Mechanical alloying synthesis and spark plasma sintering consolidation of CoCrFeNiAl high-entropy alloy

Journal of Alloys and Compounds 589 (2014) 61–66

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Mechanical alloying synthesis and spark plasma sintering consolidation of CoCrFeNiAl high-entropy alloy Wei Ji, Zhengyi Fu ⇑, Weimin Wang, Hao Wang, Jinyong Zhang, Yucheng Wang, Fan Zhang 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

Article history: Received 13 August 2013 Received in revised form 19 November 2013 Accepted 22 November 2013 Available online 1 December 2013 Keywords: Entropy Mechanical alloying Sintering Microstructure Phase transitions Mechanical properties

a b s t r a c t An equiatomic CoCrFeNiAl high-entropy alloy was synthesized by mechanical alloying, and phase evolutions, microstructure, thermal properties and annealing behaviors were investigated. It was found that a body-centered cubic structured solid solution with refined microstructure of 20 nm in grain size could be obtained after 30 h milling. As-milled powder exhibited good phase stability until 500 °C, and transformed into a face-centered cubic phase above 500 °C. The as-milled powder was subsequently consolidated by spark plasma sintering at 900 °C. BCC phase and FCC phase coexisted in the consolidated HEA, which had excellent properties in Vickers hardness of 625 HV and compressive strength of 1907 MPa. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Traditional metallurgical theory suggests that complex phases or intermetallics will be easily formed in alloy systems with multiple principal components and lead to poor mechanical properties of the alloys [1]. So for centuries, the design concept of alloy systems has been based on utilizing one or two elements as the principal components, and sometimes, minor amounts of other elements are used to enhance the properties, such as steels and NiAl intermetallics [2]. However, this paradigm has been broken by the theory of high-entropy alloys (HEA) 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, with a small difference in atom radii (<15%) and concentration of each element varying from 5 to 35 at.%. The high mixing entropy of multi-principle elements will induce lattice distortion and sluggish cooperative diffusion. As a consequence, HEA often possess simple solid-solutions and amorphous structure rather than intermetallics [4]. With proper composition designing, the HEA exhibits high hardness, excellent ductility as well as promising resistances to wear, oxidation and corrosion [5]. Among the HEA systems, CoCrCuFeNi with a face-centered cubic (FCC) structure and CoCrFeNiAl with a body-centered cubic (BCC) structure have been most studied [3,4,6,7]. CoCrFeNiAl system is a simple but fundamental system for the analysis of ⇑ Corresponding author. Tel.: +86 027 87865484; fax: +86 027 87215421. E-mail address: [email protected] (Z. Fu). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.11.146

the relationship between the microstructure and properties of the HEA. For instance, Wang et al. investigated the microstructure and compressive properties of the CoCrFeNiAl HEA prepared by vacuum arc melting [6]. Wang et al. studied cooling rate and size effect on the microstructure and mechanical properties of CoCrFeNiAl HEA [7]. Moreover, the CoCrFeNiAlx (0 6 x 6 2) HEAs fabricated by casting have been investigated systematically [8–10]. Apparently, the CoCrFeNiAl HEA system has been prepared mainly by arc melt/casting. The fabrication routes are unsuitable for industrial manufacturing due to the disadvantages of diseconomy and limitations in shape and size of final products [11]. 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 scope of HEA [12,13]. Combined with the novel spark plasma sintering (SPS) technique, high-entropy alloys can be easily obtained from the as-milled powders [14–16]. In this work, we focus on the high-entropy alloy system of CoCrFeNiAl synthesized by mechanical alloying and subsequently prepared by spark plasma sintering of the powder.

2. Experiment details Metal powders of Co, Cr, Fe, Ni, Al with purities of higher than 99.5 wt.% and particle size 645 lm were used as starting materials. The elemental powders were mixed in equiatomic composition and milled in a planetary ball-miller for 60 h at 250 rpm in an argon atmosphere. High performance stainless steel vials and balls were utilized as the milling media with a ball-to-powder mass ratio of 15:1.

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W. Ji et al. / Journal of Alloys and Compounds 589 (2014) 61–66 of the as-sintered samples were measured using the Archimedes principle. Bulk hardness of the sectioned and polished specimens was measured using Vickers hardness tester (Wolpert-430SV) with a load of 98 N at a loading rate of 0.05 mm/s and dwell time of 15 s. The compressive properties at room temperature were measured by a MTS810 testing machine at a loading rate of 1 mm/min, and the features of corresponding fracture surface were observed by SEM. A thin foil of sintered material was prepared by mechanical thinning followed by ion milling and subsequently observed using TEM. At least five measurements were performed to obtain the average value for both Vickers hardness and compressive strength.

3. Results and discussion 3.1. Synthesis by mechanical alloying

Fig. 1. XRD patterns of equiatomic CoCrFeNiAl HEA powder with different milling time from 6 h to 60 h.

Table 1 The crystallite size (CS), lattice strain (LS) and lattice parameter (LP) of equiatomic CoCrFeNiAl high-entropy alloy powder with milling time. Milling time (h)

CS (nm)

LS (%)

LP (Å)

0 6 18 30 42 60

– 20 16 14 14 14

– 0.24 0.35 0.47 0.61 0.67

– 2.878-BCC 2.882-BCC 2.876-BCC 2.877-BCC 2.881-BCC

N-heptane acts as the processing controlling agent (PCA) to avoid cold welding as well as to prevent the alloy from oxidizing. The MA process was monitored by powder extraction at a regular interval of 6 h. The mixture after milling for 60 h further experienced annealing at different temperatures (from 500 to 1000 °C, with a stepsize of 100 °C) in a flowing argon atmosphere with a soaking time of 1 h and free cooled to the room temperature. Subsequently, the as-milled powder was consolidated by spark plasma sintering (Dr. Sinter-3.20MK II, SCM) at a 900 °C for 10 min under 50 MPa unxial pressure in argon atmosphere, and cooled to 600 °C within 5 min and then free cooled to room temperature. The crystal structure of the as-preserved alloy was characterized by X-ray diffractometer (XRD, Rigaku Ultima III) with Cu Ka radiation. The microstructure of the powders was observed using scanning electron microscopy (SEM, Hitachi 3400) and transmission electron microscopy (TEM, Philips M12). The chemical composition was analyzed by X-ray fluorescence spectroscopy (XRF, PANalytical Axios Advanced) and energy-dispersive X-ray spectroscopy (EDS, EDAX). The thermal analysis was carried out by differential thermal analyzer (DTA, NETZSCH 449C) up to 1500 °C at a heating rate of 5 °C/min, in flowing argon atmosphere. Densities

3.1.1. X-ray analysis XRD patterns shown in Fig. 1 reveal that one major phase of CoCrFeNiAl HEA is formed within a milling time of 30 h. The primitive 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, peak broadening is obvious and some peaks become invisible after 18 h of milling. As the milling time increases to 30 h, only the 3 most intensive peaks of a BCC structure ((1 1 0), (2 0 0), (2 1 1)) can be identified, by which is deduced the formation of a simple solid solution. The BCC solid-solution reveals a lattice parameter of 2.876 Å. Further extended MA process time up to 60 h results in no obvious change in the XRD patterns. 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 [17,18]. The crystallite size and lattice strain after different milling times have been calculated by Scherrer’s formula after eliminating the instrumental and the strain contributions [19,20]. As shown in Table 1, the crystallite size in the BCC phase is refined as the milling duration increases, reaching 14 nm after 30 h MA. Further increase of the milling time has no vital influence on the crystallite size, which indicates that the equilibrium between crystallite

Table 2 The result of elemental analysis by XRF for 60 h milled CoCrFeNiAl high-entropy alloy. Element

At.%

Al Cr Fe Co Ni O

19.86 19.77 20.43 20.12 19.70 0.12

Fig. 2. SEM image and corresponding EDS analysis of 60 h milled CoCrFeNiAl HEA powder: (a) SEM image and (b) EDS spectrum with elemental analysis.

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Fig. 3. TEM images and SAED patterns of 60 h milled CoCrFeNiAl HEA powder: (a) low magnification TEM bright field image and SAED patterns and (b) high magnification TEM image.

Table 3 The mixing enthalpy (J K1 mol1) of atomic-pairs in CoCrFeNiAl system [27,28].

Table 4 The calculated Hmix and Smix values of some HEA solid solutions and intermetallics.

Element

Co

Cr

Fe

Ni

Al

System

Hmix (kJ mol1)

Smix (J K1 mol1)

Co Cr Fe Ni Al

0

4 0

1 1 0

0 7 2 0

19 10 11 22 0

CoCrFeNiAl CoCrFeNiTiAl CoCrFeNiMn CoCrFeNiMnAl CuAl AlFe Cr2Al CrFe NiFe3

12.32 20.44 4.80 11.89 1.00 11.00 8.89 1.00 1.50

1.61R 1.79R 1.61R 1.79R 0.69R 0.69R 0.69R 0.69R 0.69R

refinement and cold welding has been achieved. The lattice strain of milled powders increases gradually as the milling time extends and reaches 0.67% when the milling time is 60 h [21].

3.1.2. Microstructure and chemical composition analysis The microstructure and corresponding EDS spectrum of as milled CoCrFeNiAl HEA powders for 60 h are shown in Fig. 2. The alloy powder has an average particle size of less than 3 lm, as seen in Fig. 2(a), and the elliptical shaped particles have evolved to the shape of sheets with a thickness of less than 1 lm. In the mixture of raw materials, the nominal composition of each element is 20%. The result of elemental analysis by XRF, as seen in Table. 2, indicates that the equiatomic composition is maintained after MA. Comparing with the XRF, the result of EDS shown in Fig. 2(b) reveals good chemical homogeneity in the as-milled HEA powder.

The nanocrystalline nature of CoCrFeNiAl HEA after 60 h MA has been further characterized by the bright field image of TEM and the selected area electron diffraction (SAED) pattern, as shown in Fig. 3. The crystal size measured from Fig. 3(b) (approximately 20 nm) is in good consistency with the result of Scherrer’s formula. The existence of nanoscaled crystallite indicates that ball milled microscaled alloy particles observed in the SEM image are agglomerations of nanosized grains. The rings in the SAED pattern of Fig. 3(a) reveal that the nanocrystalline HEA powder prepared by milling for 60 h has a crystal structure of BCC, which is in agreement with the XRD result. This result confirms that the CoCrFeNiAl high-entropy alloy with a structure of simple BCC solid solution has been successfully fabricated by mechanical alloying.

Fig. 4. Thermal analysis of CoCrFeNiAl HEA: (a) DTA curve of the 60 h milled CoCrFeNiAl HEA powder and (b) XRD patterns of the 60 h milled CoCrFeNiAl HEA powder under varying annealing temperatures.

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Fig. 5. XRD patterns of 60 h milled, 900 °C heat-treated and 900 °C SPS-ed CoCrFeNiAl HEA.

In previous studies of Zhang and Guo, the solid solution formation criteria and the phase stability of high-entropy alloy fabricated by casting have been investigated [3,22–26]. According to their results, the as-calculated values of DSmix (J K1 mol1), DHmix (kJ mol1) and d for CoCrFeNiAl HEA are 1.61R, 12.32 and 5.77, respectively, which is highly consistent with the criteria (Table. 3 shows the mixing enthalpies of atomic-pairs in the CoCrFeNiAl alloy system [27,28], the atomic and electronic configuration for Co, Cr, Fe, Ni and Al, as can be found in references [29–31]). Moreover, the extension of solid solubility is the main advantage of MA. Therefore it is reasonable to infer that the formation of simple solid solution in the as-milled HEA is easier than that in the as-cast HEA. These calculated data suggest that the simple solid solution is likely to be formed during the process of MA. 3.1.3. Thermal analysis: DTA curve and annealing behavior Fig. 4(a) shows the DTA results of the CoCrFeNiAl high-entropy alloy powder after milled for 60 h. In the DTA curve, the first endo-

Fig. 7. Room temperature compressive strain–stress curve of 900 °C SPS-ed CoCrFeNiAl HEA bulk.

thermic peak at 92.4 °C is related to the energy absorption of the PCA evaporation. The wide exothermic range from 100 to 650 °C is associated with the release of internal stresses introduced by lattice strain and structural deformation. The exothermic peaks at 669.2 °C and 837.7 °C are related to the energy release of the phase transformation. The long endothermic line after the exothermic peaks can be associated with the gradual collapse of the crystalline structure under high temperature. The two endothermic peaks at 1307.1 °C and 1389.8 °C are regarded as the melting points of two different phases respectively [32]. The XRD patterns of 60 h mechanical alloyed CoCrFeNiAl powder after annealing for 1 h at increasing temperatures are shown in Fig. 4(b). The result evidences the high thermal stability of the BCC structural phase up to 500 °C. Phase transformation starts at 600 °C as the diffraction peaks corresponding to a FCC structure appear. The FCC structural phase acts as the major phase after the sample being annealed at 800 °C and the intensity of the diffraction peaks increases significantly. Besides, the intensity of BCC phase diffraction peaks decreases gradually with increasing temperature.

Fig. 6. TEM images and SAED patterns of the 900 °C SPS-ed CoCrFeNiAl HEA bulk: (a) TEM bright field image of bulk CoCrFeNiAl HEA after SPS, its corresponding SAED patterns (b) and (c) indicate region A with a BCC phase and region B with an FCC phase, respectively.

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Fig. 8. Two different kinds of fracture morphology of 900 °C SPS-ed CoCrFeNiAl HEA bulk: (a) cleavage fracture morphology and (b) slip fracture morphology.

The XRD pattern of the alloy powder annealed at 1000 °C exhibits its simple solid solution phase, which depicts that the alloy is composed mainly of an FCC structural phase and to a minor extend of a BCC phase. The phase transition under different annealing temperatures is owed to the metastable state of the solid solution caused by mechanical alloying. This metastable structure does not change at low temperature and converts to a more stable phase in the final stage of annealing [33]. Based on above results, we can draw a conclusion that both the as-milled and the as-annealed CoCrFeNiAl high-entropy powders mainly have a simple solid solution structure. The interesting phenomenon can be explained by the Gibbs free energy of mixing defined in Eq. (1).

Gmix ¼ Hmix  TSmix

ð1Þ

where Gmix is the Gibbs free energy of the mixture, Hmix is the mixing entropy, T is absolute temperature and Smix is the mixing entropy. Table. 4 shows some calculated Hmix and Smix values of HEA solid solutions and intermetallics. The entropies of the solid solution phases are much higher than those of the intermetallics. According to thermodynamics, the increase of mixing entropy significantly reduces the Gibbs free energy and thus the solid solution phases preferentially form rather than intermetallics and other complex phases, especially at high temperature [34]. 3.2. Consolidation by spark plasma sintering The 60 h mechanically alloyed CoCrFeNiAl high-entropy alloy powder was subsequently consolidated by spark plasma sintering, at 900 °C with a pressure of 50 MPa in argon atmosphere for 10 min. The obtained HEA bulks achieve relative densities of above 99% and Vickers hardness of 625 HV. The value of the hardness is higher than that of most commercial available hard facing alloys (e.g. stellite, approximately 500 HV) [35]. XRD patterns of the densified HEA bulk are shown in Fig. 5. Both FCC and BCC solid solution structures can be characterized in the sample. The crystallite structure is distinguishingly different from that of the as-milled or as-annealed alloy, and seems to be an intermediate state. The unexpected phenomenon may be associated with the critical conditions of the non-equilibrium process of rapid sintering in SPS, and the large pulsed electric current of SPS might also lead to uncertain phase evolution [36]. Fig. 6 illustrates the TEM bright field image and corresponding SAED patterns of bulk CoCrFeNiAl HEA after SPS. It is obvious that there are two different sizes of grains, one is larger than 200 nm and the other one is smaller than 50 nm. Corresponding SAED patterns seen in Fig. 6(b) and (c) indicate that the smaller particles have a BCC structure while the larger ones have an FCC structure. The structure agrees well with the XRD results in Fig. 5.

The room-temperature compressive true stress strain curve of the consolidated high-entropy alloy prepared by SPS is shown in Fig. 7. It can be seen that the bulk specimen exhibits a relatively higher compressive strength of 1907 MPa, which is similar to the as-cast CoCrFeNiAl HEAs [6]. It is acknowledged that Al with its larger atomic radius can strengthen the HEA by improving the effect of solid solution strengthening [37]. Moreover, the BCC phase often exhibits a higher strength and a lower plasticity than an FCC phase. The CoCrFeNiAl HEA fabricated by MA-SPS method displays a BCC structure with ultrafine grains (<100 nm) as shown in Fig. 6(b). As a consequence, the excellent properties of hardness and compressive strength depend on the solid solution strengthening and BCC structured ultrafine grains. In Fig. 7, some plastic deformation (11.16%) can also be found, which is less pronounced than that of the sample made by the process of arc-melt [6]. As discussed above, the plasticity of the alloy is mainly contributed by the FCC phase and restrained by the BCC phase. Fig. 8 shows the corresponding fractographic feature of the compressive strength tested alloy. Cleavage steps can be observed in Fig. 8(a) while slip lines appear in Fig. 8(b). It can be concluded that the fracture mechanism of the consolidated CoCrFeNiAl highentropy alloy is cleavage fracture and slip separation. 4. Conclusion The equiatomic CoCrFeNiAl High-entropy alloy powder has been successfully synthesized by mechanical alloying. A BCC structured solid solution with a grain size of 20 nm was obtained after 30 h MA process. The BCC phase exhibited high phase thermal stability up to 500 °C, and gradually transformed into an FCC solid solution above 500 °C. TEM results indicated that BCC phase and FCC phase coexisted in the SPS-consolidated HEA. The bulk specimens showed high Vickers hardness of 625 HV and compressive strength of 1907 MPa, which are due to the solid solution strengthening and BCC structured ultrafine grains. Acknowledgments The authors would like to acknowledge the financial support by the National Natural Science Foundation of China under Granted Nos. 50772081 and 50821140308, and the Ministry of Education of China under Granted No. PCSIRT0644. References [1] A.L. Greer, Nature 366 (1993) 303–304. [2] A. Inoue, X.M. Wang, Acta. Mater. 48 (2000) 1383–1395. [3] J.W. Yeh, S.K. Chen, S.J. Lin, J.Y. Gan, T.S. Chin, T.T. Shun, C.H. Tsau, S.Y. Chang, Adv. Eng. Mater. 6 (2004) 299–303. [4] J.W. Yeh, Ann. Chim. Sci. Mat. 31 (2006) 633–648.

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