metals Article
The Al Effects of Co-Free and V-Containing High-Entropy Alloys Songqin Xia 1 , Xiao Yang 1,2, *, Mingbiao Chen 3 , Tengfei Yang 4 and Yong Zhang 1, * 1 2 3 4
*
State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China;
[email protected] State Key Laboratory for Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China College of Mechanical Engineering, Qinghai University, Xining 810016, China;
[email protected] State Key Laboratory of Nuclear Physics and Technology, Center for Applied Physics and Technology, Peking University, Beijing 100871, China;
[email protected] Correspondence:
[email protected] (X.Y.);
[email protected] (Y.Z.); Tel.: +86-10-6233-3073 (Y.Z.)
Academic Editor: Vincent Fournée Received: 19 December 2016; Accepted: 5 January 2017; Published: 10 January 2017
Abstract: In this study, five-component high-entropy alloys (HEAs) Alx CrFeNiV (where x denotes the molar ratio, x = 0, 0.1, 0.3, 0.5, 0.75, 1, and 1.5) were prepared using an arc-melting furnace. The effects of the addition of the Al on the crystal structures were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Also, two non-equiatomic ratio HEAs, Alx CrFeNiV (x = 0.3, and 0.5), were systematically studied through the use of various characterization methods in the as-cast state. The Al0.3 CrFeNiV alloy displayed typical duplex body-centered cubic (BCC) structures, including disordered BCC (A2), and NiAl-type ordered BCC (B2) phases. Meanwhile, in regard to the Al0.5 CrFeNiV alloy, this alloy was found to contain an unknown phase which was enriched in Cr and V, as well as the coherent A2/B2 phases. Both of these alloys displayed very high yield and fracture strengths. However, their compression fracture strains were approximately 10%. Also, the fracture surfaces showed mainly cleavage fracture modes. Keywords: high-entropy alloys; microstructure; compressive properties
1. Introduction High-entropy alloys (HEAs) are a new class of materials which are defined as alloys consisting of four or more base elements with equal or non-equal fractions. The high configurational entropy which is induced by the number of base elements is able to prevent the formation of intermetallic phases, and thereby avoids the disadvantages of conventional multicomponent alloys. HEAs mainly consist of simple solid solution structures, such as body-centered cubic (BCC), face-centered cubic (FCC), or hexagonal close-packed (HCP) structures [1–3]. However, in many cases, more complicated structures may exist in some HEAs [4–7], including ordered-solid solutions and/or intermetallic phases, which indicates that the high configurational entropy may not be sufficient to prevent the formation of ordered solid solutions and intermetallic phases in some HEAs [8]. Many HEA systems reported are based on transition metals—namely Co, Cr, Fe, and Ni—with the additions of such elements as Al and Mn. The widely studied Alx CoCrFeNi HEAs are demonstrated from FCC to BCC structures through a mixture of both phases with increased Al content [9]. In recent studies, the HEAs have been reported to exhibit promising irradiation resistance properties, which make them potential cladding material candidates for components of the next-generation nuclear reactors, as well as other high-radiation resistant materials [10–12]. However, considering the
Metals 2017, 7, 18; doi:10.3390/met7010018
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radioactivity of Cobalt in most HEAs under neutron irradiation, exploring these novel HEAs without Co-element is necessary. In this study, the V-element was added to the widely studied Alx CoCrFeNi HEAs to replace Co. The two main objectives which were focused on were as follows: (i) To gain new understanding of the Al effects on phase formations in both Co-free and V-containing HEAs; and (ii) To explore the microstructures and mechanical properties of the Alx CrFeNiV alloys. 2. Experimental Procedures In this research study, HEAs samples with nominal compositions of Alx CrFeNiV (where x denotes the molar ratio, x = 0, 0.1, 0.3, 0.5, 0.75, 1, and 1.5) were prepared by the arc-melting of the pure elements in a Ti-gettered high-purity argon atmosphere inside a water-cooled copper mold (SKY Technology Development Co., Ltd. Chinese Academy of Science, Shenyang, China). The purities of the alloying elements were determined to be above 99.95 atomic percent (at %). In order to ensure chemical homogeneity, the ingots were flipped over and re-melted at least five times. The produced ingots of the Alx CrFeNiV alloys had dimensions of approximately Φ 30 mm × 10 mm. Table 1 lists the composition of the alloys of this work. Table 1. The composition of the alloys used in the experiment (at %). Alloy No.
Al
Cr
Fe
Ni
V
Atomic Ratio
1 2 3 4 5 6 7
0 2.4 7.0 11.1 15.8 20 27.3
25 24.4 23.3 22.2 21.1 20 18.2
25 24.4 23.3 22.2 21.1 20 18.2
25 24.4 23.3 22.2 21.1 20 18.2
25 24.4 23.3 22.2 21.1 20 18.2
CrFeNiV Al0.1 CrFeNiV Al0.3 CrFeNiV Al0.5 CrFeNiV Al0.75 CrFeNiV Al1 CrFeNiV Al1.5 CrFeNiV
The microstructures of the as-cast samples were characterized by XRD (TTRIII, Tokyo, Japan) at 40 kV and 40 mA, with a scanning rate of 2◦ per minute from 20◦ to 90◦ . Then, the prepared samples of Φ 3 mm × 6 mm for the compressive tests were prepared and investigated at room temperature, with a strain rate of 2 × 10−4 per second. Meanwhile, Vickers microhardness, HV, was measured on polished cross-section surfaces of the Alx CrFeNiV alloys (x = 0.3, 0.5, 0.75, 1, and 1.5) using a 136-degree Vickers diamond pyramid (HV-1000, Shanghai, China) under a 500 g load applied for 15 s. The fracture surface and morphology of the alloys were examined using a scanning electron microscope (SEM, Zeiss Supra 55, Oberkochen, Germany) equipped with an energy-dispersive X-ray (EDX) spectrometer. The transmission electron microscopy (TEM, Technai F30, FEI, Hillsboro, OR, USA)-EDX characterizations were performed under Scanning TEM (STEM) nano-probe conditions, with a beam size of approximately 1 nm. The TEM samples were prepared by mechanical polishing to approximately 40 µm in thickness, followed by the utilization of ion milling. Also, the samples for the SEM/Back Scattered Electron (BSE) study were prepared by electro-polishing at room temperature, in a solution of HClO4 (15%), CH3 OH (60%), and CH3 (CH2 )3 OH (25%). 3. Results 3.1. Results of the XRD and SEM Figure 1a shows the XRD patterns of the Alx CrFeNiV HEAs, and Figure 1b shows the macroscopic samples of the three representative alloys, including CrFeNiV, Al0.3 CrFeNiV, and AlCrFeNiV, respectively. The crystal structures of those alloys with lower Al content (for example, x = 0 and 0.1) were quite different from that for the alloys with higher Al content (for example, x ≥ 0.3). For CrFeNiV alloy, the diffraction patterns might be identified with FCC or L12 structure. Meanwhile, with regard
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to the Al0.1 CrFeNiV alloy, an FCC solid solution and a BCC solid solution coexisted in the alloy, but there were still some unidentified peaks for the two lower Al content alloys. For the x = 0.3, all of the diffraction patterns could be identified with typical duplex BCC structures, including the Metals 2017, 7, 18 3 of 12 A2 and B2 phases. For x ≥ 0.5, along with the similar duplex BCC structures, there were minor unidentified at small 2θpatterns angles,could which an ‘unknown phase’ in the including present study. = 0.3, all peaks of the diffraction becorresponded identified withto typical duplex BCC structures, A2 and B2 phases.that Forthe x ≥ increase 0.5, alongof with the similar duplex BCC structures, there were Thesethe results confirmed Al content could affect the structures of the Alminor x CrFeNiV unidentified peaks at small 2θ angles, which corresponded an ‘unknown phase’ in the 1b. present alloys. The representative macroscopic features of the as-casttoingots are shown in Figure Both the study. These results confirmed that the increase of Al content could affect the structures of the Al0.1 CrFeNiV and CrFeNiV alloys broke into many pieces immediately following the arc-melting. AlxCrFeNiV alloys. The representative macroscopic features of the as-cast ingots are shown in Figure The alloys of the Al0.75 CrFeNiV, AlCrFeNiV, and Al1.5 CrFeNiV broke into two pieces. In contrast, 1b. Both the Al0.1CrFeNiV and CrFeNiV alloys broke into many pieces immediately following the arcthe as-cast ingots of the Al0.3 CrFeNiV and Al0.5 CrFeNiV samples did not break (as shown in Figure 1b), melting. The alloys of the Al0.75CrFeNiV, AlCrFeNiV, and Al1.5CrFeNiV broke into two pieces. In whichcontrast, might the have indicated a better ductility. Their microstructure and mechanical properties were as-cast ingots of the Al0.3CrFeNiV and Al0.5CrFeNiV samples did not break (as shown in also further investigated in this present study. Figure 1b), which might have indicated a better ductility. Their microstructure and mechanical The SEM/BSE images of the Al CrFeNiV and Al0.5 CrFeNiV HEAs are shown in Figure 2. A large 0.3 properties were also further investigated in this present study. number of nano-precipitations were observed in the Al0.3 VCrFeNi XRD and The SEM/BSE images of the Al0.3commonly CrFeNiV and Al0.5CrFeNiV HEAs are shown inalloy. FigureThe 2. A large of nano-precipitations commonly in the the Al 0.3VCrFeNi alloy. The XRD and with SEM number results verified that the Al0.3were CrFeNiV alloy observed was one of typical duplex BCC structures SEMB2 results verified theCrFeNiV Al0.3CrFeNiV alloy2b) waspresented one of the typical BCC structures with A2 and phases. Thethat Al0.5 (Figure a moreduplex complicated microstructure, A2 and B2 phases. The Al 0.5CrFeNiV (Figure 2b) presented a more complicated microstructure, with with a periodic fine-scale (approximately 100 nm in width) structure consisting of obvious alternating a periodic fine-scale (approximately 100 nm in width) structure consisting of obvious alternating bright and dark interconnected phases. These were found to be similar to the inter-dendritic and bright and dark interconnected phases. These were found to be similar to the inter-dendritic and inter-plate region structures in the selected Alx CoCrFeNi alloys [13], which were believed to have inter-plate region structures in the selected AlxCoCrFeNi alloys [13], which were believed to have been been formed by a spinodal decomposition mechanism [14]. Similar structures were also reported in formed by a spinodal decomposition mechanism [14]. Similar structures were also reported in the Althe [15][15] andand AlFeMnNi HEAs confirmthe themicrostructures microstructures of x CoCrCuFeNi AlxCoCrCuFeNi AlFeMnNi HEAs[16]. [16].In Inorder order to to further further confirm both of theboth HEAs, TEM and analyses were performed ininthis the HEAs, TEMSTEM and STEM analyses were performed thisresearch research study. study.
Figure 1. (a) XRD patterns of the as-cast AlxCrFeNiV HEAs; (b) Macroscopic samples of the three
Figure 1. (a) XRD patterns of the as-cast Alx CrFeNiV HEAs; (b) Macroscopic samples of the three representative alloys CrFeNiV, Al0.3CrFeNiV, and AlCrFeNiV. representative alloys CrFeNiV, Al0.3 CrFeNiV, and AlCrFeNiV.
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Figure 2. Back-scattering SEM images of: (a) Al0.3CrFeNiV HEAs; and (b) Al0.5CrFeNiV HEAs. Figure 2. Back-scattering SEM images of: (a) Al0.3 CrFeNiV HEAs; and (b) Al0.5 CrFeNiV HEAs.
3.2. Results of the TEM and STEM 3.2. Results of the TEM and STEM Figure 3a,b shows the bright field (BF) and STEM images of the Al0.3CrFeNiV. Also, the Figure 3a,b shows the bright field (BF) and STEM images of the Al CrFeNiV. Also, corresponding selected area electron diffraction (SAED) pattern is presented in the0.3 inset of Figure 3a. the corresponding selected area electron diffraction (SAED) pattern is presented in the inset of Figure 3a. It can be observed in the figures that a large number of spherical precipitates, with average diameters It can be observed in the figures that a large number of spherical precipitates, with average diameters of approximately 100 nm, were distributed throughout the matrix. The SAED patterns suggested that of approximately 100 nm, were distributed throughout the matrix. The SAED patterns suggested the precipitates were a B2 phase, and the matrix was an A2 phase, as illustrated in Figure 3a. Figure that the precipitates were a B2 phase, and the matrix was an A2 phase, as illustrated in Figure 3a. 3b shows the STEM image of the Al0.3CrFeNiV, and a clearer duplex phase structure can be clearly Figure 3b shows the STEM image of the Al0.3 CrFeNiV, and a clearer duplex phase structure can be observed, which agreed with the results of the XRD and SEM. clearly observed, which agreed with the results of the XRD and SEM. In addition, the chemical compositions of the five different regions were measured by TEM-EDX. In addition, the chemical compositions of the five different regions were measured by TEM-EDX. The average chemical compositions of the A2 and B2 phases in the Al0.3CrFeNiV alloy are presented The average chemical compositions of the A2 and B2 phases in the Al0.3 CrFeNiV alloy are presented in in Figure 3c. The TEM-EDX line scanning across a grain with a B2 structure was a representative Figure 3c. The TEM-EDX line scanning across a grain with a B2 structure was a representative region, region, which is marked in Figure 3b. The TEM-EDX results showed that the A2 phase was enriched which is marked in Figure 3b. The TEM-EDX results showed that the A2 phase was enriched in Fe, Cr, in Fe, Cr, and V, while the B2 phase was enriched in Al and Ni. Therefore, based on the results of the and V, while the B2 phase was enriched in Al and Ni. Therefore, based on the results of the TEM-EDX TEM-EDX and the representative binary B2 intermetallic compounds AB (such as FeAl and NiAl) and the representative binary B2 intermetallic compounds AB (such as FeAl and NiAl) [17], it was [17], it was expected that, for the B2 phase in the Al0.3CrFeNiV, the Al atoms preferentially occupied expected that, for the B2 phase in the Al0.3 CrFeNiV, the Al atoms preferentially occupied the center the center sites of the unit cell where the A1 could be replaced by V since the V had a closer atomic sites of the unit cell where the A1 could be replaced by V since the V had a closer atomic radius to the radius to the Al. Meanwhile, the Ni and Fe randomly occupied the corner sites, and formed a (Ni, Fe) Al. Meanwhile, the Ni and Fe randomly occupied the corner sites, and formed a (Ni, Fe) (Al, V) B2 (Al, V) B2 structure similar to the NiAl due to the strongly negative formation enthalpy of the Al-Ni structure similar to the NiAl due to the strongly negative formation enthalpy of the Al-Ni and V-Ni. and V-Ni. In regard to the Al0.5 CrFeNiV alloy, a representative TEM image is shown in Figure 4, and the In regard to the Al0.5CrFeNiV alloy, a representative TEM image is shown in Figure 4, and the corresponding SAED pattern is presented in the inset of Figure 4a. The red circles in the SAED pattern corresponding SAED pattern is presented in the inset of Figure 4a. The red circles in the SAED pattern are the unidentified diffraction spots which suggested that, along with the coherent A2/B2 phases, are the unidentified diffraction spots which suggested that, along with the coherent A2/B2 phases, another phase appeared in the alloy. The crystal structure of the phase was not identified in the current another phase appeared in the alloy. The crystal structure of the phase was not identified in the study. The STEM image of the Al0.5 CrFeNiV alloy is shown in Figure 4b. Two findings were observed current study. The STEM image of the Al0.5CrFeNiV alloy is shown in Figure 4b. Two findings were in this study: (1) numerous spherical precipitates (the blue square in the inset of Figure 4b) with observed in this study: (1) numerous spherical precipitates (the blue square in the inset of Figure 4b) unknown structures and average diameters of approximately 80 nm, were found to be distributed with unknown structures and average diameters of approximately 80 nm, were found to be throughout the matrix; (2) the typical inter-weaved short rods aligned at approximately 90◦ angles distributed throughout the matrix; (2) the typical inter-weaved short rods aligned at approximately (the red circle thecircle inset in of the Figure It should that smaller spherical nano-precipitates 90° angles (theinred inset4b). of Figure 4b).beItnoted should be the noted that the smaller spherical nanowere not uniformly in the matrix, and the coherent A2/B2 phase was also a precipitates were notdistributed uniformly distributed in the matrix, and the coherent A2/B2mixture phase mixture was characteristic feature of the spinodal structure, which may have been formed by periodic composition also a characteristic feature of the spinodal structure, which may have been formed by periodic modulations during the solidification [18]. composition modulations during the solidification [18]. Moreover, it was determined that of the Moreover, it was determined that the the chemical chemical compositions compositions of the five five different different grains grains were were almost identical, and a representative TEM-EDX line scanning across the nano-precipitates can be seen almost identical, and a representative TEM-EDX line scanning across the nano-precipitates can be in Figure 4b. The results of the TEM-EDX is shown in Figure 4c, where the spherical Crand Vrich seen in Figure 4b. The results of the TEM-EDX is shown in Figure 4c, where the spherical Cr- and Vnano-precipitates are shown as being dispersedly distributed inin the rich nano-precipitates are shown as being dispersedly distributed thealloy. alloy.These Theseresults results indicated indicated that higher contents of Al can cause another phase formation in the Al CrFeNiV HEAs, namely 0.5 that higher contents of Al can cause another phase formation in the Al0.5CrFeNiV HEAs, namely an an unknown phase. This resulted in the crystal structure of the phase not being identified in the current study. Therefore, based on the results of the chemical compositions of the unknown phase, it was
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unknown phase. This resulted in the crystal structure of the phase not being identified in the current Metals 2017, 7, 18 5 of 12 study. Therefore, based on the results of the chemical compositions of the unknown phase, it was assumed in Cr and V V due to the of Cr Cr (30 (30 at at %) %) and and V V assumed that that the the unknown unknown phase phase was was enriched enriched in Cr and due to the content content of (50 at %), as shown in Figure 4c. Furthermore, the unknown phase observed by the STEM may have (50 at %), as shown in Figure 4c. Furthermore, the unknown phase observed by the STEM may have corresponded angles, which appeared in the XRD plotplot for corresponded to to the theminor minorunidentified unidentifiedpeaks peaksatatsmall small2θ2θ angles, which appeared in the XRD the Al0.5 alloy, as shown in Figure 1a. For the B2 phase in Al0.50.5CrFeNiV for the AlCrFeNiV 0.5CrFeNiV alloy, as shown in Figure 1a. For the B2 phase in Al CrFeNiV alloy, alloy, itit may may have have been a (Ni, Fe) Al type B2 structure, which was similar to the NiAl. been a (Ni, Fe) Al type B2 structure, which was similar to the NiAl.
Figure 3. AlAl 0.3CrFeNiV alloy and corresponding SAED patterns (zone axis Figure 3. (a) (a)BF BFTEM TEMimage imageofofthe the 0.3 CrFeNiV alloy and corresponding SAED patterns (zone [100]) showing the alloy consisted of a and anand A2 an phase; (b) STEM-HAADF image, image, where axis [100]) showing the alloy consistedB2 ofphase, a B2 phase, A2 phase; (b) STEM-HAADF the redthe line indicates the EDX Compositional profiles acrossacross the nano-precipitates. where red line indicates thescan EDXline; scan(c) line; (c) Compositional profiles the nano-precipitates.
Figure 3. (a) BF TEM image of the Al0.3CrFeNiV alloy and corresponding SAED patterns (zone axis Metals[100]) 2017, 7, 18 showing the alloy consisted of a B2 phase, and an A2 phase; (b) STEM-HAADF image, where6 of 12
the red line indicates the EDX scan line; (c) Compositional profiles across the nano-precipitates.
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Figure 4. 4. (a) AlAl 0.5CrFeNiV alloy and corresponding SAED patterns (zone axis Figure (a)BF BFTEM TEMimage imageofofthe the 0.5 CrFeNiV alloy and corresponding SAED patterns (zone [100]) showing the alloy consisted of a third a B2with phase andphase an A2and phase; axis [100]) showing the alloy consisted of aphase, third along phase,with along a B2 an (b) A2 STEMphase; HAADF image of the Al 0.5CrFeNiV alloy which clearly reveals the three different phases; (c) (b) STEM-HAADF image of the Al0.5 CrFeNiV alloy which clearly reveals the three different phases; Compositional profiles across thethe unknown be (c) Compositional profiles across unknownphase phaseininthe theAl Al0.50.5CrFeNiV CrFeNiValloys alloyswere were found found to to be significantly enriched in Cr and V. significantly enriched in Cr and V.
3.3. Mechanical Mechanical Properties Properties 3.3. Figure 55 shows shows the the compression compression stress-strain stress-strain curves curves of of the the Al AlxCrFeNiV CrFeNiV HEAs HEAs at at the the different different Al Al Figure x content levels, levels, and and the the values values of of the the fracture fracturestrength strength(σ (σb),), yield yield strength strength(σ (σ0.2), ), and plastic strain limit content 0.2 and plastic strain limit b (ε p) are listed in Table 2. The two AlxCrFeNiV alloys (x = 0.3 and x = 0.5) exhibited fairly high fracture (εp ) are listed in Table 2. The two Alx CrFeNiV alloys (x = 0.3 and x = 0.5) exhibited fairly high fracture strengths (>2700 (>2700 MPa) MPa) and and yield yield strengths strengths (>2000 (>2000 MPa). MPa). When When xx = = 0.3, and plastic plastic strengths 0.3, the the fracture fracture strength strength and strain achieved achievedthe themaximum maximumlevels, levels,namely namely 3073 MPa and 9.2%, respectively. When thecontent Al content strain 3073 MPa and 9.2%, respectively. When the Al was was increased to x = 0.5, the yield strength was found to be increased, while the fracture strength and increased to x = 0.5, the yield strength was found to be increased, while the fracture strength and plastic plastic strain limit decreased. It was interesting to note that the stress-strain curve of the Al 0.3CrFeNiV strain limit decreased. It was interesting to note that the stress-strain curve of the Al0.3 CrFeNiV alloy alloy exhibited a serration behavior, which wasinabsent the Al0.5VCrFeNi. The observed serration exhibited a serration behavior, which was absent the Alin 0.5 VCrFeNi. The observed serration behavior behavior suggested that additional deformation mechanisms became in Al0.3CrFeNiV alloy, suggested that additional deformation mechanisms became active in Alactive 0.3 CrFeNiV alloy, which may whichbeen mayrelated have been related to theofformation slip onsurfaces the sample [10]. The serration have to the formation slip bandsofon thebands sample [10].surfaces The serration behavior at behavior at room temperature presented a very interesting topic to be studied in regard to the room temperature presented a very interesting topic to be studied in regard to the HEAs, as HEAs, it was as it was at observed at temperatures cryogenic temperatures as welltemperatures as elevated temperatures [20]. These observed cryogenic [19], as well[19], as elevated [20]. These findings may findings may potentially provide a new understanding of the mechanical-deformation behavior of HEAs at room temperature. Also, the results of more focused investigations will be reported in future publications. The comparisons which were completed with other typical HEAs and bulk metallic glasses (BMGs) are summarized in Table 2. It appeared that the Al0.3CrFeNiV and Al0.3CrFeNiV alloys had the desirable strength, but limited ductility among the listed materials. Moreover, the Young’s
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In addition, the microhardness values of the as-cast AlxCrFeNiV alloys (x = 0.3, 0.5, 0.75, 1, and 1.5) are given in Figure 7. The as-cast Al0.3CrFeNiV alloy has a microhardness of 708 HV. The Metals 2017, 7, 18 7 of 12 Al0.5CrFeNiV alloy has a slightly lower microhardness of 680 HV. A further increase in the Al content results in a significant decrease first, then an increase in microhardness. For example, Al0.75CrFeNiV has microhardness 582 understanding HV and Al1.5CrFeNiV has microhardness of 694 HV. Combined the potentially provide aofnew of the mechanical-deformation behavior of HEAs with at room result of the XRD 1a),of it more can befocused deduced that the volume fraction of unknown phase might temperature. Also,(Figure the results investigations will be reported in future publications. affect the microhardness of Alcompleted xCrFeNiV alloys due to the similar phase’ for Al0.5CrFeNiV, The comparisons which were with other typical HEAs‘unknown and bulk metallic glasses (BMGs) Al0.75 CrFeNiV, and respectively. are summarized in AlCrFeNiV Table 2. It alloys, appeared that the Al0.3 CrFeNiV and Al0.3 CrFeNiV alloys had the desirable strength, but limited ductility among the listed materials. Moreover, the Young’s modulus (E) 2. Room based temperature test results for (Figure the AlxCrFeNiV = 0.3, and 0.5) HEAs couldTable be calculated on thecompression true stress-strain curves 5b). The(x Young’s moduli (E) of the compared with the other alloys from the material big-data. Al0.3 CrFeNiV and Al0.5 CrFeNiV alloys were determined to be 113.5 GPa, and 111.8 GPa, respectively.
No.
Alloy
0.2 (MPa)
p (MPa)
p (%)
Ref.
Table 2. Room temperature compression test results for the Alx CrFeNiV (x = 0.3, and 0.5) HEAs 1 Al0.3CrFeNiV 2066.8 3072.8 9.2 This work compared with the other alloys from the material big-data.
2 3 No. 4 1 5 2 36 47 58 6 8 7 10 8 811 1012 1113 12 14 13 1415 1516 1617 17 18 18 1919 2020 2121
Al0.5CrFeNiV 2295.7 Al0.5CrFeNiTi0.25 1880 Alloy σ0.2 (MPa) AlCoCrFeNi 1500 Al0.3 CrFeNiV 2066.8 AlCoCrFeNiTi 0.5 2260 Al0.5 CrFeNiV 2295.7 AlCoCrFeNiTi 1860 1880 Al 0.5 CrFeNiTi0.25 CoCrCuFeNi 230 AlCoCrFeNi 1500 2260 AlCoCrFeNiTi0.5 0.5 CoCrCuFeNiTi 700 AlCoCrFeNiTi 1860 CoCrCuFeNiTi0.8 1042 CoCrCuFeNi 230 CoCrCuFeNiTi 1227 700 CoCrCuFeNiTi0.5 Al4Cu48Zr480.8 1199 CoCrCuFeNiTi 1042 CoCrCuFeNiTi 1227 Al0.3CoFeNiSi0.3 938 AlMoNbTaW 1199 4 Cu48 Zr48 1058 Al0.3 CoFeNiSi0.3 938 MoNbTaVW 1246 MoNbTaW 1058 CoCrFeMnNiV 0.75 740 MoNbTaVW 1246 740 CoCrFeMnNiV CoCrFeMnNiV 1660 0.75 CoCrFeMnNiV 1660 AlNbTiV 1020 AlNbTiV 1020 AlCrCoCuFeNiV 1469 AlCrCoCuFeNiV 1469 AlCrCoCuFeNi 1303 AlCrCoCuFeNi 1303 AlCrCoCuFeMnNi 1005 AlCrCoCuFeMnNi 1005 AlCrCoCuFeNiTi 1234 AlCrCoCuFeNiTi 1234
2766.4 3475 σp (MPa) 2830 3072.8 3140 2766.4 2580 3475 888 2830 3140 1650 2580 1848 888 1272 1650 1882 1848 1272 2857 1882 1211 2857 1270 1211 1325 1270 1325 1845 1845 1318 1318 1970 1970 2081 2081 1480 1480 1356 1356
8.9 This work 40 [21] εp (%) Ref. 26.9 [22] 9.2 This work 23.3 [22] 8.9 This work 8.8 [22][21] 40 50.2 [23][22] 26.9 23.3 21.6 [23][22] 8.8 2.11 [23][22] 50.2 [23] 0 [23][23] 21.6 5.3 [24][23] 2.11 330 [25][23] 5.3 1.5 [20][24] 33 [25] 0.5 [20][20] 1.5 7.8 [26][20] 0.5 7.8 0.5 [26][26] 0.5 5 [27][26] 5 [27] 16 [28] 16 [28] 2424 [28][28] 1515 [28][28] 99 [28][28]
Figure 5.5. (a) (a) Compressive Compressive engineering engineering stress-strain stress-strain curves; curves; and and (b) (b) Compressive Compressive true true stress-strain stress-strain Figure curves of the Al xCrFeNiV HEAs (x = 0.3, and x = 0.5) cylindrical samples with 3 mm diameter × 6 mm curves of the Alx CrFeNiV HEAs (x = 0.3, and x = 0.5) cylindrical samples with 3 mm diameter × 6 mm height(with (withan anaspect aspectratio ratioof of2). 2). height
Figure 6 shows the morphologies of the fractographs of the Alx CrFeNiV (x = 0.3, and x = 0.5) HEAs samples. Both of the fractographic morphologies of the two alloys exhibited typical river-like patterns and cleavage steps, which indicated that a cleavage fracture mechanism dominated the fracture process of both alloys. Furthermore, there were many microcracks observed (indicated by the
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yellow arrows in the Figure 6) which prevailed on the cleavage fracture surface. These findings were Metals 2017, 7, 18 the poor plasticity shown in Figure 5. 8 of 12 consistent with
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Figure CrFeNiV; and (b) Al 0.5CrFeNiV CrFeNiV alloys alloys following following Figure 6. 6. SEM SEM images images of of fracture fracture surfaces surfacesof: of: (a) (a) Al Al0.3 0.3CrFeNiV; and (b) Al0.5 the the compression compression deformation deformation at at room room temperature; temperature; the the yellow yellow arrows arrowsindicate indicatethe themicrocracks. microcracks.
In addition, the microhardness values of the as-cast Alx CrFeNiV alloys (x = 0.3, 0.5, 0.75, 1, and 1.5) are given in Figure 7. The as-cast Al0.3 CrFeNiV alloy has a microhardness of 708 HV. The Al0.5 CrFeNiV alloy has a slightly lower microhardness of 680 HV. A further increase in the Al content results in a significant decrease first, then an increase in microhardness. For example, Al0.75 CrFeNiV has microhardness of 582 HV and Al1.5 CrFeNiV has microhardness of 694 HV. Combined with the result of the XRD (Figure 1a), it can be deduced that the volume fraction of unknown phase might Figure 6. SEM images of fracture surfaces of: (a) Al0.3CrFeNiV; and (b) Al0.5CrFeNiV alloys following affect the microhardness of Alx CrFeNiV alloys due to the similar ‘unknown phase’ for Al0.5 CrFeNiV, the compression deformation at room temperature; the yellow arrows indicate the microcracks. Al0.75 CrFeNiV, and AlCrFeNiV alloys, respectively.
Figure 7. Dependence of microhardness of the as-cast AlxCrFeNiV alloys on the Al molar ratio.
4. Discussion In regard to the Co-Cr-Fe-Ni system HEAs, it has been proven that the addition of Al or V could promote the formation of an ordered phase. In particular, the effects of Al addition on the microstructure of the AlxCoCrFeNi HEA system have been widely investigated [29–31]. It has been revealed that the microstructure of the AlxCoCrFeNi HEAs evolved from a single A1, to A1 + B2, and B2 + A2 structures with increases in Al content [12]. Moreover, the alloying element V can also cause Figure 7. of of Al alloys on molar intermetallic formation in the CoCrFeMnNi and CoCrFeNi HEA systems [26]. Figurephase 7. Dependence Dependence of microhardness microhardness ofthe theas-cast as-cast AlxxCrFeNiV CrFeNiV alloys on the the Al Al molar ratio. ratio. In the present study, the alloy composition contained both V and Al elements, and thus the 4. ordered phase was unavoidable. For the Al0.3CrFeNiV alloy, it was observed that the alloy had typical 4. Discussion Discussion duplex BCC structures, including A2 and B2 phases. Also, the proven B2 phase wasthe enriched in Ni Al andorAl, In to the theCo-Cr-Fe-Ni Co-Cr-Fe-Ni system HEAs, it has been addition V In regard to system HEAs, it has been proven thatthat the addition of Alof or V could and the A2 matrix was enriched in V and Cr. When the Al content was increased to x = 0.5, the the could promote thephase formation an ordered phase. In particular, effects Al addition addition promote the formation of anof ordered phase. In particular, thethe effects ofofAl on the typical coherentofA2/B2 phases and nano-precipitates could widely be observed. Otto et[29–31]. al. [8] investigated microstructure microstructure of the the Al AlxxCoCrFeNi CoCrFeNi HEA HEA system system have have been been widely investigated investigated [29–31]. It It has has been been various factors which affect the phase stability of HEAs, and concluded that multiple phased revealed microstructureof ofthe theAl AlxCoCrFeNi HEAs evolved from a single to +A1 B2, revealed that the microstructure HEAs evolved from a single A1,A1, to A1 B2,+ and x CoCrFeNi microstructures, as wellwith as intermetallic phases, would formed when the enthalpy hadcan greater and A2 structures increases Al content [12]. be Moreover, the alloying element also B2 +B2 A2+structures with increases in Alin content [12]. Moreover, the alloying element V canValso cause influences than configurational entropy. Therefore, basedand on the viewpoints and results[26]. of the present cause intermetallic formation the CoCrFeMnNi CoCrFeNi systems intermetallic phasephase formation in thein CoCrFeMnNi and CoCrFeNi HEA HEA systems [26]. study, it was expected that as the Al content increased, the Al and Ni elements would more easily be In the present study, the alloy composition contained both V and Al elements, and thus the combined to form a B2 phase, due to the strongly negative formation enthalpy of B2 structure for Alordered phase was unavoidable. For the Al0.3CrFeNiV alloy, it was observed that the alloy had typical Ni [32]. When x = 0.5, along with the coherent A2/B2 phases, the unknown phases which contained duplex BCC structures, including A2 and B2 phases. Also, the B2 phase was enriched in Ni and Al, large amounts of V and Cr were distributed in the alloy in the form of nano-precipitates. and the A2 matrix phase was enriched in V and Cr. When the Al content was increased to x = 0.5, the typical coherent A2/B2 phases and nano-precipitates could be observed. Otto et al. [8] investigated
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In the present study, the alloy composition contained both V and Al elements, and thus the ordered phase was unavoidable. For the Al0.3 CrFeNiV alloy, it was observed that the alloy had typical duplex BCC structures, including A2 and B2 phases. Also, the B2 phase was enriched in Ni and Al, and the A2 matrix phase was enriched in V and Cr. When the Al content was increased to x = 0.5, the typical coherent A2/B2 phases and nano-precipitates could be observed. Otto et al. [8] investigated various factors which affect the phase stability of HEAs, and concluded that multiple phased microstructures, as well as intermetallic phases, would be formed when the enthalpy had greater influences than configurational entropy. Therefore, based on the viewpoints and results of the present study, it was expected that as the Al content increased, the Al and Ni elements would more easily be combined to form a B2 phase, due to the strongly negative formation enthalpy of B2 structure for Al-Ni [32]. When x = 0.5, along with the coherent A2/B2 phases, the unknown phases which contained large amounts of V and Cr were distributed in the alloy in the form of nano-precipitates. In order to assist in analyzing the phase stability of the Alx CrFeNiV alloys, certain derived thermodynamic parameters such as: δ (atomic size difference); ∆Hmix (enthalpy of mixing); Ω (ratio of entropy to enthalpy values); ∆χ (Pauling electronegativity difference); and VEC (valence electron concentration) were calculated based on the following expressions [33]: s
n
∑ ci (1 − ri /r)2
δ=
(1)
i =1
n
∑
∆Hmix =
Ωij ci c j
(2)
i =1,i 6= j
Ω=
Tm ∆Smix |∆Hmix |
(3)
n
VEC =
∑ ci (VEC)i
(4)
i =1
s ∆χ =
n
∑ c i ( χi − χ ) 2
(5)
i =1
where VEC is the average valence electron concentration; VECi is the valence electron concentration of element i; ∆χ is the Pauling electronegativity difference; χi is the Pauling electronegativity of element i [34]; ci , ri , and χi are the atom fraction, radius, and electronegativity of the i-th element, respectively; and ra and χa are the average radius and electronegativity of the elements in the alloys, respectively. Table 3 presents the corresponding values of δ, ∆Hmix , ∆Smix , Tmix , Ω, VEC, and ∆χ for the Alx CrFeNiV alloys (x = 0.3, and 0.5). It has been determined in previous studies that the small δ (≤6.6%), near-zero values of the absolute ∆Hmix (−22 KJ/mol~5 KJ/mol), large values of Ω (~≥1.1), and small ∆χ (~≤17.5%) effectively favor the formation of solid solutions, rather than that of intermetallic compounds. Also, the δ–∆Hmix , δ–Ω, and δ–∆χ schemes could be adopted to predict the solid solution formations in HEAs [33,35,36]. Moreover, Guo et al. [37] concluded that the VEC could effectively predict the stability of BCC and FCC solid solutions in HEAs, and found that the BCC phase was stable at a lower VEC (
