U.P.B. Sci. Bull., Series , Vol. , Iss. , 20 1 ISSN 1454 -2331 [622437]

U.P.B. Sci. Bull., Series …, Vol. …, Iss. …, 20 1 ISSN 1454 -2331
RECENT ADVANCES OF N ANOINDENTATION IN HI GH
ENTROPY ALLOYS
Sushil KUMAR1, Satpal SHARMA2
HEAs are equiatomic or non equiatomic alloys which contained five or more
metallic or non -metallic elements without any single base material and highly
investigated in recent years to replace traditional alloys or composite materials
which are currently being in use. The precise calculation of engineering properties
of high entropy alloys on micro scale and nano scale proves nanoindentation an
innovative and efficient to ol. Nanoindentation technique is now broadly adopted for
assessment of mechanical characterization of high entr opy alloys for measurement
nano hardness, young’s modulus, fracture toughness of the equiatomic and non
equiatomic high entropy alloys or high ent ropy nanostructured materials.

Keywords : Nanoindentation; High Entropy Alloys; Mechanical Properties .
1. Introduction and overview of nanoindentation
In the recent years, significant efforts have been made in mechanical
characterization to eva luate the mechanical properties of the high entropy
materials with high precision on small scale or nano scale. Although the idea of
HEAs was reported before 2004, the research was accelerated after 2010 when
Jien-Wei Yeh and Brian Cantor started investiga ting them [1 -2]. The scientific
development in technology now makes it possible to calculate mechanical
properties precisely of homogeneous or heterogeneous engineering materials.
Nano indentation or depth sensing indentation is now becoming very popular
technique for characterization of engineering materials and nanomaterials.
Moreover, nanoindentation can be implemented to measure the fracture toughness
of coatings which is difficult to compute by other conventional techniques [3].The
indenter tip with ve ry precise geometry is penetrating into the test specimen with
a specific load and measurement of load and displacement with an increasing load
up to a specific value. During the process, the load and displacement are recorded
and analyzed to determine the indentation area. Nanoindentation normally does
not require any sample preparation for testing of various types of materials
ranging from hard metals to soft metals.

1 Research Scholar ., Dept. of Mechanical Engineering , Gautam Buddha University , Greater Noida,
201308, India , e-mail: [anonimizat]
2 Faculty. , Dept. of Mechanical Engineering, Gautam Buddha University, Greater Noida, 201308,
India , e-mail: [anonimizat]

Sushil Kumar, Satpal Sharma

Fig. 1 Indentation geometry at maximum load for conical indenter [4]

Scanning prob e microscopy (SPM) and atomic force microscope (AFM) have
been used to study the properties of the nano size materials. The measurement of
nano indentation load and displacement curve generated by indenter is based on
the technique invented by Oliver and P harr [4]. The nanoindentation is very useful
fastest technique of evaluation to establish a relation into crystalline structure and
composition of high entropy alloys by knowing which phase holding high
hardness and reduced modulus.

Fig. 2 Load displace ment curve measured by indentation technique [5]

The nanoindentation technique came into possession from conventional
indentation tests but with advancement of the technology, size of tips was
considerably reduced and the accuracy and resolution of depth were enhanced The
elastic and plastic deformation produced when indenter is pressed into the
specimen during the measurement and analysis of mechanica l properties [5]. The

Recent advances of nanoindentation in high entropy alloys
hardness and reduced modulus can be measured by examining the load –
displacement cur ve. There is three things should be taken into consideration while
performing the nanoindentation. The first is to select the nanoindentation load,
secondly, to analyze the effect of different phases of the test specimen during
nanoindentation test. The la st is that there should be minimum distance in
between two indents to nullify work hardening .

2. Mechanical Behavior – Hardness and Young's Modulus

The nanoindentation technique calculates the hardness and young’s modulus
by employing indenter tip at low load at different temperatures with high accuracy
and precision. The pronounced modulation in the amount of Young’s modulus is
as a result of evolution of additional metallic and intermetallic phases which
contained larger modulus as compared to single -phase crystalline structure [3, 6].
Liu et al [7] examined and measured the mechanical properties of CoCrFeNi and
AlCrFeNiTi high entropy alloy at room temperature and high temperature through
nanoindentation equipped with laser heating system. The evalua tion of
mechanical and microstructural properties of the high entropy alloys were
performed by implementation of continuous stiffness method. Sun et al [8]
evaluated the microstructural properties and phase formation of AlxCoCrCuFeNi
high entropy alloy by varying the percentage of aluminium in HEA matrix by
utilizing nanoindentation technique. It is observed that increase of addition of
aluminium contents significantly improved the hardness of the BCC phase of the
solid solution. Li and Bhushan [9] evaluate d the substrates of thin films which
greatly influenced at high loads and optimize the properties of thin films and
coatings for discrete applications. Kiener et al [10] utilized nanoindentation to
assess mechanical properties of the nanostructured high en tropy alloys of different
grain sizes. The investigation exhibited that grain sizes does not have control the
young’s modulus but largely rely on existence or absence of intermetallic phases
in the matrix of high entropy alloys. Tolstolutskaya et al [11] s tudied the ion
irradiation effect on hardening of heat treated CrFeNiMn high -entropy alloys of
different configurations and evaluated nanohardnes, secondary phases.
The nanoindentation equipped with laser heating system is an efficient
instrument for me asuring the of high -temperature mechanical properties of
heterogeneous engineering materials composed several phases for better
interpretation and practical understanding of the compositional and
microstructural changes in such materials at elevated tempe ratures and their
responses to mechanical deformation. The nanoindentation technique is now
widely considered fast and efficient approach to understand the chemical
compositions, phase stability and configuration details of nanostructured HEAs.
The precise characterization and evaluation of reduced elastic modulus and

Sushil Kumar, Satpal Sharma

nanohardness in each phase by nanoindentation imparts new insights to design
new HEAs with optimum performance.

3. Incipient Plasticity and Dislocation Nucleation

The origin of instrument ed nanoindentation has permitted the analysis of
deformation behavior of crystalline structure, where the nucleation and motion of
dislocations or defects can be ascertained [12 -13]. Sun et al [14] implemented
nanoindentation technique to characterize the deformation on the fcc and bcc
phases of AlCrCuFeNi 2 high entropy alloys. It is observed that at high indentation
loads, the dissipation of plastic energy increased. The outcome analysis
demonstrate that elastic modulus in the fcc phase is higher compared to bcc phase
while implementing a load range from 100 μN to 2000 μN on high entropy alloys.
Fang et al [15] evaluated the deformation performance of Cu 29Zr32Ti15Al5Ni19 high
entropy alloys with spherical indenter. The mechanical properties, shear strain,
surface textur es indentation force and radial distribution function were evaluated
through nanoindentation technique of the high entropy bulk metallic glasses
materials. Results revealed that atomic size difference provides better
understanding of amorphous formation ab ilities and mechanical properties of high
entropy bulk metallic glass materials. Jiao et al [16] evaluated the plastic
deformations of high entropy alloy Al0 .5CoCrFeNi at different strain rates by
nanoindentation technique at room temperature. The results demonstrate that at
different strain rates contact stiffness and elastic modulus does not change but
hardness decreases due to increase in indentation depth and size of the indenter.
The results demonstrated serrated behavior due to indentation rate and h igh
localized plastic deformation observed during nanoindentation. Li et al [17]
utilized spherical rigid indenter to study the both elastic and plastic deformations
of indentation in FeCrCuAlNi high -entropy. The effects of shear strain,
indentation force , radial distribution function, load displacement relationship,
severe lattice distortion on the deformation processes were evaluated.
Nanoindentation results show that addition of equal amount of element can
significantly enhance the mechanical properties of high entropy alloy as compared
to traditional alloy. Low stacking fault energy and the dense atomic arrangement
are responsible for improved mechanical and microstructural properties of the
high entropy alloy. Muthupandi et al [18] examined the nanoind entation behavior
and microstructures of annealed AlCoCrFeNi high entropy alloy. Electron
microscopy revealed different nanoindentation behaviors are due to the presence
of multiple phases and pile -up and sink -in characteristics in the grain boundary
and g rain regions. The major dislocation activities observed under the pile up and
minor dislocation activities were found to under the sink in or confined to indenter
tip. The susceptibility to elastic and plastic deformation for every phase of the

Recent advances of nanoindentation in high entropy alloys
AlCoCrFeNi HEA was studied at different hardness -to-modulus ratio. The study
susceptibility of plasticity can be proved a useful technique in finding the pile -up
and sink -in characteristics of the other high entropy alloys. Pi et al [19] used
continuous stiffness mea surement mode of the nanoindentation to evaluate the
Cu 29Zr32Ti15Al5Ni19 high entropy metallic glass. A good combination of excellent
plasticity and homogeneity observed in glassy Cu 29Zr32Ti15Al5Ni19 high entropy
alloys. The creep was observed at constant load at room temperature and mean
values of nanohardness and modulus were found 7.45 GPa and 105.4 GPa,
respectively.
The new mechanisms of dislocation and their propagation in new combinations of
high entropy alloys and high entropy composites are yet to be known.
Nanoindentation testing can enhance understanding in the deformation related
mechanisms with their effects on mechanical properties of the high entropy alloys.
The implementation of simulation techniques and their comparison with different
exper imental models may discover the outstanding mechanical properties of high
entropy alloys.

4. Fracture Toughness and Creep Behaviour

It is challenging to precisely evaluate the fracture toughness, cracks
propagation of brittle metals or alloys at a mi cro range. Repeated and accurately
measurements of micro/nano properties makes nanoindentation a unique and
efficient tool for nano fracture toughness for quality control and R&D of
advanced materials. Ma et al [20] studied nanoindentation creep behaviors of a
CoCrFeCuNi HEA deposited and annealed films synthesized by magnetron
sputtering were investigated with a spherical tip. The calculation of strain rate
sensitivity was obtained from steady -state creep and the creep deformation. Study
revealed that int ernal crystalline structure and loading rate create a difference in
the creep behavior. Wang et al [21] studied internal mechanism and behavior of
the crossover in the initial creep stage during nanoindentation of CoFeNi high
entropy alloy. The stress and holding time explained the different mechanism of
the crossover before and after the crossover point. The analysis of attributes and
conduct of the crossover point can be a useful technique to evaluate the creep rate
in different engineering materials. Ma et al [22] investigated creep behavior of
CoCrFeNiCu high entropy alloy films composed of fcc and bcc structures was
investigated at room temperature by nanoindentation technique. The results
showed that creep deformation of high entropy films can be impro ved by
accelerating the loading rate. The activation volume, dislocation nucleation and
strain rate sensitivity of nanostructured HEA films were evaluated. At different
loads, the creep behavior of Ti 16.7Zr16.7Hf16.7Cu 16.7Ni16.7Be 16.7 was evaluated by
nano indentation technique and outcomes were compared with different high

Sushil Kumar, Satpal Sharma

entropy alloys. Study revealed that high entropy bulk metallic glass materials
contain small strain rate leads to good creep resistance. Kelvin model was adopted
to explain the creep curv es, amorphous structure and complex configuration of
high entropy bulk metallic glass [23]. Wang et al [24] measured notch fracture
toughness of arc -melted TiZrNbTaMo high -entropy alloys at room temperature.
The analysis showed that the increase of Mo conc entration in HEAs results in an
appreciable reduction in toughness. Gong et al [25] evaluated creep behavior of
Ti20Zr20Hf20Be 20Cu 20Ni10 high entropy bulk metallic glasses at different loading
rates by nanoindentation technique. Kelvin model was adopted to explain the
experimental creep curves by the use of strain rate sensitivity, retardation spectra
and creep compliance. Replacement of Cu with Ni, microstructure gets denser
which improves the hardness and Young’s modulus. After the study of related
mecha nism and the pronounced high entropy effect, experimental results
demonstrated that addition of Ni effectively boost the creep resistance of high
entropy alloy.
The studies provide new insights into the understanding the observed
creep characteristics in HEA, HEA films, distinct lattice structures, kinetics of
plastic deformation in HEA at the nanoscale. Not much work has been reported
on fracture and creep allied effects like oxidation, irradiation on the properties
high entropy alloys which are yet to b e explored.

5. Scratch Test, Coefficient of Friction and Wear Behavior

Nanoindentation scratch tests were executed to know the adhesion of thin
films and coatings under a ramping load and the point of failure to know the
surface wear properties [26 -27]. The Nanoindentation and nanoscratch techniques
calculates applied load and depth of indentation or scratch cycle to discover the
coefficient of friction of films and coatings of an engineering material which is
important to measures its tribological per formance [28 -29]. Varughese et al [30]
effectively used nanoindentation in scratch testing by moving the test specimen
relative to the indenter tip. The coefficient of friction evaluated by measuring
lateral force and normal force. The adhesion of the coa tings is efficiently
evaluated by nano scratch test by scratch by applying constant load or glide with
respect to the sliding distance. Friction and wear behavior of TiZrHfNb high
entropy alloy were investigated by Nanoscratch technique under both ramping and
constant load. The coefficient of friction (COF) reduced rapidly when the normal
load increases in elastic regime. The applied load is proportional to the wear rate
of the TiZrHfNb high entropy alloy and wears resistance scales linearly.
TiZrHfNb high entropy alloy exhibit improved hardness/strength and wear
resistance on ramping and constant load modes. Results demonstrated that
TiZrHfNb high entropy alloy with low coefficient of friction and high wear

Recent advances of nanoindentation in high entropy alloys
resistance can be utilized for tribological applic ations [31] Nanoindentation
technique was implemented on the Al 0.5CoCrCuFeNi high entropy alloy to
analyze serration behavior and creep characteristics at two separate temperatures.
The interaction of active dislocations and obstacles produced serrated flo w at
different temperatures. The creep was observed during the holding period and
underneath the indenter due to pronounced dislocation activities [32].
The few reported studies and analysis of friction and wear behavior of HEA
showed enhanced wear result s and it will be interesting to investigate in the field
of nanostructured high entropy alloys and high entropy nanocomposites at
different loads and temperature ranges.

6. Conclusions

Due to remarkable and excellent results, the conventional indentat ion methods
are now completely replaced by nanoindentation in many areas in last few years.
Recent developments in high entropy alloys, nanomaterials science and
nanotechnology make nanoindentation an efficient tool. The Nanoindentation
technique provides the more useful information regarding properties of
homogenous and heterogeneous materials and subjected to intensive research
which recently extended to nanostructured materials. Recent advancement in
testing tools and high temperature nanoindentation pro ves it as a novel technique
for investigation of advanced high entropy alloys and high entropy nanomaterials.

Sushil Kumar, Satpal Sharma

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