**2.1 Obtaining of biocompatible high entropy alloys in the RAV MRF ABJ 900 furnace**

High entropy alloys can be obtained in optimum conditions in RAV furnaces working in high purity argon-controlled environments. The concept for the design of experimental alloy recipes from the CrFeMoNbTaTiZr system was based on the choice of chemical elements having extremely low biotoxicity, currently used as an alloying base for classical alloys used to manufacture medical devices. In order to produce high entropy alloys in the CrFeMoNbTaTiZr system in the MRF ABJ 900 vacuum arc remelting equipment within the **ERAMET**—SIM, UPB Laboratory, seven classes of different alloys were chosen in which the chemical composition varied, maintaining the equiatomic proportion in each of them, as follows: HEAB 1 —CrFeMoNbTaTiZr; HEAB 2—CrFeMoNbTaTi; HEAB 3—CrFeMoNbTaZr; HEAB 4—CrFeMoTaTiZr; HEAB 5—CrFeTaNbTiZr; HEAB 6—CrTaNbTiZrMo; HEAB 7 —FeTaNbTiZrMo. Raw materials consisting of elements with purity greater than 99.5 wt % were mechanically processed to be introduced into the RAV equipment, then weighed, and dosed in equiatomic reports (**Table 1**).

number of successive remelting was imposed by the fact that the load contained high melting temperature elements, such as: Fe—1538°C; Ti—1668°C; Zr—1855°C;

The buttons made from biocompatible HEA alloys (**Figure 1b**) were weighed in order to determine the production coefficient. The losses were due to the fact that, during the process of producing the alloys, there were small drips in the working area, under the action of the electric arc, without their excessive vaporization.

Samples were taken from the biocompatible high entropy alloys produced in order to carry out the microstructural analysis. The samples taken by abrasive disk cutting under cooling liquid jet were then subjected to the metallographic preparation procedure, using abrasive paper with grit sizes ranging between 360 and 2500, followed by polishing using alpha alumina suspension with grit sizes ranging between 3 and 0.1 μm. The experimental alloys were not chemically etched using metallographic reagents so as to also perform localized chemical composition analyses using the EDAX detector. The microstructural analysis was carried out by optical and scanning electron microscopy, using an Olympus GX 51 optical microscope and a SEM Inspect S electron microscope equipped with an EDAX type Z2e

Following the metallographic examination, a significant part of the chemical elements included in the matrix of the HEAB 1 (CrFeMoNbTaTiZr alloy) and HEAB 2 (CrFeMoNbTaTi alloy) was dissolved, forming a solid homogeneous solution

Elements such as Cr, Zr, Fe, and Nb formed a common solid solution in which intermetallic compounds precipitated. Nonetheless, a series of compounds of hardto-melt elements (Mo and Nb), dispersed relatively uniformly in the base matrix (**Figure 3**), were visible. High melting temperature chemical elements, such as Ta and Mo, were not completely melted, the rounded grains being partially adherent to

(**Figure 3a**, **b**). In the case of the HEAB 3 (CrFeMoNbTaZr alloy), the tantalum was impossible to melt within the volume of the designed batch, which was too small. The big difference between the melting temperatures of the chemical elements constituting the alloy, corroborated with the rapid cooling in the water cooled copper base plate, also generated fracture effects in the metal matrix at the interface

Even though the HEAB 4 (CrFeMoTaTiZr alloy) contained only two hard-tomelt elements (Mo and Ta), it was still impossible to completely dissolve its metallic grains (**Figure 4a**). Furthermore, between the hard-to-melt metal grain (Ta) and the embedded metal matrix, some micro-cracks were formed perpendicular to the interface, as a result of solidification stresses (in other words, as a result of the inability of the high-hardness metal matrix to disperse the stresses generated by

The HEAB 5 (CrFeTaNbTiZr alloy) and HEAB 6 (CrTaNbTiZrMo alloy) ran a similar course with the other alloys presented above, the dissolution of the Ta particles being impossible in this case as well (**Figure 4**). A possible cause could be the volume of the grains (having a mass of approx. 10.42 g), which was too big in relation to the total volume of the batch (having a total mass of approx. 29.22 g), which was too small. The very short time in which the alloy was in liquid state did

In the case of the HEAB 7 (FeTaNbTiZrMo alloy), the issue of dissolving hardto-melt metal particles (Ta and Nb) could not be solved during the melting stage

the solid solution existing between the other chemical elements of the alloy

Cr—1907°C; Nb—2477°C; Mo—2623°C; Ta—3017°C.

*High Entropy Alloys for Medical Applications DOI: http://dx.doi.org/10.5772/intechopen.89318*

detector from the LAMET, UPB laboratory.

with the Ta or Mo grains (**Figure 3a**, **b**).

not allow for the complete dissolution of larger grains.

**2.2 Microstructure**

(**Figure 2a**, **b**).

rapid cooling).

**187**

For each experimental alloy sample, a metal load constant of approx. 30 g was maintained. The raw materials were deposited on the copper plate of the RAV equipment in an order meant to ensure the quickest possible formation of a metal bath under the action of the electric arc (**Figure 1**). This mode of operation is very important when working with refractory chemical elements. After coupling to the cooling system, the process continued with successive suctions until a pressure of <sup>5</sup> <sup>10</sup><sup>3</sup> mbar was obtained in the working area.

Argon atmosphere (5.3 purity levels) was then introduced, and approx. 8–10 homogenization melts were performed, by rotating the samples, in order to ensure a uniform distribution of the chemical elements in the alloys produced. The high


#### **Table 1.**

*Mass and production efficiency of biocompatible high entropy alloy batches.*

**Figure 1.** *HEAB 1 to HEAB 7 batches prepared for melting on the copper plate of the RAV equipment (a) and after melting (b).*

### *High Entropy Alloys for Medical Applications DOI: http://dx.doi.org/10.5772/intechopen.89318*

number of successive remelting was imposed by the fact that the load contained high melting temperature elements, such as: Fe—1538°C; Ti—1668°C; Zr—1855°C; Cr—1907°C; Nb—2477°C; Mo—2623°C; Ta—3017°C.

The buttons made from biocompatible HEA alloys (**Figure 1b**) were weighed in order to determine the production coefficient. The losses were due to the fact that, during the process of producing the alloys, there were small drips in the working area, under the action of the electric arc, without their excessive vaporization.
