Symposium F.SF06—High-Entropy and Compositionally Complex Alloys

Cantor alloy, which is a face-centered cubic (fcc) high entropy alloy composed of the equiatomic ratio of Co, Cr, Fe, Mn, and Ni, has been attracting attentions as structural materials because of its good strength–ductility balance. By introducing a secondary strengthening phase to the Cantor-based fcc matrix, the development of materials with an excellent balance of strength and ductility is expected. We selected Ti and Si as additive elements. We investigated the phase equilibrium of Ti or Si added Cantor alloy and found that these elements introduce some secondary phases such as Laves phase, αMn structure phase, and βMn structure phase. However, as the basis of alloy designing with these phases as precipitates, it is quite important to reveal the effect of additive elements on the mechanical properties of the matrix phase. In the present study, the effect of Ti or Si addition on mechanical properties of Cantor-based fcc single-phase alloys were investigated.
Several (Co, Cr, Fe, Mn, Ni) -Si and (Co, Cr, Fe, Mn, Ni) -Ti alloys were prepared by arc melting or induction melting. The ratio of Co, Cr, Fe, Mn, and Ni was selected and kept to be equiatomic. Each alloy sealed in evacuated silica tubes was annealed at 1000 ^°C for 168 hours. After homogenization, all the alloys were identified as fcc single-phase alloys by using XRD and FE-EPMA. The Vickers hardness was investigated with a load of 4.9 N. After machining from the cold-rolled sheets, the dog-bone-shaped specimens for tensile tests were annealed at 1000 ^°C for 15 minutes, and fully recrystallized fcc single-phase specimens with a grain size of ~40 μm were obtained, regardless of the composition. The tensile test was performed with an engineering strain rate of 1.0 × 10^-3 s^-1 at room temperature.
It was found that the Vickers hardness increased with either element addition. The increase in hardness of the Ti-added alloy was greater than that of the Si-added alloy. Furthermore, the ductility was improved by Si addition, while Ti addition does not show significant improvement. It was suggested that Ti is an alloying element that improves strength, while Si improves both strength and elongation of the matrix phase.

Medium/high entropy alloys are a class of complex compositional solid solution alloys with equiatomic or nearly equiatomic compositions, which prefers to form single-phase solid solution. Some of these alloys exhibit exceptional mechanical properties. For example, their strength, ductility and fracture toughness increase with decreasing temperature. Cr-Fe-Co-Ni equiatomic solid solution alloy is one such alloy with face-centered cubic (FCC) structure, which has been reported to exhibit exceptional strength in polycrystalline materials. However, the detailed mechanisms behind the exceptional mechanical properties of Cr-Fe-Co-Ni equiatomic alloy remain unclear in many aspects, primarily because of the lack of fundamental studies using single crystals. In this study, we prepared single crystals of the Cr-Fe-Co-Ni equiatomic solid solution alloy by directional solidification in an optical floating zone furnace, and investigated the deformation behavior of the [-123]-oriented single crystals under uniaxial tensile and compressive loading over a wide temperature range (13K to 1073K).
Temperature dependence of the activation volume were investigated by strain rate change compression tests (1×10^-5 to 5×10^-3 s^-1). Stress-strain curves obtained from tensile tests at low temperatures indicate the presence of twinning deformation, also found in Cr-Mn-Fe-Co-Ni and Cr-Co-Ni equiatomic solid solution alloys. This is considered to reflect a relatively low stacking fault energy for the Cr-Fe-Co-Ni equiatomic alloy. We also compared the critical resolved shear stress (CRSS) of the Cr-Fe-Co-Ni equiatomic alloy with Cr-Mn-Fe-Co-Ni and Cr-Co-Ni equiatomic alloys. The CRSS value increases significantly with decreasing temperature down to 0 K, and with increasing temperature above 873 K. Activation volume of the Fe-Cr-Co-Ni equiatomic alloy is found to be much lower than those of binary FCC solid solution alloys, which is consistent with quinary HEA and ternary MEA.

The high entropy alloys (HEAs) have unique properties such as a high radiation resistance and corrosion resistance at elevated temperatures compared with conventional nuclear component materials. This attractive property could make HEAs candidates for high temperature nuclear reactor components. However, the studies on their microstructural evolution and mechanical property change under irradiation, especially at elevated temperatures, is very limited. FCC-type nuclear fission and/or fusion materials have lower stacking fault energies (SFE), and these materials would form stacking fault-type defects, such as stacking fault tetrahedron (SFT) especially in Cu alloy and Ni base alloys and Frank loops in matrix, when irradiated in a wide temperature range. Those stacking fault-type defects, especially Frank loops, would increase the yield strength and decrease the elongation of materials. Therefore, the control of SFE would be a key of less degradation of FCC-type materials under irradiation.In this study, we investigated the effect of Mn and Ni concentration on the stacking fault energy (SFE) in CoFeCrNi_xMn_y high entropy alloys. The TEM observation of the deformed alloys indicated that the SFE of CoFeCrNi_xMn_y seemed to be increased with increasing Mn and Ni concentration. Furthermore, the tensile properties of the alloys appeared to have a relationship with SFE, especially in the yield strength and total elongation. From these results, it is suggested that CoFeCrNi_xMn_y could be well-designed by controlling the stacking fault energy with optimized Mn and Ni concentration.

Recently, high-entropy alloys (HEAs) with the body-centered cubic (BCC) structure have received considerable attentions as new structural materials because of their attractive mechanical properties, such as superior strength retention at elevated temperatures of V-Nb-Mo-Ta-W equiatomic alloy and its derivatives and good low-temperature ductility observed for Ti-Zr-Nb-Hf-Ta equiatomic alloy. However, the underlying mechanisms endowing these BCC-HEAs with attractive mechanical properties are largely unknown mostly because of the lack of experimental results for these BCC-HEAs. It is thus quite important to investigate fundamental deformation behavior of these BCC-HEAs. In the present study, we have prepared polycrystalline ingots of the Ti-Zr-Nb-Hf-Ta equiatomic alloy by arc-melting, cold-rolling and subsequent heat-treatment. Compression tests of polycrystalline specimens were carried out at room temperature and 77 K and orientation dependence of slip plane was investigated through trace analysis of apparent slip planes appeared on the surfaces of compressed polycrystalline specimens. In addition, micropillar compression tests of single crystals fabricated from the polycrystalline ingots were carried out as a function of specimen size and loading axis orientations in order to clarify the size and orientation dependence of critical resolved shear stress.

In order to operate advanced nuclear power reactors safely and efficiently, it is essential to develop new structural materials with high irradiation resistance, especially at higher temperatures. The development of structural materials for nuclear reactors has been mainly focused on highly reliable steel materials such as austenitic stainless steels and low alloy steels. While, in recent years, high entropy alloys (HEAs) have attracted attention as a high radiation resistance material at high temperatures. In this study, we investigated the effects of Mn and Ni composition on the change of mechanical property and microstructure development in FeCrNiMn-based alloys under irradiation.
The results of the present study indicate that the stacking fault energy of FeCrNiMn-based alloys seemed to be increased with increasing Mn and Ni concentration. Furthermore, the ion-irradiation to the alloys resulted in the formation of plane defects such as stacking fault tetrahedra and frank loops. The number density and the average size of these defects appeared to have a strong relationship with stacking fault energy of each alloy. From these results, it is suggested that FeCrNiMn-based alloys could be improved to a higher irradiation resistant alloys with an appropriate stacking fault energy by controlling Mn and Ni concentration.

Lattice distortion in high entropy alloys is postulated to have major effects on these alloys and their properties. There are limited studies that look at the effect of lattice distortion on entropy stabilized oxides. In this study, the entropy stabilized oxide, MgCoNiCuZnO₅ is explored to understand the lattice distortion in this system. This work uses molecular dynamics to identify the explicit distances that each atom and atom type distorts from its parent rocksalt crystal structure. Through manipulation of the Buckingham interatomic potentials used to define the structure, effectively changing the alloy composition, changes in the lattice distortion are understood. This entropy stabilized oxide can be optimized to either increase or decrease the total lattice distortion in the system by changing the atomic composition or by replacing certain elements with alternative elements. The effective bond length between metal and oxygen as opposed to bond strength, charge, or mass, is the main factor that causes the distortion of this system.

The alloys have been traditionally held using a base metal and others in lesser proportion in order to improve the properties of the former. However, at the end of the 20th century a new concept of alloys emerged totally revolutionary based on the mixing of multiple components in fractions identical or similar molars and it was not until 1996 when the concepts of "high-entropy alloys" (HEAs) and "multi-principal-element alloys" (MPEAs), which were perfected and extended in 2004 in publications by Jien-Wei Yeh (Taiwan) and Brian Cantor (UK) (Yeh et al., 2004)(Zhang, 2019)
The behavior of two new high entropy alloys named BioHEA 1 (FeMoTaTiZr) and BioHEA 2 (NbMoTaTiZr) containing chemical elements that exhibit relatively low bio-toxicity for the human body has been obtained and characterized in order to use them for medical instruments, such as surgical blades, saws or cutters.
Vacuum arc remelting (VAR) process was used for the manufacture of the high-entropy alloys by using the MRF ABJ 900 VAR at ERAMET Laboratory (Romania). Highly pure powders of Mo, Ta, Ti, Zr, Nb and Fe (99.9%) were used and classified according to ASTM B214-16 . Eventual losses of material by vaporization are to be heed, so as the theoretical chemical elements assimilations degree into the melt. Both conditions are taken into account for the charge. The alloys obtained were melted in VAR unit up to seven times making use of Argon as inert atmosphere, so that an adequate homogeneity could be achieved.
The experimental alloys were microstructurally characterized to highlight the phase’s types and the distribution of chemical elements in the dendritic formations.
The corrosion properties of the HEAs were evaluated using a potentiodynamic polarization method, open circuit potential, pitting potential and repassivation potential. Also the electrochemical impedance spectroscopy (EIS) technique was used. The alloys were immersed in SBS (Simulated Body Fluid) during one week and the corrosion parameters were registered.
Analysis of the impedance spectra was carried out by fitting different equivalent circuits to the experimental data. Two equivalent circuits, with one time constant and two time constants respectively, can be satisfactory used for fitting the spectra: one time constant represents the characteristics of the passive film and the second one is for the charge transfer reactions.
We concluded that the low corrosion rates, low corrosion currents and high polarization resistance attest the good stability of these high entropy alloys in simulated biological environment.

References:
Yeh, J. W. et al. (2004) “Nanostructured high-entropy alloys with multiple principal elements: Novel alloy design concepts and outcomes,” Advanced Engineering Materials, 6(5), pp. 299-303+274. doi: 10.1002/adem.200300567.
Zhang, Y. (2019) High-Entropy Materials A Brief Introduction. doi: 10.1007/978-981-13-8526-1.
ASTM B214-16, Standard Test Method for Sieve Analysis of Metal Powders, ASTM International, West Conshohocken, PA, 2016. (2016) doi:10.1520/B0214-16.

Since the discovery of the face-centered-cubic (fcc) single-phased CoCrFeMnNi and the proposal of the concept of high entropy alloys (HEA) in 2004, the understanding of the thermodynamic and mechanical behavior of those alloys has greatly increased. Indeed, it is now possible to reliably map the stable phases in the Co-Cr-Fe-Mn-Ni system using the CALPHAD method [1] and to calculate the solid-solution strengthening [2]. So today, modelling tools are available to “investigate the unexplored central region of multi-component space” [3]. It is time to go pass single-phase materials and/or equimolar quinary compositions and look for complex concentrated alloys. To do so, two different approaches will be presented.
First, the objective is to identify compositions of solid-solution to resist to severe environment (like high temperature and corrosive environment). The strategy consists in: (i) identifying criteria which are relevant for the targeted properties (like the yield strength, the Cr content or the cost) ; (ii) mapping those criteria for the entire composition space ; (iii) selecting compositions ; (iv) performing an experimental validation by processing, microstructural characterization and properties measurements. This will be presented for the Co-Cr-Fe-Mn-Ni and Co-Cr-Fe-Mo-Ni systems.
Second, the objective is to create a chemical architecturation within the CoCrFeMnNi alloy in order to strengthen it while preserving its ductility. More specifically, a mixture of pure Ni and CoCrFeMnNi powders is sintered in order to create chemical gradients at the interfaces of both phases [4]. The microstructure of these innovative chemically architectured alloys will be characterized in detail. Then it will be shown how the width of the gradient can be controlled by varying the processing parameters and how it influences the mechanical properties. Finally, the different strengthening contributions will be sorted out and analyzed.


[1] Bracq, G., Laurent-Brocq, M., Perrière, L., Pirès, R., Joubert, J.-M., and Guillot, I., Acta Materialia, 2017. 128: p. 327-336.
[2] Bracq, G., Laurent-Brocq, M., Varvenne, C., Perrière, L., Curtin, W.A., Joubert, J.M., and Guillot, I., Acta Materialia, 2019. 177: p. 266-279.
[3] Cantor, B., Chang, I.T.H., Knight, P., and Vincent, A.J.B., Materials Science and Engineering: A, 2004. 375–377: p. 213-218.
[4] Laurent-Brocq, M., Mereib, D., Garcin, G., Monnier, J., Perrière, L., and Villeroy, B., Journal of Alloys and Compounds, 2020. 835: p. 155279.

The corrosion resistance of several single phase, non-equiatomic, NiFeCrMnCo compositionally complex alloys (CCAs) containing a range of Cr content from 6-22% was investigated in chloride solutions. Comparison was made to NiCr and FeCr binary alloys with similar Cr contents as the CCAs. The oxide film character and the localized corrosion performance, including susceptibility to crevice and pitting corrosion, were studied utilizing in-situ electrochemical and ex-situ surface-sensitive techniques. An NiFeCrMnCo CCA with only 6% Cr was found to exhibit passivity and breakdown by pitting corrosion. Crystallographic pit morphology was observed in most of these CCAs, suggesting a difficulty in precipitation of a salt film, which would result in polished pit surfaces. The crystallographic pit morphology indicates that the pit growth was under charge-transfer/ohmic control. The passive film composition, thickness, and elemental valence states, and fate of each element were studied for the CCA with 22% Cr by in-situ atomic emission spectro-electrochemistry, ex-situ X-ray photoelectron spectroscopy, and atom probe tomography. It demonstrated slightly better corrosion resistance than the binary Ni-24Cr alloy. Passive films on the HEA contained primarily Cr, and small amounts of Ni, Fe and Mn, while dissolution of Ni, Fe, Co was observed. Metallic Ni was enriched at the passive film-metal interface. Enrichment of Cr in the passive film occurred to a greater extent in the HEA than for the Ni-Cr binary alloy. Enrichment factors were determined and the origins of enrichment were assessed.

Ti-V-Cr based hydrogen absorbing alloys these have BCC structure absorb hydrogen up to 3 wt% that is the highest among materials working under ambient conditions [1]. However, they have a potential to store hydrogen around 4 wt% because more than 1wt% of hydrogen in the hydrogenated alloys cannot be released at room temperature. To improve hydrogen capacity, the number of constituent elements is increased to make multi component alloys. Equimolar multi component alloys are called as high entropy alloys or multi principal element alloys. It has been reported a five-element equimolar high entropy alloy TiZrNbHfTa absorbed hydrogen at 300°C [2]. We tried to prepare four or five element Ti-V-Cr based alloys with equimolar or near-equimolar compositions because they are lighter than the reported high entropy alloys consisted of heavy elements and expected to absorb hydrogen under ambient conditions. Hydrogen absorbing performance was measured at room temperature. Some of four and five element alloys with BCC structure prepared in this study reversibly absorb and desorb hydrogen at room temperature.
[1] E. Akiba, H. Iba, Intermetallics, 6, 461-470 (1996).
[2] C. Zlotea, M.A. Sow, G. Ek, J. -P. Couzinié, L. Perrière, I. Guillot, J. Bourgon, K. T. Møller, T. R. Jensen, E. Akiba, M. Sahlberg, J. Alloys Comp., 775, 667-674 (2019).

Reports on the unique properties achieved with HEAs, including improved mechanical properties, has motivated the application of the multi-component approach to oxide materials, expanding the available compositional space and providing greater flexibility to meet the demands of today’s advanced materials. We have applied this methodology to an alumina based spinel oxide, resulting in the synthesis of polycrystalline (Mg_0.2Ni_0.2Co_0.2Cu_0.2Zn_0.2)Al₂O₄. Samples were made by either the solid state reaction method or the polymeric steric entrapment (PSE) technique, and in-situ High Temperature X-Ray Diffraction (HTXRD) was used to compare the phase transformation behavior and investigate the high temperature phase stability. Resonant Ultrasound Spectroscopy (RUS) has been used to evaluate the elastic behavior of a dense polycrystalline pellet sample, allowing for a comparison of the mechanical properties of the multicomponent (Mg_0.2Ni_0.2Co_0.2Cu_0.2Zn_0.2)Al₂O₄ to those of the traditional MgAl₂O₄ spinel.

High-entropy alloys (HEAs) are a new class of metallic materials because of their unique properties; high strength and ductility at low temperatures, texture stability due to sluggish diffusion of atoms at elevated temperatures, and so on. HEAs are solid solution and the constitute atoms form a lattice but the chemical atomic arrangement is totally disordered. It is pointed out that, however, some local inhomogeneity or structure are existed in HEAs to accommodate the large local strains and hence assist the stability of HEAs [1]. Amorphous alloys are another type of random system where the atom positions are chemically and topologically disordered. Existence of local structures in amorphous alloys and the characteristic dynamic response were suggested by several studies. We found that the resistivity of amorphous (a-) ZrCu alloys was decreased to less than 60% of the initial value after passing an electric pulse(electrpulsing). In the experiment, the current exponentially decayed with the time constant of ~3 ms from the initial current density of ~0.6 GA/m² [2]. Further, nanocrystallites of ZrCu, the high-temperature intermetallic compound phase, were revealed from the X-ray diffraction measurement and transmission electron microscopy. These observations suggested that that crystalline-like local structures in a-ZrCu are embedded in the amorphous alloys and transformed to nanocrystallites through collective motions of the local structures excited by electropulsing. The information on the local structure and its dynamical behavior can be surveyed from the electropulsing experiment.
In the present study, the electropulsing experiments were performed for CrMnFeCoNi HEA in the as-prepared state and annealed state. The phase separation of CrMnFeCoNi HEA was reported after annealing below 1073 K [3]. It is expected that the local structure is developed by annealing below 1073 K and the texture change or phase separation are accelerated by electropulisng. For CrMnFeCoNi HEA in the as-prepared state, no decrease in the resistivity and no secondary phase formation were observed after electropulsing up to 1.4 GA/m². By annealing at 1023 K for 1 hour, the resistivity of CrMnFeCoNi HEA showed an increase by 10 % from that in the as-prepared state. The resistivity increased by annealing showed a slight recovery by electropulsing up to 1.2 GA/m²; the resistivity became 8% larger than that in the as-prepared state. It is noted that no texture change and no new phase formation were detected by the XRD measurements after the present annealing and subsequent electropulsing. The increase in the resistivity by annealing indicates some local structure formation toward the phase separation, whereas the slight recovery of the increased resistivity by electropulsing disappearance of the local structure induced by annealing. These results suggest that in CrMnFeCoNi HEA, local structures which are dynamically unstable and easily transform to other states are not exited or considerably stable if existed.

Reference
J. Dinga et. al., PNAS 115, (2018), 8919–8924.
H. Tanimoto et. al., Mater. Trans. 61 (2020) 878-883.
B. Schuh et. al., Acta Mater, 96 (2015) 258.

High entropy oxides (HEOs) offer great potential to innovate fields involving applied materials. However, the development of HEOs for use as heterogeneous catalysts is still lacking, due in part to current HEO production methods. Common synthetic techniques involve physically mixing individual metal oxides and heating the mixture to high temperatures (>1000 °C), which allows for interdiffusion and achievement of phase pure structures. However, high temperature synthetic routes sinter HEOs into low surface area materials, rendering them inactive in catalyzed gas-phase reactions. Here, we explore a sol-gel synthesis of HEOs conducted at much lower temperatures (500 °C). The resulting HEOs contain primarily rare earth elements and are made with uniform structure, high surface area, and high catalytic activity for CO oxidation. Incorporation of multiple dopant elements into the fluorite phase HEOs facilitates the transfer of reactive lattice oxygen to CO reactant via a Mars van Krevelen mechanism. Further, the high surface area of these materials leads to a high number of active sites for CO oxidation. The sol-gel synthesis presented herein is broadly applicable and enables the design of HEOs as effective catalysts.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525.

Thus far, soft magnetic behavior in high-entropy alloys has only been observed in disordered single-phase microstructures (e.g., FCC or BCC) or a mixture of disordered and ordered structures (e.g., FCC/BCC + B2/L2₁/L1₂). Here, we have reported, for the first time, a novel single-phase B2-ordered Fe₃₀Co₄₀Mn₁₅Al₁₅ medium-entropy alloy with potential use as a soft magnet. The microstructure and deformation mechanisms were examined using transmission electron microscopy. The effects of annealing at 600, 800 and 1000 °C on phase stability, mechanical behavior and magnetic behavior were studied. Outstanding soft magnetic properties were obtained for the as-cast alloy, i.e., a high saturation magnetization of ~156 emu/g, a high Curie temperature of ~810 °C, and a very low coercivity of ~2.6 Oe, which are superior to values previously reported for medium- and high-entropy alloys. The alloy exhibited excellent microstructural, mechanical, and magnetic stability upon long time (> 3 days) annealing at up to 600 °C. Furthermore, high temperature magnetic measurements revealed a gentle decrease in the saturation magnetization and almost unchanged coercivity at temperatures up to 700 °C, suggesting its potential for high-temperature applications as a soft magnet. The work was supported by NIST grant 60NANB2D0120.

Compositionally complex pyrochlore oxide solid solutions with and without uranium incorporation were fabricated by solid state reaction and consolidated by sparking plasma sintering. The as-sintered compositionally complex pyrochlore oxide with distinct mixing entropy varies from medium to high were considered and their thermal and mechanical properties were compared to baseline single component rare-earth titanate pyrochlore (A₂Ti₂O₇). The medium entropy pyrochlore oxides have lower thermal conductivity than its high entropy counterpart. The measured thermal conductivity increases by increasing the A-site mixing entropy and decreasing a modified size disorder parameter, and thus the size disorder and mixing entropy could be good indicators for predicting thermal conductivity of multicomponent pyrochlore solid solutions. Uranium incorporation in general reduces hardness and fracture toughness as compared with single component compositions due to the significant low hardness and fracture toughness for UO₂. High entropy pyrochlore with uranium displays the highest thermal conductivity among all of the multicomponent pyrochlore solid solutions with significantly better mechanical properties than UO₂. This work opens up the possibility of designing multicomponent oxide solid solutions by controlling their chemical disorder/mixing entropy to achieve acceptable thermal-mechanical properties and desired radiation and corrosion performance for potential nuclear waste form and inert matrix fuel applications.

Short range ordering (SRO) in Cr-Co-Ni medium entropy alloy is considered as an important phenomenon for understanding its physical properties. Recent works indicate that slowly cooled alloy from 1273 K exhibits relatively higher yield stresses and higher stacking fault energies. The knowledges of the thermodynamic condition and kinetics of the formation of SRO are required to understand the physical and mechanical properties of the alloy, but are still open question. In the present study, we have successfully confirmed that an atomic scale rearrengement occurs by annealings at the temperatures below 873 K, which may be attributed as the formation of SRO. The SRO disappears when the specimen is annealed at the temperatures of 973 K or higher. We will show experimental results indicating the developement of SRO in Cr-Co-Ni medium entropy alloy and variations of other physical properties such as lattice constant, elastic properties and yeild stress.

High-entropy alloys are promising materials for the high strength and ductility. As the origin of these important properties, the lattice distortion, sluggish diffusion, and cocktail effect have been discussed. The previous studies suggested that the yield strength is proportional to the atomic displacement theoretically. Thus, it is essential to clarify the local structure in this class of alloys experimentally. We investigated the local structure in the medium-entropy alloy CrCoNi by the measurements of the EXAFS (extended x-ray absorption fine structure) for the Cr, Co, Ni-K edges, whose spectra were obtained accurately by recording the transmitted x-ray beam. We found that the Debye-Waller factor in the Cr atom is larger than those in the Co and Ni atoms, suggesting that the Cr atom has a guest-like character inducing the local structural disorder in CrCoNi. Our result is consistent with the previous studies suggesting that the atomic displacement tends to be larger in the atom having the lower atomic number. We will discuss the local structure and the annealing effect on the basis of the EXAFS results.

High entropy ceramics- a chemically disorder crystal with four or more metals randomly distributed in the cation sublattice and either carbon, oxygen or boron populates the anion sublattice. These new materials displayed remarkable properties such as high hardness, low thermal conductivity and reversible electrochemical energy storage behavior [1-3]. The local structure in the chemically disorder crystal affects the properties which demands a nanoscale probe to establish the structure property relationship. Scanning transmission electron microscopy (STEM) provides the opportunity to map the local structure in high entropy materials with variation in local chemistry or in presence of point, line and planar defects in the crystal.

High entropy carbide thin films were synthesized by using reactive radio frequency (RF) magnetron sputtering and carbon content in the samples were regulated by flowing 99.99% methane gas during deposition. Two thin films having a 10% C vacancy and a stoichiometric high entropy carbide were characterized by a combination of STEM imaging and spectroscopy techniques. The low magnification TEM and STEM images reveals formation of stacking faults in the carbon deficient sample. The clustering of carbon vacancies instigated the stacking faults in the materials which is substantiated by EELS mapping in STEM mode. Segregation of carbon vacancies are preferred due to the reduced energy penalty by the formation of stacking faults compared to the random distribution as explored by first principle calculation in binary transition metal carbides [4]. In comparison, the stoichiometric high entropy samples are free from any kind of point or line defects. We observed a broad distribution of the cation-cation bond distances in both samples which are corroborated by the pair distribution generated from density function theory calculation of the chemically disorder crystal. The atomic size mismatch among the constituents of the high entropy carbide cations contributed to the wider distribution. High-angle annular dark field (HAADF) STEM imaging of stoichiometric sample show variation in the intensity of the metal atomic columns with alternate plane of higher and lower intensity. Such variation can be associated with the presence of nanoscale cation ordering in the sample. We will further discuss using statistical tools how such variation in elemental distribution lead to change in local strain which may have profound effect on strengthening of the materials.
References:
[1] P. Sarker et al., Nature Communications 9 (2018)
[2] X. Yan et al., Journal of the American Ceramic Society 101 (2018)
[3] A. Sarkar et al., Nature Communications 9 (2018)
[4] H. Ding et al., Journal of the European Ceramic Society 34 (2014)

Compared with many other steel alloys, highly alloyed FeMnAlC lightweight steels have been shown to have a unique combination of strength and ductility, which has gained interest for automotive applications [1-3]. One of the potential reasons for these enhanced properties has been attributed to the growth of ordered κ-carbides in the austenitic matrix upon aging [3-5]. A particularly important deformation mechanism for high strain-hardening rate alloys is short-range order (SRO)-linked planar glide [6]. Thus, in order to understand the role that local atomic structure may ultimately have on material properties, it becomes crucial to characterize and correlate short-range ordering with deformation mechanisms.

In this presentation, we will show the direct imaging of atomically ordered clusters within the austenite matrix with scanning transmission electron microscopy (STEM). Using Revolving STEM (RevSTEM) imaging, atom column indexing, intensity correlation, and analysis [7,8], we first show direct evidence for nanosized ordered clusters in FeMnAlC, suggesting L1₂-type ordering of the substitutional atoms in agreement with κ-carbide. We then map the extent of ordering with respect to the introduced plastic strain, and the evolution of crystallographic defects such as dislocations and twin boundaries, to provide insights on the behavior of defects in relation to short-range ordering. Finally, we will examine the effect of aging treatments on these phenomena [9].

References:
[1] H. Kim et al., Sci. Technol. Adv. Mater. 14 (2013), 014205.
[2] J.D. Yoo et al., Mater. Sci. Eng. A 508 (2009), p. 234-240.
[3] H. Kim et al., Sci. Technol. Adv. Mater., vol. 14, no. 1 (2013).
[4] W. K. Choo et al., Acta Mater., vol. 45, no. 12 (1997), p. 4877–4885.
[5] S.-D. Kim et al., Sci. Rep. 9 (2019), 15171.
[6] V. Gerold and H.P. Karnthaler, Acta Metall. 37 (1989), p. 2177–83.
[7] C. Niu et al., Appl. Phys. Lett. 106 (2015), 161906.
[8] X. Sang and J. M. LeBeau, Ultramicroscopy 138 (2014), p. 28.
[9] We acknowledge support for this work from the National Science Foundation (CMMI-1922206).

Solid solution strengthening is the major strengthening mechanism for its yeild stress in high entropy alloys (HEA). In order to understand the solid solution strengthening mechanism, lattice distortions caused by the difference in effective atomic radii of the constituent elements have recently been discussed theoretically such as first-principles calculations and Goldschmidt radius. However, no experimental study has been reported to determine the effective atomic radii of the constituent elements in HEA. In this study, the effective atomic radii of the constituent elements in the Cr-Mn-Fe-Co-Ni alloys have been experimentally determined. The effective atomic radii are estimated by applying a rigid sphere model to the effective atomic volumes of the constituent elements determined by an X-ray diffraction. As a result of comparing the derived effective atomic radii with the previously reported theoretical ones, the experimentally determined effective atomic radii show different trends in terms of the absolute values and the order. It has been reported that there is a correlation between the yield stress and the mean square atomic displacement (MSAD) parameter caused by the difference in the atomic radii. However the present study shows there is no correlation between the yield stress and the lattice strains estimated from the experimentally determined atomic radii. These results suggest that the difference in atomic radii is not directly related to the solution strengthening mechanism of HEAs.

High Entropy Alloys (HEAs) are inherently complex and span a vast composition space, across which single-phase solid solutions (SPSS) potentially form. Using combinatorial co-sputtering and high-throughput EDX and synchrotron XRD, we consistently fabricate and characterize large numbers of distinct quinary HEAs. By mining a first data set comprising ~2,500 quinary HEAs based on the elements Al, Cr, Mn, Fe, Co, Ni, and Cu, we show that resulting crystal structures are determined by the combination of the BCC/FCC element content and the atomic size difference. We further reveal a BCC preference effect: The BCC structure becomes increasingly favorable with increasing atomic size difference, because of its ability to accommodate atoms of various sizes more efficiently than FCC. Finally, we present our new expanded data set comprising 10,000 ternary and quinary compositions based on 20 different elements. We place special emphasis on the metastable character of these alloys: Given the high cooling rates involved (~10¹⁰ K/s), SPSS and glasses are obtained through polymorphic solidification and can be studied across exceptionally wide composition ranges.

The presence, nature, and impact of chemical short-range order in the CrCoNi multi-principal element alloy are all topics of current interest and debate, especially in the context of the system’s role as a model for related high-entropy alloys. While the ordering of this material has been previously analyzed in terms of purely chemical relationships, first-principles calculations reveal that its origins are fundamentally magnetic. Specifically, the dominant interaction is found to be repulsion among like-spin Cr nearest neighbors, which is complemented by a preference to form a second-nearest neighbor sublattice of Cr with moments opposing those of ferromagnetic Co. Magnetically aligned pairs of Co and Cr atoms are additionally calculated to be unfavorable, emphasizing the need to analyze the ordering of this material in spin-polarized terms. Models of order following these principles are used to explain anomalous experimental measurements concerning both net magnetization and atomic volumes across a range of compositions, demonstrating the effects of both short-range order and magnetic interactions with implications for a broader class of high-entropy alloys containing multiple principal 3d elements.

The much larger compositional space afforded by high-entropy alloys (HEAs) compared to traditional alloys, opens new opportunities to develop high-performance materials. Here the CALculation of PHAse Diagrams (CALPHAD)-based high-throughput computational method (HTCM) is used to screen lightweight HEAs in the Al-Cr-Fe-Mn-Ti system that contain nanoscale L2₁ precipitates within the body-centered-cubic (BCC) matrix for cost-effective high-temperature applications. The order-disorder transition behaviors and their effects on the microstructures and mechanical properties of these newly-designed lightweight high-entropy alloys (LWHEAs) are understood by in-situ neutron scattering and advanced microcopies, ab-initio molecular dynamics (AIMD), and Monte-Carlo (MC) simulations. The fundamental understanding of the order-disorder transition behaviors of these LWHEAs explains the different microstructural and mechanical responses among them, which provides in-depth insights into the discovery of advanced precipitation-strengthened structural materials by the HEA concept.

Acknowledgment
P.K.L. very much appreciates the support of the U.S. Army Research Office project numbers of W911NF-13-1-0438 and W911NF-19-2-0049 and the support from the National Science Foundation under grant DMR-1611180 and 1809640. M.C.G. acknowledges the support of the US Department of Energy’s Fossil Energy Cross-Cutting Technologies Program at the National Energy Technology Laboratory (NETL) under the RSS contract, 89243318CFE000003. M.W. was supported through DOE grant SC-0014506 for the development of simulation methods. Atom probe tomography (APT) was conducted at Oak Ridge National Laboratory (ORNL)’s Center for Nanophase Materials Sciences (CNMS), which is a U.S. Department of Energy (DOE) Office of Science User Facility. The Neutron-scattering work was carried out at the Spallation Neutron Source (SNS), which is the U.S. DOE user facility at ORNL, sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences.

Lattice distortion has been regarded as one of the four core effects of complex concentrated and high entropy alloys. Recently, it was shown that atomic-level pressure (or the misfit volume) in 3d transition metal element (V, Cr, Mn, Fe, Co, Ni)-based complex concentrated alloys (3d CCAs) with face-centered cubic structure originates from charge transfer between neighboring atoms, which suggests the electronic origin of lattice distortion, rather than a classical mechanical view mainly based on atomic size arguments. Here we show that the magnitude of the local lattice distortion of a specific element is strongly affected by its electronegativity. The trend is only little affected in the presence of short-range order . This study provides an important link between atomistic properties (electronegativity) and physical properties in 3d CCA, and rationalizes the proposed relation between lattice distortion and complexity-induced properties in 3d CCAs.

High entropy alloys (HEAs) have attracted the major interest due to their novel mechanical and structural properties. Local chemical ordering (LCO) plays an important role in determining thermal and electrical conductivity of solid solutions, diffusion, and passivity of alloys containing elements that are electrochemically active. Here, we describe that the change in LCO may promote formation of a distinct-uncommon crystal orientation relationship (OR) with the matrix. In this study, we examined the role of LCO on the OR of BCC precipitates in annealed Al0.3CoCrFeNi via using in-situ and ex-situ TEM heating techniques, where we report a new BCC-ORs due to local chemical fluctuations. These studies were coupled with Extended Electron Energy Loss Fine Structure (EXELFS) and Energy-dispersive X-ray spectroscopy (EDS). Overall, the LCO associated with ORs offers new opportunities to tune properties, enabling a more predictive view of phase transformation in this class of alloys.

Compositionally complex alloys, often termed high-entropy alloys (HEAs), are alloy systems with at least five principal elements in (near-)equiatomic ratios that are promising candidates for the development of materials with enhanced damage tolerance, i.e., that is a good combination of strength and toughness, which is key for the increasing complexity of energy-efficient and safety-critical applications. Despite containing elements with different crystal structures, the most studied HEA to date, the Cantor alloy CrMnFeCoNi, is a single-phase face-centered cubic (FCC) material that has been shown to exhibit strength levels above 1 GPa and fracture toughness values well above 200 MPa.m^½ at room temperature. Interestingly, the alloy’s strength, ductility and fracture toughness improve simultaneously with decreasing temperatures, a trend that is the opposite for most other materials. This is achieved through a combination of sequentially triggered deformation mechanisms including planar dislocation slip, rapid motion of partial dislocations, near-tip crack bridging and deformation-induced nano-twinning. Based on our work on this alloy we will examine compositionally modified variations of this material that were designed with the aim to gradually lower stacking-fault energy. We will show how their failure resistance develops in the same temperature range when compositionally triggered deformation mechanisms such as transformation induced plasticity (TRIP) or twinning induced plasticity (TWIP) are enabled and additionally compare their behavior to a similar alloy that contains a second phase. Using a combination of advanced electron microscopy imaging techniques, we will show how deformation modes such as planar dislocation slip, deformation-induced nano-twinning, and transformation-induced plasticity control mechanical properties and highlight the importance of controlled triggering of each individual mechanism.

We report on tribological investigations with an additively manufactured (AM) equiatomic CoCrFeMnNi high entropy alloy. As this work highlights, AM HEAs are a promising combination of advanced and processing techniques for next-generation structural and multi-functional devices, including applications where high-consequence and low volume drive design and qualification. The tribological, mechanical, and microstructural properties for this HEA in a range of environments are studied, including ultra-high vacuum (UHV), to highlight environment-dependent frictional and wear properties. We showed that low friction (mu< 0.4) and high wear resistance (K ~ 10^-6 mm³/Nm) are achievable with unlubricated high entropy alloys in inert environments and establish evidence of inverse Hall-Petch behavior by a recently proposed process of dynamic amorphization. We also discuss the origin of impressive wear resistance, linked to extreme grain refinement, that was achievable even in inert environments.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy?s National Nuclear Security Administration under contract DE-NA0003525. -or- if you have a character limit, you may use: SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.

Most of high entropy alloys (HEAs) with body centered cubic (BCC) structure suffer from limited plasticity at room temperature, which hinders their practical applications. To tackle this challenge, a Fe_49.5Mn₃₀Co₁₀Cr₁₀B_0.5 high entropy alloy with body centered cubic (BCC) structure was prepared by closed field unbalanced magnetron sputtering. The coating exhibits an excellent combination of strength and ductility (~ 2.8 GPa yield strength and ~ 40% compressive strain), which can be attributed to the synergy of multiple strengthening mechanisms, including transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) effect, compositional fluctuations, grain boundary engineering, grain refinement, and nanolaminate strain partitioning. A two-step phase transformation; namely, BCC to hexagonal closed packed (HCP) and then to face centered cubic (FCC) structure, was identified under applied load. On one hand, the phase transformation from BCC to HCP and then to FCC structure, along with the formation of heterogeneous, bi-modal microstructure, works to absorb the strain and mitigate the stress concentration in the alloy. On the other hand, the increased phase boundaries generated by phase transformation pose as additional barriers to dislocation movements, providing extra strain hardening. The results of this work demonstrates that the two-step phase transformation is an effective mechanism that imparts the new HEA with appreciable damage tolerance while maintaining high strength.

The equiatomic, face centered cubic (fcc), single phase high and medium entropy alloys CrMnFeCoNi and CrCoNi are perfect candidates for the study of solid solution hardening in a statistically distributed crystal lattice. Both alloys show extraordinary properties compared to conventional alloys like nickel-based superalloys or steels, which is not expected for single face fcc materials. Previous investigations on CrMnFeCoNi and CrCoNi_. alloys have concentrated almost exclusively on polycrystalline structures.

This work focuses on the determination of the entropy effects of mechanical deformation on single crystalline (SX) single phase fcc alloys with 5, 3 and 1 component. Any influences such as grain size, grain boundary sliding and diffusion or effects due to precipitation can be isolated. Targeted creep tests under vacuum made it possible to eliminate the influence of oxidation at higher temperatures. These ideal conditions allow a dined comparison to study the solid solution strengthening effect towards to a higher configurational entropy depending on the temperature. Therefore, the alloys with different configurational entropy CrMnFeCoNi (Sconf = 1.61 J/(mol*K), CrCoNi (Sconf = 1.10 J/(mol*K), and pure nickel (Sconf = 0 J/(mol*K), were chosen.

All alloys were cast in single crystalline state by using induction casting under argon atmosphere for further characterization. For the mechanical testing the miniature specimens are manufactured by electrical discharge machining (EDM) and tested in a radiation-heated vacuum creep device with a maximum temperature tolerance less than ± 1 °C. A main part of this work concentrated on the investigation of the dislocations in the alloys. This was realized over a wider temperature range from 700 °C to 1200 °C.Using stress levels of 2 MPa, the results of the creep tests provide quite different behavior of sluggish diffusion in the three tested alloys. The reason for this discrepancy was examined in more detail by electron channelling contrast imaging (ECCI) and transmission electron microscopy (TEM). Especially for this purpose some creep tests were interrupted and cooled down to room temperature under load, to examine the dislocation structure in the samples.

Fatigue resistance is a crucial requirement to the novel high-entropy alloys when they come to engineering applications, as many metal structures used in practice are failed by cyclic loading. Here, a thorough analysis of the information on the low-cycle fatigue, high-cycle fatigue, crack-growth rates, and fatigue mechanisms in the high-entropy alloy literature unveils a guideline through which the discovery and design of fatigue-resistant high-entropy alloys can be facilitated. Overall, multi-phase alloys, particularly the metastable ones, are favorable to fatigue resistance over single-phase alloys. Suggestions are proposed in the end to accelerate the discovery and design of candidates for fatigue-resistant applications.

High-entropy alloys (HEAs) can either be defined as solid solution alloys containing at least 5 elements in equiatomic or near-equiatomic composition, or as alloys with high configurational entropies (larger than 1,5R), regardless of the number of elements involved [1]. In the present study we report on an alloy design route for refractory high-entropy alloys (RHEAs) based on equimolar Mo-V-Nb alloys with additions of W and Ti. In general, our work was motivated by Senkov et al. [2, 3] who investigated refractory HEAs based on Mo-Nb-W-Ta and Mo-V-Nb-W-Ta. Also to be noted is the US Patent published by Bei [4] in which a multi-component solid solution alloy with high entropy of mixing, namely Mo-V-Nb-W-Ti, has been mentioned. Senkov et al. investigated their alloys experimentally and showed that they have a single-phase body-centered cubic (bcc) structure. It is assumed that the patented alloy Mo-V-Nb-W-Ti also consists of a single-phase bcc structure. The aim of our present experimental work was to produce and analyze a refractory high-entropy alloy of aforesaid composition with a single-phase structure and a reduced density, compared to Senkov’s alloys, by replacing one and/or both of the heavy elements W and Ta with Ti. For this purpose, a systematic alloy design involving four- and five-element compositions was used. SEM (scanning electron microscopy) analysis have shown that Mo-V-Nb-xW-yTi (x=0, 20; y=5, 10, 15, 20, 25) is in fact a RHEA with a bcc dendritic structure. Furthermore, the Ti-concentration of the experimental alloys was varied within the composition interval between 5 at.% and 35 at.% as it is defined by Yeh [5], for each principle elements in HEAs, to obtain the influence of Titanium on the microstructure evolution. Additionally, compressive tests at elevated temperatures (in a range of 400-1100 °C) will be carried out to evaluate the influence of the different alloying elements and the Ti-fraction, on the mechanical properties. The observations of the present work are then compared to the published results on similar alloys from the group of K. F. Yao et al. and critically discussed [6, 7].


[1] M. C. Gao, P. K. Liaw, J. W. Yeh, and Y. Zhang, High-entropy alloys: Fundamentals and applications, book, 2016.
[2] O. N. Senkov, G. B. Wilks, D. B. Miracle, C. P. Chuang, and P. K. Liaw, “Refractory high-entropy alloys,” Intermetallics, vol. 18, no. 9, pp. 1758–1765, 2010, doi: 10.1016/j.intermet.2010.05.014.
[3] O. N. Senkov, G. B. Wilks, J. M. Scott, and D. B. Miracle, “Mechanical properties of Nb25Mo25Ta 25W25 and V20Nb20Mo 20Ta20W20 refractory high entropy alloys,” Intermetallics, vol. 19, no. 5, pp. 698–706, 2011, doi: 10.1016/j.intermet.2011.01.004.
[4] H. Bei, “Multi-component solid solution alloys having high mixing entropy,” USA Patent No US 2013/0108502 A1. 2013.
[5] J. W. Yeh, “Alloy design strategies and future trends in high-entropy alloys,” Jom, vol. 65, no. 12, pp. 1759–1771, 2013, doi: 10.1007/s11837-013-0761-6.
[6] Z. D. Han et al., “Effect of Ti additions on mechanical properties of NbMoTaW and VNbMoTaW refractory high entropy alloys,” Intermetallics, vol. 84, pp. 153–157, 2017, doi: 10.1016/j.intermet.2017.01.007.
[7] Z. D. Han et al., “Microstructures and mechanical properties of TixNbMoTaW refractory high-entropy alloys,” Mater. Sci. Eng. A, vol. 712, no. December 2017, pp. 380–385, 2018, doi: 10.1016/j.msea.2017.12.004.

High impact resistance of the annealed and pre-deformed equiatomic CrMnFeCoNi high-entropy alloys (HEAs) is expected from their excellent high-strain-rate plasticity. Its underlying deformation mechanisms under dynamic loading have been identified by both experiments and simulation. The microstructures of alloys after dynamic loading have been revealed by (high-resolution) transmission electron microscopy. The high strain-hardening ability of the CrMnFeCoNi HEA (enabled by solid-solution hardening, forest dislocation hardening, twinning hardening, and phase transformation), a marked strain-rate sensitivity and modest thermal softening, result in an excellent resistance to shear localization. For the annealed CrMnFeCoNi alloy, a shear strain of ~7 has been discovered as required for the shear-band propagation (using a hat-shaped specimen that enables one single shear band to initiate and grow). Additionally, the pre-deformed CrMnFeCoNi alloy obtains an even higher absorbed impact strain energy (under similar loading conditions). The remarkable high-strain-rate plasticity is significant for its structural applications.

Crystalline defects are of fundamental importance in understanding and thereby tailoring the macroscopic mechanical responses of complex concentrated alloys (CCAs). Particularly in the context of microstructural metastability engineering, appreciable interest has aggregated in transformation-induced plasticity or twinning effect. However, the inherent operative unit for the foregoing deformation mechanism, stacking faults, has not yet drawn abundant attention. By investigating a CoCrNiW CCA, we will reveal in this presentation a lesser-explored deformation “faulting” response. Through coupled in-situ synchrotron and electron channeling contrast imaging (ECCI) experiments, we will show that it is the nucleation of extensive stacking faults that is acting as the major plasticity carrier within this CCA, providing macroscopic strain hardenability, and thereby suppressing the formation of the blocky HCP-martensite. We will demonstrate through computational thermodynamic approach that this sort of faulting mechanism is largely ascribed to a negative intrinsic stacking fault energy, for which our in-situ ECCI deformation experiment will reveal a direct validation. Broader insights into the role of multi-layer generalized stacking fault energy landscape will also be discussed in a sense to complement the current thermodynamic-guided design strategy of metastable CCAs or high-entropy alloys.

High entropy alloys (HEAs) can exhibit good combinations of mechanical properties, thermal stability, and corrosion resistance that are critical for high-temperature applications. However, their suitability for service at elevated temperatures under creep conditions has not been explored in detail. In addition to understanding how the special attributes of the concentrated solid solution matrix influence creep, knowledge of the creep properties and deformation mechanisms of the HEA is a prerequisite for further consideration of engineering the material for creep resistance through precipitation or dispersion strengthening. In the present study, constant stress creep tests on an FCC single-phase CrMnFeCoNi were conducted under vacuum between 1023 K and 1173 K. The stress exponent was found to be 3.7±0.1 over all temperatures and stresses. The apparent activation energy of creep was determined to be lower than that for lattice self-diffusion. No significant change in grain size and texture was observed during creep deformation. The steady-state creep microstructure is characterized by highly entangled dislocations networks without subgrain boundary formation. This structure resembles that of low and room temperature deformation at high plastic strain for CrMnFeCoNi and is unlike the creep microstructure of conventional alloys. This observation suggests that the dislocations in CrMnFeCoNi feature more rapid multiplication than in Class A alloys and more sluggish recovery than in Class M alloys.

We report our investigation of high entropy alloys (HEA) for use in the range 600 – 900C, for turbine engine applications. The goal was to evaluate whether the high configurational entropy of HEAs would permit high temperature strengths that were competitive with superalloys while offering a lower density. CALPHAD based alloy design relied on forming a high volume fraction of L1₂ precipitates in an fcc matrix for high temperature strength and creep resistance, while minimizing brittle phases and reducing density. Samples of screened alloys were prepared by arc melting and suction cast into 3 mm diameter rods. Compression tests of the final alloy (ΔS_config=1.62) up to 871 C revealed that it had a 0.2% offset yield strength of 950 MPa at 750 C, which is not only competitive with most superalloys (ΔS_config≤1.3) but bettered the best high temperature HEA to date. The density was approximately 7.9 gm/cc, giving the alloy specific strength advantage over existing superalloys. TEM revealed important composition trends in the precipitate phase, and possibly a result of high alloy content influencing atomic partitioning. The microstructure and mechanical properties will be discussed.

FCrMnFeCoNi high-entropy alloy, known as Cantor alloy, exhibits enhanced ductility and high fracture toughness at cryogenic temperatures. This anomalous damage tolerance was attributed to the formation of nanotwins at lower temperatures. Such study motivated the extensive investigations on fcc high/medium entropy alloys, but major effort has been on the search for new compositions and mechanical properties, while the effect of phase transformation on mechanical properties or microstructure formation is not well-understood.
In the present study, a series of high-entropy alloys with different fcc/hcp phase stability was designed and the deformation behavior and microstructures were investigated. They were also subjected to severe plastic deformation (SPD) by high-pressure torsion (HPT). The process of grain refinement was compared for different alloys.
Ingots of Cr20Mn20Fe20Co40-xNix (x=0~20) was prepared by high-frequency melting. They were then hot forged and rolled into bars. They are heat treated at 1473 K for 48 hrs followed by water quench. In as-quenched condition, the samples with 0~10mol%Ni were composed of hcp phase and fcc phase, while others are in a fcc single phase state.
Room temperature tensile tests revealed that yield stress does not depend on Ni content while tensile strength and total elongation depends strongly on Ni content. The best strength-elongation balance was obtained in the 10Ni and 15Ni samples. X-ray diffraction and EBSD revealed deformation induced fcc-hcp transformation (5Ni) and pronounced twinning (10Ni), both of them led to pronounced work hardening. Sample with hcp phase exhibited higher strength but lower ductility than those of fcc single phase samples.
The 10 mmΦ disks of 0~20Ni alloys were deformed by HPT under an applied compressive stress of 5 GPa up to 10 rotations of the anvils. For the samples with 15~20Ni, HPT led to a drastic increase in the value of micro Vickers hardness (Hv). Hv was about 125 in an as-quenched sample; it increased to about 320Hv in the center of the disk and to about 420Hv at peripheral positions even after 0.25 rotation. This can be attributed to rapid progress of grain refinement. BSD-SEM observations revealed that pronounced formation of lamellar structures with about 100 nm width separated by nanotwins. The formation of lamellar limits the mean-free path of dislocations and accelerate dislocation density increase which leads to formation of equiaxed nanograins (~50 nm) by continuous dynamic recrystallization. The observed behavior was similar to low stacking fault materials such as SUS316 or Cu-Al alloys. It was found the with decreasing Ni content and volume fraction of fcc phase resulted in less pronounced hardening and grain refinement by HPT deformation, which may be due to limited number of slip systems in hcp. <gdiv></gdiv>

In just ten years, research that evolved from equiatomic combinations of refractory metal elements has led to the discovery of a new class of materials that has great potential for high temperature service. The so called refractory metal high entropy superalloys (RMHES) exhibit fine-scale two-phase microstructures comprising small coherent cuboidal precipitates of one phase within a matrix of the other. Visually, these microstructures are similar to those of Ni-based superalloys, and the similarities continue in that both phases have cubic structures with similar lattice parameters and one phase is a superlattice structure of the other. However, there are two key differences between RMHES and conventional superalloys. First, RMHES are based on a body centred unit cell and not the face centred form, which may have implications for a range of material properties. Second, in current RMHES, the matrix phase has the ordered CsCl (B2) structure whilst the precipitates are a disordered bcc solid solution phase, the inverse configuration to that found in Ni-based systems. The presence of an ordered matrix leads to concerns with respect to ductility and toughness, both of which are key parameters for many industrial applications. Despite this, initial mechanical property data from these alloys is extremely promising with the state-of-the-art material, AlMo_0.5NbTa_0.5TiZr, exhibiting a 0.2% proof stress of 2 GPa at room temperature and ∼1.6 GPa at 800°C. In addition, the incorporation of a significant quantity of Al, Ti and Zr means that the density of this alloy is 7.4 g.cm^-3, well below that of Ni-base superalloys. As such, RMHES show great potential as an alternative or complementary option to conventional Ni-base superalloys for elevated temperature service.

However, there is still a huge amount to learn about these new materials and several aspects of their metallurgy require further investigation. In particular, work is required to understand the microstructural formation pathway, to elucidate the mechanism by which the two phases form and evaluate whether their configuration can be inverted. Similarly, it is critical to assess the stability of the fine-scale two-phase microstructure during prolonged exposure to likely service temperatures, particularly given the detrimental effect that topologically close packed phases are known to have on the properties of other high temperature alloys.

One of the challenges in studying the microstructural formation and stability of these materials is their inherent compositional complexity, which makes it difficult to identify the effects of individual alloying elements. As such, our approach has been to elucidate the critical mechanisms through the characterisation of key compositionally simpler constituent systems. Consequently, this talk will present data from a number of systematically varying alloy series, that build in complexity from ternary to senary systems. The results of these studies will be used to discuss how the fine-scale microstructure observed in these alloys arises, identify the role of specific key elements and determine the phase equilibria, and hence stability, of the microstructure following long duration thermal exposures. Such information will be critical in the continued development of RMHES and the determination of a commercially viable high temperature alloy.

The identification of promising solid-solution regions in High Entropy Alloy composition space is hindered by the lack of suitable tools to visualize stability relationships in high-component systems. Traditional phase diagrams use barycentric coordinates to represent composition axes, requiring D = (N –1) spatial dimensions to visualize an N component system. This means that systems with ≥4 components cannot be visualized on traditional phase diagram axes. We propose forgoing barycentric composition axes in favor of two energy axes: a formation-energy axis and a ‘reaction energy’ axis. These two dimensional visualizations are successful in capturing complex stability relationships in N ≥ 5 component systems. By constructing temperature-dependent phase stability graphs, we show that these visual tools can both rationalize high-level qualitative statements regarding the quench-ability of metastable HEA solid-solutions as well as illuminate details about compositional phase-separation.

In the high-entropy alloy (HEA) community, it is often a goal to find compositions that are stable as single phases. This single-phase label is often applied based on interpretation of powder x-ray diffraction (XRD), specifically the lack of a detectable peak not indexable to the HEA phase. While scientifically important in general, accuracy of these labels is particularly crucial for accurate machine learning based models in these systems. Depending on many factors, including background levels, signal-to-noise ratio (SNR), grain size and other peak broadening effects, and peak overlap, detection of these secondary phase peaks can be quite difficult. For instance, we show that in a compositionally varying NbTiTa film, the portion of the film in which we can identify the presence of a secondary phase varies considerably based on XRD collection time (and thereby SNR). To quantify this effect in a broad way, we simulated XRD patterns from a common HEA phase with a distinct secondary phase, varying sample and measurment factors including SNR, phase fraction, peak spacing, peak widths, and background level. Each pattern is fit with two models, an HEA specific model and a two phase model having at least one extra peak compared to the HEA model. We then use a Bayesian information criterion based on these fits to apply a single-phase and dual-phase label to the simulated data, so that we can clarify the role of each nuisance factor on the detection limit for secondary phases. We demonstrate that under realistic sample and diffraction conditions, even with idealized simulated data, a secondary phase can remain obscured even at the several percent phase fraction level. We hope this data set can act as a guide for understanding a reasonable lower limit for phase detection and will facilitate a conversation leading to improved standards for data quality when applying phase labels in the HEA space and beyond.

Refractory complex concentrated alloys (RCCAs) open new opportunities for high-temperature structural applications that require a combination of high strength, high melting point, good ductility, low density etc. Despite the vast compositional space due to the presence of multiple principal elements, current design of RCCAs is mostly focused on equiatomic compositions to increase stability of single solid solution BCC phase, and to avoid formation of brittle intermetallic phases. Nevertheless, the knowledge of what competing phases are and their stability dependence on each element is yet to be broadened to optimize the development of RCCAs. In this work, we start with an equiatomic NbVZr alloy, which is one of the two multi-phase-forming ternary compositions in the Nb-Ti-V-Zr system. Besides a BCC majority phase, we identified two Laves phases, cubic C15 and hexagonal C14 phases, of which C14 was previously reported to be unstable. With first principles calculations, we determine the stable composition for each phase, and a good consistency is found with the experimental results. The Laves phases are more compliant than BCC, but they strengthen the material, as measured by nanoindentation. Ongoing work expands the scope to non-equiatomic compositions to better understand elemental effects on the alloy’s phase selection and properties. We apply combinatorial methods to synthesize Nb-Ti-V-Zr compositional libraries via direct laser deposition, which facilitates the high-throughput identification of different phases, microstructures and mechanical properties as a function of composition. This work also provides guidelines for exploration of compositional effects in all RCCA systems; a complete knowledge of the role each element plays will significantly expedite the design of RCCA candidates for high-temperature applications.

High-entropy alloys (HEA) were initially regarded as stable single-phase disordered solid solutions with fcc or bcc structures stabilized by increased configurational entropy [1,2]. Later, a question about possible metastability of these phases arose [3], and currently the most of the known HEAs recognized as metastable materials [4]. Decomposition of the metastable HEA phases results in (i) precipitation of new phases (intermetallic, σ-, η-, etc.) and (ii) crystal structure changes in the initial major multicomponent phase. Both these effects are equally important for understanding stability and metastability of HEAs. However, most of the known up-to-now works derive metastability of HEAs from the experimentally observed precipitation of secondary phases after annealing, and less attention was paid to evolution of the matrix phase, which commonly remains multicomponent. Thus, a question arises concerning “final destination” of major fcc and bcc high-entropy phases after long-term annealing: will they finally transform into stable HEA or decay down to simple ordered phases, such as intermetallic and binary (ternary) solid solutions?
A task of this work is to reveal in situ fine crystal structure transformations in the major fcc and bcc phases of the 3d-transition metals high-entropy alloys CoCrFeNiX, where X = Al, Ti, Cu, or Mn.
A uniform high entropy alloys were produced in a powdered form by fast (90-120 min) mechanical alloying in the planetary mill. The powders were annealed at different temperatures, up to 1473 K in vacuum, with simultaneous X-ray diffraction analysis. Compacted bulk samples were obtained using Spark Plasma Sintering (SPS) method. Structure and properties of the powders and bulk samples were studied with XRD, SEM, EDS, TEM and other methods.
Several specific transformations of crystal structure and microstructure were detected for different studied HEAs. They involve shrinking and expansion of crystal lattice, co-existence of two FCC phases, transformations of BCC into FCC phase, and, possibly, martensitic-like transformation of cubic into tetragonal phase. Shrinking of the atomic structures were detected for all phases after annealing in the temperature range 873 – 1273 K for 5 – 6 h. A drift of the (111) peak regarding to (200) peak of the fcc phase CoCrFeNiAl at 1273 K allow assumption of weak martensitic transformations: high-temperature cubic phase turns into slightly tetragonal (c/a = 1.00123) phase at room temperature. Despite of the structural transformations, the alloys remains multi-principal elemental (high-entropy) after annealing.
This research was done at the expense of grant from Russian Science Foundation (Project No. 20-13-00277). Use of the Spark Plasma Sintering set-up was possible due to support from the Ministry of Education and Science of the Russian Federation under the Competitiveness Enhancement Program of NUST MISiS (Grant no. K2-2020-015).
References.
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[2] B. Cantor, I.T.H. Chang, P. Knight, A.J.B. Vincent, Mater. Sci. Eng. A 375-377 (2004) 213-218.
[3] B. Cantor, Ann. Chim. Sci. Mat. 32(3) (2007) 245-256.
[4] S.A. Kube, J. Schroers, Scripta Mater. 186 (2020) 392-400.

High-entropy alloys possess unique properties such as long decomposition time of their metastable random solution phase. Being favorable from the point of view of applications, this property also manifests a challenge to their experimental study: it requires long annealing times to observe alloy decomposition. This opens avenues for application of ab initio methods which, in their turn, encounter the problem of large supercells and long simulation times which are beyond the capabilities of modern supercomputers. Machine learning hence comes to the stage as a tool to accelerate ab initio calculations through learning from a limited number or relatively small-scale calculations and using the efficient machine-learned model in full-scale simulations.

Application of machine learning, in its turn, faces difficulties originating from the vast configurational space of high-entropy alloys. Indeed, there are about 300 different nearest-neighbor atomic environments in an fcc-based binary alloy (after applying symmetries), while this number for a five-component alloy is about 25 million. Such a large configurational space is harder to cover in interatomic interaction models, which typically manifests in degradation of their accuracy. For instance, when using cluster expansion - a data-driven model used to model confrontational lattice entropy - it gets harder to maintain the meV accuracy of the model with adding more and more elements into the model. Also, the accuracy of interatomic potentials that are used to model alloys at vibrational time scales, degrade with increasing chemical complexity.

In my talk I will present some of the latest developments in machine learning-assisted ab initio modeling of entropic effects in high-entropy alloys. Namely, I will show how one can use machine-learning interatomic potentials to accurately study elastic properties, thermal expansion, short-range order, and stability of phases of prototypical medium-entropy and high-entropy alloys.

Recently, high-entropy alloys (HEAs) have attracted attention as a new category of advanced metal materials, and large amount of effort are concentrated to explore the solid solution single phase of HEAs because of their excellent mechanical properties. Latest development of HEAs extends not only to structural materials but also high-performance functional materials. Although attention is often paid on the single phase HEAs, the most important contribution of HEAs to physical metallurgy and alloy engineering must be its application to the matrix phase of multi-phase microstructures, i.e. the HEAs are utilized as a matrix phase of structural materials with multi-phase microstructures. High strength and good ductility of HEAs provide ideal properties as a matrix phase of microstructure, and there is a great choice because of a large variety of HEAs, the element combination in HEAs with considering the composition variation is almost infinite. Up until now, alloy systems have been categorized into Fe-based alloys, Ni-based alloys and so on, focusing on the principle element in the matrix phase. Therefore, so to speak, "HEA-based alloys" must be the ultimate alloy system in the history of metallurgy.
In order to carry forward the development of "HEA-based alloys", it is convenient that we are able to utilize the conventional approaches for alloy design based on phase diagrams and phase transformations. However, we always encounter the difficulty because the phase diagram of multi-component alloy systems, in particular high composition region in the phase diagram, is unknown in many cases, and the most of phase transformations observed in multi-component alloy systems is quite complicated. Therefore, the fundamental issues to boost up materials design in the field of HEAs are (1) the effective and accurate assessment of phase diagrams and Gibbs energies in the wide composition range of multi-component alloy systems, and (2) the rapid optimization of materials properties and microstructures within the diverse space of materials parameters and process conditions. In order to overcome these issues, we focused on the following three targets in this study: (i) First-principals calculation on the phase diagram of HEAs and re-assessment of the Gibbs energy database with Calphad approach, (ii) Comprehensive evaluation of materials parameters in Gibbs energy functions and diffusion mobility functions based on the phase-field method with data assimilation techniques by using the kinetic data of microstructures, and (iii) Effective optimization of materials properties and microstructures accelerated by several machine learning techniques. The objective of this study is to provide the efficient strategy in materials science and engineering to handle HEAs on a daily basis.
In particular, as the phase-field method coupled with data science technique is an interesting topic in this approach, the recent results obtained from this study are demonstrated in the presentation. We applied one of the data assimilation techniques "Adjoint method" to the phase-field theory, which derives the adjoint equations in conjunction with phase-field method, and the thermodynamic parameters in Gibbs energy function and the atom diffusivities in HEAs are estimated from the experimental data of composition profiles of HEAs, which is obtained by the conventional diffusion couple investigation on HEAs. As a result, materials parameters are reasonably and efficiently determined in this methodology. It should be emphasized that the inverse problem implemented with phase-field method is quite useful because there are a lot of materials parameters and process conditions utilized in phase-field simulations, and the large amount of experimental microstructure data required for data assimilation process is now available due to the recent development of material characterization devices and techniques.

In traditional body-centered cubic (bcc) metals, the core properties of screw dislocations play a critical role in plastic deformation at low temperatures. Recently, much attention has been focused on refractory high-entropy alloys (RHEAs), which also possess bcc crystal structures. In this presentation we present results on dislocation core structures and their mobilities in bcc HEAs, including effects of chemical short-range order (SRO) in these multiple principal element alloys. Specifically, using density functional theory (DFT), we investigate the distribution of dislocation core structures and energies in MoNbTaW RHEAs alloys, and how they are influenced by SRO. The average values of the core energies in the RHEA are found to be larger than those in the corresponding pure constituent bcc metals, and are relatively insensitive to the degree of SRO. However, the presence of SRO is shown to have an effect on narrowing the distribution of dislocation core energies and decreasing the spatial heterogeneity of these energies in the RHEA. It is argued that the consequences for the mechanical behavior of HEAs is a change in the energy landscape of the dislocations which would likely heterogeneously inhibit their motion. We also present results of molecular dynamics simulations using a machine-learning potential for MoNbTaW RHEAs, that is shown to reproduce well the DFT results for distribution of core energies. The simulations investigate dislocation motion for both screw and edge dislocations, and are analyzed in terms of the nature of the mechanisms of motion and pinning induced by spatial heterogeneities in the RHEA. This research was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, under contract no. DE-AC02-05-CH11231 within the Damage-Tolerance in Structural Materials (KC 13) program.

The motion of dislocations and their interactions with obstacles control the mechanical behavior of metals and alloys. In disordered random alloys such as binary solid solutions or high entropy alloys, the random distribution of atoms generates internal stresses that impede the motion of dislocations, leading to improved mechanical properties. Understanding the interactions between dislocations and this structural environment is crucial to better predict the mechanical properties of the alloys.

In this context, we investigated the behavior of a dislocation evolving in a random solid solution by means of a continuous elastic model based on previous work [1]. In a first step, using micro-elasticity theory, we derived general expressions for the spatial correlations of the stress and displacement fields emerging from elastic interactions of different atoms in random alloys. This approach is based on the definition of eigenstrains associated with each atomic lattice site in a linear elastic medium. In particular, we show that in the case of isotropic elasticity the spatial correlations of the displacement and stress fields follow 1/d and 1/d³ behaviors respectively.

In a second step, the influence of the random solid solution is assessed by relaxing a dislocation line in a noisy stress environment. The nature of the noise (amplitude and spatial correlations) is carefully implemented using the developed analytic expressions to mimic the stress environment of atomistic random solid solutions. This approach allows us to characterize the shape of the dislocation represented as a power spectrum, as well as model quantitatively the dislocation behavior in a random structural environment without integrating the dynamics of individual atoms. Finally, the results obtained from the continuous model are compared with atomistic simulations performed in Al₅₀-Mg₅₀ random alloys, and the assumptions used in previous models in the literature [2] are discussed in light of the present findings.

REFERENCES
[1] P.A. Geslin, D. Rodney. "Thermal fluctuations of dislocations reveal the interplay between their core energy and long-range elasticity." Physical Review B 98 (2018), 174115.
[2] C. Varvenne, A. Luque, W.A. Curtin. "Theory of strengthening in fcc high entropy alloys." Acta Materialia 118 (2016),164-176.

Strengthening high entropy alloys (HEAs) via secondary intermetallic phases has been proven to be an effective approach experimentally. However, the design of intermetallic phases in HEAs for property enhancement is challenging due to the limited understanding of intermetallics in compositionally complex space. While density-functional-theory (DFT)-based methods have promoted design of binary and ternary intermetallics, they are not amenable for rapidly screening the vast combinatorial space of HEAs. In this work, we have developed a machine learning model to accelerate the discovery of intermetallics by predicting their formation enthalpy given only the composition. The model uses easily accessible elemental properties as descriptors and has a mean absolute error (MAE) of ∼ 0.044 eV/atom for a testing set of binary alloys. We use the ML model to successfully identify new binary intermetallics that are subsequently confirmed using DFT. The model trained with binary intermetallics can predict ternary intermetallics with a MAE of ~0.057 eV/atom without further training. We further extend this model to multi-element systems and guide the prediction of compositionally complex intermetallics that may form in HEAs.

Acknowledgements: This work was supported by the National Science Foundation through grant number DMR-1809571.

Multi- principal-element alloys (high-entropy alloys) have attracted worldwide attention because they open up a vast compositional space in which different kinds of materials may be discovered [1] and the understanding of the diffusion kinetics in HEAs is of fundamental significance [2]. Weexamine the interdiffusion in multicomponent systems using the approach analogically to one developed earlier for description of interdiffusion in binary alloys [3-5]. In opposite to traditional theory this approach takes into consideration an active role of vacancies, equilibrium distribution of which is not supposed, therefore there are contributions in equations for component fluxes, conditioned by vacancy concentration gradient. We have developed the system of diffusion equations for components and vacancies after substituting the expressions for fluxes in the equations of a continuity. Solutions of the equations show that the vacancy concentration’s deviation from the equilibrium one equalize the fluxes of components [3] and component profiles are determined by interdiffusion coefficient. This coefficient [3-5] differs from traditional one (Darken's approach).
A direct generalization of the theory to the case of multicomponent systems meets with serious mathematical complexities. Therefore, we developed a new version of the equations linearization of the original system. The system of linearized equations is solved and we have found a relation between the interdiffusion coefficients and the corresponding tracer diffusion coefficients. Interdiffusion coefficient equations significantly differ from traditional one (Darken's approach). Then we numerically simulate interdiffusion in some alloys and analyze interdiffusion in high-entropy alloys, a possibility of sluggish diffusion in these alloys and the reasons of this effect. Using results the applicability of the quasi-binary approximation is analyzed in calculating the diffusion coefficients in multicomponent alloys.
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