Powder characteristics of Al0.5CoCrFeMnNi high-entropy alloys fabricated by gas atomisation method

The fabrication of high-quality metallic powders has been attracted considerable attention in terms of producing advanced materials through additive manufacturing (AM) technology. The efficiency of AM products mainly dependent on the initial powder, therefore, characterisation of pre-alloyed powders become important. In this study, we explored the fabrication of Al_0.5CoCrFeMnNi high-entropy alloy (HEA) powders by gas atomisation process and investigated their powder characteristics according to particle size distribution. X-ray diffraction results revealed the formation of major face-centred cubic and minor body-centred cubic phase by the addition of Al content. The detailed characterisations such as scanning electron microscopy, Auger electron spectroscopy and XPS analysis was carried out to examine the chemical composition of HEA powders. The relationship between secondary dendrite arm spacing and cooling rate was evaluated. The results demonstrated that produced alloy powders are spherical in shape with smooth surfaces, compositional homogeneity, indicating the gas-atomised powders are widely acceptable for additive manufacturing applications.

Keywords: High-entropy alloys; gas atomisation; powder characteristics; microstructure; cooling rate

1. Introduction

Nowadays, high-entropy alloys (HEAs) have drawn scientific interest in materials engineering for various structural applications, due to their outstanding mechanical properties such as high strength, fracture toughness, good thermal stability, excellent wear and corrosion resistance [[1]]. The concept of HEAs independently developed by Yeh and Cantor et al. in 2004, based on the mixing of five or more principal elements with nearly equiatomic concentration consists inbetween 5 and 35 at.-% [[6]]. Despite containing greater number of components in HEAs, which exhibits simple crystal structures such as face-centred cubic (FCC), body-centred cubic (BCC) and hexagonal-close packed structures instead of ordered phases and intermetallic compounds resulted in high configurational entropy [[8]]. Besides, fabrication of HEAs shows a significant influence on microstructure and mechanical properties [[10]]. Until today, most of the HEAs fabricated using conventional arc melting and casting methods, which requires additional thermal-mechanical procedures to eliminate crystalline defects and homogenise the microstructure. These methods imposing enormous constraints to produce HEAs with complex geometry in terms of price and efficiency for structural applications.

In contrast to the conventional methods, powder metallurgy associated additive manufacturing (AM) technology has emerged as a new transformative technology with many industrial advantages including, short processing time, high-dimensional accuracy and less material wastage, indicating a potential method to produce complex metallic parts [[11]]. Therefore, fabrication of high-quality feedstock powders for AM technology has become significant attention, because the physical, chemical and mechanical properties of AM products directly influenced by the initial powders [[13]]. For instance, a spherical-shaped powder with good morphology and narrow size distribution are considerable factors to improve the powder flowability during material processing [[14]]. The rapid solidified gas atomisation process is a well-established method for producing various alloy powders with excellent advantages including good morphology, spherical shape, compositional homogeneity, low oxidation risks and mass production ability [[15]]. Very recently, considerable efforts have been invested to produce complex HEA parts by AM technology via pre-alloyed gas-atomised powders [[16]]. Wang et al. reported the fabrication of CoCrFeMnNi HEAs by AM technology using gas-atomised HEA powders and detailed characterisations were invested to examine the gas-atomised powders suitable for AM processes [[18]]. Li et al. studied the microstructure and mechanical properties of CoCrFeMnNi HEAs produced by selective laser melting technique and improved the mechanical properties as compared to the as-cast CoCrFeMnNi HEA, due to pile up of dislocations and lattice distortion are the main strengthening of AM-HEAs [[7]]. Eißmann et al. reported a single-phase CoCrFeMnNi HEAs fabricated by electron beam melting (EBM) technology using gas-atomised pre-alloy powders. However, the significance of powder quality on the final product of EBM–HEA has not been investigated [[19]]. Therefore, analysis of powders characteristics such as morphology (shape and satellites), particle size distribution, compressibility, uniformity, tap density, surface oxides and impurity concentrations are considerable factors for AM applications. To the best of my knowledge, there are no reports have been focused on the production of Al0.5CoCrFeMnNi HEA powders by gas atomisation method and investigation of powder characteristics, which are suitable for AM technology.

In this study, Al0.5CoCrFeMnNi HEA powders fabricated by a rapid solidified gas atomisation process and studied their powder characteristics according to particle size distribution. The crystal structure of HEAs examined by X-ray diffraction (XRD) technique and chemical composition of HEAs were investigated by scanning electron microscopy (SEM)–EDS, Auger electron spectroscopy (AES) and XPS analysis. Furthermore, we estimated the secondary dendrite arm spacing and cooling rates of as-atomised powders according to particle size distribution.

2. Experimental procedure

2.1. Materials and methods

High-purity metallic components of Co, Cr, Fe, Mn, Ni and Al were used as starting materials and weighed according to stoichiometric ratio to achieve the desired Al0.5CoCrFeMnNi HEA. The mixture of raw elements was melted in high-frequency induction furnace and the melt stream was poured through a boron nitride melt delivery nozzle with an orifice diameter of 5 mm and atomised in high-purity argon atmosphere. The rapidly solidified atomised powders collected from the bottom chamber and classified according to different size distribution, < 20, 20–63, 63–106 and 106–150 μm. The particle size distribution of as-atomised powders was analysed by laser diffraction particle size analyser. The obtained alloy powders were carried out to examine the powder characteristics by powder characteristic tester. The crystal structure of all the powders was examined using the XRD analysis (Rigaku diffractometer, Miniflex-600, Japan, CuKα radiation with a wavelength of λ = 0.154 Å). The oxygen content of HEA powders was analysed by Eltra ONH-2000 Oxygen/Nitrogen/Hydrogen determinator according to the particle size distribution. The morphology and cross-sectional microstructure of as-atomised powders observed by scanning electron microscopy (SEM MIRA LMH II TEKSAN). While the chemical composition of alloy powder was examined using SEM-equipped EDS analysis, Auger electron spectroscopy and X-ray photoelectron spectroscopy analysis. In addition, microhardness measurements were carried out on the polished surface of as-atomised powders according to the particle size distribution under an applied load of 50 gf with dwell time of 10 s using micro-Vickers indenter measurement system. For each range of particle size distribution, at least 10 hardness measurements were carried, and the average values were considered to maintain the accuracy.

2.2. Powder characteristics

The analysis of powder characteristics for the as-atomised powders is very important because it can influence the powder flowability and packing properties.

Tap density: It is the ratio of powder mass to the final volume occupied by powder when it has been mechanically tapped for a certain period of time. In general, tap density provides information about interparticle flowability with external pressure and packing behaviour of powders. Table 1 shows the measured tap density of as-atomised powders is about 4.77 g cm−3, indicates that produced powders have good compaction properties.

Table 1. Powder characteristics of Al0.5CoCrFeMnNi HEA produced by gas atomisation process.

Composition (at.-%)	Tap density (g cm−3)	Compressibility (%)	Angle of repose in (°)	Flat plate angle in (°)
Al0.5CoCrFeMnNi	4.77	8.39	23.57	25.33

Compressibility: The compressibility of a powder can be defined as the thickness reduction of powder by applying normal force. It can be influenced by several factors including, particle size distribution, shape, satellites and surface roughness of powders. It is the most important factor for compacting the metallic powders [[20]]. The measured compressibility of 8.39% for the as-atomised powders (data shown in Table 1), indicates that powders have excellent compressibility.

Angle of repose: The angle of repose (AOR) can be measured when the powder falls freely through a funnel onto a horizontal plate by gravity and powder piled as cone shape, the formation of angle between slope of a powder pile and horizontal plane is called the angle of repose [[21]]. It is the most important powder characterisation technique to realise the powder flowability. In general, the smaller AOR indicates that the better flowability of powders. Table 1 shows the measurement of angle of repose for the as-atomised powders, it showed the lowest angle of repose about 23.57°, due to its spherical shape and fewer satellites on powder surface, indicating the powders have excellent flowability and reliable for AM applications. This methodology was recommended by ASTM for characterising the metal powder for AM applications [[22]]. In addition, Schulze et al. demonstrated the time taken to discharge powders through funnel can be considered as a measurement factor for evaluating the flowability [[23]]. Carr et al. [[24]] and Raymus et al. [[25]] demonstrated that angle of repose with below 30° shows exceptional flowability.

Flat plate angle: After free fall of powder through a funnel on the flat plate, immerse a plane in the piled powder and pull up the plane in vertical direction, we can get some powder on the plane. The angle formed between the plane and slope of powder, then apply some external pressure on the powder to make another angle and average those two angles is called flat plate angle. The measured flat plate angle of powder shown in Table 1. The lowest flat plate angle indicates higher flowability and it is usually greater than angle of repose. The measured flat angle of as-atomised powders about 25.33°, indicating produced alloy powder reveals excellent flowability.

3. Results and discussion

3.1. Powder morphology and particle size distribution of as-atomised HEA powder

Figure 1(a) illustrates the schematic representation of gas atomisation process and it can be able to produce metallic powders with spherical shape and homogeneous chemical composition. Initially, the raw materials placed in top chamber called as heating furnace, where the raw elements are melted. Then molten metal was poured through a ceramic nozzle by spraying high pressure gas, as a result metal droplet rapidly solidifies and collected from the bottom chamber. The obtained as-atomised powders were sieved according to particle size distribution such as < 20, 20–63, 63–106 and 106–150 μm and the corresponding powder morphology can be displayed in Figure 1(b–e), respectively. The analysis of powder morphology is an important factor that can influence the flowability and packing density during metal deposition. The obtained as-atomised powder shows a spherical in shape with smooth surface morphology regardless of particle size distribution. While there are few satellites appeared on the powder surface, as shown in Figure 1(b–e). The formation of satellites on powder surface was due to the difference in solidification rate among powders. The rapidly solidified droplets triggered by the turbulent flow in the atomisation chamber, as a result fine particle attached on the powder surface and this kind of satellite formation is very common in gas-atomised powders [[26]]. However, the presence of higher satellites leads to a decrease in the flowability and tap density. Therefore, optimisation of cooling rate is essential to minimise the satellite particles and improve the powder flowability during processing. The spherical-shaped powders showed higher packing density rather than irregular powder, leading to achieve products with uniform powder bed density and good quality of final product. The obtained results indicating the produced Al0.5CoCrFeMnNi HEA powders having higher quality in terms of shape, smooth surfaces, fewer irregularities and satellites, which are beneficial for the AM applications.

PHOTO (COLOR): Figure 1. Schematic diagram of rapid solidified gas atomisation system. The obtained as-atomised Al0.5CoCrFeMnNi HEA powders were classified according to particle size and corresponding powder morphology examined by SEM, as shown in Figure (b) < 20 μm, (c) 20–63 μm, (d) 63–106 μm and (e) 106–150 μm.

In addition, particle size distribution shows significant influence on powder flowability, compressibility and packing density during laser melt deposition. As shown in Figure 2, the particle size distribution of as-atomised and different size range of Al0.5CoCrFeMnNi HEA powders. For instance, the median diameter of D50 is about 16, 49, 100 and 143 μm was observed for the particle size ranges of <20, 20–63, 63–106 and 106–150 μm, respectively. Furthermore, the measurement of particle size distribution results revealed that all the powders showed narrow size distribution, indicating the particular range of powders are homogeneous.

PHOTO (COLOR): Figure 2. Particle size distribution of as-atomised Al0.5CoCrFeMnNi HEA powders according to particle size.

3.2. Cross-sectional microstructure of as-atomised Al0.5CoCrFeMnNi HEA powders

Figure 3(a–d) shows the cross-sectional microstructure of as-atomised Al0.5CoCrFeMnNi HEA powders according to particle size distribution. It can be seen that all the powders have dendritic microstructure, while the dendrite arm spacing has been increased with increasing particle size. In particular, below 20 μm powder showed relatively small dendritic cells, which is about 1–2 μm, whereas increasing particle size increased the dendritic cell. The microstructural variation with particle size is purely related to the cooling behaviour of HEAs during atomisation. In order to understand the microstructural variation with particle size, we evaluated the dendrite arm spacing and cooling rates according to particle size distribution.

Graph: Figure 3. SEM cross-sectional micrographs shows the formation of dendrite structure initiating from surface nucleation site in gas-atomised Al0.5CoCrFeMnNi HEAs according to particle size distribution (a) < 20 μm, (b) 20–63 μm, (c) 63–106 μm and (d) 106–150 μm.

3.3. Determination of secondary dendrite arm spacing

The relationship between secondary dendrite arm spacing and cooling rate can be shown by the following equation.

Graph

${\lambda }_2=37.7(GV{)}^{-0.422}$ (1)

λ 2 is the secondary dendrite arm spacing, G is the temperature gradient and V is the growth rate; GV is the cooling rate. As mentioned above, the secondary dendrite arm spacing mainly dependent on cooling rate during processing. In order to estimate the dendrite arm spacing according to particle size distribution, we examined the SEM analysis and selected number of particles and averaged them to maintain accuracy. The relationship between measured dendritic arm spacing and particle size can be shown in Figure 4(a). The calculated results revealed that secondary dendrite arm spacing substantially decreased with decreasing particle size distribution. The obtained dendrite arm spacing results were used to approximate the cooling rate of HEAs according to particle size, indicates that increase of cooling rate significantly decreased the particle size and the results shown in Figure 4(a). For instance, when the powder size of below 20 μm, higher cooling rate of 23523.47°C s−1 was obtained and for 106–150 μm range of powders showed lower cooling rate of 332.27°C s−1. However, the cooling rate of atomised powders lies within the range of 102 to 105°C s−1. Owing to the increase of cooling rate during atomisation shows fine-grained microstructure than that of conventional methods [[27]]. On the other hand, the mechanical properties of rapid solidified alloys described by calculating the secondary dendrite arm spacing (SADS), which is the most important factor that influences the mechanical properties of materials [[28]]. For instance, a decrease in SADS value significantly increased the strength and elongation and SADS can be controlled by changing the cooling rate. Figure 4(b) shows the hardness measurements of Al0.5CoCrFeMnNi HEA powders according to particle size distribution. The results revealed the hardness increased with decreasing particle size. The maximum hardness of 227 Hv was achieved for the particle size of 30 µm and then gradually decreased to 159 Hv, whereas increasing particle size to 150 µm. However, the obtained hardness measurements showed error range about 5% regardless of particle size distribution. The results demonstrated the reduction of dendrite arm spacing improve the Vickers hardness of HEAs produced by atomisation process.

PHOTO (COLOR): Figure 4 (a). Measurement of secondary dendrite arm spacing as a function of particle size and estimated the cooling rates of for the as-atomised Al0.5CoCrFeMnNi high entropy alloy powders according to particle size and (b) variation of microhardness as function of particle size.

3.4. X-ray diffraction patterns

Figure 5 shows the XRD patterns of as-atomised Al0.5CoCrFeMnNi HEA powders according to particle size distribution. The resultant diffraction peaks corresponding to two crystalline peaks, including a major FCC phase and minor BCC phase was observed. The existence of BCC phase is due to the addition of 0.5 at.-% of Al into CoCrFeMnNi HEA can be able to form a minor BCC phase. While the diffraction peaks of HEAs become sharper and full width at half maximum slightly increased, indicating the reduction of crystallite size. Furthermore, there are no impurity peaks were identified in XRD patterns of all the powders.

PHOTO (COLOR): Figure 5. XRD patterns of gas-atomised Al0.5CoCrFeMnNi HEA powders according to particle size distribution.

Figure 6 shows the presence of oxygen concentration in as-atomised powders according to particle size distribution was examined by ONH analyser. The obtained results indicating the below 20 μm powders showed 315 ppm and significantly decreased to 100 ppm for the particle size of 106–150 μm. The increased oxygen concentration for the small size powders mainly due to larger surface area when compared to the large powders. In addition, fine powders are less than 1 μm and fewer satellite-type powders also contributed to the increase in oxygen content. However, the obtained results indicating the presence of low-oxygen quantities leads to reduce the metallurgical defects during AM process.

Graph: Figure 6. Oxygen content of as-atomised Al0.5CoCrFeMnNi HEA powders according to particle size distribution.

3.5. Chemical composition of as-atomised HEAs examined by SEM-EDS, AES and XPS analysis

Figure 7 shows the SEM-EDS colour mapping of as-atomised HEA powders with different size distribution. The results revealed that all the nominal components of Al, Co, Cr, Fe, Mn and Ni were uniformly distributed in the matrix regardless of particle size, indicating GA enable to produce HEAs with homogeneous chemical composition. Furthermore, the clear compositional information can be examined using XPS analysis. Figure 8 shows the auger electron spectroscopy results for the Al0.5CoCrFeMnNi HEA powders with different particle size. The results indicating the distribution of chemical elements from the surface to certain depth of powders. At the beginning, only oxygen was detected on the powder surface after sputtering at 10 min, the nominal elements of Co, Cr, Fe, Mn and Ni were stabilised. With increasing depth from powder surface, the oxygen content is decreased for all the powder sizes and except Mn remaining other elemental concentration is well-matched with the standard composition. Furthermore, the Al contents could not identify by AES analysis due to the formation of AlNi secondary phase, which can be confirmed by XRD analysis (shown in Figure 5).

PHOTO (COLOR): Figure 7. The cross-sectional SEM images shows the elemental distribution in as-atomised Al0.5CoCrFeMnNi HEA powders with different particle size distribution (a) < 20 μm and (b) 106–150 μm.

PHOTO (COLOR): Figure 8. Auger electron spectroscopy results of as-atomised Al0.5CoCrFeMnNi HEA powder with different particle sizes (a) < 20 μm, (b) 20–63 and (c) 63–106 μm.

The high-resolution XPS analysis was carried out to examine the surface compositional behaviour of as-atomised Al0.5CoCrFeMnNi HEA powders. The high-resolution Al 2p, Co 2p, Cr 2p, Fe 2p, Mn 2p and Ni 2p spectra for each element of the Al0.5CoCrFeMnNi alloy powder surfaces are shown in Figure 9(a–f), respectively. The XPS results revealed the formation of metal oxides such as AlO, CoO and NiO satellite peaks were observed on the surface of as-atomised powders. The obtained XPS peaks are identified using the NIST photoelectron spectroscopy database (http://srdata.nist.gov/xps/). The presence of oxides in the feedstock powders nucleate the metallurgical defects such as pores, cracks resulting in poor powder packing density. Meanwhile, higher oxides alter the surface tension of the molten pool, as a result melt flowability decreased during printing, leading to deteriorates the mechanical properties of final products [[29]]. Though, the produced HEA powders by atomisation showed low oxygen contents, which can be able to reduce the defects during the AM process.

Graph: Figure 9. The XPS high-resolution spectra of as-atomised Al0.5CoCrFeMnNi HEA powder, (a) Al 2p, (b) Co 2p, (c) Cr 2p, (d) Fe 2 p, (e) Mn 2p and (f) Ni 2p.

4. Conclusion

In this report, Al0.5CoCrFeMnNi HEA powders fabricated by gas atomisation process and investigated the powder characteristics according to particle size distribution. The obtained alloy powders showed spherical in shape with smooth surfaces and fewer satellite powders. The measured powder characteristics such as tap density (4.7 g cm−3), compressibility (8.39%), AOR (23.57°) and flat plate angle (25.33°), indicate that it can be able to improve powder flowability during printing. The SEM–EDS results indicating the alloy powders exhibiting homogeneous chemical composition regardless of particle size. The secondary dendrite arm spacing decreased with decreasing particle size, while increasing the cooling rate, as a result increased the hardness of HEA powders. This study demonstrated that produced HEA powders by gas atomisation possess excellent morphology, flowability and homogeneous chemical composition, which enables a potential evidence for utilising in AM technology.

Acknowledgements

This work was supported by the Basic Research Laboratory Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (MOE) (No: 2019R1A4A1026125). Part of this research work was supported by Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by Ministry of Science and ICT (No: 2018M3D1A1025730).

Disclosure statement

No potential conflict of interest was reported by the author(s).

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By Yeeun Lee; Cheenepalli Nagarjuna; Jun-Woo Song; Kwang Yong Jeong; Gian Song; Jinkyu Lee; Jong-Hyeon Lee and Soon-Jik Hong

Reported by Author; Author; Author; Author; Author; Author; Author; Author

Yeeun Lee completed her bachelor degree in Advanced Materials Engineering, Kongju National University, Republic of Korea. Currently, she is studying Master's degree in Advanced Materials Engineering, Kongju National University, Republic of Korea. She has hugely interest in powder metallurgy, high entropy alloys and additive manufacturing process.

Cheenepalli Nagarjuna completed his Master's degree in Department of Physics at Sri Venkateswara University, Tirupati, India. Currently, he is studying PhD in Materials Science and Engineering, Kongju National University, South Korea. His major research interests in Powder Metallurgy include the fabrication of high entropy alloy powders by gas atomisation and mechanical alloying process, followed by sintering and investigate their microstructure and mechanical properties for structural applications.

Jun-Woo Song completed his PhD in Materials Science and Engineering, Kongju National University, South Korea. Currently, he is working as Postdoctoral researcher in Materials Science and Engineering, Kongju National University, South Korea. His major interests in powder metallurgy include the fabrication of various alloy powders by gas atomisation, high energy ball milling and consolidated by spark plasma sintering processes.

Kwang Yong Jeong completed his Master's degree in Materials Science and Engineering, Kongju National University, South Korea. Currently, he is doing PhD in Materials Science and Engineering, Kongju National University, South Korea. His Major research interest in powder metallurgy, such as fabrication of metallic powders by gas or water atomisation and high energy ball milling process followed by various sintering techniques.

Professor Gian Song obtained his Ph.D in materials Science and Engineering at University of Tennessee, Knoxville, followed by worked as Post-Doctoral Research Associate at Oak-ridge national lab. Then, he finally joined the division of advanced materials engineering at Kongju national university. He has been focusing on the development of novel metallic materials with excellent mechanical properties and has published about 64 scientific papers in peer-reviewed international journals.

Professor Jinkyu Lee is working in Advanced Materials Engineering at Kongju National University. He received his PhD degree from the Department of Metallurgical Engineering, Yonsei University, Seoul, Korea, in 2002. He has pioneered research in bulk metallic glasses, nano structured materials and powder metallurgy, and has published more than 100 research papers in peer-reviewed international journals.

Professor Jong-Hyeon Lee obtained his Ph.D in Metallurgical Engineering at Chungnam National University, Korea, followed by he worked as Senior Researcher at Korea Atomic Energy Research Institute. Currently, he is working in Department of Materials Science and Engineering at Chungnam National University, where he has been focusing on the development of novel electrochemical oxide reduction processes and published more than 180 scientific papers in peer-reviewed international journals.

Professor Soon-Jik Hong is completed his PhD in Materials Science and Engineering in Chungnam National University, South Korea, followed by he worked as Post-Doctoral researcher in University of Central Florida, USA. Currently, he is working as professor in Materials Science and Engineering, Kongju National University, South Korea. His major research interests in Powder Metallurgy include the fabrication of various metal alloy powders by gas or water atomisation and mechanical alloying processes, followed by various sintering techniques, investigation of microstructure and mechanical properties. He has published more than 250 scientific papers in peer-reviewed international journals.