**1. Introduction**

Throughout recent years, high entropy alloys (HEAs) have arisen as a new reality in the advanced materials domain. The motivation behind this innovative class of engineering materials was based on the purpose of broadening the boundaries of knowledge in what concerns alloys with more than one main constituent. This concept can be traced back to 2004, when the first papers concerning this subject were published [1,2], whereas, at the present time, several other works [3–12] have also been presented to extend the available information regarding HEAs.

Currently, a common definition for these alloys was introduced by one of the precursor works, where high entropy alloys are described as metallic compounds with at least five principal elements, with the percentage of each component varying between 5 and 35 at.% [2]. However, other definitions include a wider range of materials, with different component amounts, which can also be considered high entropy alloys [3]. Several review papers address and discuss the different nomenclatures regarding high entropy alloys [3,13–17], and as such, we refrain from addressing this topic in this paper.

As established by Yeh [18], these innovative multi-element compositions confer to these materials four distinct core effects:

The "high entropy effect", associated to the high configurational entropy that stabilizes the formation of simple solid-state phases such as body-centered-cubic (BCC) or face-centered-cubic (FCC), while inhibiting the development of brittle intermetallic compounds. However, it must be noted that high entropy alloys can still exhibit brittle-like behavior, as evidenced for the AlCoCrCuFeNi system [19].

The "lattice distortion effect", that occurs due to the lack of a dominant element in the composition of the alloy, resulting in different atoms of different sizes occupying the lattice positions of the crystal structure, promoting its distortion and affecting the physical and mechanical properties of the alloy. Depending on the selected atomic elements and their concentration, distinct phases, with potentially different mechanical properties can form as evidenced in the work of Wu et al. [20]. As such, the lattice distortion effect can be used to promote a phase over the other for specific alloy systems. Recent work by He et al. [21] has shown that not only the atomic ratio of the element that compose the high entropy alloy affect the lattice distortion, but the alloy Poisson ration must also be considered.

The "sluggish diffusion effect", that constrains the atomic diffusion of the elements and inhibits the phase transformations that require such phenomenon to occur. As a result, higher recrystallization temperatures can be achieved, and formation of nano-precipitates and amorphous structures are susceptible to occur, as such, second-phase precipitation only occurs after extremely long periods [22]. Bhattacharjee et al. [23] showed that heat treatments with the duration of 1 h at or below 800 ◦C in severely deformed CoCrFeNiMn high entropy alloys would not result in any significant grain growth. However, when the temperature was of 900 or 1000 ◦C, massive grain growth was observed. Regarding the extremely long times and temperatures required for second-phase precipitation, Pickering et al. [22] showed that annealing at 700 ◦C for times above 500 h would lead to the formation of M23C6 and σ phase.

The "cocktail effect", which refers to the enhancement of the established properties of the alloy, which cannot be attributed independently to any of the elements that compose the material [24].

Considering the above-mentioned features and the prospect of customizing the composition of high entropy alloys, a new path for a wide range of applications can be expected. As such, owing to these properties, the outstanding performance of HEAs to operate under extreme conditions is subjected to intensive research. Depending on the composition of the alloy, the corrosion resistance [25–28], the ability to sustain high cyclical loading [29,30], wear resistance [31–33], and the good performance at both high [34,35] and cryogenic temperatures [36–38] are some of the key features that these alloys exhibit towards being novel solutions for structural and functional applications [39]. Miracle et al. [40] suggest that high entropy alloys can be used as structural materials in transportation and energy sectors. The same author [41] proposes that some high entropy alloys can be used in functional applications that require resistance to radiation damage or when in need of diffusion barriers, as in the microelectronics sector. Replacement of conventional materials by entropy alloys is suggested to occur in the future, and materials to be replaced include stainless steels, Al-, Ti-, and Ni-based alloys. This is related to the fact that high entropy alloys can be fine-tuned to simultaneously present, if desired, low density and high mechanical strength or other combination of properties. These properties will be dictated by the elements that compose the high entropy alloy system. Table 1 summarizes the mechanical properties (yield stress, ultimate tensile stress, and elongation) for multiple high entropy alloy systems at different temperatures.


**Table 1.** Mechanical properties of various high entropy alloys (HEAs) under different temperatures.

To ensure the viability of these alloys to be used in complex shaped structures, their weldability is an important issue that needs to be addressed. Any advanced engineering alloy will require welding to either obtain complex shape structures or to couple its properties to those from another material. As such, evaluating the weldability of novel alloys is fundamental to further expand its potential applications. Additionally, because these alloys are now being more studied, it is possible to adjust their chemistry or microstructure to avoid weldability issues, such as liquation cracking. The sooner these potential issues are found, the easier and more cost-effective it is to find a solution.

This paper analyses the overall progress achieved using well-known welding techniques and weldability studies on HEAs. First, focus is given to fusion-based techniques, in a second instance, a review of solid-state ones is provided.
