- Email: [email protected]
Microstructure and mechanical properties of refractory HfMo0.5NbTiV0.5Six high-entropy composites
Accepted Manuscript Microstructure and mechanical properties of refractory HfMo0.5NbTiV0.5Six highentropy composites Yuan Liu, Yan Zhang, Heng Zhang, Naijuan Wang, Xiang Chen, Huawei Zhang, Yanxiang Li PII:
S0925-8388(16)33121-8
DOI:
10.1016/j.jallcom.2016.10.014
Reference:
JALCOM 39180
To appear in:
Journal of Alloys and Compounds
Received Date: 1 July 2016 Revised Date:
2 September 2016
Accepted Date: 1 October 2016
Please cite this article as: Y. Liu, Y. Zhang, H. Zhang, N. Wang, X. Chen, H. Zhang, Y. Li, Microstructure and mechanical properties of refractory HfMo0.5NbTiV0.5Six high-entropy composites, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.10.014. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Microstructure and mechanical properties of refractory HfMo0.5NbTiV0.5Six high-entropy composites Yuan Liu a,b *, Yan Zhang a, Heng Zhang a,b, Naijuan Wang a, Xiang Chen a,b, Huawei Zhang a,b and
RI PT
Yanxiang Li a,b
a School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China
b Key Laboratory for Advanced Materials Processing Technology, Ministry of Education, PR China ∗ Corresponding author, Tel: +8610 62789328. E-mail: [email protected]
SC
Abstract:
HfMo0.5NbTiV0.5Six (x=0, 0.3, 0.5, 0.7) high-entropy alloys are synthesized by induction levitation melting with the aim of achieving a balanced combination of excellent strength at elevated temperature
mechanical properties of the alloys from 20
M AN U
and reasonable ductility at room temperature (RT). The microstructure, phase evolution and compression to 1200
are reported in this paper. It is found that the
HfMo0.5NbTiV0.5 matrix forms a simple disordered body-centered cubic (BCC) phase. After adding the Si element, multi-component silicide (Hf, Nb, Ti)5Si3 is generated inside the alloys and exhibits a transition from hypoeutectic structure to eutectic structure and then to hypereutectic structure as the Si content increases. The addition of Si significantly improves the hardness and strength but reduces the ductility. At room temperature, The HfMo0.5NbTiV0.5 and HfMo0.5NbTiV0.5Si0.7 alloys show yield strengths of
TE D
1260MPa and 2134MPa, respectively, and the compressive mechanism transitions from ductile deformation to brittle fracture from x=0 to x=0.7. Strain softening and silicide segmentation are found to be typical during compression deformation of these alloys at elevated temperatures. In these conditions, the alloys survive at least 35% of engineering compression strain without fracture. During deformation at , the yield strengths of HfMo0.5NbTiV0.5 and HfMo0.5NbTiV0.5Si0.7 alloys are 60MPa and 235MPa,
EP
1200
respectively. The attractive strength of the Si-containing alloys at elevated temperatures is strongly
AC C
dependent on the strengthening effect caused by the silicides.
Keywords: Refractory alloy; High-entropy composite; Metals and alloys; Microstructure; Mechanical properties
1. Introduction
High entropy alloys (HEAs) are a new research direction in the development of advanced materials.
They are loosely defined as alloys containing five or more principal elements in equal or near-equimolar ratios [1-4]. This type of alloy usually possesses a simple solid-solution phase along with excellent strength and ductility [5-8]. Recently, some refractory high-entropy alloys based on refractory elements have been extensively proposed and aimed at applications in advanced gas turbines or airplane engines due to their excellent elevated temperature properties, such as MoNbTaW, MoNbTaVW, HfNbTaTiZr,
ACCEPTED MANUSCRIPT HfMoNbTaTiZr and HfMoTaTiZr [9-15]. However, these refractory alloys have rather high densities. The intentional addition of a simple second phase in the solid solution to obtain a composite structure has been attempted to achieve superior elevated temperature mechanical properties. At the same time, this reduces the density [16-26]. As a result, mutual interactions between the matrix and strengthening phase lead to better composite properties. For example, the Laves phase that formed in
183MPa at 1000
6.5 g/cm3 greatly improved the yield strength to
RI PT
Cr-Nb-Ti-V-Zr system alloys with a density of
[18]. Significant improvement in strength has also been achieved by the addition of
Si into the single bcc solid solution phase [27-29]. The HfNbTiVSi0.5 alloy exhibits a noticeable ductility exceeding 10% at RT and a remarkable high yield stress of 240MPa at 1000
. However, the strength is
limited at higher temperatures due to matrix softening. Furthermore, the presence of excess V element
SC
results in precipitation of the V-rich phase, which may seriously reduce the elevated temperature strength [29].
Recently published data on Mo-containing HEAs suggest that Mo has beneficial effects on the
M AN U
mechanical properties according to the solution hardening effect [30]. It has also been noticed that Mo has better compatibility with Hf, Nb, Ti and V elements, and no extra second phase forms in these types of HEAs [13-15]. In addition, the binding energy between Mo and Si is generally lower than that between Hf, Nb, Ti, V and Si, suggesting that the Mo atom may concentrate in the matrix phase rather than in the silicide [12, 30, 31]. Therefore, it is reasonable to deduce that the intentional addition of Mo can lead to further enhancement of the heat resistance of the matrix. Based on the consideration above, a HfMo0.5NbTiV0.5 matrix with excellent RT ductility is prepared by partially replacing V with Mo, and Si
TE D
is selected as the HfNbTiVSi0.5 alloy for incorporation into the matrix to generate silicide in situ to strengthen the HEAs and reduce the density. In this study, HfMo0.5NbTiV0.5Six x=0, 0.3, 0.5, 0.7 alloys are synthesized by induction levitation melting. Then their microstructure evolution and mechanical properties were analyzed and discussed. The aims of the present work are as follows: ( ) to prepare
EP
silicide reinforced composites that exhibit high mechanical properties at elevated temperatures and retain a reasonable plasticity at RT, and ( ) to analyze the effect of silicide on the strength and deformation
AC C
behavior of the alloys, providing a reference for further alloy design in order to achieve even better properties.
2. Experimental procedures Alloys with a nominal composition of HfMo0.5NbTiV0.5Six (x=0, 0.3, 0.5, 0.7) were synthesized by
induction levitation melting in high-purity argon atmosphere. The raw elements had purities higher than 99.9wt%. To ensure chemical homogeneity, all samples were flipped over and re-melted two times. The crystal structure was characterized by a Rigaku Smartlab X-ray diffractometer with Cu Kα radiation generated at 40KV and 200mA. The scanning rate was 4°/min and 2 theta ranged from 10° to 130°. The microstructure and chemical composition were analyzed by a JOEL JSM-7001F scanning electron microscope (SEM) and JOEL JXA-8230 electron probe microanalyzer (EPMA). Vickers microhardness measurements were conducted by an MH-5L microhardness tester under a load of 0.5 kg applied for 20 s.
ACCEPTED MANUSCRIPT Each sample was examined at least nine times in different areas. Room temperature, 1000
and 1200
compression tests were conducted on a Gleeble-3500D thermal-mechanical simulator under an initial strain rate of 0.001s-1. Cylindrical specimens for compression testing were 5.0 mm in diameter and 7.5 mm in height at RT and were 8.0 mm in diameter and 12.0 mm in height at 1000
and 1200
. All tests
were performed until the fracture of the specimen or until the height reduction reached 35%. After testing,
RI PT
the samples were cut into halves and the structure parallel to the compression direction was studied on plane.
3. Results 3.1 Crystal structure and density
Typical X-ray diffraction patterns of the as-cast HfMo0.5NbTiV0.5Six alloys are presented in Fig. 1.
SC
Only one BCC phase is identified for the matrix alloy of HfNbMo0.5TiV0.5. With the addition of Si, additional diffraction peaks are observed, indicating the formation of a new phase. Through calibration by JCPDS card, the multiple peaks are identified as belonging to hexagonal M5Si3-type silicide. With
M AN U
increasing Si content, the intensity of peaks corresponding to the M5Si3 phase increases, whereas, their diffraction intensities are lower than that of the BCC phase, which suggests that the matrix may dominate over the M5Si3 phase in the alloys. Meanwhile, all peaks of the BCC phase are slightly shifted rightwards with the increase in Si, indicating a reduced lattice constant. This result mainly results from the decrease in the larger radius element Hf in the matrix phase, as shown by the subsequent quantitative analysis. According to Bragg’s law, the lattice constants of all of the peaks are calculated with a high degree of accuracy: HfMo0.5NbTiV0.5 (a=3.313 Å for matrix), HfMo0.5NbTiV0.5Si0.3 (a=3.289 Å for matrix and
TE D
a=7.691 Å, c=5.285 Å for silicide), HfMo0.5NbTiV0.5Si0.5 (a=3.272 Å for matrix and a=7.698 Å, c=5.275
AC C
EP
Å for silicide), and HfMo0.5NbTiV0.5Si0.7 (a=3.258 Å for matrix and a=7.674 Å, c=5.259 Å for silicide).
Fig.1 X-ray diffraction patterns of the as-cast HfMo 0.5NbTiV 0.5 Six alloys Today, several parameters have been defined as a guideline to predict the structure stability and phase formation in HEAs. Zhang et al. [32] used the atomic-size difference (δ) and the combined effects of the mixing parameter (Ω) defined as T∆Smix/∆Hmix to predict phase formation behavior in HEAs. Simple solid-solution phase trends to be formed when δ
6.6% and Ω 1.1, while the criteria within
ACCEPTED MANUSCRIPT other scopes, intermetallics or bulk metallic glasses will easily occur. Moreover, Guo et al. [33] further proposed the valence electron concentration (VEC) to predict phase stability for solid solutions in HEAs, which indicated that the FCC solid solution is stable when VEC
8 and VEC
6.87 for BCC solid
solution. Using the approach above, relevant parameters of the proposed criteria for HfNbMo0.5TiV0.5Six alloys are calculated (the characteristics of six components are listed in Table 1), and the values are listed in Table 2. In the present study, it is evident that the formation of silicide and BCC matrix in
RI PT
HfMo0.5NbTiV0.5Six alloys is perfectly predicted by the criteria above. Although there are plenty of silicides forming in the HEAs, the parameters δ, Ω and VEC are still favorable for predicting the structural stability and phase formation.
Table 1 Melting temperature (Tm), atomic radius (r) and valence electron concentration (VEC) of Hf, Nb, Mo, Ti, V and Si pure metals [33] Nb
Mo
Tm, K
2506
2750
2896
r, Å
1.578
1.429
1.363
VEC
4
5
6
Ti
V
Si
1941
2192
1693
1.462
1.316
1.153
4
5
4
SC
Hf
M AN U
Metal
The experimentally measured densities of the alloys are also presented in Table 2. Si addition indeed leads to an effective density reduction from 9.02 to 8.49 g/cm3.
Table 2 Relevant parameters of proposed criteria for HfNbMo0.5TiV0.5Six alloys Alloy
HfMo0.5NbTiV0.5 HfMo0.5NbTiV0.5Si0.3 HfMo0.5NbTiV0.5Si0.5
δ
Ω
VEC
ߩcal
ߩexp 3
g/cm
g/cm3
-0.88
12.96
5.9%
36.07
4.63
8.98
9.02
-16.17
12.61
7.9%
1.86
4.58
8.48
8.75
-24.44
12.39
8.8%
1.19
4.56
8.17
8.60
-31.13
12.17
9.4%
0.91
4.53
7.91
8.49
EP
HfMo0.5NbTiV0.5Si0.7
△Smix
TE D
△Hmix
3.2 Microstructure and chemical compositions
AC C
Representative backscattered electron images (BEI) of the as-cast HfMo0.5NbTiV0.5Six alloys are shown in Fig. 2 and the chemical compositions of different phases as marked in Fig. 2(c) measured by quantitative analysis are listed in Table 3. It is observed from Fig. 2(a) that the HfNbMo0.5TiV0.5 alloy is made up of two areas with different contrasts, but there are no obvious boundaries between them. EPMA analysis shows that the bright area is rich in high-melting-point elements, i.e., Nb (2750K) and Mo (2896K), and the dark area is enriched with low-melting-point elements, i.e., Hf (2506K), Ti (1941K) and V (2192K). This is generally related to the tendency of high melting elements to preferably solidify as a solid phase [9, 10, 14]. By adding Si in the matrix alloy, the phase with a fine rod shape and net-like configuration is found in the HfMo0.5NbTiV0.5Si0.3 alloy (see Fig. 2(b)). The main components are Hf (31.9 at %), Nb (7.9 at %), Ti (17.0 at %) and Si (38.2 at %). Considering the XRD results, the phase can be identified as (Hf, Nb, Ti)5Si3-type silicide. Some bright matrices can also be found around the silicide,
ACCEPTED MANUSCRIPT and the chemical composition is similar to that of the matrix alloy. It can be concluded that they belong to one phase. By further increasing the Si content, the micromorphology of silicide changes significantly with the increase in volume fraction. These results are clear indications that the microstructure of silicide exhibits an evolution from hypoeutectic structure to eutectic structure and then to hypereutectic structure with the increase in Si concentration in the HfMo0.5NbTiV0.5Six alloys. Similar microstructure evolution has been
RI PT
reported in Guo N.N.’s work [27]. Thus, it is reasonable to assume that the silicide presents a hypoeutectic microstructure when x = 0.3 (Si = 6.9 at %). During solidification, the matrix is generated before the silicide and presents a typical dendritic structure, as shown in Fig. 2 (b). When x of Si content is 0.5 (Si =11.1 at %), the majority of rod-shape silicides coarsen and are replaced by a continuous mesh and lath-like structure (see Fig. 2(c)). In other words, the alloy generates the eutectic and hypereutectic
SC
structure silicide. The contents of Nb and Si in the silicide increase, while the distributions of Hf and Ti decrease. When the Si content is increased to 0.7 (Si = 14.9 at %), further reduction of the mesh like
M AN U
silicide can be observed in Fig. 2(d); the volume fraction, especially the size of the hypereutectic
AC C
EP
TE D
lath-shape silicide, increases at the same time.
Fig. 2 SEM images of the as-cast HfMo 0.5 NbTiV0.5 Si x alloys: (a) x=0, (b) x=0.3, (c) x=0.5, (d) x=0.7 Table 3 Chemical composition (in at.%) of the HfMo0.5NbTiV0.5Six constituent Alloy
element
Hf
Nb
Mo
Ti
V
Si
ACCEPTED MANUSCRIPT
HfMo0.5NbTiV0.5Si0.7
25.0
12.5
25.0
12.5
-
Dark phase
22.2
28.3
13.4
24.7
11.4
-
Bright phase
27.6
19.7
10.3
27.4
14.9
-
Nominal
23.3
23.3
11.6
23.3
11.6
6.9
Dark phase
18.6
30.1
15.4
24.1
11.5
0.3
Bright phase
25.0
19.5
9.1
28.8
17.6
0.0
Silicide
31.9
7.9
1.1
Nominal
22.2
22.2
11.1
Dark phase
15.0
31.9
19.1
Bright phase
24.3
16.4
7.7
Silicide
29.3
10.9
1.2
Nominal
21.3
21.3
Dark phase
14.2
29.7
Bright phase
22.8
16.2
Silicide
28.7
RI PT
HfMo0.5NbTiV0.5Si0.5
25.0
17.0
3.9
38.2
22.2
11.1
11.2
21.9
11.5
0.6
31.0
20.6
0.0
15.7
2.6
40.3
SC
HfMo0.5NbTiV0.5Si0.3
Nominal
10.6
21.3
10.6
14.9
17.6
24.0
13.9
0.6
7.7
31.5
21.8
0.0
16.0
2.9
40.4
M AN U
HfMo0.5NbTiV0.5
10.8
1.2
The judgment methods above given by Zhang and Guo provide a way to predict the phase formation in HEAs, and the value of the mixing enthalpies is likely to be the main cause of elements distribution in different phases. From a thermodynamic viewpoint, i.e., ∆G=∆H-T∆S, a combination reaction always tends to form a more stable structure to decrease the Gibbs free energy. A larger entropy is conducive to
TE D
the formation of solid solution, while a lower enthalpy will promote the formation of intermetallic compounds. Although the above elements can participate in forming silicides, it is evident that Hf is sufficient for overcoming the contribution of entropy with Si to decrease the Gibbs free energy due to their most negative mixing enthalpy, as shown in Table 3. Meanwhile, the mixing enthalpies of silicide
EP
between V, Mo and Si are less competitive. This demonstrates that V and Mo atoms tend to be excluded from the silicide during the solidification process. Thus, an extremely low distribution of Mo, V in the
AC C
silicide is observed. However, further research on the influence of enthalpy is needed. Table 4 Mixing enthalpies (kJ/mol) of different atomic pairs
[12, 30, 31]
△Hmix
Hf
Nb
Mo
Si
Ti
V
Hf
0
4
-4
-77
0
-2
0
-6
-56
2
-1
0
-35
-4
0
0
-66
-48
0
-2
Nb
Mo Si Ti V 3.3 Hardness
0
ACCEPTED MANUSCRIPT The Vickers microhardness values of the as-cast HfMo0.5NbTiV0.5Six alloys are shown in Fig. 3. For the matrix HfMo0.5NbTiV0.5 alloy, the hardness is 403 HV. Si addition has a promoting effect on the hardness increase. Evidently, the formation of silicide strengthens the matrix. When the content of Si is 0.7, the hardness reaches 612 HV. Meanwhile, the values of hardness have large fluctuations with the formation of the silicide because of the major difference in hardness between the matrix and silicide, as
M AN U
SC
RI PT
well as the non-uniform distribution of silicide in the microscopic area.
Fig. 3 Microhardness of the HfMo 0.5 NbTiV0.5 Si x alloys 3.4 Mechanical properties demonstrates
the
compressive
engineering
TE D
Fig.4
stress-strain
curves
of
the
as-cast
HfMo0.5NbTiV0.5Six alloys at different temperatures. Typical compression mechanical properties are summarized in Table 5. Apparently, the alloy with a lower Si content shows good capacity for plastic deformation at RT. Yet, the strength improves significantly with the addition of Si. The matrix 35% without fracture, and the yield
EP
HfMo0.5NbTiV0.5 alloy has the maximum compressive plasticity of
strength is 1260MPa. However, the HfMo0.5NbTiV0.7 alloy has the maximum yield strength (as high as 2134MPa) while still exhibiting a fracture strain of 9.2%.
AC C
By increasing the compression temperature to 1000
, the yield strengths of the alloys decrease to
values of less than 800 MPa, but the plasticity is greatly increased, i.e., the alloys are compressed to a strain of 35% without fracture. The yield stress of the matrix HfMo0.5NbTiV0.5 alloy is 368 MPa, while the HfMo0.5NbTiV0.5Si0.3, HfNbMo0.5TiV0.5Si0.5 and HfMo0.5NbTiV0.5Si0.7 alloys exhibit yield stress increases to 398MPa, 614MPa and 673MPa, respectively. It is clear that the Si content has an obvious effect on the strength improvement. At the higher temperature of 1200
the yield strength of the matrix
HfMo0.5NbTiV0.5 alloy drops to 60 MPa. At this temperature, the reinforcement effect of silicide is remarkable, i.e., the yield strengths of the HfMo0.5NbTiV0.5 Si0.3, HfMo0.5NbTiV0.5Si0.5 and HfMo0.5NbTiV0.5Si0.7 alloys are 166 MPa, 188 MPa and 235 MPa, respectively.
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 4 Compression stress-strain curves of the as-cast HfMo0.5NbTiV0.5Six alloys (a) x=0, (b) x=0.3, (c) x=0.5, (d) x=0.7 Table 5 Compressive mechanical properties of HfMo0.5NbTiV0.5Six high entropy alloys Temperature
AC C
HfMo0.5NbTiV0.5Si0.3
HfMo0.5NbTiV0.5Si0.5
HfMo0.5NbTiV0.5Si0.7
Fracture strain
MPa
Peak stress MPa
20
1260
35%
--
1000
368
35%
394
1200
60
35%
--
EP
HfMo0.5NbTiV0.5
Yield strength
TE D
Alloy
20
1617
18.5%
2016
1000
398
35%
430
1200
166
35%
187
20
1787
11.9%
2052
1000
614
35%
696
1200
188
35%
219
20
2134
1000
673
35%
741
1200
235
35%
268
9.2%
2242
4. Discussion 4.1 Comparison of mechanical properties with other alloys Fig. 5 shows the temperature dependences of the yield strength and specific yield strength (SYS) of
ACCEPTED MANUSCRIPT HfMo0.5NbTiV0.5Six and similar refractory HEAs with the nominal compositions of HfMoNbTiZr and HfNbTaTiZr. It is apparent that both the yield strength and the SYS of the HfMo0.5NbTiV0.5Six alloys highly increase with the Si content at temperatures of ≤1200 in temperature from ambient to 1200
and gradually decrease with the increase
. Comparing between previously reported HfNbTaTiZr and
HfMo0.5NbTiV0.5 obtained in this work (both with simple solid solution phase), the yield stresses of the
RI PT
Si-containing alloys are considerably higher. Although the HfMo0.5NbTiV0.5Si0.3 alloy is outperformed by the HfMoNbTiZr both at room temperature and elevated temperatures, the yield stress of HfMo0.5NbTiV0.5Si0.5 is equal to or higher than that of HfMoNbTiZr, and HfMo0.5NbTiV0.5Si0.7 has the highest yield stress in the entire temperature range from RT to 1200
. The SYS result also shows the
same trend. The SYS of HfMo0.5NbTiV0.5Si0.3 is higher than that of the HfNbTaTiZr and
SC
HfMo0.5NbTiV0.5 alloys, while it is smaller than the SYS of HfMoNbTiZr at all tested temperatures. At higher Si content, the SYS of HfMo0.5NbTiV0.5Si0.5 is nearly the same, and that of HfMo0.5NbTiV0.5Si0.7 is higher than the SYS of HfMoNbTiZr. These results show that the HfMo0.5NbTiV0.5Si0.7 alloy has the most
M AN U
attractive set of properties because of its considerably improved strength both at RT and elevated temperature, as well as its reduced density. Although the formation of silicide phase reduces the ductility of the alloy, it obviously leads to the increase in yield strength and is especially beneficial for elevated
AC C
EP
TE D
temperature properties.
Fig. 5 Temperature dependence of (a) yield strength and (b) specific yield strength of
ACCEPTED MANUSCRIPT HfMo0.5NbTiV0.5Six, HfMoNbTiZr, and HfNbTaTiZr alloys 4.2 Fracture feature Fig. 6 presents the longitudinal-section microstructure of HfMo0.5NbTiV0.5Six specimens after the compression test at RT. Although serious shear deformation occurs in HfMo0.5NbTiV0.5, the alloy does not fracture at RT with a compressive strain of 35%. In the local area of the sample, a small amount of crack
RI PT
initiation can be observed. During compression, the crack bridge is terminated under the matrix resistance effect. It is worth noting that particles maintain the integrity of the crystal structure in most areas, which shows that the alloy has a better ductility. With the addition of Si, the alloys are brittle fractured but still present high ductility. As shown in Fig. 6(b), (c) and (d), there are two types of morphologies in the
SC
fracture surface: smooth area for the plastic fracture of the matrix phase and bulky cleavage plane for the brittle fracture of the silicide phase. This suggests that the fracture occurs mainly through the coordination
M AN U
of two different deformation behaviors derived from the silicide and matrix. The silicide segments the matrix and has a strong constraint on it. During deformation, the dislocation movement is inhibited by the silicide and piles at the interfaces. This largely enhances the strength. When the volume fraction of silicide is large, local deformation is not coordinated, which will produce stress concentration and subsequently reduces the alloy plasticity. To a certain extent, the matrix phase that possesses high plasticity hinders the expansion of micro cracks, and this behavior is the main reason for the Si-added
TE D
HfMo0.5NbTiV0.5Six alloys presenting a relatively high plasticity at RT. With the increase in Si content, the cleavage area increases significantly. This finding is consistent with the compression results
AC C
EP
mentioned above.
RI PT
ACCEPTED MANUSCRIPT
SC
Fig. 6 SEM images of the microstructure of HfMo 0.5NbTiV0.5 Si x alloys after deformation at room temperature (a) x=0, (b) x=0.3, (c) x=0.5, (d) x=0.7 The microstructures of HfMo0.5NbTiV0.5Six specimens after compression at 1200
are shown in
M AN U
Fig. 7. Obvious deformation bands in the HfNbMo0.5TiV0.5 alloy can be observed from Fig. 7(a), demonstrating that the strain softens at this temperature and that the matrix remains at a certain strength due to the solution strengthening effect of the high-melting element of Mo. With the addition of Si, silicides in Si-containing alloys are severely crushed under the action of plastic flow, becoming more refined and uniform (as seen from Fig. 7(b), (c) and (d)). However, no cavity is observed at the boundaries, demonstrating a better interfacial adhesion between the matrix and silicides. It is interesting
TE D
to note that the eutectic silicides are squashed, while the hypereutectic silicides retain a relatively block morphology discretely distributed in the matrix. A possible explanation for this phenomenon is that the hypereutectic silicide has a better ability to resist boundary sliding. It is evident that the attractive strength of the Si-containing alloys at elevated temperatures is strongly dependent on the strengthening effect
EP
caused by the silicides. In addition, the formation of M5Si3-type silicide induces more Mo to segregate in the matrix, also resulting in the improvement of the alloy temperature tolerance.
AC C
Thus, the morphology and the number of silicides have a significant impact on the mechanical properties of the alloys. Therefore, for further improving the toughness at room temperature and high temperature performance of the alloy, it is essential to adjust the content of the silicide. In addition, the morphology and size of the silicide should also be considered for improving the mechanics and deformation relationship with the matrix.
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
5.
TE D
Fig. 7 SEM backscattered electron images of the microstructure of HfMo 0.5 NbTiV 0.5Si x alloys after deformation at 1200 : (a) x=0, (b) x=0.3, (c) x=0.5, (d) x=0.7 Conclusion
In this study, a refractory high-entropy alloy of HfMo0.5NbTiV0.5Six was synthesized by induction levitation melting. The microstructure and mechanical properties were characterized and discussed. Based
EP
on the obtained results and analysis, the following conclusions are summarized: (1) The HfMo0.5NbTiV0.5 matrix was mainly composed of a BCC phase. After adding the Si element,
AC C
multi-component silicide (Hf, Nb, Ti)5Si3 was generated inside the alloys. When x of Si content was 0.3, the silicide presented a hypoeutectic microstructure. With the increase in Si concentration, the microstructure of silicide exhibited an evolution from hypoeutectic structure to eutectic structure and then to hypereutectic structure.
(2) The density of the HfMo0.5NbTiV0.5Six alloys decreased with increasing of Si content. The
strength greatly increased with the Si content, while the plasticity at room temperature was reduced gradually. For example, the respective yield strength and fracture strain of the HfMo0.5NbTiV0.5 alloy were
35% and 1260MPa, while the yield strength and fracture strain of the HfMo0.5NbTiV0.5Si0.7 alloy
were 9.2% and 2134MPa, respectively. The fracture surfaces of the Si-containing alloys suggest that the fracture mode was a mixture of plastic fracture of the matrix phase and brittle cleavage fracture of the
ACCEPTED MANUSCRIPT silicide. (4) The yield strengths of HfMo0.5NbTiV0.5 and HfMo0.5NbTiV0.5Si0.7 alloys were 60MPa and 235MPa at 1200
, respectively. The attractive strength at elevated temperatures of the Si-containing
AC C
EP
TE D
M AN U
SC
RI PT
alloys was strongly dependent on the strengthening effect caused by the silicides.
ACCEPTED MANUSCRIPT Reference
[1]
J.W. Yeh, S.K. Chen, S.J. Lin, J.Y. Gan, T.S. Chin, T.T. Shun, C.H. Tsau, S.Y. Chan, Nano
structured high-entropy alloys with multiple principal elements: novel alloy design concepts and outcomes, Adv. Eng. Mater. 6 (2004) 299-303. [2]
J.W. Yeh, Recent progress in high-entropy alloys, Ann. Chim. Sci. Mat. 31 (2006) 633-648.
[3]
J.W. Yeh, Alloy Design Strategies and Future Trends in High-Entropy Alloys, JOM 65 (2013)
[4]
Y. Zhang, T.T. Zuo, Z. Tang, M.C. Gao, K.A. Dahmen, P.K. Liaw, et al. Microstructure and properties of high-entropy alloys, Prog. Mater. Sci. 61(2014) 1-93.
[5]
RI PT
1759-1771.
Y. Liu, M. Chen, Y.X. Li, X. Chen, Microstructure and mechanical performance of AlxCoCrCuFeNi high-entropy alloys, Rare Met. Mat. Eng. 38 (2009) 1602–1607.
Y.D. Wu, Y.H. Cai, T. Wang, J.J. Si, J. Zhu, Y.D. Wang, X.D. Hui, A refractory Hf25Nb25Ti25Zr25
SC
[6]
high-entropy alloy with excellent structural stability and tensile properties. Mater. Lett. 130 (2014) 277-280.
N.D. Stepanov, D.G. Shaysultanov, G.A. Salishchev, M.A. Tikhonovsky, Structure and
M AN U
[7]
mechanical properties of a light-weight AlNbTiV high entropy alloy, Mater. Lett. 142 (2015) 153– 155. [8]
M. Heidelmann, M. Feuerbacher, D.C. Ma, B. Grabowski, Structural anomaly in the high-entropy alloy ZrNbTiTaHf, Intermetallics 68 (2016) 11-15.
[9]
O.N. Senkov, G.B. Wilks, J.M. Scott, D.B. Miracle, Mechanical properties of (2011) 698-706.
[10]
TE D
Nb25Mo25Ta25W25 and V20Nb20Mo20Ta20W20 refractory high entropy alloys, Intermetallics 19 O.N. Senkov, J.M. Scott, S.V. Senkova, D.B. Miracle, C.F. Woodward, Microstructure and room temperature properties of a high-entropy TaNbHfZrTi alloy, J. Alloy. Compd. 509 (2011) 6043-6048.
O.N. Senkov, J.M. Scott, S.V. Senkova, F. Meisenkothen, D.B. Miracle, C.F. Woodward,
EP
[11]
Microstructure and elevated temperature properties of a refractory TaNbHfZrTi alloy, J. Mater. Sci. 47 (2012) 4062–4074.
O.N. Senkov, S.L. Semiatin, Microstructure and properties of a refractory high-entropy alloy after
AC C
[12]
cold working, J. Alloy. Compd. 649 (2015) 1110-1123.
[13]
N.N. Guo, L. Wang, L.S. Luo, X.Z. Li, Y.Q. Su, J.J. Guo, H.Z. Fu, Microstructure and mechanical properties of refractory MoNbHfZrTi high-entropy alloy, Materials and Design, 81 (2015) 87-94.
[14]
C.C. Juan, M.H. Tsai, C.W. Tsai, C.M. Lin, W.R. Wang, C.C. Yang, S.K. Chen, S.J. Lin, J.W. Yeh, Enhanced mechanical properties of HfMoTaTiZr and HfMoNbTaTiZr refractory high-entropy alloys, Intermetallics 62 (2015) 76-83.
[15]
N.N. Guo, L. Wang, L.S. Luo, X.Z. Li, R.R. Chen, Y.Q. Su, J.J. Guo, H.Z. Fu, Effect of composing element on microstructure and mechanical properties in Mo-Nb-Hf-Zr-Ti multi-principle component alloys, Intermetallics 69 (2016) 13-20.
[16]
O.N. Senkov, C.F. Woodward, Microstructure and properties of a refractory NbCrMo0.5Ta0.5TiZr alloy, Mater. Sci. Eng. A Struct. Mater. Prop. Microstruct. Process. 529 (2011) 311-320.
ACCEPTED MANUSCRIPT [17]
O.N. Senkov, S.V. Senkova, C. Woodward, D.B. Miracle, Low-density, refractory multi-principal element alloys of the Cr-Nb-Ti-V-Zr system: Microstructure and phase analysis. Acta Mater, 61 (2013) 1545-1557.
[18]
O.N. Senkov, S.V. Senkova, D.B. Miracle, C. Woodward, Mechanical properties of low-density, refractory multi-principal element alloys of the Cr-Nb-Ti-V-Zr system, Mater. Sci. Eng. A, 565 (2013) 51–62. É. Fazakas, V. Zadorozhnyy, L.K. Varga, A. Inoue, D.V. Louzguine-Luzgin, F.Y. Tian, L. Vitos,
RI PT
[19]
Experimental and theoretical study of Ti20Zr20Hf20Nb20X20 (X = V or Cr) refractory high-entropy alloys. Int. J. Refract. Met. Hard Mater. 47 (2014) 131-138. [20]
D.B. Miracle, J.D. Miller, O.N. Senkov, C. Woodward, M.D. Uchic, J. Tiley, Exploration and Development of High Entropy Alloys for Structural Applications, Entropy 16 (2014)
[21]
SC
494-525.
C.M. Lin, C.C. Juan, C.H. Chang, C.W. Tsai, J.W. Yeh, Effect of Al addition on mechanical properties and microstructure of refractory AlxHfNbTaTiZr alloys, J. Alloy. Compd. 624 (2015)
[22]
M AN U
100-107.
N.D. Stepanov, N.Y. Yurchenko, V.S. Sokolovsky, M.A. Tikhonovsky, G.A. Salishchev, An AlNbTiVZr0.5 high-entropy alloy combining high specific strength and good ductility, Mater. Lett. 161 (2015) 136–139.
[23]
N.D. Stepanov, N.Y. Yurchenko, D.V. Skibin, M.A. Tikhonovsky, G.A. Salishchev, Structure and mechanical properties of the AlCrxNbTiV (x= 0, 0.5, 1, 1.5) high entropy alloys. J. Alloy. Compd. 652 (2015) 266-280.
M.G. Poletti, G. Fiore, B.A. Szost, L. Battezzati, Search for high entropy alloys in the
TE D
[24]
X-NbTaTiZr systems (X = Al, Cr, V, Sn), J. Alloy. Compd. 620 (2015) 283–288. [25]
N.N. Guo, L. Wang, L.S. Luo, X.Z. Li, R.R. Chen, Y.Q. Su, J.J. Guo, H.Z. Fu, Microstructure and mechanical properties of in-situ MC-carbide particulates-reinforced refractory high-entropy Mo0.5NbHf0.5ZrTi matrix alloy composite. Intermetallics 69 (2016) 74-77. H. Chen, A. Kauffmann, B. Gorr, D. Schliephake, C. Seemüller, J.N. Wagner, H.J. Christ, M.
EP
[26]
Heilmaier. Microstructure and mechanical properties at elevated temperatures of a new Al-containing refractory high-entropy alloy Nb-Mo-Cr-Ti-Al. J. Alloy. Compd. 661 (2016)
[27]
AC C
206-215.
N.N. Guo, L. Wang, L.S. Luo, X.Z. Li, R.R. Chen, Y.Q. Su, J.J. Guo, H.Z. Fu. Microstructure
and mechanical properties of refractory high entropy (Mo0.5NbHf0.5ZrTi)BCC/M5Si3 in-situ compound. J. Alloy. Compd. 660 (2016) 197-203.
[28]
X.T Liu, W.B. Lei, L.J. Ma, J. Liu, J.L. Liu, J.Z. Cui, On the microstructures, phase assemblages and properties of Al0.5CoCrCuFeNiSix high-entropy alloys, J. Alloy. Compd. 630 (2015) 151– 157.
[29]
Y. Zhang, Y. Liu, Y.X. Li, X. Chen, H.W. Zhang. Microstructure and mechanical properties of a refractory HfNbTiVSi0.5 high-entropy alloy composite, Mater. Lett. 174 (2016) 82-85.
[30]
L. Jiang, Z.Q. Cao, J.C. Jie, J.J. Zhang, Y.P. Lu, T.M. Wang, T.J. Li, Effect of Mo and Ni elements on microstructure evolution and mechanical properties of the CoFeNixVMoy high entropy alloys, J. Alloy. Compd. 649 (2015) 585-590.
ACCEPTED MANUSCRIPT [31]
Akira Takeuchi, Akihisa Inoue, Calculations of mixing enthalpy and mismatch entropy for ternary amorphous alloys, Mater. Trans. 41 (2000) 1372-1378.
[32]
X. Yang, Y. Zhang, Prediction of high-entropy stabilized solid-solution in multi-component alloys. Mater. Phy. Chem. 132 (2012) 233-238. S. Guo, C.T. Liu, Phase stability in high entropy alloys: Formation of solid-solution phase or
EP
TE D
M AN U
SC
RI PT
amorphous phase, Prog. Nat. Sci: Mater. Int. 21(2011) 433−446.
AC C
[33]
ACCEPTED MANUSCRIPT Highlights:
RI PT
SC M AN U TE D EP
Refractory HfMo0.5NbTiV0.5Six high-entropy composites are prepared successfully. Multi-component silicide (Hf, Nb, Ti)5Si3 is generated with adding Si. The addition of Si reduces the density and improves the hardness and strength. Silicides play a significant role on strength increment. The composite structure is beneficial to the comprehensive properties.
AC C
S0925-8388(16)33121-8
DOI:
10.1016/j.jallcom.2016.10.014
Reference:
JALCOM 39180
To appear in:
Journal of Alloys and Compounds
Received Date: 1 July 2016 Revised Date:
2 September 2016
Accepted Date: 1 October 2016
Please cite this article as: Y. Liu, Y. Zhang, H. Zhang, N. Wang, X. Chen, H. Zhang, Y. Li, Microstructure and mechanical properties of refractory HfMo0.5NbTiV0.5Six high-entropy composites, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.10.014. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Microstructure and mechanical properties of refractory HfMo0.5NbTiV0.5Six high-entropy composites Yuan Liu a,b *, Yan Zhang a, Heng Zhang a,b, Naijuan Wang a, Xiang Chen a,b, Huawei Zhang a,b and
RI PT
Yanxiang Li a,b
a School of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China
b Key Laboratory for Advanced Materials Processing Technology, Ministry of Education, PR China ∗ Corresponding author, Tel: +8610 62789328. E-mail: [email protected]
SC
Abstract:
HfMo0.5NbTiV0.5Six (x=0, 0.3, 0.5, 0.7) high-entropy alloys are synthesized by induction levitation melting with the aim of achieving a balanced combination of excellent strength at elevated temperature
mechanical properties of the alloys from 20
M AN U
and reasonable ductility at room temperature (RT). The microstructure, phase evolution and compression to 1200
are reported in this paper. It is found that the
HfMo0.5NbTiV0.5 matrix forms a simple disordered body-centered cubic (BCC) phase. After adding the Si element, multi-component silicide (Hf, Nb, Ti)5Si3 is generated inside the alloys and exhibits a transition from hypoeutectic structure to eutectic structure and then to hypereutectic structure as the Si content increases. The addition of Si significantly improves the hardness and strength but reduces the ductility. At room temperature, The HfMo0.5NbTiV0.5 and HfMo0.5NbTiV0.5Si0.7 alloys show yield strengths of
TE D
1260MPa and 2134MPa, respectively, and the compressive mechanism transitions from ductile deformation to brittle fracture from x=0 to x=0.7. Strain softening and silicide segmentation are found to be typical during compression deformation of these alloys at elevated temperatures. In these conditions, the alloys survive at least 35% of engineering compression strain without fracture. During deformation at , the yield strengths of HfMo0.5NbTiV0.5 and HfMo0.5NbTiV0.5Si0.7 alloys are 60MPa and 235MPa,
EP
1200
respectively. The attractive strength of the Si-containing alloys at elevated temperatures is strongly
AC C
dependent on the strengthening effect caused by the silicides.
Keywords: Refractory alloy; High-entropy composite; Metals and alloys; Microstructure; Mechanical properties
1. Introduction
High entropy alloys (HEAs) are a new research direction in the development of advanced materials.
They are loosely defined as alloys containing five or more principal elements in equal or near-equimolar ratios [1-4]. This type of alloy usually possesses a simple solid-solution phase along with excellent strength and ductility [5-8]. Recently, some refractory high-entropy alloys based on refractory elements have been extensively proposed and aimed at applications in advanced gas turbines or airplane engines due to their excellent elevated temperature properties, such as MoNbTaW, MoNbTaVW, HfNbTaTiZr,
ACCEPTED MANUSCRIPT HfMoNbTaTiZr and HfMoTaTiZr [9-15]. However, these refractory alloys have rather high densities. The intentional addition of a simple second phase in the solid solution to obtain a composite structure has been attempted to achieve superior elevated temperature mechanical properties. At the same time, this reduces the density [16-26]. As a result, mutual interactions between the matrix and strengthening phase lead to better composite properties. For example, the Laves phase that formed in
183MPa at 1000
6.5 g/cm3 greatly improved the yield strength to
RI PT
Cr-Nb-Ti-V-Zr system alloys with a density of
[18]. Significant improvement in strength has also been achieved by the addition of
Si into the single bcc solid solution phase [27-29]. The HfNbTiVSi0.5 alloy exhibits a noticeable ductility exceeding 10% at RT and a remarkable high yield stress of 240MPa at 1000
. However, the strength is
limited at higher temperatures due to matrix softening. Furthermore, the presence of excess V element
SC
results in precipitation of the V-rich phase, which may seriously reduce the elevated temperature strength [29].
Recently published data on Mo-containing HEAs suggest that Mo has beneficial effects on the
M AN U
mechanical properties according to the solution hardening effect [30]. It has also been noticed that Mo has better compatibility with Hf, Nb, Ti and V elements, and no extra second phase forms in these types of HEAs [13-15]. In addition, the binding energy between Mo and Si is generally lower than that between Hf, Nb, Ti, V and Si, suggesting that the Mo atom may concentrate in the matrix phase rather than in the silicide [12, 30, 31]. Therefore, it is reasonable to deduce that the intentional addition of Mo can lead to further enhancement of the heat resistance of the matrix. Based on the consideration above, a HfMo0.5NbTiV0.5 matrix with excellent RT ductility is prepared by partially replacing V with Mo, and Si
TE D
is selected as the HfNbTiVSi0.5 alloy for incorporation into the matrix to generate silicide in situ to strengthen the HEAs and reduce the density. In this study, HfMo0.5NbTiV0.5Six x=0, 0.3, 0.5, 0.7 alloys are synthesized by induction levitation melting. Then their microstructure evolution and mechanical properties were analyzed and discussed. The aims of the present work are as follows: ( ) to prepare
EP
silicide reinforced composites that exhibit high mechanical properties at elevated temperatures and retain a reasonable plasticity at RT, and ( ) to analyze the effect of silicide on the strength and deformation
AC C
behavior of the alloys, providing a reference for further alloy design in order to achieve even better properties.
2. Experimental procedures Alloys with a nominal composition of HfMo0.5NbTiV0.5Six (x=0, 0.3, 0.5, 0.7) were synthesized by
induction levitation melting in high-purity argon atmosphere. The raw elements had purities higher than 99.9wt%. To ensure chemical homogeneity, all samples were flipped over and re-melted two times. The crystal structure was characterized by a Rigaku Smartlab X-ray diffractometer with Cu Kα radiation generated at 40KV and 200mA. The scanning rate was 4°/min and 2 theta ranged from 10° to 130°. The microstructure and chemical composition were analyzed by a JOEL JSM-7001F scanning electron microscope (SEM) and JOEL JXA-8230 electron probe microanalyzer (EPMA). Vickers microhardness measurements were conducted by an MH-5L microhardness tester under a load of 0.5 kg applied for 20 s.
ACCEPTED MANUSCRIPT Each sample was examined at least nine times in different areas. Room temperature, 1000
and 1200
compression tests were conducted on a Gleeble-3500D thermal-mechanical simulator under an initial strain rate of 0.001s-1. Cylindrical specimens for compression testing were 5.0 mm in diameter and 7.5 mm in height at RT and were 8.0 mm in diameter and 12.0 mm in height at 1000
and 1200
. All tests
were performed until the fracture of the specimen or until the height reduction reached 35%. After testing,
RI PT
the samples were cut into halves and the structure parallel to the compression direction was studied on plane.
3. Results 3.1 Crystal structure and density
Typical X-ray diffraction patterns of the as-cast HfMo0.5NbTiV0.5Six alloys are presented in Fig. 1.
SC
Only one BCC phase is identified for the matrix alloy of HfNbMo0.5TiV0.5. With the addition of Si, additional diffraction peaks are observed, indicating the formation of a new phase. Through calibration by JCPDS card, the multiple peaks are identified as belonging to hexagonal M5Si3-type silicide. With
M AN U
increasing Si content, the intensity of peaks corresponding to the M5Si3 phase increases, whereas, their diffraction intensities are lower than that of the BCC phase, which suggests that the matrix may dominate over the M5Si3 phase in the alloys. Meanwhile, all peaks of the BCC phase are slightly shifted rightwards with the increase in Si, indicating a reduced lattice constant. This result mainly results from the decrease in the larger radius element Hf in the matrix phase, as shown by the subsequent quantitative analysis. According to Bragg’s law, the lattice constants of all of the peaks are calculated with a high degree of accuracy: HfMo0.5NbTiV0.5 (a=3.313 Å for matrix), HfMo0.5NbTiV0.5Si0.3 (a=3.289 Å for matrix and
TE D
a=7.691 Å, c=5.285 Å for silicide), HfMo0.5NbTiV0.5Si0.5 (a=3.272 Å for matrix and a=7.698 Å, c=5.275
AC C
EP
Å for silicide), and HfMo0.5NbTiV0.5Si0.7 (a=3.258 Å for matrix and a=7.674 Å, c=5.259 Å for silicide).
Fig.1 X-ray diffraction patterns of the as-cast HfMo 0.5NbTiV 0.5 Six alloys Today, several parameters have been defined as a guideline to predict the structure stability and phase formation in HEAs. Zhang et al. [32] used the atomic-size difference (δ) and the combined effects of the mixing parameter (Ω) defined as T∆Smix/∆Hmix to predict phase formation behavior in HEAs. Simple solid-solution phase trends to be formed when δ
6.6% and Ω 1.1, while the criteria within
ACCEPTED MANUSCRIPT other scopes, intermetallics or bulk metallic glasses will easily occur. Moreover, Guo et al. [33] further proposed the valence electron concentration (VEC) to predict phase stability for solid solutions in HEAs, which indicated that the FCC solid solution is stable when VEC
8 and VEC
6.87 for BCC solid
solution. Using the approach above, relevant parameters of the proposed criteria for HfNbMo0.5TiV0.5Six alloys are calculated (the characteristics of six components are listed in Table 1), and the values are listed in Table 2. In the present study, it is evident that the formation of silicide and BCC matrix in
RI PT
HfMo0.5NbTiV0.5Six alloys is perfectly predicted by the criteria above. Although there are plenty of silicides forming in the HEAs, the parameters δ, Ω and VEC are still favorable for predicting the structural stability and phase formation.
Table 1 Melting temperature (Tm), atomic radius (r) and valence electron concentration (VEC) of Hf, Nb, Mo, Ti, V and Si pure metals [33] Nb
Mo
Tm, K
2506
2750
2896
r, Å
1.578
1.429
1.363
VEC
4
5
6
Ti
V
Si
1941
2192
1693
1.462
1.316
1.153
4
5
4
SC
Hf
M AN U
Metal
The experimentally measured densities of the alloys are also presented in Table 2. Si addition indeed leads to an effective density reduction from 9.02 to 8.49 g/cm3.
Table 2 Relevant parameters of proposed criteria for HfNbMo0.5TiV0.5Six alloys Alloy
HfMo0.5NbTiV0.5 HfMo0.5NbTiV0.5Si0.3 HfMo0.5NbTiV0.5Si0.5
δ
Ω
VEC
ߩcal
ߩexp 3
g/cm
g/cm3
-0.88
12.96
5.9%
36.07
4.63
8.98
9.02
-16.17
12.61
7.9%
1.86
4.58
8.48
8.75
-24.44
12.39
8.8%
1.19
4.56
8.17
8.60
-31.13
12.17
9.4%
0.91
4.53
7.91
8.49
EP
HfMo0.5NbTiV0.5Si0.7
△Smix
TE D
△Hmix
3.2 Microstructure and chemical compositions
AC C
Representative backscattered electron images (BEI) of the as-cast HfMo0.5NbTiV0.5Six alloys are shown in Fig. 2 and the chemical compositions of different phases as marked in Fig. 2(c) measured by quantitative analysis are listed in Table 3. It is observed from Fig. 2(a) that the HfNbMo0.5TiV0.5 alloy is made up of two areas with different contrasts, but there are no obvious boundaries between them. EPMA analysis shows that the bright area is rich in high-melting-point elements, i.e., Nb (2750K) and Mo (2896K), and the dark area is enriched with low-melting-point elements, i.e., Hf (2506K), Ti (1941K) and V (2192K). This is generally related to the tendency of high melting elements to preferably solidify as a solid phase [9, 10, 14]. By adding Si in the matrix alloy, the phase with a fine rod shape and net-like configuration is found in the HfMo0.5NbTiV0.5Si0.3 alloy (see Fig. 2(b)). The main components are Hf (31.9 at %), Nb (7.9 at %), Ti (17.0 at %) and Si (38.2 at %). Considering the XRD results, the phase can be identified as (Hf, Nb, Ti)5Si3-type silicide. Some bright matrices can also be found around the silicide,
ACCEPTED MANUSCRIPT and the chemical composition is similar to that of the matrix alloy. It can be concluded that they belong to one phase. By further increasing the Si content, the micromorphology of silicide changes significantly with the increase in volume fraction. These results are clear indications that the microstructure of silicide exhibits an evolution from hypoeutectic structure to eutectic structure and then to hypereutectic structure with the increase in Si concentration in the HfMo0.5NbTiV0.5Six alloys. Similar microstructure evolution has been
RI PT
reported in Guo N.N.’s work [27]. Thus, it is reasonable to assume that the silicide presents a hypoeutectic microstructure when x = 0.3 (Si = 6.9 at %). During solidification, the matrix is generated before the silicide and presents a typical dendritic structure, as shown in Fig. 2 (b). When x of Si content is 0.5 (Si =11.1 at %), the majority of rod-shape silicides coarsen and are replaced by a continuous mesh and lath-like structure (see Fig. 2(c)). In other words, the alloy generates the eutectic and hypereutectic
SC
structure silicide. The contents of Nb and Si in the silicide increase, while the distributions of Hf and Ti decrease. When the Si content is increased to 0.7 (Si = 14.9 at %), further reduction of the mesh like
M AN U
silicide can be observed in Fig. 2(d); the volume fraction, especially the size of the hypereutectic
AC C
EP
TE D
lath-shape silicide, increases at the same time.
Fig. 2 SEM images of the as-cast HfMo 0.5 NbTiV0.5 Si x alloys: (a) x=0, (b) x=0.3, (c) x=0.5, (d) x=0.7 Table 3 Chemical composition (in at.%) of the HfMo0.5NbTiV0.5Six constituent Alloy
element
Hf
Nb
Mo
Ti
V
Si
ACCEPTED MANUSCRIPT
HfMo0.5NbTiV0.5Si0.7
25.0
12.5
25.0
12.5
-
Dark phase
22.2
28.3
13.4
24.7
11.4
-
Bright phase
27.6
19.7
10.3
27.4
14.9
-
Nominal
23.3
23.3
11.6
23.3
11.6
6.9
Dark phase
18.6
30.1
15.4
24.1
11.5
0.3
Bright phase
25.0
19.5
9.1
28.8
17.6
0.0
Silicide
31.9
7.9
1.1
Nominal
22.2
22.2
11.1
Dark phase
15.0
31.9
19.1
Bright phase
24.3
16.4
7.7
Silicide
29.3
10.9
1.2
Nominal
21.3
21.3
Dark phase
14.2
29.7
Bright phase
22.8
16.2
Silicide
28.7
RI PT
HfMo0.5NbTiV0.5Si0.5
25.0
17.0
3.9
38.2
22.2
11.1
11.2
21.9
11.5
0.6
31.0
20.6
0.0
15.7
2.6
40.3
SC
HfMo0.5NbTiV0.5Si0.3
Nominal
10.6
21.3
10.6
14.9
17.6
24.0
13.9
0.6
7.7
31.5
21.8
0.0
16.0
2.9
40.4
M AN U
HfMo0.5NbTiV0.5
10.8
1.2
The judgment methods above given by Zhang and Guo provide a way to predict the phase formation in HEAs, and the value of the mixing enthalpies is likely to be the main cause of elements distribution in different phases. From a thermodynamic viewpoint, i.e., ∆G=∆H-T∆S, a combination reaction always tends to form a more stable structure to decrease the Gibbs free energy. A larger entropy is conducive to
TE D
the formation of solid solution, while a lower enthalpy will promote the formation of intermetallic compounds. Although the above elements can participate in forming silicides, it is evident that Hf is sufficient for overcoming the contribution of entropy with Si to decrease the Gibbs free energy due to their most negative mixing enthalpy, as shown in Table 3. Meanwhile, the mixing enthalpies of silicide
EP
between V, Mo and Si are less competitive. This demonstrates that V and Mo atoms tend to be excluded from the silicide during the solidification process. Thus, an extremely low distribution of Mo, V in the
AC C
silicide is observed. However, further research on the influence of enthalpy is needed. Table 4 Mixing enthalpies (kJ/mol) of different atomic pairs
[12, 30, 31]
△Hmix
Hf
Nb
Mo
Si
Ti
V
Hf
0
4
-4
-77
0
-2
0
-6
-56
2
-1
0
-35
-4
0
0
-66
-48
0
-2
Nb
Mo Si Ti V 3.3 Hardness
0
ACCEPTED MANUSCRIPT The Vickers microhardness values of the as-cast HfMo0.5NbTiV0.5Six alloys are shown in Fig. 3. For the matrix HfMo0.5NbTiV0.5 alloy, the hardness is 403 HV. Si addition has a promoting effect on the hardness increase. Evidently, the formation of silicide strengthens the matrix. When the content of Si is 0.7, the hardness reaches 612 HV. Meanwhile, the values of hardness have large fluctuations with the formation of the silicide because of the major difference in hardness between the matrix and silicide, as
M AN U
SC
RI PT
well as the non-uniform distribution of silicide in the microscopic area.
Fig. 3 Microhardness of the HfMo 0.5 NbTiV0.5 Si x alloys 3.4 Mechanical properties demonstrates
the
compressive
engineering
TE D
Fig.4
stress-strain
curves
of
the
as-cast
HfMo0.5NbTiV0.5Six alloys at different temperatures. Typical compression mechanical properties are summarized in Table 5. Apparently, the alloy with a lower Si content shows good capacity for plastic deformation at RT. Yet, the strength improves significantly with the addition of Si. The matrix 35% without fracture, and the yield
EP
HfMo0.5NbTiV0.5 alloy has the maximum compressive plasticity of
strength is 1260MPa. However, the HfMo0.5NbTiV0.7 alloy has the maximum yield strength (as high as 2134MPa) while still exhibiting a fracture strain of 9.2%.
AC C
By increasing the compression temperature to 1000
, the yield strengths of the alloys decrease to
values of less than 800 MPa, but the plasticity is greatly increased, i.e., the alloys are compressed to a strain of 35% without fracture. The yield stress of the matrix HfMo0.5NbTiV0.5 alloy is 368 MPa, while the HfMo0.5NbTiV0.5Si0.3, HfNbMo0.5TiV0.5Si0.5 and HfMo0.5NbTiV0.5Si0.7 alloys exhibit yield stress increases to 398MPa, 614MPa and 673MPa, respectively. It is clear that the Si content has an obvious effect on the strength improvement. At the higher temperature of 1200
the yield strength of the matrix
HfMo0.5NbTiV0.5 alloy drops to 60 MPa. At this temperature, the reinforcement effect of silicide is remarkable, i.e., the yield strengths of the HfMo0.5NbTiV0.5 Si0.3, HfMo0.5NbTiV0.5Si0.5 and HfMo0.5NbTiV0.5Si0.7 alloys are 166 MPa, 188 MPa and 235 MPa, respectively.
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
Fig. 4 Compression stress-strain curves of the as-cast HfMo0.5NbTiV0.5Six alloys (a) x=0, (b) x=0.3, (c) x=0.5, (d) x=0.7 Table 5 Compressive mechanical properties of HfMo0.5NbTiV0.5Six high entropy alloys Temperature
AC C
HfMo0.5NbTiV0.5Si0.3
HfMo0.5NbTiV0.5Si0.5
HfMo0.5NbTiV0.5Si0.7
Fracture strain
MPa
Peak stress MPa
20
1260
35%
--
1000
368
35%
394
1200
60
35%
--
EP
HfMo0.5NbTiV0.5
Yield strength
TE D
Alloy
20
1617
18.5%
2016
1000
398
35%
430
1200
166
35%
187
20
1787
11.9%
2052
1000
614
35%
696
1200
188
35%
219
20
2134
1000
673
35%
741
1200
235
35%
268
9.2%
2242
4. Discussion 4.1 Comparison of mechanical properties with other alloys Fig. 5 shows the temperature dependences of the yield strength and specific yield strength (SYS) of
ACCEPTED MANUSCRIPT HfMo0.5NbTiV0.5Six and similar refractory HEAs with the nominal compositions of HfMoNbTiZr and HfNbTaTiZr. It is apparent that both the yield strength and the SYS of the HfMo0.5NbTiV0.5Six alloys highly increase with the Si content at temperatures of ≤1200 in temperature from ambient to 1200
and gradually decrease with the increase
. Comparing between previously reported HfNbTaTiZr and
HfMo0.5NbTiV0.5 obtained in this work (both with simple solid solution phase), the yield stresses of the
RI PT
Si-containing alloys are considerably higher. Although the HfMo0.5NbTiV0.5Si0.3 alloy is outperformed by the HfMoNbTiZr both at room temperature and elevated temperatures, the yield stress of HfMo0.5NbTiV0.5Si0.5 is equal to or higher than that of HfMoNbTiZr, and HfMo0.5NbTiV0.5Si0.7 has the highest yield stress in the entire temperature range from RT to 1200
. The SYS result also shows the
same trend. The SYS of HfMo0.5NbTiV0.5Si0.3 is higher than that of the HfNbTaTiZr and
SC
HfMo0.5NbTiV0.5 alloys, while it is smaller than the SYS of HfMoNbTiZr at all tested temperatures. At higher Si content, the SYS of HfMo0.5NbTiV0.5Si0.5 is nearly the same, and that of HfMo0.5NbTiV0.5Si0.7 is higher than the SYS of HfMoNbTiZr. These results show that the HfMo0.5NbTiV0.5Si0.7 alloy has the most
M AN U
attractive set of properties because of its considerably improved strength both at RT and elevated temperature, as well as its reduced density. Although the formation of silicide phase reduces the ductility of the alloy, it obviously leads to the increase in yield strength and is especially beneficial for elevated
AC C
EP
TE D
temperature properties.
Fig. 5 Temperature dependence of (a) yield strength and (b) specific yield strength of
ACCEPTED MANUSCRIPT HfMo0.5NbTiV0.5Six, HfMoNbTiZr, and HfNbTaTiZr alloys 4.2 Fracture feature Fig. 6 presents the longitudinal-section microstructure of HfMo0.5NbTiV0.5Six specimens after the compression test at RT. Although serious shear deformation occurs in HfMo0.5NbTiV0.5, the alloy does not fracture at RT with a compressive strain of 35%. In the local area of the sample, a small amount of crack
RI PT
initiation can be observed. During compression, the crack bridge is terminated under the matrix resistance effect. It is worth noting that particles maintain the integrity of the crystal structure in most areas, which shows that the alloy has a better ductility. With the addition of Si, the alloys are brittle fractured but still present high ductility. As shown in Fig. 6(b), (c) and (d), there are two types of morphologies in the
SC
fracture surface: smooth area for the plastic fracture of the matrix phase and bulky cleavage plane for the brittle fracture of the silicide phase. This suggests that the fracture occurs mainly through the coordination
M AN U
of two different deformation behaviors derived from the silicide and matrix. The silicide segments the matrix and has a strong constraint on it. During deformation, the dislocation movement is inhibited by the silicide and piles at the interfaces. This largely enhances the strength. When the volume fraction of silicide is large, local deformation is not coordinated, which will produce stress concentration and subsequently reduces the alloy plasticity. To a certain extent, the matrix phase that possesses high plasticity hinders the expansion of micro cracks, and this behavior is the main reason for the Si-added
TE D
HfMo0.5NbTiV0.5Six alloys presenting a relatively high plasticity at RT. With the increase in Si content, the cleavage area increases significantly. This finding is consistent with the compression results
AC C
EP
mentioned above.
RI PT
ACCEPTED MANUSCRIPT
SC
Fig. 6 SEM images of the microstructure of HfMo 0.5NbTiV0.5 Si x alloys after deformation at room temperature (a) x=0, (b) x=0.3, (c) x=0.5, (d) x=0.7 The microstructures of HfMo0.5NbTiV0.5Six specimens after compression at 1200
are shown in
M AN U
Fig. 7. Obvious deformation bands in the HfNbMo0.5TiV0.5 alloy can be observed from Fig. 7(a), demonstrating that the strain softens at this temperature and that the matrix remains at a certain strength due to the solution strengthening effect of the high-melting element of Mo. With the addition of Si, silicides in Si-containing alloys are severely crushed under the action of plastic flow, becoming more refined and uniform (as seen from Fig. 7(b), (c) and (d)). However, no cavity is observed at the boundaries, demonstrating a better interfacial adhesion between the matrix and silicides. It is interesting
TE D
to note that the eutectic silicides are squashed, while the hypereutectic silicides retain a relatively block morphology discretely distributed in the matrix. A possible explanation for this phenomenon is that the hypereutectic silicide has a better ability to resist boundary sliding. It is evident that the attractive strength of the Si-containing alloys at elevated temperatures is strongly dependent on the strengthening effect
EP
caused by the silicides. In addition, the formation of M5Si3-type silicide induces more Mo to segregate in the matrix, also resulting in the improvement of the alloy temperature tolerance.
AC C
Thus, the morphology and the number of silicides have a significant impact on the mechanical properties of the alloys. Therefore, for further improving the toughness at room temperature and high temperature performance of the alloy, it is essential to adjust the content of the silicide. In addition, the morphology and size of the silicide should also be considered for improving the mechanics and deformation relationship with the matrix.
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
5.
TE D
Fig. 7 SEM backscattered electron images of the microstructure of HfMo 0.5 NbTiV 0.5Si x alloys after deformation at 1200 : (a) x=0, (b) x=0.3, (c) x=0.5, (d) x=0.7 Conclusion
In this study, a refractory high-entropy alloy of HfMo0.5NbTiV0.5Six was synthesized by induction levitation melting. The microstructure and mechanical properties were characterized and discussed. Based
EP
on the obtained results and analysis, the following conclusions are summarized: (1) The HfMo0.5NbTiV0.5 matrix was mainly composed of a BCC phase. After adding the Si element,
AC C
multi-component silicide (Hf, Nb, Ti)5Si3 was generated inside the alloys. When x of Si content was 0.3, the silicide presented a hypoeutectic microstructure. With the increase in Si concentration, the microstructure of silicide exhibited an evolution from hypoeutectic structure to eutectic structure and then to hypereutectic structure.
(2) The density of the HfMo0.5NbTiV0.5Six alloys decreased with increasing of Si content. The
strength greatly increased with the Si content, while the plasticity at room temperature was reduced gradually. For example, the respective yield strength and fracture strain of the HfMo0.5NbTiV0.5 alloy were
35% and 1260MPa, while the yield strength and fracture strain of the HfMo0.5NbTiV0.5Si0.7 alloy
were 9.2% and 2134MPa, respectively. The fracture surfaces of the Si-containing alloys suggest that the fracture mode was a mixture of plastic fracture of the matrix phase and brittle cleavage fracture of the
ACCEPTED MANUSCRIPT silicide. (4) The yield strengths of HfMo0.5NbTiV0.5 and HfMo0.5NbTiV0.5Si0.7 alloys were 60MPa and 235MPa at 1200
, respectively. The attractive strength at elevated temperatures of the Si-containing
AC C
EP
TE D
M AN U
SC
RI PT
alloys was strongly dependent on the strengthening effect caused by the silicides.
ACCEPTED MANUSCRIPT Reference
[1]
J.W. Yeh, S.K. Chen, S.J. Lin, J.Y. Gan, T.S. Chin, T.T. Shun, C.H. Tsau, S.Y. Chan, Nano
structured high-entropy alloys with multiple principal elements: novel alloy design concepts and outcomes, Adv. Eng. Mater. 6 (2004) 299-303. [2]
J.W. Yeh, Recent progress in high-entropy alloys, Ann. Chim. Sci. Mat. 31 (2006) 633-648.
[3]
J.W. Yeh, Alloy Design Strategies and Future Trends in High-Entropy Alloys, JOM 65 (2013)
[4]
Y. Zhang, T.T. Zuo, Z. Tang, M.C. Gao, K.A. Dahmen, P.K. Liaw, et al. Microstructure and properties of high-entropy alloys, Prog. Mater. Sci. 61(2014) 1-93.
[5]
RI PT
1759-1771.
Y. Liu, M. Chen, Y.X. Li, X. Chen, Microstructure and mechanical performance of AlxCoCrCuFeNi high-entropy alloys, Rare Met. Mat. Eng. 38 (2009) 1602–1607.
Y.D. Wu, Y.H. Cai, T. Wang, J.J. Si, J. Zhu, Y.D. Wang, X.D. Hui, A refractory Hf25Nb25Ti25Zr25
SC
[6]
high-entropy alloy with excellent structural stability and tensile properties. Mater. Lett. 130 (2014) 277-280.
N.D. Stepanov, D.G. Shaysultanov, G.A. Salishchev, M.A. Tikhonovsky, Structure and
M AN U
[7]
mechanical properties of a light-weight AlNbTiV high entropy alloy, Mater. Lett. 142 (2015) 153– 155. [8]
M. Heidelmann, M. Feuerbacher, D.C. Ma, B. Grabowski, Structural anomaly in the high-entropy alloy ZrNbTiTaHf, Intermetallics 68 (2016) 11-15.
[9]
O.N. Senkov, G.B. Wilks, J.M. Scott, D.B. Miracle, Mechanical properties of (2011) 698-706.
[10]
TE D
Nb25Mo25Ta25W25 and V20Nb20Mo20Ta20W20 refractory high entropy alloys, Intermetallics 19 O.N. Senkov, J.M. Scott, S.V. Senkova, D.B. Miracle, C.F. Woodward, Microstructure and room temperature properties of a high-entropy TaNbHfZrTi alloy, J. Alloy. Compd. 509 (2011) 6043-6048.
O.N. Senkov, J.M. Scott, S.V. Senkova, F. Meisenkothen, D.B. Miracle, C.F. Woodward,
EP
[11]
Microstructure and elevated temperature properties of a refractory TaNbHfZrTi alloy, J. Mater. Sci. 47 (2012) 4062–4074.
O.N. Senkov, S.L. Semiatin, Microstructure and properties of a refractory high-entropy alloy after
AC C
[12]
cold working, J. Alloy. Compd. 649 (2015) 1110-1123.
[13]
N.N. Guo, L. Wang, L.S. Luo, X.Z. Li, Y.Q. Su, J.J. Guo, H.Z. Fu, Microstructure and mechanical properties of refractory MoNbHfZrTi high-entropy alloy, Materials and Design, 81 (2015) 87-94.
[14]
C.C. Juan, M.H. Tsai, C.W. Tsai, C.M. Lin, W.R. Wang, C.C. Yang, S.K. Chen, S.J. Lin, J.W. Yeh, Enhanced mechanical properties of HfMoTaTiZr and HfMoNbTaTiZr refractory high-entropy alloys, Intermetallics 62 (2015) 76-83.
[15]
N.N. Guo, L. Wang, L.S. Luo, X.Z. Li, R.R. Chen, Y.Q. Su, J.J. Guo, H.Z. Fu, Effect of composing element on microstructure and mechanical properties in Mo-Nb-Hf-Zr-Ti multi-principle component alloys, Intermetallics 69 (2016) 13-20.
[16]
O.N. Senkov, C.F. Woodward, Microstructure and properties of a refractory NbCrMo0.5Ta0.5TiZr alloy, Mater. Sci. Eng. A Struct. Mater. Prop. Microstruct. Process. 529 (2011) 311-320.
ACCEPTED MANUSCRIPT [17]
O.N. Senkov, S.V. Senkova, C. Woodward, D.B. Miracle, Low-density, refractory multi-principal element alloys of the Cr-Nb-Ti-V-Zr system: Microstructure and phase analysis. Acta Mater, 61 (2013) 1545-1557.
[18]
O.N. Senkov, S.V. Senkova, D.B. Miracle, C. Woodward, Mechanical properties of low-density, refractory multi-principal element alloys of the Cr-Nb-Ti-V-Zr system, Mater. Sci. Eng. A, 565 (2013) 51–62. É. Fazakas, V. Zadorozhnyy, L.K. Varga, A. Inoue, D.V. Louzguine-Luzgin, F.Y. Tian, L. Vitos,
RI PT
[19]
Experimental and theoretical study of Ti20Zr20Hf20Nb20X20 (X = V or Cr) refractory high-entropy alloys. Int. J. Refract. Met. Hard Mater. 47 (2014) 131-138. [20]
D.B. Miracle, J.D. Miller, O.N. Senkov, C. Woodward, M.D. Uchic, J. Tiley, Exploration and Development of High Entropy Alloys for Structural Applications, Entropy 16 (2014)
[21]
SC
494-525.
C.M. Lin, C.C. Juan, C.H. Chang, C.W. Tsai, J.W. Yeh, Effect of Al addition on mechanical properties and microstructure of refractory AlxHfNbTaTiZr alloys, J. Alloy. Compd. 624 (2015)
[22]
M AN U
100-107.
N.D. Stepanov, N.Y. Yurchenko, V.S. Sokolovsky, M.A. Tikhonovsky, G.A. Salishchev, An AlNbTiVZr0.5 high-entropy alloy combining high specific strength and good ductility, Mater. Lett. 161 (2015) 136–139.
[23]
N.D. Stepanov, N.Y. Yurchenko, D.V. Skibin, M.A. Tikhonovsky, G.A. Salishchev, Structure and mechanical properties of the AlCrxNbTiV (x= 0, 0.5, 1, 1.5) high entropy alloys. J. Alloy. Compd. 652 (2015) 266-280.
M.G. Poletti, G. Fiore, B.A. Szost, L. Battezzati, Search for high entropy alloys in the
TE D
[24]
X-NbTaTiZr systems (X = Al, Cr, V, Sn), J. Alloy. Compd. 620 (2015) 283–288. [25]
N.N. Guo, L. Wang, L.S. Luo, X.Z. Li, R.R. Chen, Y.Q. Su, J.J. Guo, H.Z. Fu, Microstructure and mechanical properties of in-situ MC-carbide particulates-reinforced refractory high-entropy Mo0.5NbHf0.5ZrTi matrix alloy composite. Intermetallics 69 (2016) 74-77. H. Chen, A. Kauffmann, B. Gorr, D. Schliephake, C. Seemüller, J.N. Wagner, H.J. Christ, M.
EP
[26]
Heilmaier. Microstructure and mechanical properties at elevated temperatures of a new Al-containing refractory high-entropy alloy Nb-Mo-Cr-Ti-Al. J. Alloy. Compd. 661 (2016)
[27]
AC C
206-215.
N.N. Guo, L. Wang, L.S. Luo, X.Z. Li, R.R. Chen, Y.Q. Su, J.J. Guo, H.Z. Fu. Microstructure
and mechanical properties of refractory high entropy (Mo0.5NbHf0.5ZrTi)BCC/M5Si3 in-situ compound. J. Alloy. Compd. 660 (2016) 197-203.
[28]
X.T Liu, W.B. Lei, L.J. Ma, J. Liu, J.L. Liu, J.Z. Cui, On the microstructures, phase assemblages and properties of Al0.5CoCrCuFeNiSix high-entropy alloys, J. Alloy. Compd. 630 (2015) 151– 157.
[29]
Y. Zhang, Y. Liu, Y.X. Li, X. Chen, H.W. Zhang. Microstructure and mechanical properties of a refractory HfNbTiVSi0.5 high-entropy alloy composite, Mater. Lett. 174 (2016) 82-85.
[30]
L. Jiang, Z.Q. Cao, J.C. Jie, J.J. Zhang, Y.P. Lu, T.M. Wang, T.J. Li, Effect of Mo and Ni elements on microstructure evolution and mechanical properties of the CoFeNixVMoy high entropy alloys, J. Alloy. Compd. 649 (2015) 585-590.
ACCEPTED MANUSCRIPT [31]
Akira Takeuchi, Akihisa Inoue, Calculations of mixing enthalpy and mismatch entropy for ternary amorphous alloys, Mater. Trans. 41 (2000) 1372-1378.
[32]
X. Yang, Y. Zhang, Prediction of high-entropy stabilized solid-solution in multi-component alloys. Mater. Phy. Chem. 132 (2012) 233-238. S. Guo, C.T. Liu, Phase stability in high entropy alloys: Formation of solid-solution phase or
EP
TE D
M AN U
SC
RI PT
amorphous phase, Prog. Nat. Sci: Mater. Int. 21(2011) 433−446.
AC C
[33]
ACCEPTED MANUSCRIPT Highlights:
RI PT
SC M AN U TE D EP
Refractory HfMo0.5NbTiV0.5Six high-entropy composites are prepared successfully. Multi-component silicide (Hf, Nb, Ti)5Si3 is generated with adding Si. The addition of Si reduces the density and improves the hardness and strength. Silicides play a significant role on strength increment. The composite structure is beneficial to the comprehensive properties.
AC C