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Rapid solidification and liquid-phase separation of undercooled CoCrCuFexNi high-entropy alloys
Intermetallics 72 (2016) 44e52
Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Rapid solidification and liquid-phase separation of undercooled CoCrCuFexNi high-entropy alloys N. Liu a, *, P.H. Wu a, P.J. Zhou a, Z. Peng a, X.J. Wang a, **, Y.P. Lu b a b
School of Materials Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang, Jiangsu, 212003, PR China School of Materials Science and Engineering, Dalian University of Technology, Dalian, 116024, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 September 2015 Received in revised form 29 November 2015 Accepted 25 January 2016 Available online xxx
Fluxing and cyclic superheating technique was used to investigate the rapid solidification behavior of CoCrCuFexNi (x ¼ 1.0, 1.5, 2.0, molar concentration) high-entropy alloys in this study. The microstructures of CoCrCuFexNi (x ¼ 1.0, 1.5, 2.0) high-entropy alloys solidified at different undercoolings (DT) were investigated. Liquid-phase separation leading to Cu-rich and Cu-depleted regions, were obtained in the solidified microstructure from highly undercooled melt. This occurs when the melt undercooling exceeds a critical undercooling (DTcrit) of 160 K for CoCrCuFeNi, 190 K for CoCrCuFe1.5Ni and 293 K for CoCrCuFe2Ni alloy. However, typical dendrites and interdendritic regions were observed in rapid-solidified CoCrCuFexNi alloys prepared from melts with a small undercooling (DT < DTcrit). Conversely, a large amount of Cu-rich spheres and even egg-type structures were observed in alloys solidified from melts with large degree of undercooling, DT in excess of the critical value, DTcrit. A large amount of Cu-rich nano-phases were found in the matrix, possibly, due to the precipitation of Cu-rich phase from the supersaturated solid solution obtained during solidification. The positive enthalpies of mixing between Cu and the other elements in the multi-component alloys resulted in the occurrence of liquid-phase separation prior to the liquidesolid transformation starts. The sluggish diffusion effect of high-entropy alloys and rapid solidification play an important part in the precipitation of nanophase during the solid-state transformation in the Cu-based matrix. Similar to other immiscible alloys, liquid-phase separation occurred when a critical undercooling was exceeded. Differently, nanophases were found in the microstructures of multi-component CoCrCuFexNi (x ¼ 1.0, 1.5, 2.0) alloys. © 2016 Elsevier Ltd. All rights reserved.
Keywords: C. rapid solidification A. high-entropy alloys D. microstructure B. phase transformation D. nanocrystalline structure
1. Introduction Multi-component alloys [1], or high-entropy alloys (HEAs) [2], with at least five principal metallic elements having atomic percentages between 5% and 35%, are newly emerged metallic materials systems. Generally, the majority of as-solidified HEAs tend to solidify as simple solid solutions (FCC and/or BCC). FeNiCoCrMn equiatomic alloy, with simple FCC-structure, was first explored by Cantor [1]. Recently, Z. Wu et al. performed detailed studies of recovery, recrystallization, grain growth and phase stability of quaternary, ternary and binary equiatomic alloys made from the constituent elements of FeNiCoCrMn alloy [3]. Furthermore, the mechanical properties of Co-free FeNiCoCr18 and equiatomic
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (N. Liu), [email protected] (X.J. Wang). http://dx.doi.org/10.1016/j.intermet.2016.01.008 0966-9795/© 2016 Elsevier Ltd. All rights reserved.
FeNiCoCrMn alloys were investigated at large temperature ranges showing that the yield and ultimate strengths of this high-entropy alloy increase as the temperature is decreased [4e6]. Studies of ascast microstructure of another FCC-structured CoCrCu0.5FeNi high entropy alloy were carried out by Lin et al. [7]. However, more attention has been paid on the effect of heat treatment on the microstructure and property of CoCrCu0.5FeNi high entropy alloy [8,9]. Recently, a liquid-phase separation phenomenon was found in as-cast CoCrCuFe0.5Ni high-entropy alloy [10]. This suggests that CoCrCuFe0.5Ni high-entropy alloy is probably a metastable immiscible alloy. Liquid-phase separation was firstly studied by Nakagawa in CueCo and CueFe alloys in 1958 [11]. These alloy systems have a nearly flat liquidus and a positive deviation from the Raoult's law. All exhibit a definite thermodynamics tendency for liquid immiscibility as the undercooling or cooling rate exceeds a critical value. For such alloys, once a homogeneous liquid is undercooled into the metastable immiscibility gap, it will decompose into two liquids
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Fig. 1. Microstructure and surface scanning image of the undercooled CoCrCuFeNi alloy at △T ¼ 80 K.
Fig. 2. macroscopic images of CoCrCuFeNi alloy solidified at different undercoolings (a) △T ¼ 160 K, (b) △T ¼ 230 K, (c) △T ¼ 380K.
Table 1 The values of DHmix AB (kJ/mol) by Miedema's model for atomic pairs between the elements [20]. Element (atomic radius)
Co
Cr
Cu
Fe
Ni
Co(1.25) Cr(1.28) Cu(1.28) Fe(1.27) Ni(1.25)
e
4 e
þ6 þ12 e
1 1 þ13 e
0 7 þ4 2 e
phases, Cu-depleted (L1) and Cu-rich (L2) phases. Previously, liquid-phase separated microstructures of rapid-solidified binary CoeCu and FeeCu alloys and ternary FeeCoeCu alloy [12e17] have been investigated thoroughly. However, research on the liquidphase separation of multi-component alloys is still limited. Up to now, many questions are still open: Does liquid-phase separation occur in rapid-solidified CoCrCuFexNi (x ¼ 1.0, 1.5, 2.0) highentropy alloys? Are the microstructures of multi-component CoCrCuFexNi high-entropy alloys similar or different to those found in the abovementioned binary and ternary alloys? The purpose of this work is to provide a basic understanding of the solidification behavior of these high entropy alloys at different degree of undercoolings.
2. Experimental procedures 5 g of CoCrCuFexNi (x ¼ 1.0, 1.5, 2.0) alloys were prepared using high purity (99.9wt %) raw elements of cobalt, chromium, copper,
iron and nickel. Specimens were undercooled using fluxing purification and cyclic superheating technique. The detailed experimental procedures were described elsewhere [13]. To prevent the specimens from being oxidized, they were covered by borate glass. The heat released from the metal during heating can melt the glass purifier, which can thus enwrap the alloy and protect alloy from the surrounding atmosphere. It had been proved that the glass purifier did not react with the alloy melt, so the compositions of alloys remained the same. The crucible was placed in the middle position of the high-frequency induction coil located in a vacuum chamber. After the vacuum chamber was evacuated down to 1 106 mbar and then backfilled with a high-purity Ar gas to a chamber pressure of 600 mbar. The alloys were melted, superheated, and solidified several times, to obtain different degree of undercooling. It has been shown that the cyclic times and the superheated temperature (i.e., the difference of the highest heated temperature and the melting point temperature) before solidification can be correlated with the undercooled temperature; the undercooling was increased with the increasing superheated temperature and cyclic times. Thus, various undercoolings were achieved in the alloy melt to study its influence on the solidified microstructures. At the same time, the heating and cooling cycles of each sample were monitored by an infrared pyrometer with 5 K relative accuracy and 1 ms response time. The undercooling can be calculated using DT ¼ TmTn, where Tm is the melting temperature of the alloys, Tn is the nucleation temperature of the alloys. The rapid-solidified specimens of undercooled CoCrCuFexNi alloys were sectioned and polished, and then etched with chloroazotic acid. The microstructures of the alloys were examined using
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Fig. 3. Microstructures of the undercooled CoCrCuFeNi alloy at △T ¼ 160 K (aec) and △T ¼ 230 K (def). (a) and (d) the separated boundary, (b) and (e) the Cu-rich region, (c)and (f) the Cu-depleted region.
the scanning electron microscope (SEM, JEOL-JSM-6480), equipped with an X-ray energy-dispersive spectrometer (EDS).
3. Results and discussion 3.1. Microstructures of undercooled CoCrCuFexNi alloy (x ¼ 1) Dendrites and interdendritic structures were found in CoCrCuFeNi alloy solidified at △T ¼ 80 K, as shown in Fig. 1. According to the EDS measurement the dendrites comprised of Fe, Co and Ni elements, while the interdendritic region consisted of a large amount of Cu element. A large amount of Cr-rich rod-like phases were observed at the grain boundary of the dendrites, as well as in the Cu-rich regions, as shown in Fig. 1. At undercooling of 160 K, the as-solidified microstructure consisted of a liquid-phase separated structure, as shown in Fig. 2. Several spherical Cu-rich phase were found at the center of the sample, as shown in Fig. 2 a. It suggests that liquid-phase separation has occurred in the original homogeneous liquids and Cu-rich
droplets are formed prior to the occurrence of liquidesolid transformation. As the undercooling increased to 230 K, egg-type structure was obtained [18]. It consisted of two regions: namely Cu-rich and Cu-depleted regions, as labeled in Fig. 2 b. It demonstrates that multi-component CoCrCuFexNi high-entropy alloys are similar to CoeCu, FeeCu and FeeCoeCu alloys, and liquid-phase separation occurs when the undercooling of the melt exceeds a critical value. It is investigated that the occurrence of liquid-phase separation is related to the positive enthalpies of mixing between Cu and other elements [19]. As illustrated in Table 1 [20], the mixing enthalpies between CueFe, CueCo, CueCr, and CueNi are þ13, þ6, þ12 and þ 4 kJ/mol respectively. It means that the binding forces between Cu and Cr, Co and Fe elements are weak, and Cu cannot mix with the other elements thermodynamically, thereby Cr, Co and Fe elements are rejected from the Cu-rich phase, and vice versa. Generally, the liquideliquid phase transformation in metastable immiscible alloys begins with the nucleation of the liquid minority phase in the form of spheres [19]. In this work, the
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Fig. 4. Microstructures of the undercooled CoCrCuFe1.5Ni high-entropy alloys at different undercoolings (a) △T ¼ 40 K, (b) △T ¼ 160 K, (c) Line scanning of Fig. 4a.
Fig. 5. Macroscopic images of CoCrCuFe1.5Ni alloy solidified at different undercoolings (a) △T ¼ 190 K, (b) △T ¼ 350 K.
Fig. 6. Microstructures of the undercooled CoCrCuFe1.5Ni high-entropy alloys at different undercoolings (a) △T ¼ 190 K; (b) △T ¼ 350 K.
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Fig. 7. Nano-structures of the undercooled CoCrCuFe1.5Ni high-entropy alloys at different undercoolings. (a) △T ¼ 40 K; (b) △T ¼ 160 K; (c) △T ¼ 190 K; (d) △T ¼ 350 K.
minority phase is the Cu-rich liquids according to the experiment results, and the neighbouring liquid is Cu-depleted relatively. The Cu-rich droplets are not fixed in space but move around owing to various forces, such as gravity, natural convection and forced convection. The as-solidified microstructure of immiscible alloys strongly depends on the liquideliquid decomposition and the spatial separation behavior within the molten state [19]. If the time interval between the formation of the Cu-rich droplets and the solidification of the Cu-depleted liquids is long enough, then these Cu-rich droplets came together at the bottom of the melt before solidification, leading to the solidification of these two separated liquid phases. Especially, near the liquid-phase separation boundary, several Cu-depleted spheres, enriched with (Fe, Co, Ni) elements, were found in the Cu-rich region, as shown in Fig. 3aed, indicating the second liquid-phase separation occurred in the Cu-rich liquid. This suggests that Cu-depleted droplets form in the Cu-rich liquid before the liquidesolid transformation. The smaller the distance between the phase separation boundaries of the two regions, the more the Cu-depleted spheres exist. Furthermore, irregular structures can be seen at the center of the spheres, labeled as “A” in Fig. 3 b. EDS result shows that, Fe and Cr are rich in region A, while in region B the contents of Fe and Ni are higher than the nominal value. Meanwhile, many Cu-rich nanosized grains were also observed in the Cu-rich matrix, as shown in Fig. 3 b. Additionally, many (Co, Cr, Fe) rich rod-like precipitates were found at the grain boundary of the granular dendrite in the Cu-depleted region of Fig. 3 c. Liquid-phase separation becomes more evident when undercooling increases to 230 K. In the Cu-depleted region, Fig. 3 d, many Cu-rich spheres were observed near the separation boundary. At the same time several Cu-depleted spheres were seen in the Cu-rich region. Surprisingly, many Cu-rich nanosized precipitates were found in the Cu-depleted spherical structures when DT ¼ 230 K, as shown in Fig. 3e. In the Cu-depleted region, a similar
microstructure to those found in alloy undercooled to DT ¼ 160 K were observed (Fig. 3f). 3.2. Microstructure of undercooled CoCrCuFe1.5Ni alloy For CoCrCuFe1.5Ni alloy, a typical dendritic morphology and some interdendritic Cu-rich regions are observed at small undercoolings of 40 K and 160 K, as shown in Fig. 4a and b. Simultaneously, rod-like structures were found at the grain boundary, as well as in the Cu-rich region. According to EDS, the dendrites contained Fe, Co and Ni elements, while the rod-like structures comprised of Cr element (Fig. 4 c). Moreover, lots of Cu-rich phases distributed randomly in the trunk of the dendrites. This is believed to be a result of Cu precipitation from the supersaturated solid solution during the cooling cycle after rapid solidification. It is clear that liquid-phase separation did not occur in CoCrCuFe1.5Ni alloy until DT ¼ 160 K. A large amount of Cu-rich spheres were found in the CoCrCuFe1.5Ni alloy when DT¼190 K (Fig. 5 a ), indicating the occurrence of liquid-phase separation before solidification. At an undercooling of 350 K, a small region of Cu-rich phase were seen at the bottom of the sample, as labelled in Fig. 5 b. Some petal-shaped dendrites were seen in the Cu-rich spheres, which contained Fe, Co and Ni elements (Fig. 6 a ). A rough boundary divides the microstructure of undercooled CoCrCuFe1.5Ni alloy into two regions when undercooling increased to 350 K (Fig. 6 b). Similar microstructure to Fig. 3 f was observed in the Cu-depleted region of CoCrCuFe1.5Ni alloy solidified at an undercooling of 350 K. Large amount of white cubic phases, corresponded to Cu-rich nano-phases, were observed in the regions of interdendritic (i.e. small undercooling) and the Cu-rich dendritic regions (large undercooling) of undercooled CoCrCuFe1.5Ni alloys (Fig. 7). The sizes of nano-precipitates are found to be 110 nm, 100 nm, 140 nm and 100 nm with the increase of undercooling from 40 K to 350 K,
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Fig. 8. Microstructures of the undercooled CoCrCuFe2.0Ni high-entropy alloys solidified at (a) △T ¼ 100 K; (b) △T ¼ 157 K; (c) △T ¼ 293 K; (d) the liquid-phase separation boundary (e) the Cu-rich region and (f) the Cu-depleted region △T ¼ 398 K.
Fig. 9. Macroscopic images of CoCrCuFe2.0Ni alloy solidified at different undercoolings (a) △T ¼ 293 K, (b) △T ¼ 398 K.
respectively. Moreover, the amount of nano-precipitates increases with increasing undercooling from 40 to 350 K. Due to the positive enthalpies of mixing between Cu and the other component, solid
solubility between Cu and the other component is relative small. Supersaturated solid solutions were formed during rapid solidification, and then a large amount of nano-phases were precipitated
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Fig. 10. Nano-structures of the undercooled CoCrCuFe2.0Ni high-entropy alloys at different undercoolings (a) △T ¼ 100 K; (b) △T ¼ 157 K; (c) △T ¼ 293 K; (d) △T ¼ 398 K.
from the supersaturated solid solutions during solid-state transformation. 3.3. Microstructures of undercooled CoCrCuFe2.0Ni alloy As shown in Fig. 8, similar to CoCrCuFe1.5Ni alloy, typical dendritic and interdendritic structures were obtained in CoCrCuFe2.0Ni alloy at small undercoolings. Liquid-phase separated microstructure was observed only when the undercooling reached to a critical value 293 K, as shown in Fig. 9 a . When DT < 293 K, Fe, Co and Ni elements are still rich in the dendrites, and rod-like structures at grain boundary are rich in Cr and Fe element. Moreover, lots of Curich regions also exist at the interdendritic region. With the increase of undercooling, the shape of the Cu-rich region at the interdendrite changes from irregular to spheric (Fig. 8aec). It supports that the Cu-rich droplets form primarily in the original liquid, and liquid-phase separation occurs at DT ¼ 293 K before solidification. The macroscopic phase separation structure (Fig. 9 b) is obtained at DT ¼ 398 K, and microstructures of the phase separation boundary and the different regions of Cu-rich and Cu-depleted are shown in Fig. 8def. Rod-like Cr-rich precipitates are found at the phase separation boundary (Fig. 9d) and grain boundary of the Cudepleted regions (Fig. 9 f), as well as at the grain boundary of the Cu-rich region (the inserted graph of Fig. 9 e). Furthermore, nanosized Cu-rich phases were seen in the Cu-rich region, and the grain size decreases, but the number density increases with increasing undercooling, as shown in Fig. 10. Different to CoCrCuFeNi high-entropy alloys, nano-precipitates were not observed in the undercooled CoeCu, FeeCu and FeeCoeCu alloys. The lattice diffusion in the FeCoNiCrMn alloys had been studied systematically, and it was proposed that the diffusion in HEAs is sluggish. Diffusion couple experiments showed that the activation energy of diffusion in the FeCoNiCrMn alloys is higher than those in other FCC ternary alloys and pure elements
[21]. It also showed that for Ni atoms diffusing in FeCoNiCrMn alloys, the mean potential energy difference between lattice sites was 50% higher than that in FeeCreNi alloys [21]. This sluggish diffusion in HEAs could be the reason for formation of supersaturated solid solution and nano-sized precipitates. With the increased undercooling, the solidification process is far away from equilibrium solidification, and supersaturated solid solution forms during rapid solidification, possibly, due to the sluggish diffusion of high-entropy alloy. Especially for this multicomponent alloy, the solid solubility, to a large extent, exceeds the equilibrium value. As a result, nanosized precipitates form during the solid-state transformation. In summary, nonequilibrium supersaturated solid solution is obtained due to the sluggish atomic diffusion in high-entropy alloys during liquidesolid transformation. Subsequently, nanosized precipitates are formed in Cu-rich regions and Cu-depleted sphere during the cooling process.
3.4. Phase constituent of undercooled CoCrCuFexNi alloys The XRD patterns of undercooled CoCrCuFexNi alloys solidified at different undercoolings are presented in Fig. 11. The undercooled CoCrCuFexNi alloy mainly comprised of FCC structure, including FCC1 and FCC2, where FCC1 is similar to that of Fe0.64Ni0.36 phase with a lattice constant of 0.3583 nm, and FCC2 is Cu-rich phase with lattice constant of 0.3595 nm. At the same time, a small diffraction peak was found at about 2q ¼ 44 , corresponding to a cubic structural phase, which is possibly the Cr and Fe rich phase at the interdendrites. Moreover, the diffraction peak of Cr and Fe rich cubic structural phase becomes stronger with increasing Fe content. Combined with microstructure and composition analysis, FCC1 is dendritic phase and FCC2 phase corresponds to Cu-rich phase.
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Fig. 11. The XRD patterns of undercooled CoCrCuFexNi alloys (a) CoCrCuFeNi; (b) CoCrCuFe1.5Ni; (c) CoCrCuFe2Ni.
4. Conclusions Highly undercooled multi-component CoCrCuFexNi (x values in molar ratio, x ¼ 1.0, 1.5, 2.0) high-entropy alloys were studied for the first time. The results show that microstructures are changed with the increasing degree of undercooling. Based on the current study, the following conclusions can be drawn. (1) Liquid-phase separations occur in the undercooled multicomponent CoCrCuFexNi (x ¼ 1, 1.5, 2) high-entropy alloys. The critical undercooling (DT crit) value for liquid-phase separation is 160 K for CoCrCuFeNi, 190 K for CoCrCuFe1.5Ni and 293 K for CoCrCuFe2Ni. This indicates that multicomponent CoCrCuFexNi high-entropy alloys are metastable immiscible alloys. (2) When DT < DTcrit, the undercooled microstructures of undercooled CoCrCuFexNi alloys comprised of (Fe, Ni and Co)-rich dendrite and Cu-rich inter-dendrite region. When DT DTcrit, several Cu-rich spheres and even egg-type structures were obtained in the as-solidified microstructures of CoCrCuFexNi alloys with the increase of undercooling, indicating the occurrence of liquid-phase separation prior to solidification. The liquid-phase separation became more evident as the degree of undercooling increases. (3) The presence of supersaturated solid solution and Cu-rich nanosized phases in this multi-component CoCrCuFeNi high-entropy alloy is believed to be synergistic effects of high
entropy of mixing and sluggish atomic diffusion in solid state. Acknowledgments This work is supported by the National Natural Science Foundation of China Nos. (51201072 and 51471079) and the Qinglan Project of Jiangsu Provence. P.H. Wu is grateful to the financial support of Graduate Student Innovation Projects of Jiangsu University of Science and Technology (YSJ14S-15) and Graduate Student Innovation Projects of Jiangsu Province (SJLX15-0530). This work is supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] B. Cantor, I.T.H. Chang, P. Knight, et al., Microstructural development in equiatomic multicomponent alloys, Mater. Sci. Eng. A 375e377 (2004) 213. [2] J.W. Yeh, S.K. Chen, S.J. Lin, J.Y. Gan, T.S. Chin, T.T. Shun, Nanostructured highentropy alloys with multiple principal elements: novel alloy design concepts and outcomes, Adv. Eng. Mater. 6 (2004) 299e303. [3] Z. Wu, H. Bei, G.M. Pharr, E.P. George, Temperature dependence of the mechanical properties of equiatomic solid solution alloys with face-centered cubic crystal structures, Acta Mater. 81 (2014) 428e441. [4] Z. Wu, H. Bei, F. Otto, G.M. Pharr, E.P. George, Recovery, recrystallization, grain growth and phase stability of a family of FCC-structured multi-component equiatomic solid solution alloys, Intermetallics 46 (2014) 131e140. [5] Z. Wu, H. Bei, Microstructures and mechanical properties of compositionally complex Co-free FeNiMnCr18 FCC solid solution alloy, Mater. Sci. Eng. A 640 (2015) 217e224. [6] Z. Wu, Y.F. Gao, H. Bei, Single crystal plastic behavior of a single-phase, facecenter-cubic-structured, equiatomic FeNiCrCo alloy, Scr. Mater. 109 (2015)
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108e112. [7] C.M. Lin, H.L. Tsai, Equilibrium phase of high-entropy FeCoNiCrCu0.5 alloys at elevated temperature, J. Alloy. Comp. 489 (2010) 30e35. [8] C.M. Lin, H.L. Tsai, H.Y. Bor, effect of annealing treatment on microstructure and properties of high-entropy Cu0.5CoCrFeNi alloys, Intermetallics 18 (2010) 1244e1250. [9] C.M. Lin, H.L. Tsai, H.Y. Bor, effect of aging treatment on microstructure and properties of high-entropy Cu0.5CoCrFeNi alloys, Mater. Chem. Phys. 128 (2011) 50e56. [10] P. H. Wu, N. Liu, P.J. Zhou, Z. Peng, W.D. Du, X. J. Wang, Y. Pan, Microstructures and Liquid-Phase Separation in Multi-component CoCrCuFeNi High-entropy Alloys, accepted by Mater. Sci. Tech. doi: http://dx.doi.org/10.1179/ 1743284715Y.0000000127. [11] Y. Nakagawa, liquid immiscibility in Copper-iron and Copper-Cobalt in the supercooled state, Acta Metall. 6 (1958) 704. [12] N. Liu, Investigation on the phase separation in undercooled Cu-Fe melts, J. Non-crystalline Solids 358 (2012) 196. [13] Y.Z. Chen, F. Liu, G.C. Yang, X.Q. Xu, Y.H. Zhou, Rapid solidification of bulk undercooled hypoperitectic FeeCu alloy, J. Alloy. Comp. 427 (2007) L1eL5. [14] W. Yang, Z.F. Xu, W.J. Li, C.C. Cai, S. Li, F. Liu, G.C. Yang, Comparisons of grain refinement and recalescence behavior during the rapid solidification of
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Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Rapid solidification and liquid-phase separation of undercooled CoCrCuFexNi high-entropy alloys N. Liu a, *, P.H. Wu a, P.J. Zhou a, Z. Peng a, X.J. Wang a, **, Y.P. Lu b a b
School of Materials Science and Engineering, Jiangsu University of Science and Technology, Zhenjiang, Jiangsu, 212003, PR China School of Materials Science and Engineering, Dalian University of Technology, Dalian, 116024, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 September 2015 Received in revised form 29 November 2015 Accepted 25 January 2016 Available online xxx
Fluxing and cyclic superheating technique was used to investigate the rapid solidification behavior of CoCrCuFexNi (x ¼ 1.0, 1.5, 2.0, molar concentration) high-entropy alloys in this study. The microstructures of CoCrCuFexNi (x ¼ 1.0, 1.5, 2.0) high-entropy alloys solidified at different undercoolings (DT) were investigated. Liquid-phase separation leading to Cu-rich and Cu-depleted regions, were obtained in the solidified microstructure from highly undercooled melt. This occurs when the melt undercooling exceeds a critical undercooling (DTcrit) of 160 K for CoCrCuFeNi, 190 K for CoCrCuFe1.5Ni and 293 K for CoCrCuFe2Ni alloy. However, typical dendrites and interdendritic regions were observed in rapid-solidified CoCrCuFexNi alloys prepared from melts with a small undercooling (DT < DTcrit). Conversely, a large amount of Cu-rich spheres and even egg-type structures were observed in alloys solidified from melts with large degree of undercooling, DT in excess of the critical value, DTcrit. A large amount of Cu-rich nano-phases were found in the matrix, possibly, due to the precipitation of Cu-rich phase from the supersaturated solid solution obtained during solidification. The positive enthalpies of mixing between Cu and the other elements in the multi-component alloys resulted in the occurrence of liquid-phase separation prior to the liquidesolid transformation starts. The sluggish diffusion effect of high-entropy alloys and rapid solidification play an important part in the precipitation of nanophase during the solid-state transformation in the Cu-based matrix. Similar to other immiscible alloys, liquid-phase separation occurred when a critical undercooling was exceeded. Differently, nanophases were found in the microstructures of multi-component CoCrCuFexNi (x ¼ 1.0, 1.5, 2.0) alloys. © 2016 Elsevier Ltd. All rights reserved.
Keywords: C. rapid solidification A. high-entropy alloys D. microstructure B. phase transformation D. nanocrystalline structure
1. Introduction Multi-component alloys [1], or high-entropy alloys (HEAs) [2], with at least five principal metallic elements having atomic percentages between 5% and 35%, are newly emerged metallic materials systems. Generally, the majority of as-solidified HEAs tend to solidify as simple solid solutions (FCC and/or BCC). FeNiCoCrMn equiatomic alloy, with simple FCC-structure, was first explored by Cantor [1]. Recently, Z. Wu et al. performed detailed studies of recovery, recrystallization, grain growth and phase stability of quaternary, ternary and binary equiatomic alloys made from the constituent elements of FeNiCoCrMn alloy [3]. Furthermore, the mechanical properties of Co-free FeNiCoCr18 and equiatomic
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (N. Liu), [email protected] (X.J. Wang). http://dx.doi.org/10.1016/j.intermet.2016.01.008 0966-9795/© 2016 Elsevier Ltd. All rights reserved.
FeNiCoCrMn alloys were investigated at large temperature ranges showing that the yield and ultimate strengths of this high-entropy alloy increase as the temperature is decreased [4e6]. Studies of ascast microstructure of another FCC-structured CoCrCu0.5FeNi high entropy alloy were carried out by Lin et al. [7]. However, more attention has been paid on the effect of heat treatment on the microstructure and property of CoCrCu0.5FeNi high entropy alloy [8,9]. Recently, a liquid-phase separation phenomenon was found in as-cast CoCrCuFe0.5Ni high-entropy alloy [10]. This suggests that CoCrCuFe0.5Ni high-entropy alloy is probably a metastable immiscible alloy. Liquid-phase separation was firstly studied by Nakagawa in CueCo and CueFe alloys in 1958 [11]. These alloy systems have a nearly flat liquidus and a positive deviation from the Raoult's law. All exhibit a definite thermodynamics tendency for liquid immiscibility as the undercooling or cooling rate exceeds a critical value. For such alloys, once a homogeneous liquid is undercooled into the metastable immiscibility gap, it will decompose into two liquids
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Fig. 1. Microstructure and surface scanning image of the undercooled CoCrCuFeNi alloy at △T ¼ 80 K.
Fig. 2. macroscopic images of CoCrCuFeNi alloy solidified at different undercoolings (a) △T ¼ 160 K, (b) △T ¼ 230 K, (c) △T ¼ 380K.
Table 1 The values of DHmix AB (kJ/mol) by Miedema's model for atomic pairs between the elements [20]. Element (atomic radius)
Co
Cr
Cu
Fe
Ni
Co(1.25) Cr(1.28) Cu(1.28) Fe(1.27) Ni(1.25)
e
4 e
þ6 þ12 e
1 1 þ13 e
0 7 þ4 2 e
phases, Cu-depleted (L1) and Cu-rich (L2) phases. Previously, liquid-phase separated microstructures of rapid-solidified binary CoeCu and FeeCu alloys and ternary FeeCoeCu alloy [12e17] have been investigated thoroughly. However, research on the liquidphase separation of multi-component alloys is still limited. Up to now, many questions are still open: Does liquid-phase separation occur in rapid-solidified CoCrCuFexNi (x ¼ 1.0, 1.5, 2.0) highentropy alloys? Are the microstructures of multi-component CoCrCuFexNi high-entropy alloys similar or different to those found in the abovementioned binary and ternary alloys? The purpose of this work is to provide a basic understanding of the solidification behavior of these high entropy alloys at different degree of undercoolings.
2. Experimental procedures 5 g of CoCrCuFexNi (x ¼ 1.0, 1.5, 2.0) alloys were prepared using high purity (99.9wt %) raw elements of cobalt, chromium, copper,
iron and nickel. Specimens were undercooled using fluxing purification and cyclic superheating technique. The detailed experimental procedures were described elsewhere [13]. To prevent the specimens from being oxidized, they were covered by borate glass. The heat released from the metal during heating can melt the glass purifier, which can thus enwrap the alloy and protect alloy from the surrounding atmosphere. It had been proved that the glass purifier did not react with the alloy melt, so the compositions of alloys remained the same. The crucible was placed in the middle position of the high-frequency induction coil located in a vacuum chamber. After the vacuum chamber was evacuated down to 1 106 mbar and then backfilled with a high-purity Ar gas to a chamber pressure of 600 mbar. The alloys were melted, superheated, and solidified several times, to obtain different degree of undercooling. It has been shown that the cyclic times and the superheated temperature (i.e., the difference of the highest heated temperature and the melting point temperature) before solidification can be correlated with the undercooled temperature; the undercooling was increased with the increasing superheated temperature and cyclic times. Thus, various undercoolings were achieved in the alloy melt to study its influence on the solidified microstructures. At the same time, the heating and cooling cycles of each sample were monitored by an infrared pyrometer with 5 K relative accuracy and 1 ms response time. The undercooling can be calculated using DT ¼ TmTn, where Tm is the melting temperature of the alloys, Tn is the nucleation temperature of the alloys. The rapid-solidified specimens of undercooled CoCrCuFexNi alloys were sectioned and polished, and then etched with chloroazotic acid. The microstructures of the alloys were examined using
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Fig. 3. Microstructures of the undercooled CoCrCuFeNi alloy at △T ¼ 160 K (aec) and △T ¼ 230 K (def). (a) and (d) the separated boundary, (b) and (e) the Cu-rich region, (c)and (f) the Cu-depleted region.
the scanning electron microscope (SEM, JEOL-JSM-6480), equipped with an X-ray energy-dispersive spectrometer (EDS).
3. Results and discussion 3.1. Microstructures of undercooled CoCrCuFexNi alloy (x ¼ 1) Dendrites and interdendritic structures were found in CoCrCuFeNi alloy solidified at △T ¼ 80 K, as shown in Fig. 1. According to the EDS measurement the dendrites comprised of Fe, Co and Ni elements, while the interdendritic region consisted of a large amount of Cu element. A large amount of Cr-rich rod-like phases were observed at the grain boundary of the dendrites, as well as in the Cu-rich regions, as shown in Fig. 1. At undercooling of 160 K, the as-solidified microstructure consisted of a liquid-phase separated structure, as shown in Fig. 2. Several spherical Cu-rich phase were found at the center of the sample, as shown in Fig. 2 a. It suggests that liquid-phase separation has occurred in the original homogeneous liquids and Cu-rich
droplets are formed prior to the occurrence of liquidesolid transformation. As the undercooling increased to 230 K, egg-type structure was obtained [18]. It consisted of two regions: namely Cu-rich and Cu-depleted regions, as labeled in Fig. 2 b. It demonstrates that multi-component CoCrCuFexNi high-entropy alloys are similar to CoeCu, FeeCu and FeeCoeCu alloys, and liquid-phase separation occurs when the undercooling of the melt exceeds a critical value. It is investigated that the occurrence of liquid-phase separation is related to the positive enthalpies of mixing between Cu and other elements [19]. As illustrated in Table 1 [20], the mixing enthalpies between CueFe, CueCo, CueCr, and CueNi are þ13, þ6, þ12 and þ 4 kJ/mol respectively. It means that the binding forces between Cu and Cr, Co and Fe elements are weak, and Cu cannot mix with the other elements thermodynamically, thereby Cr, Co and Fe elements are rejected from the Cu-rich phase, and vice versa. Generally, the liquideliquid phase transformation in metastable immiscible alloys begins with the nucleation of the liquid minority phase in the form of spheres [19]. In this work, the
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Fig. 4. Microstructures of the undercooled CoCrCuFe1.5Ni high-entropy alloys at different undercoolings (a) △T ¼ 40 K, (b) △T ¼ 160 K, (c) Line scanning of Fig. 4a.
Fig. 5. Macroscopic images of CoCrCuFe1.5Ni alloy solidified at different undercoolings (a) △T ¼ 190 K, (b) △T ¼ 350 K.
Fig. 6. Microstructures of the undercooled CoCrCuFe1.5Ni high-entropy alloys at different undercoolings (a) △T ¼ 190 K; (b) △T ¼ 350 K.
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Fig. 7. Nano-structures of the undercooled CoCrCuFe1.5Ni high-entropy alloys at different undercoolings. (a) △T ¼ 40 K; (b) △T ¼ 160 K; (c) △T ¼ 190 K; (d) △T ¼ 350 K.
minority phase is the Cu-rich liquids according to the experiment results, and the neighbouring liquid is Cu-depleted relatively. The Cu-rich droplets are not fixed in space but move around owing to various forces, such as gravity, natural convection and forced convection. The as-solidified microstructure of immiscible alloys strongly depends on the liquideliquid decomposition and the spatial separation behavior within the molten state [19]. If the time interval between the formation of the Cu-rich droplets and the solidification of the Cu-depleted liquids is long enough, then these Cu-rich droplets came together at the bottom of the melt before solidification, leading to the solidification of these two separated liquid phases. Especially, near the liquid-phase separation boundary, several Cu-depleted spheres, enriched with (Fe, Co, Ni) elements, were found in the Cu-rich region, as shown in Fig. 3aed, indicating the second liquid-phase separation occurred in the Cu-rich liquid. This suggests that Cu-depleted droplets form in the Cu-rich liquid before the liquidesolid transformation. The smaller the distance between the phase separation boundaries of the two regions, the more the Cu-depleted spheres exist. Furthermore, irregular structures can be seen at the center of the spheres, labeled as “A” in Fig. 3 b. EDS result shows that, Fe and Cr are rich in region A, while in region B the contents of Fe and Ni are higher than the nominal value. Meanwhile, many Cu-rich nanosized grains were also observed in the Cu-rich matrix, as shown in Fig. 3 b. Additionally, many (Co, Cr, Fe) rich rod-like precipitates were found at the grain boundary of the granular dendrite in the Cu-depleted region of Fig. 3 c. Liquid-phase separation becomes more evident when undercooling increases to 230 K. In the Cu-depleted region, Fig. 3 d, many Cu-rich spheres were observed near the separation boundary. At the same time several Cu-depleted spheres were seen in the Cu-rich region. Surprisingly, many Cu-rich nanosized precipitates were found in the Cu-depleted spherical structures when DT ¼ 230 K, as shown in Fig. 3e. In the Cu-depleted region, a similar
microstructure to those found in alloy undercooled to DT ¼ 160 K were observed (Fig. 3f). 3.2. Microstructure of undercooled CoCrCuFe1.5Ni alloy For CoCrCuFe1.5Ni alloy, a typical dendritic morphology and some interdendritic Cu-rich regions are observed at small undercoolings of 40 K and 160 K, as shown in Fig. 4a and b. Simultaneously, rod-like structures were found at the grain boundary, as well as in the Cu-rich region. According to EDS, the dendrites contained Fe, Co and Ni elements, while the rod-like structures comprised of Cr element (Fig. 4 c). Moreover, lots of Cu-rich phases distributed randomly in the trunk of the dendrites. This is believed to be a result of Cu precipitation from the supersaturated solid solution during the cooling cycle after rapid solidification. It is clear that liquid-phase separation did not occur in CoCrCuFe1.5Ni alloy until DT ¼ 160 K. A large amount of Cu-rich spheres were found in the CoCrCuFe1.5Ni alloy when DT¼190 K (Fig. 5 a ), indicating the occurrence of liquid-phase separation before solidification. At an undercooling of 350 K, a small region of Cu-rich phase were seen at the bottom of the sample, as labelled in Fig. 5 b. Some petal-shaped dendrites were seen in the Cu-rich spheres, which contained Fe, Co and Ni elements (Fig. 6 a ). A rough boundary divides the microstructure of undercooled CoCrCuFe1.5Ni alloy into two regions when undercooling increased to 350 K (Fig. 6 b). Similar microstructure to Fig. 3 f was observed in the Cu-depleted region of CoCrCuFe1.5Ni alloy solidified at an undercooling of 350 K. Large amount of white cubic phases, corresponded to Cu-rich nano-phases, were observed in the regions of interdendritic (i.e. small undercooling) and the Cu-rich dendritic regions (large undercooling) of undercooled CoCrCuFe1.5Ni alloys (Fig. 7). The sizes of nano-precipitates are found to be 110 nm, 100 nm, 140 nm and 100 nm with the increase of undercooling from 40 K to 350 K,
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Fig. 8. Microstructures of the undercooled CoCrCuFe2.0Ni high-entropy alloys solidified at (a) △T ¼ 100 K; (b) △T ¼ 157 K; (c) △T ¼ 293 K; (d) the liquid-phase separation boundary (e) the Cu-rich region and (f) the Cu-depleted region △T ¼ 398 K.
Fig. 9. Macroscopic images of CoCrCuFe2.0Ni alloy solidified at different undercoolings (a) △T ¼ 293 K, (b) △T ¼ 398 K.
respectively. Moreover, the amount of nano-precipitates increases with increasing undercooling from 40 to 350 K. Due to the positive enthalpies of mixing between Cu and the other component, solid
solubility between Cu and the other component is relative small. Supersaturated solid solutions were formed during rapid solidification, and then a large amount of nano-phases were precipitated
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Fig. 10. Nano-structures of the undercooled CoCrCuFe2.0Ni high-entropy alloys at different undercoolings (a) △T ¼ 100 K; (b) △T ¼ 157 K; (c) △T ¼ 293 K; (d) △T ¼ 398 K.
from the supersaturated solid solutions during solid-state transformation. 3.3. Microstructures of undercooled CoCrCuFe2.0Ni alloy As shown in Fig. 8, similar to CoCrCuFe1.5Ni alloy, typical dendritic and interdendritic structures were obtained in CoCrCuFe2.0Ni alloy at small undercoolings. Liquid-phase separated microstructure was observed only when the undercooling reached to a critical value 293 K, as shown in Fig. 9 a . When DT < 293 K, Fe, Co and Ni elements are still rich in the dendrites, and rod-like structures at grain boundary are rich in Cr and Fe element. Moreover, lots of Curich regions also exist at the interdendritic region. With the increase of undercooling, the shape of the Cu-rich region at the interdendrite changes from irregular to spheric (Fig. 8aec). It supports that the Cu-rich droplets form primarily in the original liquid, and liquid-phase separation occurs at DT ¼ 293 K before solidification. The macroscopic phase separation structure (Fig. 9 b) is obtained at DT ¼ 398 K, and microstructures of the phase separation boundary and the different regions of Cu-rich and Cu-depleted are shown in Fig. 8def. Rod-like Cr-rich precipitates are found at the phase separation boundary (Fig. 9d) and grain boundary of the Cudepleted regions (Fig. 9 f), as well as at the grain boundary of the Cu-rich region (the inserted graph of Fig. 9 e). Furthermore, nanosized Cu-rich phases were seen in the Cu-rich region, and the grain size decreases, but the number density increases with increasing undercooling, as shown in Fig. 10. Different to CoCrCuFeNi high-entropy alloys, nano-precipitates were not observed in the undercooled CoeCu, FeeCu and FeeCoeCu alloys. The lattice diffusion in the FeCoNiCrMn alloys had been studied systematically, and it was proposed that the diffusion in HEAs is sluggish. Diffusion couple experiments showed that the activation energy of diffusion in the FeCoNiCrMn alloys is higher than those in other FCC ternary alloys and pure elements
[21]. It also showed that for Ni atoms diffusing in FeCoNiCrMn alloys, the mean potential energy difference between lattice sites was 50% higher than that in FeeCreNi alloys [21]. This sluggish diffusion in HEAs could be the reason for formation of supersaturated solid solution and nano-sized precipitates. With the increased undercooling, the solidification process is far away from equilibrium solidification, and supersaturated solid solution forms during rapid solidification, possibly, due to the sluggish diffusion of high-entropy alloy. Especially for this multicomponent alloy, the solid solubility, to a large extent, exceeds the equilibrium value. As a result, nanosized precipitates form during the solid-state transformation. In summary, nonequilibrium supersaturated solid solution is obtained due to the sluggish atomic diffusion in high-entropy alloys during liquidesolid transformation. Subsequently, nanosized precipitates are formed in Cu-rich regions and Cu-depleted sphere during the cooling process.
3.4. Phase constituent of undercooled CoCrCuFexNi alloys The XRD patterns of undercooled CoCrCuFexNi alloys solidified at different undercoolings are presented in Fig. 11. The undercooled CoCrCuFexNi alloy mainly comprised of FCC structure, including FCC1 and FCC2, where FCC1 is similar to that of Fe0.64Ni0.36 phase with a lattice constant of 0.3583 nm, and FCC2 is Cu-rich phase with lattice constant of 0.3595 nm. At the same time, a small diffraction peak was found at about 2q ¼ 44 , corresponding to a cubic structural phase, which is possibly the Cr and Fe rich phase at the interdendrites. Moreover, the diffraction peak of Cr and Fe rich cubic structural phase becomes stronger with increasing Fe content. Combined with microstructure and composition analysis, FCC1 is dendritic phase and FCC2 phase corresponds to Cu-rich phase.
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Fig. 11. The XRD patterns of undercooled CoCrCuFexNi alloys (a) CoCrCuFeNi; (b) CoCrCuFe1.5Ni; (c) CoCrCuFe2Ni.
4. Conclusions Highly undercooled multi-component CoCrCuFexNi (x values in molar ratio, x ¼ 1.0, 1.5, 2.0) high-entropy alloys were studied for the first time. The results show that microstructures are changed with the increasing degree of undercooling. Based on the current study, the following conclusions can be drawn. (1) Liquid-phase separations occur in the undercooled multicomponent CoCrCuFexNi (x ¼ 1, 1.5, 2) high-entropy alloys. The critical undercooling (DT crit) value for liquid-phase separation is 160 K for CoCrCuFeNi, 190 K for CoCrCuFe1.5Ni and 293 K for CoCrCuFe2Ni. This indicates that multicomponent CoCrCuFexNi high-entropy alloys are metastable immiscible alloys. (2) When DT < DTcrit, the undercooled microstructures of undercooled CoCrCuFexNi alloys comprised of (Fe, Ni and Co)-rich dendrite and Cu-rich inter-dendrite region. When DT DTcrit, several Cu-rich spheres and even egg-type structures were obtained in the as-solidified microstructures of CoCrCuFexNi alloys with the increase of undercooling, indicating the occurrence of liquid-phase separation prior to solidification. The liquid-phase separation became more evident as the degree of undercooling increases. (3) The presence of supersaturated solid solution and Cu-rich nanosized phases in this multi-component CoCrCuFeNi high-entropy alloy is believed to be synergistic effects of high
entropy of mixing and sluggish atomic diffusion in solid state. Acknowledgments This work is supported by the National Natural Science Foundation of China Nos. (51201072 and 51471079) and the Qinglan Project of Jiangsu Provence. P.H. Wu is grateful to the financial support of Graduate Student Innovation Projects of Jiangsu University of Science and Technology (YSJ14S-15) and Graduate Student Innovation Projects of Jiangsu Province (SJLX15-0530). This work is supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions. References [1] B. Cantor, I.T.H. Chang, P. Knight, et al., Microstructural development in equiatomic multicomponent alloys, Mater. Sci. Eng. A 375e377 (2004) 213. [2] J.W. Yeh, S.K. Chen, S.J. Lin, J.Y. Gan, T.S. Chin, T.T. Shun, Nanostructured highentropy alloys with multiple principal elements: novel alloy design concepts and outcomes, Adv. Eng. Mater. 6 (2004) 299e303. [3] Z. Wu, H. Bei, G.M. Pharr, E.P. George, Temperature dependence of the mechanical properties of equiatomic solid solution alloys with face-centered cubic crystal structures, Acta Mater. 81 (2014) 428e441. [4] Z. Wu, H. Bei, F. Otto, G.M. Pharr, E.P. George, Recovery, recrystallization, grain growth and phase stability of a family of FCC-structured multi-component equiatomic solid solution alloys, Intermetallics 46 (2014) 131e140. [5] Z. Wu, H. Bei, Microstructures and mechanical properties of compositionally complex Co-free FeNiMnCr18 FCC solid solution alloy, Mater. Sci. Eng. A 640 (2015) 217e224. [6] Z. Wu, Y.F. Gao, H. Bei, Single crystal plastic behavior of a single-phase, facecenter-cubic-structured, equiatomic FeNiCrCo alloy, Scr. Mater. 109 (2015)
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