Introduction
The development of human civilization has been accompanied by the desire to discover new materials, including revolutionary metals and principally new alloys. More than 5000 years ago, the first alloy was created by diluting copper with a tiny amount of tin. The name of it is bronze, which in such a manner revolutionized the way people of that time lived. Despite its simplicity, its developer’s approach has been used by people from then onwards. The name traditionally used to depict this strategy is a ‘base element’ paradigm, which means that one, sometimes two, principle elements with specific attractive properties are utilized as a base to which other minor elements are added to improve the characteristic properties. While the initial idea represents a rather simple concept, the further discovery of new metallic elements has led to the added complexity of the paradigm.
As compared to the original approach, today, the additions of alloys can be regulated to hundredths of a percent, making it possible for alloys to contain as many as a dozen minor elements. However, despite the transformation of the base element concept, the lack of new principle elements, as new stable metallic elements have not been discovered for almost a century, does not allow new materials to be developed. Materials development has been even rendered a mature technology due to the reason, as mentioned earlier. However, in 2004 pursuing two different research goals, two groups came up with a novel paradigm for designing alloys by mixing numerous elements in either equimolar or near-equimolar composition.
The key focus of the two groups was on the unexamined central regions of the phase diagrams, where all-alloy elements amass without any apparent base element. Yeh and his colleagues gave these newly designed alloys comprised of multiple elements the following term “HEA”, which stands for high-entropy alloys. The term ‘HEA’ implies that its definition is premised on the magnitude of entropy, in this case, high configurational molar entropy: SSS,ideal>1.61R, where SSS,ideal represents the total configurational molar entropy in an ideal SS, i.e. solid solution, and R is the gas constant.
These two pioneering papers are particularly significant for the discovery of a new realm of alloy compositions. Although the results of both papers were published during the same year, the work on both pieces of research started earlier and at different points in time. The work by Cantor and his colleagues began in the 1970s as an undergraduate thesis on multi-principal elements alloys (MPEAs), while the other founding paper stems from the series of theses in 1996. Although the focus of both groups was similar, their stated intentions varied in terms of their motivations. The original formulation of the research intention in Cantor’s and colleagues’ paper does not provide either a mention of entropy or a determined attempt to find single-phase, disordered SS phases. On the other hand, it is often agreed that the rationale behind the studies of HEAs is that high configurational entropy might be conducive to single-phase SS phases rather than to IM, i.e. intermetallic, phases, which makes them relatively stabilized. Although both founding papers attempt to explore the same thing, it is Yeah and his colleagues who sought to find single-phase solid solutions due to their ability to be synthesized, processed, and made use of.
In the original paper, HEAs definition can be described as composition-based which implies that there is no limit regarding the entropy magnitude, but only of the concertation of the elements. Moreover, in the founding paper from 2004, HEAs were described as a mixture of five or more principal elements, the concentration of each being between 5 and 35 atom percent. However, today there are materials that contain as few as three principal elements with the maximum number of element concentrations exceeding 35 atom percent. The aim of the novel concept is the development of new alloy systems with path-breaking functional and mechanical properties.
It should be noted that each HEA represents a new alloy base because each of them can be altered with the addition of minor elements, which results in a large number of new alloy bases. Apart from a vast number of these new alloy bases, other significant results that the research on HEAs can lead to are related to some of the HEAs mechanical responses to traditional materials. They can demonstrate superior hardness values, exceptional yield strength and ductility, high fatigue resistance, and also great fracture toughness. There have been researching conducted on the behavior of HEAs under irradiation, predominantly for FCC (face-centered cubic) crystalline structures [8].
The state of art, the current advancement in research, results
As stated previously, 2004 has been considered the point of departure for the studies on high-entropy alloys, although the background research in the area happened before this. During the past decade and a half, HEAs have turned into one of the most aspirational fields in material science, and overall, the field can be described as a fast-evolving one. Despite their differences, the reports mentioned above by Cantor and Yeh’s groups opened the opportunities for exploring a substantial compositional space of multicomponent for the search of new metallic materials.
From its early development, it was believed that the HEAs possessed a different set of properties that distinguished them from the conventional systems. Yeh summarized these properties into the “four core effects”, which are still considered the dominant driving force in developing high-entropy alloys. The first core effect is the high entropy effect, which is the most significant effect of its ability to aggravate the solution phases formation. The second core effect is severe lattice distortion, which not only influences properties but also reduces the thermal effect on them by making the thermal diffusivity of high-entropy alloys insensitive to temperature compared with conventional metals, which are sensitive to it. The third core effect is the so-called cocktail effect, which is related to predicting how well the composing element interact by using the mixture rule. The last one of four core effects is sluggish diffusion, which is about the HEAs kinetics of diffusion that is hindered if compared with conventional alloys and metals.
The very existence of these four core effects is the possibility of discovering new materials that will be superior to the existing ones, but it does not imply that the effects cannot be argued against. For instance, there has been collected much evidence that the previously undoubted stabilizing effect of configuration entropy might have been exaggerated. The experimental experience has supported the argument that there are just a few high-entropy alloys that are proved to be stable as single solid solutions up to a melting point. Moreover, data from various sources demonstrates that just configurational entropy itself is not the critical factor determining the process of shaping single-phase SS microstructures with crystal structures. Along with the advancement of the core effects understanding among the scientific community, the evolution of the HEAs definition criteria is happening. However, even though the discussed above definition of the HEAs might modify with the flow of time, the chances that the term will be substituted with any other circulating in the scientific community are rather low.
In the early years of the HEAs research, the majority of the HEAs examined were multi-phase alloys, while today, there has been an increased interest in single-phase alloys to test their behavior in-depth and disclose their fundamental mechanisms without any influence of secondary phases. The HEA research as a field is motivated to a great extent by the single-phase SS microstructures, which implies that both elements and alloys are not picked at random but are carefully selected to produce the microstructures. A noticeable focus in the field is on the single-alloy family called ‘3d transition metal HEAs’ using four or even more elements from the palette of 9 Al, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu.
The CoCrFeMnMi SS phase has the name of the ‘Cantor alloy’ after the author of one of the pioneering papers in the respective field. Alloys studied up to 2010 came from this single-alloy family, while the alloys studied later up until 2015 were also predominantly from the same family. Hence, there is a particular motivation to bring up the bias related to the usage of just a few elements to produce the HEAs, as in the case of the trinity of Cr-Fe-Ni, which appeared in more than 80% of the above-mentioned single-alloy family, and in three-quarters of all the alloys reported by 2015. Thus, it can be deduced that the alloys produced until today cannot be viewed as a random dataset due to the limitation of the elements used in their production.
The first new HEA family was presented in 2010, which was propelled by the desire to develop high-temperature structural alloys, which can be used at temperature exceeding 1000°C. This new family is called refractory HEA (RHEA) and originally was centered on the following refractory metals: Mo, Nb, Ta, V, W. It should be mentioned that today the list has grown to include other refractory metals as well as 3d transition metals and compound-forming elements. The introduction of refractory HEA was followed by the design of other new alloys families such as the 4f TM (the 4f transition metal) alloy family, and HEA brasses, as well as bronzes, were taken to a new level by improving the balance of properties in them. The selection of elements for these new designs occurs on what can be described as an intuitive approach, which is logical and quite simple.
Another significant point in the development of the HEAs is that although initially, they included only metals, today, the list has expanded to incorporate ceramic and intermetallic compounds. For instance, ceramic ones can offer structural properties at rather high temperatures, and they usually have either two or three crystal sublattices. Hence, while the traditional alloying approach for this type of material is mainly limited to just one elemental substitution on either one or two sublattices simultaneously, the studies in the field of the HEAs demonstrate that a lot of elements can be substituted on one sublattice. Moreover, the HEA studies showcase that a host structure can be preserved when new elements are added and consequently form different crystal structures.
Discussion about the possibilities of future research with a focus on investigating irradiation effect
As mentioned earlier in the introduction, among the essential findings of the HEA studies, one can find such valuable ones as superior yield strength and ductility, excellent fatigue resistance, great fracture toughness, and unique irradiation resistance proposed mechanism of self-healing. This finding implies that the HEAs can be utilized to endurance the irradiation threats posed to the structural materials used in nuclear systems. Since conventional materials for these systems include steels, alloys, composites, and ceramics that are likely to be rather limited in enduring these high irradiation risks, new structures such as HEAs have to be developed further.
Due to the configurational entropy contribution to the total free energy in alloys, even the elements possessing various crystal structures can crystalize as a comparatively simple phase. Through the exceptional self-healing mechanisms, the process of blending different elements in the HEA results in probable irradiation resistance. What might stabilize the discorded solid-solution state is high entropy rather than multi-phase structures. Compared to conventional alloys, high-entropy ones are inclined to form a simple solid solution structure, the examples of which are FCC or BCC, based on the high entropy of the solution phases.
It is emphasized by the researchers studying the subject of HEAs irradiation behavior that it is essential to study what role and how the primary damage plays its role in the high-entropy alloys. They also highlight that the principle question has a two-fold character, which means that they regard two types of the multicomponent system effect. The first question is about how this system affects the radiation cascades, and the second is about its effect on the recrystallization process.
The prospects of future research regarding the irradiation behavior of alloys can bring rather fruitful results for the nuclear industry, which, in turn, is expected to play an ever-bigger role in the years to come. This growing impact of nuclear energy is related to increasing energy demand and more concern about the state of the environment. Improving the safety and effectiveness of fission reactors is an important aspect of making the nuclear industry more sustainable. It is also a necessity, which is based on the fact that the materials that have been traditionally used for nuclear reactors are only able to sustain irradiation up to a certain level of DPA, which does not possess the capacity of resisting a rather severe environment in the prospective nuclear systems.
In terms of future research in this direction, the modeling work is significant as it leads to evolving stability at high temperatures. Apart from the experimental part of the research, the focus should be on acquiring complete empirical data on irradiation damage. Moreover, the equivalence among three types of irradiation, namely electron, ion, and neutron, and modeling study, should be further studied.
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