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

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 technological operations pursued in this case are presented below.

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

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

*Engineering Steels and High Entropy-Alloys*

military applications.

activities.

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

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

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].

**Alloy Yield strength,** 

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

**MPa**

AlCrFeCoNi 1250.96 2004.23 32.7 CrFeCoNiCuTi 1272 1272 1.6 AlCrMnFeCoNiCuTiV 1862 2431 0.95 CrFeCoNiCuTi0.5 700 1650 21.26

**Compressive strength, MPa**

**Plastic deformation, %**

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

**140**

**Table 1.**

**Figure 1.**

*Vacuum arc remelting equipment MRF ABJ 900 and working chamber, in ERAMET Laboratory, University Politehnica of Bucharest, Romania.*

operations performed at least three times. The vacuuming is performed using preliminary vacuum systems and diffusion pumps, which can provide pressure levels of about 3.5–4 × 10<sup>−</sup><sup>4</sup> mbar. Pure argon (Ar 5.3, 99.99%) is used for purging and melting. These operations provide a maximum oxygen content of 40–60 ppm in the working chamber. The final stage of the process is the argon purging of the working chamber and the setting of a working pressure level slightly above the atmospheric pressure.

The process of producing high-entropy alloys consists in melting the load materials, followed by the remelting of the samples for five to seven times, turning them on opposite sides to ensure a fully alloyed state and to improve the chemical homogenization of the mini-ingots. The entire preparation process is carried out by electric arc remelting in argon-controlled atmosphere.

The melting mode of the vacuum arc remelting furnace must be adapted to the type of alloy being prepared. These parameters vary during the preparation process, depending on the stages and the activities being carried out. The thorium tungsten electrode must be shifted during melting so that it is approximately 1/4" away from the copper plate electrode, and the electric arc formed sweeps the entire surface of the load for complete homogenization. Following melting and solidification, mini-ingots of weights almost constant compared to that of the load introduced into the VAR are produced.

Using this method, experimental high-entropy alloys have been instantaneously cooled by forced cooling of the water pumped at the base of the copper plate (the crystallizer of the furnace).

The melted material solidifies ultrafast in the water-cooled copper shell. The mini-ingots produced can have different shapes, depending on the shape of the cavity in the copper plate of the furnace. The samples were analyzed, according to the chemical composition, in order to define their physical, chemical, mechanical, etc. properties.

**143**

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

The chemical compositions of the metallic materials used are as follows:

Cu = 0.14 wt%; Al = 0.12 wt%; Fe = ballance wt%

equipment, both from the ERAMET Laboratory, Bucharest.

• Metallic chromium with 99 wt% Cr

*Metallic molds for casting Al0.8CrFeCoNi alloy.*

**Figure2.**

• Metallic cobalt with 99.5 wt% Co

**vacuum arc remelting furnace**

losses, is as follows (Eq. (1)):

• Electrolytic nickel with 99.5 wt% Ni

• Electrolytic aluminum with 98.5 wt% Al

• Extra soft steel, MK3 grade: C = 0.02 wt%; Si = 0.04 wt%; Mn = 0.21 wt%; S = 0.02 wt%; P = 0.015 wt%; Ni = 0.2 wt%; Cr = 0.15 wt%; Mo = 0.07 wt%;

**2.2 Obtaining of AlCrFeCoNi alloy in duplex system including induction and** 

A vacuum induction melting-vacuum arc remelting (VIM-VAR) duplex technology was selected to produce the high-entropy alloy AlCrFeCoNi in order to increase the purity of the alloy and to improve its mechanical properties. The experimental batches were prepared in the vacuum induction melting furnace Balzers, HU-40-25- 40-04 type, with a capacity of 12 kg and in the MRF ABJ 900 vacuum arc remelting

The high-entropy alloy used to make the ballistic protection plates in the vacuum induction furnace was produced according to a classical preparation technology, using highly pure materials, and metallic molds made of iron Fc 250 (**Figure 2**) were used for casting to ensure the production of 150 × 100 × 10 mm rectangular plates. The load calculation (based on the molar mass of the chemical elements, Mmol) for obtaining the Al0.8CrFeCoNi alloy, taking into account the estimated elemental

The concentration (% weight) of the alloying elements in the alloy was Al = 8.72 wt%; Cr = 21 wt%; Fe = 22.62 wt%; Co = 23.83 wt%; and Ni = 23.83 wt%. The alloy mass calculated based on the volume and density of the alloy cast in a plate and

Mmol = 27x0.8 + 52 + 56 + 59 + 59 = 247.6 g (1)

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

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

#### **Figure2.** *Metallic molds for casting Al0.8CrFeCoNi alloy.*

*Engineering Steels and High Entropy-Alloys*

operations performed at least three times. The vacuuming is performed using preliminary vacuum systems and diffusion pumps, which can provide pressure levels of

*Vacuum arc remelting equipment MRF ABJ 900 and working chamber, in ERAMET Laboratory, University* 

These operations provide a maximum oxygen content of 40–60 ppm in the working chamber. The final stage of the process is the argon purging of the working chamber and the setting of a working pressure level slightly above the atmospheric pressure. The process of producing high-entropy alloys consists in melting the load materials, followed by the remelting of the samples for five to seven times, turning them on opposite sides to ensure a fully alloyed state and to improve the chemical homogenization of the mini-ingots. The entire preparation process is carried out by

The melting mode of the vacuum arc remelting furnace must be adapted to the type of alloy being prepared. These parameters vary during the preparation process, depending on the stages and the activities being carried out. The thorium tungsten electrode must be shifted during melting so that it is approximately 1/4" away from the copper plate electrode, and the electric arc formed sweeps the entire surface of the load for complete homogenization. Following melting and solidification, mini-ingots of weights almost constant compared to that of the load introduced into the VAR are produced. Using this method, experimental high-entropy alloys have been instantaneously cooled by forced cooling of the water pumped at the base of the copper plate (the

The melted material solidifies ultrafast in the water-cooled copper shell. The mini-ingots produced can have different shapes, depending on the shape of the cavity in the copper plate of the furnace. The samples were analyzed, according to the chemical composition, in order to define their physical, chemical, mechanical,

electric arc remelting in argon-controlled atmosphere.

mbar. Pure argon (Ar 5.3, 99.99%) is used for purging and melting.

**142**

etc. properties.

about 3.5–4 × 10<sup>−</sup><sup>4</sup>

*Politehnica of Bucharest, Romania.*

**Figure 1.**

crystallizer of the furnace).

The chemical compositions of the metallic materials used are as follows:

