**2.2 Microstructure**

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,

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

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

**Alloy Element, g Production efficiency, g Cr Fe Mo Nb Ta Ti Zr**

HEAB 1 CrFeMoNbTaTiZr 2.55 2.73 4.68 4.54 8.77 2.37 4.38 29.85 HEAB 2 CrFeMoNbTaTi 2.97 3.19 5.47 5.30 10.32 2.74 — 29.91 HEAB 3 CrFeMoNbTaZr 2.74 2.95 5.06 4.90 9.54 — 4.80 29.42 HEAB 4 CrFeMoTaTiZr 2.98 3.20 5.50 — 10.36 2.75 5.21 29.69 HEAB 5 CrFeTaNbTiZr 3.00 3.22 — 5.36 10.42 2.76 5.24 29.72 HEAB 6 CrTaNbTiZrMo 2.78 — 5.13 4.98 9.68 2.56 4.87 29.94 HEAB 7 FeTaNbTiZrMo — 2.97 5.10 4.94 9.61 2.55 4.83 29.95

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

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

then weighed, and dosed in equiatomic reports (**Table 1**).

<sup>5</sup> <sup>10</sup><sup>3</sup> mbar was obtained in the working area.

*Engineering Steels and High Entropy-Alloys*

**Table 1.**

**Figure 1.**

*melting (b).*

**186**

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 detector from the LAMET, UPB laboratory.

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

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 the solid solution existing between the other chemical elements of the alloy (**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 with the Ta or Mo grains (**Figure 3a**, **b**).

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 rapid cooling).

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 not allow for the complete dissolution of larger grains.

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

#### **Figure 2.**

*Cross-section of biocompatible HEABs upon the same magnification. Optical microscopy (1000 magnification) (a) CrFeMoNbTaTiZr; (b) CrFeMoNbTaTi; (c) CrFeMoNbTaZr; (d) CrFeMoTaTiZr; (e) CrFeTaNbTiZr; (f) CrTaNbTiZrMo.*

(**Figure 5**). The FeTaNbTiZrMo alloy shares characteristics similar to the ones previously mentioned. The Ta particle failed to dissolve in this case as well, a fracture being developed from its interface with the base matrix (**Figure 6**).

The remaining elements formed a fairly homogenous solid solution, with a dendritic appearance, in which a series of intermetallic compounds were dispersed and uniformly distributed. The dendritic appearance of the metallic matrix was also highlighted on the breaking surfaces of the mini ingots (**Figure 7a**). The images obtained with the help of SEM electron microscopy show micro-dendrites alternating with surfaces of cleavage fractures and micro-fractures, which explain the high brittleness of these alloys in cast state. The localized chemical composition analyses

(**Table 2**) highlighted the distribution of elements into the dendritically metallic matrix and the segregation of some elements (Nb, Ti, and Ta) near the interface

*Undissolved blocks present in the experimental HEAB alloys. (a) CrFeMoNbTaTiZr; (b) CrFeMoNbTaZr.*

*Undissolved tantalum fragments in the high entropy alloys. (a) CrFeTaNbTiZr; (b) CrTaNbTiZrMo.*

with undissolved blocks of Nb and Ta (**Figure 8**).

*Undissolved Ta and Nb fragments in the FeTaNbTiZrMo alloy.*

**Figure 3.**

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

**Figure 4.**

**Figure 5.**

**189**

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

#### **Figure 3.**

*Undissolved blocks present in the experimental HEAB alloys. (a) CrFeMoNbTaTiZr; (b) CrFeMoNbTaZr.*

#### **Figure 4.**

*Undissolved tantalum fragments in the high entropy alloys. (a) CrFeTaNbTiZr; (b) CrTaNbTiZrMo.*

**Figure 5.** *Undissolved Ta and Nb fragments in the FeTaNbTiZrMo alloy.*

(**Table 2**) highlighted the distribution of elements into the dendritically metallic matrix and the segregation of some elements (Nb, Ti, and Ta) near the interface with undissolved blocks of Nb and Ta (**Figure 8**).

(**Figure 5**). The FeTaNbTiZrMo alloy shares characteristics similar to the ones previously mentioned. The Ta particle failed to dissolve in this case as well, a fracture being developed from its interface with the base matrix (**Figure 6**). The remaining elements formed a fairly homogenous solid solution, with a dendritic appearance, in which a series of intermetallic compounds were dispersed and uniformly distributed. The dendritic appearance of the metallic matrix was also highlighted on the breaking surfaces of the mini ingots (**Figure 7a**). The images obtained with the help of SEM electron microscopy show micro-dendrites alternating with surfaces of cleavage fractures and micro-fractures, which explain the high brittleness of these alloys in cast state. The localized chemical composition analyses

*Cross-section of biocompatible HEABs upon the same magnification. Optical microscopy (1000 magnification) (a) CrFeMoNbTaTiZr; (b) CrFeMoNbTaTi; (c) CrFeMoNbTaZr; (d) CrFeMoTaTiZr; (e) CrFeTaNbTiZr;*

**Figure 2.**

**188**

*(f) CrTaNbTiZrMo.*

*Engineering Steels and High Entropy-Alloys*

#### **Figure 6.**

*Undissolved tantalum fragment in the FeTaNbTiZrMo alloy.*

#### **Figure 7.**

*Microstructure (a) and semi-quantitative composition spectrum (b) of FeTaNbTiZrMo alloy on the breaking surface.*

the formation of which is suppressed during rapid cooling. The final result of annealing treatments is to reduce the level of micro- or macroscopic residual stresses [3, 4]. Thus, homogenization annealing combined with rapid cooling was shown to allow for increased values of mechanical characteristics. In some cases, depending on the chemical composition of the alloy, hardness values decreased if successive heat treatments were applied at different temperatures [1, 22]. Some of the heat treatments applied to high entropy alloys can either lead to hardening effects or to an increased plasticity and toughness, depending on the value of the heating temperature, on the actual duration of holding at those temperatures as well as on the cooling mode. The effects of heat treatments on macro- and microstructure depending on temperature are very interesting and suggestive [22–30]. The dendritic morphology specific to cast alloys is unaffected if the holding temperature does not exceed 1040°C. On the contrary, when the temperature exceeds 1200°C, phases rich in certain chemical elements occur (e.g., Cu) [20]. The formation of HC-rich in BC-Ta-Nb and hexagonal-packed close (HCP) was highlighted during the heat treatments, depending on the annealing time at 700°C [31].

*SEM image of interface between two undissolved blocks and dendritically metallic matrix (Nb, a), (Nb and Ta, b), (Ta, c) and semi-quantitative composition spectrum for the FeTaNbTiZrMo alloy (d) for the*

**Figure 8.**

**191**

*micro-zones (spot 1 to spot 6).*

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

When it comes to the experimental alloys developed in this chapter, a series of heat treatments were applied in order to achieve the microstructural homogenization and the dissolution of hard-to-melt particles. The samples underwent heat treatment for aging at 600°C for 4 hours and then at 900°C for 6 hours. The heat treatments were performed in the Nabertherm LT 15/12/P320 furnace with a programmable chart for the heat regime. The heating speed was of 20°C/min, and the samples kept for 4 hours were cooled in air, while the samples kept for 6 hours were

cooled in the furnace. The evolution of the new alloy microstructure was

highlighted by optical and electron microscopy. Thus, before applying heat treatments, the hard-to-melt elements (Ta, Nb, and Mo) did not completely dissolve in the metal melt; they were only diffused at the level of the separation boundaries, on short distances. After the heat treatment, an increased tendency toward oxidation was noticed in the elements located in the superficial layer of the samples, as well as


#### **Table 2.**

*The chemical composition of micro-zones which corresponds to Figure 8.*

#### **2.3 Heat treatments**

Given the fact that HEAs are metallic materials with a high degree of chemical heterogeneity resulted from the association of chemical elements with significant differences in atomic diameters and different mutual solubility, the majority of the researchers in the field resorted to applying heat treatments after producing the alloys [20, 21]. The homogenization heat annealing treatments can reduce or eliminate the segregation effects of chemical elements which take place during casting in the case of high entropy alloys [20]. Microstructures near-equilibrium thus results, either by dissolving the metastable phases or by nucleating the equilibrium phases,

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

**Figure 8.**

*SEM image of interface between two undissolved blocks and dendritically metallic matrix (Nb, a), (Nb and Ta, b), (Ta, c) and semi-quantitative composition spectrum for the FeTaNbTiZrMo alloy (d) for the micro-zones (spot 1 to spot 6).*

the formation of which is suppressed during rapid cooling. The final result of annealing treatments is to reduce the level of micro- or macroscopic residual stresses [3, 4]. Thus, homogenization annealing combined with rapid cooling was shown to allow for increased values of mechanical characteristics. In some cases, depending on the chemical composition of the alloy, hardness values decreased if successive heat treatments were applied at different temperatures [1, 22]. Some of the heat treatments applied to high entropy alloys can either lead to hardening effects or to an increased plasticity and toughness, depending on the value of the heating temperature, on the actual duration of holding at those temperatures as well as on the cooling mode. The effects of heat treatments on macro- and microstructure depending on temperature are very interesting and suggestive [22–30]. The dendritic morphology specific to cast alloys is unaffected if the holding temperature does not exceed 1040°C. On the contrary, when the temperature exceeds 1200°C, phases rich in certain chemical elements occur (e.g., Cu) [20]. The formation of HC-rich in BC-Ta-Nb and hexagonal-packed close (HCP) was highlighted during the heat treatments, depending on the annealing time at 700°C [31].

When it comes to the experimental alloys developed in this chapter, a series of heat treatments were applied in order to achieve the microstructural homogenization and the dissolution of hard-to-melt particles. The samples underwent heat treatment for aging at 600°C for 4 hours and then at 900°C for 6 hours. The heat treatments were performed in the Nabertherm LT 15/12/P320 furnace with a programmable chart for the heat regime. The heating speed was of 20°C/min, and the samples kept for 4 hours were cooled in air, while the samples kept for 6 hours were cooled in the furnace. The evolution of the new alloy microstructure was highlighted by optical and electron microscopy. Thus, before applying heat treatments, the hard-to-melt elements (Ta, Nb, and Mo) did not completely dissolve in the metal melt; they were only diffused at the level of the separation boundaries, on short distances. After the heat treatment, an increased tendency toward oxidation was noticed in the elements located in the superficial layer of the samples, as well as

**2.3 Heat treatments**

**Figure 6.**

**Figure 7.**

*surface.*

**Table 2.**

**190**

*Undissolved tantalum fragment in the FeTaNbTiZrMo alloy.*

*Engineering Steels and High Entropy-Alloys*

Given the fact that HEAs are metallic materials with a high degree of chemical heterogeneity resulted from the association of chemical elements with significant differences in atomic diameters and different mutual solubility, the majority of the researchers in the field resorted to applying heat treatments after producing the alloys [20, 21]. The homogenization heat annealing treatments can reduce or eliminate the segregation effects of chemical elements which take place during casting in the case of high entropy alloys [20]. Microstructures near-equilibrium thus results, either by dissolving the metastable phases or by nucleating the equilibrium phases,

*Microstructure (a) and semi-quantitative composition spectrum (b) of FeTaNbTiZrMo alloy on the breaking*

**Spot Chemical elements, wt% Location**

 — — 100 ——— Nb block — 24.74 67.99 7.28 — — Interface with Nb 3 24.19 10.26 14.30 14.25 28.81 8.20 Matrix 4 3.66 25.7 30.00 7.98 5.49 26.90 Matrix — 42.14 48.27 9.59 — — Interface with Ta — 100 ———— Ta block

**Fe Ta Nb Ti Zr Mo**

*The chemical composition of micro-zones which corresponds to Figure 8.*

#### **Figure 9.**

*The oxidized layer of CrFeMoTaTiZr alloy after heat treatment at 800°C/24 hours/slow cooling in the furnace with fractures (a) and peeling (b).*

#### **Figure 10.**

*The interface with undissolved Ta particle (a) and semi-quantitative composition spectrum (b) of the heat-treated CrFeMoTaTiZr alloy.*

the formation of a homogenized alloy band, with a dendritic microstructure, located immediately below the complex oxide layer (**Figure 9**).

The effects of the heat treatment are also highlighted by means of an EDS analysis performed with an AMETEC Z2e analyzer on micro-zones located both at the center of the CrFeMoTaTiZr samples as well as at the edges, in order to quantify the oxidation and diffusion effects. An image of the boundary between an undissolved Ta particle and the embedding metal matrix is illustrated in **Figure 10**.

microns), where the average hardness value was 882 HV0.2, that is, approx. 66 HRC, as well as the formation of a homogenization band of approx. 45 microns, where the hardness increased to 1337 HV0.2. The microstructural aspects are in

5 have very close values of microhardness, with the average being around

hardness in some areas (under the oxidized layer), which came to about 1337 HV0.2. This development, which represents an increase of over 150%, can be exploited in terms of obtaining high-wear-resistant surfaces for medical

As can be seen from the data presented in **Table 3** and **Figure 11**, HEAB 1–HEAB

750 HV0.2. A notable difference was obtained with the HEAB 6 alloy, which had the lowest hardness value (544 HV0.2) before heat treatment. Also, for the HEAB 7 alloy, the microhardness was between the minimum and maximum values of the

After heat treatment, the HEAB 6 alloy recorded a considerable increase in

accordance with the microhardness values.

*Evolution of microhardness for experimental biocompatible HEA.*

*Microhardness values for biomedical HEA samples.*

**Alloy Microhardness values in points**

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

CrFeMoNbTiZr Central zone Marginal zone

Heat-treated CrFeMoTaTiZr Central zone Marginal zone homogenized layer

**Table 3.**

**Figure 11.**

**of HV0.2/10**

791; 832; 817; 775; 772 801; 809; 822; 798; 815

974; 1349; 1406; 1286; 1434 906; 876; 893; 893; 840 1296; 1429; 1305; 1339; 1316

CrFeMoNbTaTiZr 749; 739; 700; 743; 747 736 2.76 CrFeMoNbTaTi 750; 732; 747; 804; 736 754 3.85 CrFeMoNbTaZr 775; 748; 719; 725; 803 754 4.66 CrFeMoTaTiZr 697; 764; 795; 790; 712 752 5.97 CrFeMTaNbTiZr 749; 700; 748; 736; 705 728 3.24 CrTaNbTiZrMo 581; 567; 591; 590; 544 575 3.42 FeTaNbTiZrMo 626; 686; 655; 657; 651 655 3.26

**Average value, HV 0.2**

> 797 809

1290 882 1337 **Variation coefficient of hardness**

> 3.30 1.22

14.38 2.88 4.03

other materials (655 HV 0.2).

instruments.

**193**

#### **3. Microhardness**

The heat treatment effects on mechanical hardness characteristics are highlighted by the different microhardness values determined with the help of the Shimadzu HMV 2 T microhardness tester, presented in **Table 3**.

The analysis of the microhardness values resulted from applying a homogenization treatment showed that hardness increases in the highly alloyed metal matrix up to the average value of 1290 HV0.2 in case of the CrFeMoTaTiZr alloy. The diffusion of the chemical elements during the treatment determined a reduced hardness in the marginal zone adjacent to the surface (located at a distance of approx. 200
