**1. Introduction**

High-entropy alloys (HEAs) are known for their special mechanical properties: high tensile strength resistance even at high temperature, high hardness, toughness exceeding that of most pure metals and alloys, comparable strength to that of structural ceramics and some metallic glasses, exceptional ductility and fracture toughness at cryogenic temperatures, and corrosion resistance. These specific properties were mainly attributed to complex concentrated solid solutions formed by the suppression of fragile intermetallic compounds as a result of high mixing entropy and enthalpy values [1]. HEAs generally tend to form single-phase solid solutions in the case of low mixing enthalpy and atomic size difference. Generally speaking, the formation of a single-phase solid solution corresponds to the mixing enthalpy values (DHmix) situated in domain of −15 kJ/mol < DHmix <5 kJ/ml and 0 < δ < 5 [2].

As a result of these special features, HEA is currently an alternative to use as material for a number of special areas, such as structural applications; aerospace engineering and civil transportations; superconducting electromagnets such as magnetic resonance imaging, scanners, nuclear magnetic resonance machines, and particle accelerators; high-temperature applications such as gas turbines, rocket nozzles, and nuclear construction; cryogenic applications such as rocket casings, pipework, and liquid O2 or N2 equipment; refractory elements such as Nb, Mo, and Ta that can maintain their high strength even above 1200°C, superior to traditional super alloys such as Inconel 718 and Haynes 230; hardfacing applications; and military applications.

The specifications on the security of collective protection equipment and structures in the military field set forth enhanced requirements for the resistance of the protection panels/floors/elements against the penetration by various types of projectiles, due to the diversification of the types of interventions in the military activities.

The main characteristics of the materials intended for the manufacture of protection components are as follows: the highest possible breaking and yield strength values, the highest possible hardness and impact resistance, and the highest possible elongation at break and energy absorbed by a notched specimen while breaking under an impact load (the Charpy test) at temperatures down to minus 40°C. The current military specifications recommend hardness values of at least 540–600 BHN (Brinell hardness) or 55–60 HRC (Rockwell hardness) and, for strength characteristics, such as the yield strength, values above 1500 and 1700 MPa for the breaking strength. In the case of the impact fracture energy using the Charpy test, the values must be of approximately 13 J at −40°C, with elongations of at least 6% [3].

Such requirements have been met by designing metallic alloys of various compositions; the most widely used is the high-strength microalloyed steels, used to produce reinforcement elements of thicknesses between 8.5 and 30 mm. Some research papers in the military field [4] have shown that the hardness of the plating material is not a sufficient factor to provide maximum resistance against penetration by projectiles, considering that the values of the mechanical strengths (yield and breaking) are much more important in the behavior process under dynamic stress.

Accurate measurements of dynamic stresses during impact stress have detected mechanical stress values of 28 GPa in the case of bainitic microstructure steels, while in the case of static stress, the measured stresses were of no more than 2 GPa [5].

A ballistic performance index (BPI) [6, 7] was also proposed to estimate the ballistic strength of armored plates, containing data on steel density, the elastic modulus, yield and tensile strength, Poisson's coefficient, and the constriction or elongation during impact tests. There are terms that contain elastic and plastic deformation components, as well as terms that take into account the kinetic energy of the target-projectile system after impact. Therefore, the main characteristics that


**141**

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

metallic materials used in special military applications need to feature for the best

• The highest possible hardness, as a measurement of the solid material's resistance against the penetration of its surface by various types of penetrators, with permanent shape changes, when a static or dynamic force is applied to them; the macroscopic hardness is generally characterized by the nature and strength of the intermolecular bonds, and the behavior of the solid material

• Toughness, which describes the capacity of a metallic material to absorb the breaking energy, the strength of its metallic matrix when various cracks occur and propagate, and the energy of forming the breaking surfaces, considering that the breaking occurs by consuming the impact stress energy, with local

• Impact strength, which is the relative susceptibility to breaking by the action

High-entropy alloys from the AlCrFeCoNi system feature very good mechanical properties for military applications, as shown in **Table 1**. Thus, the yield stress, the compressive strength, and the plastic deformation of these alloys reach unexpected values [8–20], making them usable as composite structures, resistant to dynamic stresses with high deformation velocity, applicable in the field of collective protection [9–20].

The most commonly used method for successfully producing high-entropy alloys is the electric arc melting of the load materials in a vacuum arc remelting equipment (with current values of up to 500 A) in controlled atmosphere. The main

Special type of materials chosen for making the ballistic target is experimental alloys AlCrFeCoNi, obtained in vacuum arc remelting equipment (MRF ABJ 900, in ERAMET Laboratory, University Politehnica of Bucharest, Romania). For calculating the metallic load, the theoretical degrees of assimilation of the elements in the melt and of the possible vaporization losses during the metallurgical process in vacuum or in argon-controlled atmosphere must be taken into account (**Figure 1**). Mass losses are estimated on the basis of the literature data thereon, the degree of oxidation of the metallic load materials, the characteristics of the elements in the load, their positioning in the series of electrochemical potentials, the characteristics of the preparation aggregate, and the experience in the preparation process. It should be noted that the metallic load used to obtain high-entropy alloys must be of high quality, low in phosphorus and sulfur, and degreased and properly machined (in terms of granulometry). The degree of purity of the elements the alloy is over 99%. The elements dosed in equimolecular or quasi-equimolecular proportions are introduced following an order determined according to the type of alloy to be prepared, into the crucibles in the copper plate, water-cooled throughout the metallurgical process. In order to produce high-entropy alloys under high-purity conditions, the working chamber must be suitably prepared by successive vacuuming and argon purging

**2. Obtaining of AlxCryFezCovNiw class of high-entropy alloys**

technological operations pursued in this case are presented below.

**2.1 Obtaining AlCrFeCoNi alloys in vacuum arc remelting furnace**

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

under the action of force is complex.

of forces applied at high velocity.

impact behavior are as follows:

plastic deformation.

#### **Table 1.**

*Mechanical properties of some high-entropy alloys [8].*

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

metallic materials used in special military applications need to feature for the best impact behavior are as follows:


High-entropy alloys from the AlCrFeCoNi system feature very good mechanical properties for military applications, as shown in **Table 1**. Thus, the yield stress, the compressive strength, and the plastic deformation of these alloys reach unexpected values [8–20], making them usable as composite structures, resistant to dynamic stresses with high deformation velocity, applicable in the field of collective protection [9–20].
