Structure and properties of HEA surface layer after electron-ion-plasma processing

Introduction

The development and study of high-entropy alloys (HEAs) are of significant scientific interest due to their unique microstructure [1; 2], composite composition [3], and mechanical properties [4 – 6]. In contrast to traditional alloys, which typically consist of one or two basic elements, HEAs are composed of several major elements (at least five) in equimolar or near-equimolar ratios. The original findings in the field of HEAs are extensively discussed in analytical reviews [7 – 9] and monographs (e.g., [10]). These publications describe the microstructure, properties, and thermodynamics, review the results of structural modeling, and explore novel methods for producing multi-component alloys. Substantial efforts have been dedicated to addressing the challenge of enhancing the mechanical properties of five-component alloys like MnCoCrFeNi and AlCoCrFeNi through grain boundary strengthening [11; 12], solid solution hardening [13 – 16], and interstitial solid-solution hardening [17]. Relevant theoretical developments are also in progress [18]. The paper [13] proposes a method to enhance strength through partial amorphization, as this structure lacks grain boundaries or dislocations. To improve surface properties, HEAs undergo various types of surface treatments. For example, the paper [19] reviews various processing methods and their impact on the surface of CoCrFeMnNi HEA obtained through selective laser melting. The considered treatments include electrolytic polishing, electro-erosion machining, milling, grinding, mechanical polishing with abrasives, and combinations of these methods. The results demonstrate that grinding smoothes the surface and increases microhardness, however, it leaves tool marks and residual stresses due to microstructure deformation. Mechanical polishing using abrasives enables the creation of an ultra-smooth surface without subsurface damage. Electro-erosion machining results in surface melting, leading to increased residual stresses and microhardness. When electrolytic polishing is combined with other methods, it removes residual stresses and damage caused by previous processing, thereby smoothing the surface. However, when electrolytic polishing is used in isolation from other methods, a micrometer-level surface roughness cannot be achieved. The paper [20] addresses the challenge of low strength and wear resistance in the CoCrFeMnNi alloy with an FCC crystal lattice by employing the powder-pack boriding method. This treatment results in a double layer enriched with silicon and boron, leading to increased microhardness and wear resistance in the borated samples. One of the most promising and highly efficient methods for surface hardening is electron-beam treatment [9; 10]. This treatment ensures ultrahigh heating rates of the surface layer (up to 10⁶ K/s) to the specified temperatures, followed by cooling rates of 10⁴ – 10⁹ K/s through heat extraction into the bulk of the material. As a result, non-equilibrium submicro- and nanocrystalline structural-phase states emerge in the surface layer.

The aim of this research is to analyze the elemental and phase compositions, as well as the defect substructure of the HEA surface layer formed as a result of complex treatment, combining film deposition (B + Cr) and pulsed electron beam irradiation.

Materials and methods

The study material utilized was a HEA with the elemental composition AlCrFeCoNi, which was obtained using cold metal transfer technology [20]. The samples had dimensions of 15×15×5 mm. The surface treatment of the HEA samples was carried out in two steps: 1 – formation of a “film/substrate” system: a 0.5 μm thick boron film was deposited, and on top of it, a 0.5 μm thick chromium film was added; 2 – irradiation of the “film (B) + film (Cr)/HEA (substrate)” system with a pulsed electron beam.

The boron film was deposited onto the surface of HEA samples using plasma-assisted radio frequency sputtering (RF sputtering) of boron powder cathode. The following process parameters were applied: power W = 800 W; frequency f = 13.56 MHz; duration of processes t = 35 min (resulting in a 0.5 μm thick boron film); current of PINK plasma generator I_p = 50 A; heating current I_h = 145 A; bias voltage U_b = 50 V; duty cycle 75 %; bias frequency 50 kHz. Before applying the boron film, the surface of the HEA samples, following placement in the installation chamber and subsequent evacuation, underwent a brief 15-minute etching process with argon plasma. The 0.5 µm thick chromium film was deposited onto the samples with the boron film using an arc evaporator. The following process parameters were employed: samples with the boron film were placed opposite the arc evaporator, without rotation; arc evaporator current I_a = 80 A; plasma generator current I_p = 20 A; heating current I_h = 135 A; duty cycle 75 %; bias voltage U_b = 35 V; pressure p = 0.3 Pa; chromium deposition t = 10 min. The “film/substrate” system was then irradiated with an intense pulsed electron beam on the SOLO installation, employing the following process parameters: energy of accelerated electrons is 18 keV, energy density of the electron beam is 20 – 40 J/cm²; pulse duration is 200 µs; number of pulses is 3; pulse repetition rate is 0.3 s\(^–\)¹; pressure of the working gas (argon) is 0.02 Pa. Based on estimations [10], under these irradiation parameters, the temperature of the surface layer of the “film (B + Cr)/(HEA) substrate” system surpasses the melting point of the HEA. This leads to the expectation that the process of forming a molten surface layer of HEA samples alloyed with boron and chromium atoms (during rapid heating) and the subsequent development of a submicro- and nanocrystalline multiphase structure strengthened with metal borides (during rapid cooling) will occur.

The phase composition and the condition of the crystal lattice in the main phases of the sample’s surface layer were investigated through X-ray phase and X-ray diffraction analysis. This analysis was conducted using a Shimadzu XRD 6000 X-ray diffractometer located in Kyoto, Japan. X-ray shooting utilized copper-filtered CuK_α1 radiation and a CM-3121 monochromator. The phase composition was determined with reference to the PDF 4+ databases, and full-profile analysis was performed using the POWDER CELL 2.4 program. To achieve the desired film thickness, the deposition modes for the boron and chromium films were selected through experiments employing the Calotest CAT-S-0000 device, designed to determine film thickness. Material hardness was assessed using the Vickers method on a PMT-3 microhardness tester with a 0.5 N load. The tribological characteristics, including the friction coefficient and wear parameter of the material, were examined using a Pin on Disc and Oscillating TRIBOtester tribometer by TRIBOtechnic in Clichy, France. The tests were conducted with a ceramic Al₂O₃ ball, 6 mm in diameter, a 2 mm radius for the friction track, a 100 m path for the counter body’s travel, a sample rotation speed of 25 mm/s, and an indenter load of 2 N. These tribological tests were performed under dry friction conditions at room temperature.

Results and discussion

The HEA produced through additive technology exhibits a dendritic structure. Dendrites are polycrystalline aggregates with an average grain size of 12.3 μm. X-ray microanalysis revealed that the HEA consists of the chemical elements Al, Cr, Fe, Co, and Ni in the following proportions at. %: Al 33.4; Cr 8.3; Fe 17.1; Co 5.4; Ni 35.7.

A mapping technique was employed to visualize the distribution of atoms within the alloy’s bulk. The results indicate that the boundaries of grains and dendrites are enriched with chromium and iron atoms, while the volume of the grains contains higher concentrations of aluminum and nickel. Cobalt atoms are distributed in a quasi-homogeneous manner throughout the alloy’s bulk.

X-ray phase analysis confirmed that the alloy in question possesses a simple cubic crystal lattice, with a crystal lattice parameter is 0.28795 nm.

Irradiation of the “film/substrate” system with a pulsed electron beam induces significant alterations in the mechanical and tribological properties of HEA samples. Firstly, there is a substantial increase in microhardness, with the maximum value achieved following irradiation of the “film/substrate” system with a pulsed electron beam having an electron beam energy density (E_s) of 20 J/cm² (Fig. 1, a). Secondly, the wear resistance of the samples improves, and the friction coefficient decreases, with the most favorable outcomes observed after irradiation of the “film/substrate” system with a pulsed electron beam having an energy density of 20 J/cm² (Fig. 1, b, c).

The alterations in the mechanical and tribological properties of the alloy are evidently attributed to changes in the structure of the surface layer of the samples. It was observed that when the “film/substrate” system is exposed to an electron beam with an energy density of 20 J/cm², the sample’s surface undergoes fragmentation, forming a network of microcracks (Fig. 2, а). These microcracks vary in size from 40 to 200 μm, with an average size of 104 μm. Within the fragments, a grain structure becomes apparent (Fig. 2, c). The average grain size within the fragments measures 2.7 μm, which is 4.5 times smaller than the average grain size of the initial HEA.

With an increase in electron beam energy density, the average grain size of the HEA surface layer also increases, reaching 19 μm when E_s = 40 J/cm². it reaches 19 μm. It is evident that the significant reduction in the average grain size of the surface layer when E_s = 20 J/cm² is one of the reasons for the improvement in the alloy’s strength properties, a phenomenon known as the Hall–Petch effect.

Irradiation of the “film/substrate” system with a pulsed electron beam at E_s = 20 J/cm² does not result to the complete dissolution of the film. Instead, we observe extended film interlayers within the bulk and along the boundaries of the fragments, as well as film islands situated at the junctions of the fragments (Fig. 2, b, c).

With the increase in electron beam energy density, reaching 30 J/cm² and subsequently 40 J/cm², the (B + Cr) film completely dissolves (Fig. 3). In these cases, the surface of the samples is once again fragmented by a network of microcracks, which signifies the formation of significant tensile stresses within the surface layer of the alloy as it is irradiated.

The rapid solidification of the surface layer results in the creation of a sub-grain structure, often referred to as a rapid solidification structure (Fig. 3, c). When E_s = 20 J/cm², the subgrain structure is infrequently observed, at E_s = 30 J/cm², it forms at the junctions of grain boundaries and fragments; and at E_s = 40 J/cm², subgrains are formed across the entire surface of the sample. The size of these subgrains remains consistent and ranges from 150 to 200 nm, independent of the energy density of the electron beam.

X-ray microanalysis was employed to demonstrate that the sections of the film that remain after irradiation of the “film/substrate” system with a pulsed electron beam at 20 J/cm² are enriched with chromium, boron, and oxygen atoms.

Additionally, extended interlayers along the fragment boundaries are observed to be enriched with oxygen and aluminum.

The islands that form on the HEA surface when the “film/substrate” system is irradiated with an electron beam, with an energy density of 30 and 40 J/cm², are found to be enriched with chromium, aluminum, and oxygen atoms. This indicates that, as a result of irradiation of the “film/substrate” system with a pulsed electron beam, chromium and aluminum oxiborides are formed on the HEA surface. The quantity of these oxiborides decreases as the energy density of the electron beam increases. The formation of oxiborides contributes to the enhancement of HEA microhardness and wear resistance.

The phase composition of the HEA surface layer, modified due to irradiation of the “film/substrate” system with a pulsed electron beam, was investigated through X-ray phase analysis. Regardless of the value of E_s, the alloy remains a single-phase material with a simple cubic crystal lattice.

The lattice parameter exhibits an ambivalent dependence on the value of E_s (Fig. 4). One of the reasons for the variation in the crystal lattice parameter of the alloy is the incorporation of boron atoms into the samples, and their concentration within the bulk of the alloy increases as the energy density falls within the range E_s = (20 – 30) J/cm². It is worth noting that the boron atoms within the HEA crystal lattice will occupy interstitial positions, leading to an increase in the lattice parameter. The formation of a solid interstitial solution is another physical mechanism contributing to the hardness of the alloy. The absence of detectable strengthening phases in the alloy under study by X-ray phase analysis may be attributed to their limited quantity.

Conclusions

HEA samples with non-equiatomic elemental composition AlCrFeCoNi were manufactured using cold metal transfer technology. A comprehensive treatment of the HEA samples’ surface layer was carried out, combining the creation of the “film (Cr + B)/(HEA) substrate” system with subsequent irradiation using a pulsed electron beam at various electron beam energy density levels (ranging from 20 – 40 J/cm². The optimized irradiation mode (electron-beam energy density of 20 J/cm², irradiation duration of 200 μs, three pulses, pulse frequency of 0.3 s\(^-\)¹) was identified, resulting in a significant increase in microhardness (almost double), enhanced wear resistance (over fivefold improvement), and a 1.3-fold reduction in the friction coefficient. The conducted studies of the structure and phase composition suggested that the improvement in HEA strength and tribological properties is linked to a considerable reduction in average grain size (4.5 times smaller), the formation of chromium and aluminum oxyboride particles, and the incorporation of boron atoms into the HEA crystal lattice.