**1. Introduction**

#### **1.1 Background**

HEAs are defined as alloys comprising more than five main elements mixed in an equiatomic or near-equiatomic fraction [1–14]. Many HEAs have been reported to have: superior mechanical properties, such as ultrahigh fracture even at high temperatures, high hardness, toughness exceeding that of most pure metals and alloys, excellent comparable strength to that of structural ceramics and some metallic glasses, exceptional ductility, and fracture toughness at cryogenic temperatures [1, 3], and good physical properties, such as superconductivity, supermagnetism, and significant resistance to corrosion [5].

By replacing one or several elements in the composition of high entropy alloys, properties that significantly differ from the initial ones can be obtained. Furthermore, the decrease or increase in the ratio of additional elements can generate different metallographic structures with significant influences on the properties of alloys [6–10]. While high-strength conventional alloys are based mainly on the controlled distribution of one or two high-hardness phases at most, in high entropy alloys, the exceptional properties are based on the quenching effect of the supersaturated solid solution and on the suppression of the intermetallic phases [1, 4–12]. The complex distribution of the various chemical elements within the crystalline network of high entropy alloys appears to be the main cause of their special characteristics when compared to the classical or bi-component alloys. Choosing the combination of chemical elements could allow to simultaneously cumulate superior mechanical properties

as well as to ensure special corrosion resistance and biocompatibility, making their use as a new class of biocompatible metallic materials suitable [10, 12].

According to the most recent evaluations, the criteria for forming simple solid solutions in high entropy alloys must comply with the following conditions [13, 14]:

• Configuration entropy (ΔSam), which in cases of high entropy alloys must be higher than 11 J/mol K. The entropy of mixing [Eq. (1)] is calculated using Boltzmann's formula:

$$
\Delta \mathbf{S}\_{\text{mix}} = -\mathbf{R} \sum c\_i \text{Inc}\_i \tag{1}
$$

where χ<sup>i</sup> is the electronegativity of the element i, and *χ* is the average

*ΔHam*

<sup>1</sup> <sup>¼</sup> <sup>1</sup> � *TAΔSam*

• The critical correlation ratio to obtain only solid solutions is determined by the

where IM index refers to intermetallic compounds, while TA is the homogenization temperature; k2 index is considered to be 0.6 and represents the ratio between the entropy of the formation of the compounds and that of the formation of solid

Even though some inconsistencies can be noticed between the above-mentioned

High entropy alloys with different compositional characteristics have attracted a

After developing refractory high entropy alloys composed of a single BCC phase in W-Nb-Mo-Ta and W-Nb-Mo-Ta-V alloy systems, it was necessary to produce alloys that comprised transitional metals such as Nb-Mo-Ta-W, V-Nb-Mo-Ta-W, Ta-Nb-Hf-Zr-Ti, Hf-Nb-Ta-Ti-Zr, Mo-Nb-Ta-V-W and the equiatomic Hf-Mo-Nb-Ta-Ti-Zr alloys [3, 15–18]. From a biocompatibility perspective, it is interesting to note that the majority of these elements are biocompatible, with the exception of vanadium. By combining the HEA concept with the need to ensure alloy biocompatibility, biocompatible high entropy alloys have been designed from the abovementioned systems, with potential use for orthopedic implants. Results are reported

The high entropy alloys in the CrCoFeMoMnNiNb system microalloyed with Ta,

criteria, they are useful to evaluate the conditions under which solid phases are

lot of attention due to their potentially interesting properties for special fields. Meanwhile, this area provides vast opportunities for new compositions and micro-

as regards the production and testing of high entropy alloys in the system TiNbTaZrMo, TiNbTaZrFe, TiNbTaZrW, TiNbTaZrCr, and TiNbTaZrHf having deformability and biocompatibility characteristics superior to pure titanium, con-

Ti or Zr have special mechanical characteristics (compression strength above 2000 MPa, good deformation capacity under severe conditions and good dynamic impact behavior), excellent passive film chemical stability (corrosion potential in simulated biological environment), and reduced cytotoxicity (determined within the MTT test, ISO 10993) compared with the classical alloys used in highly demanding medical devices (CoCr or CoCrMo alloys), which have recorded side effects (tissue necrosis and release of metal ions in the body exceeding the

The data for the VEC parameter (valence atom concentration) from the TiNbTaZr and TiNbTaZrX alloys (where X was the element replaced, in turn, with Cr, V, Mo, W, and Fe) were located around value 5, indicating the formation of a BCC structure as well as the tendency to form a solid solution phase. In accordance with the above-mentioned criteria, the TiNbTaZr and TiNbTaZrX1 alloys (where X1 can be one of the Mo or W elements) show a reduced possibility of solid solution formation due to the high value of the δ parameter (difference of the

ð Þ 1 � *k*<sup>2</sup> >

*ΔHIM ΔHam*

(7)

electronegativity.

solutions.

**1.2 State of the art**

expression Eq. (7):

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

*kcr*

formed in multi-component alloys [13, 14].

structures, especially for complex alloys [15, 16].

sidered to be the least cytotoxic of all metals.

acceptable limits) [15].

atomic radius) [18].

**183**

where R is the gas constant (8.314 J/mol K) and ci is the molar fraction of element i.

• The enthalpy of mixing (ΔHam) of the alloy must be between �11.6 and 3.2 kJ/mol, and it is calculated using the derived formula [Eq. (2)] from Miedema's macroscopic model:

$$
\Delta \mathbf{H}\_{\text{am}} = \sum c\_i c\_j \,\Delta \mathbf{H}\_{\text{ij}} \tag{2}
$$

where ΔHij is the binary enthalpy of the formation of the elements i and j.

• The atomic radius difference criterion (δ), which claims that the phases that contain predominantly solid solutions are formed for values lower than 6.6%, and at values lower than 4%, only solid solutions are formed. The calculation formula Eq. (3) for δ was defined as follows:

$$\delta = 100\sqrt{\sum c\_i \left(1 - \frac{r\_i}{\overline{r}}\right)}2\tag{3}$$

where *ri* is the atomic radius of element i and *r* is the average atomic radius.

• The derived parameter Ω includes ΔSam and ΔHam and is taken into consideration only together with δ. If Ω > 1.1 and δ < 3.6%, only solid solutions are formed. If 1.1 < Ω < 10 and 3.6% < δ < 6.6%, only solid solutions and intermetallic compounds are formed, and if Ω > 10, only solid solutions are formed. The calculation formula for Ω Eq. (4) is:

$$
\Delta = \mathbf{T}\_{\text{top}} \,\Delta \mathbf{S}\_{\text{am}} / |\Delta \mathbf{H}\_{\text{am}}| \tag{4}
$$

where Ttop is the melting temperature calculated using the expression Eq. (5):

$$\mathbf{T\_{top}} = \Sigma \mathbf{c\_i} \mathbf{T\_{top}} \tag{5}$$

where Ttop i is the melting temperature of element i.

• The difference in electronegativity Δχ (according to Allen) of the various components of the alloy must be comprised between 3 and 6% in order to form only solid solutions. The calculation formula was deduced in a similar way to the one used to calculate the difference in atomic radius Eq. (6):

$$
\Delta \chi = \mathbf{100} \sqrt{\Sigma c\_i \left( 1 - \frac{\chi\_i}{\overline{\chi}} \right)} 2 \tag{6}
$$

where χ<sup>i</sup> is the electronegativity of the element i, and *χ* is the average electronegativity.

• The critical correlation ratio to obtain only solid solutions is determined by the expression Eq. (7):

$$k\_1^{cr} = 1 - \frac{T\_A \Delta S\_{am}}{\Delta H\_{am}} (1 - k\_2) > \frac{\Delta H\_{IM}}{\Delta H\_{am}} \tag{7}$$

where IM index refers to intermetallic compounds, while TA is the homogenization temperature; k2 index is considered to be 0.6 and represents the ratio between the entropy of the formation of the compounds and that of the formation of solid solutions.

Even though some inconsistencies can be noticed between the above-mentioned criteria, they are useful to evaluate the conditions under which solid phases are formed in multi-component alloys [13, 14].

### **1.2 State of the art**

as well as to ensure special corrosion resistance and biocompatibility, making their use

According to the most recent evaluations, the criteria for forming simple solid solutions in high entropy alloys must comply with the following conditions [13, 14]:

• Configuration entropy (ΔSam), which in cases of high entropy alloys must be higher than 11 J/mol K. The entropy of mixing [Eq. (1)] is calculated using

where R is the gas constant (8.314 J/mol K) and ci is the molar fraction of element i.

• The enthalpy of mixing (ΔHam) of the alloy must be between �11.6 and 3.2 kJ/mol, and it is calculated using the derived formula [Eq. (2)] from

where ΔHij is the binary enthalpy of the formation of the elements i and j.

r

• The derived parameter Ω includes ΔSam and ΔHam and is taken into

where *ri* is the atomic radius of element i and *r* is the average atomic radius.

consideration only together with δ. If Ω > 1.1 and δ < 3.6%, only solid solutions are formed. If 1.1 < Ω < 10 and 3.6% < δ < 6.6%, only solid solutions and intermetallic compounds are formed, and if Ω > 10, only solid solutions are

where Ttop is the melting temperature calculated using the expression Eq. (5):

• The difference in electronegativity Δχ (according to Allen) of the various components of the alloy must be comprised between 3 and 6% in order to form only solid solutions. The calculation formula was deduced in a similar way to

s

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi *<sup>Σ</sup>ci* <sup>1</sup> � *<sup>χ</sup><sup>i</sup>*

*χ* � �

2

the one used to calculate the difference in atomic radius Eq. (6):

Δχ ¼ 100

• The atomic radius difference criterion (δ), which claims that the phases that contain predominantly solid solutions are formed for values lower than 6.6%, and at values lower than 4%, only solid solutions are formed. The calculation

> ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi <sup>X</sup>*ci* <sup>1</sup> � *ri*

*r* � �

2

Ω ¼ Ttop ΔSam*=*∣ΔHam∣ (4)

Ttop ¼ ΣciTtop i (5)

(3)

(6)

<sup>X</sup>*ci*lnci (1)

<sup>Δ</sup>Ham <sup>¼</sup> <sup>X</sup>*cicj* <sup>Δ</sup>Hij (2)

ΔSmix ¼ �R

as a new class of biocompatible metallic materials suitable [10, 12].

Boltzmann's formula:

*Engineering Steels and High Entropy-Alloys*

Miedema's macroscopic model:

formula Eq. (3) for δ was defined as follows:

formed. The calculation formula for Ω Eq. (4) is:

where Ttop i is the melting temperature of element i.

**182**

δ ¼ 100

High entropy alloys with different compositional characteristics have attracted a lot of attention due to their potentially interesting properties for special fields. Meanwhile, this area provides vast opportunities for new compositions and microstructures, especially for complex alloys [15, 16].

After developing refractory high entropy alloys composed of a single BCC phase in W-Nb-Mo-Ta and W-Nb-Mo-Ta-V alloy systems, it was necessary to produce alloys that comprised transitional metals such as Nb-Mo-Ta-W, V-Nb-Mo-Ta-W, Ta-Nb-Hf-Zr-Ti, Hf-Nb-Ta-Ti-Zr, Mo-Nb-Ta-V-W and the equiatomic Hf-Mo-Nb-Ta-Ti-Zr alloys [3, 15–18]. From a biocompatibility perspective, it is interesting to note that the majority of these elements are biocompatible, with the exception of vanadium. By combining the HEA concept with the need to ensure alloy biocompatibility, biocompatible high entropy alloys have been designed from the abovementioned systems, with potential use for orthopedic implants. Results are reported as regards the production and testing of high entropy alloys in the system TiNbTaZrMo, TiNbTaZrFe, TiNbTaZrW, TiNbTaZrCr, and TiNbTaZrHf having deformability and biocompatibility characteristics superior to pure titanium, considered to be the least cytotoxic of all metals.

The high entropy alloys in the CrCoFeMoMnNiNb system microalloyed with Ta, Ti or Zr have special mechanical characteristics (compression strength above 2000 MPa, good deformation capacity under severe conditions and good dynamic impact behavior), excellent passive film chemical stability (corrosion potential in simulated biological environment), and reduced cytotoxicity (determined within the MTT test, ISO 10993) compared with the classical alloys used in highly demanding medical devices (CoCr or CoCrMo alloys), which have recorded side effects (tissue necrosis and release of metal ions in the body exceeding the acceptable limits) [15].

The data for the VEC parameter (valence atom concentration) from the TiNbTaZr and TiNbTaZrX alloys (where X was the element replaced, in turn, with Cr, V, Mo, W, and Fe) were located around value 5, indicating the formation of a BCC structure as well as the tendency to form a solid solution phase. In accordance with the above-mentioned criteria, the TiNbTaZr and TiNbTaZrX1 alloys (where X1 can be one of the Mo or W elements) show a reduced possibility of solid solution formation due to the high value of the δ parameter (difference of the atomic radius) [18].

Refractory element Mo is preferred in manufacturing metallic biomaterials due to the fact that it can be found in several conventional metallic biomaterials, such as Ti-15Mo-5Zr-3Al and Co-Cr-Mo [15–19]. Cast and quenched HEA TiNbTaZrMo had breaking strength values exceeding 1000 MPa, higher than those of TiNbTaZrHf and Ti6Al4V refractory alloys, but also good deformability. Quenching led to the improvement of TiNbTaZrMo deformability, which was attributed to coarse granulation and/or redistribution of constituent elements in the dendritic and interdendritic regions [15, 18]. The distribution of cells formed on different types of substrates plays a significant role in cellular functions that involve protein migration, proliferation, and synthesis. In osteointegration tests, the osteoblasts formed on cast and quenched HEA TiNbTaZrMo surfaces showed a widespread morphology, fairly similar to the morphology of the cells on the CP-Ti titanium alloy. On the other hand, the osteoblasts formed on the 316 L austenitic stainless steel were smaller and had a less widespread morphology. The results obtained indicated that osteoblasts had a better tendency to develop, contributing significantly to forming the bone matrix in the case of HEA TiNbTaZrMo, with or without heat quenching treatments, the effects being similar to CP-Ti alloys [15, 16].

dendritic microstructure suffers from a rolling finish, reducing interdendritic spaces and eliminating pores and casting defects [17, 20, 21]. The Al0.1CoCrFeNi alloy contains 2.44 at % Al and 24.4 at % of elements Co, Cr, Fe, and Ni, respectively, being single-phased with FCC structure. While casting, the mechanical characteristics of this alloy are modest; the yield strength is 140 MPa, the tensile strength is 370 MPa, and the elongation reaches 65%. By applying combined thermo-mechanical treatments (cold rolling with 60% reduction and homogenization at 1000°C for 24 hours) increases in mechanical properties and microstructure

Combining a Ti oxide layer with a Zr oxide layer in a TiNbTaZr alloy demonstrated an excellent biocompatibility. From this viewpoint, obtaining a high corrosion resistance of the implanted metals, which work under physiological corrosion conditions over long periods of time, became a major concern. The excellent corrosion resistance of HEA TiZrNbTaMo in corrosive environments, comparable to that of the Ti6Al4V, was due to the passivation effects of the surface and to achieve high stability to the pitting phenomenon [21–26]. In general, the biomaterial-made implantable elements are aimed at improving and extending the patients' lives. After using orthopedic prostheses made from bio inert materials for a long time, the emphasis is now laid on using materials that can activate tissue repair mechanisms, called bioactive materials, through phenomena aimed at increasing proliferation and differentiation of osteoblasts, resulting in in situ reformation of bone architecture [27, 28]. Regarding the biocompatibility of implantable alloys, it is mandatory to ensure an increased corrosion resistance in the corrosive physiological environ-

This chapter presents a series of results regarding the obtaining and characteri-

CrTaNbTiZrMo, and FeTaNbTiZrMo alloying systems. All the alloys were produced at laboratory scale, in an electric-arc remelting furnace in an inert argon atmosphere. Given that the alloys contain easily fusible elements, some of the metal components were not completely melted during the primary processing. The microstructural characteristics and the microhardness of the alloys suffered changes following the application of homogenization heat treatments. Some of these experimental alloys underwent corrosion resistance tests in simulated biological environments, the results obtained being encouraging. The microscopic investigation of cell viability in direct contact with FeMoTaTiZr alloy in a 1:1:1:1:1 ratio, in which the in vitro cultivation of mesenchymal stem cells isolated from human bone tissue was carried out, demonstrated the biocompatibility of this type of alloy [28]. The conditions of adhesion to the implanted metallic material can be improved by depositing hydroxyapatite-based layers on its surfaces using the magnetron sputtering method. This method allows controlling the deposition parameters so as to obtain thin, flawless, uniform layers, with a very good adhesion to the substrate, low roughness, resistant to corrosion and wear, low stress layers, which are essential

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

High entropy alloys can be obtained in optimum conditions in RAV furnaces working in high purity argon-controlled environments. The concept for the design

zation of high entropy biocompatible alloys from the CrFeMoNbTaTiZr, CrFeMoNbTaTi, CrFeMoNbTaZr, CrFeMoTaTiZr, CrFeTaNbTiZr,

modification from dendritic to polygonal are obtained [17, 18].

ment compared to the classical alloys [18].

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

properties to be used in medical applications [29].

**2. Experimental results**

**furnace**

**185**

Further research is necessary in order to clarify the origin of the excellent biocompatibility of HEA TiNbTaZrMo. These results clearly indicate that these alloys are a new class of metallic biomaterials with exceptional characteristics. To conclude, the new TiNbTaZrMo equiatomic biocompatible alloy contains two BCC solid solution-type phases with a fine equiaxial dendritic structure and an excellent biocompatibility compared to pure Ti, together with superior mechanical properties, indicating the possibility of being used as a new class of metallic biomaterials.

Over the last few years, there has been an increased need to manufacture stents to take over blood vessel functions. Alongside the classical titanium alloys used in this respect, some high entropy alloys from the CoCrFeNiMn and Al0.1CoCrFeNi systems were also investigated [16, 20].

CoCrFeNiMn alloy is equiatomic and was developed for the first time by Cantor [17]. He found the alloy to be very stable, to have higher configuration entropy than melting entropy, being made up of a single phase solid solution with FCC crystalline structure [17, 18]. Moreover, it possesses remarkable mechanical properties, such as high plasticity and ductility, as well as significant tear resistance. This alloy's microstructure is dendritic, and its diffusion rates are very slow. This conclusion was reached after carrying out analyses on heat treated specimens at 700 or 900°C for one-hour periods, when it was discovered that the diffusion of elements was extremely low regardless of the value of the holding temperature [20].

The heat treatments led to a moderate increase of the breaking strength from 447 to 515 MPa (in the case of homogenization to 900°C, maintaining the elongation at break at the same value of 51%) for AlCrFeNiMn alloys [17, 18, 20]. The increase of the homogenization time to 48 hours and of the temperature to 1000°C did not produce major changes in the mechanical characteristics, there even being a slight decrease in the breaking strength to 475 MPa and in elongation to 50%. This behavior led to the conclusion that these alloys are not substantially consolidated by precipitating intermetallic compounds during heat treatments. However, different results were obtained by applying a combined treatment that consisted of annealing + rolling + annealing. The heat and mechanical processing parameters were: annealing at 1000°C for 4 hours, cold rolling with a 50% thickness reduction, from 5.8 to 2.9 mm, and a reduction speed of 0.2 mm per passing, followed by a new annealing at 1000°C for 4 hours.

Following the initial annealing treatment, the material loses its hardness, acquires even greater plasticity, and partial diffusion phenomena occur, while the

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

Refractory element Mo is preferred in manufacturing metallic biomaterials due to the fact that it can be found in several conventional metallic biomaterials, such as Ti-15Mo-5Zr-3Al and Co-Cr-Mo [15–19]. Cast and quenched HEA TiNbTaZrMo

had breaking strength values exceeding 1000 MPa, higher than those of TiNbTaZrHf and Ti6Al4V refractory alloys, but also good deformability. Quenching led to the improvement of TiNbTaZrMo deformability, which was attributed to coarse granulation and/or redistribution of constituent elements in the dendritic and interdendritic regions [15, 18]. The distribution of cells formed on different types of substrates plays a significant role in cellular functions that involve protein migration, proliferation, and synthesis. In osteointegration tests, the osteoblasts formed on cast and quenched HEA TiNbTaZrMo surfaces showed a widespread morphology, fairly similar to the morphology of the cells on the CP-Ti titanium alloy. On the other hand, the osteoblasts formed on the 316 L austenitic stainless steel were smaller and had a less widespread morphology. The results obtained indicated that osteoblasts had a better tendency to develop, contributing significantly to forming the bone matrix in the case of HEA TiNbTaZrMo, with or without heat quenching treatments, the effects being similar to CP-Ti alloys

Further research is necessary in order to clarify the origin of the excellent biocompatibility of HEA TiNbTaZrMo. These results clearly indicate that these alloys are a new class of metallic biomaterials with exceptional characteristics. To conclude, the new TiNbTaZrMo equiatomic biocompatible alloy contains two BCC solid solution-type phases with a fine equiaxial dendritic structure and an excellent biocompatibility compared to pure Ti, together with superior mechanical properties, indicating the possibility of being used as a new class of metallic biomaterials. Over the last few years, there has been an increased need to manufacture stents to take over blood vessel functions. Alongside the classical titanium alloys used in this respect, some high entropy alloys from the CoCrFeNiMn and Al0.1CoCrFeNi

CoCrFeNiMn alloy is equiatomic and was developed for the first time by Cantor [17]. He found the alloy to be very stable, to have higher configuration entropy than melting entropy, being made up of a single phase solid solution with FCC crystalline structure [17, 18]. Moreover, it possesses remarkable mechanical properties, such as high plasticity and ductility, as well as significant tear resistance. This alloy's microstructure is dendritic, and its diffusion rates are very slow. This conclusion was reached after carrying out analyses on heat treated specimens at 700 or 900°C for one-hour periods, when it was discovered that the diffusion of elements was

The heat treatments led to a moderate increase of the breaking strength from 447 to 515 MPa (in the case of homogenization to 900°C, maintaining the elongation at break at the same value of 51%) for AlCrFeNiMn alloys [17, 18, 20]. The increase of the homogenization time to 48 hours and of the temperature to 1000°C did not produce major changes in the mechanical characteristics, there even being a slight decrease in the breaking strength to 475 MPa and in elongation to 50%. This behavior led to the conclusion that these alloys are not substantially consolidated by precipitating intermetallic compounds during heat treatments. However, different results were obtained by applying a combined treatment that consisted of annealing

extremely low regardless of the value of the holding temperature [20].

+ rolling + annealing. The heat and mechanical processing parameters were: annealing at 1000°C for 4 hours, cold rolling with a 50% thickness reduction, from 5.8 to 2.9 mm, and a reduction speed of 0.2 mm per passing, followed by a new

Following the initial annealing treatment, the material loses its hardness, acquires even greater plasticity, and partial diffusion phenomena occur, while the

[15, 16].

systems were also investigated [16, 20].

*Engineering Steels and High Entropy-Alloys*

annealing at 1000°C for 4 hours.

**184**

dendritic microstructure suffers from a rolling finish, reducing interdendritic spaces and eliminating pores and casting defects [17, 20, 21]. The Al0.1CoCrFeNi alloy contains 2.44 at % Al and 24.4 at % of elements Co, Cr, Fe, and Ni, respectively, being single-phased with FCC structure. While casting, the mechanical characteristics of this alloy are modest; the yield strength is 140 MPa, the tensile strength is 370 MPa, and the elongation reaches 65%. By applying combined thermo-mechanical treatments (cold rolling with 60% reduction and homogenization at 1000°C for 24 hours) increases in mechanical properties and microstructure modification from dendritic to polygonal are obtained [17, 18].

Combining a Ti oxide layer with a Zr oxide layer in a TiNbTaZr alloy demonstrated an excellent biocompatibility. From this viewpoint, obtaining a high corrosion resistance of the implanted metals, which work under physiological corrosion conditions over long periods of time, became a major concern. The excellent corrosion resistance of HEA TiZrNbTaMo in corrosive environments, comparable to that of the Ti6Al4V, was due to the passivation effects of the surface and to achieve high stability to the pitting phenomenon [21–26]. In general, the biomaterial-made implantable elements are aimed at improving and extending the patients' lives. After using orthopedic prostheses made from bio inert materials for a long time, the emphasis is now laid on using materials that can activate tissue repair mechanisms, called bioactive materials, through phenomena aimed at increasing proliferation and differentiation of osteoblasts, resulting in in situ reformation of bone architecture [27, 28]. Regarding the biocompatibility of implantable alloys, it is mandatory to ensure an increased corrosion resistance in the corrosive physiological environment compared to the classical alloys [18].

This chapter presents a series of results regarding the obtaining and characterization of high entropy biocompatible alloys from the CrFeMoNbTaTiZr, CrFeMoNbTaTi, CrFeMoNbTaZr, CrFeMoTaTiZr, CrFeTaNbTiZr, CrTaNbTiZrMo, and FeTaNbTiZrMo alloying systems. All the alloys were produced at laboratory scale, in an electric-arc remelting furnace in an inert argon atmosphere. Given that the alloys contain easily fusible elements, some of the metal components were not completely melted during the primary processing. The microstructural characteristics and the microhardness of the alloys suffered changes following the application of homogenization heat treatments. Some of these experimental alloys underwent corrosion resistance tests in simulated biological environments, the results obtained being encouraging. The microscopic investigation of cell viability in direct contact with FeMoTaTiZr alloy in a 1:1:1:1:1 ratio, in which the in vitro cultivation of mesenchymal stem cells isolated from human bone tissue was carried out, demonstrated the biocompatibility of this type of alloy [28]. The conditions of adhesion to the implanted metallic material can be improved by depositing hydroxyapatite-based layers on its surfaces using the magnetron sputtering method. This method allows controlling the deposition parameters so as to obtain thin, flawless, uniform layers, with a very good adhesion to the substrate, low roughness, resistant to corrosion and wear, low stress layers, which are essential properties to be used in medical applications [29].
