High Entropy Alloy Thin Films Research Articles

Microstructural stability and mechanical property of AlCrFeMoTi high-entropy amorphous alloy thin films under He+ ions irradiation

High-entropy amorphous materials have recently proved exceptional radiation resistance, showing potential as a candidate materials for nuclear industry applications. This study synthesized nearly equi-atomic AlCrFeMoTi high-entropy amorphous thin films with a thickness of about 499 nm using magnetron sputtering. The prepared films were subjected to low dose, 0.5 MeV He+ ion irradiation, with varying irradiation damage ranging from 0.01 to 0.1 displacement per atom (dpa). Increasing the irradiation dose led to a decrease in the radius of the inner diffraction halo (r) and an increase in the crystal-like order, indicating an irradiation-induced structural relaxation process within the amorphous matrix. This relaxation results in more efficient atomic packing, yielding a denser amorphous structure and enhanced hardness of AlCrFeMoTi high-entropy amorphous thin films. This study demonstrates that AlCrFeMoTi high-entropy amorphous thin films exhibit remarkable radiation resistance, with stable microstructure and mechanical properties under He+ irradiation, making them promising candidates for applications in the nuclear industry.

Structural Parameters and Behavior in Simulated Body Fluid of High Entropy Alloy Thin Films.

The structure, composition and corrosion properties of thin films synthesized using the Pulsed Laser Deposition (PLD) technique starting from a three high entropy alloy (HEA) AlCoCrFeNix produced by vacuum arc remelting (VAR) method were investigated. The depositions were performed at room temperature on Si and mirror-like polished Ti substrates either under residual vacuum (low 10-7 mbar, films denoted HEA2, HEA6, and HEA10, which were grown from targets with Ni concentration molar ratio, x, equal to 0.4, 1.2, and 2.0, respectively) or under N2 (10-4 mbar, films denoted HEN2, HEN6, and HEN10 for the same Ni concentration molar ratios). The deposited films' structures, investigated using Grazing Incidence X-ray Diffraction, showed the presence of face-centered cubic and body-centered cubic phases, while their surface morphology, investigated using scanning electron microscopy, exhibited a smooth surface with micrometer size droplets. The mass density and thickness were obtained from simulations of acquired X-ray reflectivity curves. The films' elemental composition, estimated using the energy dispersion X-ray spectroscopy, was quite close to that of the targets used. X-ray Photoelectron Spectroscopy investigation showed that films deposited under a N2 atmosphere contained several percentages of N atoms in metallic nitride compounds. The electrochemical behavior of films under simulated body fluid (SBF) conditions was investigated by Open Circuit Potential (OCP) and Electrochemical Impedance Spectroscopy measurements. The measured OCP values increased over time, implying that a passive layer was formed on the surface of the films. It was observed that all films started to passivate in SBF solution, with the HEN6 film exhibiting the highest increase. The highest repassivation potential was exhibited by the same film, implying that it had the highest stability range of all analyzed films. Impedance measurements indicated high corrosion resistance values for HEA2, HEA6, and HEN6 samples. Much lower resistances were found for HEN10 and HEN2. Overall, HEN6 films exhibited the best corrosion behavior among the investigated films. It was noticed that for 24 h of immersion in SBF solution, this film was also a physical barrier to the corrosion process, not only a chemical one.

The effect of Cr-content on the corrosion behavior of Ti0.5Mo0.5CoNiMnCrx high-entropy alloy thin films deposited by direct current magnetron sputtering

The present study aims to investigate the impact of Cr-content on the corrosion behavior of Ti0.5Mo0.5CoNiMnCrx high-entropy alloy thin films (HEATFs) fabricated through direct current magnetron sputtering. The microstructure and chemical composition of the HEATFs were characterized using grazing incidence X-ray diffraction (GIXRD), field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS), and transmission electron microscopy (TEM). The corrosion performance of the HEATFs was evaluated through electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests in 1 M NaCl and 0.5 M H2SO4 solutions. The GIXRD, FESEM, and TEM results confirmed a uniform amorphous structure of the HEATFs. EIS analysis and assessment of fitted parameters indicated the formation of a protective passive film during immersion in 1 M NaCl at the open circuit potential (OCP), in contrast to the behavior observed in 0.5 M H2SO4. Moreover, all the HEATFs exhibited superior corrosion resistance compared to the 316 L stainless steel substrate, as evidenced by the potentiodynamic polarization curves obtained in both 1 M NaCl and 0.5 M H2SO4 environments.

Nickel content-dependent microstructure and mechanical properties of TiZrNbHfNi high entropy alloy thin films

Alloying is known as an effective strategy to improve the mechanical properties of metallic materials. Here we show that the microstructure and mechanical behavior of a TiZrNbHf1.6 high entropy alloy film can be highly tuned by Ni addition. The newly designed TiZrNbHf1.6Nix (x = 0, 0.4, 1.2, 1.8, 2.5, 3.0, 4.6) alloy films are synthesized by co-sputtering Ti, Zr, Nb, Hf and Ni elemental targets. The microstructure and mechanical behavior of the alloy films are investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), and nano/micro-indentations. The microstructural analysis indicates that the TiZrNbHf1.6 film has a single body-centered cubic with columnar-nanograined structure. As Ni content increases to 0.4 or above, the films show a complete amorphous structure. Our theoretical analysis shows that the transition from the crystalline to the amorphous structure can be attributed to the large atomic radius difference and the change of the thermodynamic parameters (e.g., mixing entropy and mixing enthalpy) due to the Ni addition. Compared to the TiZrNbHf1.6 film with a hardness of 4.8 GPa, a hardness increase of 8–35% is achieved in the Ni-doped alloy films (5.2–6.5 GPa) as measured by nanoindentation. The elevated hardness may be attributed to the severe lattice distortion and the amorphous structure induced by the Ni addition. Furthermore, microindentation reveals a diversity of deformation behavior in those films with different Ni content. A homogeneous deformation (slight plastic pileup) occurred in the films with Ni content of 1.2 and 4.6, which is close to the deformation behavior of the crystal TiZrNbHf1.6 film. While the film with a Ni content of 1.8 exhibits a catastrophic cracking deformation. Finally, multiple shear banding dominates the deformation of these films with Ni content of 0.4, 2.5 and 3.0. Our findings demonstrate that ultrastrong and highly deformable amorphous high entropy alloy thin films can be developed by appropriate element addition, making them promising materials for corrosion and wear-resistant coatings.

Design of high-entropy films as ultra-violet light reflector

This study proposes the design and implementation of high-entropy alloy (HEA) thin films as materials for the reflector of deep ultraviolet light-emitting devices and marks the first application of HEA in optical field. In the material design phase, an Al-Co-Zn-Ni alloy is designed by considering the reflectivity, phase stability and binary formation energy. Reverse Monte Carlo calculation is employed to establish stable atomic structure. Frequency-dependent dielectric equations are derived from first principles calculations, with dielectric constants incorporating contributions from ions and electrons and combining classical Drude and Lorentz models. The dielectric equations yield optical constants that are used to calculate the reflectance. In the experimental phase, we systematically vary the aluminum content and prepare Al-Co-Zn-Ni thin films ranging from pure aluminum to equimolar compositions. In the deep ultraviolet region, the reflectance of thermally treated Al65Zn12Co12Ni11 and Al50Zn16Co17Ni17 thin films is found superior to that of gold by three times. Structural and crystalline changes with varying aluminum content are found, as well as intriguing phase separation and superlattice formation that persist until equimolar compositions. This unique nanoscale structural variation causes the Young's modulus and hardness of Al-Co-Zn-Ni thin films to initially increase and then decrease with decreasing aluminum content. There is a significant increase in the corrosion potential difference when reducing Al contents in Al-Co-Zn-Ni, indicating improved passivation effects as the equimolar composition is approached. By modulating the aluminum content, the nanoscale structure of Al-Co-Zn-Ni can be adjusted, thereby improving its optical, mechanical, corrosion resistance, and thermal stability properties. This demonstrates the high versatility of this material for practical optical applications.

Tuning microstructure and properties of MoNbTaWZr high entropy alloy films by adjusting the parameters in high power impulse magnetron sputtering

Within this study a series of MoNbTaWZr thin films were synthesized using high power impulse magnetron sputtering (HiPIMS) with varying the deposition parameters: duty cycle from 0.5 to 100 % (i.e. DC magnetron sputtering (DCMS)), Ar pressure from 0.5 to 3 Pa and pulse frequency from 25 to 200 Hz. The structure of the deposited films was analyzed by X-ray diffraction and scanning and transmission electron microscopy. In addition, the electrical resistivity and the residual stress of the films was determined. The influence of the deposition parameter variation on structure and properties of the MoNbTaWZr films is discussed on the established framework of film growth conditions achievable with HiPIMS and DCMS and how they can be influenced by adapting the deposition parameters. The performed work is intended to contribute to a comprehensive understanding about synthesis-structure-property relations for refractory high entropy alloy thin films while using MoNbTaWZr films as a model system.

Effect of nitrogen pressure on the fabrication of AlCrFeCoNiCu0.5 high entropy nitride thin films via cathodic arc deposition

In the past two decades, high entropy alloy (HEA) coatings have attracted great attention due to their superior mechanical properties, outstanding corrosion and oxidation resistance, and exceptionally high thermal stability. In comparison to HEA thin films, high entropy nitrides (HENs) exhibit higher mechanical strength and chemical inertness. In this work, AlCrFeCoNiCu0.5 HEA and HEN thin films were fabricated using a filtered cathodic arc. By regulating the deposition pressure from 0.0005 Pa (HEA thin film) to 0.05 Pa, the nitrogen concentration in each thin film was precisely controlled to tune the mechanical properties. Scanning transmission electron microscopy-energy dispersive spectroscopy revealed that the nitrogen concentration of the films was up to 21.2 at. % at the pressure of 0.05 Pa. The reduced effect of preferential sputtering increased aluminum concentration from 8.3 ± 1.5 to 12.9 ± 2.2 at. % as pressure increased up to 0.05 Pa. X-ray photoelectron spectroscopy further confirmed the formation of AlN and CrN at pressures of 0.01–0.05 Pa. The highest hardness and elastic modulus of the HEN film were 12.4 ± 0.6 and 347.3 ± 17.7 GPa, respectively, which were 84.8% and 131.4% higher than those of the HEA thin film.

Phase decomposition of AlCrFeCoNiCu0.5 HEA thin films by vacuum annealing

The most attractive advantages of high entropy alloy (HEA) thin films are their excellent properties, such as high strength and high corrosion and oxidation resistance, with a lower material cost. However, the thermal stability of the HEA thin films is always controversial. This is critical since the thermal stability of metals and alloys is closely related to the material's performance at high temperatures and also determines the application field of the material. The question of how will the nanocrystalline high entropy alloy thin film perform in terms of thermal stability needs to be clarified urgently. In this research, the thermal stability of AlCrFeCoNiCu0.5 HEA thin film fabricated by cathodic arc deposition was investigated at 500 °C as a function of time during vacuum annealing. X-Ray Diffraction (XRD), High-Resolution Transmission Electron Microscopy (HRTEM) and Transmission Kikuchi Diffraction (TKD) were applied to analyze the chemistry and microstructure of HEA thin films in detail. The results showed that there was no obvious change in the elemental compositions of the films after annealing, but the elemental distribution of the annealed films was different from the as-deposited film, especially at the film surface and film-substrate interface for the 24 h-annealed sample. The existence of the FeCo phase with B2 structure was a significant sign to demonstrate that the spinodal decomposition occurred in the single face-centered-cubic (FCC) phase matrix of the film during annealing. Simultaneously, the segregation of Cu and Cr near the film-substrate interface and the penetration of Ni and Cu towards the Si substrate were clearly observed. These unusual phenomena indicated that the thermal stability of HEA was not always excellent. Also, it was noticed that a few Kirkendall voids forming at the film-substrate interface after 24 h of annealing would affect the adhesion of the film. Additionally, heat recovery and recrystallization were achieved by 500 °C annealing, which were confirmed by the decrease of lattice parameter and the increase of grain size in the 24 h-annealed thin film. Furthermore, the hardness and elastic modulus of the thin films before and after annealing were measured by nanoindentation. The results showed all the thin films exhibited excellent mechanical properties, and the optimal hardness and elastic modulus of 7.7 ± 0.3 GPa and 183.9 ± 4.4 GPa were obtained after 3 h-annealing.

Introducing a new heterogeneous nanocomposite thin film with superior mechanical properties and thermal stability

High entropy alloy (HEA) films offer excellent mechanical properties due to their random solid solution structure. However, their large grain boundary volume fraction can cause thermal instability, resulting in phase decomposition that affects their high-temperature performance. Nevertheless, it remains an interesting question whether phase decomposition can be used as a processing tool to create new HEA materials. In this study, AlCrFeCoNiCu0.5 HEA thin films were fabricated and annealed at 500 °C for up to 72 h. The 72-hour annealed thin film exhibited a 30 % increase in mechanical properties, exceeding most other HEA thin films. Characterization using X-ray and electron microscopy revealed a decomposition-induced phase transformation, which produced four new phases, including Cu-rich FCC phase, Cr-rich BCC phase, and ordered B2 phase of AlNi and FeCo. The enhanced mechanical properties were due to back stress strengthening and BCC + B2 phase strengthening. The 72-hour annealed thin film also showed excellent thermal stability through a new round of annealing, exhibiting a low variation in microstructure and chemical composition after being annealed at 500 °C for 100 h.

Direct electrodeposition of CoFeNiMoW high entropy alloy thin films from aqueous medium

The direct electrodeposition of CoFeNiMoW high entropy alloy (HEA) thin films from aqueous medium is demonstrated. By employing a citrate-containing bath and varying the applied current density (ic), HEAs with different compositions are obtained. Specifically, the composition of the HEA thin film obtained at ic = 50 mA cm−2 is Co30Fe21Ni19Mo25W5. The physical characterization of the HEA thin films is performed by energy dispersive x-ray spectrometry (EDS), grazing incidence x-ray diffraction (GIXRD), and 57Fe Mössbauer spectroscopy. Results confirm the presence and composition of the multicomponent HEA thin films and highlight galvanostatic electrodeposition as a simple, versatile, and cost-effective technique to create these HEA thin films.

Utilizing cluster percolation theory to analyze electrical resistivity jumps in high-entropy alloy thin films containing small atoms

The high-entropy alloy thin films (HEATFs) with non-metal small atoms have obvious characteristics of adjustable properties. However, the mismatch phenomenon of phase-transition and electrical resistivity jump usually occur during the interstitial-filling. To explain this phenomenon, a new method is proposed that transforms the multi-color percolation of HEATFs to two-color percolation. The [N/O-M6] clusters with high electrical resistivity and [▫-M6] clusters with low electrical resistivity in (NbMoTaW)100-xNx and (NbMoTaWV)100-xOx films are equivalent to black and white spheres, respectively (M and ▫ ▫-M6]) reaches the face-centered-cubic and amorphous percolation thresholds of 0.198 and 0.27, respectively, a conductive percolation path is formed, contributing to a significant reduction in electrical resistivity. This work enables it possible to predict the composition at sudden change in electrical resistivity of HEATFs with non-metallic small atoms, which has great significance for the functional composition design.

Flexomagnetic Effect Enhanced Ferromagnetism and Magnetoelectrochemistry in Freestanding High-Entropy Alloy Films.

Freestanding thin films of functional materials enable the tuning of properties via strain and strain gradients, broadening their applications. Here, a systematic approach is proposed to fabricate freestanding CrMnFeCoNi high-entropy alloy (HEA) thin films by pulsed laser deposition using expansion-contraction of NaCl substrates and weak van der Waals interaction of the interface, which form wrinkles with inhomogeneous strain gradients when transferred to a viscous handle. We demonstrate that the nonuniform gradients of external strain (flexomagnetic effect) can induce the partial structural phase transition from FCC to BCC in the wrinkled HEA film, resulting in a 10-fold increase in its room-temperature saturation magnetization compared with the unstrained flat HEA film. Furthermore, after applying an external magnetic field, due to the different electron transfer behavior caused by the electron scattering in wrinkled and flat HEA films, their electrocatalytic magnetic responses showed a diametrically opposite picture. Our work provides a promising strategy for tuning physical and chemical properties via complex strained geometries.

Preparation of graphene-coated high entropy alloy nanoparticles by double pulse carbothermal shock

High entropy alloy (HEA) nanoparticles have wide application prospects in electrocatalysis due to their unique structure. However, the surface of HEA nanoparticles has strong chemical activity due to numerous suspended and unsaturated bonds, which poses serious challenges to storage and transportation. To solve this problem, we used a double pulse carbothermal shock process to coat the surface of HEA nanoparticles with graphene. The formation mechanism of HEA nanoparticles and the graphene coating mechanism were discussed. The results showed that the HEA thin films were curled into nanoparticles by the change of orientation of twin structure under the thermal stress of carbothermal shock. After the complete HEA nanoparticles were formed, the carbon dissolved in the HEA nanoparticles will gradually precipitate out during a second carbothermal shock and form multiple layers of graphene on the surface, and forming a structure of graphene-coated HEA nanoparticles.

Compositional modulation through galvanic displacement in electrochemically deposited FeCoNiCuZn high entropy alloy thin films

After completing the aqueous electrodeposition of FeCoNiCuZn nanocrystalline high entropy (HEA) alloy system consisting of elements with significant differences in melting points and reduction potentials, unwanted galvanic displacement reactions (GDR) were observed on the alloy system when the film was held back in the electrolyte. This phenomenon, observed in alloys electroplated from aqueous baths containing a noble metal (currently Cu), leads to preferential deposition of the noble elements and changes the composition progressively at open circuit potential. A change in composition occurs in the electrodeposited alloy where Cu (from ∼13 to 58 at. %) was deposited on the alloy due to its nobler reduction potential causing displacement of Zn > Fe ∼ Co > Ni (order of displacement) from the deposited alloy with the Cu2+ ions present in the electrolyte. This work is the first observational report of galvanic displacement in HEAs composed of five elements. We categorically state that this GDR is crucial when designing/synthesizing HEAs alloys through electrodeposition.

CoFeNiTix and CrFeNiTix high entropy alloy thin films microstructure formation

High entropy alloys (HEA) composition-structure relationships are crucial for guiding their design and applications. Here, we use a combined experimental and molecular dynamics (MD) approach to investigate phase formation during physical vapor deposition (PVD) of CoFeNiTix and CrFeNiTix HEA thin films. We vary titanium molar ratio from 0 to 1 to understand the role of a larger element in the alloy mixture. The experiments show that a high titanium content favors amorphous phase formation in the samples produced by magnetron co-sputtering. In contrast, a low titanium content results in the formation of a face-centered cubic (FCC) structure in both HEA families. This effect of titanium content on the stability of the amorphous and FCC phases is reproduced in PVD MD simulations. The threshold titanium molar ratio is identified to be ∼0.53 and ∼0.16 for the CoFeNiTix and CrFeNiTix films in the experiments, and ∼0.53 and ∼0.53 in the MD simulations. In addition, the atomistic modeling allows for energy versus volume calculations with increasing titanium content, which demonstrate the stabilization of the amorphous phase with respect to crystalline structures. To isolate the effect of atomic sizes, additional simulations are performed using an average-atom model, which disregards differences in atomic radii while preserving the average properties of the alloy. In these simulations, the energetic stability of the amorphous phase disappears. The combined experimental and simulation results demonstrate that the formation of the amorphous phase in HEA thin films generated by PVD is directly caused by the atomic size difference.

Enhanced strength of AlCoCrCu0.5FeNi high entropy alloy thin films reinforced by multi-phase hardening and nanotwins

High entropy alloy (HEA) thin films have become increasingly popular because they exhibit favorable properties but with lower material consumption. Here, this research demonstrates that annealing of AlCoCrCu0.5FeNi thin films results in multiphase hardening and the formation of nanotwins. HEA thin films were atmospherically annealed at different temperatures (300 – 700 °C) for 5 h. A thin dense oxide layer consisting of Al2O3 and Cr2O3 appeared at the top surface of the 700 °C annealed film. This oxide layer endows the HEA thin film with excellent antioxidant properties. As such, vacuum and inert gas might not be required for HEAs annealing process at up to 700 °C. A phase transformation was observed in the film annealed at 700 °C for 5 h from X-ray diffraction patterns. Results from transmission Kikuchi diffraction and energy dispersive X-ray spectroscopy mapping indicated the formation of three new phases, including the FeCo phase, Cr-rich phase and Cu nano-clusters. Variations also happened in HEA FCC matrix where two types of texture existed in the HEA FCC grains i.e., {012} <110> and {112} <110>. Of particular interest was that high-resolution transmission electron microscopy revealed a high density of annealing nanotwins among the HEA FCC grains, including lamellar and co-axial twins. The influence of these structural phenomena on mechanical properties was evaluated. The results showed that the hardness and modulus of 700 °C annealed HEA thin film increased by 25% and 24% to 8.4 ± 0.2 GPa and 149.4 ± 6.1 GPa when compared to the properties of as-deposited films.

Hierarchical FeCoNiCr high entropy alloy thin films with combined high strength and excellent corrosion resistance

A two-step temperature-controlled magnetron sputtering procedure was adopted to obtain a bilayer hierarchical Fe25.2Co25.2Ni27.4Cr22.2 high entropy alloy (HEA) thin film consisting of the external nano-lamellar layer and internal equiaxed zone, which displayed combined advantages of high hardness, good modulus and excellent corrosion resistance. The roles of heterogeneous nanostructure on the mechanical and corrosion behavior were revealed by nano-indentation and Scanning Kelvin probe (SKP) methods. Compared to the substrate 304 ASS, the hierarchical HEA film not only shows much higher nano-hardness and modulus values, but also possesses a higher electron transfer resistance (12158 vs 4567 Ω.cm2) and lower overall corrosion rate (6.28*10−4 vs 6.84*10−3 mm·year−1). Moreover, the outer nano-lamellar layer shows higher nano-hardness (∼8.1 GPa) but lower surface potential values (∼-261 mV) than those of the inter equiaxed zone (∼5.9 GPa and ∼−150 mV). The former arises from the significant grain refinement strengthening effect, while the latter is attributed to an obvious increase in the high energy and chemically active areas such as grain boundaries and triple junctions. This work demonstrates the capability of hierarchy to adjust the comprehensive performance of multi-functional films, which possess superior mechanical property and comparable corrosion performance than those of the as-cast bulk HEA alloys.

High entropy alloy thin films on SS304 substrates: Evolution of microstructure and interface modulated by energetic condensation in nanoscale

High entropy alloys (HEAs), as a novel material in the 21st century, possess several advantages, such as excellent corrosion & oxidation resistance and high mechanical properties. HEA thin films show these favourable properties with lower material costs than their bulk counterparts. Studying the HEA film-substrate interface represents challenges but is of extreme importance for the understanding of growth mechanisms with important implications for film adhesion. However, most HEA films were deposited on monocrystalline silicon substrates with limited practical applicability. Further, where commercial stainless steel, aluminium or titanium alloy substrates were used, the microstructure and chemistry at the interface were neglected. Here, we deposited AlCrFeCoNiCu0.5 HEA thin films on stainless steel 304 (SS304) substrates using cathodic arc deposition with different substrate biases. The crystallography and microstructure were investigated using an X-ray and electron-microscopy based chatacterization. A transition of an incoherent to semi-coherent interface was observed from 0 V to -50 V of the substrate bias. Energy dispersive spectroscopy demonstrated a transition of Cr2O3 to aluminum oxide across the interface. The nanoindentation tests revealed the significant improvement of mechanical properties of SS304 with HEA coatings. High-strength HEA (8.0 ± 0.2 GPa) thin films with semi-coherent interfaces were manufactured on SS304.

A new design rule for high entropy alloy diffusion barriers in Cu metallization

High entropy alloy (HEA) thin films are potential candidates to act as diffusion barrier between Cu and Si in microelectronics. To evaluate the suitability of refractory HEAs, six different HEA thin films based on the MoNbTaW system and alloyed with Ti, V, Cr, Mn, Zr or Hf have been deposited by high power impulse magnetron sputtering with a thickness of 20 nm on Si substrates. The bi-layer architecture was completed by a 150 nm thick top-layer of Cu. The microstructure of the bi-layers before and after annealing in vacuum up to 800 °C was investigated by X-ray diffraction to observe possible phase changes or formations. In addition, resistance measurements and confocal laser scanning microscopy were employed to check for potential barrier failure. The lowest onset temperature for barrier failure of 600 °C was found for MoNbTaWZr, whereas the highest of 700 °C was observed for CrMoNbTaW. The obtained results for the failure temperature are discussed in terms of microstructure, strain and cohesive energy of the HEA films as well as the enthalpy of mixing between the elements in the HEA and Cu. A direct correlation between the mixing enthalpy of the fifth alloying element with Cu and the barrier failure temperature was observed.

Micro-scale tribological study of a Ni-Cr-Fe-Ti-Al-V high entropy alloy thin film by magnetron co-sputtering of Inconel-718 and Ti-6Al-4V

In this study, we fabricated high entropy alloy (HEA) thin films by RF magnetron co-sputtering of Inconel 718 and Ti-6Al-4V targets. Their coefficient of friction (CoF) and specific wear rate (K0) were investigated to evaluate their potential as a material for MEMS moving parts. By placing different number of Ti-6Al-4V pellets (samples abbreviated as IT2, IT4, IT6) on an Inconel 718 target (abbreviated as IT0), Ni-Cr-Fe-Ti-Al-V HEA thin films with configurational entropy of mixing (∆Sconf) >1.5R were successfully deposited. Electron microscopy and diffraction confirms that IT0 possessed a mixed nanocrysalline/amorphous structure, while the Ti-added IT2, IT4 and IT6 films were amorphous. They also have a significantly reduced root-mean-square roughness. In order to evaluate their micro-scale wear behavior, various tribological studies including indentation, scratch test and scanning wear test were conducted. From IT0 to IT6, film hardness H increases while reduced modulus Er remains similar. Plastic criterion Rw is also reduced, showing that the films IT2, IT4 and IT6 are more prone to elastic deformation. Scratch test also reveals that IT6 can afford the highest normal load before the transition of wear mode occurs. Lastly, a scanning wear test was conducted to evaluate the specific wear rate (K0) of each HEA film. It was found that IT6 had the lowest value of K0 = 2.26 × 10−4 mm3 N−1 m−1, which was also a lot lower than Si-to-Si (K0 = 0.0183 mm3 N−1 m−1) wear in other studies. The excellence of IT6 can be attributed to the fact that alloying of Ti effectively reduced the average interatomic distance in the HEA films, resulting in a densely packed, very elastic microstructure. Therefore, IT6 is expected to have longer service life than silicon if it were made into MEMS moving parts.