- Email: [email protected]
Alloying behavior and novel properties of CoCrFeNiMn high-entropy alloy fabricated by mechanical alloying and spark plasma sintering
Intermetallics 56 (2015) 24e27
Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Short communication
Alloying behavior and novel properties of CoCrFeNiMn high-entropy alloy fabricated by mechanical alloying and spark plasma sintering Wei Ji, Weimin Wang, Hao Wang, Jinyong Zhang, Yucheng Wang, Fan Zhang, Zhengyi Fu* State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 June 2014 Received in revised form 10 August 2014 Accepted 24 August 2014 Available online
An equiatomic CoCrFeNiMn high-entropy alloy was synthesized by mechanical alloying (MA) and spark plasma sintering (SPS). During MA, a solid solution with refined microstructure of 10 nm which consists of a FCC phase and a BCC phase was formed. After SPS consolidation, only one FCC phase can be detected in the HEA bulks. The as-sintered bulks exhibit high compressive strength of 1987 MPa. An interesting magnetic transition associated with the structure coarsening and phase transformation was observed during SPS process. © 2014 Elsevier Ltd. All rights reserved.
Keywords: A. High-entropy alloys B. Mechanical properties B. Magnetic properties C. Mechanical alloying and milling D. Microstructure F. Electron microscopy, transmission
1. Introduction For centuries, the design concept of alloy systems has been based on utilizing one or two elements as the principal components, with minor amounts of other elements for property enhancement, such as steels and NiAl intermetallics [1,2]. However, this paradigm has been broken by the suggestion of high-entropy alloys (HEAs) developed by Yeh et al. [3]. A HEA is originally defined as an alloy system composed of at least five principal elements in an equimolar or near equimolar ratio (varying 5e35 at.%) with a small difference in atom radii (<15%). The high mixing entropy of multi-principle elements will induce lattice distortion and sluggish cooperative diffusion. As a consequence, HEAs often possess simple solid-solutions or amorphous structure rather than intermetallics [4] and exhibit high hardness, excellent strength as well as promising resistances to wear, oxidation and corrosion [5]. Among HEA systems, the equi/non-equiatomic CoCrFeNiMn has attracted great interest for its unique characteristics. Liu et al. investigated the grain growth behavior of CoCrFeNiMn HEA during annealing [6]. Yao and his colleagues studied the exceptional phase stability and tensile ductility of Co5Cr2Fe40Ni26Mn27 HEA [7]. Moreover, the microstructure, texture evolution and dislocation nucleation of CoCrFeNiMn HEAs fabricated by vacuum arc-melting
* Corresponding author. Tel.: þ86 027 87865484; fax: þ86 027 87215421. E-mail address: [email protected] (Z. Fu). http://dx.doi.org/10.1016/j.intermet.2014.08.008 0966-9795/© 2014 Elsevier Ltd. All rights reserved.
have been investigated systematically [8,9]. However, the CoCrFeNiMn HEA system has been prepared mainly by arc melt/casting. But those fabrication routes are unsuitable for industrial manufacturing due to the disadvantages of diseconomy and limitations in shape and size of final products [10]. By contrast, mechanical alloying (MA) is a more convenient way, which has been widely used for the synthesis of nanocrystalline materials with uniform microstructure. Thus MA is expected to reduce the cost of preparing nanocrystalline materials and widen the application of HEA [11,12]. Combined with the new spark plasma sintering (SPS) technique, high-entropy alloys can be easily obtained from the asmilled powders [13e15]. What's more, the novel magnetic properties, which can be usually observed in traditional alloy systems containing Mn element [16e18], have not been investigated for the CoCrFeNiMn HEA. This scope is also an academic issue and required to be revealed and discussed. In this work, we focused on the high-entropy alloy system of CoCrFeNiMn synthesized by mechanical alloying and spark plasma sintering, and studied on the alloying behavior, microstructure, mechanical and magnetic properties.
2. Experiment details High purity (>99.5 wt.%) Co, Cr, Fe, Ni and Mn powders with particle size less than 45 mm were used as starting materials. The elemental powders were mixed in equiatomic composition and
W. Ji et al. / Intermetallics 56 (2015) 24e27
milled in a planetary ball-miller for 60 h at 250 rpm in an argon atmosphere. Stainless steel vials and balls were utilized as the milling media with a ball-to-powder mass ratio of 15:1. N-heptane was introduced as the processing controlling agent (PCA) to avoid cold welding as well as preventing metal oxidation. The MA process was monitored by regular powder extraction at an interval of 6 h. Subsequently, the as-milled powder was consolidated by SPS (Dr. Sinter-3.20 MKII, SCM) at 800 C for 10 min under 50 MPa uniaxial pressure in argon atmosphere. The crystal structure of the as-preserved alloy prepared was examined by X-ray diffractometer (XRD, Rigaku Ultima III) with CuKa radiation. The microstructure of the powders was observed using scanning electron microscopy (SEM, Hitachi 3400) and transmission electron microscopy (TEM, JEOL JEM-2010HT). Density of the bulk HEA was calculated using the Archimedes principle. Bulk hardness of the sectioned and polished specimens was measured using Vickers hardness tester (Wolpert-430SV). The compressive properties at room temperature were measured by a MTS810 testing machine. The magnetic properties of the HEAs were characterized on a Physical Property Measurement System (PPMS, Quantum Design PPMS-9T). 3. Results and discussion The XRD patterns of as-milled CoCrFeNiMn HEAs powders and consolidated bulks were shown in Fig. 1. The primary blending powder includes diffraction patterns of all alloying elements. After 6 h MA, the diffraction peaks of the principle elements can still be observed with a dramatic decrease in intensity. With prolonged milling time to 30 h, peak broadening is obvious and some peaks become invisible. As the milling time increases to 60 h, only peaks belongs to a BCC structure ((110), (200), (211)) and an FCC structure ((111), (200)) can be identified, by which is deduced the formation of a simple solid solution. Throughout the milling process, the decrease in intensity, broadening of the peak and its subsequent disappearance may result from the three following factors: refined crystal size, high lattice strain and decreased crystallinity. The crystallite sizes (CS) during MA were present in Table 1 calculated by Scherrer's formula after eliminating the instrumental and the strain contributions. As shown, the CS of the BCC phase is significantly refined to 13.4 nm after 42 h MA and then slightly decreases to 12.7 nm after 60 h milling. The results reveal that the balance between crystalline refinement and cold welding
Fig. 1. XRD patterns of CoCrFeNiMn HEAs powders milled under different time and CoCrFeNiMn HEA bulk consolidated by SPS.
25
Table 1 The crystalline size (CS) and lattice parameter (LP) of the BCC and FCC phases with different milling time. Milling time (h)
CS (nm) BCC
12 30 42 60
21.3 16.1 13.4 12.7
LP (Å) FCC
± ± ± ±
0.02 0.02 0.01 0.01
21.1 16.5 13.9 9.8
BCC ± ± ± ±
0.02 0.02 0.02 0.01
2.866 2.871 2.876 2.878
FCC ± ± ± ±
0.01 0.02 0.01 0.01
3.519 3.524 3.534 3.536
± ± ± ±
0.01 0.01 0.01 0.02
of BCC phase might have been achieved when the milling time reaches 42 h. Further increase of the milling time has no vital influence on the crystallite size of the BCC phase, but the FCC phase can still be refined after 42 h of milling. The CS of the FCC phase for 42 h MA and 60 h MA are 13.9 nm and 9.8 nm, respectively. Table 1 also lists the lattice parameters (LP) of the BCC and FCC phases with varying milling time. The lattice parameters of both BCC and FCC phases increase as the milling time prolongs. At the initial stage of milling, the LP of BCC (2.866 Å) and FCC (3.519 Å) phases are rather closed to that of highly pure iron (2.866 Å) and nickel (3.524 Å), respectively. As MA processing, the simple solidsolutions are gradually formed from the principle components. The corporation of elements with larger atomic size, e.g. Cr and Mn, results in the enlargement in lattice parameter. The XRD pattern of the SPSed HEA bulk shows only an FCC solid solution structures, seen in Fig. 1. The crystallite structure is different from that of the as-milled alloy but similar with the ascast one [7]. Compared with the XRD patterns of as-milled HEA powder, a peak shift can be clearly observed for the as-sintered bulk. The peak shifting towards lower Bragg angle (2q) indicates that the lattice parameter of as-sintered HEA bulk is larger than that of as-milled HEA powder. The phase transition is resulted from the converting to more stable phases of as-milled alloy powder during subsequent consolidation at higher temperatures [19,20]. Moreover, due to the non-equilibrium state of the MA process, a large amount of internal stress would be stored in the lattice, as well as defects, i.e. lattice distortion and twins. The internal stress was released when the HEA was sintered at high temperature and the metastable state transformed to stable one. Both would contribute to the expansion of the crystal lattice. The microstructures of obtained CoCrFeNiMn HEA powders after 60 h MA were shown in Fig. 2(a)e(b). As shown in Fig. 2(a), asmilled powder agglomerates into elliptical shape of ~2.36 mm and less than 1 mm in thickness. The nanocrystalline nature of CoCrFeNiMn HEA powder has been further characterized by the TEM bright field image and the SAED patterns, seen in Fig. 2(b). The average grains with size of ~10 nm can be observed in the bright field TEM image, and the rings in the SAED pattern of Fig. 2(b) reveal that the nanocrystalline HEA powder consists of a BCC phase and a FCC phase, which is in good agreement with the XRD analysis. The results confirm that the CoCrFeNiMn high-entropy alloy with a structure of simple solid solution has been successfully fabricated by mechanical alloying. The TEM bright field image and corresponding SAED patterns of bulk CoCrFeNiMn HEA by SPS are shown in Fig. 2(c). It can be observed that there are two different sizes of grains, one is 100e200 nm with twin crystals and the other one is approximate 50 nm. Corresponding SAED patterns indicate that the grains both have a structure of FCC, with similar calculated lattice parameters of 3.589 Å and 3.590 Å, respectively. Available literatures indicate that only a single FCC phase exist in the CoCrFeNiMn high-entropy alloy [8,9] and so the grains with different size can be considered as one FCC phase. The result can better account for the only FCC crystal
26
W. Ji et al. / Intermetallics 56 (2015) 24e27
Fig. 2. Microstructures of as-preserved CoCrFeNiMn HEAs: (a)e(b) SEM and TEM images of 60 h milled HEA powder, respectively; and (c) TEM bright field image of SPSed CoCrFeNiMn HEA.
structure characterized in Fig. 1, because these grains with different size possess the same lattice parameter that cannot be distinguished by the X-ray diffractometer. The room-temperature compressive properties of the SPS-ed HEA are shown in Fig. 3. It can be observed that the bulk specimen exhibits a high compressive strength of 1987 MPa, which is higher than most of the as-reported HEAs [21]. The density of 7.85 g/cm3 and Vickers hardness of 646 HV are achieved for the bulks. The value of hardness is higher than that of most commercial available hard facing alloys (e.g. stellite, approximately 500 HV [22]) and some HEAs with similar element composition (e.g. CoCrFeNiTiAl HEA, 432 HV [23]). The excellent properties of hardness and compressive strength depend to the solid solution strengthening and nano-scale grains. Fig. 4 presents the magnetic hysteresis curves of the aspreserved CoCrFeNiMn high-entropy alloys measured at room temperature. The HEA powder milled for 60 h exhibits soft magnetic properties and the saturated magnetizations (Ms), remanence ratio (Mr/Ms) and coercivity force (Hc) are 94.29 emu/g, 2.95% and 175.68 Oe, respectively. Compared with the magnetic properties of
Fig. 3. Compressive stressestrain curve of SPSed CoCrFeNiMn HEA.
W. Ji et al. / Intermetallics 56 (2015) 24e27
27
project from the Ministry of Science and Technology of the People's Republic of China. References
Fig. 4. Magnetic hysteresis curves of the as-preserved CoCrFeNiMn high-entropy alloys measured at room temperature.
HEAs in the available literatures [24], the CoCrFeNiMn alloy powder shows higher saturated magnetizations and lower remanence ratio. This result indicates that the as-milled powder possesses an excellent characteristic feature of soft magnetic and can be utilized as soft magnetic materials. On the other hand, the alloy powder shows a similar superparamagnetic property. As previously stated, the as-milled HEA powder was consisted of nanoparticles with the size of ~10 nm, which can easily lead to the superparamagnetic behavior. It is well known that superparamagnetic behavior is observed for magnetic nanoparticles with sizes less than 10 nm [25,26]. However, after consolidated by SPS at 800 C, the HEA bulk has a paramagnetic curve with the Ms of 32.49 emu/g and Hc of 0 Oe under 20,000 Oe magnetic field. The interesting phenomenon of magnetic evolution during SPS was resulted from the structure coarsening and phase transformation [27]. As shown in Fig. 2(c), the grain size of SPS-ed HEA is larger than 50 nm and even up to 200 nm. Nanoparticles in this size could not exhibit superparamagnetic properties again [28]. In addition, Zhang and coworkers [29] had investigated the magnetic properties of TixCoCrCuFeNi alloys and they shows magnetic transition with the increasing of Ti content. But in the present alloy system, there is no ratio change for the metal elements. So the result is irradiative for further application in some special condition. 4. Conclusion The alloying behavior and magnetic evolution in an equiatomic CoCrFeNiMn HEA during MA-SPS process was studied. The simple structure with a BCC phase and a FCC phase after 60 h milling reveals the synthesis of the HEA. TEM results indicate that two FCC phases with similar lattice parameters and different grain sizes coexisted in the SPS-consolidated HEA. The bulks exhibit high compressive strength of 1987 MPa. An interesting magnetic transition during SPS was discovered, which is associated with the structure coarsening and phase transformation after sintering. Acknowledgment This work was financially supported by the National Natural Science Foundation of China and the international cooperation
[1] Greer AL. Confusion by design. Nature 1993;366:303e4. [2] Inoue A, Wang XM. Bulk amorphous FC20 (FeeCeSi) alloys with small amounts of B and their crystallized structure and mechanical properties. Acta Mater 2000;48:1383e95. [3] Yeh JW, Chen SK, Lin SJ, Gan JY, Chin TS, Shun TT, et al. Nanostructured highentropy alloys with multiple principal elements: novel alloy design concepts and outcomes. Adv Eng Mater 2004;6:299e303. [4] Yeh JW. Recent progress in high-entropy alloys. Ann Chim Sci Mat 2006;31: 633e48. [5] Chen YY, Duval T, Hung UD, Yeh JW, Shih HC. Microstructure and electrochemical properties of high entropy alloys-a comparison with type-304 stainless steel. Corros Sci 2005;47:2257e79. [6] Liu WH, Wu Y, He JY, Nieh TG, Lu ZP. Grain growth and the HallePetch relationship in a high-entropy FeCrNiCoMn alloy. Scr Mater 2013;68:526e9. [7] Yao MJ, Pradeep KG, Tasan CC, Raabe D. A novel, single phase, non-equiatomic FeMnNiCoCr high-entropy alloy with exceptional phase stability and tensile ductility. Scr Mater 2014;72e73:5e8. [8] Zhu C, Lu ZP, Nieh TG. Incipient plasticity and dislocation nucleation of FeCoCrNiMn high-entropy alloy. Acta Mater 2013;61:2993e3001. [9] Bhattacharjee PP, Sathiaraj GD, Zaid M, Gatti JR, Lee C, Tsai CW, et al. Microstructure and texture evolution during annealing of equiatomic CoCrFeMnNi high-entropy alloy. J Alloy Compd 2014;587:544e52. [10] Suryanarayana C, Ivanov E, Boldyrev VV. The science and technology of mechanical alloying. Mater Sci Eng A 2001;304e306:151e8. [11] Zhang KB, Fu ZY, Zhang JY, Shi J, Wang WM, Wang H, et al. Nanocrystalline CoCrFeNiCuAl high-entropy solid solution synthesized by mechanical alloying. J Alloy Compd 2009;485:31e4. [12] Chen WP, Fu ZQ, Fang SC, Xiao HQ, Zhu DZ. Alloying behavior, microstructure and mechanical properties in a FeNiCrCo0.3Al0.7 high entropy alloy. Mater Des 2013;51:854e60. [13] Li Q, Wang G, Song XP, Fan L, Hu WT, Xiao FR, et al. Ti50Cu23Ni20Sn7 bulk metallic glasses prepared by mechanical alloying and spark-plasma sintering. J Mater Process Tech 2009;209:3285e8. [14] Fu ZQ, Chen WP, Xiao HQ, Zhou LW, Zhu DZ, Yang SF. Fabrication and properties of nanocrystalline Co0.5FeNiCrTi0.5 high entropy alloy by MA-SPS technique. Mater Des 2013;44:535e9. [15] Bouad N, Marin-Ayral RM, Tedenac JC. Mechanical alloying and sintering of lead telluride. J Alloy Compd 2000;297:312e8. [16] Khovaylo VV, Skokov KP, Gutfleisch O, Miki H, Takagi T, Kanomata T, et al. Peculiarities of the magnetocaloric properties in NieMneSn ferromagnetic shape memory alloys. Phys Rev B 2010;81:214406. [17] Wu ZG, Liu ZH, Yang H, Liu YN, Wu GH. Metamagnetic phase transformation in Mn50Ni37In10Co3 polycrystalline alloy. Appl Phys Lett 2011;98:061904. [18] Yao CZ, Zhang P, Liu M, Li GR, Ye JQ, Liu P, et al. Electrochemical preparation and magnetic study of BieFeeCoeNieMn high entropy alloy. Electrochim Acta 2008;53:8359e65. [19] Ji W, Fu ZY, Wang WM, Wang H, Zhang JY, Wang YC, et al. Mechanical alloying synthesis and spark plasma sintering consolidation of CoCrFeNiAl highentropy alloy. J Alloy Compd 2014;589:61e6. [20] Yavari AR, Desre PJ, Benameur T. Benameur, mechanically driven alloying of immiscible elements. Phys Rev Lett 1992;68:2235e8. [21] Wang YP, Li BS, Ren MX, Yang C, Fu HZ. Microstructure and compressive properties of AlCrFeCoNi high entropy alloy. Mater Sci Eng A 2008;491: 154e8. [22] Kapoor S, Liu R, Wu XJ, Yao MX. Temperature-dependence of hardness and wear resistance of stellite alloys. Eng Technol 2012;67:964e73. [23] Zhang KB, Fu ZY, Zhang JY, Wang WM, Lee SW, Niihara K. Characterization of nanocrystalline CoCrFeNiTiAl high-entropy solid solution processed by mechanical alloying. J Alloy Compd 2010;495:33e8. [24] Zhang KB, Fu ZY, Zhang JY, Shi J, Wang WM, Wang H, et al. Annealing on the structure and properties evolution of the CoCrFeNiCuAl high-entropy alloy. J Alloy Compd 2010;502:295e9. [25] Shinde SR, Ogale SB, Higgins JS. Co-occurrence of superparamagnetism and anomalous Hall effect in highly reduced cobalt-doped rutile TiO2d films. Phys Rev Lett 2004;92:166601. [26] Fan XB, Tan FY, Zhang GL, Zhang FB. A novel strategy to fabricate gammaFe2O3-MWCNTs hybrids with selectively ferromagnetic or superparamagnetic properties. Mater Sci Eng A 2007;454:37e42. [27] Zhang Y, Zuo TT, Tang Z, Gao MC, Dahmen KA, Liaw PK, et al. Microstructures and properties of high-entropy alloys. Prog Mater Sci 2014;61:1e93. [28] Vucinic-Vasic M, Bozin ES, Bessais L, Stojanovic G, Kozmidis-Luburic U, Abeykoon M, et al. Thermal evolution of cation distribution/crystallite size and their correlation with the magnetic state of Yb-substituted zinc ferrite nanoparticles. J Phys Chem C 2013;117:12358e65. [29] Wang XF, Zhang Y, Qiao Y, Chen GL. Novel microstructure and properties of multicomponent CoCrCuFeNiTix alloys. Intermetalllics 2007;15:357e62.
Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Short communication
Alloying behavior and novel properties of CoCrFeNiMn high-entropy alloy fabricated by mechanical alloying and spark plasma sintering Wei Ji, Weimin Wang, Hao Wang, Jinyong Zhang, Yucheng Wang, Fan Zhang, Zhengyi Fu* State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 June 2014 Received in revised form 10 August 2014 Accepted 24 August 2014 Available online
An equiatomic CoCrFeNiMn high-entropy alloy was synthesized by mechanical alloying (MA) and spark plasma sintering (SPS). During MA, a solid solution with refined microstructure of 10 nm which consists of a FCC phase and a BCC phase was formed. After SPS consolidation, only one FCC phase can be detected in the HEA bulks. The as-sintered bulks exhibit high compressive strength of 1987 MPa. An interesting magnetic transition associated with the structure coarsening and phase transformation was observed during SPS process. © 2014 Elsevier Ltd. All rights reserved.
Keywords: A. High-entropy alloys B. Mechanical properties B. Magnetic properties C. Mechanical alloying and milling D. Microstructure F. Electron microscopy, transmission
1. Introduction For centuries, the design concept of alloy systems has been based on utilizing one or two elements as the principal components, with minor amounts of other elements for property enhancement, such as steels and NiAl intermetallics [1,2]. However, this paradigm has been broken by the suggestion of high-entropy alloys (HEAs) developed by Yeh et al. [3]. A HEA is originally defined as an alloy system composed of at least five principal elements in an equimolar or near equimolar ratio (varying 5e35 at.%) with a small difference in atom radii (<15%). The high mixing entropy of multi-principle elements will induce lattice distortion and sluggish cooperative diffusion. As a consequence, HEAs often possess simple solid-solutions or amorphous structure rather than intermetallics [4] and exhibit high hardness, excellent strength as well as promising resistances to wear, oxidation and corrosion [5]. Among HEA systems, the equi/non-equiatomic CoCrFeNiMn has attracted great interest for its unique characteristics. Liu et al. investigated the grain growth behavior of CoCrFeNiMn HEA during annealing [6]. Yao and his colleagues studied the exceptional phase stability and tensile ductility of Co5Cr2Fe40Ni26Mn27 HEA [7]. Moreover, the microstructure, texture evolution and dislocation nucleation of CoCrFeNiMn HEAs fabricated by vacuum arc-melting
* Corresponding author. Tel.: þ86 027 87865484; fax: þ86 027 87215421. E-mail address: [email protected] (Z. Fu). http://dx.doi.org/10.1016/j.intermet.2014.08.008 0966-9795/© 2014 Elsevier Ltd. All rights reserved.
have been investigated systematically [8,9]. However, the CoCrFeNiMn HEA system has been prepared mainly by arc melt/casting. But those fabrication routes are unsuitable for industrial manufacturing due to the disadvantages of diseconomy and limitations in shape and size of final products [10]. By contrast, mechanical alloying (MA) is a more convenient way, which has been widely used for the synthesis of nanocrystalline materials with uniform microstructure. Thus MA is expected to reduce the cost of preparing nanocrystalline materials and widen the application of HEA [11,12]. Combined with the new spark plasma sintering (SPS) technique, high-entropy alloys can be easily obtained from the asmilled powders [13e15]. What's more, the novel magnetic properties, which can be usually observed in traditional alloy systems containing Mn element [16e18], have not been investigated for the CoCrFeNiMn HEA. This scope is also an academic issue and required to be revealed and discussed. In this work, we focused on the high-entropy alloy system of CoCrFeNiMn synthesized by mechanical alloying and spark plasma sintering, and studied on the alloying behavior, microstructure, mechanical and magnetic properties.
2. Experiment details High purity (>99.5 wt.%) Co, Cr, Fe, Ni and Mn powders with particle size less than 45 mm were used as starting materials. The elemental powders were mixed in equiatomic composition and
W. Ji et al. / Intermetallics 56 (2015) 24e27
milled in a planetary ball-miller for 60 h at 250 rpm in an argon atmosphere. Stainless steel vials and balls were utilized as the milling media with a ball-to-powder mass ratio of 15:1. N-heptane was introduced as the processing controlling agent (PCA) to avoid cold welding as well as preventing metal oxidation. The MA process was monitored by regular powder extraction at an interval of 6 h. Subsequently, the as-milled powder was consolidated by SPS (Dr. Sinter-3.20 MKII, SCM) at 800 C for 10 min under 50 MPa uniaxial pressure in argon atmosphere. The crystal structure of the as-preserved alloy prepared was examined by X-ray diffractometer (XRD, Rigaku Ultima III) with CuKa radiation. The microstructure of the powders was observed using scanning electron microscopy (SEM, Hitachi 3400) and transmission electron microscopy (TEM, JEOL JEM-2010HT). Density of the bulk HEA was calculated using the Archimedes principle. Bulk hardness of the sectioned and polished specimens was measured using Vickers hardness tester (Wolpert-430SV). The compressive properties at room temperature were measured by a MTS810 testing machine. The magnetic properties of the HEAs were characterized on a Physical Property Measurement System (PPMS, Quantum Design PPMS-9T). 3. Results and discussion The XRD patterns of as-milled CoCrFeNiMn HEAs powders and consolidated bulks were shown in Fig. 1. The primary blending powder includes diffraction patterns of all alloying elements. After 6 h MA, the diffraction peaks of the principle elements can still be observed with a dramatic decrease in intensity. With prolonged milling time to 30 h, peak broadening is obvious and some peaks become invisible. As the milling time increases to 60 h, only peaks belongs to a BCC structure ((110), (200), (211)) and an FCC structure ((111), (200)) can be identified, by which is deduced the formation of a simple solid solution. Throughout the milling process, the decrease in intensity, broadening of the peak and its subsequent disappearance may result from the three following factors: refined crystal size, high lattice strain and decreased crystallinity. The crystallite sizes (CS) during MA were present in Table 1 calculated by Scherrer's formula after eliminating the instrumental and the strain contributions. As shown, the CS of the BCC phase is significantly refined to 13.4 nm after 42 h MA and then slightly decreases to 12.7 nm after 60 h milling. The results reveal that the balance between crystalline refinement and cold welding
Fig. 1. XRD patterns of CoCrFeNiMn HEAs powders milled under different time and CoCrFeNiMn HEA bulk consolidated by SPS.
25
Table 1 The crystalline size (CS) and lattice parameter (LP) of the BCC and FCC phases with different milling time. Milling time (h)
CS (nm) BCC
12 30 42 60
21.3 16.1 13.4 12.7
LP (Å) FCC
± ± ± ±
0.02 0.02 0.01 0.01
21.1 16.5 13.9 9.8
BCC ± ± ± ±
0.02 0.02 0.02 0.01
2.866 2.871 2.876 2.878
FCC ± ± ± ±
0.01 0.02 0.01 0.01
3.519 3.524 3.534 3.536
± ± ± ±
0.01 0.01 0.01 0.02
of BCC phase might have been achieved when the milling time reaches 42 h. Further increase of the milling time has no vital influence on the crystallite size of the BCC phase, but the FCC phase can still be refined after 42 h of milling. The CS of the FCC phase for 42 h MA and 60 h MA are 13.9 nm and 9.8 nm, respectively. Table 1 also lists the lattice parameters (LP) of the BCC and FCC phases with varying milling time. The lattice parameters of both BCC and FCC phases increase as the milling time prolongs. At the initial stage of milling, the LP of BCC (2.866 Å) and FCC (3.519 Å) phases are rather closed to that of highly pure iron (2.866 Å) and nickel (3.524 Å), respectively. As MA processing, the simple solidsolutions are gradually formed from the principle components. The corporation of elements with larger atomic size, e.g. Cr and Mn, results in the enlargement in lattice parameter. The XRD pattern of the SPSed HEA bulk shows only an FCC solid solution structures, seen in Fig. 1. The crystallite structure is different from that of the as-milled alloy but similar with the ascast one [7]. Compared with the XRD patterns of as-milled HEA powder, a peak shift can be clearly observed for the as-sintered bulk. The peak shifting towards lower Bragg angle (2q) indicates that the lattice parameter of as-sintered HEA bulk is larger than that of as-milled HEA powder. The phase transition is resulted from the converting to more stable phases of as-milled alloy powder during subsequent consolidation at higher temperatures [19,20]. Moreover, due to the non-equilibrium state of the MA process, a large amount of internal stress would be stored in the lattice, as well as defects, i.e. lattice distortion and twins. The internal stress was released when the HEA was sintered at high temperature and the metastable state transformed to stable one. Both would contribute to the expansion of the crystal lattice. The microstructures of obtained CoCrFeNiMn HEA powders after 60 h MA were shown in Fig. 2(a)e(b). As shown in Fig. 2(a), asmilled powder agglomerates into elliptical shape of ~2.36 mm and less than 1 mm in thickness. The nanocrystalline nature of CoCrFeNiMn HEA powder has been further characterized by the TEM bright field image and the SAED patterns, seen in Fig. 2(b). The average grains with size of ~10 nm can be observed in the bright field TEM image, and the rings in the SAED pattern of Fig. 2(b) reveal that the nanocrystalline HEA powder consists of a BCC phase and a FCC phase, which is in good agreement with the XRD analysis. The results confirm that the CoCrFeNiMn high-entropy alloy with a structure of simple solid solution has been successfully fabricated by mechanical alloying. The TEM bright field image and corresponding SAED patterns of bulk CoCrFeNiMn HEA by SPS are shown in Fig. 2(c). It can be observed that there are two different sizes of grains, one is 100e200 nm with twin crystals and the other one is approximate 50 nm. Corresponding SAED patterns indicate that the grains both have a structure of FCC, with similar calculated lattice parameters of 3.589 Å and 3.590 Å, respectively. Available literatures indicate that only a single FCC phase exist in the CoCrFeNiMn high-entropy alloy [8,9] and so the grains with different size can be considered as one FCC phase. The result can better account for the only FCC crystal
26
W. Ji et al. / Intermetallics 56 (2015) 24e27
Fig. 2. Microstructures of as-preserved CoCrFeNiMn HEAs: (a)e(b) SEM and TEM images of 60 h milled HEA powder, respectively; and (c) TEM bright field image of SPSed CoCrFeNiMn HEA.
structure characterized in Fig. 1, because these grains with different size possess the same lattice parameter that cannot be distinguished by the X-ray diffractometer. The room-temperature compressive properties of the SPS-ed HEA are shown in Fig. 3. It can be observed that the bulk specimen exhibits a high compressive strength of 1987 MPa, which is higher than most of the as-reported HEAs [21]. The density of 7.85 g/cm3 and Vickers hardness of 646 HV are achieved for the bulks. The value of hardness is higher than that of most commercial available hard facing alloys (e.g. stellite, approximately 500 HV [22]) and some HEAs with similar element composition (e.g. CoCrFeNiTiAl HEA, 432 HV [23]). The excellent properties of hardness and compressive strength depend to the solid solution strengthening and nano-scale grains. Fig. 4 presents the magnetic hysteresis curves of the aspreserved CoCrFeNiMn high-entropy alloys measured at room temperature. The HEA powder milled for 60 h exhibits soft magnetic properties and the saturated magnetizations (Ms), remanence ratio (Mr/Ms) and coercivity force (Hc) are 94.29 emu/g, 2.95% and 175.68 Oe, respectively. Compared with the magnetic properties of
Fig. 3. Compressive stressestrain curve of SPSed CoCrFeNiMn HEA.
W. Ji et al. / Intermetallics 56 (2015) 24e27
27
project from the Ministry of Science and Technology of the People's Republic of China. References
Fig. 4. Magnetic hysteresis curves of the as-preserved CoCrFeNiMn high-entropy alloys measured at room temperature.
HEAs in the available literatures [24], the CoCrFeNiMn alloy powder shows higher saturated magnetizations and lower remanence ratio. This result indicates that the as-milled powder possesses an excellent characteristic feature of soft magnetic and can be utilized as soft magnetic materials. On the other hand, the alloy powder shows a similar superparamagnetic property. As previously stated, the as-milled HEA powder was consisted of nanoparticles with the size of ~10 nm, which can easily lead to the superparamagnetic behavior. It is well known that superparamagnetic behavior is observed for magnetic nanoparticles with sizes less than 10 nm [25,26]. However, after consolidated by SPS at 800 C, the HEA bulk has a paramagnetic curve with the Ms of 32.49 emu/g and Hc of 0 Oe under 20,000 Oe magnetic field. The interesting phenomenon of magnetic evolution during SPS was resulted from the structure coarsening and phase transformation [27]. As shown in Fig. 2(c), the grain size of SPS-ed HEA is larger than 50 nm and even up to 200 nm. Nanoparticles in this size could not exhibit superparamagnetic properties again [28]. In addition, Zhang and coworkers [29] had investigated the magnetic properties of TixCoCrCuFeNi alloys and they shows magnetic transition with the increasing of Ti content. But in the present alloy system, there is no ratio change for the metal elements. So the result is irradiative for further application in some special condition. 4. Conclusion The alloying behavior and magnetic evolution in an equiatomic CoCrFeNiMn HEA during MA-SPS process was studied. The simple structure with a BCC phase and a FCC phase after 60 h milling reveals the synthesis of the HEA. TEM results indicate that two FCC phases with similar lattice parameters and different grain sizes coexisted in the SPS-consolidated HEA. The bulks exhibit high compressive strength of 1987 MPa. An interesting magnetic transition during SPS was discovered, which is associated with the structure coarsening and phase transformation after sintering. Acknowledgment This work was financially supported by the National Natural Science Foundation of China and the international cooperation
[1] Greer AL. Confusion by design. Nature 1993;366:303e4. [2] Inoue A, Wang XM. Bulk amorphous FC20 (FeeCeSi) alloys with small amounts of B and their crystallized structure and mechanical properties. Acta Mater 2000;48:1383e95. [3] Yeh JW, Chen SK, Lin SJ, Gan JY, Chin TS, Shun TT, et al. Nanostructured highentropy alloys with multiple principal elements: novel alloy design concepts and outcomes. Adv Eng Mater 2004;6:299e303. [4] Yeh JW. Recent progress in high-entropy alloys. Ann Chim Sci Mat 2006;31: 633e48. [5] Chen YY, Duval T, Hung UD, Yeh JW, Shih HC. Microstructure and electrochemical properties of high entropy alloys-a comparison with type-304 stainless steel. Corros Sci 2005;47:2257e79. [6] Liu WH, Wu Y, He JY, Nieh TG, Lu ZP. Grain growth and the HallePetch relationship in a high-entropy FeCrNiCoMn alloy. Scr Mater 2013;68:526e9. [7] Yao MJ, Pradeep KG, Tasan CC, Raabe D. A novel, single phase, non-equiatomic FeMnNiCoCr high-entropy alloy with exceptional phase stability and tensile ductility. Scr Mater 2014;72e73:5e8. [8] Zhu C, Lu ZP, Nieh TG. Incipient plasticity and dislocation nucleation of FeCoCrNiMn high-entropy alloy. Acta Mater 2013;61:2993e3001. [9] Bhattacharjee PP, Sathiaraj GD, Zaid M, Gatti JR, Lee C, Tsai CW, et al. Microstructure and texture evolution during annealing of equiatomic CoCrFeMnNi high-entropy alloy. J Alloy Compd 2014;587:544e52. [10] Suryanarayana C, Ivanov E, Boldyrev VV. The science and technology of mechanical alloying. Mater Sci Eng A 2001;304e306:151e8. [11] Zhang KB, Fu ZY, Zhang JY, Shi J, Wang WM, Wang H, et al. Nanocrystalline CoCrFeNiCuAl high-entropy solid solution synthesized by mechanical alloying. J Alloy Compd 2009;485:31e4. [12] Chen WP, Fu ZQ, Fang SC, Xiao HQ, Zhu DZ. Alloying behavior, microstructure and mechanical properties in a FeNiCrCo0.3Al0.7 high entropy alloy. Mater Des 2013;51:854e60. [13] Li Q, Wang G, Song XP, Fan L, Hu WT, Xiao FR, et al. Ti50Cu23Ni20Sn7 bulk metallic glasses prepared by mechanical alloying and spark-plasma sintering. J Mater Process Tech 2009;209:3285e8. [14] Fu ZQ, Chen WP, Xiao HQ, Zhou LW, Zhu DZ, Yang SF. Fabrication and properties of nanocrystalline Co0.5FeNiCrTi0.5 high entropy alloy by MA-SPS technique. Mater Des 2013;44:535e9. [15] Bouad N, Marin-Ayral RM, Tedenac JC. Mechanical alloying and sintering of lead telluride. J Alloy Compd 2000;297:312e8. [16] Khovaylo VV, Skokov KP, Gutfleisch O, Miki H, Takagi T, Kanomata T, et al. Peculiarities of the magnetocaloric properties in NieMneSn ferromagnetic shape memory alloys. Phys Rev B 2010;81:214406. [17] Wu ZG, Liu ZH, Yang H, Liu YN, Wu GH. Metamagnetic phase transformation in Mn50Ni37In10Co3 polycrystalline alloy. Appl Phys Lett 2011;98:061904. [18] Yao CZ, Zhang P, Liu M, Li GR, Ye JQ, Liu P, et al. Electrochemical preparation and magnetic study of BieFeeCoeNieMn high entropy alloy. Electrochim Acta 2008;53:8359e65. [19] Ji W, Fu ZY, Wang WM, Wang H, Zhang JY, Wang YC, et al. Mechanical alloying synthesis and spark plasma sintering consolidation of CoCrFeNiAl highentropy alloy. J Alloy Compd 2014;589:61e6. [20] Yavari AR, Desre PJ, Benameur T. Benameur, mechanically driven alloying of immiscible elements. Phys Rev Lett 1992;68:2235e8. [21] Wang YP, Li BS, Ren MX, Yang C, Fu HZ. Microstructure and compressive properties of AlCrFeCoNi high entropy alloy. Mater Sci Eng A 2008;491: 154e8. [22] Kapoor S, Liu R, Wu XJ, Yao MX. Temperature-dependence of hardness and wear resistance of stellite alloys. Eng Technol 2012;67:964e73. [23] Zhang KB, Fu ZY, Zhang JY, Wang WM, Lee SW, Niihara K. Characterization of nanocrystalline CoCrFeNiTiAl high-entropy solid solution processed by mechanical alloying. J Alloy Compd 2010;495:33e8. [24] Zhang KB, Fu ZY, Zhang JY, Shi J, Wang WM, Wang H, et al. Annealing on the structure and properties evolution of the CoCrFeNiCuAl high-entropy alloy. J Alloy Compd 2010;502:295e9. [25] Shinde SR, Ogale SB, Higgins JS. Co-occurrence of superparamagnetism and anomalous Hall effect in highly reduced cobalt-doped rutile TiO2d films. Phys Rev Lett 2004;92:166601. [26] Fan XB, Tan FY, Zhang GL, Zhang FB. A novel strategy to fabricate gammaFe2O3-MWCNTs hybrids with selectively ferromagnetic or superparamagnetic properties. Mater Sci Eng A 2007;454:37e42. [27] Zhang Y, Zuo TT, Tang Z, Gao MC, Dahmen KA, Liaw PK, et al. Microstructures and properties of high-entropy alloys. Prog Mater Sci 2014;61:1e93. [28] Vucinic-Vasic M, Bozin ES, Bessais L, Stojanovic G, Kozmidis-Luburic U, Abeykoon M, et al. Thermal evolution of cation distribution/crystallite size and their correlation with the magnetic state of Yb-substituted zinc ferrite nanoparticles. J Phys Chem C 2013;117:12358e65. [29] Wang XF, Zhang Y, Qiao Y, Chen GL. Novel microstructure and properties of multicomponent CoCrCuFeNiTix alloys. Intermetalllics 2007;15:357e62.