An Insight into the Development of Light Weight High Entropy Alloys

High Entropy Alloys (HEAs) have been emerged as conceptually innovative alloys that defied the conventional alloying concepts. The concept of HEAs is based on designing the alloys with multiple principal elements ranging from 5 to 35 atomic percent with a target to form single phase solid solution arising from high entropy of the system. The initial development of HEAs was based on the high density principal elements. Recently, low density elements such as Mg, Al, Li, Si and Ca were used to develop light weight high entropy alloy (LWHEA) systems to address the issues related to greenhouse gas emissions. LWHEAs with equiatomic and non-equiatomic compositions were developed successfully. However, single phase solid solution was seldom formed in LWHEAs. In view of current development on LWHEAs, chemical compositions, microstructure and mechanical properties are reviewed and presented in this study.

Initial work on development of HEAs used principal elements with relatively high density such as Hf, Ti, V, Ta, Nb, Zr etc [3][4][5][6]. Promising properties were reported including superior hardness, wear, oxidation and corrosion resistance. The density values of most of the HEAs initially developed ranged from ~3g/cc [7] to 11.49g/cc [3].
In order to preserve and enhance the quality of environment and human health, researchers are currently focussing on sustainable technologies that do not pollute land, air and water thus leading to an ecosystem that is preferably superior to what we are living in. One of the major issue faced by humanity is global warming. Global warming arises principally from the burning of fossil fuels that releases greenhouse gases such as carbon dioxide and transportation sector is one of the main culprit. Global warming is near its tipping point and the clmate change will be irreversible if the temperature rises to 2 °C beyond that of pre-industrial time [8]. One of the methods to arrest and reduce the emissions of green house gases is by reducing the weight of vehicles/automobiles. For example, every 10kg weight reduction of vehicle translates into 1.25g/km carbon dioxide reduction [9]. Among the different metallic materialsaluminum with a density of 2.7g/cc dominates currently as light weight material in automobile, sports, aerospace,

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Res Dev Material Sci electronics and space sectors. Magnesium with a density of 1.74g/ cc is the relatively new entry in recent years triggered by the discovery of huge reserves both in land and water bodies [10]. With a potential of ~33% additional weight saving over aluminum, magnesium is currently the cynosure for materials developers. Besides, magnesium is a nutritional element required by human body and plants for healthy growth unlike aluminum which is widely established as neurotoxic element. Accordingly, recycling of magnesium is not an issue and its entry into food chain will only be beneficial. Thus magnesium has dual advantages of being lightweight and a nutritional element.
In view of the key advantages of magnesium, researchers are also attempting to develop HEAs by integrating light weight elements for providing alternatives to materials selectors for weight critical applications. Some of these light weight elements that can lead to low density HEAs are Mg, Li, Al, Si, Ca and Sc. They are often present in dominance in light weight HEAs (LWHEAs) which can be defined as one with overall density<3g/cc and preferably below the density of aluminum (2.7g/cc). A list of some LWHEAs reported in public domain are listed in Table 1 along with their processsing routes.   Table 1 indicates that both liquid and solid pahse routes are successfully employed to synthesize LWHEAs. To note is that many of these LWHEAs are not equiatomic and often shows the presence of secondary phases [7,11] (Figure 2).Comparison with a commercial AZ31 Mg alloy shows that LWHEAs hardness values are almost 3-6 times higher [13].
Hardness and compressive properties are commonly reported for HEAs in general. In general, HEAs including LWHEAs reveal high hardness values attributed primarily to solid solution and precipitation effects. Table 2 shows hardness values of LWHEAs reported in literature.  The compressive properties of LWHEAs are shown in Table  3. The results revealed appreciable levels of yield and peak stress and failure strains limited to <5%. The relatively high yield stresses of LWHEAs compared to commercial AZ31 Mg alloy indicate difficulty in initiation of motion of dislocations due to inherent microstructural differences. The peak stress, however, was similar with lower strain values suggesting an early crack nucleation due the inability of dislocations to move beyond certain extent and accumulation of stress levels.
LWHEAs research work so far does not report tensile properties and the lowest density is 2.20. While the density levels less than that of currently used aluminum alloys alloys are realized they are still far away from that of magnesium alloys. There are many challenges faced in developing LWHEAs as most of the equiatomic compositions tried are very brittle or reactive with environment due to the selected number of elements that can be mixed maintaining the low density. It is hoped that dedicated research work will be conducted by research community to develop new LWHEAs that can be found suitable for wide spectrum of weight critical applications. The challenge stays.

Conclusion
Current research on LWHEAs development indicated that designing HEAs is still challenging as multiple phases appear in most of the alloy systems. Based on the reported data, LWHEAs revealed high hardness which is almost 3-6 times higher than that of conventional AZ31 alloy. Arising from the formation of multiple phases in LWHEAs, the compressive yield strength was relatively higher as compared to AZ31 alloy. However, the fracture strain was <5% showing limited plasticity. To date, the most crucial challenge in developing LWHEAs sets on the inevitable formation of secondary phases. Further research efforts are required to contain the types and amount of secondary phases through smart compositional control to realize a better combination of hardness, strength and plasticity.