Journal of Materials Science & Technology  2019 , 35 (10): 2331-2335 https://doi.org/10.1016/j.jmst.2019.05.050

Orginal Article

Microstructure and mechanical properties of FexCoCrNiMn high-entropy alloys

Tao Zhanga, Lijun Xina, Fufa Wua*, Rongda Zhaoa, Jun Xianga, Minghua Chena, Songshan Jiangb, Yongjiang Huangb*, Shunhua Chenc

aSchool of Materials Science and Engineering, Liaoning University of Technology, Jinzhou, 121001, China
bSchool of Materials Science and Engineering, Harbin Institute of Technology, Harbin, 150001, China
cSchool of Mechanical Engineering, Hefei University of Technology, Hefei, 230009, China

Corresponding authors:   *Corresponding authors.E-mail addresses: ffwooxy@163.com (F. Wu), yjhuang@hit.edu.cn(Y. Huang).*Corresponding authors.E-mail addresses: ffwooxy@163.com (F. Wu), yjhuang@hit.edu.cn(Y. Huang).

Received: 2019-03-22

Revised:  2019-04-28

Accepted:  2019-05-31

Online:  2019-10-05

Copyright:  2019 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

More

Abstract

The microstructure and tensile properties of FexCoCrNiMn high-entropy alloys (HEAs) were investigated. It was found that the FexCoCrNiMn HEA has a single face-centered cubic (fcc) structure in a wide range of Fe content. Further increasing the Fe content endowed the FexCoCrNiMn alloys with an fcc/body-centered cubic (bcc) dual-phase structure. The yield strength of the FexCoCrNiMn HEAs slightly decreased with the increase of Fe content. An excellent combination of strength and ductility was achieved in the FexCoCrNiMn HEA with higher Fe content, which can be attributed to the outstanding deformation coordination capability of the fcc/bcc dual phase structure.

Keywords: High-entropy alloys ; Microstructure ; Mechanical properties ; Strength ; Ductility

0

PDF (2424KB) Metadata Metrics Related articles

Cite this article Export EndNote Ris Bibtex

Tao Zhang, Lijun Xin, Fufa Wu, Rongda Zhao, Jun Xiang, Minghua Chen, Songshan Jiang, Yongjiang Huang, Shunhua Chen. Microstructure and mechanical properties of FexCoCrNiMn high-entropy alloys[J]. Journal of Materials Science & Technology, 2019, 35(10): 2331-2335 https://doi.org/10.1016/j.jmst.2019.05.050

1. Introduction

The traditional method for the design of metallic alloys is that one or two principal elements combine with minor other elements to achieve the desired properties. Usually, the development and application of these alloys are seriously limited by the formation of brittle intermetallic phases, which significantly deteriorate their mechanical properties. High-entropy alloys (HEAs), as advanced structural materials with high performance properties, were put forward and extensively investigated [[1], [2], [3], [4], [5], [6], [7]]. Especially, HEAs, which consist of at least five principal elements, each contributing 5-35 at.%, were developed by Yeh et al. [[8], [9], [10], [11]]. The excellent physical, chemical and mechanical properties of HEAs are mainly related to four high-entropy effects in thermodynamics, sluggish diffusion in structure, severe lattice distortion effect in dynamics, and cocktail effect in properties [7,8,12]. The structures of HEAs are mainly composed of face-centered cubic (fcc), body-centered cubic (bcc) and hexagonal closed-packed (hcp) solid solution, and they exhibit different mechanical properties. At room temperature, the HEAs with a single-phase fcc structure exhibit excellent plasticity, whereas the HEAs with a single-phase bcc structure possess high strength and relatively limited ductility. However, it is still challenging to simultaneously achieve both high strength and large ductility in HEAs [13,14].

Recently, HEAs with multi-phase structure have been reported, including dual-phase HEAs (DP HEAs), eutectic HEAs (EHEAs) and transformation-induced plasticity-assisted, dual-phase HEAs (TRIP-DP HEAs) [[15], [16], [17], [18], [19], [20], [21], [22]]. He et al. and Wang et al. studied the effects of Al on the structure and mechanical properties of FeCoCrNiMn HEAs [15,16], and found that as Al content increased, the crystalline structure of (FeCoNiCrMn)100-xAlx HEAs changed from a single-phase fcc structure to an fcc/bcc dual-phase structure, and then to a single-phase bcc structure, resulting in the increase of strength but the trade-off of ductility [15,16]. However, the modulated fcc/bcc lamellar structures can provide an excellent strength-ductility combination in the AlCoCrFeNi2.1 eutectic HEA [17,18,23]. Li et al. reported non-equiatomic TRIP-DP HEAs with phase instability and complicated dual-phase structures [[19], [20], [21]], and attributed their high strength and excellent ductility to interface hardening and transformation-induced hardening [21]. Most recently, precipitation-hardened HEAs with good match of strength-ductility were designed in non-equiatomic Al0.5Cr0.9FeNi2.5V0.2, (FeCoNi)86Al7Ti7, and (FeCoNi)86Al8Ti6 alloys, which exhibited not only high strength more than 1 GPa but also large ductility [24,25]. Therefore, fine tuning the composition away from equiatomic composition can effectively modify the structure of HEAs, and enhance their mechanical properties.

Due to the element and properties limitation of equiatomic HEAs, interstitial alloyed HEAs [20,[26], [27], [28], [29], [30]] and non-equiatomic HEAs were proposed. They seem to be more excellent in the strength-ductility combination, such as the iron-rich HEAs and the nickel-rich HEAs [[19], [20], [21],24,25]. As far as the Cantor alloy FeCoCrNiMn HEA concerned, if the Fe content is increased and the mechanical properties are still as good as or better than the equiatomic one [[19], [20], [21]], the chemical composition and mechanical properties of the these HEAs will be closer to the iron-based alloys or steels, resulting in the enlargement of the processing window of HEAs and being useful for reducing the production cost of HEAs by increasing the content of inexpensive Fe element, i.e. decreasing expensive elements of Co and Ni. Meanwhile, it is envisioned that increasing Fe content will decrease the mixing entropy of FeCoCrNiMn, leading to the decrease of the stability of fcc phase and promoting the formation of bcc phase.

In this work, the FeCoCrNiMn HEA was selected as the model material. The main goal is to study the microstructure and mechanical properties of the Fe-rich HEAs with increased Fe content.

2. Experimental

The chemical composition of the designed HEAs is FexCoCrNiMn. Here x is 1 (FeCoCrNiMn, i.e. Fe20Co20Cr20Ni20Mn20), 2.67 (Fe2.67CoCrNiMn, i.e. Fe40Co15Cr15Ni15Mn15), 6 (Fe6CoCrNiMn, i.e. Fe60Co10Cr10Ni10Mn10), 6.53 (Fe6.53CoCrNiMn, i.e. Fe62Co9.5Cr9.5Ni9.5Mn9.5), and 8.25 (Fe8.25CoCrNiMn, i.e. Fe67Co8.25Cr8.25Ni8.25Mn8.25), hereafter labelled as F20, F40, F60, F62, and F67, respectively. The HEAs were prepared by arc melting pure raw materials (>99.9 wt%) under an argon atmosphere. The alloy ingots were re-melted for five times to ensure the chemical homogeneity, followed by drop casting the alloy melt into a copper mold to fabricate cylindrical samples with a dimension of 6 mm in diameter and 70 mm in length.

The microstructure and chemical compositions of the as-cast samples were examined by a Carl Zeiss optical microscope (OM), and a Sigma 500 Scanning Electron Microscope (SEM) equipped with an energy disperse spectrometer (EDS) and an electron back-scattered diffraction (EBSD) detector. The phases of the alloys were characterized by X-ray diffraction (XRD) using a Rigaku diffractometer with Cu radiation.

The dog-bone-shaped tensile specimens, with a gauge size of 10 mm (length) ×1 mm (width) ×1 mm (thickness), were cut from as-cast samples through electrical discharge machining (EDM). The surfaces of the tensile specimens were mechanically ground and polished to a mirror finish before the tensile tests, which were conducted on a SANS CMT5305 testing machine at room temperature with a constant initial strain rate of 1 × 10-4 s-1. A strain gauge of 10 mm was applied during tensile loading. The tensile test for each alloy was repeated for 5 times. The deformed specimens after tensile tests were examined by OM and SEM to investigate the deformation and fracture features.

3. Results and discussion

3.1. Microstructure of FexCoCrNiMn HEAs

Fig. 1 shows the typical microstructure features of the Fe(CoCrNiMn)x alloys. All studied alloys showed randomly oriented grains, as shown in the EBSD inverse pole figure (IPF) map in Fig. 1(a)-(c). The average grain size of the F20, F40, F60, and F62 alloys is nearly similar, ranging from 25 to 150 μm. However, the average grain size of the F67 alloy is 220 μm, which is remarkably larger than those of the F20, F40, F60 and F62 alloys. All alloys experienced dendrite growth process during solidification, as shown in the OM images in the insets of Fig. 1(a)-(c). With increasing x to 8.25 (the F67 alloy), a dendritic bcc phase structure was formed and imbedded in the fcc matrix, as shown in Fig. 1(c) and (d). The volume fraction of the dendritic bcc phase is estimated to be 30.9%. The chemical composition of the dendrites (region A in Fig. 1(e)) and the fcc matrix (region B in Fig. 1(e)) was determined to be Fe67.8Co8.3Ni7.6Cr8.3Mn8 and Fe59.3Co8.5Ni10.1Cr9.1Mn13, respectively, by EDS detection. Clearly, Fe element is enriched in the dendrite region, whereas Mn and Ni elements are enriched in the matrix region (Fig. 2(f)). Co and Cr elements are almost uniformly distributed in both the bcc dendrite and the fcc matrix (Fig. 2(f)). The element concentration and distribution are consistent with the previous studies in FeCoCrMnNi alloys [31,32].

Fig. 1.   EBSD images showing the microstructure of the FexCoCrNiMn alloys: (a) F20, (b) F40, and (c) F67, with insets showing the corresponding OM images, (d) EBSD image showing the phase composition of the F67 alloy, (e) SEM image of the F67 alloy showing the dendrite structure (Points A and B denote the dendrite and matrix, respectively) and (f) EDS results of the F67 alloy showing the chemical composition difference between the dendrite and the matrix along the dotted line in Fig. 1(e).

Fig. 2.   XRD patterns of the FexCoCrNiMn alloys, with inset showing the main diffraction peaks of the fcc and bcc phases.

Fig. 2 shows the XRD patterns of the Fe(CoCrNiMn)x HEAs. When the amount of Fe is 20 at.% (F20), 40 at.% (F40) and 60 at.% (F60), the alloys were composed of a single fcc phase, as shown in Fig. 2. However, with increasing Fe content to 62 at.%, an obvious peak, which was indexed as bcc (110) phase, occurred on the right of the fcc (111) peak in the XRD pattern. With further increasing Fe content to 67 at.%, the bcc (110) peak becomes much stronger. Clearly, as Fe content increases, the structure of the HEAs transformed from a single fcc phase to an fcc/bcc dual-phase structure. The lattice parameters of fcc and bcc phases of the F67 alloy are a1 = 3.604 Å and a2 = 2.883 Å, respectively. From F62 to F67 alloy, the intensity of the bcc (110) peak was significantly enhanced whereas the fcc (111) peak becomes weaker, indicating that the volume fraction of the bcc phase increased but the fcc phase decreased. The whole volume fraction of the fcc phase was still higher than that of the bcc phase, which is consistent with the EBSD results in Fig. 1(c) and (d).

3.2. Mechanical property of FexCoCrNiMn HEAs

Fig. 3(a) shows the representative tensile nominal stress-strain curves of the FexCoCrNiMn alloys. When the Fe content is 20 at.% (F20), the alloy yielded at 245 MPa, followed by a high strain hardening to 521 MPa at the plastic strain of 56.3% prior to failure, which is relatively smaller than that of the rolled and recrystallized FeCoCrNiMn HEA [2,4]. When the Fe content increased to 40 at.%, the alloy F40 yielded at 236 MPa, followed by a large strain hardening to 498 MPa at the plastic strain of 55.1% prior to failure. When the Fe content increased to 60 at.%, the yield strength and ultimate tensile strength of the F60 alloy decreased to 190 MPa and 450 MPa, respectively. However, the homogeneous plastic strain of the F60 alloy increased to 64.3%. Further increasing the amount of Fe to 62 at.%, the yield strength of the F62 alloy decreased to 172 MPa, however, the ultimate tensile strength increased to 496 MPa which is larger than that of the F60 alloy. The final plastic strain of the F62 alloy reached up to 86.8%, which is the largest value among the studied HEA samples. The yield strength of F67 was 148 MPa, then followed by strong strain-hardening to 604 MPa, and the whole plastic strain was 61%. All alloys displayed strong plastic instability after the ultimate tensile stress, showing strong necking phenomena. It was clear that the yield strength of all alloys decreased with the increase of Fe amount (Fig. 3(b)), which actually reflected the solid-solution strengthening effect weakened with the amount of CoCrNiMn decreasing, as shown in Fig. 3(b). Though the ultimate tensile strength of the FexCoCrNiMn alloys decreased with increasing Fe content from 20 at.% to 60 at.%, it increased reversely in the F62 and F67 alloys when the Fe content increased to 62 at.% and 67 at.%, respectively. The area reduction at fracture for all alloys also displayed the same tendency as the elongation (Fig. 3(b)).

Fig. 3.   Tensile properties of the FexCoCrNiMn alloys: (a) nominal tensile stress-strain curves; (b) statistics of the ultimate tensile strength and the ductility of the alloys with the Fe content; (c) true stress-strain curves; (d) strain-hardening rate with respect to the true strains.

In order to further understand the plastic deformation behaviour of the HEAs, the true stress-strain curves and strain-hardening rates were calculated and analyzed as following. Fig. 3(c) shows the true stress-strain curves of the alloys corresponding to the engineering ones in Fig. 3(a). With increasing Fe content, the ultimate true tensile strength decreased from 851 MPa for F20 to 798 MPa for F40, and to 764 MPa for F60. However, the ultimate true strength began to increase from 978 MPa for F62 to 1067 MPa for F67, which is almost 25.4% higher than that of F20, and 39.7% higher than that of F60. Fig. 3(d) is the strain-hardening rate, which was calculated by differentiating the true stress over true strain. In the initial plastic deformation region between true strains of 10%-20%, the strain-hardening rate decreases with the increase of Fe content. In this region, the average strain-hardening rate decreased from 1322 MPa for F20, to 1175 MPa for F40, and finally to 1077 MPa for F60, respectively. It is interesting that the strain-hardening rate of F20 decreased continuously and had strain-hardening rate valleys, which are 1158 MPa and 1059 MPa for F40 and F60, respectively. After the true strain of 30%, the strain-hardening rates of F20, F40, and F60 decreased significantly. The results indicate that the solid solution strengthening effect is weakened by increasing Fe content, which can be attributed to the decrease of the CoCrNiMn content.

It should be noticed that the strain-hardening rate of the F62 alloy continuously increased from 1059 MPa at the true strain of 12% to 1361 MPa at the true strain of 50%, which is consistent with its relatively high ultimate tensile strength and the large homogeneous plastic deformation capability, as shown in Fig. 3(a) and (c), respectivley. However, among the studied HEA samples, though the F67 alloy exhibits the smallest initial strain-hardening rate, it increased sharply from 957 MPa at the true strain of 7.5% to 2649 MPa at a true strain of 40%. This is more than twice of those for the F20, F40, F60, and F62 alloys. After the true strain of 40%, the strain-hardening rate of the F67 alloy began to decrease, but it was still larger than other alloys before the true strain of 55%, as shown in Fig. 3(d). Therefore, though the F67 alloy yielded at a relatively lower stress, it underwent continuously and gradually strengthened strain-hardening to exhibit high tensile strength and large homogeneous plastic deformation. The plastic deformationbehaviour of the F67 alloy is more like that of TRIP and TWIP steels [[33], [34], [35]]. In a word, the strain-hardening capability of the F67 alloys with dual-phase structure is obviously better than that of single-phase F20, F40, and F60 alloys.

3.3. Comprehensive understanding of the strength and ductility of FexCoCrNiMn HEAs

The above experimental results show that the microstructure and mechanical properties of the FexCoCrNiMn HEAs are significantly dependent upon the Fe content. For the alloys with a single fcc solid solution phase (F20, F40, and F60 alloys), the lattice distortion decreases with increasing Fe content. Both the yield strength and the ultimate tensile strength decrease with increasing Fe content, suggesting a solid-solution strengthening effect induced by lattice distortion [15]. Though the valence electron concentration keeps almost unchanged for the F20, F40, and F60 alloys, the average atomic radius of the CoCrNiMn group is 0.1274 nm, which is 2.66% larger than 0.1241 nm of Fe [36]. Therefore, there should be a weak solid solution strengthening effect by changing Fe or CoCrNiMn concentration. For the alloys of F40 and F60, their CoCrNiMn contents are about 75% and 50% of that of the F20 alloy, therefore the solid-solution strengthening effect is smaller than that of the F20 alloy, which can be reflected on the decrease of the yield strength and ultimate tensile strength of F40 and F60 alloys, as shown in Fig. 2(a) and (b). However, by further increasing the Fe content or decreasing the CoCrNiMn content, both fcc and bcc phases are formed in the F62 and F67 alloys. Due to the excellent plastic deformation capability of the fcc phase and the relatively higher strength of the bcc phase, both F62 and F67 alloys exhibit good combination of high ultimate tensile strength and large homogeneous plastic deformation, as shown in Fig. 3.

It should be noted that although the ultimate tensile strength of the F62 and F67 alloys are larger than the F20, F40, and F60 alloys, the yield strength of the F62 and F67 alloys is still smaller than that of the F20, F40, and F60 alloys, which implies that the yield strength of the alloys obeys the rule of solid-solution strengthening. It is reasonable that the yield strength of the fcc phase still decreases with increasing Fe content or decreasing the CoCrNiMn concentration, and the yield strength increment, induced by the bcc phase, cannot balance the yield strength reduction caused by the fcc phase, resulting in the low yield strength of the F62 and F67 alloys. However, the bcc phase helps increase the strain-hardening rate of the F67 alloys in the following plastic deformation, leading to the high ultimate tensile strength and large ductility of the F67 alloy. Furthermore, in dual-phase or heterogeneous structure, the plastic deformation is microscopically nonhomogeneous, leading to strain gradient, strain partitioning, and large internal stress [37]. This contributes to the extra back stress-induced hardening during the tensile deformation. Therefore, the strain hardening rate of the F67 alloy is sustained to large uniform strains. The underlying strengthening and toughening mechanism for the F62 and F67 alloys needs further deeply comprehensive investigations.

Fig. 4 shows the strength-ductility combination of some explored steels and HEAs [20,38], together with the studied HEAs. It indicates that for single fcc HEAs, such as F20, F40, and F60 alloys, they possess relatively high ductility but low strength, as shown in the upper-left corner of Fig. 4. After dual phase is introduced to the HEAs, such as F67 alloy, which locates at the upper right of single fcc HEAs, as shown in Fig. 4, both the strength and ductility are simultaneously improved. Due to the sample size limitation and the as-cast microstructure by copper mould, the mechanical properties of the present alloys shown in Fig. 4 is just for a schematic illustration, which mostly aims to compare the mechanical properties between the F20, F40, F60 alloys and the Fe62, F67 alloys.

Fig. 4.   Strength-ductility relationship of some classes of metallic materials, including HEAs.

Therefore, by modifying the amount of Fe element, the FexCoCrNiMn HEAs can exhibit improved mechanical properties. The alloys can exhibit a single fcc phase and possess similar mechanical properties within a wide Fe or CoCrNiMn content range, implying a wide processing window for single fcc HEAs by adjusting the Fe content. Moreover, the structure of HEAs can also be tailored to be bcc/fcc dual phase, and the strength-ductility combination can be significantly optimized. The results also demonstrate a potential economic feasibility, since less expensive elements of Co and Ni are used by increasing the Fe content, i.e. decreasing the CoCrNiMn content, which makes the HEAs more like Fe-based alloys or steels with better mechanical performance.

4. Conclusion

This investigation indicates that the microstructure and mechanical properties of the FexCoCrNiMn HEAs can be significantly tuned by increasing the Fe content. Within a wide composition range of20-60 at.% Fe, the HEAs exhibit relatively lower strength and larger ductility with a slight tensile strength decreasing with decreasing Fe content. Further increasing the Fe content endows the FexCoCrMnNi alloy with a bcc/fcc dual phase structure, which exhibits a good combination of strength and ductility. These findings are helpful for designing HEAs with good mechanical performance by elements and structure adjusting in a large processing window.

Acknowledgments

This work was supported financially by the National Natural Science Foundation of China (Nos. 51571006, 51702143 and 51805236) and the Program for Liaoning Distinguished Professor.


/