**6. Conclusions**

High-entropy alloys are well suited for production in vacuum arc remelting furnaces due to low material losses and to obtaining high purity by working in vacuum and argon. Moreover, they can also be produced in induction furnaces, for superior purity, using the VIM-VAR duplex process.

The hardness decrease of the AlCrFeCoNi class high-entropy alloys is proportional to the aluminum content decrease, from 500 HV1 for HEA 1 (AlCrFeCoNi) at 400 HV1 for HEA 5 (Al0.8CrFeCoNi) and 224 HV1 for HEA 6 (Al0.6CrFeCoNi).

**153**

*Characterization and Testing of High-Entropy Alloys from AlCrFeCoNi System for Military…*

The hardness decrease can be explained by the reduction in the quantity and number of hard precipitates (Fe-Al compounds) in the metallic matrix.

The hardness further decreases by increasing the nickel content, which allows the formation of face-centered cubic (FCC) solid solutions of low hardness and high tenacity. In this type of solid solution the chemical elements have a good solubility, with a low tendency of separation of hard and brittle compounds.

The fracture energy values are in the range of 62–67 J for all five types of alloys, as the hardness oscillates in the range 200–500 HV0.1. As a result, the hardening effect is not manifested by decreasing the metal matrix toughness in the case of

The microstructure of high-entropy alloys is virtually "frozen" at the melt, with the solution retaining a conglomerate of chemical elements which are oftentimes very different (iron-related elements such as Cr, Ni, and Co, which together form solid solutions along with aluminum, a transition metal whose solubility greatly differs from that of Fe, Cr, Ni, and Co). The cooling condition creates entropy with very high values and explains the obtaining of completely different characteristics

Depending on the share of the alloying elements, one or two types of solid solutions are formed, partially embedding the other alloying elements. The dendritic microstructure predominates, separating into acicular compounds or globular precipitates, depending on the chemical composition and the cooling rate.

The tests on the behavior of high-entropy alloys at strong impacts were conducted under identical conditions. In the case of ballistic packages resistant at high-velocity penetration impacts, the best option is the HEA-steel system. Highentropy alloys, according to their composition, can be used in various sectors: medical engineering, earthwork equipment, and ballistic packages for individual

This paper was supported by the Romanian National Authority for Scientific Research CNDI-UEFISCDI, project number PN-III-P1-1.2-PCCDI-2017-0875 –

The authors want to thank the team of researchers coordinated by Professor PhD Tudor Cherecheş for collaborating in the dynamic tests and the processing of

*DOI: http://dx.doi.org/10.5772/intechopen.88622*

high-entropy alloys in the analyzed alloy class.

compared to the alloy cooled at usual rates.

and collective protection.

**Acknowledgements**

**Thanks**

experimental data.

PCCDI 20-2018 (HEAPROTECT).

#### *Characterization and Testing of High-Entropy Alloys from AlCrFeCoNi System for Military… DOI: http://dx.doi.org/10.5772/intechopen.88622*

The hardness decrease can be explained by the reduction in the quantity and number of hard precipitates (Fe-Al compounds) in the metallic matrix.

The hardness further decreases by increasing the nickel content, which allows the formation of face-centered cubic (FCC) solid solutions of low hardness and high tenacity. In this type of solid solution the chemical elements have a good solubility, with a low tendency of separation of hard and brittle compounds.

The fracture energy values are in the range of 62–67 J for all five types of alloys, as the hardness oscillates in the range 200–500 HV0.1. As a result, the hardening effect is not manifested by decreasing the metal matrix toughness in the case of high-entropy alloys in the analyzed alloy class.

The microstructure of high-entropy alloys is virtually "frozen" at the melt, with the solution retaining a conglomerate of chemical elements which are oftentimes very different (iron-related elements such as Cr, Ni, and Co, which together form solid solutions along with aluminum, a transition metal whose solubility greatly differs from that of Fe, Cr, Ni, and Co). The cooling condition creates entropy with very high values and explains the obtaining of completely different characteristics compared to the alloy cooled at usual rates.

Depending on the share of the alloying elements, one or two types of solid solutions are formed, partially embedding the other alloying elements. The dendritic microstructure predominates, separating into acicular compounds or globular precipitates, depending on the chemical composition and the cooling rate.

The tests on the behavior of high-entropy alloys at strong impacts were conducted under identical conditions. In the case of ballistic packages resistant at high-velocity penetration impacts, the best option is the HEA-steel system. Highentropy alloys, according to their composition, can be used in various sectors: medical engineering, earthwork equipment, and ballistic packages for individual and collective protection.
