**7. Research and development on HEA in microjoining**

HEAs are the novel and advanced alloys which are composed of 5–35 at.% where all the elements serve as principal elements. They are unique because of their attractive properties compared to their conventional alloys such as strength and ductility trade-off, high thermal stability, high wear, and corrosion resistance. HEAs are first discovered in 1995 by Yeh et al. and coined as multicomponent alloys in 2004 by Cantor et al. [32–33]. Previous studies have shown that most of the HEAs are composed of simple face-centered cubic (FCC), body-centered cubic (BCC), or FCC + BCC solid solutions owing to their high configurational entropy. The composition of these HEAs can be tailored to obtain promising properties such as high hardness and ductility as well as high resistance to wear, oxidation, and corrosion.

#### **7.1 Thermodynamic calculations for fabrication of HEA**

In accordance with the Hume-Rothery rules, the factors affecting the binary solid solutions, i.e., atomic size difference, valence electron concentration (VEC), the crystal structure of the solute and solvent atoms, and the difference in electronegativity, will be used to design the HEAs. The criteria adopted for the solid solution phase formation is given by various parameters according to the laws of thermodynamics. According to Zhang et al., the various parameters for HEA formation are given as follows [41, 42]:

$$
\Delta S\_{\text{mix}} = -R \sum\_{i=1}^{n} \mathbf{C}\_{i} \ln \mathbf{C}\_{i} \tag{3}
$$

$$
\Delta H\_{\text{mix}} = 4 \sum\_{i=1, i \neq j}^{n} \mathbf{C}\_{i} \mathbf{C}\_{j} \Delta H\_{\text{mix}}^{AB} \tag{4}
$$

$$
\Delta \mathcal{Q} = \left| T \Delta \mathcal{S}\_{\text{mix}} / \Delta H\_{\text{mix}} \right| \tag{5}
$$

$$r = \sum\_{i=1}^{n} \mathbf{C}\_i r\_i \tag{6}$$

**231**

**Figure 7.**

*Schematic for arc melting.*

*High-Entropy Alloys for Micro- and Nanojoining Applications*

There are various reasons for using HEA as fillers:

pounds compared to the fusion bonding processes.

**8. Future and current status of HEA fillers**

1.High ductility of HEAs will lead to a robust tough joint. It can be fabricated

2.HEAs will minimize the formation of IMCs in the joint and ensure a safe joint.

3.The cracking and distortion are minimized due to the IMCs and reaction com-

4.A single solid solution of HEAs will have a narrow or even zero solidus-liquidus range which is beneficial for brazing to avoid distortion and thermal damage.

These fillers are generally prepared by using various methods such as arc melting, powder metallurgy, sputtering, laser cladding, and electrodeposition. Most of the HEAs are produced initially by arc melting starting from Cantor alloy and its

Arc melting consists of melting the pure elements by an arc in a vacuum or argon environment followed by a cooling approach [46]. However, some vaporizable elements are difficult to use in this method and lead to porosity. The schematic

There is another variation of the solidification method for obtaining single crystals of HEA, known as Bridgman solidification method [46]. In this case the polycrystalline HEA is melted and cooled from the liquid state (usually an eutectic

*DOI: http://dx.doi.org/10.5772/intechopen.91166*

**7.3 Advantages of HEA as filler**

easily into wires and rods.

derivatives.

**8.1 Arc melting**

diagram is shown in **Figure 7**.

**8.2 Bridgman solidification**

$$
\boldsymbol{r} = \sum\_{i=1}^{n} \mathbf{C}\_{i} \boldsymbol{r}\_{i} \tag{6}
$$

$$
\boldsymbol{\Theta}^{2} = \sum\_{i=1}^{n} \mathbf{C}\_{i} \left[ \mathbf{1} - \frac{\boldsymbol{r}\_{i}}{\sum\_{l=1}^{n} \mathbf{C}\_{l} \boldsymbol{r}\_{i}} \right]^{2} \tag{7}
$$

δ is the atomic size difference; *Ci* is the atomic fraction of ith element with radius *ri*. *T* denotes the temperature. The average radius of all elements is *r*. The enthalpy change before and after mixing of A and B elements is ∆*Hmix AB* , and *ΔHmix* and *ΔSmix* are the enthalpy change and mixing entropy of the HEA. The parameter Ω is the interaction parameter of the HEA. For HEA formation, δ ≤6% and Ω ≥1.1.

#### **7.2 Applications of HEA**

These novel HEAs find applications in various fields such as aerospace, automobiles, submarines, and nuclear and power plant industries [43–45]. Therefore, it is of utmost importance to develop a reliable and feasible technique of microjoining using these HEAs as fillers. The important conventional brazing methods cause cracking issues in joining metal-ceramic components due to the presence of various interfacial compounds, in addition to the inherent residual stress in fusion welding approaches. In contrast, brazing avoids the cracking issues and distortion, but still, the creation of numerous IMCs at the interface is a serious concern. HEAs with an optimal balance of strength and ductility and mainly solid solution phases may overcome these interfacial compounds and can be an excellent tool for brazing industries. There is not enough work done in this area related to high-entropy brazing. Few studies talk about the laser brazing of Ni superalloy. Gao and his coworkers used FeCoNiMnCu non-equiatomic HEA to braze the Ni superalloy [46]. In this study, they also evacuated the effect of brazing time and foil thickness on the shear strength and found a remarkable improvement.
