Microstructures and mechanical properties of mechanically alloyed and spark plasma sintered Al0.3CoCrFeMnNi high entropy alloy

Materials Chemistry and Physics 210 (2018) 62e70

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Microstructures and mechanical properties of mechanically alloyed and spark plasma sintered Al0.3CoCrFeMnNi high entropy alloy Rizaldy M. Pohan a, 1, Bharat Gwalani b, 1, Junho Lee a, Talukder Alam b, J.Y. Hwang c, Ho Jin Ryu d, Rajarshi Banerjee b, *, Soon Hyung Hong a a

Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Daejeon, 305- 701, South Korea Department of Materials Science and Engineering, University of North Texas, Denton, TX 76207, USA Institute of Advanced Composite Materials, Korea Institute of Science and Technology, Jeonbuk, 565-905, South Korea d Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology, Daejeon, 305- 701, South Korea b c

h i g h l i g h t s
Al0.3CoCrFeMnNi high entropy alloy was prepared using Spark Plasma Sintering.
Optimum density and grain size is obtained at 900  C sintering temperature.
Microstructure consisted of fcc matric, ordered bcc (B2) and chromium carbides.
Alloy showed high compressive strength of 979 MPa and failure strain of 39%.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 April 2017 Received in revised form 6 September 2017 Accepted 10 September 2017 Available online 11 September 2017

The present study focuses on phase evolution in Al0.3CoCrFeMnNi high entropy alloys (HEAs) during mechanical alloying and after spark plasma sintering. Aluminium addition hardens and induces ordered precipitates in a soft fcc alloy based on CoCrFeMnNi. Mechanical alloying of the alloy powders resulted in a single fcc phase. However, ordered B2 precipitates and chromium carbide precipitates were observed after spark plasma sintering. Sintering temperature optimization was done and maximum densification and hardness were obtained at 900  C. High compressive yield strength of 979 ± 20 MPa and compressive ductility of 39 ± 3% were observed for the SPS processed alloy. Significant contributions from grain boundary strengthening coupled with dispersion strengthening via carbides and B2 particles appear to be major contributors to alloy strengthening. These hard intermetallic particles not only keep the grain growth in check but also increase the cumulative (fcc þ B2) strength of the material. © 2017 Elsevier B.V. All rights reserved.

Keywords: High entropy alloy Powder metallurgy Sintering Mechanical characterization

1. Introduction High-entropy alloys (HEAs) is a new class of metallic alloys, defined by Yeh et al. [1] as alloys consisting of five or more metallic elements ranging from 5 to 35 at.%. As the name suggests, highentropy alloys have high configurational entropy, which favors the formation of solid solution instead of intermetallic compounds. Properties that attract interest towards HEA research include but are not limited to high strength, thermal stability, wear resistance, and corrosion resistance, among other interesting properties.

* Corresponding author. E-mail address: [email protected] (R. Banerjee). 1 These authors contributed equally. http://dx.doi.org/10.1016/j.matchemphys.2017.09.013 0254-0584/© 2017 Elsevier B.V. All rights reserved.

Of the myriad possibilities HEA systems offer, CoCrFeMnNi is one of the more intensively researched alloy system, due to its physical properties such as cryogenic mechanics [2], thermodynamic stability [3], malleability, to name a few. Originally reported by Cantor et al. [4], the microstructure was known to consist of a single fcc solid solution. Although this alloy has many interesting traits, its initial mechanical properties were quite low [5]. Most studies on this alloy have been based on synthesis via conventional melting and solidification. Benefits attributed to this method include complete densification, since no gases are trapped within the slab and contamination of other elements can be removed. However, melting tends to create coarse grains (>4 mm) with heterogeneous grain structure during cooling [6]. To improve mechanical

R.M. Pohan et al. / Materials Chemistry and Physics 210 (2018) 62e70

properties, different methods that have been applied include rolling at room, cryogenic and elevated temperature; high pressure torsion [3] and swaging [7] after arc melting. Alternative methods of synthesizing HEAs are by powder metallurgy, mechanical alloying (MA) and spark plasma sintering (SPS). Target material in the form of powder is subjected to repetitive cold welding, fracturing, and re-welding to achieve solid state alloying. The alloyed powders are then subjected to pressure and are rapidly heated, thereby consolidating the powders. Previous research confirmed that CoCrFeMnNi follows the Hall-Petch equation [8]. The powder metallurgy process enables achieving nanosized grains, which in turn improve mechanical properties. Comparing arc melting and powder metallurgy reveals that powder metallurgy is the more superior process. During the powder metallurgy process, grain refinement is obtained during mechanical alloying, while spark plasma sintering can maintain these small grains; whereas in arc melting, the grains are coarse and inhomogeneous. The addition of aluminum may increase mechanical properties by further increasing the lattice distortion effect due to its larger size. A study by He et al. [9] confirmed that limited addition of Al will increase yield strength, decrease density due to solid solution strengthening and the formation of a harder bcc phase with further addition of aluminum [10,11]. Furthermore a study by Gwalani et al. [12] showed the effect of various ordered precipitates (L12 and B2), stabilized by addition of Al, on the tensile properties of CoCrFeNi HEA system. This study focuses on the microstructures and mechanical properties of Al0.3CoCrFeMnNi HEA produced by mechanical alloying and spark plasma sintering.

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analysis were carried out using CAMECA IVAS® 3.6.8 software.

3. Results and discussion 3.1. SEM and XRD results during MA Fig. 1 (a) shows secondary electron (SE) SEM images demonstrating the change in size scale and morphology of the powder agglomerates as a function of milling time during mechanical alloying. The mixtures of elemental powders of Al, Co, Cr, Fe, Mn, and Ni were cold-welded together by the repeated collisions of milling media. The average size of the powder agglomerates increased rapidly in the early stages of milling up to 6 h. After 6 h, agglomerate size did not grow any further due to the balance between cold-welding and fracture of the milled powder. Fig. 1(b) shows the effect of milling time on crystallinity of the Al0.3CoCrFeMnNi powders. Initially, elemental peaks of the respective elements were observed. However, as milling time increased, intensity of elemental peaks decreased, thereby denoting the alloying process. After 36 h of milling, only FCC peaks were observed. These FCC peaks of low intensity and relative broadening indicate a decrease in crystallite size according to the Scherrer formula. Crystallite size vs. milling time plot is shown in Fig. 2. Associating crystallite size and XRD plot validates the inference that the alloying process was complete after 36 h of milling. Thus, high entropy alloy phases in the alloy can be obtained by MA.

2. Experimental A sequence of processing steps was followed to make the Al0.3CoCrFeNi alloy. High purity powders of Al, Co, Mn, Ni of <15 mm, Cr of <45 mm (Kojundo Co, Ltd.), Fe of <25 mm, were used (expressed in molar ratio) in the planetary ball mill (Pulverisette 5/ 4, Fritzsch) for 36 h for mechanical alloying. Stainless steel vials and Ø 1.1 cm stainless steel balls were used with a milling speed of 200 rmp. A ball-to-powder mass ratio of 15:1 was maintained and process was done in argon atmosphere with addition of n-heptane as process control agent (PCA) to reduce cold welding. As-milled powders were then consolidated using spark plasma sintering (Dr. Sinter Lab. SPS-515S) at 800, 900, 1000  C in medium vacuum atmosphere (1.5  105 Bar) for 10 min under 50 MPa uniaxial pressure with a heating rate of 100  C/min. Crystal structure of the milled powders and sintered alloys were examined by X-ray diffractometer (XRD, Rigaku D/Max-2500) with CuKa radiation. Microstructural characterization was done using Scanning Electron Microscopy (SEM, Phillips XL30SFE) and Transmission Electron Microscope (Tecnai G2 F30 S-Twin). Densities of the sintered alloys were measured using the Archimedes principle. Hardness of the sintered alloy samples was measured by the Vickers hardness testing machine (Mitutoyo HM-124). Compressive properties were measured using INSTRON 5583 with Ø3 mm  6 mm cylindrical specimens at 0.2 mm/min crosshead speed. Samples from the SPS processed alloys were prepared for Atom probe Tomography (APT) using Focused Ion Beam (FIB). Standard lift-out techniques were used for APT sample preparation in the FIB before mounting the small sections of the samples on suitable holders for analysis. APT experiments were conducted on a CAMECA local electrode atom probe 3000X HR instrument. All experiments were performed in the temperature range of 40e60 K with target evaporation of 0.5% and pulse fraction of 20% of a steady-state applied DC voltage. APT data reconstruction and

Fig. 1. (a) SEM micrographs of Al0.3CoCrFeMnNi powders as a function of milling time. (b) XRD peaks of Al0.3CoCrFeMnNi as a function of milling time.

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Fig. 2. Crystallite size of Al0.3CoCrFeMnNi as a function of milling time.

3.2. Effect of sintering temperature To optimize the sintering temperature, a series of experiments were carried out at 800, 900 and 1000  C. The relative density and hardness measurements of the alloy were done after sintering at these temperatures (Fig. 3). The relative density increases considerably from 800  C to 900  C but does not change much from 900 to 1000  C. On the contrary, hardness decreases as temperatures go from 900 to 1000  C. This could be due to faster grain growth occurring at 1000  C, thereby causing the drop in hardness. The grain size obtained after 800  C was 0.091 ± 0.002 mm, after 900  C was 0.117 ± 0.004 mm and that after 1000  C was 0.953 ± 0.044 mm. Joo et al. [13] sintered CoCrFeMnNi at temperatures ranging from 900  C to 1100  C by SPS. At 900  C, they reported grain sizes 0.28 mm for 60 min MA but 1.87 mm after sintering at 1100  C for the same 60 min MA powder. In the current alloy the authors think that the grain boundary pinning by the B2 phase plays an important role in restricting the grain sizes at lower temperature. But at higher temperatures the B2 stability is possibly reducing and hence grain boundary pinning might not be playing an as crucial role. Based on density and hardness 900  C appears to be the optimum temperature for sintering of this alloy, the alloys prepared at 900  C were analyzed further. 3.3. Characterization of sintered alloys 3.3.1. X-ray diffraction That the XRD peaks from the sintered alloy display high intensity and narrow width compared to before sintering indicates that a well-developed recrystallized microstructure is obtained after spark plasma sintering (Fig. 4 (a)). Al0.3CoCrFeMnNi displayed a combination of fcc and minor B2/bcc peaks. This reveals the formation of two different phases during spark plasma sintering.

3.3.2. Stability and phase prediction A mechanism to predict formation of solid solution HEA and fcc/ bcc phase stability was proposed by Yang [14] and Guo et al. [15,16], respectively. The formation of solid solution HEA is predicted by competition between entropy and enthalpy using a unitless entity symbolized as U:

U¼

Tm DSmix jDHmix j

(1)

where Tm is the melting temperature, DSmix is the mixing entropy, and DHmix is the mixing enthalpy. The value of U should be greater than or equal to 1.1 to form solid solutions. The atomic size mismatch between elements (d), is calculated using:

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u n   uX ci ð1­ri =rÞ2 d¼t

(2)

i¼1

where ci is the atomic ratio of the ith element, ri is the radius of the ith element, and r is the average atomic radius. The atomic radii of the atomic species should also be similar in order to promote solid solution phase. To calculate mixing enthalpy, the equation below is used:

DHmix ¼

n X

4DHmix ij ci cj

(3)

i¼1;isj

where DHmix is the mixing enthalpy between the ith and jth ij element. Melting temperature is calculated using:

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Fig. 3. (a) Relative density of the sintered Al0.3CoCrFeMnNi alloy as a function of sintering temperature (b) Vickers hardness of the sintered Al0.3CoCrFeMnNi alloy as a function of sintering temperature.

Tmix ¼

n X

ci ðTm Þi

DSmix ¼ R (4)

n X ðci lnci Þ

(5)

i¼1

i¼1

where ðTm Þi is the melting point of the ith element. While entropy is calculated using:

where R is the gas constant (8.314 J/K.mol). The above equations confirmed that HEAs will be stable if several conditions are met: 15 < DHmix < 5 kJ/mol [14], U  1.1, and

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Fig. 4. (a) XRD peaks of SPS processed Al0.3CoCrFeMnNi alloy sintered at 900  C. (b) Room temperature stress-strain plot of the SPS processed Al0.3CoCrFeMnNi alloy sintered at 900  C.

d  6.6% [15]. The addition of Al in CoCrFeMnNi is known to promote the formation of an ordered bcc phase within the fcc microstructure [9]. Phase stability of these phases was predicted by means of Valence Electron Concentration (VEC). According to the equation:

VEC ¼

n X i¼1

ci ðVECÞi

(6)

fcc phase is stable when VEC  8 and bcc phase is stable when VEC  6.87 [15]. A mixture of fcc and bcc is found between these two values. From the results displayed in Table 1, Al0.3CoCrFeMnNi is predicted to be stable as a solid solution HEA with a mixture of fcc and bcc phase in the microstructure, and consistent with the XRD results. 3.3.3. Compressive test Compressive stress-strain curve at ambient temperature for

R.M. Pohan et al. / Materials Chemistry and Physics 210 (2018) 62e70 Table 1 Summarized HEA stability and phase prediction. Summarized Prediction Data

DHmix (kJ/mol) Tm mix ( C) DSmix (J/K) Omega Delta (%) VEC

7.16 1306.518 14.397 2.627 4.44 7.739

Al0.3CoCrFeMnNi is displayed in Fig. 4(b). Al0.3CoCrFeMnNi has yield strength of 979 ± 20 MPa and strain to failure of 39 ± 3%. The tensile strength of CoCrFeMnNi processed by melting has been shown to be ~500 MPa, and addition of Al increased it to ~600 MPa for Al0.3CoCrFeMnNi and ~800 MPa for Al0.5CoCrFeMnNi [9]. In the current study the maximum strength achieved is 1.8 GPa under compressive loading. Testing this alloy under tensile loading will be part of future study. The primary strengthening mechanisms operative in this alloy appear to be grain size strengthening (HallPetch effect) and dispersion strengthening.

3.3.4. Microstructural examination using scanning electron microscopy Microstructure of the SPS processed Al0.3CoCrFeMnNi alloy is

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shown in Fig. 5. The measured relative density of >99% of the sample indicates full densification has been achieved. The microstructure shows a light gray phase, a dark gray phase, and a dark phase. As discussed previously, based on XRD and simple phase stability prediction, Al0.3CoCrFeMnNi consists of fcc and bcc phases. The Electron Back Scattered Diffraction (EBSD) orientation mapping results (Fig. 5(bed)) show the presence of a dual phase mixture in the microstructure. Fig. 5 (b) shows the fcc þ bcc map, whereas Fig. 5 (c) and (d) show the isolated fcc and bcc maps, respectively. Some regions of the microstructure are not indexed in the orientation maps due to ultra-fine grain sizes and the presence of a third carbide phase, which will be discussed in detail in subsequent sections. 3.3.5. Phase identification and compositional characterization using transmission electron microscopy Fig. 6 (a) is a Bright Field (BF) Transmission Electron Microscopy (TEM) image showing the fine-scale grains, which are highlighted by dotted lines. Selected Area Electron Diffraction (SAED) pattern shows a ring formation due to the small grain sizes. The rings corresponding to {001}bcc and {111}fcc are marked and labeled in the figure. Spot scans using Energy Dispersive X-ray Spectroscopy (EDS) were used to reveal the composition of different regions in the microstructure. The compositions from three different spots (Spot I, II and III) are labeled in the BFTEM images shown in Fig. 5

Fig. 5. Back scattered SEM image of the SPS processed Al0.3CoCrFeMnNi alloy (b) EBSD map from the region marks by the yellow box in Fig. 5(a). (ced) EBSD maps highlighting only BCC and only FCC regions, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 6. Bright Field TEM image from the SPS processed Al0.3CoCrFeMnNi alloy. Different nano-grains are marked with different colored dotted lines. The inset shows the SADP from the matrix. (b) A grain is highlighted and the EDS composition corresponding to this grain is shown in Table 2 (spot I) (c) The EDS composition corresponding to grain II and III (bcc/ B2 and Cr Carbide) are shown in Table 2 (spot I and spot III). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

(b) and (c) and in Table 2. Spot I corresponds with the matrix composition (HEA composition); Spot II shows high concentration of Al and Ni can be expected to be from the B2 phase; and the Spot III region is rich in Cr, which could be from a Cr carbide. Carbon enrichment in the SPS process normally happens from the graphite presses used in the process. As the diffusion of carbon occurs quite

rapidly, carbide phases are often observed in SPS processed alloys [17,18]. While the fcc phase has an even distribution of elements, with a low amount of Al, the B2 phase has aluminum as the major element. This phenomenon was also observed in cast AlxCoCrFeMnNi, where the large atomic size of Al cannot accommodate an

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Table 2 TEM-EDS compositions taken from three different spots marked in Fig. 6. Element

AlK CrK MnK FeK CoK NiK

Spot I

Spot II

Spot III

Wt%

At%

Wt%

At%

Wt%

At%

1.72 11.31 19.45 21.72 23.09 22.68

3.54 12.06 19.64 21.57 21.73 21.43

28.11 8.59 10.69 13.05 18.08 21.47

45.14 7.16 8.43 10.13 13.29 15.84

3.15 58.06 6.34 19.32 9.44 3.7

6.08 58.23 6.02 18.04 8.35 3.29

fcc structure, causing a B2 phase to be formed [19,20]. VEC calculation further strengthened the evidence of the fcc þ bcc phase. 3.3.6. Compositional homogeneity via atom probe tomography Al0.3CoCrFeMnNi was prepared after optimizing parameters like mechanical alloying time, sintering temperature etc. Hence atom probe tomography was used to check the compositional homogeneity of the alloy at sub-micron levels. Figs. 7 and 8 show the APT results from the SPS processed Al0.3CoCrFeMnNi alloy. A 5 nm slice of 3-D ion maps from the matrix region is provided in Fig. 7. All the ions appear to be uniformly distributed, thus confirming that the solid solution phase constitutes the matrix region. 100% ion map for Al and 50% ion maps for all other elements are provided for better visualization due to the relatively high concentration of other alloying elements. In addition, Fig. 8 captures an interface between the carbide and the matrix; the carbide is rich in Cr and C ions (Fig. 8 (a) and (b)). A 25% Cr iso-concentration surface is created (Fig. 8 (c)) to generate a proximity histogram across the interface (Fig. 8 (d)) of the two phases. The compositional change on going

Fig. 8. Atom probe tomography reconstructions from SPS processed Al0.3CoCrFeMnNi alloy. (a), (b) and (c) show the ion maps of Cr, C and Ni respectively. (d) Proximity histogram showing the compositional partitioning across a carbide and matrix interface.

Fig. 7. Atom probe tomography reconstructions from SPS processed Al0.3CoCrFeMnNi alloy, showing the ion maps of various elements shown in different colors (as marked). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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from matrix to carbide (shown by the red arrow in Fig. 8 (c) is shown in Fig. 8 (d)). Note that the carbide contains about 60% Cr, 10%Fe and 10%Mn. The carbon content is about 15%, and the rest (5%) is distributed in Co and Ni (all % in at. %). 4. Conclusion In this study, Al0.3CoCrFeMnNi were prepared by planetary ball milling followed by spark plasma sintering. Process parameters like milling time and sintering temperature were optimized to achieve a single phase high entropy alloy. SEM and XRD examination of mechanically alloyed powders after various milling durations in a planetary mill was used to optimize the milling time. Density and hardness measurements of the sintered powders at different sintering temperature were identified to be the most optimum condition for processing this alloy temperatures were used to optimize the sintering temperature. 36 h milling time and 900  C for. The sub micron compositional homogeneity of the matrix phase in the optimized condition was shown using atom probe tomography. Mechanical strength under compressive loading was as high as 979 MPa, with a failure strain of about 39%. Microstructural examination showed the presence of an ultra-fine grained structure comprised of grains in the range of 50e200 nm. The high strength of this SPS processed HEA can be attributed to the refined grain size as well as the presence of secondary strengthening phases like B2 and chromium carbides in the microstructure.

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Acknowledgments This work was supported by the National Research Foundation of Korea grant funded by the Ministry of Science, ICT & Future Planning (MSIP) and (No. NRF-2015R1A5A1037627 & NRF2015R1A2A2A01002436). The authors also acknowledge the Materials Research Facility (formerly Center of Advanced Research and Technology) at the University of North Texas for microscopy facilities. References [1] J.W. Yeh, S.K. Chen, S.J. Lin, J.Y. Gan, T.S. Chin, T.T. Shun, C.H. Tsau, S.Y. Chang, Nanostructured high-entropy alloys with multiple principal elements: novel alloy design concepts and outcomes, Adv. Eng. Mater. 6 (5) (2004) 299e303. þ274. [2] N. Stepanov, M. Tikhonovsky, N. Yurchenko, D. Zyabkin, M. Klimova, S. Zherebtsov, A. Efimov, G. Salishchev, Effect of cryo-deformation on structure and properties of CoCrFeNiMn high-entropy alloy, Intermetallics 59 (2015) 8e17. €lker, E.P. George, H. Clemens, R. Pippan, [3] B. Schuh, F. Mendez-Martin, B. Vo

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