Advanced Materials Research Vols. 97-101 (2010) pp 1408-1411
© (2010) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/AMR.97-101.1408
Online: 2010-03-02
Synthesis and Characterization of NiCoFeCrAl3 High Entropy
Alloy Coating by Laser Cladding
Hui Zhang1,2,a, Yizhu He2,b*, Ye PAN 1,c, Yinsheng He3,d and Keesam Shin3,e
1
2
Jiangsu Key Lab of Advanced Metallic Material, School of Materials Science and Engineering,
Southeast University, Nanjing 211189, Jiangsu, P. R. China
School of Materials Science and Engineering, Anhui University of Technology, Maanshan 243002,
Anhui, P. R. China
3
School of Nano and Advanced Materials Engineering, Changwon National University,
Changwon 641-773, Republic of Korea
a
b
c
d
huizhang@ahut.edu.cn, heyizhu@ahut.edu.cn, panye@seu.edu.cn, hisen@changwon.ar.kr,
e
keesam@changwon.ac.kr
*To whom correspondence should be addressed
Keywords: NiCoFeCrAl3; High entropy alloy; Laser cladding; Coating
Abstract. The NiCoFeCrAl3 high entropy alloy coating with a little addition of C, Si, Mn, Mo has
been succesively synthesized by laser cladding. The results show that simple solution phases of
ordered BCC and a small fraction of FCC are obtained with fine equaixed dendrites morphology.
Because the fine grain strengthening obtained by rapid solidification and the additived small atomic
elements like C, Si further increase the distortion of the solid solution lattice, The microhardness of
the coating reached above 800 HV and is 50 % higher than previous study on the similar composition
by arc melting technique.
Introduction
Traditional alloy systems have been typically based on the use of one principal element as the matrix.
Substantial additions of other elements may be incorporated into these alloys to improve some aspects
of properties, while often result in an obvious composition segregation or a formation of multiple
brittle intermetallic compounds. This restricts the number of alloys that can be studied. Recently, this
paradigm has been broken by high-entropy alloys(HEAs) proposed by Yeh, et al in 2004[1-2]. HEAs
are defined as an alloy that contains at least five principle elements with each elemental concentration
between 5 and 35 at.%. Because of the high mixing entropy, these alloys usually form simple solid
solution like FCC or BCC crystal structure rather than many complex phases after solidification. With
proper composition design, HEAs can possess multiple excellent properties such as high strength,
good ductility, and good resistances to wear, oxidation and corrosion, etc[3-4].
So far, most published papers concerned on HEAs have adopted arc melting technique to obtain
bulk ingots[5-7]. However, this preparation method caused high production cost due to many
precious metals being contained in HEAs. In present work, laser cladding technique is proposed to
prepare high properties of HEAs coating on the surface of low cost iron substrate. It is supposed that
the rapid cooling rate(104-106 ℃/s) in laser molten pool could further improve the mixing entropy of
the HEAs and the forming ability of simple solid solution in the coating. Meanwhile, some previous
studies[8-9] have identified that the addition of Al can obviously improve the mechanical properties
of HEAs due to its large atomic radius. Consequently, NiCoFeCrAl3 composite powders with a little
addition of small atomic C, Si elements were selected as the coating composition for this reasearch.
Experimental procedure
The powders for laser cladding with the mole ratio of NiCoFeCrAl3 were mixed up with pure
element. To enhance the effects of solid solution strengthening, C, Si, Mn, Mo were added in the
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Advanced Materials Research Vols. 97-101
1409
coating with the total amount less than five mole percent. Then the mixed-powders were uniformly
milled and preplaced onto the surface of Q235 steel substrate to form 1.5 mm thick powder bed.
The equipment used for cladding is a 5 kW TJ-HLT5000 type continuous wave CO2 laser system.
The laser cladding parameters were given as follows: 2.0 kW laser power, 4.5 mm beam diameter and
300 mm/min scanning speed. After laser scanning, about 1.2 mm thick coatings together with a thin
layer melted substrate were obtained. Microstructures and phases of the coatings were investigated by
XRD and SEM. The microhardness of the coatings was measured by Vickers hardness tester with a
load of 4.9N and loading time of 30 s. Each clad layer was tested by six points from the interface to the
top with an equal interval.
Results and analysis
30
(200)
(111)
▲
●
40
50
60
▲
(220)
▲
(211)
▲ BCC ordered
● FCC
▲
(100)
Intensity [arb.units]
(110)
Phases of the iCoFeCrAl3 coating. Fig.1 presents the X-ray diffraction spectra of the clad layer
which is sampled in the middle of the coating. It is interesting to note that although the main additived
elements like Ni, Co, Al are all FCC structure, the phase structure of NiCoFeCrAl3 alloy coating is
identified to be simple solid solution based on ordered BCC as main phase and a small fraction of
FCC structure with weak diffraction peaks. This phenomenon is in accordance with those in most
previous works by other prepared techniques concerned on Al element[8], and can be explained by
using the atomic packing efficiency of the FCC and the BCC. As BCC structure (68%) has lower
atomic packing efficiency than FCC structure(74%), the additived large atomic aluminium element
will lead to larger lattice strain and higher lattice distortion energy in the FCC solid solution. To relax
the lattice distortion energy, the metastable FCC phase prefers to transform to relatively stabilized
BCC structure.
●
70
80
90
Degree [2θ]
Fig.1. XRD parttern of NiCoFeCrAl3 clad coating.
Microstructure of the iCoFeCrAl3 coating. Fig. 2 shows the cross section images at the different
areas in the NiCoFeCrAl3 coating. In Fig.2a, there is a 30-40 µm wide white layer of planar
crystallization at the interface, which was hard to corrosion for metallographic examination,
indicating a good combination bonding between the clad layer and the steel substrate[10]. During the
laser cladding process, because the substrate acts as a heat sink, there exists positive gradient at the
beginnning of solidification on the side of liquid phase. Therefore, the planar crystallization is formed
because of high G/R (G is temperature gradient and R is solidification speed of crystal). Subsequently
fine dendrites growing opposite to the heat flux is formed near planar crystal because of sharp
decreasing of G and gradual increasing of V, then fine equiaxed dendrite is observed at the central
region of the coatings in Fig.2b.
Meanwhile, it should be noted that the typical directional growth dendrites in conventional nickle
or cobalt alloy coatings prepared by laser cladding[11] have almost been wholly transformed to fine
equiaxed dendrites in NiCoFeCrAl3 HEAs coating and the width of planar crystallization in
NiCoFeCrAl3 coating is also much wider. It can be explained from the viewpoint of kinetics, a
long-range diffusion for phase separation is sluggish in solid high-entropy alloys with multi-principle
elements. With aluminium, carbon and silicon etc added in the alloy, the mixing of large and small
atomic elements further decreases the substitutional diffusion of elements and slows the rate of crystal
1410
Manufacturing Science and Engineering I
growth, leading to the formation of wide planar crystal and enhancing the transition from columnar to
equiaxed dendrites in the coating.
(a)
(b)
100µm
100µm
Fig.2. The cross section image of NiCoFeCrAl3 clad coating. (a) interface; (b) central region.
Fig.3a presents the SEM image of the equiaxed dendrites at the central region in the coating. As
previously reported and discussed[8], the dendrite region is BCC phase and the FCC phase exists in
the interdendrites. This is well in consistance with the relative peak intensities of the BCC and FCC
phases in the XRD pattern. It looks like the long strip shaped interdendrite network of FCC phase is
not continuous in some local areas, and there are some needle-like finer FCC phase precipitated in the
dendrites, as shown in Fig.3b. The EDS of the arrow marking in Fig.3b are summarized in Table 1. It
shows that the Al and Cr elements are a little segregated and other additived elements are relatively
uniformly distributed in microstructure. This elemental segregation can be explained by the mixing
enthalpies among the principle elements as papers[7,12] suggested.
(a)
(b)
A
B
Fig.3. SEM of NiCoFeCrAl3 clad coating .(a) central region; (b) magnification image.
Table 1 Content of elements at marking area in Fig.3b, in at. %.
Areas
Fe
Ni
Cr
Al
Co
Si
Mn Mo
A
13.53 13.31 8.73 46.32 13.11 1.17 1.04 2.79
B
13.68 12.57 17.54 35.54 15.67 1.20 1.19 2.61
Microhardness of the iCoFeCrAl3 coating. Fig. 4 shows microhardness distribution from top to
the interface of the layer. The microhardness of the coating is about three times compared with that of
Q235 substrate and the hardness profile of the HEA coating is uneven. Due to the higher cooling rate
at the top of the coating, the effects of fine grain and solid solution strengthening are increased, the
microhardness decreased from 810 HV at the top area to 770 HV at the interface of the coating.
Furthermore, the result presents that with a little addition of C, Si, Mn, Mo elements in the HEAs
alloy and with rapid solidification laser cladding technique being utilized, the multiple carbide
compounds were not precipitated in the coating as the previous work[13] demonstrated and the
microhardness of the coating is about 50% higher compared with that of the Li et al work[14] on the
similiar composition of NiCoFeCrAl3 with 506 HV.
Advanced Materials Research Vols. 97-101
1411
Microhardness [HV]
900
800
700
600
500
400
300
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Distance from the coating surface [mm]
Fig.4. Microhardness distribution of NiCoFeCrAl3 clad coating.
Conclusions
1. The NiCoFeCrAl3 high entropy alloy coating with a little addition of C, Si, Mn, Mo elements has
been succesively synthesized by laser cladding, and the simple solid solution phases with fine
equaixed dendrites are obtained.
2. Due to the increasing effects of fine grain and solid solution strengthening, the microhardness of
the coating reach above 800 HV and is 50% higher than that in previous study on the similar
composition by arc melting technique.
Acknowledgments
This work was supported by Youth Teachers Foundation of Anhui Education Department
(2007jq1027). The authors would also like to thank“The Ministry of Science & Technology of
People’s Repubic of China” for financing this INTER-GOVERNMENTAL S&T COOPERATION
PROJECT (Project No. 2002009).
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Manufacturing Science and Engineering I
10.4028/www.scientific.net/AMR.97-101
Synthesis and Characterization of NiCoFeCrAl3 High Entropy Alloy Coating by Laser Cladding
10.4028/www.scientific.net/AMR.97-101.1408
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