Microstructure and Compressive Properties of NbTiVTaAlx High Entropy Alloys

Abstract

The novel refractory high entropy alloys with the compositions of NbTiVTaAlx were prepared under a high-purity argon atmosphere and their microstructure and compressive properties at room temperature were investigated. Despite containing many constituents, all alloys had a single solid solution phase with body-centered cubic (BCC) structure, and possessed high compressive yield strength and ductility, which should be attributed to solid solution strengthening. (C) 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of MRS-Taiwan

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Procedia Engineering 36 ( 2012 ) 292 – 298
1877-7058 © 2012 Published by Elsevier Ltd.
doi: 10.1016/j.proeng.2012.03.043
IUMRS-ICA 2011
Microstructure and Compressive Properties of NbTiVTaAlx
High Entropy Alloys
X. Yanga, Y. Zhanga,b,*, and P.K. Liawb
aHigh-entropy Alloys Research Center, State Key Laboratory for Advanced Metals and Materials, University of Science and
Technology Beijing, Beijing 100083, China
b Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996-2200, USA
Abstract
The novel refractory high entropy alloys with the compositions of NbTiVTaAlx were prepared under a high-purity
argon atmosphere and their microstructure and compressive properties at room temperature were investigated.
Despite containing many constituents, all alloys had a single solid solution phase with body-centered cubic (BCC)
structure, and possessed high compressive yield strength and ductility, which should be attributed to solid solution
strengthening.
© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of MRS-Taiwan
Keywords: High entropy alloy; solid solution; yield strength; ductility; solid solution strengthening
1. Introduction
Recently, high-entropy alloys (HEAs), defined as alloys that generally have at least 5 major metallic
elements and each of which has an atomic percentage between 5 % and 35% [1], have attracted increasing
attentions. According to the regular solution model, the alloys have very high entropy of mixing, which
makes HEAs usually form FCC and/or BCC solid solutions rather than intermetallic compounds or other
complex ordered phases, and the total number of phases is well below the maximum equilibrium number
allowed by the Gibbs phase rule [1-4]. In the past decade, a number of these HEAs have been explored
* Corresponding author. Tel.: Tel.: +86 10 62334927; fax: +86 10 62333447.
E-mail address: drzhangy@ustb.edu.cn
Available online at www.sciencedirect.com

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X. Yang et al. / Procedia Engineering 36 ( 2012 ) 292 – 298
with excellent properties such as high strength, better room temperature ductility, good resistances to wear
and high thermal stability, but few HEAs can meet the requirements of aerospace industry [4-10].
According to empirical rule, a rapid decrease in strength of HEAs occurs at temperatures above
׽0.6Tm, where Tm is the melting temperature. To date, HEAs research has emphasized systems mainly
based on the transition metals such as Cr, Mn, Fe, Co, Ni, Ti and Cu, and hardly any HEAs can be used at
the temperature above 1273K [4, 10]. In view of the excellent softening resistance, it is reasonable to
explore high entropy alloys with high melting point. Recently American researchers have explored some
refractory HEAs, composed of some transition metal elements with high melting temperature such as Ta,
W, Nb, Mo and V [11, 12]. However, these alloys exhibit high density and low plastic strain. In this work,
the lighter elements, Al and Ti, are selected to decrease the density and improve the ductility, thus the
novel refractory HEAs, NbTiVTaAlx, are explored and their microstructure and mechanical properties are
investigated.
2. Experimental procedure
Alloy ingots with nominal composition of NbTiVTaAlx (x values in molar ratio, x = 0, 0.2, 0.5, and
1.0, denoted by Al0, Al0.2, Al0.5 and Al1.0, respectively) were prepared by arc melting the mixtures of
high-purity metals with the purity better than 99 wt% under a Ti-gettered high-purity argon atmosphere
on a water-cooled Cu hearth. The alloys were remelted several times and flipped each times in order to
improve homogeneity. The prepared alloy buttons with about 11 mm thick and 30 mm in diameter were
cut into appropriate form for investigating their microstructure and compressive properties.
Microstructure investigations of alloys were carried out by X-ray diffraction (XRD) using a PHILIPS
APD-10 diffractometer with Cu KĮ radiation. Cylindrical samples of ĭ3 mm × 6 mm were prepared for
room compressive tests and investigated using MTS 809 materials testing machine at room temperature
with a strain rate of 2 × 10í4 sí1. The morphologies of cross sections and fracture surfaces were examined
using a ZEISS SUPRA 55 scanning electron microscope (SEM) with energy dispersive spectrometry
(EDS).
3. Results and discussion
Fig. 1 (a) XRD patterns of the as-solidified NbTiVTaAlx (x=0, 0.25, 0.5, and 1) alloys; (b) the detailed scans for the peaks of (110)
of BCC solid solutions.

294 X. Yang et al. / Procedia Engineering 36 ( 2012 ) 292 – 298
Figure 1(a) and (b) show the XRD patterns of the specimens for NbTiVTaAlx alloy series. From Fig.
1(a), only BCC solid solution phase can be detected in all alloys, indicating that the addition of Al has
little effect on the phase constitution of this type of HEAs, and the crystal-plane indices of BCC structure
corresponding to diffraction peaks can be identified and marked in XRD patterns. This may be ascribed to
the high entropy effect. For these alloys, high entropy of mixing (ǻSmix) caused by multi-principal
elements significantly lower ǻGmix, which will make random solid solution easily form and more stable
than intermetallic compounds or other ordered phases during solidification.
The magnified image of (110) for BCC reflections is shown in Fig. 1(b). It is noticed that the (110)
peak shifts towards lower 2ș as the Al contents increase, which indicates that the Al addition can cause
the decrease of lattice parameters of NbTiVTaAlx alloys. Obviously, Al element has similar atomic radius
to three elements of Nb, Ti and Ta, but Al has a distinctly larger atomic radius than V (see Table 1),
which will affect the extent of lattice distortion and change the lattice constants of alloys.
Table1. The crystal structure, atomic radius (r), melting temperature (Tm) and density (ȡ) of high purity Nb, Ti, V, Ta and Al metals.
The investigated crystal structure and the calculated melting temperature for the NTiVTaAlx alloy series are also given here.
Metal Nb Ti V Ta Al Al0 Al0.25 Al0.5 Al1.0
Crystal structure BCC HCP BCC BCC FCC BCC BCC BCC BCC
r (pm) 147 146 135 147 143 ʊ ʊ ʊ ʊ
Tm (K) 2750 1946 2202 3293 933.5 2548 2453 2368 2225
Fig. 2 SEM backscatter images of the (a) Al0, (b) Al0.25, (c) Al0.5and (d) Al1.0 alloys.
Figure 2 shows the microstructures of NbTiVTaAlx alloys. It can be seen that the microstructure of Al0
alloy consists of equiaxial dendritic-like grains, and Al0.25, Al0.5, and Al1.0 alloys exhibit typical cast

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dendritic microstructure with the Al contents increase. For all four alloys, the grain sizes are rather lager,
and the different regions show different morphology and crystal orientation due to the nonuniform
temperature gradient during solidification.
In our previous work [13], the parameter ȍ and the atomic size difference į have been proposed to
predict the solid solution formation in multicomponent alloys, and ȍ  1.1, į  6.6 % should be expected
as the criteria for forming HE stabilized solid-solution phase. The parameter ȍ and į are expressed as[13]:
mmix
mix
TS
H
'
: ' (1)
2
1
(1 / )
n
ii
i
crr
G

¦ (2)
Where ǻHmix is the enthalpy of mixing and ǻSmix is the entropy of mixing; Tm is the melting point of
alloy, which is calculated using the rule of mixtures (
1
()
n
mimi
i
TcT
¦), n is the elemental number of the
alloys, ci is the atomic percent of the ith element, ri is the atomic radius of the ith element, (
1
n
ii
i
rcr
¦) is
the average atomic radius of the alloy. In this work, the parameters ȍ and į of NbTiVTaAlx alloys are
calculated according to Eq. (1) and (2), and the corresponding results are listed in Table 2. The
calculation required physicochemical and thermodynamic parameters for the constituent elements are
obtained from Ref. [14], some of them are given in Table 1. Clearly, the parameters ȍ and į of studied
alloys are met the criteria for forming HE stabilized solid solution phase (ȍ  1.1, į  6.6 %), thus the
studied alloys are inclined to form solid solution phase during solidification. The curves of ȍ and į as a
function of Al contents are plotted in Fig. 3. It can be seen that the ȍ values, reflecting the competitive
relationship between ǻSmix and ǻHmix, decrease from 117.461 to 2.215 with Al addition, which indicates
that the effect of mixing of entropy on solid solution weaken; while the atomic size difference reaches the
maximum (3.424 %) as x = 0.25, subsequently, į values decrease with Al contents increase as x > 0.25.
Table 2. The calculated parameters based on Eq. (1) and (2).
Alloys ǻHmix (KJ/mol) ǻSmix (J/k.mol) į (%) ȍ
NbTiVTa -0.25 11.53 3.34 117.46
NbTiVTaAl0.25 -4.82 12.71 3.42 6.47
NbTiVTaAl0.5 -8.40 13.15 3.30 3.71
NbTiVTaAlV1 -13.44 13.38 3.16 2.22
Moreover, in NbTiVTaAlx alloys, three constituent elements of Nb, V, Ta exhibit BCC structure (see
Table 1), and Al element usually is considered as a BCC stabilizer, all of which facilitate the formation of
BCC structure in the studied alloys. Guo et al. [15] proposed that valence electron concentration (VEC)
can control the phase stability for BCC or FCC solid solution; FCC phases are found to be stable at higher
VEC ( 8) and BCC phases are stable at lower VEC (< 6.87). The VEC for multicomponent alloys can be
defined as:
1
VEC ( )
n
ii
i
cVEC
¦ (3)
Where (VEC)i is the VEC of the ith element. The VEC for constituent elements of the studied alloys
are taken from Ref. [15] and the calculated VEC of Al0, Al0.25, Al0.5 and Al1.0 are 4.75, 4.65, 4.56 and
4.40, respectively. The relationship between VEC and Al contents of alloys is shown in Fig. 3(b). All
studied alloys possess lower VEC, thus BCC phase is stable.

296 X. Yang et al. / Procedia Engineering 36 ( 2012 ) 292 – 298
Fig. 3 (a) The curves of ȍ and į as a function of Al contents for NbTiVTaAlx (x=0, 0.25, 0.5and 1) alloys; (b) the relation between
VEC and Al contents of alloys.
Fig. 4 (a) The compressive engineering stress-strain curves of NbTiVTaAlx (x = 0, 0.25, 0.5 and 1.0) alloys; (b) compressive test
results for these alloys.
Figure 4(a) shows the compressive engineering stress-strain curves of room temperature test for
NbTiVTaAlx alloys. All alloys exhibit high yield compressive strength and good plastic deformation. The
yield strength of Al0, Al0.25, Al0.5 and Al1.0 are 1092 MPa, 1330 MPa, 1012 MPa and 991 MPa,
respectively. After yielding, the strength of alloys increase continuously and the samples of alloys do not
break under about 50% compressive strain. The relations between yield strength, elastic modulus and Al
contents for studied alloys are shown in Fig. 4(b). It can be seen that when the Al content increases to x =
0.25, the maximum yield strength increase to 1330 MPa; while the further addition of Al element can
make yield strength decrease. This variation tendency of yield strength is similar to that of atomic size
difference. Moreover, the elastic modulus of studied alloys changes slightly with the Al addition. The
high yield strength of these alloys likely is attributed to solid solution strengthening.
For NbTiVTaAlx alloys with single BCC structure, each atom can be expected as a solute atom and it
can randomly occupy the crystal lattice site of alloy. Nevertheless these solute atoms with different sizes
and properties can interact with each other and elastically distort the crystal lattice, which induce the

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X. Yang et al. / Procedia Engineering 36 ( 2012 ) 292 – 298
formation of local elastic stress field. The interactions between these local elastic stress fields and the
stress field of dislocations in alloy will hinder dislocation movements, and cause the increase of strength.
In general, the relation between the strengthening ('ı) and solute concentration (c) is expressed as [16]:
n
c
V
'v (4)
Here n § 0.5. Compared with the traditional crystal materials, the concentration of solute for
NbTiVTaAlx HEAs is extremely high, so these alloys exhibit high compressive strength. On the other
hand, the strengthening caused by atomic size mismatch will increase with the increase of atomic size
difference. In NbTiVTaAlx HEAs, the atomic size difference reach a maximum as x = 0.25, the
corresponding compressive yield strength of Al0.25 alloy also is the highest, which implies that the effect
of atomic size mismatch on the strength is very notable in NbTiVTaAlx HEAs.
Figure 5 shows the morphologies of the fractographs of the deformed sample for Al0 alloy. The lateral
surface of the deformed sample under 50% compressive strains is shown in Fig. 5(a). It can be seen that
the sample with the barrel-like form does not fracture after compressive deformation, but some surface
cracks, marked by the arrows, are observed. Besides, the sample exhibits slightly nonuniform
deformations, which may be ascribed to inhomogeneous microstructures. As a contrast, the secondary
electron image and backscatter electron images of the deformed sample with 30% compressive strains are
shown in Fig. 5 (b) and (c), respectively, no cracks can be observed in both surface and insider. Deformed
grains that were elongated in the radial direction can be seen in Fig. 5 (c). The similar morphologies have
also been observed in Al0.25, Al0.5 and Al1.0 alloys.
Fig. 5 The morphologies of the fractographs of the deformed sample for Al0 alloy. (a) The lateral surface of the deformed sample
under 50% compressive strains; the secondary electron image and backscatter electron images of the deformed sample with 30%
compressive strains are shown in (b) and (c), respectively.
4. Conclusions
A new series of NbTiVTaAlx high entropy alloys have been successfully prepared, which all have
simple phase structures and exhibit obvious dendrite structures. The phase formation rule of them has

298 X. Yang et al. / Procedia Engineering 36 ( 2012 ) 292 – 298
been discussed, based on the parameters of ȍ, į and VEC. It is concluded that these alloys possess
excellent BCC solid-solution formation ability. All alloys have high compressive yield strength and
ductility (no fracture under 50% strains), which should be attributed to solid solution strengthening.
Acknowledgements
The authors would like to acknowledge financial support by the National Natural Science Foundation
of China (NNSFC, No.50971019).
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