Microstructural and Mechanical properties

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PROJECT REPORT
SURFACE ENGINEERING
Topic: – Microstructural and Mechanical properties
analysis of Two different High Entropy Alloys (HEA)
coated using Thermal Spray Process
Name – AKSHAY PRAKASHBHAI JOTANI
Enrolment – 102872492
1
INDEX: –
1) Abstract and Summary 2
2) Introduction to thermal spray process 3
• 2.1) Working in detail 4
• 2.2) Classification 4
• 2.3) D-Gun Process 5
• 2.4) Plasma Transferred Arc Process 6
• 2.5) Surface Treatments 6
3) High Entropy Alloys 7
4) Microstructural and mechanical properties of 2
HEA’s coated using Thermal spray process 8
• 4.1 Alloy 1 FeCoCrNiMo0.2 9
• 4.2 Setup parameters 9
• 4.3 Results 10
• 4.4 Mechanical Properties 12
• 4.5 Alloy 2 AlCoCrFeNiTi 13
• 4.6 Setup parameters 14
• 4.7 Results 14
5) Conclusion and Discussion 16
• Alloy 1 FeCoCrNiMo0.2 16
• Alloy 2 AlCoCrFeNiTi 16
6) References 17
2
Abstract and Summary
As we know that in engineering we try to design something which is not only given
100% thought process but also considered it’s all aspects and different possibility. In
engineering when designing something right from its atomic structure to its simulation
in real life is considered as equally important. By doing these we are not only improving
overall quality but also its ability to outperform. There are many aspects of designing
the part in engineering for example geometrical (aesthetic and ergonomics), material
selection, durability etc. now if we see one way there’s lot riding on which material we
select for any application, because it will determine its cost and overall performance.
In this study we will focus on two major aspects of materials. First are new types of
materials which are called High Entropy Alloys and Second is material coating using
the thermal spray process.
Now question arises why not use just a simple material and go ahead with our
manufacturing part. Due to advanced development in materials we are now able to
combine more than 2 materials to customize the material outcome as per our
requirement. Earlier in iron age we developed a technique which taught us to prevent
corrosion and now we can improve specific properties of material as per our
requirement for example if in a material we want higher thermal conductivity and
hardness we can easily achieve such properties. For example, the carbon ceramics
tiles which were used in Space Shuttle to prevent it from burning upon re-entry in to
the earth atmosphere can withstand extreme heat and force, but it was brittle at the
same time.
Changes to the material can be made using 2 major approaches. Internally and
externally. As discussed earlier changes internally altering its chemical composition
and externally it can be made by a technique called thermal spray process. Thermal
spray process is an external process which allows us to develop new layer of material
on top of existing material to improve certain aspects in terms of properties. Thermal
process allows us to externally customise material such as a coating of certain material
can provide ability to withstand higher thermal temperature, protection against
corrosion, increase surface hardness and many more. Basically, it is a clever
technique to customise certain material. For, example if the material is not able to with
stand certain hardness due to its internal chemical composition we can just simply
apply a layer of coating which will be able to withstand certain level of hardness.
Brief explanation is provided below regarding different types of thermal spray
processes and different approaches as per requirement. Also, various techniques are
explained to give better understanding of the spray process.
Further in this study/paper literature review or in other words analysis is done based
on Mechanical and Microstructural Properties of 2 different high entropy alloys coated
using different thermal spray process.
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2. Introduction to Thermal Spray
Thermal spray is defined as the coating or spraying metallic and non-metallic material
on to the prepared substrate in molten or powder form to form a separate layer. The
material which are sprayed can be in any form such as ceramics, powder, rod or wire.
Thermal spray process is divided into many sub-processes but first we need to
understand the basic working.
Fig 1: – basic concept of thermal spray process 4
In the above figure we can observe and understand the basic working. Firstly, the
substrate is prepared (cleaned). The equipment’s required are Gas, Powder or feed
material and spray torch. The (feed)material is deposited or loaded and then passed
through the spray torch in which due to combustion or electrical discharge the material
is melted then streamed towards the substrate with the help of high velocity gas stream
gets deposited. The torch is moved at certain angel to achieve a quality layer. Molten
particles stick onto the surface due to the temperature and in cold spraying due to its
velocity.
Fig 2: – Shows the Plasma Spray Torch from thermal spray family 4
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2.1 Working in Detail: –
It can it easily understood with the help of sub-steps listed below: –
1) Torch which can be considered as the most important part of any process which
helps to generate high velocity stream of jet which includes energy and power
supply
2) Second major player is powder. Some cases powder is to be prepared and
according to the specific grain size and injection into high velocity gas stream,
so it can melt before being deposited.
3) Surroundings also matter in some of the process which means they might need
controlled environment in terms of humidity, specific gases, dust proof etc.
4) Substrate preparation which includes cleaning the surface, holding it position
at certain angle and safe surroundings
5) Lastly controlling the equipment which controls the motion of the torch, it can
be hand held in some cases.
The steps listed above can vary according to the requirement and selected criteria.
Fig 3: – Simple Classification tree of Thermal Spray Processes 4
2.2 Classification: –
Thermal spray processes are classified into several sub-processes which further
divided according to their way of working. First and foremost, they are divided
according to their discharge i.e. combustion and electric discharge. In (1)
combustion process, combustion takes place inside the torch and feed material in
terms of powder or a wire gets melted then it is sprayed with the help of high
velocity stream. (2) in electric discharge process the feed material in terms of
powder or a wire is melted due to high voltage current. An arc is generated due to
which the material melts and then with the help of high velocity stream it gets
deposited.
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Under combustion processes in flame thermal spray process comes and it is one
of the oldest techniques and simplest to understand. The powder is fed through a
simple hopper then with the help of oxy-fuel combustion it generates high
temperature and powder, or wire gets melted and due to its own pressure and
velocity it gets deposited on to the substrate. Secondly HVOF or High Velocity OxyFuel comes in which by using a specially designed nozzle because as the name
suggests high velocity which means the particles speed coming out of the nozzle
is subsonic to supersonic 4. Thirdly D-Gun or detonation gun comes which works
according to its name. A mixture of explosive materials is exploded inside the Dgun and then sprayed through the nozzle and during that powder is heated and
sprayed onto the substrate 4.
In electric discharge method as described earlier electric arc is generated due to
high voltage current. In wire arc the feed material is wire and it is melted with the
help of arc and then deposited to the part. The arc is generated between two
consumable electrodes and gas is included which helps in atomising and
depositing the substrate. In plasma transferred arc the feed material is fed in to the
plasma device and then heated and sprayed into the substrate. In PTA the spraying
operation is performed in horizontal calibration due to its equipment position 4.
In flame spray process it may affect the uppermost layer of the surface and might
change its granular structure which may affect later duration its application. As far
as plasma process go it falls under the same category but it does a slightly better
job because it melts the layer like in welding process by creating a pool from
plasma and then depositing the coating material.
2.3 G-Gun process in Detail: –
Fig 4: – Structural diagram of D-Gun 4
As we can see in the figure above and understand the working of the D-gun is
simple to understand. It was developed in Russia in early 1950’s. In working it is
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like the HVOF, the variation in this process is that a controlled detonation takes
place which includes oxygen combined with fuel and powder coating material. As
we can see that gun is open from one side and closed from the other it’s for a
reason for how the overall process works. Oxygen along with the fuel gas which is
most likely to be acetylene is induced to the barrel as we can see in the figure and
spark plug generates the spark that ignites the mixture which then creates an
explosion. Pressure which is created by that explosion is about 2Mpa and it is
utilized, or it automatically creates a flow towards the open side of the barrel. Along
that powder is fed which heats up and in some cases melts and then gets deposited
to the substrate. This is a cyclic process which is carried out again and again inside
the D-gun at the rate of 4 to 8 times per seconds. Now due to detonation and the
gas velocity the spraying speed is around or can reach 1050 to 3050 m/s. It can
produce very high-quality coatings with smooth surfaces. If one has the
requirement to produce a high-quality coating with smooth surface and properties
such as high hardness, high wear resistance, high corrosion resistance, high
bonding strength along with cohesive strength the D-gun is capable of spraying.
2.4 Plasma-Transferred Arc deposition (Electric discharge)
Fig 5: – Working of Plasma Transferred Arc 4
This process is multi-functional process as far as its working goes. Feed material
is powder and fed into plasma arc which gets heated and then defused to the
substrate. It basically works like welding process as a result it only employs metallic
material as a feeding material. It uses argon gas as plasma forming gas with gas
flow rate of 0.5 g/s and the distance is relatively shorter between the nozzle and
the workpiece. As it works like welding process it creates a pool of molten metal
which is protected by shielding gas and then powder is heated and then melted
with the help of transferred arc. The coating deposition rates depends on the flow
rate of the gas and plasma. PTA can also coat 2 different types of powder such as
1 which melts in the process and other ceramic based which is sprayed as a top
layer which drastically increases its area of application, but the material range is
limited to some specific material. It produces a smooth and uniform layer of coat.
2.5 Surface Treatments: –
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Before starting thermal spray process, it is equally important to do surface
treatment. It includes preparing the substrate cleaning the surface and making sure
it is properly done because if not prepared properly then unwanted material might
get stuck between the coat layer and the layer of the substrate creating a separate
layer which later can be a medium to let the coated surfaces peel away. Many
methods such as strain hardening, surface hardening, thermo chemical etc. are
used. In surface hardening the part is heated to certain austenitic temperature and
rapidly cooled down which helps in hardening the surface of that part hence the
name surface hardening. Other way is to treat the substrate chemically in which
includes methods such as carburizing, nitriding, carbonitriding etc. all these
methods involves forming a layer of carbon which is done in very controlled
environment and surroundings. Chemically treating a material takes a bit longer
then surface hardening.
3. High Entropy alloys
High entropy alloys are defined as the materials with mixture of 5 or more principle
materials. When we look at the conventional materials we know that they have 1
or 2 primary materials present them. If there’s one major element it is called as a
material for example iron. If more than 1 material is present such as in stainless
steel which is chromium and carbon it is called as an alloy of that mother material.
Similarly, high entropy alloy consists of the more than 5 materials with each
containing their own concentration between 5 % to 35 % along with the base
material.
As we know that in conventional materials the microstructure and the chemical
composition is simple and easy to understands because of less entropy but due to
high concentration of the elements present in the HEA’s the entropy increases
because of complex microstructure and chemical composition. The microstructure
and mechanical properties are highly customizable in HEA’s as there are many
elements present. Due to high entropy it generates complex BCC and FCC
structure. Some elements are FCC based yet others are BCC based. With
presence several elements it produces alloys with unique and special mechanical
properties with enrich in high hardness, high strength, extreme wear resistance,
corrosion resistance and many more. While it is easy to produce HEA’s it
automatically widens the area of application due to its properties.
HEA’s are different than conventional materials as a result it has its own unique
effects which are exclusive to HEA’s they are often called “core effects”.
Core Effects: –
• High Entropy Effect
• Sluggish diffusion Effect
• Severe Lattice-distortion Effect
• Cocktail Effect
2 effects are explained below
1) High Entropy effect: –
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It is defined as the effect which helps in the formation of phase during the solid
formation state. HEA’s have greater than normal entropy rate as we know it and
this effect tends to stabilize that phase and helps in overall process.
Fig 6: – XRD diagram of Alloys added one after another to record Structure 5
The concrete evidence of this effect can be proved from the XRD pattern shown
in the figure above. As we can observe that almost all the materials consist of
2 major phase structure which are clearly FCC and BCC with minor phases
which did not appeared during the XRD scan 3.
2) Severe Lattice distortion effect: –
As we know in HEA’s many elements are present which creates a different type
of lattice structure as compared to conventional materials where it contains only
one major or principle elements and it forms uniform and continuous grain or
lattice structure 3, 5. However, in HEA’s some elements atom may be larger
than the other elements alloy which pushes one another upon settlement or
solidification and hence create a distortion in a lattice structure and called as
an effect.
Fig 7: – Lattice structures (a)same size atoms in pattern & (b) Irregular atom
with distorted pattern 3
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In the figure above (a) shows near perfectly aligned atoms which creates a good
uniform pattern and (b) shows distortion due to imperfect atom size which
creates irregular pattern upon solidification and as a result we get distorted
lattice structure.
4 Microstructural and mechanical
properties of 2 HEA’s coated using
Thermal spray process
About: –
Below described are reviewed experiments in which High Entropy alloys were
coated using thermal spray process. The process is described in detail right
from start to finish which includes material preparation to the results discussed
as achieved during the experiment. Both the materials were coated using
different methods such as HVOF (High Velocity Oxy-Fuel) and Air Plasma
Spray.
High Entropy alloy used are: –
• FeCoCrNiMo0.2
• AlCoCrFeNiTi
4.1 Alloy 1 FeCoCrNiMo0.2
4.1 Introduction: –
The coating material of FeCoCrNiMo0.2 was prepared by gas atomization
process and further it was coated using the High Velocity Oxy-fuel (HVOF) and
Air Plasma Spray (APS). All the microstructural and mechanical properties were
investigated in detail such as phase formation, wear resistance (mechanical)
and many other. Also, the comparison of both the spray process is described
in detail. In shorter version the wear resistance in APS technique was noted a
notch higher than the HVOF process. But it was noted that both processes can
affect the microstructure due to their extreme temperature thus affecting all the
other properties such as oxidation, mechanical and chemical.
4.2 Setup and Parameters: –
Powder preparation was done using high quality materials with purity of 99.9%
FeCoCrNiMo0.2. The material was melted using nearly pure argon and the
parameters were Gas flow rate was 0.25 m^3/s, atomization pressure 4 MPa
and metal flow rate was at 50 g/s 2. The powder was formed using a regular
technique with melted droplets dropping directly into atomization chamber and
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getting solidified on cooling. The spherical shaped powder particles were of
certain range between 15 to 45 μm 2. The oxygen content was calculated
using the oxygen/nitrogen determinator.
The substrate of 100mm X 30mm X 20mm was pre-cut and polished using the
sand paper to 140 grit level 2. Steel was used as a primary material for this
experiment. Layer of coating was coated using the APS and HVOF as stated
earlier. The exact parameters used are shown in image below.
Fig 8: – Setup parameters for HVOF and APS 2
4.3 Results: –
Feedstock powder was noted to be spherical shaped as it was manufactured
during the gas atomization process. Characteristics such as satellite formation
on that spherical shaped elements were due to rapid cooling during the
solidifying process. Other properties were nominal to the previous results.
Fig 9: – XRD pattern of Gas atomization process and coating along with
oxides Fe3O4, Fe2O3 and AB2O4 5
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As we can observe from the XRD graph above we can clearly that both the
coatings were primarily formed based on FCC. In both the processes slight
oxidation might have occurred in the feedstock powder. As we know from
previous studies that HEA’s displays excellent properties in terms of thermal
conductivity and stability which is the result of the high entropy effect and
sluggish effect which are 2 of the core effects of the HEA’s. As stated earlier
the feedstock powder was subjected to oxidation during gas atomization
process it was reported that it was even more affected by that and due to rapidly
cooling droplets of molten metals. Later it was also observed that it was different
reaction in both the methods. For example, in APS oxidation was more as
compared to HVOF and it affected the grain structure while the graph shows
sudden rise at certain level which clearly states generating a good granular
structure.
Fig 10: – image from SEM containing low and high magnification of (a)&(b)
APS and (c)&(d) HVOF coating, along with pore, (A)HEA & (B)Oxide phase
2
AS we can observe in the figure above where both the materials are displaying
a fine and uniform lamellar structure. In APS thermal spraying process, it was
observed that the structure had more defects because of previous oxidation
effect which was observed during the gas atomization process. This resulted
into the structural defects such as Splats, pores and cracks. During the
operation due to extreme (>10,000C) in APS it is possible that the feed powder
was completely melted and resulted in high oxidation rate before getting
deposited onto the substrate. While in HVOF temperature is relatively low at
about 3000 C and the powder is accelerated at high speeds 190 to 1100 m/s
2. Feedstock powder which has been through such temperature and velocity
may not be melted completely or just partially melted which again results into
low oxidation when compared to one another. In images c & d it can be
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observed and understood based on the oxidation process which might have
occurred during the travel process from spray gun to substrate it produces
microstructural defects such as interlamellar gaps, pores, splats and not
creating enough lamellar structures.
4.4 Mechanical Properties: –
The microhardness was tested to determine the hardness of the coating. The
coating coated using the APS thermal spray process exhibited the average of
356 HV (0.2) and the coating coated using the HVOF process shown 390 HV
(0.2) 2
Fig 11: – Microhardness of both the coatings 2
As we can observe and understand from the graph above which is of
microhardness to the distance of interface we can clearly see that the hardness
of the APS process varies more than the HVOF process in comparison. Due to
the oxidation phase that occurred during the preparation of powder through gas
atomization process it resulted into the varying result of the microhardness test.
Because of that phase it formed the non-uniformed structure with all the defects
that stated earlier such as pores, gaps etc. while other study shows that coating
coated from APS should have much higher HV(hardness) value. However,
many studies also state that it degrades overtime and decreases its overall
value. In HVOF due to high velocity spray and a hospitable temperature it
produces the coating with lesser defects on oxidation and forms a uniform
lamellar structure which holds up uniformly in the hardness test.
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Fig 12: – Result after wear in friction test with different duration (a&b) Steel,
(c-e) APS and (f-h) HVOF 2
Scratch test was performed during this experiment and it resulted into a fine
data analysis. The image/figure above clearly shows worn out surfaces in both
the spraying methods in 3 different stages in terms of time interval of 3, 5 and
15 min. In APS scratch test the layer of oxide formed and acted as an adhesive
particle which only grew on the 5 and 15 min test. Specifically, it was observed
that in 15min scratch test the layer of oxide formed a crack which later became
chip and flew of the original position which resulted into relatively large amount
of pit like structure and produces a huge derby. While in HVOF only small
amount of oxides were formed due to friction and several cracks were observed
within but it showed noticeable amount of fight against the scratch.
4.5 Alloy 2 AlCoCrFeNiTi
4.6 Introduction: –
It is inevitable to avoid the fact that aluminium and titanium are used in
application where weight matters along with their superior mechanical and
physical properties. Thus, the study of special alloy AlCoCrFeNiTi was
undertaken to create an experiment which later resulted in producing noticeable
results such as high hardness, high wear resistance, high ductility, high strength
and many other qualities. The coating was coated using the HVOF (high
velocity oxy-fuel) thermal spraying process due to its capability of producing
denser coats at relatively lower temperature. Hardness test along with the
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scratch was performed in which the material exceeded the expected results and
performed well.
4.7 Setup and parameters: –
Proposed material was prepared by melting the entire batch so that it can create
a homogeneous mixture and produced to be the feed stock material from gas
atomization with using 99.9 % pure argon gas to avoid any foreign particle. The
particle size was kept at -60+20 μm. Before coating the substrate was cleaned
with Ethanol. The substrate with having the diameter of 40 mm was used and
the particles were blasted sizing -600+425 μm at 3.5 bar pressure from an angle
of 60 degrees and 100 mm distance from actual substrate. 1
HVOF parameters was such as Oxygen was 810 L/min, nitrogen 11L/min,
powder feed rate was 80 g/min, no. of layers 9 and spray distance was 360mm
from the substrate. 3 wear tests were done 1) ball on disk test (20N), 2)
oscillating wear test (26N) and 3) scratch test (1-200N). 1
4.8 Results: –
Feedstock powder particles were noted to be spherical in shape and without
satellite structure. Particles sizing 20 μm were observed during this with
homogeneous mixture of elements.
Fig 13: – XRD pattern of powder (a) full & (b) zoomed view of peak of BCC 1
Surprisingly in this thermal spray process a BCC based structured was formed
and with furthermore investigation of the peak it was known that it produces
some fine grained and coarse particles. This resulted into significantly changing
of the BCC due to low intensity radiation which reacted during the formation.
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Fig 14: – SEM images of coating (a) Overview (b) detailed view with (I)spray,
(II)oxides and (III)pores 1
As we can observe from the image above which clearly shows that structure or
layer of coat formed during the spraying process was homogeneous. Also, we
know from the previous experiment with ALLOY 1 that oxidation during the
HVOF process is significantly less as compared to APS. In this experiment the
oxidation occurred less, and the coating was lamellar lattice structure and less
defects. However, the pores and lamellar oxides are still visible in the image
above as (II) & (III). 1
Fig 15: – (a)Ball-a-disk, (b)oscillating and (c) scratch test with (I)chrome plated
& (II) HVOF Coating 1
In all 3 tests the material revealed significant results as compared to APS
thermal spray process. In ball disk test it revealed that coating was productive
and showed very little wear depth in HVOF as compared to hard chromed
material. In oscillating wear test the results are very similarly as we can see
from the bar graph above. Lastly in scratch test the depth of wear was
measured little bit lower.
Fig 16: – (a)progressive load scratch (b) wear depth profile 1
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The coating coated using the HVOF process produced fine layer of coat as a
result it only had minor cracks as expected due to no oxidation and lower
temperature upon spraying. Above in the wear depth VS scratch length diagram
we can observe that the wear is continuous along with the scratch length.
5 Conclusion and Discussion: – (Both the Alloys)
The experiments which were conducted were exceptional in terms of data
which we got after conducting the experiment. Both the alloys experiment
provided tons of data to study and discuss such as their microstructural and
mechanical properties.
5.1 Alloy 1 FeCoCrNiMo0.2
For alloy 1 FeCoCrNiMo0.2 coating was done with 2 different methods APS
and HVOF. On both the methods APS and HVOF the hardness of 356.4 and
390.09 HV0.2 was achieved with microstructure being mostly homogeneous in
HVOF and lamellar in both the processes.
In both the FCC structure was dominant. Though the oxide formation was
heavily noted in the APS method which later created cracks during the scratch
test and formed derbies. While in HVOF due to comparatively lower
temperature the oxidation was far less and exhibited good hardness against
scratch test. As a result, the coating from APS got value of 3.9 × 10-5 mm3/ N·m
and HVOF of 4.8 × 10-4 mm3/N·m.
5.2 Alloy 2 AlCoCrFeNiTi
For alloy 2 the coating was only done with the HVOF method. The
microhardness of 730 ± 82 HV 0.1 was determined. The structure was formed
homogeneous and lamellar 1.
Surprisingly in this experiment BCC structure was formed. In terms of
microstructure a fine grained and less defective structure was formed with
defects being pores, gaps and splats.
Lastly 3 tests were performed to determine harness 1) ball on disk, 2)
Oscillating wear test and 3) scratch test. In all 3 tests it out performed and
exhibited good mechanical properties.
Both the alloys FeCoCrNiMo0.2 & AlCoCrFeNiTi displayed significant
microstructural and mechanical properties with High strength, high wear
resistance, high corrosion resistance and many more. Which proves that High
Entropy alloys exhibits far better properties which are also customizable.
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REFERANCES: –
1.1.1 1 Microstructure and Wear Resistance of AlCoCrFeNiTi HighEntropy Alloy Coatings Produced by HVOF
M. Löbel
ISSN: 2079-6412 , 2079-6412; DOI: 10.3390/coatings7090144
Coatings , 2017, Vol.7(9), p.144
1.1.2
2Microstructure and Wear Behavior of FeCoCrNiMo0.2 High Entropy Coatings
Prepared by Air Plasma Spray and the High Velocity Oxy-Fuel Spray Processes
Li, Tianchen; Liu, Yong; Liu, Bin; Guo, Wenmin; Xu, Liyou
ISSN: 2079-6412 , 2079-6412; DOI: 10.3390/coatings7090151
Coatings , 2017, Vol.7(9), p.151
1.1.3 3 High-Entropy Alloys: A Critical Review
Tsai, Ming-Hung; Yeh, Jien-Wei
ISSN: 2166-3831 , 2166-3831; DOI: 10.1080/21663831.2014.912690
4 Materials research letters. , 2014, Vol.2(3), p.107-123
1.1.4 Thermal spray fundamentals: From powder to part
Maher I Boulos
ISBN: 9780387689913 , 9780387689913; DOI: 10.1007/978-0-387-68991-3
Thermal Spray Fundamentals From Powder to Part / , 2014, p.1-1566
5High-Entropy Alloys: A Critical Review
Ming-Hung Tsai & Jien-Wei Yeh
To cite this article: Ming-Hung Tsai & Jien-Wei Yeh (2014) High-Entropy Alloys: A Critical Review,
Materials Research Letters, 2:3, 107-123, DOI: 10.1080/21663831.2014.912690
Other minor references: –
1 Recent progress in antireflection and self-cleaning
technology – From surface engineering to
functional surfaces
Lin Yao, Junhui He Article history:Received 6 March 2013, Received in revised form 30 August 2013,Accepted 17 September
2013, Available online 14 December 2013
1.1.5
2 The microstructure and strengthening mechanism of thermal spray coating
NixCo0.6Fe0.2CrySizAlTi0.2 high-entropy alloys
Wang,
LM;
Chen,
CC;
Yeh,
JW;
Ke,
ST
ISSN: 0254-0584
,
1879-
3312; DOI: 10.1016/j.matchemphys.2010.12.022

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