Equiatomic Alloy Research Articles

Crystallization kinetics of GdYScAlCo high-entropy bulk metallic glass

The thermal stability and non-isothermal crystallization of a new bulk-amorphous high-entropy (HE-BMG) equiatomic GdYScAlCo alloy were studied by differential scanning calorimetry (DSC). The alloy shows a four-stage crystallization process. The kinetic parameters (activation energy (Eα)), the pre-exponential factor (logA) and glass-forming ability indicators (kinetic fragility index, characteristic temperatures) for the GdYScAlCo alloy were obtained. The Eα values obtained by isoconversional methods indicate a nonlinear Arrhenian behaviour and a complex process. The Avrami equation modification proposed by Jeziorny and the multivariate nonlinear regression method were applied on the nonisothermal crystallization. In the case of primary crystallization of the amorphous GdYScAlCo alloy under nonisothermal conditions, the kinetics of the nucleation process is best described by an autocatalytic reaction.

Precipitation growth of σ-phase along the interface in CoCrFeMnNi high-entropy alloy

The equiatomic CoCrFeMnNi high-entropy alloy (HEA) has a face-centered cubic (FCC) single-phase structure with a good balance of yield strength and elongation due to the twinning-induced plasticity (TWIP). In order to increase the usability of FCC-structured HEA, it is necessary to develop its mechanical properties. The hard precipitation of the σ-phase might be a promising strengthening mechanism. Furthermore, developing an advanced manufacturing process is necessary to apply the σ-phase-containing HEAs. This study focused on σ-phase formation during annealing after hot compression of a CoCrFeMnNi HEA. The σ-phase was primarily observed near the high-angle grain boundaries and acted as the obstacle for the boundary migration, resulting in grain growth during annealing. In addition, the development of the σ-phase according to the interface type was investigated. This microstructure increased the hardness of the alloy and may contribute to the further development of high-strength HEAs.

Depth distribution analysis of alloying and impurity elements in Ni-Ti alloy thin films using secondary ion mass spectrometer

Ni-rich Ni-Ti thin films of various compositions and thickness (69 nm - 141 nm) were deposited on Si (111) substrate by magnetron sputtering system using equiatomic NiTi alloy sputtering target. X-ray reflectivity analysis of the thin films deposited at room temperature showed the presence of a layered structure as: NiO/Ni-Ti/NiO/SiO2 on Si substrate. Secondary ion mass spectrometry (SIMS) was used to determine the qualitative (intensity) depth distribution of elements Ni, Ti, O and Si in the films using MCs+ approach. Relative sensitivity factor values were utilized to convert the qualitative depth profiles to concentration depth profiles. Surface and depth distribution of alloying elements in the deposited films were investigated. It was observed that the films deposited at high sputtering power and working pressure resulted in more non-uniform depth distribution of alloying elements compared to films deposited at low sputtering power and working pressure. SIMS depth profile data showed the presence of high oxygen contaminations at the surface and at the interface of the films. The Ti concentration in the deposited films depends upon the background oxygen contamination present in the deposition chamber during sputtering process. The concentration of Ni in the films deposited under similar working pressure increases with increase in the sputtering power. However, with increase in working pressure during deposition process results in decrease in the concentration of both Ni and Ti in the deposited films. In order to study the effect of insitu substrate heating on the distribution of alloying and impurity elements, one of the film was deposited on heated Si substrate. The film deposited on heated substrate represents the layer structure as NiTiO3 on Si substrate. The concentration of oxygen in the film deposited on heated substrate is comparatively higher than those deposited at room temperature.

Design and development of low density, high strength ZrNbAlVTi high entropy alloy for high temperature applications

Superior high temperature strength, better thermodynamic stability and good workability are some of the desirable attributes of structural materials for next generation nuclear reactors. In this regard, the present study involves the development of a low density, equiatomic ZrNbVTiAl High Entropy Alloy (HEA) for nuclear applications. The alloy was designed using a combination of thermodynamic simulations and first principle techniques and later synthesized using levitation melting to ensure better purity and compositional homogeneity. The melted alloy possessed lower density (5.76 g/cm3) and higher hardness (5.7 GPa) in comparison to conventional structural materials like austenitic steel. Detailed characterization of the alloy showed the presence of bcc type phase along with small fractions of Al- Zr type intermetallics in as-solidified condition. The mechanical properties of the equiatomic ZrNbVTiAl alloy were studied using compression testing at room temperature and up to 1000 °C. The alloy exhibited superior specific yield strength as well as dynamic recrystallization at high temperatures resulting in the breaking of cast structure, thereby reflecting its appreciable workability. The alloy also showed a satisfactory elastic modulus (128 GPa) when measured using ultrasonic technique. The experimentally measured values of elastic moduli of alloy were compared with the values obtained through Density Functional Theory (DFT) simulations and the results matched within acceptable range. Results from the present study indicates that the equiatomic ZrNbVTiAl alloy could be a suitable candidate for light weight, high temperature and high strength structural applications.

Effect of local chemical order on the irradiation-induced defect evolution in CrCoNi medium-entropy alloy

High- (and medium-) entropy alloys have emerged as potentially suitable structural materials for nuclear applications, particularly as they appear to show promising irradiation resistance. Recent studies have provided evidence of the presence of local chemical order (LCO) as a salient feature of these complex concentrated solid-solution alloys. However, the influence of such LCO on their irradiation response has remained uncertain thus far. In this work, we combine ion irradiation experiments with large-scale atomistic simulations to reveal that the presence of chemical short-range order, developed as an early stage of LCO, slows down the formation and evolution of point defects in the equiatomic medium-entropy alloy CrCoNi during irradiation. In particular, the irradiation-induced vacancies and interstitials exhibit a smaller difference in their mobility, arising from a stronger effect of LCO in localizing interstitial diffusion. This effect promotes their recombination as the LCO serves to tune the migration energy barriers of these point defects, thereby delaying the initiation of damage. These findings imply that local chemical ordering may provide a variable in the design space to enhance the resistance of multi-principal element alloys to irradiation damage.

Assessing the high concentration of vacancies in refractory high entropy alloys

Accurate determination of monovacancy formation enthalpy is vital for work on diffusion, melting point determination of high temperature materials, radiation damage, and thermophysical stability of alloys. These enthalpies take on a single value in pure metals but is made more complex in concentrated solid solutions due to the local chemical environments possible around each vacancy. Herein, using first-principles density functional theory, we report the distributions of vacancy formation enthalpies in 21 equiatomic 5-component solid-solution alloys from the Hf-Mo-Nb-Ta-Ti-W-Zr system. Chemical disorder is treated using the special quasi-random structure method, and the chemical potential of vacant element is treated by approximating it to its total energy in its standard state. We find that the highest vacancy formation enthalpies belong to the MoNbTaTiW alloy (which incidentally is the most reported in literature) and the lowest belongs to HfNbTaTiZr, with other alloys in-between. We use the whole distribution of formation enthalpies to estimate the equilibrium concentration of vacancies as a function of temperature.

Using concentration gradients to examine the effects of Al, Ga and Sn additions on the low-activation VCrMnFe system

A critical design criterion for future fusion reactor components is low activation. The equiatomic multi-principal element alloy VCrMnFe is comprised solely of low activation elements and forms a single-phase solid solution at temperatures over 1000 °C. However, at lower temperatures it forms detrimental sigma phase. In this work, compositional gradients of Ga, Sn or Al were induced in VCrMnFe using only a furnace to investigate their effect on intermetallic formation. By examining how the microstructure changed across a region with varying composition, phase stability limits could be assessed. For example, all three elements were found to prevent sigma phase from forming within the alloy when they were present at relatively low concentrations (2–5 at%). Al was found to be the most promising addition (in terms of not causing embrittlement), and the approach used enabled the characterisation of the VCrMnFe–Al pseudo binary phase diagram up to 50 at% Al after heat treatment of 800 °C/240 h followed by ageing at 600 °C/240 h, with numerous ordered phases found using electron diffraction. The level of Al addition required to suppress the sigma phase has been identified more precisely, which will be useful for future alloy development work.

Formation of chemical short-range orders of two kinds and the co-existence with medium-range orders in an equiatomic VFeCoNi alloy

The chemical short-range orders (CSROs) have aroused the growing concern in the emerging multi-principal-element alloys. The CSROs as the built-in nanoscale heterogeneities cause the difference in the microstructures from conventional alloys and are expected to have an effect on mechanical properties. The formation of CSROs relates to elemental species and recent verifications of CSROs are mainly in ternary alloys. Here, the CSROs are investigated in an as-annealed, quaternary, and equiatomic VFeCoNi alloy of face-centered cubic-structured single phase before and after tensile deformation. Both the lattice structure and elemental species occupation are investigated in detail by means of electron diffraction, dark-field imaging, and atomic-resolution chemical mapping in a high-resolution transmission electron microscope. The CSROs are characterized crystallographically by two M (M: specific element)-enriched planes to sandwich one M-depleted plane. Particularly, M can be either V or Fe, instead of single elemental species reported previously. This indicates that two kinds of CSROs have been simultaneously formed in terms of the elemental species occupation, while both having the same lattice structure. Further, these two kinds of CSROs may grow up into the medium-range orders in sizes larger than 1 nm. Meanwhile, both short- and medium-range orders are mechanically stable during tensile deformation to maintain an almost unchangeable average sizes and areal fractions as well. The Moran's index analyses predict the presence of more V-enriched CSROs and CMROs than Fe-enriched ones. The present results provide the insights into understanding the production of CSROs and their growth as a result of the complex and diverse chemical species in the multi-principal-element alloys.

Fabrication of FeCoNi medium-entropy alloy nanoparticles by high-repetition-rate UV picosecond laser ablation in water

Laser ablation in liquid (LAL) is a rapid heating and cooling process that enables the formation of a variety of nanomaterials. Extensive LAL studies for metals, semiconductors, insulators, and carbons have been reported. Further, alloys are worthwhile target to investigate the applicability of LAL. We report the LAL of equiatomic FeCoNi medium-entropy alloy (MEA) in water using a high-repetition-rate UV picosecond laser with a galvanometric scanner. The extinction spectra of the colloidal solution, morphology, elemental composition, and size distribution of MEA nanoparticles (NPs) were investigated as a function of laser-irradiation time, laser power, and scanning speed. Electron microscopy revealed that chain-like superstructures longer than 10 µm consisting of Ni-rich FeCoNi MEA-core Fe-shell spherical NPs of approximately 50-nm diameter were formed. These superstructures were not originated from the NP agglomeration during sample preparation for microscopy or the successive laser irradiation of colloids, but from the overstriking ablation on the target surface. Generally, the LAL target was irradiated by spatially and temporally separated near-infrared or visible laser pulses to efficiently produce NPs. Overstriking of UV ultrashort laser pulses was shown to be a valuable strategy for modifying the elemental composition of MEA NPs and for forming their superstructures.

Characteristic strengthening mechanisms in body-centered cubic refractory high/medium entropy alloys

The present work reports characteristic strengthening mechanisms of HfNbTaTi, HfNbTiZr, HfNbTi, HfTaTi, and NbTiZr equi-atomic refractory medium entropy alloys (RMEAs). The alloys were processed by high-pressure torsion and subsequent annealing to obtain microstructures with average grain sizes ranging from several hundred nanometers to several tens of micrometers. Their mechanical properties were evaluated by tensile tests at room temperature. Precise Hall–Petch relationships of the RMEAs were acquired based on the tensile yield strength data. Small slopes of the Hall–Petch relationships and weak grain refinement strengthening were clarified to be attributed to their low elastic modulus. In addition, the friction stresses in the RMEAs were higher than those of conventional BCC metals and alloys. By comparing the experimental data and a theoretical model, it was suggested that interaction between severe lattice distortion and elastic field of edge dislocations can largely contribute to the high friction stress of the RMEAs at room temperature.

Magnetostriction reduction and magnetic permeability enhancement of Ti added Fe0.7Co0.3 alloy

The current work presents the study of three alloys, (1) the binary Fe0.7Co0.3 alloy (atomic fraction), (2) the addition of 3 at. % of Ti, and (3) the addition of 3 at. % of Ti and 1% of Nb. The interest is to increase the permeability of this binary alloy due to the additions of Ti and Nb, since Fe–Co alloys with Co content in the range of 27–35 at. % have much lower permeability than the commercial equiatomic alloy. The advantage of low Co Fe–Co alloys is the lower price compared to the equiatomic one. The alloys were produced by arc melting and were subsequently hot rolled and annealed in the final thickness at 1200 °C over 24 h. Microstructure characterization revealed single α-phased alloys with body centered cubic structures. Positive results were observed in the magnetic and electric properties of the (Fe0.70Co0.30)97Ti3 alloy compared to Fe0.7Co0.3, meaning the decreasing saturation magnetostriction λs, and the growth of the maximum relative permeability by 24% and of the resistivity by 63%, occurred without a change in the coercive field and in the maximum magnetic induction. The (Fe0.7Co0.3)97Ti3 alloy's maximum magnetic relative permeability is μrmax = 855 and occurs at a field of only 160 A/m, this field being more than two times lower compared to the one of the binary alloy. On the other hand, the addition of Ti + Nb has improved the deformation performance of the (Fe0.7Co0.3)97Ti3 alloy at high temperatures and did not change the magnetostriction, nor the resistivity. The quaternary alloy presents a maximum magnetic relative permeability of μrmax = 270 at 1960 A/m and a maximum magnetic induction reduced by 14% compared to the other two alloys.

Dislocation plasticity in equiatomic NiCoCr alloys: Effect of short-range order

Equiatomic NiCoCr solid solutions have been shown by recent experiments and atomistic simulations to display exceptional mechanical properties that have been suggested to be linked to nanostructural short-range order (SRO) features that may arise from thermal treatments, such as annealing or/and aging. Here we use hybrid Monte Carlo--molecular dynamics simulations to gain further insights of thermal effects on the SRO formation as well as the edge dislocation plasticity mechanisms of equiatomic NiCoCr face-centered cubic solid solution. For that purpose, we utilize two well-known NiCoCr interatomic potentials, one of which displays well-documented SRO, believed to be linked to experimental evidence and labeled as the Li-Sheng-Ma potential, while the other (Farkas-Caro) does not. We use these two potentials to discern short-range ordering (from inherent randomness in random solid solutions) and understand how SROs influence dislocation depinning dynamics in various thermal annealing scenarios. In this context, we used robust, scale-dependent metrics to infer a characteristic SRO size in the Li-Sheng-Ma case by probing local concentration fluctuations which otherwise indicate uncorrelated patterns in the Farkas-Caro case in a close agreement with random alloys. Our Voronoi-based analysis shows meaningful variations of local misfit properties owing to the presence of SROs. Using relevant order parameters, we also report on the drastic increase of chemical ordering within the stacking fault region. More importantly, we find that the Li-Sheng-Ma potential leads to excellent edge dislocation depinning strength with low stacking fault width. Our findings indicate an enhanced roughening mechanism due to the SROs-misfit synergy that leads to significant improvements in dislocation glide resistance. We argue that the improvements in alloy strength have their atomistic origins in the interplay between nanoscopic SROs and atomic-level misfit properties.

Effect of Segregation on Deformation Behaviour of Nanoscale CoCrCuFeNi High-Entropy Alloy

A molecular dynamics (MD) simulation method is used to investigate the effect of grain boundary (GB) segregation on the deformation behavior of bicrystals of equiatomic nanoscale CoCrCuFeNi high-entropy alloy (HEA). The deformation mechanisms during shear and tensile deformation at 300 K and 100 K are analyzed. It is revealed that upon tensile deformation, the stacking fault formation, and twinning are the main deformation mechanisms, while for the shear deformation, the main contribution to the plastic flow is realized through the GB migration. The presence of the segregation at GBs leads to the stabilization of GBs, while during the shear deformation of the nanoscale CoCrCuFeNi HEA without the segregation at GBs, GBs are subject to migration. It is found that the GB segregation can differently influence the plasticity of the nanoscale CoCrCuFeNi HEA, depending on the elemental composition of the segregation layer. In the case of copper and nickel segregations, an increase in the segregation layer size enhances the plasticity of the nanoscale CoCrCuFeNi HEA. However, an increase in the thickness of chromium segregations deteriorates the plasticity while enhancing maximum shear stress. The results obtained in this study shed light on the development of HEAs with enhanced mechanical properties via GB engineering.

Evaluation of Equiatomic CrMnFeCoNiCu System and Subsequent Derivation of a Non-Equiatomic MnFeCoNiCu Alloy.

Investigation into non-equiatomic high-entropy alloys has grown in recent years due to questions about the role of entropy stabilization in forming single-phase solid solutions. Non-equiatomic alloys have been shown to retain the outstanding mechanical properties exhibited by their equiatomic counterparts and even improve electrical, thermal, and magnetic properties, albeit with relaxed composition bounds. However, much remains to understand the processing-structure-property relationships in all classes of so-called high-entropy alloys (HEAs). Here, we are motivated by the natural phenomena of crystal growth and equilibrium conditions to introduce a method of HEA development where controlled processing conditions determine the most probable and stable composition. This is demonstrated by cooling an equiatomic CrMnFeCoNiCu alloy from the melt steadily over 3 days (cooling rate ~4 °C/h). The result is an alloy containing large Cr-rich precipitates and an almost Cr-free matrix exhibiting compositions within the MnFeCoNiCu system (with trace amounts of Cr). From this juncture, it is argued that the most stable composition is within the CrMnFeCoNiCu system rather than the CrMnFeCoNi system. With further optimization and evaluation, a unique non-equiatomic alloy, Mn17Fe21Co24Ni24Cu14, is derived. The alloy solidifies and recrystallizes into a single-phase face-centered cubic (FCC) polycrystal. In addition to possible applications where Invar is currently utilized, this alloy can be used in fundamental studies that contrast its behavior with its equiatomic counterpart and shed light on the development of HEAs.

Interdiffusion in Refractory Metal System with a BCC Lattice: Ti/TiZrHfNbTaMo

Interdiffusion of the elements in a diffusion pair consisting of Ti and an equiatomic high-entropy alloy (HEA) TiZrHfNbTaMo in the temperature range of 1473-1673 K has been studied. A calculated results phase diagram of the alloy by Thermo-Calc 2021-B software as used to determine the temperature stability range of the β-phase in the alloy. Ti-HEA diffusion pairs were obtained by low = temperature welding and then diffusion annealing was carried out at temperatures of 1473, 1573, and 1673 K during 12, 9, and 6 h, respectively. The interdiffusion zone was profiled using electron probe microanalysis (EPMA). The diffusion parameters of the HEA's elements were obtained using Hall's method. An experimental results discussion is given.

Effects of Heat Treatment on the Interface Microstructure and Mechanical Properties of Friction-Stir-Processed AlCoCrFeNi/A356 Composites

Equiatomic AlCoCrFeNi high-entropy alloy (HEA) has gained significant interest in recent years because of its excellent mechanical properties. A356 aluminum alloy reinforced by AlCoCrFeNi HEA particles was fabricated by friction stir processing (FSP) and subsequent heat treatment. Solution and aging treatments were specially performed for the composites to control the interface microstructure, and interfacial microstructure and tensile properties were explored at different conditions. The interface between the matrix and HEA particles showed a dual-layered core-shell structure and the thickness of the shell region increased with the solution time. The microstructure located in the shell layers consisted of a solid solution with increasing aluminum content, in which a radial-shaped solid solution phase formed in the region close to the core of the HEA particle and scattered solid solution grains with high Ni content formed in the region close to the matrix alloy. The gradient of composition and microstructure across the HEA/Al interface can be obtained through heat treatment, and an optimal interface bonding state and mechanical property were obtained after solution treatment for 2 h. Compared with FSPed A356 aluminum alloy, the FSPed composite enhanced the tensile stress by 60 MPa and the stain by 5% under the optimized conditions. The overgrowth of the shell layer decreased both the tensile strength and the ductile greatly due to the formation of a radial-shaped solid solution phase in the shell region.

Yield and flow properties of ultra-fine, fine, and coarse grain microstructures of FeCoNi equiatomic alloy at ambient and cryogenic temperatures

The current study reveals the Hall-Petch relationship, or mean grain size control, and flow properties of equiatomic FeCoNi alloy having different grain sizes at room temperature (298 K) and cryogenic temperature (77 K). For the first time, various mean grain sizes ranging from the ultra-fine regime (0.7 μm) to the coarse grain regime (145.3 μm) were achieved in a FeCoNi alloy by high-pressure torsion and subsequent annealing. The tensile yield strength depends strongly on temperature and grain size. Hall-Petch plots demonstrate that the grain boundary strength coefficient is insensitive to the temperature, whereas friction stress increases as the temperature decreases. For all the grain sizes both strength and ductility were increased with decreasing the temperature down to 77 K. Dislocation slip is responsible for the room temperature mechanical properties. At cryogenic temperature, on the other hand, nano twinning appears as an additional deformation mechanism in addition to the dislocation slip.

Pressure dependent electronic, spintronic, optical and mechanical properties of equiatomic quaternary Heusler alloys NbMnVAl and NbFeCrAl

The pressure dependent physical properties of Nb-based equiatomic quaternary Heusler alloys NbMnVAl and NbFeCrAl have been investigated using full-potential linearized augmented plane wave (FP-LAPW) method with exchange and correlation potentials of the PBEsol scheme. Half-metallic behavior in NbMnVAl (NbFeCrAl) with band gap of 0.32 (0.29) eV in spin-down (spin-up) state at ambient pressure is observed. The total magnetic moments are derived as 2.0 μΒ/f.u. for NbMnVAl and NbFeCrAl alloys, which reconciles with Slater-Pauling rule. Phonon frequencies are computed using Vienna Ab initio Simulation Package (VASP). The exchange interactions and Curie temperatures are determined using spin-polarized relativistic Korringa-Kohn-Rostoker (SPR-KKR) method. Moreover, the computations of mechanical properties revealed the stability and ductile nature of both compounds. The optical properties proved the applicability of these compounds in optoelectronic devices. Pressure dependent half-metallic properties are discussed to show spintronic applications of both the quaternary alloys.

Untra-fine-grained equiatomic CoCrNi medium entropy alloys with high density stacking faults and strengthening mechanisms

This study introduces a powder metallurgy route (a combination of mechanical alloying (MA) and spark plasma sintering) to enhance the mechanical properties of equiatomic CoCrNi medium-entropy alloys (MEAs) using multiple strengthening mechanisms. The yield strength of MEA with an average grain size of 0.41 μm produced by MA is 777 ± 21 MPa, which is 121% higher than that of MEA with grains of 4.55 μm prepared from commercial powders. Additionally, the yield strength of the untra-fine-grained alloy is 231% higher than that of the one produced by casting. Such an enhancement of the strength is attributed to the coordination of the ultra-fine grains, twins, and high-density stacking faults. Moreover, the influence of twins and stacking faults was investigated using molecular dynamic simulations. The intersection of ultra-high density stacking faults generates many immobile dislocation locks and separates the grains into several small parts, leading to a dynamic grain-refinement effect. These findings indicate that MA is an effective way to produce MEAs with superior strength.

Fabrication of Equiatomic CoCrFeMnNi High-Entropy Alloy by Metal Injection Molding Process Using Coarse-Sized Powders

Fabrication of Equiatomic CoCrFeMnNi High-Entropy Alloy by Metal Injection Molding Process Using Coarse-Sized Powders