into three types, a positive, a negative and an immobile, and coupled evolution equations are employed to describe the complex interactions between the different types of dislocations. Furthermore, the effect of the lattice distortion is considered by introducing stochastic terms in the evolution equations. The effect of temperature, grain size, and residual dislocation density is investigated.

Recently, a kind of multi-component alloys which consist of five or more elements in equal or near equal atomic ratio, together with high mixing entropy, have been developed and named as high entropy alloy (HEA). These HEAs have attracted lots of attentions due to that they usually possess simple crystalline structures and many excellent properties. It is known bulk metallic glasses (BMGs) usually possess much better properties than their crystalline counterparts. Up to date, lots of BMGs, which constitute one dominant metallic element and some other alloying elements just like most crystalline alloys do, have been developed. Then developing high entropy BMGs (HE-BMGs) would be interesting. Here, some research progresses in HE-BMGs have been reviewed. It shows that HEAs could also possess high glass-forming ability despite of that they possess no principal element. Some of them could exhibit a critical size larger than 20 mm. In addition, the effects on glass-forming ability of high-entropy alloys will be discussed.

High entropy alloy (HEA, also known as multi-principal element alloy or compositionally complex alloy) refers to simple-phase solid solution alloy that contains multiple principal components in equimolar or near-equimolar ratios. To computationally address the complexities of this type of high-order alloy systems, we have recently performed atomistic modeling and evaluated the solid-solution stability for CoCrFeNi and AlCoCrFeNi HEAs. In our simulations, the interatomic interactions were described using a set of modified embedded atom method (MEAM) interatomic potentials for the two alloy systems. First, we used atomistic simulation methods to examine solid-solution phase formation rules for CoCrFeNi high entropy alloy. Using the Monte Carlo (MC) simulations based on the developed MEAM potentials, we sampled the thermodynamically equilibrium structures of the CoCrFeNi alloy and further predicted that the CoCrFeNi alloy could form a solid solution phase with high configurational entropy of 1.329R at 1373 K. Furthermore, we examined the stability of this solid solution phase of the CoCrFeNi alloy against the well-recognized solid-solution phase formation rules by varying the MEAM potentials and thus tuning the atom size and mixing enthalpy in the alloy. Our simulation results revealed that it required atom size difference effect δ<0.05 and mixing enthalpy effect -10 kJ/mol < ΔH = 0 kJ/mol for the modeled CoCrFeNi alloy to remain a single solid solution phase. Furthermore, we studied the stability of solid-solution phase of AlxCoCrFeNi HEAs using the developed MEAM potentials and the atomistic MC simulation method. In our MC simulations, different component elements were allowed to exchange their positions and thus the modelled HEAs were relaxed to their thermodynamic equilibrium states after several millions MC steps at 1300 K. The mixing Gibbs free energy of the HEAs was calculated using adiabatic switching thermodynamic integration method. We predicted that the AlxCoCrFeNi HEAs would form a single fcc solid-solution phase when x<0.44, a single bcc solid solution phase when x>1.75, whereas a mixture of fcc and bcc phases when 0.44 less than x less than1.75. Our theoretical results are quite consistent with experimental observation. Consequently, we have demonstrated that atomistic modeling techniques to be useful methods for understanding the composiotin-structure-property relations of novel high entropy alloys.

Equiatomic or nearly equiatomic solid solution alloys, consisting of five or more elements, may exist as a stable single-phase crystallizing in simple crystal structures such as the BCC and FCC structures, because the contribution of configurational entropy to the Gibbs free energy may be high enough to suppress compound formation and phase separation. These solid solution alloys have recently been called “high-entropy alloys (HEAs)”. Although the solid-solution strengthening (SSS) will be one of the most important mechanisms that dominate the mechanical strength of HEAs, the classical theories of SSS, such as Fleischer’s, Labusch’s and so on, which were established mostly for dilute solid solutions, cannot be applied to HEAs because the definition of “solvent” or “solute” in HEAs is difficult and also because the “solute” concentration is as high as ~20 at.%. In dilute solid-solutions, a spherically symmetric strain, generated around solute atoms due to the atomic size/modulus misfits between solute and solvent atoms, acts as obstacles for moving dislocations and is a determining factor in the degree of SSS. In HEAs, on the other hand, the lattice is locally distorted everywhere because multiple principal elements with different sizes are randomly distributed and interact with one another so that all atoms are displaced from the regular lattice positions. Therefore, it is not easy to estimate the magnitude of the local lattice strain, hence to predict the degree of SSS in HEAs. In the present study, we have experimentally measured the magnitude of atomic displacements averaged over the constituent elements in the quinary equiatomic CrMnFeCoNi HEA with the FCC structure by single-crystal synchrotron X-ray diffraction as well as deduced the magnitude of atomic displacements for each of the constituent elements by first-principles calculations. In addition, we have deduced the magnitude of the atomic displacements in some of the derivative quaternary and ternary equiatomic alloys to investigate the correlation between the degree of SSS and the magnitude of the atomic displacements. We have found that the atomic displacement averaged over the constituent elements can be a good scaling parameter to predict the degree of SSS in equiatomic alloys

Elastic constants are fundamental parameters that directly reflect interatomic bonding and stability of crystal lattice. Though, polycrystalline elastic constants and their temperature dependence of fcc-CrMnFeCoNi high entropy alloy have already been reported, mono-crystalline elastic constants have only been theoretically calculated. In this report, experimentally determined mono-crystalline elastic constants are presented. A monocrystal of an equiatomic quinary high entropy alloy of fcc-CrMnFeCoNi was prepared by a modified Bridgeman method. The ingot was subjected to a heat treatment for homogenization at 1150°C for 1 week. After determining the crystallographic orientation, specimen with a rectangular parallelepiped shape having its orthogonal faces parallel to the crystallographic {100} planes was cut from the ingot. Elastic constants have been determined by an ultrasound resonance method. The values of determined elastic constants indicate that the bulk modulus of fcc-CrMnFeCoNi high entropy alloy is in a distribution of conventional fcc-metals when the melting point is taken into account. The elastic anisotropy factor for cubic crystal defined as 2C₄₄/( C₁₁ -C₁₂) of fcc-CrMnFeCoNi high entropy alloy is slightly larger than the conventional fcc-metals, which indicates the relatively strong directionality of the interatomic bondings.

Some of the refractory high entropy alloys (HEA) exhibit high-temperature mechanical properties that exceed modern Ni-based super alloys. However, little is known about their fundamental physical properties such as, e.g. ground states or degree of chemical ordering at elevated temperatures. We present a combined first-principles and Monte Carlo (MC) study to investigate ordering at elevated temperatures in the prototype BCC HEA NbMoTaW [1]. Chemical interactions have been computed from first-principles by means of the coherent potential approximation in combination with the screened generalized perturbation method. Surprisingly, the NbMoTaW solid-solution at elevated temperatures does not originate from the small magnitude but rather from the highly frustrated long-ranged nature of the chemical interactions. The role of the chemical interactions on thermodynamic properties is examined in detail.

[1] F. Körmann, A.V. Ruban, and M.F.H. Sluiter, Materials Research Letters (2016), DOI 10.1080/21663831.2016.1198837

Predicting intrinsic properties of single-phase high entropy alloys (HEAs) requires appropriate treatment of their atomic structures, which generally are disordered in nature. Here we have used hybrid Monte Carlo / molecular dynamics (MC/MD) simulations and special quasi-random structure (SQS) model to mimic the disordered structures of HEAs in FCC, BCC and HCP structures for quaternary, quinary, senary, septenary, and octonary equimolar compositions. Utilizing first-principles density functional theory (DFT), we have calculated the lattice parameters, enthalpies of formation, and elastic properties of hundreds of equimolar compositions with the FCC, BCC and HCP structures. Whether they may form single-phase solid solution upon routine laboratory arc/induction melting is examined using a variety of empirical models. The entropy sources (e.g., configurational entropy, vibrational entropy, and electronic entropy) of HEAs are predicted based on DFT and MC/MD simulations. The predicted total and excess entropies are compared with experimental heat capacity data where available as well as CALPHAD modeling.

Two high-entropy alloys (HEAs) with equiatomic concentrations formed by adding Cu and Al elements into NiFeCoCr alloy were produced by the spray casting. It is found that phase compositions have the large difference for these two HEAs: two FCC phases observed from NiFeCoCrCu HEAs and two BCC phases obtained from NiFeCoCrAl HEAs. The fusion enthalpy and entropy of primary phases are all smaller than that of any elements in those two HEAs. The densities of those two alloys indicate that the experimental result of NiFeCoCrCu HEAs is close to the calculated result, but the experimental value of NiFeCoCrAl HEAs is higher than the calculated result. The Vickers hardness of adding Al element in NiFeCoCr alloy is about 2.7 times larger than adding the Cu element. Furthermore, the thermal expansion and diffusion coefficients of both HEAs illustrate that their values of NiFeCoCrCu HEAs are higher than that of NiFeCoCrAl HEAs.

The high-entropy-alloy-design strategy of combining multiple elements in near-equimolar ratios has become a hot research topic, and is being implemented to produce materials with exceptional engineering properties. The goal of the present research is to gain better understanding of the elemental distributions and the evolution of the configurational entropy during the solidification of high-entropy alloys. Ultimately, we wish to understand how the atomic-level configurational properties affect the macroscopic engineering properties. Model alloys in the Al_xCoCrCuFeNi family will be discussed, with an emphasis on integrated theoretical and experimental studies, such as ab initio molecular dynamics simulations, neutron scattering, synchrotron X-ray diffraction, high-resolution electron microscopy, and atom-probe tomography. It will be shown that even when these materials undergo cooling transformations involving chemical ordering and elemental segregation, a significant amount of disorder remains, due to the distributions of multiple elements in the major phases. The results suggest that the high-entropy-alloy-design strategy may be used to develop a wide range of complex materials, which are not limited to single-phase solid solutions.

This work reports on a promising new class of irradiation-tolerant materials: High-entropy alloys (HEAs). Comparing the irradiation-induced/enhanced precipitation behavior of HEA with single phase (Al_0.1CoCrFeNi) and multi-phase (Al_0.75- and Al_1.5CoCrFeNi) structure, we found the single-phase solid solution exhibits an excellent phase stability against precipitation under room temperature ion irradiation. No precipitate was observed even at the 65 dpa. In contrast, numerous coherent precipitates are present in both multi-phase HEAs. Then the single phase HEA Al_0.1CoCrFeNi was irradiated by Au ions at elevated temperatures ranging from 250 °C to 650 °C to investigate the structural damage and phase stability of single phase HEA under high temperature irradiation. It was found that Al_0.1CoCrFeNi HEA exhibits good structural stability against irradiation at elevated temperatures, although some weak nanoscale phase decomposition or segregation was found by APT characterization after a dose of 54 dpa. The irradiation-induced dislocation defect characteristics are similar with that of conventional alloys with fcc structure, but no void can be observed by TEM at four different temperatures, suggesting good resistance to volume swelling. The phase stability and defect characteristics of HEAs under ion irradiation are discussed based on the high configurational entropy and low atomic diffusion. This study demonstrates the structural stability of single-phase HEAs under irradiation and provides important implications for searching for HEAs with high irradiation tolerance.

The phase structures of senary and septenary refractory High Entropy Alloys (HEAs) are examined in this study. The studied HEAs include CrMoNbTaVW and MoNbTaTiVW, etc [1-4]. CALPHAD calculations were performed first to examine the possible existence of different phases within such HEAs. HEA specimens were then synthesized and characterized experimentally.
Different phases in traditional alloys usually possess different crystal structures and lattice constants, and are thus easily distinguishable through X-ray diffraction (XRD) measurements. In contrast, within the refractory HEAs examined, we find multiple phases with significantly different chemical compositions possessing the same crystal structure and extremely similar lattice constants, thus showing “single-phase-like” XRD patterns. Experimental characterizations using XRD, scanning electron microscopy (SEM), and transmission electron microscope (TEM) were conducted to verify the existence of multiple phases within the tested refractory HEA specimens. The issue of phase separation in HEAs poses interesting challenges to alloy modeling, and have implications on their mechanical properties.

References:
1. Zhang, B., Gao, M.C., Zhang, Y., Guo, S.M., Senary refractory high-entropy alloy CrxMoNbTaVW. Calphad, 2015. 51: p. 193-201.
2. Zhang, B., Gao, M.C., Zhang, Y., Yang, S., Guo, S.M., Senary refractory high entropy alloy MoNbTaTiVW. Materials Science and Technology, 2015. 31(10): p. 1207-1213.
3. Zhang, B., Gao, M.C., Zhang, Y., Guo, S.M., Supporting data for senary refractory high-entropy alloy Cr x MoNbTaVW. Data Brief, 2015. 5: p. 730-735.
4. Gao, M.C., Zhang, B., Guo, S.M., Qiao, J.W., Hawk, J.A., High-Entropy Alloys in Hexagonal Close-Packed Structure. Metallurgical and Materials Transactions A, 2015, 47: 3322-3332.

High entropy alloys (HEAs) which containing five or more components in equimolar or near-equimolar ratios, have been considered as a new class of advanced materials in recent years. Due to a combination of high entropy, lattice distortion, sluggish diffusion and Cocktail effects, HEAs hold promises for a variety of applications, such as high strength alloys, corrosion resistant alloys, wear resistant alloys, and diffusion barriers.
However, most of the high entropy alloys are brittle, which limited their applications. For instance, NbMoTaW and VNbMoTaW alloys show superior mechanical properties at elevated temperatures, but they are brittle at room temperature with very limited compressive plasticity (~ 2%). Alloying is proved to be an effective method to improve mechanical properties of alloys, and it can be used to enhance the ductile of NbMoTaW and VNbMoTaW alloys. By evaluate the mixture enthalpy, melt temperature, atomic radius and other physical properties, it was expected that alloying with Ti element would improve the grain boundary mechanical properties of the NbMoTaW and VNbMoTaW alloys. The TiNbMoTaW and TiVNbMoTaW alloys were produced in present work and the effect of Ti additions on their structural stability and mechanical properties were investigated at both room temperature and elevated temperature.
Both of them exhibit single body-centered cubic (BCC) crystal structures, which remain stable even after annealing at 1273 K for 10 h. In comparison with the mechanical properties of the NbMoTaW and VNbMoTaW alloys, Ti additions are beneficial in enhancing both the strength and the compressive ductility at room-temperature. The room-temperature yield strength values of the TiNbMoTaW and TiVNbMoTaW alloys are as high as ~1343 and ~1515 MPa, respectively. In addition, their compressive plasticities are significantly enhanced to be larger than 10% at room temperature compared with the limited compressive plasticity of ~2% for the NbMoTaW and VNbMoTaW alloys. In particular, they also show very promising high temperature mechanical performance. During deformation at 1473 K, their yield strength values are as high as ~586 and ~659 MPa, respectively. The combination of high strength and good ductility at elevated temperatures enables the present refractory high entropy alloys to be used as high temperature structure materials for engineering applications.

In-situ neutron diffraction measurements of CrCoFeNiMo_x (x=0-0.2) were carried out on TAKUMI beamline in J-PARC. Diffraction patterns from loading and transverse directions were collected using Time-Of-Flight (TOF) method by two detectors located at 2θ=±90°. Tensile deformation behaviors up to 40% of plasticity are studied. For all CrCoFeNiMo_x (x=0-0.2) HEAs, neutron diffraction patterns show a face-centered cubic (FCC) crystal structure. The addition of Mo enhances the strength of CrCoFeNiMo_x (x=0-0.2) HEAs without sacrificing much ductility. Local dislocation density was investigated by the Convolutional Multiple Whole Profile (CMWP) method. Lattice strain of the grains parallel to the tensile loading direction with different crystallographic hkl orientations was determined as a function of applied stress using the Z-Rietveld software. All HEAs show significant elastic anisotropy dependent on different grain orientations regardless of the Mo concentrations. The dislocation density shows an increase with increasing plasticity. In addition, a strong texture change is also observed. The effect of Mo additions on the microscopic deformation behaviors will be discussed.

High-entropy alloys (HEAs), composed of five or more elements of near-equal molar percentage in random solution, have excellent thermomechanical properties. However, HEAs deform via sudden slips under certain temperature and strain-rate regimes, which are seen as “serrations” in the stress-strain curve. The cause of serrations is unknown; ex-situ experiments suggest large crystal slips. Here, with help of in-situ transmission electron microscopy (TEM), we demonstrate the spatiotemporal dislocation dynamics leading to planar slips in the Al_0.1CoCrFeNi nanopillars. In situ compression tests of the HEA nanopillars were performed using a Hysitron picoindenter in a JEOL 2010 LaB6 TEM operated at 200 keV. The compression tests were performed in displacement-controlled mode. Using a transducer sensor, the load and displacement were measured while the nanopillar deformation was monitored by diffraction contrast electron imaging. A video of each test was recorded using a charge-coupled device (CCD) camera running at 10 frames per second. The experimental studies here have revealed physical details of slowly-deformed HEAs and provided insights on the creations and multiplication of dislocations and slip avalanches in HEAs during deformation.

We present finite-temperature magnetic properties of four- and five-component FCC CoFeNiCr-based high entropy alloys (HEA). Particular emphasis is put on CoCrFeNiPd HEAs, which have been suggested as potential magneto-caloric materials recently [1]. Curie temperatures are computed employing density functional theory and a magnetic mean-field model [2]. Our theoretical results are in excellent agreement with experimental data revealing high predictive power of the employed theoretical method. The computational framework is used to explore the dominant mechanisms that determine the Curie temperature. Finally we propose alternative alloying strategies for tuning the Curie temperature towards room temperature. A wide range of ferromagnetic properties and Curie temperatures near room temperature in hitherto unexplored alloys are identified. Our predicted ‘treasure maps’ [2] narrow down the enormous configuration space for distinct magnetic properties of these multi-component alloys to a well-defined set of alloy compositions.

[1] Belyea, D. D. et al., Sci. Rep. 5, 15755 (2015); Lucas, M. S. et al., J. Appl. Phys. 109, 07E307 (2011); ibid., J. Appl. Phys. 113, 17A923 (2013).
[2] Körmann, F. et al., Appl. Phys. Lett. 107, 142404 (2015).

High Entropy Alloys (HEAs) are a new class of multi-component alloys some of which exhibit surprising characteristics including, very large strain hardening rates, large fracture toughness at room temperature, and a strong temperature dependence of yield strength at or below room temperatures. These properties are closely linked to nano-twinning and dislocation-mediated plasticity, yet little experimental work has explored dislocation dissociation, stacking fault energy, or core structures in these alloys [3]. Building on an earlier study [1], an HEA containing 5 elements (Cr, Co, Mn, Fe, and Ni) with equiatomic composition [2] was deformed to 5% plastic strain at room temperature. Post-test observations using diffraction contrast scanning transmission electron microscopy (DC-STEM) analysis provide evidence for numerous planar slip bands composed of 1/2<110> dislocations. More detailed analyses of dislocation separation distances were performed using high-order diffraction vector DC-STEM and atomic resolution high angle annular dark field (HAADF) STEM on 1/2<110> dislocations in 60° orientation. Large variations in dissociation distances are found, leading to the concept of a local stacking fault energy (SFE). This finding is supported through embedded-atom-method (EAM) calculations of a model, concentrated, three-element solid solution. For the first time, the Nye tensor and center of symmetry analysis were used collectively to accurately determine dissociation distance. Using high-resolution energy dispersive X-ray spectroscopy, no ordering or segregation was observed, indicating that this alloy is a true solid solution down to the atomic scale in the recrystallized and lightly deformed state. To further explore the 3-dimensional configuration of dislocations, a novel through-focal HAADF-STEM imaging has also been employed, coupled with simulations of HAADF-STEM contrast using the μSTEM software package [3]. Both planar and heavily jogged configurations are identified with this approach. These results will be discussed in the context of the concept of a “local” SFE in this HEA, and in relationship to the unique macro-behavior exhibited by these alloys.
References:
[1] F. Otto et al. Acta Mater. 61 (2013) 5743–5755.
[2] B. Cantor et al. Mater. Sci. Eng. A. 375-377 (2004) 213–218.
[3] B.D. Forbes, et al., Phys. Rev. B, 82 (2010) 1-8.

High-entropy alloys (HEAs) are new alloys that contain five or more elements in roughly- equal proportion. We present new experiments and theory on the deformation behavior of HEAs under slow stretching (straining), and observe differences, compared to conventional alloys with fewer elements. For a specific range of temperatures and strain-rates, HEAs deform in a jerky way, with sudden slips that make it difficult to precisely control the deformation. An analytic model explains these slips as avalanches of slipping weak spots and predicts the observed slip statistics, stress-strain curves, and their dependence on temperature, strain-rate, and material composition. The ratio of the weak spots’ healing rate to the strain-rate is the main tuning parameter, reminiscent of the Portevin-LeChatellier effect and time-temperature superposition in polymers. Our model predictions agree with the experimental results. The proposed widely-applicable deformation mechanism is useful for deformation control and alloys design.

Reference: Robert Carroll, Chi Lee, Che-Wei Tsai, Jien-Wei Yeh, James Antonaglia, Braden Brinkman, Michael LeBlanc, Xie Xie, Shuying Chen, Peter K. Liaw, and Karin A. Dahmen, Experiments and Model for Serration Statistics in Low-Entropy, Medium-Entropy, and High-Entropy Alloys, Scientific Reports 5, Article number: 16997 (2015), doi:10.1038/srep16997

High entropy alloys (HEAs) continue to attract substantial interest particularly their phase selection or simple crystal structures such as FCC and BCC. There have been a wide variety of different theoretical models applied to identify the characteristics of this phase selection but full understanding remains elusive.

We analyse phase selection using classical molecular dynamics. By combining the low computational cost of classical methods with a genetic algorithm for sampling we are able to generate thousands of large configurations and then analyse their significance to the overall thermodynamic average by using statistical mechanics. With this we can extract the enthalpy, free energy and entropy of mixing for these HEAs along with structural features. We apply this to the CoCrFeNi alloyed with Al and Ti. Our results indicate that entropy may be playing the crucial role in phase selection of BCC versus FCC.

"High-entropy alloys" are an interesting new class of multi-component alloys. Equimolar alloys CrMnFeCoNi or NiCrCo are representatives of single-phase FCC equimolar multicomponent alloys (EMAs) that have attracted considerable attention recently. They are believed to be characterized by a low stacking fault energy (SFE). However, as of yet, the SFE in these alloys is not well understood. Using molecular dynamics simulations with realistic EAM potentials, here we determine the magnitude of the SFE and analyse the SFE variations from location to location in the alloy, as well as the temperature dependence of the average SFE and its distribution. We also show variations of the separation between partial dislocations and found that this dissociation distance depends not only on local SFE, but also on the lattice friction stress which is large and variable in these highly-concentrated solutions. Our results shed light on the complex and unconventional behavior of SFE in EMAs, and demonstrate that computer simulation is particularly powerful in mapping out the wide spectrum of SFE variations, their manifestation in partial separation distance, and the important role on dissociation played by lattice friction. Our findings also provide guidance for interpreting future experimental measurements of SFE and the dissociation width in EMAs.

Most metallurgical mechanisms for increasing strength lead to loss in ductility. To overcome this trade-off, we developed a metastability-engineering strategy to design bulk high-entropy alloys with multiple compositionally equivalent high-entropy phases. High-entropy alloys were originally thought to benefit from phase stabilization through entropy maximization. Yet here, motivated by recent work that relaxes the strict restrictions on high-entropy alloy compositions by demonstrating the weakness of this connection, the concept is overturned. We decrease phase stability to achieve two key benefits, i.e., interface hardening due to a dual-phase microstructure and transformation-induced hardening. This produces two desirable strength characteristics: extensive hardening as known from advanced steels (due to decreased phase stability) and massive solid-solution strengthening of high-entropy alloys. In our transformation-induced, plasticity-assisted, dual-phase high-entropy alloy (TRIP-DP-HEA), ductility is also improved through an increased strain hardening capacity enabled by sequentially activated dislocation hardening of the stable phase and transformation-induced hardening of the metastable phase. This combined increase in strength and ductility distinguishes the TRIP-DP-HEA alloy from other recently developed structural materials. This metastability-engineering strategy should thus usefully guide the design of high-entropy alloys in the near-infinite compositional space. (Li et al., Nature, 2016)

The effects of grain size and temperature on the mechanical properties of both undoped and 1.1 at. % carbon-doped FeNiMnAlCr high entropy alloys (HEA) are presented. Atom probe tomography revealed carbon segregation to the grain boundaries for the carbon-doped HEA, which retarded both recrystallization and grain growth in the cold-rolled material. The yield strength-grain size relationship can be described by the Hall-Petch equation with the carbon addition producing a higher lattice friction stress, but a lower value of the Hall-Petch slope. The yield strength of the HEAs increased rapidly with decreasing temperature from 973 K to 77 K with a yield strength of over 1100 MPa obtained in the fine-grained carbon-doped HEA. The influence of carbon, grain size, and temperature on the fracture behavior and dislocation substructure evolution was determined by scanning electron microscopy and transmission electron microscopy, respectively.
This research was supported by the US Department of Energy (DOE), Office of Basic Energy Sciences grant DE-FG02-07ER46392. APT was conducted at ORNL's Center for Nanophase Materials Sciences (CNMS), which is a U.S. DOE Office of Science User Facility.

High Entropy alloys (HEAs) are solid solution alloys containing five or more principal elements in equal or near equal atomic percent (at %). We synthesized Al_0.7CoCrFeN HEA by vacuum arc melting and homogenized it at 1250°C for 50 hours. The microstructure shows the presence of two phases: the Body-Centered Cubic (BCC: A2+B2) and the Face-Centered Cubic (FCC). Using the Focused Ion Beam, we fabricated single-crystalline cylindrical nano-pillars from each phase within individual grains in the Al_0:7CoCrFeNi HEA. These nano-pillars had diameters ranging from 400 nm to 2 microns and were oriented in [001] direction in the BCC phase and in [324] direction in the FCC phase. Uniaxial compression experiments revealed the yield strength of 2.2 GPa for the 400nm diameter samples in the BCC phase and 1.2 GPa for the equivalent diameter samples in the FCC phase. We observed the presence of a size effect in both phases, with smaller pillars having substantially greater strength compared with bulk and with larger-sized samples. The size effect power exponent for BCC phase was -0.28, which is lower than that for most pure BCC metals, and the FCC phase had the exponent of -0.66, equivalent to most pure FCC metals. We discuss these results in the framework of nano-scale plasticity and intrinsic lattice resistance through the interplay of the internal (microstructural) and external (dimensional) size effects

In general, high entropy alloys are considered as a regular solution in calculation. In this study, the free energy of high entropy alloys was calculated by using the cluster variation method(CVM) and first-principles calculation, and from the free energy, bulk modulus and coefficient of thermal expansion(CTE) were estimated.In addition to the simulation work, bulk modulus and CTE were experimentally measured for a Ni-11Fe-11Cr-20Co-8Al-8Ti (at. %) high entropy alloy. The sample was prepared by arc-melting followed by homogenization heat treatment at 1200°C for 30h.CTE calculated by CVM that was equilibrated at each temperature shows a large negative value at low temperature. However, using the free energy calculated by fixing cluster probability at homogenized temperature, CTE increases monotonically. Compared with the measured CTE, the calculated CTE was underestimated especially at high temperature. The difference between calculated and measured bulk moduli is less than 15 % and the temperature dependence of both the calculated and measured bulk moduli shows similar trend. Pair-cluster probabilities obtained by the CVM calculation indicates the probabilities different from that of regular solution, but the pair-cluster probabilities had a small extent of deviation caused by the interaction energy of the pair atoms. These results suggest that not only concentration but also pair or tetrahedron cluster contribute to bulk modulus and other thermodynamic property of high entropy alloys.

"High-entropy alloys", or in general equimolar multi-component alloys (EMAs), are an emerging class of alloys that have attracted considerable attention recently. CrMnFeCoNi or NiCrCo are representatives of single-phase FCC EMAs, exhibiting an unusual combination of high tensile strength and ductility. It is believed that this extraordinary strength-ductility synergy is closely related to the hierarchical deformation twinning microstructures developed in these alloys, which are characterized by an unsually low stack fault energy (SFE). However, while it is expected that the low SFE would promote partial dislocations to create numerous stacking faults, it is unclear how the latter would lead to profuse deformation twins, considering the lack of pole dislocations or grain boundaries that emit twinning dislocations. Using molecular dynamics simulations with realistic EAM potentials, we demonstrate a new deformation twinning (DT) mechanism that takes advantage of the intersection between faults. These intersections are of glide-like symmetry due to the Burgers vectors carried by the intersecting faults, in lieu of the screw symmetry in the conventional pole mechanism. The translational component (along the slip plane normal direction) of this glide-like symmetry plays the role of promoter to mediate DT, constituting the source of twinning dislocations that run on consecutive atomic planes. We also show transmission electron microscopy evidence of widespread fault intersections and twins that evolve from them. This new mechanism is especially prolific at high driving stresses, explaining the enhanced occurrence of complex and hierarchical twin structures in EMAs at cryogenic deformation temperatures.

Multicomponent high-entropy alloys (HEAs) with severe lattice distortions have been found to present better mechanical properties than and different deformation behavior from traditional alloys with one principle element. However, their mechanical behavior at the nanoscale as well as the correlation between their distorted structure and special mechanical deformation remain unclear. Therefore in this work, face-centered cubic (fcc) low-entropy binary, medium-entropy ternary and high-entropy quinary alloys with similar levels of cohesive energy and lattice distortion were prepared; a pure unitary metal and a less-solute binary alloy with lower degrees of lattice distortion were also prepared for comparison. The structures of these alloys were characterized, and the nanoscopic mechanical behavior was examined by instrumented nanoindentations and in-situ compressions in a transmission electron microscope (TEM). Structural characterizations indicated that, for the binary alloys with an increased content of solute, the measured volumetric density increased (the unit volume shrank) as compared to the theoretical values, well following the Danisch’s random-packing model of different-size balls. On the contrary, for the high-entropy quinary alloys, the measured volumetric density decreased (the unit volume expanded). Nanoindentation measurements indicated that, for the binary alloys with an increased content of solute, the mechanical properties increased and showed a typical anisotropy for different-orientation grains. However, for the quinary alloys, the mechanical properties decreased in a specific orientation, and the mechanical anisotropy became unobvious. In-situ TEM compressions suggested that the change of dislocation activities from full dislocations to stacking faults might be responsible for the decrease of mechanical properties in the specific direction.

Over the last decade a brand-new alloy design concept of high-entropy alloys (HEAs) has attracted significant attention in the metal community. Contrary to traditional alloy design concepts, HEAs are equiatomic, multi-principal element systems in which the configurational entropy contribution to the total free energy can stabilize solid-solution state instead of forming intermetallic compounds. It is important to have concrete knowledge of thermal conductivity (κ) for structural applications of alloys, especially in the extreme environments. Although it is reported that several HEAs exhibit relatively low κ compared with other FCC solid-solutions, how the increase of configurational entropy of mixing (ΔSmix) in FCC solid-solutions with different numbers of principal elements (NPEs) is connected with κ variation and what the dominant contribution for the κ variation and its temperature dependence is have not been investigated yet. In the present study, we investigate how to manipulate the ratio between thermal conductivity (κ) and yield strength (σy) in face-centered cubic solid-solutions by varying the number of principal elements (NPEs) and temperature. With increasing NPEs, NiCoFeCrMn high-entropy alloy (HEA) exhibits the lowest κ with positive Δκ/ΔT and the highest κl/κe ratio due to severe lattice distortion and compositional complexity. One the one hand, σy increases with increasing NPEs and decreasing temperature. Thus, NiCoFeCrMn HEA exhibits the highest σy/κ ratio, higher than those of representative cryogenic alloys, which can be distinctively enlarged with decreasing temperature. These results would give us a guideline on how to manipulate properties using HEA design concept in order to develop novel cryogenic materials.

Over the last few years compositionally complex alloys consisting of five or more principal elements, also known as high entropy alloys (HEA), gained large attention within the materials science community. Refining the microstructure of a five-element HEA (CoCrFeMnNi) by applying high pressure torsion, the grain size can be reduced to less than 50 nm paired with a significant increase of strength to 1950 MPa and a hardness of ~520 HV. During further isochronal and isothermal annealing treatments an additional hardness increase was found, and by applying high resolution microscopy techniques such as TEM and ATP the occurrence of newly formed nanophases could be detected.
To gain further insights into the microstructure-property relationships, nanoindentation testing was used to compare hardness, Young’s modulus, as well as strain rate sensitivity between these different microstructures and annealing states. By applying simple strain rate controlled testing protocols it was found that the formation of these nanophases directly leads to a significant increase in the Young’s modulus from 205 GPa in the as-deformed state to ~255 GPa after 1 h at 550 °C. Afterwards the modulus decreases down to the coarse-grained (cg) and ufg-value. Moreover, a strong fluctuation of the modulus of the cg-samples was found leading to the conclusion that the investigated HEA is highly elastically anisotropic. Additionally, strain rate sensitivity values for all microstructural states and annealing treatments could be deduced by nanoindentation strain rate jump tests. As expected for an fcc nc-structure, strain rate sensitivity was increased in the ultrafine grained states. However, also the single crystalline and cg-states show some significant influences on the hardness upon changing the strain rate. This can be directly related to the high lattice distortions proposed to be present in HEAs, causing a high lattice friction. Overall, it was shown that nanoindentation is an elegant and very versatile tool to shed more light onto the microstructure-property relationships of complex materials and was proven to be beneficial as a high throughput testing technique.

Positron annihilation spectroscopy is a particularly useful set of techniques for studying vacancy defects in crystalline solids [1]. This is based on the special property of the positron to efficiently localize at open volume defects thanks to the strong repulsion by the positive ion cores, and on the ensuing modifications in the positron-electron annihilation characteristics (lifetime, Doppler broadening). By utilizing a variable-energy positron beam, the probing depth of positrons can be tuned from a few nanometers to a few microns, allowing for analysis of near-surface ion implantation damage. Importantly, the vacancy defect concentration sensitivity of the positron methods is in the 10¹⁵ - 10¹⁹ cm^-3 range and the vacancy size sensitivity is at its best with mono- and di-vacancies. Hence these methods are able to provide information on the very early stages of radiation damage.

In this work, we present results obtained in Ni, NiFe, NiCo, NiCoCr, NiCoFe and FeNiCrCo [2] implanted with 2-16 MeV Ni ions to doses 0.003 – 0.3 dpa at room temperature. No amorphization occurs at these low doses. It has been recently shown that these materials exhibit significant differences in terms of damage build-up at early stages of irradiation: the multi-atomic alloys may be significantly more resistant to radiation damage than the corresponding pure elements, possibly due to reduction of dislocation mobility [3]. Our preliminary results in these alloys suggest that the efficiency of clustering of vacancies into multi-vacancy-complexes larger than a di-vacancy strongly is strongly modified in the single-phase equiatomic alloy system, pointing towards enhanced vacancy mobility in the multi-atomic alloys. Detailed understanding of defect dynamics and reactions in these alloys is key to designing new radiation-resistant materials.


[1] F. Tuomisto and I. Makkonen, Rev. Mod. Phys. 85, 1583 (2013).
[2] Z. Wu, Y. F. Gao, and H. Bei, Scr. Mater. 109, 108 (2015).
[3] F. Granberg et al., Phys. Rev. Lett. 116, 135504 (2016).

Multiprincipal element alloys, also termed High Entropy Alloys (HEAs), emerged as a new topic in metallurgical research during the last ten years. The development of new materials was attempted in unexplored regions of multicomponent phase diagrams. HEAs imply the material is constituted by a single solid solution with only possible minor presence of secondary phases. These can be designed for properties meeting the requirements of improved performance in industrial applications.

The high number of elements in HEAs formulations stimulated the elaboration of predictive models to guide their synthesis. These consider electronic, mechanical and thermodynamic parameters such as: electronegativity, valence electron concentration (VEC), itinerant electron concentration (e/a), element size mismatch. A discussion on the role of these quantities is presented and combined with a thermodynamic approach to the formation of solid solutions in multicomponent systems using the regular solution and computing the temperature at which the free energy hypersurface changes curvature at spinodal points. Operative maps are obtained to formulate new HEAs.

Examples of new syntheses are provided for both bcc and fcc solid solutions in X-NbTaTiZr and Y-FeCoCrNi, respectively, together with assessment of some relevant mechanical properties.

The fracture toughness and fatigue crack growth behavior of the Al_0.3CoCrFeNi high entropy alloy was determined. Microstructure examination revealed a single FCC phase structure. The notched and fatigue precracked toughness values were significantly higher than that of those reported in the literature and comparable to a recent report on a single phase fcc-HEA that was deformation processed. Fatigue crack growth experiments revealed high fatigue thresholds that decreased significantly with an increase in load ratio, while Paris law slopes exhibited metallic-like behavior at low R. Fatigue thresholds at all R were much higher than most conventional alloys. Fracture surface examinations of the fatigue crack growth samples revealed ductile features at various regions of the fatigue curve, with some evidence of fatigue striations in the Paris law regime.

High-entropy alloys (HEAs) composed of certain elements with nearly equi-atomic ratios have shown attractive high-temperature mechanical properties. To understand the intrinsic behaviors of the alloys, single-crystal specimens with different crystallographic orientations are demanded for various tests. In the background of understanding the fundamental deformation mechanisms of the compositionally-disorder but structurally-order HEAs, an Al_0.3CoCrFeNi single crystal ingot of 21 mm diameter and 60 mm length was grown for mechanical tests using the Bridgman method. The concentrations of Al, Cr, and Ni increase along the solidification direction, whereas the concentrations of Co and Fe decrease along the same direction. The compositional changes are within 1 at.% for each element over a length of about 55 mm. [1 0 0] and [1 1 0] rods of nominal 6 mm diameter 25 mm length were obtained from the ingots to prepare specimens for creep tests. The orientation difference from one end to the other is 1° - 3° for each rod, as determined by back Laue X-ray diffraction. Creep behaviors and detailed microstructures of the alloys will be studied.

High entropy alloys (HEAs) are a new class of materials with very unique microstructure and properties. They contain four or more components in equal or near equal atomic percent concentration. High configurational entropy associated with mixing several elements favours multicomponent random solid solutions and inhibits the formation of intermetallic phases. However, in reality, there is a very small number of multicomponent alloys that are ideal single-phase solid solutions. In most cases, the microstructure of alloys is not homogeneous, showing non-uniform distribution of atoms. This is due to preferential interactions between atoms of different types.
The phase stability of Mo-Nb-Ta-V-W alloys was investigated using Cluster Expansion combined with Monte Carlo (MC) simulations. The chemical and configurational entropies, which are the key parameters responsible for the formation of disordered solid solutions, are analysed as functions of temperature and concentration of constituents. The effective cluster interactions are determined from ab initio calculations [1].
Representative structures of Mo-Nb-Ta-V-W alloys generated using MC simulations are used as input for ab initio simulations of basic properties of HEAs such as elastic and phonon properties, energies of formation and migration of vacancies and the interatomic spacing distribution.

[1] I. Toda-Caraballo, J.S. Wróbel, S.L. Dudarev, D. Nguyen-Manh, P.E.J. Rivera-Díaz-del-Castillo, Acta Mater. 97 (2015) 156.

The material-design strategy of combining multiple elements in near-equimolar ratios has spearheaded the emergence of high-entropy alloys (HEAs), an exciting class of materials with exceptional engineering properties. Elastic properties are important criteria for engineering material design. The elastic constants of a material provide a complete description of the response of the material to external stresses in the elastic limit, which provides fundamental insight into the nature of the bonding in the material, and is known to correlate with many mechanical properties. We employed first-principles approaches to study the elastic properties of prototype HEA systems, such as Al0.3CrFeCoNi. Computed elastic constant results from Special Quasi-random Structures (SQS) and Coherent Potential Approximations (CPA) will be compared with experimental results. We will also evaluate the validity of a previously developed statistical learning model of elastic constants for HEA systems.

CrMnFeCoNi is a prototypical FCC high-entropy alloy (HEA) that is one of the most thoroughly investigated of the HEAs. Over the last three years, it has been shown to exhibit fascinating mechanical properties, including increasing strength and tensile ductility with decreasing temperature coupled with moderate strain rate sensitivity, and high fracture toughness down to cryogenic temperatures. We review here recent progress in understanding these interesting aspects from a microstructural viewpoint. Our focus will be on quantifying, to the extent possible, its detailed microstructural evolution with processing and strain and correlating it with strength, work-hardening rate, ductility, and twinning stress. Similarities and differences between this HEA and its derivative medium-entropy alloys will be discussed that enable a better understanding of the mechanical behavior of compositionally complex alloys having the FCC structure. Funding from the German Research Foundation (DFG) through projects LA 3607/1-1 (GL) and GE 2736/1-1 (EPG) is gratefully acknowledged.

High-entropy alloys (HEAs) containing Molybdenum (Mo) are known for their high-temperature strength and superior corrosion resistance. Molybdenum is known to segregate to inter-dendritic regions in the microstructure of HEAs. We combine the special quasi-random structure (SQS) approach within density functional theory (DFT) to investigate structural, mechanical and electronic properties of Mo-based alloys. The SQS approximates short-ranged pair correlations of a random solid solution, although chemically it is an ordered, layered set of configurations in the crystal lattice. We focus our study on HEA, e.g., Mo-based quaternary{(Ti/Zr/Nb/V)-W-Ta}_{1-x}-Mo_{x} and quinary,{Cr-Co-Fe-Ni}_{1-x}-Mo_{x}. Calculated properties (e.g., lattice constants, bulk modulus, elastic constants, and formation enthalpies) are compared with classical molecular dynamics (MD) simulations. Through analysis of MD results, we also explore the short-range order (SRO) effects from assessed pair probabilities. Initial results show that weak Mo-driven short-range order that decreases with increased Mo content. These results provide insights on the clustering/ordering behavior in Mo-based refractory high-entropy alloys.

In comparison to conventional alloys, high entropy alloys potentially provide a fundamentally new way to enhance the irradiation damage tolerance by means of increasing compositional complexity. Here we explored the defect evolution in equimolar ratio single-phase concentrated solid solution alloys (NiCoCr, NiCoFeCr, NiCoFeCrPd) and pure Ni under electron irradiation (1250 kV, 600 kV) and ion irradiation (1 MeV Kr⁺⁺) inside a transmission electron microscope. Defects produced by ion irradiation include perfect dislocation loops, Frank loops and stacking fault tetrahedra. The ratio of perfect loops to Frank interstitial loops decreases as does the defect density in the order of Ni, NiCoFeCr, NiCoCr. Under electron irradiation, only Frank loops were found in Ni while Frank and perfect interstitial loops in NiCoFeCr, and large faulted loops in NiCoFeCrPd. The loop growth rate irrespective of loop type decreased in the following order Ni, NiCoFeCrPd, NiCoFeCr, NiCoCr and the total defect population in the order NiCoFeCr, NiCoCr, NiCoFeCrPd, Ni. In addition, the five element alloy showed evidence for elemental segregation to the loops. The results will be explained in terms of the influence of the alloy complexity on cascade parameters, defect migration and formation energy, although it is noted the simply increasing alloy complexity cannot account for all observations.

Molecular dynamics simulations were used to study screw dislocation glide in pure Ni and an equiatomic Ni-Fe alloy with a randomly occupied face centered cubic crystal structure (fcc). The simulations show that ½<110>{111} dislocations glide smoothly in pure Ni, with the stress dependence of the dislocation velocity being linear over a wide temperature range from 300 to 900K, as is expected for pure fcc metals. However, the glide mechanism is significantly different in the Ni-Fe equiatomic alloy, where local fluctuations in chemical composition give rise to a much more complicated behavior characterized by higher strength and a complex stress-velocity relationship. The random atomic site occupation of the Ni-Fe alloy requires modeling of long dislocation lines gliding over long distances in order to adequately capture the wide spectrum of configurational fluctuations. We present results for both stress and strain controlled dislocation glide and describe important correlations between dislocation configurations and flow stress at different temperatures. The observations are discussed in terms of their potential relevance to the mechanisms of plastic deformation in single-phase high entropy alloys.

This work was supported by the US Department of Energy Office of Science, Basic Energy Sciences, Materials Science and Engineering Division.

Abstract
High-entropy alloys (HEAs), containing multiple metallic elements in equimolar or near-equimolar ratios, have generated great interest, because they possess a simple structure but with outstanding mechanical properties. Despite their long-range crystal order, locally the multiple elements in HEAs may occupy the lattice sites in a disordered or partially-ordered fashion, leading to phenomenal properties. Thus, it is essential to establish general relationships between microstructures and essential mechanical properties of strength, ductility, and creep resistance, applicable to several HEA systems. The effect of heat-treatment on strength, ductility, serration, and creep resistance is discussed. An in-situ neutron diffraction study is used to understand the atomic-structure evolution and deformation behaviors of the Al_0.3CoCrFeNi at elevated temperatures. The results shows the creep behavior of Al_0.3CoCrFeNi is superior to conventional alloys, and the heat treatment introduces secondary B2 phase into the FCC matrix, which increase the yielding strength, decrease the ductility, and diminish the serrated flow, during compression tests at high temperatures.
Acknowledgement
This work was supported by the Department of Energy (No. DE-FE0008855) with the program manager, Vito Cedro, the U.S. Army Research Office project (W911NF-13-1-0438) with the program manager, Dr. D. M. Stepp, and the National Science Foundation (CMMI-1100080) with the program director, Dr. C. Cooper.

High Entropy material possess unique properties such as 1) high entropy effect – single phase stabilization, 2) sluggish diffusion effect – slow diffusion of atoms/ions at higher temperatures, 3) severe lattice distortion – lattice distortion due to different sized dopants and 4) cocktail effect – property dependent on one element. High entropy materials include metals, oxides, nitrides and carbides. The above mentioned properties suit well with the requirement of catalysis where the support material should exhibit resistance to sintering and maintain high specific surface area at higher operating temperatures. Keeping this in mind, we report the synthesis and characterization of a single phase solid solution system consisting of five rare earth/ transition metal oxides. A facile co-precipitation- re-dispersion method was employed to synthesize two distinct solid solution nanoparticle compositions. One solid solution consists of ceria as a major constituent Ce_0.6Gd_0.1Hf_0.1La_0.1Zr_0.1O_2-δ and other consists of equiatomic ratios of metals Ce_0.2Gd_0.2Hf_0.2La_0.2Zr_0.2O_2-δ. Effect of heat treatment up to 1000°C/24h on phase stability and grain growth behavior was studied using powder X-ray Diffraction (XRD), Raman spectroscopy and Nitrogen sorption studies Brunauer-Emmett-Teller (BET) method. For both solid solution systems, XRD results for heat treated sample showed reflections corresponding to pure cubic fluorite phase of cerium oxide with uniform shift in peak positions compared to that of cerium oxide although the constituent oxides show different phase behavior in pure form. Similarly for both compositions, Raman data also supports the XRD data signifying the formation of single phase solid solution with creation of oxygen vacancies. A band corresponding to F2g mode of cubic structure was observed for both samples at lower calcination temperature whereas at higher temperature, phase transformation occurs for Ce_0.2Gd_0.2Hf_0.2La_0.2Zr_0.2O_2-δ from cubic to tetragonal t” which was evident from Raman spectrum but not from XRD. Another characteristic feature of entropy stabilized system is sluggish diffusion of atomic/ionic species at higher temperature. For both solid solution systems, particle growth was highly restricted where crystallite size increased from 4nm to 8nm and 4nm to 12nm for Ce_0.6Gd_0.1Hf_0.1La_0.1Zr_0.1O_2-δ, and Ce_0.2Gd_0.2Hf_0.2La_0.2Zr_0.2O_2-δ respectively which is in contrast with ceria nanoparticle whose crystallite size increases from 10nm to 96nm at 500°C/4h and 1000°C/24h respectively. BET results show better surface area stability at higher temperatures for both compositions when compared to pure ceria. From the above mentioned observations, we can conclude that Ce_0.6Gd_0.1Hf_0.1La_0.1Zr_0.1O_2-δ and Ce_0.2Gd_0.2Hf_0.2La_0.2Zr_0.2O_2-δ are entropy stabilized oxides whose characteristic high temperature phase stability, resistance to surface area loss and oxide ion deficiency makes them a suitable candidate for catalytic applications.

Substandard castability and thermomechanical post processing requirements for high entropy

alloys (HEAs) are common obstacles to their industrial and technological implementation.

Achieving targeted microstructures is often challenging, given the tendency for HEAs to

simply form dendritic structures during cooling from the melt. To confront these problems,

we propose a one-step processing route that causes the material to form a composite with an

HEA matrix. This strategy utilizes equilibrium solidification of a CrMnFeCoNiCu equiatomic

alloy. The resultant microstructure is composed of a soft Cr-poor matrix, with embedded hard

Cr-rich needle-shaped precipitates of random orientation. X-ray diffraction, scanning electron

microscopy and optical microscopy were used to characterize the microstructure of the

composite. Mechanical properties of the composite, including elastic modulus, yield strength

and nanoindentation hardness were also measured. This new design strategy can be further

developed and adapted to large-scale industrial production.

In this study, a thin film combinatorial approach was applied to the development of high entropy alloys (HEAs). In order to study HEA systems with different crystal structures, several HEA compositions were selected, including: OsRuWCoIr and OsRuWMoRe to obtain the hexagonal closed packed (HCP) crystal structure, and CoFeMnNiCu to achieve a face centered cubic (FCC) HEA. Thin film samples were fabricated by simultaneous magnetron sputtering of elements onto silicon wafer substrates. This arrangement yielded a chemical composition gradient in the films. The gradient in composition resulted in formation of various phases, with some regions exhibiting the desired single-phase HEA. Most of the gradient sample, and hence most of the single-phase regions, had non-stoichiometric compositions. The thin film samples were characterized using several techniques to identify promising candidate HEA compositions. Single-phase FCC and HCP HEAs were identified using this combinatorial method and successfully produced in bulk form by arc melting of metals. Characterization of the bulk alloys confirmed these HEAs each consisted of a single-phase. Using nanoindentation, the mechanical properties of the samples were measured for several alloys, and these will be discussed in relation to the screening and optimization approach presented here.

The Mo-Tc-Ru-Rh-Pd quinary alloy system is of special importance to understanding the behavior of spent nuclear fuels. The metal components are formed as byproducts of fission processes and subsequently aggregate into noble metal inclusions, typically with the hexagonal close-packed (hcp) crystal structure. Here we investigate the thermodynamic landscape of the five-component system using Density Functional Theory (DFT) simulations, with particular focus on the vibrational entropy of mixing. We find that the incorporation of the body-centered cubic (bcc) molybdenum into a close-packed coordination environment leads to softening of phonon modes and an increase in vibrational entropy commensurate with the configurational entropy of the disordered solid solution. This additional source of entropy contributes to greater thermodynamic stability of the metal precipitates and resolves lingering disagreement between reported theoretical and experimental free energies in several binary sub-systems. A careful analysis of the vibrational behavior of this system can therefore significantly improve our understanding of the key mechanisms guiding the formation, surface chemistry, and composition of metal precipitates in the harsh radiation environment typical of spent fuels.

A hHigh entropy alloy (HEA) consists of more than five alloy components with similar atomic percentage. High configurational entropy leads to formation of stable solid solution with severely distorted lattice structure. Calculation of the phase diagram of HEA based on the thermodynamics is important not only to reveal phase stability but also to search new alloy system. In addition, since the property of the alloy is closely correlated with the microstructure, it is necessary to understand the phase diagram to optimize the microstructure. Therefore, for deeper insight, hierarchical analysis from microscale down to nanoscale is required to study the microstructure. A plate type CrFeCoNiCu ingot was fabricated by arc melting and suction casting, and then homogenized at 1373 K for 24 hours. XRD, SEM, EPMA, TEM (Tecnai F20, Talos F200X @ 200 keV, Titan S @ 300 keV) and 3D-APT are utilized to identify the crystal structure and chemical composition of the constituent phases. The pseudo-binary section of the (Cr_0.25Fe_0.25Ni_0.25Co_0.25)-Cu was calculated based on the CAPHAD method[1] and the actual calculation was performed by the Thermo-Calc (version S)[2] in combination with the STGE alloy solution database (version 5.1).
CrFeCoNiCu HEA[3] has two FCC phases, CrFeCoNi-rich dendrite and Cu-rich interdendrite. Interestingly, several nanometer-sized Cu-rich particles and several tens of nanometer-sized CrFeCoNi precipitates are homogeneously distributed in the dendritic and the interdendritic regions, respectively. After homogenization, the microstructure changes dramatically but, FCC structures are sustained and the dendrite and interdendrite of the homogenized alloy were characterized by 2^nd phase of the Cu-rich phase in the dendrite and 2^nd phase of the Cr-Fe-Co-Ni phase in the interdendrite, respectively. Although phase diagram of multi-component system is difficult to predict, pseudo-binary phase diagram of CrFeCoNi/Cu HEA can be calculated since it consists of only two phases, CrFeCoNi-rich phase and Cu-rich phase. The calculated phase diagram was compared with experimental data and finally phase transformation path during solidification and annealing was found.
[1] Kaufman, H. Bernstein, Computer Calculation of Phase Diagrams, Academic Press, New York, 1970.
[2] http://www.thermocalc.com
[3] N Park et al., Met. Mater. Trans. A (2014) vol. 46, pp. 1481-1487

Multicomponent alloys and ceramics have been a very long time interesting topic, only when scientists believe that the formation of disordered phases, e.g., solid solution or amorphous phase, is due to the high-entropy of configuration or entropy of mixing, then the conception of high-entropy alloys and ceramics were proposed. Usually the criteria of high-entropy is defined about 1 R, here R is the gas constant. According to the thermodynamic theory, G = H - TS, here G is Gibbs free energy, H is enthalpy, S is entropy, T is absolute temperature, thus high-entropy will lead to the low Gibbs free energy G, and high phase stability; Kinetically, mobility of the constitute elements in high-entropy alloys is slow, and also means high stability. In the paper, the phase stability of high-entropy alloys and ceramics will be compared with the low and medium-entropy alloys and ceramics, in the forms of bulk, fibers, and films.

The fracture toughness and fatigue crack growth behavior of various high entropy alloys were determined. In addition to testing two multi-phase alloys, an FCC Al_0.3CoCrFeNi HEA was also prepared and tested. Notched and fatigue precracked toughness values were significantly higher for the FCC single phase HEA in comparison to the multi-phase HEA and higher than those reported in the literature. Fatigue crack growth experiments in 3-point bending revealed high fatigue thresholds that decreased significantly with an increase in load ratio, with low Paris law slopes. Fatigue thresholds at all R were much higher than most conventional alloys. Fatigue fracture surface examinations of the multi-phase samples revealed ductile and brittle features at various regions of the fatigue curve, with some evidence of fatigue striations in the Paris law regime. The FCC single-phase samples only exhibited ductile features, also with some evidence of fatigue striations in the Paris law regime.

Single-phase concentrated solid-solution alloys (CSAs) have received considerable attention since the discovery of high entropy alloys. With two or more equiatomic principle elements situated in a simple lattice, CSAs are fundamentally different from conventional dilute alloys. In particular, the defect energies, such as formation energies and migration barrier, exhibit distributions rather than single values as a result of the underlying compositionally disordered state. First-principles calculations, based on a supercell approach, are employed to characterize these energy distributions for vacancy and interstitial defects in binary and ternary CSAs. The results reveal a broad distribution of migration barriers for vacancies and interstitials. The distributions of formation energies for different interstitial dumbbells are separated, suggesting preferable binding of interstitials. Combined with ab initio molecular dynamics, a preferable diffusion mechanism is found in CSAs, which is closely related to the formation energy distributions. The origins of these observations result from the unique disordered structure of CSAs, in which elements are slightly displaced from the ideal lattice site. As a consequence, those elements with small atomic size have high diffusivities. These results provide insight into the defect energetics and diffusion mechanism in these novel alloys that have direct relevance to the element segregation and phase separation phenomena observed in experiment.

The rapid development of modern industries demands new materials exhibiting greatly improved performances available at extreme environments of high temperature, intense radiation flux, and high stress. High entropy alloys (HEAs) have attracted worldwide attention as strong candidates to resolve the challenges because of their useful performances including high toughness, corrosion resistance, high temperature strength, as well as a good irradiation resistance. Unlike most traditional alloys involving one major element with small additives, HEAs consist of five or more principal elements at equal or roughly equal concentrations in a single phase or simple structured microstructure. Intense work revealed that the complex compositional environment of the HEAs lead to novel properties, which are entropic stabilization of solid solutions, severe lattice distortion, and sluggish diffusion, resulting in high performances mentioned above.
In this letter, we discuss the phase stability of AlxCrFeCoNi HEAs under extreme condition by high pressure torsion and parabolic flight viscosity measurement, with interpretation of thermodynamic states regarding to Ballistic diffusion model. We show that effective temperatures, imaginary temperature which corresponds to thermodynamic states of the system under high pressure torsion, of present HEAs are in the high temperature range that high entropy phase become stable. Understanding of the actual mechanism of phase stability in the HEAs under high pressure torsion will provide the guideline of both processing routes including severe plastic deformation and development for extreme environments materials.

In a non-equiatomic HEA from CoCrFeMnNi family, so-called A3S® alloy (austenitic superstainless steel), exceptional mechanical properties are observed. High yield strength (800 MPa) associated with significant elongation to fracture (35 %) and a stable austenitic paramagnetic structure (down to LN2 temperature) may be obtained in hot forged materials. Mechanical resistance (YS) of A3S is at least 200 MPa higher than that measured in equiatomic alloy (EA) of the same family. These properties come from a nanostructure, easily formed in the material after classical hot thermomechanical treatment (forging). Yet, mechanical resistance of A3S decreases strongly after a post-forging high temperature annealing: in this state, identical behaviors of A3S and EA are noticed.
Effects of thermal conditions of forging and recrystallization annealing have been investigated in A3S. Only slight effect of temperature of forging (between 900 and 1100°C) has been shown. Surprising effects of recrystallization annealing have been found. Low temperature (up to 600°C for 48h) treatment leads to recrystallization with formation of fine (1 µm) grains. At higher temperatures (700-1100°C), recrystallization is blocked: only recovery followed by grain growth and formation of numerous twins is observed. Moreover, in this state, very high density of dislocations is conserved: they present ordered configurations with alignments in {111} planes.
Formation of twins has major effect on mechanical behavior of both A3S and EA. Their absence stabilizes nanostructures and leads to high YS values while decrease of YS is accompanied by twins presence. Relative difficulty to form these defects in A3S (as compared to EA) is explained by high value of stacking fault energy, evaluated from TEM measurements of dislocations dissociation. Moreover, strong dependence of SFE on temperature has been shown: its lower value at high temperatures is in agreement with numerous twins observed.

A new kind of equiatomic DyGdYHoTb solid solution high entropy alloy was synthesized by arc-melting. The hexagonal close packed (HCP) lattice of this HEA was revealed by X-ray diffraction (XRD) pattern. A series of discs, 2 mm in thickness and 10 mm in diameter, were applied to semi-constraint high pressure torsion under a pressure of 5 GPa with 0, 0.5, 2.5 and 5 turns, respectively. Vickers hardness of the twisted, discs showing great relevance to the number of turns, complies well with the strain-hardening theory. On the other hand, compression curves of 3 samples, one as-cast, one just pressured under 5GPa and one annealed at 1000°C under 5 GPa, were plotted after the compression test. Slight fluctuation in the compression strength and ductility observed through the curves could be explicated by the theory of alloys heat treatments.

Between Cu wires and dielectric layers in integrated circuits (ICs), an ultrathin robust diffusion barrier is needed to inhibit the rapid diffusion of Cu atoms into Si devices. Recent studies suggested that (1) multicomponent high-entropy alloy barriers with severe lattice distortions would provide a high resistance to Cu/Si interdiffusion, and (2) a “self-formation” method by the segregation of solutes from Cu alloy films to Cu/dielectric interfaces during annealing would yield an ultrathin barrier layer. In combination of the two suggestions, self-forming multicomponent high-entropy alloy layer was accordingly considered in this work for producing an effective diffusion barrier for IC application. Five Cu alloy films with one to five solute elements (immiscible in Cu, i.e. with a large positive mixing enthalpy with Cu), including Cu(V) – Cu(V,Nb,Mo,Ta,Cr), were prepared and annealed. The segregation of the solute elements and the formation of unitary V – quinary (V,Nb,Mo,Ta,Cr) alloy layers were examined. Experimental results indicated that, in the five Cu alloy films, different solute segregation behaviors were observed under the competition of mixing enthalpy and mixing entropy. For the Cu(V) alloy film, the solute segregated to the Cu/Si interface and formed a V layer, dominated by the large positive mixing enthalpy of V and Cu. However, for the Cu(V,Nb), the Cu(V,Nb,Mo) and the Cu(V,Nb,Mo,Ta) alloy films, the solutes formed intermetallic compounds at the grain boundaries of the Cu alloy films due to the negative mixing enthalpies and the low-to-medium mixing entropies of the solute elements. In comparison, for the Cu(V,Nb,Mo,Ta,Cr) alloy film, the solutes again segregated to the Cu/Si interface and formed a quinary (V,Nb,Mo,Ta,Cr) alloy layer owing to the high mixing entropy of the five solute elements.

We experimentally and theoretically investigate the phase stability of non-equiatomic CoCrFeMnNi based high entropy alloys. Deviation from the equiatomic stoichiometry is motivated by the fact the course of the entropy as a function of composition is rather flat, enabling probing a larger range of alloy compositions. The theory methods used are CALPHAD and ab initio simulations. Experimental characterization is conducted by using electron microscopy, atom probe tomography and mechanical probing. Bulk combinatorial synthesis over a wide compositional spectrum is realized by metallurgical rapid prototyping in conjunction with thermomechanical processing. The aim of this study is to systematically and critically assess and apply the predictive capability of the various thermodynamic approaches particularly with respect to assessing and tuning phase stability for such high entropy alloy systems [1-3]. We find that the thermodynamic simulations provide a consistent prediction of phase stability yielding good agreement with experimental observations. These include the equilibrium phase formation at high temperatures, the constituent phases and the trends for deformation-induced transformation effects. Based on these thermodynamic guidelines we pursue a metastability-design strategy instead of the well-established phase stabilization through entropy maximization [2-4]. The results are used for synthesizing twinning-induced high entropy alloys, transformation-induced high entropy alloys as well as dual phase high entropy alloys with high strength and ductility. These effects create a variety of additional strain hardening phenomena such as hetero-interface hardening, twin interface hardening, dual-phase hardening, transformation-induced hardening and solid-solution strengthening.


1. Li, Z., Pradeep, K.G., Deng, Y., Raabe, D., Tasan, C.C., “Metastable high-entropy dual-phase alloys overcome the strength–ductility trade-off”, Nature, June 2016
2. Ma, D., Grabowski, B., Körmann, F., Neugebauer, J. & Raabe, D. 2015, "Ab initio thermodynamics of the CoCrFeMnNi high entropy alloy: Importance of entropy contributions beyond the configurational one", Acta Materialia, vol. 100, pp. 90-97.
3. Ma, D., Yao, M., Pradeep, K.G., Tasan, C.C., Springer, H. & Raabe, D. 2015, "Phase stability of non-equiatomic CoCrFeMnNi high entropy alloys", Acta Materialia, vol. 98, pp. 288-296.
4. Deng, Y., Tasan, C.C., Pradeep, K.G., Springer, H., Kostka, A. & Raabe, D. 2015, "Design of a twinning-induced plasticity high entropy alloy", Acta Materialia, vol. 94, pp. 124-133.