High-entropy alloys (HEAs) are metallic alloys composed of at least five principal elements in near-equal concentrations. While traditionally alloys were based on a single principal element, research in the late 20th century explored multi-principal-element alloys. In 1981, Brian Cantor discovered an alloy with five equal-parts elements that formed a single FCC phase. Independent work by Jien-Wei Yeh in the 1990s developed the concept of HEAs, where high entropy of mixing favors formation of solid solution phases. Yeh's 2004 paper was the first to elucidate the HEA concept and provide experimental evidence. Research in HEAs has grown rapidly since, with its unique microstructures and properties offering

1. 1
CHAPTER 1
INTRODUCTION
High-entropy alloys or HEAs are metallic alloys composed of at least five chemical elements
in equal or near equal atomic percents (at. %). In order for an alloy to be specified as a HEA,
the concentrations of components must be between 5 to 35 at. % . Few structure models of
HEAs is presented on Figure 1. 1
Commonly used alloys are typically composed of one principal element, with only minor
additions of other elements that are added for property enhancement or easier processing.
However, it has been considered that alloys composed from a greater number of principal
elements will form complicated structures, which are difficult to analyze and engineer. It
seemed that they would not have any practical value and therefore research of these multi-
elemental alloys was very limited [1].
Experimental results yielded quite opposite conclusions. Multielemental alloys formed solid
solution phases. In 1995, Jien-Wei Yeh [2] suggested that multielemental alloys would
possess high mixing entropy, which would have an important role because it would favor
formation of simple solid solution phases. This was confirmed by several experimental results
and these alloys were then named high-entropy alloy.
Figure 1.1.Structure model of 4 × 4 × 4 AlCrCuFeTiZn HEA. . The atoms are denoted with
different colors: Al - red, Cr - blue, Cu - yellow, Fe - pink, Ti - orange, and Zn – grey[1].

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As the combinations of composition and processes for producing HEAs are numerous and as
each HEA has its own microstructure and properties to be identified and understood, the
research work is truly limitless. It becomes very important to consider the basic concepts
relating to HEAs at the very beginning, including the origin of high entropy, classification,
definition, composition notation, and the four core effects of HEA [2].
In the last few years, as a consequence of the outstanding continuing work by Jien-Wei, the
field of multicomponent and high-entropy alloys has taken off, with literally hundreds of
publications each year. Most notably, Vincent, Knight, Chang, and B.Cantor discovered in
the late 1970s a single FCC solid solution consisting of five components in equal proportions,
namely, FeCrMnNiCo. This alloy has been shown to have outstanding mechanical properties,
with high strength and high ductility. Mr. Cantor realized in the late 1970s that the
mechanical behavior of this material would be very unusual. Metal and alloy mechanical
properties depend primarily on the behavior of dislocations and how they move in response to
stress, but the concept of a dislocation as a line defect with a consistent core structure
becomes complex when there are many different components distributed on a single lattice.
Thus development of high entropy alloys has been day by days growing [1,2].

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CHAPTER 2
LITERATURE REVIEW
The history of high entropy is written on the basis of thoroughly study of research papers and
references regarding high entropy alloys.
2.1 Brief history of high entropy alloy
From the description of conventional and special alloys, historically over five millennia the
alloy design, alloy production, and alloy selection were all based on one principal-element or
one-compound concept. This alloy concept has generated numerous practical alloys
contributing to civilization and daily life. But, it still limits the degree of freedom in the
composition of the alloy and thus restricts the development of special microstructures,
properties, and applications. Consequently, materials science and engineering of alloys is not
fully explored since those alloys outside this conventional scheme have not been included.
It should be mentioned that in the late of eighteenth century, a German scientist and also
metallurgist Franz Karl Achard had studied the multicomponent equimass alloys with five to
seven elements . He could be most probably the first one to study multiprincipal-element
alloys with five to seven elements. In many ways, he is the predecessor for the researches of
Jien-Wei Yeh on HEAs. More than two centuries separate them. In 1788, Achard published a
little-known French book “Recherches sur les Propriétés des Alliages Métallique,” the first
compilation of data on alloy systems in Berlin. He disclosed the results of a laborious and
comprehensive program on over 900 alloy compositions of 11 metals, including iron, copper,
tin, lead, zinc, bismuth, antimony, arsenic, silver, cobalt, and platinum. Because of high cost,
he studied fewer compositions with silver, cobalt, and platinum[2].
Toward the end of the twentieth century two entirely independent investigations by Brian
Cantor in the United Kingdom and Jien-Wei Yeh in Taiwan made a disruptive break with the
classical tradition of alloys. A brand new alloy concept “HEAs” has been proposed and
explored and has led to a flurry of excitement. Figure 2.1 gives the number of year-wise
journal publications (until 2013) in the area of HEAs.

4. 4
Figure 2.1 year-wise publication in the area of HEAs[2].
The first work on exploring this brave new world was done in 1981 by Cantor with his
student Alain Vincent. They made several equiatomic alloys mixing many different
components in equal proportions. In particular, the world record holding multicomponent
alloy consisting of 20 different components each at 5% is held by this study. It was noticed
that only one alloy with a composition of Fe20Cr20Ni20Mn20Co20 forms a single FCC (face
centred cubic), Vincent was an undergraduate project student and the work was only written
at that time in his thesis at Sussex University. After this initial experiment there was a hiatus.
Similar studies on a wider range of alloys were repeated with another undergraduate project
student, Peter Knight, at Oxford in 1998. He achieved some similar results and some new
ones, published his results in a thesis at Oxford. Finally, Isaac Chang repeated the work again
in about 2000 at Oxford, and finally published the results in the open literature by presenting
at the Rapidly Quenched Metals conference in Bangalore in 2002, which was then published
in the journal Material Science and Engineering A in July 2004. In this paper entitled
“Microstructural development in equiatomic multicomponent alloys”, several important
conclusions were drawn. More electronegative elements such as Cu and Ge are less stable in
the FCC dendrites and are rejected into the interdendritic regions. Besides, alloy containing 20
components, that is, 5 at.% each of Mn, Cr, Fe, Co, Ni, Cu, Ag, W, Mo, Nb, Al, Cd, Sn, Pb,
Bi, Zn, Ge, Si, Sb, Mg, and another alloy consisted of 16 elements, that is, 6.25 at.% each of
Mn,

5. 5
Cr, Fe, Co, Ni, Cu, Ag, W, Mo, Nb, Al, Cd, Sn, Pb, Zn, and Mg are multiphase, crystalline
and brittle both in as-cast condition and after melt spinning. Surprisingly, however, the alloys
consisted predominantly of a single FCC primary phase, containing many elements but
particularly rich in transition metals, notably Cr, Mn, Fe, Co, and Ni. Finally, the total number
of phases is always well below the maximum equilibrium number allowed by the Gibbs phase
rule, and even further below the maximum number allowed under non-equilibrium
solidification conditions. It is also important to point out that Cantor came up with another
novel idea of equiatomic substitution later, in the early 2000s , as a method of exploring
metallic glass. These compositions are also in this vast uncharted region of materials space.
J.W. Yeh independently explored the multicomponent alloys world since 1995 . Based on his
own concept that high mixing entropy factor would play an important effect in reducing the
number of phases in such high order alloys and render valuable properties, he supervised a
master student K.H. Huang in 1996 to start the research and see the possibility of success in
the fabrication and analysis of HEAs. Around 40 equiatomic alloys with five to nine
components were prepared by arc melting. Investigations were made on microstructure,
hardness, and corrosion resistance of as-cast state and fully annealed state. The alloy design is
mainly based on commonly used elements. From those data of around 40 compositions, 20
alloys based on Ti, V, Cr, Fe, Co, Ni, Cu, Mo, Zr, Pd, and Al, with or without 3 at.% B
addition were selected as experimental alloys in the MS thesis of Huang in 1996 (Huang,
1996, published as MS thesis of National Tsing Hua Univeristy, Taiwan).
Yeh had submitted the “HEA concept” paper to Science in January 2003 but finally
unaccepted by Science. After this, he submitted the same paper to Advanced Materials and
then agreed the transfer to her sister journal, Advanced Engineering Materials for publication.
In May 2004, this paper entitled “Nanostructured high-entropy alloys with multiprincipal
elements—novel alloy design concepts and outcomes” was published. It becomes the first one
to elucidate the concept of HEAs by providing experimental results and related theory.
Besides this, another paper entitled “Multi-principal-element alloys with improved oxidation
and wear resistance for thermal spray coating” was published in Advanced Engineering
Materials in February 2004 [4]. But the term of HEA was not used in this paper. Two papers
entitled “Wear resistance and high-temperature compression strength of FCC
CuCoNiCrAl0.5Fe alloy with boron addition” and “Formation of simple crystal structures in
solid-solution alloys with multi-principal metallic elements” were published in Metallurgical
and Materials Transactions A later in the same year [5]. Before the submission of the first of

6. 6
the above paper, Professor Yeh had applied for HEAs patents in Taiwan (December 10,
1998), Japan, United States, and Mainland China.
Professor S. Ranganathan has also spent a long time to look into such multicomponent alloys
unexplored by people. Through the communications and discussions on this unknown field
with J.W. Yeh, he published a paper entitled “Alloyed pleasures—multimetallic cocktails” to
introduce three new alloy areas: bulk metallic glasses by A. Inoue, superelastic and
superplastic alloys (or gun metals) by T. Saito, and HEAs by J.W. Yeh in Current Science in
November 2003 . This becomes the first open publication in journals on HEAs, which led to
the activation of this new field. In this article, he said that the multicomponent alloys
represent anew frontier in metallurgy. They require hyperdimensions to visualize. If we use a
coarse mesh of 10 at.% for mapping a binary system, the effort involved in experimental
determination of phase diagrams rises steeply. Thus, the effort of experimental determination
of a seven component system will be 105 times that of a binary diagram and will alone need
as much effort as has been spent over the last 100 years in establishing B4000 binary and
B8000 ternary diagrams. While the computation of phase diagrams from first principles has
made impressive progress in the last decade, the calculation of higher order systems is a
daunting task. In this scenario, we have explorers like A. Inoue, T. Saito and J.W. Yeh
pointing to exciting new alloys with applications.
The open publications by the three initiators mentioned above,HEAs have become an
emerging field with many more researcher’s efforts and contributions. In a broad view, Many
aspect have been explored and researched. Fig. Shows the materials hypertetrahedron for
HEAs,which nutshell the broad spectrum of research and development that is taking place in
this field

7. 7
Figure 2.2 The material tetrahedron for high entropy alloys [2]
2.2 Definition of high entropy alloys
Each high-entropy alloy contains multiple elements, often five or more in equiatomic or near-
equiatomic ratios, and minor elements [2]. The basic principle behind HEAs is that
significantly high mixing entropies of solid solution phases enhance their stability as
compared with intermetallic compounds, especially at high temperatures. This enhancement
allows them to be easily synthesized, processed, analyzed, manipulated, and utilized by us. In
a broad sense, HEAs are preferentially defined as those alloys containing at least five
principal elements, each having the atomic percentage between 5% to 35%. The atomic
percentage of each minor element, if any, is hence less than 5% [3].
Why are such multiprincipal-element alloys called HEAs? From statistical thermodynamics,
Boltzmann’s equation can be used for calculating configurational entropy of a system:
ΔSconf = k ln w..... (2.1)
where k is Boltzmann’s constant and w is the number of ways in which the available energy
can be mixed or shared among the particles of the system. Thus the configurational entropy
change per mole for the formation of a solid solution from n elements with xi mole fraction is:
ΔSconf = -R ∑ Xi ln Xi ..... (2.2)
Let us consider an equiatomic alloy at its liquid state or regular solid solution state. Its
configurational entropy per mole can be calculated as follows

8. 8
ΔSconf = -R ln 1/n = R ln n..... (2.3)
Where R is the gas constant, 8.314 J/k mol.
Figure 2.3 shows an example illustrating the formation of quinary equiatomic alloy from five
elements. From the above equation, ΔSconf can be calculated as R ln 551.61R. For a
nonequiatomic HEA, the mixing entropy would be lower than that for an equiatomic alloy.
Consider a nonequiatomic alloy Al1.5CoCr0.5FeNi0.5 (or Al33.3 Co22.2Cr11.1Fe22.2Ni11.1
in at.%). Its configurational entropy can be calculated as 1.523R which is slightly smaller
than 1.61R of the equiatomic alloy AlCoCrFeNi.
2.3 Concept of high entropy alloys
In thermodynamics a system will try to minimize its Gibbs free energy (G) under isothermal
and isobaric conditions; that is to say, equilibrium is attained when G reaches a minimum
value. Thus, as the following relationship exists for the free energy of a system:
G == H – TS ..... (2.4)
it can be seen that the enthalpy (H) and entropy (S) of a system have a direct relationship in
determining the equilibrium state at a given temperature. For predicting the equilibrium state
of an alloy, the free energy changes from the elemental state to other states are often
compared so that the state with the lowest mixing free energy (△Gmix) can be determined.
From equation 1, it follows that the differences in free energy (△Gmix), enthalpy (△Hmix) and
entropy (△Smix) between the elemental and mixed states are related by:
△Gmix = △Hmix ﹣T△Smix ..... (2.5)
where R (8.31 J/K.mol) is the gas constant. Figure 2.4 shows the mixing entropy, calculated
by equation 3, as a function of the number of elements in the equimolar alloys. Thus, binary
and five element equimolar alloys have solution states with mixing entropies of 5.76 and
Figure 2.3 (A) Five components in equiatomic ratio before mixing and (B) mixing to form a
random solid solution. Assume their atomic sizes are same[2].

9. 9
13.37 J/K.mol, respectively. The mixing entropies for terminal solution or ordered compound
states are expected to be smaller due to a limited number of ways they can mix.
Based on the characteristics of figure 2.4, HEAs have been preferentially designated to be
alloys that comprise of five to thirteen major metallic elements. The lower limit of five
elements is imposed because it is considered to be the point at which the mixing entropy is
high enough to counterbalance the mixing enthalpy in most alloy systems and thus ensure the
formation of solid solution phases. Beyond thirteen elements there is a lebeling off of the
curve in figure 2.4, thus suggesting that little further benefit will be brought about by
composing alloys of a greater number of elements. The concentration of each element need
not be equimolar, but can be between 5 to 35 at%, therefore broadening the number of
possible HEA systems [1]. Thus, HEAs do not contain any element whose concentration
exceeds 50 at%, as is the case in the traditional alloys. Numerous alloys can therefore be
generated that satisfy the HEA criteria. J.W. YEH 6 For instance, if thirteen arbitrary elements
are selected from the periodic table, then a total of 7099 five to thirteen element alloy systems
could be obtained, as determined by the following:
C5
13+C6
13+C7
13+C8
13+C9
13+C10
13+C11
13+C12
13+C13
13 =7099 ..... (2.5)
The number of possible alloys is further increased by the fact that the alloys may or may
not be equimolar, and other minor elements could be added to modify their properties.
Examples of these three different types of HEAs are AlCoCrCuFeNi, AlCo0.5CrCuFe1.5Ni1.2
and AlCo0.5CrCuFe1.5Ni1.2B0.1C0.15.
Figure 2.4 The entropy of mixing as a function of the number of elements for equimolar
alloys in completely disordered state [2]

10. 10
Based on the above definition of HEAs, it is possible for alloys to be grouped roughly into
three categories according to their mixing entropy in the random solution state, namely (i)
low-entropy alloys (traditional alloys) with one or two major element, (ii) medium-entropy
alloys with two to four major elements and (iii) high-entropy alloys with at least five major
elements, as shown in figure 2.5.
It should be noted that the random solution states are defined as liquid solution and high-
temperature solid solution states where the thermal energy is sufficiently high to cause
different elements to have random positions within the structure. Thus, high-entropy alloy are
defined by the high entropy of the random solution state multi-principal-element alloys.
Figure 2.5 The alloy world divided by the mixing entropy of their random solution state [2].

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2.4 Four core effect of high entropy alloys
There are many factors affecting microstructure and properties of HEAs. Among these, four
core effects are most basic [1,2]. Because HEAs contain at least five major elements, and
conventional alloys are based on one or two metal elements, different basic effects exist
between these two categories. The four core effects are high entropy, severe lattice distortion,
sluggish diffusion, and cocktail effects. For thermodynamics, high-entropy effect could
interfere with complex phase formation. For kinetics, sluggish diffusion effect could slow
down phase transformation. For structure, severe lattice distortion effect could alter properties
to an extent. For properties, a cocktail effect brings excess to the quantities predicted by the
mixture rule due to mutual interactions of unlike atoms and severe lattice distortion.
2.4.1 High entropy effect
Although theoretically HEAs can form a large number of phases, only few are formed in
reality, due to the high-entropy effect. This effect plays an important role because it favors
formation of simple solid solution phases with FCC (face-centered cubic cell), BCC (body-
centered cubic cell) and HCP (hexagonal closed-packed cell) [4].
2.4.1.1 Gibbs free energy
Gibbs free energy G is a thermodynamic potential[10], which is used for calculation of work
that is performed by a thermodynamic system at a constant temperature and pressure. For the
Gibbs free energy the following applies:
𝐺=𝐻−𝑇𝑆 ..... (2.6)
where H is the enthalpy and S is entropy of the system. In stable phase the difference in the
Gibbs free energy between the elemental and the mixed state, Δ𝐺 𝑚𝑖𝑥=Δ𝐻 𝑚𝑖𝑥−𝑇 Δ𝑆 𝑚𝑖𝑥, is
minimal. We denoted Δ𝐺 𝑚𝑖𝑥 as the Gibbs free energy of mixing, Δ𝐻 𝑚𝑖𝑥 as the enthalpy of
mixing, 𝑇 as the absolute temperature and Δ𝑆 𝑚𝑖𝑥 as the entropy of mixing. It is obvious that
the temperature is of great importance for determining stable phases in HEAs. However, it
must be emphasized that it is the competition between the mixing enthalpy and the mixing
entropy that determines the formation of phases and is therefore a good parameter for
prediction of mutual solubility in solid solution phases.

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2.4.1.2 Entropy
The statistical-mechanics definition of the entropy states that entropy of the system is linearly
related to the logarithm of the number w, where w indicates the number of possible micro-
states corresponding to the macroscopic state of a system. This definition is written with the
equation ΔS=𝑘 ln w , where 𝑘 is Boltzmann’s constant.
The mixing entropy Δ𝑆 𝑚𝑖𝑥 is correlated with the possible atomic arrangements that the system
can take. It is the increase in the difference between the total entropy of several separate
systems in thermodynamic equilibrium and their partitioned, mixed without any chemical
reaction, closed system in a new thermodynamic equilibrium.
HEAs mostly consist of 5 to 13 different elements. When there are 5 different elements in a
HEA, it is predicted that the mixing entropy is already high enough to prevail over the mixing
enthalpy in most alloy systems, even if the alloys aren’t equimolar. According to the
minimization of free Gibbs energy, this ensures formation of solid solution phases. The upper
limit is set at 13 elements because there isn’t any greater benefit in composing HEAs with
more elements due to the logarithmic dependency of the mixing entropy on the number of
elements in the alloy[3].
For HEAs to form, the concentration of each element in the alloy system does not need to be
equimolar, but can range between 5 and 35 atomic %. This is shown in Figure 2.6 , where the
mixing entropy per mole for a ternary alloy system as a function of atomic ratios of all three
elements is plotted. It can be seen that the mixing entropy reaches maximum when the alloy
system is equimolar, but it doesn’t change significantly near the maximum. Widening the
range of atomic concentrations broadens the number of possible HEAs. Still, the range is
limited, and therefore HEAs do not contain any elements that have atomic concentration over
50%, as it is the case in traditional alloys [1, 2].

13. 13
Figure 2.6 Graph of the mixing entropy dependence on the atomic concentration of elements
(concentration of C element: 𝑐𝑐=1−𝑐𝐴−𝑐𝐵 ) in ternary alloy system
2.4.2 Lattice distortion effect
HEAs are composed of various elements and therefore form a lattice with/on? a multielement
basis. These elements can be of different sizes, which lead to distortion of the lattice. Larger
ions need more space, so they push away their neighbours, and small ones are surrounded by
extra space. This results in a /causes a strong internal stress-strain field, because large ions
cause compression and small ones cause tension in the lattice. In Figure 2.7, there is a
schematic representation of this effect with one-element, two-element (where elements are
very different in atomic sizes) and multielement lattice structure. However, the stress-strain
field is nott influenced only by different sizes of compound elements but also by energy of the
bonds between them. Stronger bonds tend to have smaller bonding distances than weaker
bonds.
Because of this effect, the strain energy of the lattice increases and therefore overall free
energy of the lattice also increases. Even more, stress field in the lattice is not uniform and
therefore HEAs have local stress gradients that slow down the movement of ions and are
responsible for sluggish diffusion. Lattice distortion effect is very important because it
determines whether the solid solution phases are stable. If HEAs are composed of elements
that cause the lattice distortion energy to be too high for retaining the crystal structure, it
collapses to an amorphous structure [4, 6].

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Figure 2.7 Schematic representation of a BCC lattice with a) one element (Cr), b) two
elements (Cr, V) and c) six elements (Cr, Ni, Fe, Co, Al, Ti), where atoms are distributed
randomly[2]
This effect influences mechanical, thermal, electrical, optical and chemical behaviour of the
materials. It causes a high strength for solid solutions (especially for HEAs with BCC lattice),
high thermal and electrical resistance, tensile brittleness and diffuse X-ray scattering.
2.4.3 Sluggish Diffusion Effect
Phase transformations that depend on atomic diffusion require the cooperative diffusion of
elements in order to attain the equilibrium partitioning among the phases. This, in
combination with the lattice distortion which hinders atomic movement, will limit the
effective diffusion rate in HEAs [3]. In conventional casting of HEAs, the phase separation
during cooling is often inhibited at higher temperatures and therefore delayed until lower
temperatures. This is the reason why the as-cast structures of HEAs often have nano-
precipitates in the matrix. An example of this is shown in figure 2.8. This is also the reason
for the higher recrystallization temperatures and activation energies of deformed HEAs. In
film coating technology, this can be reflected in the easier formation of amorphous structure
for a higher number of elements since the growth and even nucleation of crystalline phases
are gradually inhibited. Figure 2.8 shows the X-ray diffraction patterns of two to seven
element sputtered films, where it can be seen that for an increase in the number of elements a
nanocrystalline or even amorphous structure develops. The tendency to form nanocrystalline
or amorphous structures may be exploited to promote the mechanical, physical, and chemical
properties of the alloys.

15. 15
Figure 2.8 Nano-precipitaion in an as-cast equimolar AlCoCrCuFeNi alloy: (a) bright field
image and SAD pattern of the indicated precipitate and (b) dark field from the diffraction spot
in (a)[1]
Figure 2.9 Structural evolution of two to seven element sputtered films analyzed by x-ray[2]
2.4.4 Cocktail Effect
Since multi-principal elements are incorporated, HEAs can be viewed as an atomic-scale
composite. Therefore, they exhibit a composite effect coming from the basic features and
interactions among all the elements themselves, in addition to the indirect effects of the
various elements on the microstructure . For example, if more light elements are used, the
overall density will be reduced. If more oxidation-resistant elements are used, such as Al, Cr,
and Si, the oxidation resistance at high temperatures can be improved. If an element such as
Al is added, which has strong bonding with the other elements present, such as Co, Cr, Cu, Fe
and Ni, and promotes the formation of a BCC phase, the strength will be increased. Figure
2.11 displays the strengthening imposed by aluminum addition. Aluminum in this alloy

16. 16
Figure 2.10 Strengthening effect of aluminium addition on the cast hardness of
AlxCoCrCuFeNi alloys. A, B and C refer to the hardness, FCC lattice constant and BCC
lattice constant, respectively [1]
system has a similar effect as carbon in steels in substantially increasing the hardness,
although their strengthening mechanisms are different.
2.5 Synthesis methods of high entropy alloys
A variety of processing routes has been adopted for the synthesis of HEAs. HEAs have been
synthesized in different forms like dense solid castings, powder metallurgy parts, and films.
The processing route scan be broadly classified into three groups, namely, melting and casting
route, powder metallurgy route, and deposition techniques. Melting and casting techniques,
with equilibrium and non-equilibrium cooling rates, have been used to produce HEAs in the
shape of rods, bars, and ribbons. The most popular melt processing techniques are vacuum arc
melting, vacuum induction melting, and melt spinning.
Mechanical alloying (MA) followed by sintering has been the major solid-state processing
route to produce sintered products. Sputtering, plasma nitriding, and cladding are the surface
modification techniques used to produce thin films and thick layers of HEAs on various
substrates.

17. 17
2.5.1 Melting and Casting Route
This method is used for processing of rods , bars and ribbons.
There are three types of melting and casting route:
1.Vaccum Arc Melting
2.Vaccum Induction Melting
3.Melt Spinning
The most widely adopted route for the synthesis of HEAs is the melting and casting route.
Figure 2.11 gives an idea of the number of papers published on HEAs, grouped according to
different synthesis routes. It is very clear from Figure 2.12 that the casting route (bulk)
dominates the processing routes, with almost 75% of the papers published so faron HEAs
being produced by this route. A vast majority of HEAs that have been reported so far has been
produced by vacuum arc melting and a few by vacuum induction melting. Arc melting has
been the most popular technique for melting . HEAs as the temperatures that can be achieved
during arc melting are high (close to about 3000 c ), which is sufficient to melt most of the
metals used for making HEAs. However, the disadvantage of this technique is the possibility
of evaporation of certain low-boiling point elements during the alloy preparation thus making
compositional control more difficult. In such cases, induction and resistance heating furnaces
have been adopted for making the alloys.
Figure 2.11 The number of papers published on HEAs that were produced by different
processing routes[1]

18. 18
One of the constraints faced in the melting and casting route is the heterogeneous
microstructure developed due to various segregation mechanisms caused by the slow rate of
solidification. The typical solidification microstructure of the HEAs produced by arc melting
and casting is dendritic (DR) in nature with interdendritic (ID) segregation.
This demonstrates that faster cooling suppresses the precipitation of secondary phases leading
to the formation of predominantly single phase alloys. Among the melting and casting
techniques, those that lead to faster solidification rates such as splat quenching, melt spinning,
injection casting, suction casting, and drop casting have also shown similar microstructures
with predominantly single-phase microstructures. This brings an important point to focus
whether the single-phase structures obtained in some of the HEAs are kinetically favoured or
thermodynamically stabilized.
Laser-Engineered Net Shaping (LENS) is the technology of rapid prototyping can fabricate
HEAs in bulk form directly by injecting metal powders into the area focused with high-
powered laser beam. This technology was developed by Sandia National Laboratories for
manufacturing solid metallic components from powder using a high powered laser with a help
of computer-aided design (CAD) model Figure 2.12 shows a schematic of LENS technology.
In this technique, the metal powder is fed through a deposition head placed coaxially to a
focused laser beam. The XY table and the deposition head move with a number of degrees of
freedom in order to generate the component with the required shape and size. An inert gas is
used as a shield to prevent oxidation of the powder and the melt pool formed
Figure 2.12 Schematic diagram depicting the LENS technique [2]

19. 19
during the process. In developing HEAs, this technique has been used to produce gradient
HEA rods layer by layer with changed compositions. For example, Al content can be varied
from 0 to 3 segmentally in a grown AlxCoCrCuFeNi alloy rod Similarly, other elements
could be varied to produce segmentally gradient rods.
2.5.2 Powder Metallurgy Route
In this method the solid state processing method is used for synthesis of high entropy alloys.
2.5.2.1 Solid State Processing Route
A small fraction of about 5% of the reports on HEAs so far deal with synthesis of HEAs by
solid-state processing, which involves MA of the elemental blends followed by consolidation.
MA is a process of high-energy ball milling of elemental powder blends, which involves
diffusion of species into each other in order to obtain a homogeneous alloy. This technique
was first developed by Benjamin and his co-workers as a part of the program to produce oxide
dispersion strengthened Ni base super alloys .In 1990, Fecht and his co-workers gave a first
systematic report on the synthesis of nano crystalline metals by high-energy ball milling
Figure 2.13 shows schematically the ball to powder interaction during high-energy ball
milling that involves continuous deformation, fracture, and welding of particles finally
leading to the nano crystallization or even amorphization.
Figure 2.13 Fracture and welding phenomena during the collision of ball and powder particles
during high-energy ball milling [2]

20. 20
MA has been demonstrated over the past four decades as available processing route for the
development of a variety of advanced materials such as nanomaterials, intermetallics,
quasicrystals, amorphous materials, and nanocomposites .
The research group of Murty is the first to develop nano structured HEAs using MA and
demonstrated high thermal stability and good mechanical properties of such alloys. One of the
advantages of MA is its ability to produce excellent homogeneity in the alloy composition.
Each of the nanoparticles obtained by MA is equiatomic in its composition, which has been
confirmed by EDS and atom probe tomography.
These HEAs obtained by powder metallurgy route need to besintered to achieve dense
components. Conventional sintering of nanocrystalline alloy powders can lead to significant
grain growth during the exposure of the alloy powders to high temperatures for long periods.
2.5.3 Deposition Technique-
Figure 2.15 shows that almost 20% of the papers on HEAs reported so far have been obtained
in thin film/coating form by various techniques involving vapour and liquid.
2.5.3.1 HEA and HEA-Based Coatings From Vapour State-
Among the vapour-based surface modifications, two techniques have been quite popular,
namely, magnetron sputtering and plasma nitriding. The attempts by various investigators
were to produce thin films or layers of HEA on the surfaces of substrates such as mild steels,
Al alloys, and HEAs in order to improve corrosion resistance, oxidation resistance, and wear
resistance. Sputter deposition is a standard technique of depositing thin film onto a substrate
by sputtering away atoms from a target under the bombardment of charged gas ions. DC
sputtering shown in Figure 2.14 is the simplest of sputtering techniques wherein a DC bias is
applied between the target and the substrate to aid the deposition. The deposition rates can be
controlled by controlling power, the bias voltage, and the argon pressure. Radio frequency
(RF) sputtering shown in Figure 2.14 is used for sputter deposition of insulating materials. In
DC sputtering, if one attempts to sputter deposit an insulating film, a very high voltage to the
order of1012 V is required. This can be avoided in RF sputter deposition. In case of RF
sputter deposition, the plasma can be maintained at a lower argon pressure than in DC sputter
deposition, and hence fewer gas collisions leading to more lines of sight deposition.

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Figure 2.14 Schematic diagram showing the principle of DC and RF sputtering [2].
In magnetron sputtering, electric and magnetic fields are used to increase the electron path
length, thus leading to higher sputter deposition rates at lower argon pressures. The basic
principle of magnetron sputtering is demonstrated in Figure 2.16 Magnetron sputter
deposition uses both DC and RF for sputtering. Magnetron sputtering has been the most
widely used coating a technique for the HEAs [2]
Similarly, sputtering (both RF and DC magnetron sputtering) of AlCrSiTiV alloy nitrides on
mild steel substrate has shown a hardness of about 30 GPa and the grain size and hardness of
these coatings were found to be quite stable even at 1173 K for 5 hours. Similar results were
observed by Chang et al. (2008) in case of AlCrMoSiTi nitrides have recently developed
Figure 2.15 Schematic diagram showing the principle of magnetron sputtering [2].

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HfNbTaTiZr nitride and carbide coating on Ti6Al4V alloy by DC magnetron sputtering for
biomedical applications. They also observed that these coatings not only have excellent wear
resistance but also have good biocompatibility in simulated body fluids.
Plasma nitriding is not as widely used as magnetron sputtering for making surface hardened
layer for protection. Very few studies have been reported so far on this technique. However,
this technique has been reported to produce thicker layer (50-100 µm) than magnetron
sputtering (1µm). Plasma nitriding of Al0.3CrFe1.5MnNi0.5 alloy has led to the formation of
nitrided surface layer. The nitride layer has been analyzed as a mixture of various nitrides
(AlN, CrN, and (Mn,Fe4N) and having a peak surface hardness around 1300 HV. By pin-on-
disk adhesion wear test with an SKH-51 steel disc, the nitrided samples of HEAs with
different prior processing have higher wear resistance than the un nitrided ones by 49 to 80
times and also than nitrided samples of conventional steels by 22 to 55 times.
2.5.3.2 High entropy alloys and High entropy based coatings from liquid state
Various cladding techniques such as tungsten inert gas (TIG), also known as gas tungsten arc
welding (GTAW), and laser cladding involve melting and casting of the coating material onto
a substrate. The most common substrate for these cladding techniques has been mild steel.
Chen et al. (2008) produced equitomic AlCoCrMoNi alloy coating on low-carbon steel by
TIG cladding. In this technique, the elemental powder blend of chosen alloy is used as filler
material. During the process of TIG cladding, the filler material melts and picks up Fe from
the substrate, and forms a cladded coating containing Fein addition to the original filler
composition. Hsieh et al.(2009)produced AlCrFeMnNi HEA coating by TIG welding process.
In a similar way, deposited AlCoCrFeMoNiSi HEA on low-carbon steel by GTAW. In both
the above cases, the wear resistance of the cladded HEA was significantly higher than that of
the substrate.
Huang et al. (2011) used laser cladding to produce AlCrSiTiV coating on Ti-6Al-4V substrate
and reported that the coating resulted in an improvement in the oxidation resistance of the
alloy at 800 °C. In addition ,the coating also showed improved wear resistance due to the
presence of hard silicides (Ti,V)5Si3 in the HEA coating[2].

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2.5.4 Combinatorial Materials Synthesis
Combinatorial chemistry uses chemical synthesis methods that make it possible to prepare a
large number (up to even millions) of compositions in a single process. Combinatorial
chemistry also includes strategies that allow identification of useful components of the
libraries for such large-scale synthesis.
Over the last two decades, combinatorial chemistry has altered the drug development process
to discover new drugs . By this encouragement, materials scientists can also apply this
methodology to accelerate the discovery of new compounds for high-Tc superconductors,
luminescent materials , catalysts, and polymers (Xiang et al., 1995). They used thin-film
technology to deposit substances sequentially in different amounts layer by layer onto a
gridded substrate and then to mix the elements and create a stable compound by heating. The
physical properties of interest are then measured on each composition to find out the
outstanding composition. Basically under little guidance to predict new materials, this is a
very efficient method to discover new materials in contrast to the conventional one
composition at a time approach, which is time consuming.
2.16 Schematic diagram showing the development of alloy library coupon using
combinatorial materials science [2]

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For the development of multicomponent alloys by this method, the concept involves
development of techniques that can fabricate large number of alloy specimen with continuous
distribution of binary and ternary compositions across the surface, called the “alloy library.”
This technique saves the time, energy, and expense in alloy design and can help the
development of new HEAs with improved properties Figure 2.16 shows a schematic of the
development of alloy library coupon using combinatorial materials science. In Figure 2.16
three controlled geometry thin films are deposited and annealed to develop one coupon with
continuous distribution of elements. This high-through put synthetic route holds great promise
for further development of HEAs.
2.6 Microstructure of high entropy alloys
HEAs processed through a casting route show typical cast microstructure consisting of DR
and ID. DR region is often found to contain microstructural features like precipitates,
nanostructured phases, and modulated structure arising from SD. Elements like Cu and Ag
have been found to segregate in ID region of cast microstructure.
Figure 2.17 Depiction of phase formation sequence during cooling of AlxCoCrCuFeNi alloy
system with different aluminum contents [2]

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Figure 2.18 Bright-field TEM images showing (A) DR and ID regions, (B) DR showing
plate-like precipitates and presence of ordered B2 structure, (C) presence of rhombohedral
precipitates in DR and weak reflections of L12 phase, and (D) microstructure of ID region
and weak super lattice reflections of L12 phase for as-cast AlCoCrCuFeNi alloy[2]
2.7 Properties Of High Entropy Alloy
High entropy alloys has potential of wide range of application due to their better properties,
some of them properties are given below.
2.7.1 Stuctural Properties
HEAs have such promising properties that they are considered as potential candidates for a
wide range of applications such as high temperature, electronic, magnetic, anticorrosion, and
wear-resistant applications. Many of these properties arise out of their unique structural
feature, a multicomponent solid solution. In some cases, HEAs show nanoscale precipitates,
which further enhance some of the properties of these alloys. This chapter deals with various
structural properties of HEAs including mechanical, wear, electrochemical, and oxidation.
2.7.1.1 Mechanical properties
Mechanical properties cover hardness, elastic modulus, yield strength,ultimate strength,
elongation, fatigue, and creep. Structural applications require adequate combinations of these
properties. For high temperature applications, resistance to creep, oxidation and sulfidation
(hot corrosion) are taken into account in the material-selection requirements.

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A. Room temp mechanical properties
By considering one example of HEAs it would be possible to analyse room temp
mechanical properties of HEAs. So let consider the HEA AlxCoCrCuFeNi so by changing the
amount of Al in given HEA ,the hardness vs HEA vs crack length graph is given below
Figure 2.19 Vickers hardness and total crack length around the hardness indent of
AlxCoCrCuFeNi alloy system with different aluminum contents (x values) [2].
B. High temperature mechanical properties
Due to sluggish diffusion effect and second-phase strengthening, HEAsmight exhibit high
strength at elevated temperatures. For example, AlCoxCrFeMo0.5Ni the graph is given below
Figure 2.20 Hot hardness versus temperature plots for AlCoCrFeMo0.5Nix alloys with
varying Ni content [2]

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2.8 Applications of high entropy alloys
In the current state, due to extraordinary properties of high entropy alloys it has various
applications,
Some of them given as follows
1 .HEA Coatings for Antisticky Molds and Solar Cells :
Because HEA coatings easily form amorphous structure with very low roughness, they can
be used for antisticky coating and diffusion-barrier applications.
2. HEA Solders for Welding Hard Metal and Steel:
Because copper-based brazing alloy for welding cemented carbide and steel tends to fail due
to lower strength or excessive corrosion, a HEA brazing filler, for welding cemented carbide
and steel, having excellent strength, toughness and corrosion resistance, spreadability, and
bonding strength
3. HEAs Used as Engine materials: due to better higher elevated-temperature strength,
oxidation resistance, hot corrosion resistance, and creep resistance it can be used as engine
material .
4. HEAs Used as Nuclear materials: due to better improved elevated-temperature strength
and toughness with low irradiation damage.
5. HEAs Used as Tool materials and hard-facing materials: due to better improved room and
elevated-temperature strength and toughness, wear resistance, impact strength, low friction,
corrosion resistance, and oxidation resistance.
6. HEAs Used as Waste incinerators: due to improved elevated-temperature strength, wear
resistance, corrosion resistance, and oxidation resistance.
7. HEAs Used as Chemical plants: due to better improved corrosion resistance, wear
resistance and cavitation resistance for chemical piping systems, pumps, and mixers.
8. HEAs Used as Marine structures: due to better improved corrosion resistance and erosion
in seawater.
9. HEAs Used as Heat-resistant frames for multi floor buildings: due to better higher elevated
temperature strength which could sustain during incidences of fire.
10. HEAs Used as Light transportation materials:due to improved specific strength and
toughness, fatigue strength, creep resistance, and formability

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2.9 General discussion
In the last decade, more than 500 HEA journal and conference papers have been published,
however the understanding of the whole HEA world is still in its born phase. Several future
research trends can be foreseen .
More research on composites of HEAs with ceramic reinforcements and high-entropy ceramic
(HEC) reinforcements is required. Such a combination would generate numerous composites
among which many opportunities could be found for critical applications not easily attained
by traditional composites.
More research on medium-entropy alloys (MEAs) is also required. It is recognized that there
still exists a large space in MEAs.
Assessment of existing database to find possible applications is required.

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CHAPTER 3
CONCLUSION
High entropy alloys and high entropy-related materials have potential applications in different
fields and are expected to replace traditional materials in many sectors. In last few decades
extraordinary progress has been made. The research in field of HEAs has caught global
attention. A bright future is seen.
However more fundamental and basic studies are required. Because materials science and
solid state physics are mainly based on conventional materials with one or two principal
elements, what happens in HEAs would be interesting for better understanding of materials.

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