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Sustainable low-cost method for production of High entropy alloys from alloy scraps

In this communication,we propose a sustainable way to produce high entropy alloys from alloy scraps called Alloy mixing.We successfully demonstrate this method using a near equimolar CrCuFeMnNi HEA. Alloy scraps (304L stainless steel, Nichrome 80 and electrical wire grade Copper) obtained from various sources were melted together using vacuum arc melting along with minor additions of Mn and Cr to achieve the equiatomic composition.The alloy was characterized using X-ray Diffraction (XRD) and Scanning Electron Microscopy, which confirmed that the alloy produced through Alloy mixing exhibits a microstructure similar to that of the alloy with the same composition produced through conventional melting of pure elements


Introduction
The data published by the US Environmental Protection Agency [1] indicated that Municipal Solid Waste (MSW) in the US alone included 34.69 million tons of metallic scraps in the year 2018, of which only 34.9% was recycled. Recycling has been shown to reduce cost [2] and energy consumption [3] significantly as the primary metal production process is both cost and energy-intensive in nature. There have been several efforts in recent years to recycle 3 metallic materials, especially in the steel industry. However, large quantities of metallic scraps remain as waste. Hence, we need more avenues for recycling metallic alloy scraps.
High entropy alloys (HEAs) are a new class of alloys with five or more elements in near equal proportions. [4] [5] This new class of materials has attracted the alloying community due to its marked deviation from established norms of alloy design based on a single principal element. Over the years, researchers have found that HEAs can be tailored to possess several unique properties such as excellent fracture toughness [6], corrosion resistance [7] and catalytic properties [8]. Despite these developments on the alloy design front, the real-life application of these alloys is limited by their high cost as they are typically produced by vacuum melting of pure elements.
In this work, we propose a strategy named "Alloy mixing" to produce HEAs which can serve as a solution to the above mentioned problems. We melted alloy scraps together instead of melting pure metals to produce HEA, thereby reducing the cost of production significantly while enabling the recycling of metallic scraps. We demonstrate the feasibility of this technique using a CrCuFeMnNi HEA; the composition was chosen due to the easy availability of raw materials, and it has already been produced through the conventional process with its microstructure being well-characterized.

Materials and Methods
The precursor materials used in this study are 304L stainless steel (from "broken" tensile bars from a laboratory), Nichrome 80 (from "used" furnace coil), electrical grade Cu (from Cu wires). In addition to this, small quantities of 99.9% Mn and Cr were used to achieve the equiatomic composition. The alloys sourced from scraps were mechanically abraded on a belt grinder to remove any surface contamination.
An ingot weighing 30g was melted under a high vacuum in an Ar atmosphere on a vacuum arc furnace with a Tungsten electrode. The sample was re-melted atleast five times to ensure chemical homogeneity.
XRD analysis using a Cu-Kα source was performed for the as-cast sample for phase identification. The sample was metallographically prepared, and the microstructure was characterized using a SEM with a Field emission gun (FEG). In order to study the partitioning of elements between different phases, Energy dispersive spectroscopy (EDS) analysis was performed during the characterization under the SEM. 4 To evaluate the effect of impurities from the scrap on the yield strength of the alloy, the yield strength model included in the ThermoCalc (Version 2021b) Property calculation module.
We compared the yield strength of the alloy without any impurities (conventionally fabricated alloy) with the yield strength of alloy with impurities at 25 o C. In this study, we considered Si and C as the main impurities present in the alloy fabricated by alloy mixing.
The nominal composition of the alloy with impurities is given as the mean composition in Table 1. We also created 1000 variations in impurity concentration about the mean alloy composition using the uncertainty quantification function available on ThermoCalc. The input parameters used to create the variation is listed in Table 1. The main purpose of this analysis was to ascertain the effect of variability in scrap composition on the yield strength of the alloy. During this study, we considered face-centered cubic phase as the matrix phase, as it is the phase with the highest volume fraction as known from the data on alloy with the same composition fabricated using conventional route [9]. We used a constant grain size of 200 μm for both pure and impure variants as they are fabricated under the same solidification conditions. (i.e. vacuum arc melting) We also performed a cost analysis to compare the prices of the pure elements and the scrap to demonstrate the economic impact of this new technique.

Results and Discussion
The XRD analysis shows that the as-cast alloy has three constituent phases (2 face-centered cubic phases and a body-centered cubic phase) (Figure 1). When we compare this pattern 5 with the one reported by C. Li et al. [9] for the alloy with the same composition fabricated by the conventional method, the peaks for different phases matches well, clearly indicating that our method preserves the microstructure of the alloy. This is further confirmed by the SEM micrograph ( Figure 2) of the alloy fabricated by alloy mixing, which captures the features such as flower-pot morphology of the 2 nd phase and inter-phase boundary precipitation of the 3 rd phase, which are well-documented for the alloy fabricated through the conventional route with the same composition.  The yield strength predicted by the ThermoCalc software for the alloy without impurities (the pure alloy) is found to be 135.55 MPa, while the alloy corresponding to the mean impurity concentration is 190.21 MPa which is 50% higher than the pure alloy. The difference can be fully attributed to the difference in solid solution strengthening between the two variants of the alloy, as we don't have any precipitation in both cases, and the grain size is assumed to be the same for both alloys. We should be careful in interpreting these results because we would have a difference in work hardening between the pure and impure variants, and we should also consider the effect of the impurity elements (i.e. Si and C) on the ductility of the alloys.
However, we have results from the literature showing that Si improves work hardening rate by decreasing the Stacking fault energy (SFE) [11], and it also leads to uniform elongation and slightly enhanced ductility by the formation of refined dislocation cell-structure during 7 plastic deformation [12] [13]. The effect of carbon would be insignificant as it is present in very small amounts. Therefore, the impure alloys are expected to exhibit increased ultimate tensile strength along with good tensile ductility.
The results from the uncertainty analysis are presented below as a histogram (Figure 4a). It is interesting to note that the yield strength shows a large variation for small variation in the impurity content, which demonstrates the importance of controlling compositional variability in the scrap. It should be noted that some of the impure variants have a lower yield strength compared to that of the pure alloy, which shows that we should be careful in controlling the scrap composition. We plotted the yield strength as a function of Si content (Figure 4b), which shows that the yield strength varies linearly with Si content. This is interesting because Si, which is present as an impurity, helps in strengthening, indicating that impurities are not   Table 2 lists the market price for the alloy scraps and the pure elements. [14][15] It is evident that alloy scraps are 100 times cheaper than pure elements. However, one must be aware of the fact that there is a cost associated with pre-treating the scraps. Hence, the actual price difference will be smaller than the apparent difference in the price listed. Nevertheless, the economic benefit that comes with "Alloy mixing" is significant in the context of HEA commercialization.

Conclusions
• Alloy mixing using scraps preserves the microstructure of the alloy as confirmed by characterization using XRD and SEM.
• The yield strength of the impure alloy was 50% higher than the conventionally manufactured alloy, indicating possible additional impact on the solid solution strengthening from the impurity elements, especially Si.
• The uncertainty quantification with respect to variation in impurity content indicated that the yield strength showed a large variation (nearly 100MPa) as a function of impurity concentration. It also revealed that if we don't control the scrap composition properly, we may end up degrading the properties.
• Our cost analysis revealed that alloy mixing could lead to a significant reduction in fabrication costs.
Hence, Alloy mixing serves as a promising sustainable and cost-effective method for the fabrication of HEAs, which would enable the commercialization of HEAs while serving 9 as a feasible avenue for alloy scrap recycling. Future research must be focused on confirming the computational results with experiments and designing methods to effectively control the incoming scrap composition and establish robust scrap pretreatment cycles.

Conflict of Interest:
The authors declare that they have no conflict of interest.