**7.3 Advantages of HEA as filler**

*Engineering Steels and High Entropy-Alloys*

formation are given as follows [41, 42]:

*r* = ∑

shear strength and found a remarkable improvement.

**7.2 Applications of HEA**

**7.1 Thermodynamic calculations for fabrication of HEA**

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.

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

∆*Smix* = −*R* ∑

∆*Hmix* = 4 ∑

δ<sup>2</sup> = ∑ *i*=1

*n C <sup>i</sup>* [ 1 −

enthalpy change before and after mixing of A and B elements is ∆*Hmix*

*i*=1 *n*

*<sup>i</sup> Cj* ∆*Hmix*

\_ *ri* ∑*i*=1 *<sup>n</sup> Ci ri* ] 2

*i*=1,*i*≠*j n C*

*Ω*= |*TΔ Smix*/*Δ Hmix* | (5)

*i*=1 *n*

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

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.

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

*C <sup>i</sup> ln Ci* (3)

*C <sup>i</sup> ri* (6)

*AB* (4)

(7)

*AB* , and *ΔHmix*

**230**

There are various reasons for using HEA as fillers:

