Fabrication of CrFeCoNiSi High Entropy Alloys Dispersed with Silicon Compounds by Low-Pressure Plasma Spraying

Abstract

We aimed to fabricate CrFeCoNiSi HEA deposits dispersed with Si compounds by low-pressure plasma spraying, and we evaluated the structure and properties of the alloy deposits. A heat treatment was applied to the fabricated alloy deposits, and the formation process of precipitates in HEAs was investigated. We fabricated HEA deposits without segregation on substrates with and without water-cooling. The fcc solid solution existed in the as-sprayed deposits and heat-treated deposits. Si compounds were dispersed in the deposits. The precipitates became coarser as the heat-treatment temperature increased. The 873 K heat-treated deposits had the highest hardness both with and without water-cooling. Nanoscale precipitates were formed inside the crystal grains in the as-sprayed deposits with water-cooling. The 1273 K heat-treated deposits satisfied the definition of HEAs.

1. Introduction

The demand for metals with a longer lifespan and higher functionality has increased in recent years. In this regard, high-entropy alloys (HEAs) have attracted much attention. Cantor and Yeh et al.^1,2) first studied HEAs in 2004. Cantor discovered CrFeCoNiMn, the first HEA, and studies have since actively investigated HEAs derived from this quinary alloy.^3–5) Conventional alloys often contain a certain main element to which small amounts of other elements are added. By contrast, HEAs are multicomponent high-concentration alloys in which the main element cannot be specified, and they are generally defined as a single-phase solid-solution alloy that comprises five or more constituent elements with equal atomic fractions.⁶⁾ Recently, the concept of HEA has been expanded from this narrow definition, and the search for alloys has expanded to include alloys with non-equiatomic compositions⁷⁾ and two-phase alloys containing precipitates.⁸⁾ Studies have reported HEAs of fcc,^9,10) bcc,^11–13) fcc+bcc,^14–16) and hcp^17,18) types. However, many of these HEAs are produced by casting or arc melting, and they suffer from the problem of segregation. To suppress segregation, the authors have applied a low-pressure plasma spraying method in which the cooling rate when the molten material collides with the substrate is extremely high.¹⁹⁾ If HEAs can be formed on a substrate surface by low-pressure plasma spraying, then they can be used as a coating material. Most HEAs, like CrFeCoNiMn alloys, have been reported to form a single-phase solid solution.^20,21) Few studies have investigated HEAs containing a second phase. Therefore, in this study, we fabricated HEAs containing a Si compound as a second phase by replacing Mn with Si based on a CrFeCoNiMn alloy that forms a single-phase solid solution. A heat treatment was applied to the fabricated alloy deposits, and the formation process of precipitates in HEAs was investigated. Specifically, we aimed to fabricate CrFeCoNiSi_0.14 HEA deposits dispersed with Si compounds by low-pressure plasma spraying, and we evaluated the structure and properties of the alloy deposits.

2. Experimental Procedure

CrFeCoNiSi HEA powder obtained by the gas atomization method (22.60Cr–21.30Fe–21.50Co–21.50Ni–14.10Si (at%)) (Sanyo Special Steel, Japan) with the particle size adjusted to 32–53 µm was used as the plasma spraying material, and plasma spraying was conducted in a reduced-pressure argon atmosphere using a Metco 7 MB plasma spray gun to deposit the material. Table 1 shows the low-pressure plasma spraying conditions. To evaluate the effects of substrate temperature differences during plasma spraying on the deposit structure, deposits were prepared under the following two conditions: plasma spraying while cooling the substrate with water and plasma spraying after preheating the substrate for 60 s without water-cooling. The substrate temperature during plasma spraying was approximately 330 K with water-cooling and 1000 K without water-cooling. An SS400 plate with a diameter of 100 mm and a thickness of 3 mm was used as the substrate, and the surface of the substrate was grid-blasted using alumina. Plasma spraying was conducted with a spraying distance of 0.20 m and a spraying time of 40 s with water-cooling and 120 s without water-cooling. The obtained deposit was separated from the substrate, and only the deposit was held in a vacuum heat treatment furnace at 573 K, 773 K, 873 K, 973 K, 1073 K, and 1273 K for 12 h, after which it was cooled in the furnace. These deposits were evaluated through an X-ray diffraction (XRD) (RIGAKU, Japan, RINT-2500) test using CuKα radiation, scanning electron microscope (SEM) (JEOL, Japan, JSM-6060LV) and transmission electron microscope (TEM) (JEOL, Japan, JEM-2100F) (for structural observations), electron probe microanalyzer (EPMA) (JEOL, Japan, JXA-820) analysis, Vickers microhardness (Matsuzawa, Japan, MMT-7) test, and Suga-type wear (Suga Test Instruments, Japan, NUS-ISO-3) test (load: 19.6 N, mating material: #320 emery paper). In the Suga-type wear test, the wear wheel was rotated 0.9° when the sample was reciprocated for one cycle, so the emery paper was replaced every 400 cycles of reciprocating motion of the sample. Each sample was tested for 2000 cycles to determine the amount of wear. Additionally, the area ratio of the deposit was determined using ImageJ image processing software with an SEM image of the cross-section of the deposit.

Table 1 Plasma spraying conditions.

3. Results and Discussions

3.1 Spray material

XRD peaks of the fcc phase, Ni₂Si, and Ni₃Si were detected from the alloy powder. Figure 1 shows an SEM image of the outer surface of the alloy powder. The alloy powder had a nearly spherical shape. Figure 2 shows an EPMA elemental mapping image of the cross-section of the alloy powder. Si compounds were observed in the alloy powder.

Fig. 1

SEM image of outer surface of alloy powder.

Fig. 2

EPMA elemental mapping image of cross-section of alloy powder.

3.2 Deposit

Figures 3 and 4 show the XRD patterns of the deposits with and without water-cooling, respectively. The diffraction peaks of the fcc solid solution were detected in as-sprayed deposits with and without water-cooling. This suggests that a solid solution of HEA was formed by rapid solidification of plasma spraying. The diffraction peaks of the fcc phase were also detected in deposits with and without water-cooling that underwent heat treatment at a high temperature of 1273 K. In deposits with water-cooling, diffraction peaks of Ni₂Si were detected under the as-sprayed and 573 K and 773 K heat-treatment conditions, and a diffraction peak of Cr₃Ni₅Si₂ was detected in deposits heat-treated at over 873 K. Additionally, when the heat-treatment temperature increased, the diffraction peak of fcc shifted to the high-angle side. This is attributed to the generation of Si compounds. Unlike in the deposits with water-cooling, in the deposits without water-cooling, a diffraction peak of Cr₃Ni₅Si₂ was detected under as-sprayed conditions. This is because the substrate temperature during plasma spraying was higher in the deposits without water-cooling than in those with water-cooling. Additionally, even in the deposits without water-cooling, when the heat treatment temperature increased, the diffraction peak of fcc shifted to the high-angle side. Figures 5 and 6 show cross-sectional SEM images of the deposits with and without water-cooling, respectively. Flattened grain boundaries were observed in the deposits with and without water-cooling, and they were less clear in the deposits without water-cooling than in those with water-cooling. This is because the substrate temperature during plasma spraying was higher in the deposits without water-cooling than in those with water-cooling. Additionally, flattened grain boundaries became unclear with increasing heat-treatment temperature for deposits with and without water-cooling. In the deposits with water-cooling, precipitates were not observed under as-sprayed and 573 K and 773 K heat-treatment conditions; however, they were seen in deposits heat-treated at over 873 K. Fine precipitates with a size of approximately 0.3–0.4 µm and 0.6–1.0 µm were observed in deposits heat-treated at 873 K and 973 K, respectively. Further, precipitates with a size of approximately 1.1–2.3 µm and 3.0–4.8 µm were observed in deposits heat-treated at 1073 K and 1273 K, respectively. The area ratios of precipitates in deposits heat-treated at 873 K, 973 K, 1073 K, and 1273 K exceeded 60%, 83%, 65%, and 40%, respectively; clearly, the area ratio decreased after peaking at 973 K. Unlike in the as-sprayed deposits with water-cooling, precipitates with a size of 0.2–0.5 µm were observed in the as-sprayed deposits without water-cooling, and these precipitates became coarser as the heat-treatment temperature increased. The area ratios of the precipitates under as-sprayed and 573 K, 773 K, 873 K, 973 K, 1073 K, and 1273 K heat-treatment conditions were 50%, 50%, 61%, 81%, 64%, 55%, and 43%, respectively; clearly, the area ratio decreased after peaking at 873 K. Figure 7 shows scanning transmission electron microscopy-bright field (STEM-BF) images of the as-sprayed deposit with water-cooling. Crystal grains with different contrasts were confirmed in this deposit. They are attributed to the formation of a disordered solid solution of HEA. Additionally, in the as-sprayed deposit with water-cooling, black nanoscale particles with a size of 3–4 nm were found in grains with a size of approximately 250 nm. Nanoscale precipitates have also been reported in Al_0.3CrFeCoNi HEAs²²⁾ and in grains in CrFeCoNiNbC HEAs,²³⁾ Al_0.5CoCrFeNi HEAs,²⁴⁾ and Co_1.5CrFeNi_1.5Ti_0.5 HEAs.²⁵⁾ These reports suggest that the as-sprayed CrFeCoNiSi_0.14 HEA deposits with water-cooling in the present study formed nanoscale precipitates in the crystal grains. Furthermore, the presence of heavy elements in the STEM-BF image makes it difficult for the electron beam to penetrate; therefore, Ni in Ni₂Si detected in the XRD pattern of the as-sprayed deposit with water-cooling is thought to exist in the nanoscale precipitate particles. Nanoscale δ-Ni₂Si was generated in grains having a size of 10 µm in Cu–Ni–Si ternary alloys,²⁶⁾ and the black nanoscale precipitate particles seen in Fig. 7 were thought to be Ni₂Si. Figures 8 and 9 show the EPMA elemental mapping images of the as-sprayed deposits with and without water-cooling, respectively. No segregation was observed in the as-sprayed deposits with and without water-cooling. Therefore, the segregation that occurred during the solidification of the cast material was thought to be suppressed by rapid solidification due to plasma spraying. Figure 10 shows an EPMA elemental mapping image of the 1273 K heat-treated deposit without water-cooling. The precipitates seen in the cross-sectional SEM image were thought to be Cr₃Ni₅Si₂ from the X-ray diffraction results and EPMA elemental mapping results. EPMA point analysis revealed that the composition of the matrix of the 1273 K heat-treated deposit without water-cooling was 20.60 at% Cr, 21.66 at% Fe, 22.16 at% Co, 22.72 at% Ni, and 12.86 at% Si. ΔS_mix of the 1273 K heat-treated deposit without water-cooling was 1.591R; because it was greater than 1.5R,⁶⁾ this deposit satisfies the definition of an HEA. Figure 11 shows the Vickers microhardness test results of deposits with and without water-cooling. Among the deposits with water-cooling, the 873 K heat-treated deposit with Cr₃Ni₅Si₂ had the highest hardness. The 973 K heat-treated deposit had a higher area ratio of precipitates than the 873 K heat-treated deposit, but its hardness was lower. This was because the 973 K heat-treated deposit had coarser precipitates than those of the 873 K heat-treated deposit. Additionally, the hardness decreased with heat treatment at 1073 K and higher. This is attributed to the softening of the matrix, coarsening of the precipitates, and decrease in the area ratio of the precipitates. The hardness of the deposits without water-cooling was higher than that of deposits with water-cooling. This was because the generated deposits were different, and the adherence between the flat particles was higher in the deposits without water-cooling than in those with water-cooling. For the deposits without water-cooling, the hardness of the as-sprayed deposit and 573 K heat-treated deposit were almost the same. This was because the size and area ratio of the precipitates showed almost no difference. The 873 K heat-treated deposit had the highest hardness. This was because the precipitates were fine with a size of 0.3–0.6 µm and had a high area ratio of 81%. Additionally, the hardness decreased with heat treatment at 973 K and higher. Figure 12 shows the results of the Suga-type wear test of deposits without water-cooling. The 1273 K heat-treated deposit had the highest wear loss. And the 873 K heat-treated deposit had the highest wear resistance. This was because it had the highest hardness, as shown in Fig. 11.

Fig. 3

XRD patterns of deposits with water-cooling.

Fig. 4

XRD patterns of deposits without water-cooling.

Fig. 5

Cross-sectional SEM images of deposits with water-cooling.

Fig. 6

Cross-sectional SEM images of deposits without water-cooling.

Fig. 7

STEM-BF images of as-sprayed deposit with water-cooling.

Fig. 8

EPMA elemental mapping image of as-sprayed deposit with water-cooling.

Fig. 9

EPMA elemental mapping image of as-sprayed deposit without water-cooling.

Fig. 10

EPMA elemental mapping image of 1273 K heat-treated deposit without water-cooling.

Fig. 11

Vickers microhardness test results of deposits.

Fig. 12

Wear test results of deposits without water-cooling.

4. Conclusion

CrFeCoNiSi_0.14 HEA deposits dispersed with Si compounds were fabricated by low-pressure plasma spraying, and the structures and properties of the deposits were evaluated.

1. (1)    We fabricated HEA deposits without segregation on substrates with and without water-cooling.
2. (2)    The fcc solid solution existed in the as-sprayed deposits and heat-treated deposits.
3. (3)    Si compounds were dispersed in the deposits.
4. (4)    The 873 K heat-treated deposits had the highest hardness both with and without water-cooling.
5. (5)    Nanoscale precipitates were formed inside the crystal grains in the as-sprayed deposits with water-cooling.
6. (6)    The 1273 K heat-treated deposits satisfied the definition of HEAs.
7. (7)    Among deposits without water-cooling, the 873 K heat-treated deposit had the highest wear resistance.

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