- Email: [email protected]
The phase stability of equiatomic CoCrFeMnNi high-entropy alloy: Comparison between experiment and calculation results
Accepted Manuscript The phase stability of equiatomic CoCrFeMnNi high-entropy alloy: Comparison between experiment and calculation results Nokeun Park, Byeong-Joo Lee, Nobuhiro Tsuji PII:
S0925-8388(17)31770-X
DOI:
10.1016/j.jallcom.2017.05.175
Reference:
JALCOM 41906
To appear in:
Journal of Alloys and Compounds
Received Date: 14 February 2017 Revised Date:
13 April 2017
Accepted Date: 16 May 2017
Please cite this article as: N. Park, B.-J. Lee, N. Tsuji, The phase stability of equiatomic CoCrFeMnNi high-entropy alloy: Comparison between experiment and calculation results, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.05.175. 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.
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ACCEPTED MANUSCRIPT The Phase Stability of Equiatomic CoCrFeMnNi High-Entropy Alloy: Comparison between Experiment and Calculation Results
a
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Nokeun Parka,b*, Byeong-Joo Leec, and Nobuhiro Tsujib,d
School of Materials Science and Engineering, Yeungnam University, Gyeongsan 38541, Republic of Korea
b
Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606c
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8501, Japan
Department of Material Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea
Elements Strategy Initiative for Structural Materials, Kyoto University, Sakyo-ku, Kyoto
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d
606-8501, Japan
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Nokeun Park: School of Materials Science and Engineering, Yeungnam University, Gyeongsan 38541, Republic of Korea,
E-mail: [email protected], Tel.: +82-53-810-2534
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Byeong-Joo Lee: Department of Material Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea E-mail: [email protected], Tel.: +82-54-279-2157
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Nobuhiro Tsuji: Department of Materials Science and Engineering, Kyoto University, Sakyoku, Kyoto 606-8501, Japan
E-mail: [email protected], Tel.: +81-75-753-5462
*
Corresponding author. Email: [email protected] 1
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Abstract A comparative study about the phase stability of equiatomic CoCrFeMnNi alloy of thermodynamic calculation (Thermo-Calc) and experimental result is shown. The alloy was
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processed by through a high-pressure torsion (HPT) process and annealing treatment at temperatures ranging from 900 to 600 °C. Phase identification using X-ray diffraction and energy dispersive X-ray spectroscopy indicated that the Cr-rich sigma phase formed during annealing at temperatures lower than 800 °C after the HPT process, and fraction of sigma
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phase increased as decreasing annealing temperature as predicted by Thermo-Calc. The thermodynamic calculation based on the TCFE database was found to be suitable for
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predicting the possible phases in the CoCrFeMnNi alloy.
Keywords: High-Entropy Alloy, CoCrFeMnNi, Thermodynamics, Sigma Phase, HighPressure Torsion
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Highlights
Second phase in CoCrFeMnNi was identified as sigma phase although kinetics of sigma phase is slow.
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A number of defects generated via HPT accelerated formation of sigma phase.
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Thermo-Calc successfully predicted the formation of meta-stable sigma phase.
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1.
Introduction
Recently, a new concept of solid solution phases, so-called high-entropy alloys (HEAs), was proposed. A HEA is a mixture of at least five elements with an atomic concentration
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between 5% and 35% [1–3]. Since Yeh et al. [1] first reported HEAs, the physical properties of many kinds of HEAs, such as high strength, good ductility, corrosion resistances, etc., have been studied. Among the large number of HEAs, the CoCrFeMnNi alloy with face-centeredcubic (FCC) structure has attracted particular interest since its first report by Cantor et al. [2]. The CoCrFeMnNi alloy was reported to have a single FCC phase that maximized the effects
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of the configurational entropy of mixing. Many experimental studies of CoCrFeMnNi alloys have been carried out. Liu et al. [4] reported grain growth behaviors and Hall–Petch
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relationship in a CoCrFeMnNi alloy, in which 70% cold-rolled specimens were annealed at temperatures ranging from 850 to 950 °C. Otto et al. [5] examined the influence of temperature and microstructure on the tensile properties of the specimens annealed at temperatures ranging from 800 °C to 1150 °C. Bhattacharjee et al. [6] studied the evolution of the microstructure and texture after cold rolling and annealing over a wide temperature range. They reported an ultrafine microstructure with a mean grain size of ~1 µm at 650 °C
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and emphasized that grain coarsening is strongly restricted up to 800 °C. Laplanche et al. [7] reported the temperature dependencies of the elastic moduli and thermal expansion coefficient of the CoCrFeMnNi alloy. These reports insisted that the CoCrFeMnNi alloy
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maintains a single solid-solution phase with FCC structure. Recently, however, Otto et al. [8] found that CoCrFeMnNi after annealing at 500 or 700 °C for 500 days is not a single phase, but small fraction of other phases appears in the vicinity of grain boundary at relatively low
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temperatures. It is expected that severe plastic deformation (SPD) by the HPT accelerates the kinetics of the formation of possible phases during subsequent annealing process. Schuh et al. [9] investigated the mechanical properties, microstructure and thermal stability of a nanocrystalline CoCrFeMnNi alloy after a high-pressure torsion (HPT). They reported that the CoCrFeMnNi alloy is not a single phase at temperatures below 800 °C. They also obtained an ultrafine microstructure after the HPT process and annealing at temperatures lower than 800 °C, which is similar to that reported by Bhattacharjee et al. [6]. The microhardness increased to around 630 HV when the annealing temperature was decreased to 450 °C for 1 h. Further annealing at 450 °C induced an increase in microhardness to 910 HV 3
ACCEPTED MANUSCRIPT surprisingly even after 100 h holding. The increase in microhardness might be attributed to the formation of a NiMn FCC phase, Cr-rich BCC phase and FeCo BCC phase, like multiphase composites [9]. On the other hand, phase identification estimated by the electron diffraction pattern of transmission electron microscopy (TEM) carried out in Ref. [9] was insufficient to identify the existing phases accurately. Owing to the lack of phase
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identification based on thermodynamic calculation, the strengthening mechanism of CoCrFeMnNi annealed at lower temperatures is not yet completely understood. The aim of the present study is to confirm the formation of a second phase during annealing at temperatures, especially, lower than 800 °C in the HPT processed specimens having large
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number of defects and to determine the applicability of a thermodynamic calculation package
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previously developed for the HEA study.
Material and methods
The alloy used in the present study was an equimolar CoCrFeMnNi alloy. The equimolar high-entropy alloy was cast using a pseudo float melting process [10]. To obtain an equimolar high-entropy alloy, 0.5 at. % of Mn was additionally put into the melting process to
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compensate for Mn because of its higher vapor pressure than other elements. The as-cast HEA rod having a diameter of 10 mm was cold-rolled to a 50% reduction in thickness, and the obtained sheet 5 mm thick was homogenized at 1000 °C for 8 hours under a high vacuum
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conditions to obtain a fully recrystallized microstructure. The specimen was cooled with water after the homogenization. The chemical composition of the homogenized specimen was measured by wavelength-dispersive spectroscopy and the average value and standard
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deviation for 118 points with 100 µm interval was shown in Table 1. Hereafter, the homogenized specimen is referred to as “starting material”. Table 1Chemical composition and standard deviation for 118 points with 100 m interval of the homogenized specimen measured by wavelength-dispersive spectroscopy Co Cr Fe Mn Ni Composition
20.13
20.58
19.59
20.30
19.40
0.223
0.296
0.331
0.371
0.355
(at. %) Standard deviation
4
ACCEPTED MANUSCRIPT Disk-type specimens, 10 mm in diameter and 0.8 mm in thickness, were cut from the homogenized sheet and subjected to the HPT process. HPT was carried out at ambient temperature under a pressure of 7.5 GPa with five revolutions (a rotation angle of 1800°) per minute. The corresponding shear strain (γ) at the observation area was calculated using the following equation, γ= r·θ/t where r is the radial distance from the center of the specimen, θ
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is the rotation angle in radians and t is the thickness of the specimen [11]. The HPT-processed specimens were annealed at 600°C, 700°C, 800°C, and 900°C in a salt bath for different periods ranging from 10 s to 3.6 ks, and then cooled in water. The surface of the HPTprocessed specimens were polished mechanically using a diamond suspension with a
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diamond particle size down to 1 µm. X-ray diffraction (XRD, on a PANalytical Smart-Lab system with Cu Kα1) was used for phase identification. The longitudinal sections parallel to
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the rotation axis of HPT at a radial distance of 4.5 mm from the center of the specimens were observed by TEM. A high-angle annular dark field (HAADF) image and the chemical composition profile was acquired in scanning TEM (STEM) mode by energy-dispersive spectroscopy (EDS) operated at 200 kV. For the TEM observations, thin-foil specimens were prepared by twin-jet electro-polishing using a solution of 10% perchloric acid (HClO4) and 90% acetic acid (CH3COOH). The phase equilibrium calculations were performed using authors [12-14].
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Thermo-Calc software with the TCFE thermodynamic database upgraded by one of the
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3. Results and Discussion
Figure 1 shows XRD patterns of the starting material, specimens annealed at 600°C,
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700°C and 800°C for 3.6 ks after the HPT process. Figure 1b represents an enlarged range from 35° to 52.5° to determine the diffraction planes of the second phase. The starting material and the specimen annealed at 800 °C or at higher temperature maintained a single FCC phase with a lattice constant of approximately 0.352 nm, which is within the values previously reported [4,6,15]. The specimens annealed at 600°C and 700 °C, however, contained not only the FCC phase but also a second phase. The XRD fitting result showed that the second phase belonged to a tetragonal space group of P42/mnm with lattice constants of approximately a = b = 0.859 nm and c = 0.445 nm. As the annealing temperature decreased, the intensity of the second phase increased and sharpened, indicating an increase in the 5
ACCEPTED MANUSCRIPT volume fraction of the second phase. This trend, therefore, showed that the stable phase is not only the FCC solid-solution but also the second phase when the CoCrFeMnNi alloy is
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annealed at temperatures lower than 800 °C.
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To examine the kinetics of the formation of the second phase, the HPT-processed specimens were annealed at 700 °C for different periods. TEM images of the specimens are shown in Fig. 2. Figures 2a and 2b show the bright-field TEM image of the specimen annealed for 10 s and the diffraction pattern of an identical specimen, respectively. The arrows in Fig. 2a indicate the formation of a second phase at triple junctions of recrystallizing grains and within grains containing large number of dislocations. The size of the second
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phase was smaller than 100 nm. The diffraction pattern in Fig. 2b proved that there were at least two phases; a FCC structure and a second phase. There might be some unidentified phase that could form during TEM operation. Figure 2c represents a bright-field TEM image of the specimen annealed for 600 s and Fig. 2d shows an enlarged microstructure of the area
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surrounded by broken lines in Fig. 2c. As shown in Fig. 2c, recrystallization appears to be almost complete and both grain boundaries and twin boundaries have already developed
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clearly. The arrows in Fig. 2c and Fig. 2d indicate that the second phase is located on triple junctions of recrystallized grains, which is similar to that observed in Fig. 2a. The size of the second phase on the triple junctions is approximately 100 nm, which is larger than that shown in Fig. 2a because of the increase in the annealing period. The finer second phase on the incoherent twin boundary in Fig. 2d is smaller than 100 nm.
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Figure 3a shows a HAADF-STEM image of the specimen annealed at 700 °C for 3.6 ks.
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The white and dark contrast represents the second phase and FCC structured one, respectively. The grain boundaries and twin boundaries can be seen clearly. The coarse second phase reached to the size of approximately 300 nm but there were still fine particles as well. The fraction of the second phase was increased significantly compared to Fig. 2a or Fig. 2c. The
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chemical composition of the second phase was measured by EDS and the line profiles of each element along the line m-n shown in Fig. 3a are displayed in Fig. 3b. The distributions of Cr
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(open square) and Ni (open circle) concentrations show that Cr is enriched in the second phase while it is depleted in the FCC matrix. The other elements changed by no more than 2%. Therefore, it is reasonable to assume that the second phase in the CoCrFeMnNi specimens annealed at temperatures lower than 800 °C might be analogous to the sigma phase often existing in austenitic steels, as reported before [8].
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Thermodynamic analysis has been carried out on high-entropy alloys to predict the
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formable phases in a given alloy system. Zhang et al. [16] reported the phase diagram of CoCrFeMnNi alloy calculated using PandatTM software and its databases. Although the thermodynamic calculation for the quinary system was proposed in their study, it still shows inconsistencies with the experimental results reported in the present study and a previous work [9]. It might be due to the lack of accuracy of thermodynamic calculation for intermetallic compounds, such as sigma phase, that the stability of intermetallic compounds is
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somehow underestimated. In the present study, thermodynamic calculation for CoCrFeMnNi system was performed using Thermo-Calc software and thermodynamic database (TCFE2000 and its upgraded version) [12-14]. The computation result is shown in Fig. 4a, and Fig. 4b exhibits the result after suspending BCC phase. The formation of FeCo-rich BCC
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phase and Cr-rich BCC phase was predicted in Fig. 4a that this calculation results agree well with experimental results in Ref. [9]. Once BCC phases were excluded from the calculation
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(Fig. 4b), sigma phase appeared and the fraction of sigma phase increased with decreasing temperature. Here, sigma phase was calculated as a meta-stable phase while two BCC phases were stable. It should be mentioned here that sigma phase is also quite stable, while BCC phases are more stable compared to sigma phase. Therefore, it is reasonable for the formation of sigma phase during low temperature annealing.
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The trend that the fraction of the sigma phase increases with decreasing temperature in Fig.4b corresponds to the results shown in Fig. 1. This agrees with the previous reports about
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CoCrFeMnNi [6,9]. Schuh et al. [9] reported that the microhardness of the specimens HPT processed and subsequently annealed increased steeply with decreasing annealing temperature ranging from 800 °C to 450 °C.
Considering the present result, it is because the
fraction of the sigma phase, which is know as a very hard intermetallic compound [17-22], increases with decreasing temperature. Bhattacharjee et al. [6] showed that grain coarsening is strongly inhibited up to 800° C, which might be due to the precipitation of sigma phase.
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The results reported previously can be explained by both the formation of and the change in the fraction of sigma phase at a given temperature. The correspondence between the thermodynamic calculation using the TCFE database and the experimental reports including the previous and the current ones, suggests that the calculation using commercially released
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thermodynamic software is useful for predicting the formable phases in a given high-entropy alloy, and also for designing an alloy composition to achieve any specific property.
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Nevertheless, some adjustments in the interpretation of the results are required because the thermodynamic calculations do not fully cover the chemical compositions of HEAs and it shows an equilibrium state.
4. Conclusion The formation of Cr-rich sigma phase in the CoCrFeMnNi alloy was confirmed experimentally after the HPT process and subsequent annealing at temperatures lower than 800 °C. The formation of sigma phase was verified not only experimentally but also theoretically by the thermodynamic calculations based on the TCFE database. As the 9
ACCEPTED MANUSCRIPT annealing temperature decreased, the intensity of the XRD peak for the sigma phase increased. This suggested that both the formation of and the increase in the fraction of sigma phase can contribute to the increase in hardness of the specimens annealed at lower
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temperatures reported before [8].
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Acknowledgement This study was supported by the 2015 Yeungnam University Research Grant (215A580025), and the National Research Foundation of Korea (NRF) grant funded by the
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Korea government (MSIP) (No. NRF-2015R1C1A1A01052856). This work was also supported by the Grant-in-Aid for Scientific Research on Innovative Area, ‘‘Bulk Nanostructured Metals’’ (Area No.2201), the Grant-in-Aid for Scientific Research (A) (No.24246114), the Grant-in-Aid for Challenging Exploratory Research (No.26630365), and
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the Elements Strategy Initiative for Structural Materials (ESISM), all through the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan (Contact No.22102002).
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All these supports are gratefully appreciated.
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Captions Table 1.
Chemical composition and standard deviation for 118 points with 100 µm interval
Figure 1.
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of the homogenized specimen measured by wavelength-dispersive spectroscopy
X-ray diffraction patterns of the equiatomic CoCrFeMnNi alloy comparing the
starting material with specimens annealed at 600, 700 and 800 °C for 3.6 ks after a high-
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pressure torsion (HPT) process. An enlarged range from 35° to 52.5° to nominate diffraction planes of second phase is displayed in (b). The squares and circles indicate FCC and second
Figure 2.
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phases, respectively.
Transmission electron microscopy (TEM) images (a, c, d) and diffraction pattern
(b) of the specimen annealed at 700 °C for different periods after HPT process: (a, b) for 10 s, (c, d) for 600 s. (a, c, d) Bright-field TEM image, and (b) diffraction pattern of (a). (d) is an enlarged image of (c). The arrows in (a) indicate the formation of a second phase at the triple
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junction of the recrystallizing grains and at grains containing large number of dislocations. The arrows in (c) highlight the second phase which located on the triple junction of recrystallized grains. The arrow in (d) points the second phase on the incoherent twin
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boundary (TB).
Figure 3. High-angle annular dark field scanning TEM image (a) and chemical composition profile (b) of the specimen annealed at 700 °C for 3.6 ks after HPT process. The white and
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dark contrasts represent the second phase and FCC one, respectively.
Figure 4. Computation result of Thermo-Calc software using the TCFE database: (a) without any suspended phase, (b) after suspending BCC phase.
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S0925-8388(17)31770-X
DOI:
10.1016/j.jallcom.2017.05.175
Reference:
JALCOM 41906
To appear in:
Journal of Alloys and Compounds
Received Date: 14 February 2017 Revised Date:
13 April 2017
Accepted Date: 16 May 2017
Please cite this article as: N. Park, B.-J. Lee, N. Tsuji, The phase stability of equiatomic CoCrFeMnNi high-entropy alloy: Comparison between experiment and calculation results, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.05.175. 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.
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ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT The Phase Stability of Equiatomic CoCrFeMnNi High-Entropy Alloy: Comparison between Experiment and Calculation Results
a
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Nokeun Parka,b*, Byeong-Joo Leec, and Nobuhiro Tsujib,d
School of Materials Science and Engineering, Yeungnam University, Gyeongsan 38541, Republic of Korea
b
Department of Materials Science and Engineering, Kyoto University, Sakyo-ku, Kyoto 606c
SC
8501, Japan
Department of Material Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea
Elements Strategy Initiative for Structural Materials, Kyoto University, Sakyo-ku, Kyoto
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d
606-8501, Japan
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Nokeun Park: School of Materials Science and Engineering, Yeungnam University, Gyeongsan 38541, Republic of Korea,
E-mail: [email protected], Tel.: +82-53-810-2534
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Byeong-Joo Lee: Department of Material Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Republic of Korea E-mail: [email protected], Tel.: +82-54-279-2157
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Nobuhiro Tsuji: Department of Materials Science and Engineering, Kyoto University, Sakyoku, Kyoto 606-8501, Japan
E-mail: [email protected], Tel.: +81-75-753-5462
*
Corresponding author. Email: [email protected] 1
ACCEPTED MANUSCRIPT
Abstract A comparative study about the phase stability of equiatomic CoCrFeMnNi alloy of thermodynamic calculation (Thermo-Calc) and experimental result is shown. The alloy was
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processed by through a high-pressure torsion (HPT) process and annealing treatment at temperatures ranging from 900 to 600 °C. Phase identification using X-ray diffraction and energy dispersive X-ray spectroscopy indicated that the Cr-rich sigma phase formed during annealing at temperatures lower than 800 °C after the HPT process, and fraction of sigma
SC
phase increased as decreasing annealing temperature as predicted by Thermo-Calc. The thermodynamic calculation based on the TCFE database was found to be suitable for
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predicting the possible phases in the CoCrFeMnNi alloy.
Keywords: High-Entropy Alloy, CoCrFeMnNi, Thermodynamics, Sigma Phase, HighPressure Torsion
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Highlights
Second phase in CoCrFeMnNi was identified as sigma phase although kinetics of sigma phase is slow.
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A number of defects generated via HPT accelerated formation of sigma phase.
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Thermo-Calc successfully predicted the formation of meta-stable sigma phase.
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1.
Introduction
Recently, a new concept of solid solution phases, so-called high-entropy alloys (HEAs), was proposed. A HEA is a mixture of at least five elements with an atomic concentration
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between 5% and 35% [1–3]. Since Yeh et al. [1] first reported HEAs, the physical properties of many kinds of HEAs, such as high strength, good ductility, corrosion resistances, etc., have been studied. Among the large number of HEAs, the CoCrFeMnNi alloy with face-centeredcubic (FCC) structure has attracted particular interest since its first report by Cantor et al. [2]. The CoCrFeMnNi alloy was reported to have a single FCC phase that maximized the effects
SC
of the configurational entropy of mixing. Many experimental studies of CoCrFeMnNi alloys have been carried out. Liu et al. [4] reported grain growth behaviors and Hall–Petch
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relationship in a CoCrFeMnNi alloy, in which 70% cold-rolled specimens were annealed at temperatures ranging from 850 to 950 °C. Otto et al. [5] examined the influence of temperature and microstructure on the tensile properties of the specimens annealed at temperatures ranging from 800 °C to 1150 °C. Bhattacharjee et al. [6] studied the evolution of the microstructure and texture after cold rolling and annealing over a wide temperature range. They reported an ultrafine microstructure with a mean grain size of ~1 µm at 650 °C
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and emphasized that grain coarsening is strongly restricted up to 800 °C. Laplanche et al. [7] reported the temperature dependencies of the elastic moduli and thermal expansion coefficient of the CoCrFeMnNi alloy. These reports insisted that the CoCrFeMnNi alloy
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maintains a single solid-solution phase with FCC structure. Recently, however, Otto et al. [8] found that CoCrFeMnNi after annealing at 500 or 700 °C for 500 days is not a single phase, but small fraction of other phases appears in the vicinity of grain boundary at relatively low
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temperatures. It is expected that severe plastic deformation (SPD) by the HPT accelerates the kinetics of the formation of possible phases during subsequent annealing process. Schuh et al. [9] investigated the mechanical properties, microstructure and thermal stability of a nanocrystalline CoCrFeMnNi alloy after a high-pressure torsion (HPT). They reported that the CoCrFeMnNi alloy is not a single phase at temperatures below 800 °C. They also obtained an ultrafine microstructure after the HPT process and annealing at temperatures lower than 800 °C, which is similar to that reported by Bhattacharjee et al. [6]. The microhardness increased to around 630 HV when the annealing temperature was decreased to 450 °C for 1 h. Further annealing at 450 °C induced an increase in microhardness to 910 HV 3
ACCEPTED MANUSCRIPT surprisingly even after 100 h holding. The increase in microhardness might be attributed to the formation of a NiMn FCC phase, Cr-rich BCC phase and FeCo BCC phase, like multiphase composites [9]. On the other hand, phase identification estimated by the electron diffraction pattern of transmission electron microscopy (TEM) carried out in Ref. [9] was insufficient to identify the existing phases accurately. Owing to the lack of phase
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identification based on thermodynamic calculation, the strengthening mechanism of CoCrFeMnNi annealed at lower temperatures is not yet completely understood. The aim of the present study is to confirm the formation of a second phase during annealing at temperatures, especially, lower than 800 °C in the HPT processed specimens having large
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number of defects and to determine the applicability of a thermodynamic calculation package
2.
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previously developed for the HEA study.
Material and methods
The alloy used in the present study was an equimolar CoCrFeMnNi alloy. The equimolar high-entropy alloy was cast using a pseudo float melting process [10]. To obtain an equimolar high-entropy alloy, 0.5 at. % of Mn was additionally put into the melting process to
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compensate for Mn because of its higher vapor pressure than other elements. The as-cast HEA rod having a diameter of 10 mm was cold-rolled to a 50% reduction in thickness, and the obtained sheet 5 mm thick was homogenized at 1000 °C for 8 hours under a high vacuum
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conditions to obtain a fully recrystallized microstructure. The specimen was cooled with water after the homogenization. The chemical composition of the homogenized specimen was measured by wavelength-dispersive spectroscopy and the average value and standard
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deviation for 118 points with 100 µm interval was shown in Table 1. Hereafter, the homogenized specimen is referred to as “starting material”. Table 1Chemical composition and standard deviation for 118 points with 100 m interval of the homogenized specimen measured by wavelength-dispersive spectroscopy Co Cr Fe Mn Ni Composition
20.13
20.58
19.59
20.30
19.40
0.223
0.296
0.331
0.371
0.355
(at. %) Standard deviation
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ACCEPTED MANUSCRIPT Disk-type specimens, 10 mm in diameter and 0.8 mm in thickness, were cut from the homogenized sheet and subjected to the HPT process. HPT was carried out at ambient temperature under a pressure of 7.5 GPa with five revolutions (a rotation angle of 1800°) per minute. The corresponding shear strain (γ) at the observation area was calculated using the following equation, γ= r·θ/t where r is the radial distance from the center of the specimen, θ
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is the rotation angle in radians and t is the thickness of the specimen [11]. The HPT-processed specimens were annealed at 600°C, 700°C, 800°C, and 900°C in a salt bath for different periods ranging from 10 s to 3.6 ks, and then cooled in water. The surface of the HPTprocessed specimens were polished mechanically using a diamond suspension with a
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diamond particle size down to 1 µm. X-ray diffraction (XRD, on a PANalytical Smart-Lab system with Cu Kα1) was used for phase identification. The longitudinal sections parallel to
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the rotation axis of HPT at a radial distance of 4.5 mm from the center of the specimens were observed by TEM. A high-angle annular dark field (HAADF) image and the chemical composition profile was acquired in scanning TEM (STEM) mode by energy-dispersive spectroscopy (EDS) operated at 200 kV. For the TEM observations, thin-foil specimens were prepared by twin-jet electro-polishing using a solution of 10% perchloric acid (HClO4) and 90% acetic acid (CH3COOH). The phase equilibrium calculations were performed using authors [12-14].
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Thermo-Calc software with the TCFE thermodynamic database upgraded by one of the
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3. Results and Discussion
Figure 1 shows XRD patterns of the starting material, specimens annealed at 600°C,
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700°C and 800°C for 3.6 ks after the HPT process. Figure 1b represents an enlarged range from 35° to 52.5° to determine the diffraction planes of the second phase. The starting material and the specimen annealed at 800 °C or at higher temperature maintained a single FCC phase with a lattice constant of approximately 0.352 nm, which is within the values previously reported [4,6,15]. The specimens annealed at 600°C and 700 °C, however, contained not only the FCC phase but also a second phase. The XRD fitting result showed that the second phase belonged to a tetragonal space group of P42/mnm with lattice constants of approximately a = b = 0.859 nm and c = 0.445 nm. As the annealing temperature decreased, the intensity of the second phase increased and sharpened, indicating an increase in the 5
ACCEPTED MANUSCRIPT volume fraction of the second phase. This trend, therefore, showed that the stable phase is not only the FCC solid-solution but also the second phase when the CoCrFeMnNi alloy is
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annealed at temperatures lower than 800 °C.
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To examine the kinetics of the formation of the second phase, the HPT-processed specimens were annealed at 700 °C for different periods. TEM images of the specimens are shown in Fig. 2. Figures 2a and 2b show the bright-field TEM image of the specimen annealed for 10 s and the diffraction pattern of an identical specimen, respectively. The arrows in Fig. 2a indicate the formation of a second phase at triple junctions of recrystallizing grains and within grains containing large number of dislocations. The size of the second
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phase was smaller than 100 nm. The diffraction pattern in Fig. 2b proved that there were at least two phases; a FCC structure and a second phase. There might be some unidentified phase that could form during TEM operation. Figure 2c represents a bright-field TEM image of the specimen annealed for 600 s and Fig. 2d shows an enlarged microstructure of the area
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surrounded by broken lines in Fig. 2c. As shown in Fig. 2c, recrystallization appears to be almost complete and both grain boundaries and twin boundaries have already developed
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clearly. The arrows in Fig. 2c and Fig. 2d indicate that the second phase is located on triple junctions of recrystallized grains, which is similar to that observed in Fig. 2a. The size of the second phase on the triple junctions is approximately 100 nm, which is larger than that shown in Fig. 2a because of the increase in the annealing period. The finer second phase on the incoherent twin boundary in Fig. 2d is smaller than 100 nm.
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Figure 3a shows a HAADF-STEM image of the specimen annealed at 700 °C for 3.6 ks.
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The white and dark contrast represents the second phase and FCC structured one, respectively. The grain boundaries and twin boundaries can be seen clearly. The coarse second phase reached to the size of approximately 300 nm but there were still fine particles as well. The fraction of the second phase was increased significantly compared to Fig. 2a or Fig. 2c. The
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chemical composition of the second phase was measured by EDS and the line profiles of each element along the line m-n shown in Fig. 3a are displayed in Fig. 3b. The distributions of Cr
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(open square) and Ni (open circle) concentrations show that Cr is enriched in the second phase while it is depleted in the FCC matrix. The other elements changed by no more than 2%. Therefore, it is reasonable to assume that the second phase in the CoCrFeMnNi specimens annealed at temperatures lower than 800 °C might be analogous to the sigma phase often existing in austenitic steels, as reported before [8].
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Thermodynamic analysis has been carried out on high-entropy alloys to predict the
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formable phases in a given alloy system. Zhang et al. [16] reported the phase diagram of CoCrFeMnNi alloy calculated using PandatTM software and its databases. Although the thermodynamic calculation for the quinary system was proposed in their study, it still shows inconsistencies with the experimental results reported in the present study and a previous work [9]. It might be due to the lack of accuracy of thermodynamic calculation for intermetallic compounds, such as sigma phase, that the stability of intermetallic compounds is
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somehow underestimated. In the present study, thermodynamic calculation for CoCrFeMnNi system was performed using Thermo-Calc software and thermodynamic database (TCFE2000 and its upgraded version) [12-14]. The computation result is shown in Fig. 4a, and Fig. 4b exhibits the result after suspending BCC phase. The formation of FeCo-rich BCC
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phase and Cr-rich BCC phase was predicted in Fig. 4a that this calculation results agree well with experimental results in Ref. [9]. Once BCC phases were excluded from the calculation
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(Fig. 4b), sigma phase appeared and the fraction of sigma phase increased with decreasing temperature. Here, sigma phase was calculated as a meta-stable phase while two BCC phases were stable. It should be mentioned here that sigma phase is also quite stable, while BCC phases are more stable compared to sigma phase. Therefore, it is reasonable for the formation of sigma phase during low temperature annealing.
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The trend that the fraction of the sigma phase increases with decreasing temperature in Fig.4b corresponds to the results shown in Fig. 1. This agrees with the previous reports about
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CoCrFeMnNi [6,9]. Schuh et al. [9] reported that the microhardness of the specimens HPT processed and subsequently annealed increased steeply with decreasing annealing temperature ranging from 800 °C to 450 °C.
Considering the present result, it is because the
fraction of the sigma phase, which is know as a very hard intermetallic compound [17-22], increases with decreasing temperature. Bhattacharjee et al. [6] showed that grain coarsening is strongly inhibited up to 800° C, which might be due to the precipitation of sigma phase.
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The results reported previously can be explained by both the formation of and the change in the fraction of sigma phase at a given temperature. The correspondence between the thermodynamic calculation using the TCFE database and the experimental reports including the previous and the current ones, suggests that the calculation using commercially released
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thermodynamic software is useful for predicting the formable phases in a given high-entropy alloy, and also for designing an alloy composition to achieve any specific property.
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Nevertheless, some adjustments in the interpretation of the results are required because the thermodynamic calculations do not fully cover the chemical compositions of HEAs and it shows an equilibrium state.
4. Conclusion The formation of Cr-rich sigma phase in the CoCrFeMnNi alloy was confirmed experimentally after the HPT process and subsequent annealing at temperatures lower than 800 °C. The formation of sigma phase was verified not only experimentally but also theoretically by the thermodynamic calculations based on the TCFE database. As the 9
ACCEPTED MANUSCRIPT annealing temperature decreased, the intensity of the XRD peak for the sigma phase increased. This suggested that both the formation of and the increase in the fraction of sigma phase can contribute to the increase in hardness of the specimens annealed at lower
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temperatures reported before [8].
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Acknowledgement This study was supported by the 2015 Yeungnam University Research Grant (215A580025), and the National Research Foundation of Korea (NRF) grant funded by the
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Korea government (MSIP) (No. NRF-2015R1C1A1A01052856). This work was also supported by the Grant-in-Aid for Scientific Research on Innovative Area, ‘‘Bulk Nanostructured Metals’’ (Area No.2201), the Grant-in-Aid for Scientific Research (A) (No.24246114), the Grant-in-Aid for Challenging Exploratory Research (No.26630365), and
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the Elements Strategy Initiative for Structural Materials (ESISM), all through the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan (Contact No.22102002).
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All these supports are gratefully appreciated.
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Captions Table 1.
Chemical composition and standard deviation for 118 points with 100 µm interval
Figure 1.
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of the homogenized specimen measured by wavelength-dispersive spectroscopy
X-ray diffraction patterns of the equiatomic CoCrFeMnNi alloy comparing the
starting material with specimens annealed at 600, 700 and 800 °C for 3.6 ks after a high-
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pressure torsion (HPT) process. An enlarged range from 35° to 52.5° to nominate diffraction planes of second phase is displayed in (b). The squares and circles indicate FCC and second
Figure 2.
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phases, respectively.
Transmission electron microscopy (TEM) images (a, c, d) and diffraction pattern
(b) of the specimen annealed at 700 °C for different periods after HPT process: (a, b) for 10 s, (c, d) for 600 s. (a, c, d) Bright-field TEM image, and (b) diffraction pattern of (a). (d) is an enlarged image of (c). The arrows in (a) indicate the formation of a second phase at the triple
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junction of the recrystallizing grains and at grains containing large number of dislocations. The arrows in (c) highlight the second phase which located on the triple junction of recrystallized grains. The arrow in (d) points the second phase on the incoherent twin
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boundary (TB).
Figure 3. High-angle annular dark field scanning TEM image (a) and chemical composition profile (b) of the specimen annealed at 700 °C for 3.6 ks after HPT process. The white and
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dark contrasts represent the second phase and FCC one, respectively.
Figure 4. Computation result of Thermo-Calc software using the TCFE database: (a) without any suspended phase, (b) after suspending BCC phase.
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