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The Origins of High-Entropy Alloy Contamination Induced by Mechanical Alloying and Sintering

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The Origins of High-Entropy Alloy Contamination Induced by Mechanical Alloying and Sintering metals Article The Origins of High-Entropy Alloy Contamination Induced by Mechanical Alloying and Sintering Igor Moravcik 1,* , Antonin Kubicek 1, Larissa Moravcikova-Gouvea 1 , Ondrej Adam 1 , Vaclav Kana 2, Vaclav Pouchly 3, Antonin Zadera 2 and Ivo Dlouhy 1 1 Institute of Materials Science and Engineering, Brno University of Technology, Technicka 2896/2, 61669 Brno, Czech Republic; [email protected] (A.K.); [email protected] (L.M.-G.); [email protected] (O.A.); [email protected] (I.D.) 2 Institute of Manufacturing Technology, Brno University of Technology, Technicka 2896/2, 61669 Brno, Czech Republic; [email protected] (V.K.); [email protected] (A.Z.) 3 CEITEC BUT, Brno University of Technology, Purkynova 123, 612 00 Brno, Czech Republic; [email protected] * Correspondence: [email protected]; Tel.: +42-911-566-030 Received: 6 August 2020; Accepted: 28 August 2020; Published: 3 September 2020 Abstract: One of the prevailing problems for materials produced by powder metallurgy is contamination from various sources. This work deals with the influence of process parameters and presence of process control agents (PCA) on the contamination level of materials produced by means of mechanical alloying (MA) technology, densified with spark plasma sintering (SPS). The equiatomic CoCrFeNi high-entropy alloy (HEA) was manufactured by the said methodology. For clear comparison, the 316L austenitic steel powder was milled and densified with identical conditions as a reference material. Both materials were milled in argon and nitrogen atmospheres for various times from 5 to 30 h. Chemical analysis of contamination by carbon, oxygen, and nitrogen within the powder and bulk materials was carried out using combustion analyzers. The microstructural analysis of powders and bulk samples was carried out using scanning electron microscopy (SEM) with focus on contaminant phases. The results show that carbon contamination increases with milling time. It is caused by wear of milling vial and balls made from high-carbon steels. Increase of carbon content within consolidation using SPS was also observed. The oxygen contamination also increases with milling time. It is more pronounced in the CoCrFeNi alloy due to higher oxidation of powder surfaces prior to milling. Milling of powders using nitrogen atmosphere also causes an increase of nitrogen content in both HEA and AISI 316L. The use of PCA (ethanol) during milling even for a short time (30 min) causes significant increase of carbon and oxygen contamination. The ways to decrease contamination are discussed in the paper. Keywords: contamination; mechanical alloying; spark plasma sintering; infrared detection; high-entropy alloy; austenitic stainless steel 1. Introduction Powder metallurgy (PM), as a solid-state manufacturing technique, has advantages over traditional alloying techniques—such as casting—since it permits the controlling of the microstructure during processing and mixing of insoluble phases. This leads to the production of tailored components possessing refined microstructures and enhanced mechanical properties. As such, PM is widely used for the preparation of advanced materials such as oxide dispersion strengthened (ODS) materials, and more recently, high-entropy alloys (HEAs) [1,2]. Nevertheless, one of the biggest issues concerning materials prepared by powder metallurgy is contamination, due to a significantly larger surface area of the powders compared to the surface area of Metals 2020, 10, 1186; doi:10.3390/met10091186 www.mdpi.com/journal/metals Metals 2020, 10, 1186 2 of 15 the bulk material with similar weight. Rarely, the contamination of powder materials and the resulting formation of oxides or carbides is addressed, such as the occurrence of Cr7C3, Cr2O3 and MnCr2O4 in mechanically alloyed CoCrFeNi alloy [3], where their particles have been shown to cause strengthening of the alloy, but decrease in ductility. Moreover, the contamination may be used as an efficient feedstock alternative on the production of ODS alloys, since PM procedures enable homogeneous distribution of oxides and/or carbides into a metal matrix. As such, the nanograined modified oxides in ODS materials contribute to the overall strengthening of the material while maintaining satisfactory ductility [4]. Consequently, the presence of oxides and carbides formed in the resulting microstructure can have a significant influence on the properties of the final materials [5]. Furthermore, these particles lower the amount of important alloying elements such as Cr in a surrounding solid solution and thus may lead to loss of corrosion resistance [6]. That said, it is important to emphasize that it is very difficult to control the contamination level of the powders during mechanical alloying, as it is an inherent characteristic of the PM process. Thus, its complete avoidance becomes challenging. The contamination, however, can be greatly limited by controlling and optimizing the parameters of its possible sources. It has been previously reported that the main sources of contamination of PM are storage and transfer of powders, milling equipment, milling atmosphere, and the use of process controlling agents (PCA) [7]. Powder metallurgy has been frequently studied as a suitable technique to produce high-entropy alloys [4,8–12]—a relatively new class of multicomponent alloys with near equiatomic compositions [13,14]. Some of these HEAs have exhibited outstanding mechanical properties in a range of temperatures from cryogenic [15] to very high working temperatures [16], such as mutually high strength and ductility [17–19] and remarkable fracture resistance [20]. In this light, the present work aims on elucidating the evolution of the contamination level on PM-produced high-entropy alloy powders in comparison with traditional 316L steel powders. More specifically, the CoCrFeNi composition was chosen due to its remarkable mechanical properties when prepared by PM [3,21]. A systematic characterization of powders at different times of milling and atmospheres was performed, especially regarding the increase of the amounts of oxygen, nitrogen, and carbon in each alloy. 2. Materials and Methods Equiatomic, high-entropy alloy powders with chemical composition of CoCrFeNi (ratio of 1:1:1:1) were prepared by mechanical alloying (MA) followed by spark plasma sintering (SPS). The CoCrFeNi alloy powders were prepared from pure elemental powders, whereas the information on the powders’ characteristics are given in Tables1 and2. For reference, the 316 L alloy atomized powder was also milled with the same parameters. The powders were milled using a 10:1 ball-to-powder weight ratio, with 10 mm diameter balls. Each starting powder mixture weighed 30 g. Milling balls and powder mixtures were introduced into a high-carbon steel hardened steel milling container. The milling container was then sealed and flushed with Ar or N2 gas (purity 99.998%) to limit powder oxidation during the milling process. Dry milling was performed in Pulverisette 6 ball mill (Fritsch GmbH, Germany) with a milling rate (ratational) of 300 rounds per minute. The total milling time was selected from 5–50 h for different millings. In the end of the dry-milling process, the free powders were removed from the milling bowl and stored under protective atmosphere. The scheme of the parameters pertaining to powder material preparation is presented in Figure1. Metals 2020, 10, 1186 3 of 15 Metals 2020, 10, x FOR PEER REVIEW 3 of 16 Metals 2020, 10, x FOR PEER REVIEW 3 of 16 FigureFigure 1. Schematic 1. Schematic representation representation of variousof various powder powder materials materials prepared prepared using using a seta set of variableof variable milling parametersmilling parameters (time, atmosphere). (time, atmosphere). The 316 LThe alloy 316 atomizedL alloy atomized powder powder and CoCrFeNi and CoCrFeNi powder powder prepared Figure 1. Schematic representation of various powder materials prepared using a set of variable prepared from pure elements was used for milling experiments. Powder samples designated by the from puremilling elements parameters was (time, used foratmosphere). milling experiments. The 316 L alloy Powder atomized samples powder designated and CoCrFeNi by the spark powder plasma spark plasma sintering (SPS) arrow were subsequently densified. sinteringprepared (SPS) from arrow pure were elements subsequently was used densified.for milling experiments. Powder samples designated by the Additionally,spark plasma sintering empty bowls (SPS) arrowstill containing were subsequently part of thedensified. powders in the form of an adhesive layer Additionally, empty bowls still containing part of the powders in the form of an adhesive layer stuck to its walls (Figure 2) were filled with 200 mL of ethanol and wet-milled for 30 min. This was stuck to itsAdditionally, walls (Figure empty2) were bowls filled still with containing 200 mL part of ethanol of the powders and wet-milled in the form for of 30 an min. adhesive This waslayer done done to fully remove the powders stuck to the milling equipment. The extracted powders were then stuck to its walls (Figure 2) were filled with 200 mL of ethanol and wet-milled for 30 min. This was to fullyfiltered remove and the dried. powders The wet-milled stuck to the powders milling equipment.were measured The extractedseparately powdersonly to wereevaluate then the filtered done to fully remove the powders stuck to the milling equipment. The extracted powders were then and dried.concentration The wet-milled
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