metals
Article
The Phase Composition and Microstructure of
AlxCoCrFeNiTi Alloys for the Development of
High-Entropy Alloy Systems
Thomas Lindner *, Martin Löbel, Thomas Mehner, Dagmar Dietrich and Thomas Lampke
Materials and Surface Engineering Group, Institute of Materials Science and Engineering,
Technische Universität Chemnitz, D-09125 Chemnitz, Germany; [email protected] (M.L.);
[email protected] (T.M.); [email protected] (D.D.);
[email protected] (T.L.)
* Correspondence: [email protected]; Tel.: +49-371-531-38287
Academic Editor: Michael C. Gao
Received: 3 April 2017; Accepted: 4 May 2017; Published: 9 May 2017
Abstract: Alloying aluminum offers the possibility of creating low-density high-entropy alloys
(HEAs). Several studies that focus on the system AlCoCrFeNiTi differ in their phase determination.
The effect of aluminum on the phase composition and microstructure of the compositionally complex
alloy (CCA) system Alx CoCrFeNiTi was studied with variation in aluminum content (molar ratios
x = 0.2, 0.8, and 1.5). The chemical composition and elemental segregation was measured for the
different domains in the microstructure. The crystal structure was determined using X-ray diffraction
(XRD) analysis. To identify the spatial distribution of the phases found with XRD, phase mapping with
associated orientation distribution was performed using electron backscatter diffraction. This made
it possible to correlate the chemical and structural conditions of the phases. The phase formation
strongly depends on the aluminum content. Two different body-centered cubic (bcc) phases were
found. Texture analysis proved the presence of a face-centered cubic (fcc) phase for all aluminum
amounts. The hard η-(Ni, Co)3 Ti phase in the x = 0.2 alloy was detected via metallographic
investigation and confirmed via electron backscatter diffraction. Additionally, a centered cluster (cc)
with the A12 structure type was detected in the x = 0.2 and 0.8 alloys. The correlation of structural
and chemical properties as well as microstructure formation contribute to a better understanding of
the alloying effects concerning the aluminum content in CCAs. Especially in the context of current
developments in lightweight high-entropy alloys (HEAs), the presented results provide an approach
to the development of new alloy systems.
Keywords: HEA; high-entropy alloy; CCA; compositionally complex alloy; phase composition;
microstructure
1. Introduction
Previous material research and the development of alloy systems have been focused on
compositions with one main element providing the primary characteristics. Until now, the improvement
of these properties has mainly been achieved by alloying small amounts of other elements. A relatively
new approach is the study of multicomponent alloy systems with nearly equimolar amounts of
elements. Since the first publications in 2004, research on HEA systems, also designated as CCA,
obtains an increasing scientific interest [1,2]. HEA refers to single-phase alloys, while CCA indicates at
least two phases in the microstructure [3]. The fact that a minimum number of five different elements
can build a simple cubic crystal structure with promising material properties is the main cause of the
great number of development efforts [4]. The possibilities of alloy variations are almost unlimited.
Metals 2017, 7, 162; doi:10.3390/met7050162
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Cantor et al. were the first to develop a single fcc phase alloy system consisting of different transition
metals [1]. Further mentioned phases include bcc, hexagonal closed packed (hcp), and mixed solid
solution structures, as well as different intermetallic phases. Generally, element ratios of 5–35 atom %,
but not over 50 atom %, are characteristic of the composition of HEAs [4]. The AlCoCrCuFeNi system
and the copper-free AlCoCrFeNi system are the most studied HEA systems [5–7]. Based on these alloy
systems, different modifications have been investigated [8–11]. Hume-Rothery et al. [12] pointed out a
tendency for a definite crystal structure at a particular electron concentration. Guo et al. [13] used the
valence electron concentration (VEC) for the prediction of a preferred crystal structure in HEA where
VEC < 6.87 (bcc), VEC = 6.87–8.0 (bcc+fcc), and VEC > 8.0 (fcc). Zhu et al. [14] shifted the bcc+fcc
region to VEC = 7.3–8.1. Based on the molar ratio, the VEC is calculated according to Equation (1),
with the atomic concentration ci and the valence electron concentration (VEC)i for alloying element i:
n
VEC =
∑ ci (VEC)i
(1)
i =1
The investigation of the influence of different alloying elements on phase composition is another
area of interest. For example, a titanium addition has a large strengthening effect and usually favors the
formation of intermetallic phases due to its large negative enthalpy while mixing with other elements
in the composition. A variation in aluminum content has a significant effect on the microstructure
and physical properties [15]. The initial approach that HEAs form a simple solid solution in a bcc
or fcc structure proved to be an exception. The majority of studied HEAs seem to build a complex
structure sensitive to changes in chemical composition [16]. Furthermore, a clear identification of
the different phases with characteristic lattice parameters and chemical compositions provides the
basis for sophisticated alloy development. Wu et al. [17] found a phase in the CoCrCux FeMoNi
system with the space group of I43m, which they attributed to a simple bcc structure with the lattice
parameter of a = 8.90 Å. Feng et al. [18] also found this phase in the aluminum rich Al3–4 CrFeMnTi0.25
system and identified it as an Al8 Cr5 type. It seems that Zhang et al. [15] detected this phase in
the Al0.5–1.0 CoCrFeNiTi system as well but did not assign it as the I43m space group; they assumed
intermetallic phases of CoTi2 and FeTi. Soare et al. [19] found that the equimolar AlCoCrFeNiTi
leads to the formation of two complex phases with D03 and the A12 structure type. The A12
structure is a centered cluster (cc) phase that pertains to the group of topologically close-packed
structures (Frank–Kasper phases) characterized by brittleness and low formability. Chuang et al. [20]
found for Al0.2 Co1.5 CrFeNi1.5 Ti0.5 the presence of a hard η-(Ni, Co)3 Ti phase. Lee et al. [21] studied
Al0.5 CoCrNiTi0.5 and found similar characteristic peaks for the A12 structure type, which he assigned
as D8b . His aging experiments in the temperature range of 773 to 1473 K show that the cc phase was
dissolved after 24 h at 1473 K. Overall, the phase detection and attribution to chemical domains in
the HEA systems are current problems that need further research. A recent comprehensive review on
high-entropy alloys was provided by Gao et al. [22].
This study focuses on the alloy system Alx CoCrFeNiTi with various aluminum content values.
Phase formation and microstructural evolution depending on the change in chemical composition are
studied. The different phase types are clearly identified and lattice parameters are assigned. This will
contribute to a better understanding of the effects of aluminum alloying in HEAs. The criteria of phase
prediction are reviewed regarding their correctness.
2. Materials and Methods
High-purity elements were mixed in the intended molar ratio in order to produce three different
alloys in the Alx CoCrFeNiTi system. Granulates with a chemical purity of over 99.9% were used.
Alloy samples with a mass of 10 g and target compositions according to Table 1 were arc-melted with a
cooling rate of <50 K/s.
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Table 1. Target composition of the three alloys.
Element
Al
Co
Cr
Fe
Ni
Ti
atom %
wt %
x
atom %
wt %
x
atom %
wt %
x
3.9
1.9
0.2
13.8
7.3
0.8
23.1
12.9
1.5
19.2
21.1
1
17.2
20.0
1
15.4
18.8
1
19.2
18.7
1
17.2
17.7
1
15.4
16.6
1
19.2
20.0
1
17.2
18.9
1
15.4
17.8
1
19.2
21.1
1
17.2
19.9
1
15.4
18.7
1
19.2
17.2
1
17.2
16.2
1
15.4
15.3
1
The furnace was flooded with argon after reaching a pressure of 2 × 10−4 mbar. After the arc
was ignited with a tungsten electrode, the high purity elements were cast into a copper crucible with
a diameter of about 20 mm. The arc-furnace parameters are shown in Table 2. The samples were
remolded three times in order to achieve a homogeneous element distribution.
Table 2. Arc-furnace parameters.
Mean
Quantity (g)
Droplet Size
(mm)
Vacuum Pressure
(mbar)
Argon
Pressure (bar)
Gas Purity
(%)
Current
(A)
10
20
2 × 10−4
1.1
99.9999
250
The heat flow during solidification was recorded in a temperature range from 1800 K to
room temperature by differential scanning calorimetry (DSC) STA 409 C (Netzsch, Selb, Germany)
with a cooling rate of 20 K/min. Metallographic cross sections of the cast samples were
prepared according to standard metallographic procedures by hot mounting in conductive resin,
grinding, and polishing. The microstructure and the averaged as well as the phase selective
chemical composition were investigated by backscattered electron imaging (BSD) in conjunction
with energy-dispersive X-ray spectroscopy (EDS) measurement in a scanning electron microscope
LEO 1455VP (Zeiss, Jena, Germany) equipped with an EDS spectrometer (GENESIS, EDAX, Mahwah,
NJ, USA). The crystallographic studies were performed on all samples by X-ray diffraction (XRD)
using a D8 DISCOVER diffractometer from Bruker AXS (Billerica, MA, USA). Co Kα radiation was
used with a tube voltage of 40 kV, a tube current of 40 mA, polycap optics for beam shaping, a 1 mm
pinhole collimator, and a 1D Lynxeye XE detector. The diffraction diagrams were measured in the
diffraction angle (2θ) range from 20◦ to 129◦ with a step size of 0.01◦ , and 1.5 s/step, which corresponds
to 288 s/step due to the use of the 1D detector. The powder diffraction file (PDF) database 2014
was applied for phase identification. Texture measurements were performed utilizing a D5000
diffractometer (Bruker AXS, Billerica, MA, USA) with Co Kα radiation (tube parameters: 35 kV, 30 mA)
and a 1 mm pinhole collimator using a scintillation counter. The diffraction angle was set to 30.5◦
with tilt angles of 0◦ –80◦ and step sizes of 5◦ for both tilt and rotation angles with 2 s/step. For phase
localization, the samples were studied by electron backscatter diffraction (EBSD) using a field emission
scanning electron microscope (Zeiss NEON 40EsB, Zeiss, Jena, Germany) equipped with an EBSD
system (OIM 5.2, EDAX TSL, Mahwah, NJ, USA) operated at 15 kV in the high current mode with a
60 µm aperture. The step size was 200 nm in a field of 50 µm × 50 µm. The elemental composition was
recorded by EDS data collection simultaneously with the EBSD data acquisition. The EBSD data were
subjected to a slight cleanup procedure comprising neighbor confidence index (CI) correlation, grain CI
standardization with a minimum confidence index of 0.1, and grain dilatation with one iteration step.
Metals 2017, 7, 162
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3. Results and Discussion
3.1. Chemical Composition of the Cast Samples
The EDS measurements confirm the intended mean chemical composition. Certain deviations can
Metals 2017, 7, 162
be found in
the molar ratio of titanium (Table 3). Aluminum shows good matching, with4 ofx 10= 0.16, 0.83,
and 1.47. 3. Results and Discussion
Table
3. Average
chemical
compositions
of the three sets of samples measured by EDS.
3.1.
Chemical
Composition
of the Cast
Samples
The EDS measurements confirm the intended mean chemical composition. Certain deviations
Al of titanium
Co (Table
Cr3). Aluminum
Fe
Ni goodTi
VEC
can be foundElement
in the molar ratio
shows
matching,
with x = 0.16,
0.83, and 1.47.
atom %
3.0
19.4
19.9
20.0
20.0
17.7
wt %
x
1.5
0.16
21.2
1.0
19.2
1.0
Al
atom
%
3.0
wt %
x1.5
Co
14.3
19.4
7.6
21.2
0.83
17.4 Cr 17.3
20.2 19.9 17.7
1.0 19.2 1.0
20.7
1.0
21.7
1.0
15.7
0.9
17.9Ni
20.0
20.7
1.121.7
16.1 Ti
15.2 17.76.8
0.9 15.7
7.3
Table 3. Average chemical compositions of the three sets of samples measured by EDS.
Element
atom %
wt %
x
atom %
wt %
x
atom %
wt %
VEC was
x
0.16
1.0
atom
22.7
14.3%
17.4
wt7.6
%
12.6
20.2
x
1.47
0.83
1.0
22.7
16.2
12.6
19.7
been repeatedly
1.47
1.1
1.0
16.2 17.3 15.1
19.7 17.7 162
1.1 1.0 1.0
15.1
162
mentioned to
1.0
Fe
17.0
20.0
18.7
20.7
1.0
1.0
15.4
17.0
17.7
18.7
1.0
1.0
15.4
17.7
be an
1.0
1.0
0.9
16.7
17.9 13.8 16.1
20.2
20.7 13.7 15.26.5
1.11.1 0.9 0.9
16.7
13.8
20.2
13.7
effective criterion for
1.1
0.9
VEC
7.3
6.8
6.5
The
predicting phase
constitution in HEA/CCA. The calculated valence electron concentration according to the chemical
The VEC was in
been
repeatedly
to be
effective
criterion
forfor
predicting
composition summarized
Table
3 is 7.3mentioned
for x = 0.2,
6.8anfor
x = 0.8,
and 6.5
x = 1.5.phase
In accordance
constitution
HEA/CCA.
The
electron concentration
accordingwhile
to the chemical
with previous
studiesinfor
both x =
0.8calculated
and 1.5,valence
a bcc structure
can be predicted,
x = 0.2 fulfills the
composition summarized in Table 3 is 7.3 for x = 0.2, 6.8 for x = 0.8, and 6.5 for x = 1.5. In accordance
criterion given
by Guo et al. for a mixed bcc+fcc structure [13] and the criterion given by Zhu et al. for
with previous studies for both x = 0.8 and 1.5, a bcc structure can be predicted, while x = 0.2 fulfills
a bcc structure
[14]. given by Guo et al. for a mixed bcc+fcc structure [13] and the criterion given by Zhu et
the criterion
al. for a bcc structure [14].
3.2. Melting Behavior of the Cast Samples
3.2. Melting Behavior of the Cast Samples
Heat flow ←
The cooling curves of heat flow for the different alloys in the temperature range from 800 to
The cooling curves of heat flow for the different alloys in the temperature range from 800 to 1800
1800 K verify
thethe
formation
ofaahigh
high
temperature
phase
with an increasing
aluminum
content value
K verify
formation of
temperature
phase with
an increasing
aluminum content
value (Figure
(Figure 1).1).
x = 1.5
x = 0.8
x = 0.2
0
800
900
1000
1100
1200
1300
1400
1500
1600
1700
1800
Temperature (K)
Figure 1. DSC cooling curves of as-cast AlxCoCrFeNiTi alloy.
Figure 1. DSC cooling curves of as-cast Alx CoCrFeNiTi alloy.
In the alloy system where x = 1.5, the first exothermic reaction with nucleation of the dendrites
of
the
phase
takesxplace
at 1620
K. Theexothermic
second and last
greater peak
the heat flowof
can
be dendrites
the alloyprimary
system
where
= 1.5,
the first
reaction
withinnucleation
the
In
of the primary phase takes place at 1620 K. The second and last greater peak in the heat flow can be
found at 1550 K for x = 1.5. This corresponds to the first solidification of the x = 0.8 alloy. Lowering the
aluminum molar ratio to x = 0.2, the first solidification takes place at 1480 K corresponding with the
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found at 1550 K for x = 1.5. This corresponds to the first solidification of the x = 0.8 alloy. Lowering
the aluminum molar ratio to x = 0.2, the first solidification takes place at 1480 K corresponding with
second
peak
of xof
= x0.8.
TheThe
lastlast
exothermic
peaks
of the
x =x0.2
and
0.80.8
alloys
areare
at 1360
K. K.
In In
case
of
the
second
peak
= 0.8.
exothermic
peaks
of the
= 0.2
and
alloys
at 1360
case
x =x =0.2,
peak with
withaasmall
smallmelting
melting
temperature
range,
indicating
a eutectic
reaction.
of
0.2,ititisis aa large peak
temperature
range,
indicating
a eutectic
reaction.
The
The
different
peaks
roughly
indicate
number
formedphases,
phases,but
butsolid
solidstate
state transformations
transformations or
different
peaks
roughly
indicate
thethe
number
of of
formed
formation of precipitations can additionally occur.
occur.
3.3. Microstructure
Microstructure of
of the
3.3.
the Cast
Cast Samples
Samples
The BSD
BSD images
images(Figure
(Figure2)2)confirm
confirm
a heterogeneous
microstructure
of different
the different
alloys.
The
a heterogeneous
microstructure
of the
alloys.
The
The
material
contrast
indicates
different
phases
by
different
gray
levels.
material contrast indicates different phases by different gray levels.
(a)
(b)
(c)
Figure
2. BSD
the cast
cast Al
AlxCoCrFeNiTi
alloy with x = (a) 0.2, (b) 0.8, and (c) 1.5 in overall
Figure 2.
BSD images
images of
of the
x CoCrFeNiTi alloy with x = (a) 0.2, (b) 0.8, and (c) 1.5 in overall
view (left)
(left) and
and in
in detail
detail (right)
(right) with
with phase
phase declaration
declaration (PP:
view
(PP: primary
primary phase;
phase; IP:
IP: interdendritic
interdendritic phase;
phase;
SP:
secondary
phase;
CW:
cell
wall;
IC:
intermetallic
compound).
SP: secondary phase; CW: cell wall; IC: intermetallic compound).
Metals 2017, 7, 162
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With a low aluminum concentration, the contrast between the transition elements is relatively
low. Adding more aluminum leads to a higher contrast due to its lower electron backscattering
coefficient. Therefore, an aluminum-rich phase appears darker. Small amounts of black particles are
visible in all cross-section images. Apart from pores and breakouts formed during preparation, these
are intermetallic compound (IC) phases, which are either titanium- or aluminum-rich. As seen in
Figure 2b,c, an increasing aluminum content directly influences the primary nucleation. A dendritic
structure is formed around the IC particles. This primary phase recorded for x = 0.8 and 1.5 causes the
DSC peaks at higher temperatures with increasing aluminum amounts. The large grains of the dendritic
arms are a result of a broad melting range. Around this high temperature phase, an interdendritic
phase nucleates. Only a small amount of this phase nucleates primary around the IC particles for
x = 0.2. In the x = 0.2 and 0.8 systems, the parts around the primary structures solidify in an alternating
mixture of secondary phase and cell wall. For x = 1.5, only a small amount solidifies as a remainder.
Additionally, in the x = 0.2 system and to a lower extent in the x = 0.8 system, bright needles can be
noticed in between the cell walls.
The elemental composition of the different phases was measured by EDS. The atomic ratio was
calculated, and the results are shown in Table 4. The chemical composition of the different phases
differs significantly from the average molar ratio. The characteristically high chromium and iron
content values of the primary phase in the x = 0.2 alloy are similar to the interdendritic phase in
the x = 0.8 and 1.5 alloys. The dendrites of the primary phase for x = 0.8 and 1.5 show an elevated
aluminum content value. The dendrites of the secondary phase of the x = 0.2 alloy contain the lowest
concentration of aluminum and correlate well with the composition of the secondary dendrites for
x = 0.8 and the cell wall of x = 1.5. Similarly, the chemical compositions of the cell walls in x = 0.2 and
x = 0.8 alloys match.
Table 4. Chemical composition of the phases in atom % measured by EDS.
Phase
Structure
Al
Co
Cr
Fe
Ni
Ti
Local VEC
x = 0.2
IC
primary phase
secondary phase
cell wall
IC (η-phase)
bcc (A2)
fcc (A1)
cc (A12)
6.7
5.2
2.1
8.6
2.2
8.1
14.9
20.5
21.1
19.2
13.8
38.7
23.5
8.9
11.1
9.5
23.6
22.4
12.9
16.5
5.7
11.9
15.7
26.2
30.9
56.2
5.8
15.8
22.4
20.0
5.34
7.12
7.31
7.23
7.68
x = 0.8
IC
primary phase
interdendritic phase
secondary phase
cell wall
bcc (B2)
bcc (A2)
fcc (A1)
cc (A12)
68.3
25.8
4.5
2.6
9.9
7.7
20.6
14.5
18.0
19.2
2.5
4.3
36.7
24.7
8.8
3.6
7.5
27.8
27.4
15.4
7.8
21.9
9.7
12.5
25.9
10.1
20.0
6.8
14.8
20.9
4.37
6.47
7.11
7.22
7.20
x = 1.5
IC
primary phase
interdendritic phase
cell wall
bcc (B2)
bcc (A2)
fcc (A1)
70.5
31.1
9.6
3.6
4.8
19.7
11.1
14.3
7.6
4.9
36.6
22.3
6.0
8.4
28.5
31.0
4.4
19.8
7.5
13.5
6.7
16.1
6.6
15.2
4.20
6.30
6.78
7.18
3.4. Phase Determination
According to Figure 2, a multiphase character can be observed for all alloys, which is proven by
the results of XRD measurements shown in Figure 3. For the different alloys, the lattice parameters of
similar phases are almost identical. The major crystallographic information is summarized in Table 5.
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Metals 2017, 7, 162
Table 5. List of phases detected by X-ray diffraction (XRD) measurements.
Table 5. List of phases detected by X-ray diffraction (XRD) measurements.
Struktur-Bericht
Lattice
Struktur-Bericht
A1
A2
B2
A12
Lattice
Structure Pearson
Structure Symbol
Pearson
Type
Type
Symbol
cF4
cF4
fcc
CuCu
fcc
bcc
WW
A2
bcc
bcc
CsCl
B2
bcc
CsCl
cluster cluster
(cc) (cc) α-Mn
A12centered centered
α-Mn
A1
cc(A12)
bcc(A2)
bcc(B2)
Lattice
Fm3m
(225)
Fm
3m (225)
Detected
Lattice
Detected for x
Parameter (Å)
Parameter (Å)
for x
a = 5.88 0.2, 0.8, 1.50.2, 0.8, 1.5
a = 5.88
Im3m
(229)
Im
3m (229)
(221)
Pm3m
Pm
3m (221)
I43m
(217)
I 43
m (217)
a=
a = 2.88
2.88 0.2, 0.8, 1.50.2, 0.8, 1.5
2.94 0.8, 1.5 0.8, 1.5
a = 8.91 0.2, 0.8 0.2, 0.8
a = 8.91
a=
a = 2.94
fcc(A1)
Intensity (CPS)
x = 1.5
x = 0.8
x = 0.2
cI2
cI2
cP2
cP2
cI58
cI58
Space
Group
Space
Group
7 of 10
20
30
40
50
60
70
80
90
100
2Ɵ
Figure
XRD diffraction
diagrams
thecast
cast Al
AlxxCoCrFeNiTi
alloy
for xfor
= 0.2,
Figure 3.
XRD3. diffraction
diagrams
ofofthe
CoCrFeNiTi
alloy
x =0.8,
0.2,and
0.8,1.5.
and 1.5.
A correlation in phase presence can be found between x = 0.2 and 0.8 alloys. The low aluminum
A correlation
in phase
presence
canccbe
found
between
x = 0.2
and 0.8
alloys.
Theincreasing
low aluminum
content promotes
the presence
of the
(A12)
already
mentioned
by Soare
et al.
[19]. With
content aluminum
promotescontent,
the presence
of
the
cc
(A12)
already
mentioned
by
Soare
et
al.
[19].
With
increasing
the peak intensity decreases over x = 0.8 to absence for x = 1.5. The -phase with
space
group P6
3/mmc
(194)
and hcpdecreases
(D024) structure
nottobeabsence
clearly detected
by XRD
aluminum
content,
the
peak
intensity
over could
x = 0.8
for x = 1.5.
Theanalysis
η-phase with
similarly
mentioned
by Chuang
et 24
al.) [20].
The characteristically
high concentrations
nickel,
space group
P63as
/mmc
(194) and
hcp (D0
structure
could not be clearly
detected byofXRD
analysis
cobalt,
and
titanium
in
the
needle-shaped
IC
-phase
were
evidenced
in
the
alloys
with
similarly as mentioned by Chuang et al. [20]. The characteristically high concentrationslow
of nickel,
aluminum content (Table 4). The peak for x = 0.2 at 53.45° could not be assigned. For all three samples,
cobalt, and titanium in the needle-shaped IC η-phase were evidenced in the alloys with low aluminum
the bcc (A2) type was detected, while the x = 0.8 and 1.5 systems also show a chemically ordered bcc
◦ could not be assigned. For all three samples, the bcc
content (B2)
(Table
4). The
peak for
x is
= no
0.2evidence
at 53.45for
structure.
Whereas
there
textures of both bcc phases (A2, B2) in x = 0.2 and x =
(A2) type
detected,
while the
0.8a strong
and 1.5
systems
show({200}
a chemically
ordered
0.8was
compositions,
especially
for xx==1.5,
{200}
texture also
was found
peak at angles
aroundbcc (B2)
75°).
This can be
explained
the irradiated
area of the
1.2 mm)
in x
combination
structure.
Whereas
there
is no by
evidence
for textures
ofXRD
bothmeasurement
bcc phases(Ø(A2,
B2) in
= 0.2 and x = 0.8
with highly
local microstructural
information
preferred
For x = around
0.8, a
compositions,
especially
for x = 1.5, a strong
{200}and
texture
was solidification
found ({200}direction.
peak at angles
75◦ ).
peak at an angle of 30.6° characteristic for an fcc (A1) phase was detected. Only this peak clearly
This can be explained by the irradiated area of the XRD measurement (Ø 1.2 mm) in combination with
differs from the characteristic peaks of bcc (B2), since additional fcc peaks overlap with bcc (B2) peaks.
highly local
microstructural information and preferred solidification direction. For x = 0.8, a peak at an
This is in contrast to Soare et al. [19] who assigned a peak at 26.2° (Cu Kα radiation), corresponding
◦
angle ofthe
30.6
an fccto(A1)
phase was
this peaka clearly
differs
peakcharacteristic
at 30.6° (Co Kα for
radiation)
the complex
cubic detected.
structure D0Only
3. Nevertheless,
comparison
to from
the characteristic
peaks
bcc (B2),indicates
since additional
peaks overlap with bcc (B2) peaks. This is in
the referenced
PDFof
03-065-4682
an fcc (A1)fcc
structure.
◦ (Cu
To verify
the [19]
existence
the fcc (A1)
a texture
measurement
at thecorresponding
angle of 30.5° using
contrast to Soare
et al.
whoofassigned
a structure,
peak at 26.2
Kα radiation),
the peak
KαKα
radiation
was performed.
The recorded
figures D0
shown
in Figure 4 clearly
indicate the to the
at 30.6◦ Co
(Co
radiation)
to the complex
cubic pole
structure
.
Nevertheless,
a
comparison
3
presence of that peak for all samples by tilting the sample, although it did not appear in the
referenced PDF 03-065-4682 indicates an fcc (A1) structure.
measurements shown in Figure 3 because the corresponding lattice planes were not parallel to the
◦
To sample
verify surface.
the existence
of the fcc
(A1)that
structure,
a texture
the angle
These analyses
confirm
an fcc (A1)
structuremeasurement
is present in allatsamples.
Thisof 30.5
using Co
Kα radiation
was performed.
The
recorded
figures
in Table
Figure
4 clearly
contrasts
the prediction
criteria of Guo
et al.
[13] and pole
Zhu et
al. [14] shown
for x = 1.5.
4 shows
largeindicate
the presence of that peak for all samples by tilting the sample, although it did not appear in the
measurements shown in Figure 3 because the corresponding lattice planes were not parallel to
the sample surface. These analyses confirm that an fcc (A1) structure is present in all samples.
This contrasts the prediction criteria of Guo et al. [13] and Zhu et al. [14] for x = 1.5. Table 4 shows
large variations in chemical composition that affects the local VEC. A general prediction criterion does
not appear to be tenable for overall HEA/CCA alloy compositions.
Metals 2017, 7, 162
8 of 10
Metals 2017,
7, 162 in chemical composition that affects the local VEC. A general prediction criterion does not
variations
8 of 10
appear to be tenable for overall HEA/CCA alloy compositions.
Metals 2017, 7, 162
8 of 10
variations in chemical composition that affects the local VEC. A general prediction criterion does not
appear to be tenable for overall HEA/CCA alloy compositions.
(a)
(b)
(c)
Figure 4. XRD pole figures (in stereographic projection) at 2θ = 30.5° of the {111} lattice planes of the
Figure 4. XRD pole figures (in stereographic projection) at 2θ = 30.5◦ of the {111} lattice planes of the
fcc (A1) structure. (a) Al0.2CoCrFeNiTi; (b) Al0.8CoCrFeNiTi; (c) Al1.5CoCrFeNiTi.
fcc (A1) structure. (a) Al0.2 CoCrFeNiTi; (b) Al0.8 CoCrFeNiTi; (c) Al1.5 CoCrFeNiTi.
(a)
3.5. Phase Assignment
(b)
(c)
Figure 4. XRD pole figures (in stereographic projection) at 2θ = 30.5° of the {111} lattice planes of the
3.5. Phase Assignment
Figures
2 and 3 show
that
increasing
content
is accompanied by a significant
fcc (A1) structure.
(a) Al0.2
CoCrFeNiTi;
(b)aluminum
Al0.8CoCrFeNiTi;
(c) values
Al1.5CoCrFeNiTi.
change in microstructure. For x = 0.2, a fine structure with equal amounts of dendrites and cell walls
Figures
2 and 3 show that increasing aluminum content values is accompanied by a significant
can
be found.
In x = 1.5, the microstructural changes show large amounts of dendrites of a primary
3.5. Phase
Assignment
change in microstructure. For x = 0.2, a fine structure with equal amounts of dendrites and cell walls
phase surrounded by interdendritic parts, surrounded by cell walls that solidify as a remainder. All
Figures
show
that increasing aluminum
content
valuesamounts
is accompanied
by a significant
can be found.
In x 2=and
1.5,3which
the
microstructural
changes
show
large
of dendrites
of a primary
phase components,
can be found in the
alloys with
low
and high aluminum
content values,
change in microstructure. For x = 0.2, a fine structure with equal amounts of dendrites and cell walls
phase surrounded
by xinterdendritic
parts, surrounded by cell walls that solidify as a remainder.
are present in the
= 0.8 alloy system.
can be found. In x = 1.5, the microstructural changes show large amounts of dendrites of a primary
Figure 5 shows
the maps
of phases
orientations
distribution
primary nucleated
All phase components,
which
can be
found and
in the
alloys with
low andbetween
high aluminum
content values,
phase surrounded by interdendritic parts, surrounded by cell walls that solidify as a remainder. All
dendrites
inxthe
bccalloy
(A2) structure
in the x = 0.2 system. The secondary phase shows an fcc (A1)
are present
in
the
=
0.8
system.
phase components, which can be found in the alloys with low and high aluminum content values,
structure and similar orientation. During the formation of the cell walls with the cc (A12) structure,
Figure
5 shows
the
ofsystem.
phases and orientations distribution between primary nucleated
are present
in the
x =maps
0.8 alloy
the depletion of aluminum causes the precipitation of the -phase between two solidification fronts.
Figure
5 shows
the
maps of in
phases
orientations
betweenphase
primary
nucleated
dendrites
in
the
bcc
(A2)
structure
the xand
= 0.2
system.distribution
The secondary
shows
an fcc (A1)
Consequently, a fine-grained microstructure, which is characterized by multiple orientations, forms.
dendrites
in
the
bcc
(A2)
structure
in
the
x
=
0.2
system.
The
secondary
phase
shows
an
fcc
(A1)
structure and similar orientation. During the formation of the cell walls with the cc (A12) structure,
structure and similar orientation. During the formation of the cell walls with the cc (A12) structure,
the depletion of aluminum causes the precipitation of the
η-phase between two solidification fronts.
the depletion of aluminum causes the precipitation of the111
-phase between two solidification fronts.
Consequently,
a
fine-grained
microstructure,
which
is
characterized
multiple
orientations,
fcc (A1)
Consequently, a fine-grained microstructure,
which is characterized by by
multiple
orientations,
forms. forms.
001 101
111
fcc(A12)
(A1)
cc
001 101
111
1010
_
(A12)
ɳccphase
_ _
0001
001 2110
101
_
(a)
1010
ɳ phase
(b)
_ _
Figure 5. Al0.2CoCrFeNiTi (a) phase map and (b) orientation
distribution, color-coded according to
0001 2110
the unit triangle of the inverse pole figure (IPF).
(a)
(b)
Figure 6 shows a map comprising all revealed phases (except for the tiny and thus hardly
Figure 5.-phase)
Al0.2CoCrFeNiTi
(a) corresponding
phase map and (b)
orientation distribution, color-coded
according
discernable
and
of the
xaccording
= 0.8toalloy
Figure
5. Al0.2 CoCrFeNiTi
(a)the
phase
map and (b)orientation
orientation distribution
distribution,map
color-coded
to the
the unit triangle of the inverse pole figure (IPF).
calculated
from
Kikuchi
patterns.
Similar orientations in the dendrites of the primary phase
unit
triangle of
theEBSD
inverse
pole figure
(IPF).
are evidenced. The surrounding interdendritic phase with high chromium and iron content values
Figure 6 shows a map comprising all revealed phases (except for the tiny and thus hardly
shows good orientation matching.
discernable -phase) and the corresponding orientation distribution map of the x = 0.8 alloy
Figure 6 shows a map comprising all revealed phases (except for the tiny and thus hardly
calculated from EBSD Kikuchi patterns. Similar orientations in the dendrites of the primary phase
discernable
η-phase) and the corresponding orientation distribution map of the x = 0.8 alloy calculated
are evidenced. The surrounding interdendritic phase with high chromium and iron content values
from EBSD
Kikuchi
patterns.matching.
Similar orientations in the dendrites of the primary phase are evidenced.
shows good orientation
The surrounding interdendritic phase with high chromium and iron content values shows good
orientation matching.
Metals 2017, 7, 162
9 of 10
Metals 2017, 7, 162
9 of 10
bcc (B2)
bcc (A2)
111
001 101
111
fcc (A1)
001 101
111
cc (A12)
001 101
(a)
(b)
Figure 6. Al0.8CoCrFeNiTi (a) phase map and (b) orientation distribution, color-coded according to
Figure 6. Al0.8 CoCrFeNiTi
phase
the unit triangle (a)
of the
(IPF). map and (b) orientation distribution, color-coded according to the
unit triangle of the (IPF).
The dendrites, which primarily nucleate in the x = 0.8 and 1.5 alloys, have the highest aluminum
content and a lower concentration of chromium and iron, and are associated with a local VEC of 6.47
or 6.30, respectively, have to be assigned to the bcc (B2) phase. The phase, which is the primary phase
The dendrites,
which primarily nucleate in the x = 0.8 and 1.5 alloys, have the highest aluminum
in x = 0.2 systems and the interdendritic phase in x = 0.8 and 1.5 systems, has lower aluminum content,
content and a lower
concentration
chromium
and
are
but higher
chromium and ironof
content.
This phase,
withiron,
a local and
VEC of
7.12associated
for x = 0.2, 7.11with
for x = a local VEC of
0.8, and 6.78 for
x = 1.5,to
respectively,
can be to
identified
as a(B2)
bcc (A2)
structure.
Thephase,
local depletion
of is the primary
6.47 or 6.30, respectively,
have
be assigned
the bcc
phase.
The
which
aluminum due to the formation of the aluminum-rich bcc structures favors the formation of fcc (A1)
phase in x = 0.2insystems
and
phase
in with
x = a0.8
and
systems,
lower aluminum
the cell walls
of the
the xinterdendritic
= 1.5 alloy. This phase,
linked
local
VEC1.5
of 7.18,
shows ahas
similar
chemical
composition
as
the
secondary
phase
for
x
=
0.8
(VEC
of
7.31)
and
the
secondary
phase
for
content, but higher chromium and iron content. This phase, with a local VEC of x7.12 for x = 0.2,
= 0.2 (VEC of 7.31). This indicates a change in crystal structure from bcc to fcc for a VEC of ~7.15.
7.11 for x = 0.8,Apart
and from
6.78that,
forthex structure
= 1.5, respectively,
can be identified as a bcc (A2) structure. The local
of the cell walls of the x = 0.2 and 0.8 alloys has been confirmed to be
cc (A12), butdue
with to
local
VEC
values of 7.23of
forthe
x = 0.2
and 7.20 for x = 0.8. bcc structures favors the formation
depletion of aluminum
the
formation
aluminum-rich
of fcc (A1) in the
cell walls of the x = 1.5 alloy. This phase, linked with a local VEC of 7.18, shows a
4. Conclusions
similar chemical composition
as the secondary phase for x = 0.8 (VEC of 7.31) and the secondary phase
The effect of aluminum content on the microstructure and phase formation of the
xCoCrFeNiTi
CCAindicates
system was studied.
All systems
show multiphase
with
for x = 0.2 (VECAlof
7.31). This
a change
in crystal
structuremicrostructures
from bcc to
fccstrong
for a VEC of ~7.15.
variation in chemical composition of the different domains. Four main phases in a mixed solid
Apart from that,
the structure of the cell walls of the x = 0.2 and 0.8 alloys has been confirmed to be
solution as well as intermetallic phases were identified. By comparing the different alloys,
cc (A12), but with
local VEC
7.23
for
x = 0.2
and 7.20in for
x presence
= 0.8. and a preferred
assignments
of the values
structure of
types
were
possible.
Correlations
phase
nucleation order were found. The specific valence electron concentrations (VECs) of the different
4. Conclusionsdomains were determined and discussed in the context of known prediction criteria.
Aluminum stabilizes the high temperature phase, nucleating primarily in the bcc (B2) structure,
which is present in the x = 0.8 and 1.5 alloys. Chromium- and iron-rich phases with a bcc (A2)
The effect of
aluminum content on the microstructure and phase formation of the Alx CoCrFeNiTi
structure are present in all samples. For x = 1.5, the (B2) and (A2) phases almost entirely fill the
CCA system was
studied.A small
All systems
show inmultiphase
microstructures
strong variation in
microstructure.
remainder solidifies
the fcc (A1) structure,
which appears to with
be in contrast
to
the
phase
prediction
for
x
=
1.5
according
to
both
mentioned
prediction
criteria.
An
fcc
(A1)
chemical composition of the different domains. Four main phases in a mixed solidphase
solution as well as
with significantly lower aluminum content compared to (B2) and (A2) phases was present in all
intermetallic phases
were
identified.
By
comparing
the
different
alloys,
assignments
samples studied in this work. With decreasing aluminum content, an additional phase with a cc (A12)of the structure
structure Correlations
precipitates in x = 0.2
0.8 alloys.
With the and
furtherareduction
of aluminum
in the x order
= 0.2
types were possible.
inand
phase
presence
preferred
nucleation
were found.
alloy, the -(Ni, Co)3Ti phase precipitates. It can be noted that an increase in aluminum content
The specific valence
electron
concentrations
(VECs)
of
the
different
domains
were
determined
and
prevents the precipitation of the A12 centered cluster structure as well as the -phase.
discussed in the context of known prediction criteria.
Acknowledgments: The publication costs of this article were covered by the German Research Foundation/DFG
Aluminumandstabilizes
the
high temperature
phase,
nucleating
thethank
bcc (B2) structure,
the Technische
Universität
Chemnitz in the Open
Access Publishing
fundingprimarily
program. The in
authors
Marcin
Pügner
the XRD
and his assistanceand
duringiron-rich
their evaluation
as well as
which is present
the for
x conducting
= 0.8 and
1.5 measurements
alloys. Chromiumphases
with a bcc (A2)
Thomas Uhlig for the sample manufacturing and for conducting the DSC measurement.
structure are present in all samples. For x = 1.5, the (B2) and (A2) phases almost entirely fill the
microstructure. A small remainder solidifies in the fcc (A1) structure, which appears to be in contrast to
the phase prediction for x = 1.5 according to both mentioned prediction criteria. An fcc (A1) phase with
significantly lower aluminum content compared to (B2) and (A2) phases was present in all samples
studied in this work. With decreasing aluminum content, an additional phase with a cc (A12) structure
precipitates in x = 0.2 and 0.8 alloys. With the further reduction of aluminum in the x = 0.2 alloy,
the η-(Ni, Co)3 Ti phase precipitates. It can be noted that an increase in aluminum content prevents the
precipitation of the A12 centered cluster structure as well as the η-phase.
Acknowledgments: The publication costs of this article were covered by the German Research Foundation/DFG
and the Technische Universität Chemnitz in the Open Access Publishing funding program. The authors thank
Marc Pügner for conducting the XRD measurements and his assistance during their evaluation as well as Thomas
Uhlig for the sample manufacturing and for conducting the DSC measurement.
Author Contributions: Thomas Lindner and Martin Löbel conceived and designed the experiments;
Thomas Lindner, Thomas Mehner, and Dagmar Dietrich performed the experiments, analyzed the data, and wrote
the paper. Thomas Lampke directed the research and contributed to the discussion and interpretation of the results.
Metals 2017, 7, 162
10 of 10
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
Cantor, B.; Chang, I.; Knight, P.; Vincent, A. Microstructural development in equiatomic multicomponent
alloys. Mater. Sci. Eng. A 2004, 375–377, 213–218. [CrossRef]
Yeh, J.W.; Chen, S.K.; Lin, S.J.; Gan, J.Y.; Chin, T.S.; Shun, T.T.; Tsau, C.H.; Chang, S.Y. Nanostructured
High-Entropy Alloys with Multiple Principal Elements: Novel Alloy Design Concepts and Outcomes.
Adv. Eng. Mater. 2004, 6, 299–303. [CrossRef]
Miracle, D.; Miller, J.; Senkov, O.; Woodward, C.; Uchic, M.; Tiley, J. Exploration and Development of High
Entropy Alloys for Structural Applications. Entropy 2014, 16, 494–525. [CrossRef]
Yeh, J.W. Recent progress in high-entropy alloys. Eur. J. Control 2006, 31, 633–648. [CrossRef]
Tsai, M.; Yeh, J.-W. High-Entropy Alloys: A Critical Review. Mater. Res. Lett. 2014, 3, 107–123. [CrossRef]
Singh, S.; Wanderka, N.; Murty, B.; Glatzel, U.; Banhart, J. Decomposition in multi-component AlCoCrCuFeNi
high-entropy alloy. Acta Mater. 2011, 59, 182–190. [CrossRef]
Löbel, M.; Lindner, T.; Kohrt, C.; Lampke, T. Processing of AlCoCrFeNiTi high entropy alloy by atmospheric
plasma spraying. IOP Conf. Ser. Mater. Sci. Eng. 2017, 181, 012015. [CrossRef]
Ma, S.; Zhang, Y. Effect of Nb addition on the microstructure and properties of AlCoCrFeNi high-entropy
alloy. Mater. Sci. Eng. A 2012, 532, 480–486. [CrossRef]
Zhu, J.M.; Fu, H.M.; Zhang, H.F.; Wang, A.M.; Li, H.; Hu, Z.Q. Synthesis and properties of multiprincipal
component AlCoCrFeNiSix alloys. Mater. Sci. Eng. A 2010, 527, 7210–7214. [CrossRef]
Zhu, J.M.; Fu, H.M.; Zhang, H.F.; Wang, A.M.; Li, H.; Hu, Z.Q. Microstructures and compressive properties
of multicomponent AlCoCrFeNiMox alloys. J. Alloys Compd. 2010, 497, 52–56. [CrossRef]
Hsu, C.Y.; Juan, C.C.; Wang, W.R.; Sheu, T.S.; Yeh, J.W.; Chen, S.K. On the superior hot hardness and softening
resistance of AlCoCrx FeMo0.5 Ni high-entropy alloys. Mater. Sci. Eng. A 2011, 528, 3581–3588. [CrossRef]
Hume-Rothery, W. Research on the Nature, Properties and Conditions of Formation of Intermetallic
Compounds, with Special Reference to Certain Compounds of Tin. J. Inst. Met. 1926, 35, 295–361.
Guo, S.; Ng, C.; Lu, J.; Liu, C. Effect of valence electron concentration on stability of fcc or bcc phase in high
entropy alloys. J. Appl. Phys. 2011, 109, 103505. [CrossRef]
Zhu, Z.; Ma, K.; Wang, Q.; Shek, C. Compositional dependence of phase formation and mechanical properties
in three CoCrFeNi-(Mn/Al/Cu) high entropy alloys. Intermetallics 2016, 79, 1–11. [CrossRef]
Zhang, K.; Fu, Z. Effects of annealing treatment on phase composition and microstructure of CoCrFeNiTiAlx
high-entropy alloys. Intermetallics 2012, 22, 24–32. [CrossRef]
Yang, X.; Zhang, Y. Prediction of high-entropy stabilized solid-solution in multi-component alloys.
Mater. Chem. Phys. 2012, 132, 233–238. [CrossRef]
Wu, P.; Liu, N.; Yang, W.; Zhu, Z.; Lu, Y.; Wang, X. Microstructure and solidification behavior of
multicomponent CoCrCux FeMoNi high-entropy alloys. Mater. Sci. Eng. A 2015, 642, 142–149. [CrossRef]
Feng, R.; Gao, M.; Lee, C.; Mathes, M.; Zuo, T.; Chen, S.; Hawk, J.; Zhang, Y.; Liaw, P. Design of Light-Weight
High-Entropy Alloys. Entropy 2016, 18, 333. [CrossRef]
Soare, V.; Mitrica, D.; Constantin, I.; Badilita, V.; Stoiciu, F.; Popescu, A.-M.; Carcea, I. Influence of remelting
on microstructure, hardness and corrosion behaviour of AlCoCrFeNiTi high entropy alloy. Mater. Sci. Technol.
2015, 10, 1194–2200. [CrossRef]
Chuang, M.H.; Tsai, M.H.; Wang, W.R.; Lin, S.J.; Yeh, J.W. Microstructure and wear behavior of
Alx Co1.5 CrFeNi1.5 Tiy high-entropy alloys. Acta Mater. 2011, 59, 6308–6317. [CrossRef]
Lee, C.-F.; Shun, T.-T. Age Hardening of the Al0.5 CoCrNiTi0.5 High-Entropy Alloy. Metall. Mater. Trans. A
2014, 45A, 191–195. [CrossRef]
Gao, M.C.; Yeh, J.W.; Liaw, P.K.; Zhang, Y. High-Entropy Alloys: Fundamentals and Applications, 1st ed.;
Springer International Publishing: Cham, Switzerland, 2016.
© 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
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