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Direct laser deposited bulk CoCrFeNiNbx high entropy alloys
Intermetallics 114 (2019) 106592
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Intermetallics journal homepage: www.elsevier.com/locate/intermet
Direct laser deposited bulk CoCrFeNiNbx high entropy alloys ∗
T
Kexuan Zhou, Junjie Li, Lilin Wang, Haiou Yang, Zhijun Wang , Jincheng Wang State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an, 710072, China
A R T I C LE I N FO
A B S T R A C T
Keywords: High entropy alloys Direct laser deposition Microstructures Mechanical properties
Bulk CoCrFeNiNbx high entropy alloys (HEAs) were successfully fabricated by direct laser deposition. The microstructures, mechanical properties and work-hardening behavior of the as-deposited CoCrFeNiNbx HEAs were investigated. The uniform and fully compact microstructures were characterized in both longitudinal and transverse sections. Apparent pores and microcracks are absent in all samples. Moreover, a transition from columnar structure to equiaxed structure was observed with the increasing addition of Nb element. Room temperature tensile tests showed the excellent combination of strength and ductility compared with the as-cast counterparts. The CoCrFeNiNbx HEAs show better printability and excellent mechanical properties than other high entropy alloys within direct laser deposition processing.
1. Introduction The past decades have witnessed the rapid development of high entropy alloys, especially CoCrFeNi-based HEAs [1–5]. Compared with traditional alloys, HEAs consist of four or more principal elements with the concentration between 5 and 35 at.%, breaking through the conventional conceptions of alloy design [6]. Although there are multiple elements with different crystal structures, previous researches demonstrated that HEAs tend to form very simple solid solution phases, with more degrees of freedom for composition choice as predicted by Gibbs phase rule [1,7,8]. Unique mechanical and physical properties have been found in many HEAs [3,9–11]. Along with the development of HEAs, many researchers investigated different processing routes to fabricate HEAs, such as vacuum arc melting [3,12,13], spark plasma sintering [14], mechanical alloying [15,16], plasma spray deposition [17], sputter deposition [18], laser cladding [19,20] and additive manufacturing (AM) [21,22]. Among these methods, AM [23] is a rapid prototyping technology to fabricate fully dense metallic parts with complex near-net shape based on computer numerical control and laser cladding. AM has been shown as a promising method to achieve high degrees of shape complexity and industrial production. AM generates a non-equilibrium solidification process as a result of the rapid melting and cooling, and can enhance solute trapping and relieve compositional segregation. Therefore, AM can produce three-dimensional metallic parts with homogeneous and ultrafine microstructures efficiently and effectively. AM has been recognized as a promising technology to fabricate the high-entropy alloys in the very recent years. Selective laser melting
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(SLM) and electron beam melting (EBM) are both based on powder bed process but have different heat sources and working conditions, which include the chamber environments and the preheating procedure. In EBM, preheating procedure is needed to sinter the powder bed to prevent charging of the powders. Comparing metal AM technologies, SLM exhibits the highest solidification rate and EBM can be applied to high melting point alloys due to high absorption rate of electron beam by alloy powders, while direct laser deposition (DLD) based on powder feeding process possesses high deposition efficiency and near-rapid solidification. DLD is more applicable to the bulk component and reparations of damaged components. There is a great significance to explore the microstructures and mechanical properties of as-deposited HEAs for the potential application of additive manufacturing. Several basic researches have been done to explore the application of DLD in fabricating HEAs. Sistla et al. [24] fabricated thin walled AlxFeCoCrNi2-x HEA components but with poor formability due to the macrocracks. Joseph et al. [22] compared the microstructures and mechanical properties of the direct-laser-fabricated and arc-melted AlxCoCrFeNi HEAs, and further investigated the tensile and compress properties of the as-deposited Al0.3CoCrFeNi alloy [25]. Unfortunately, for all alloys cracking behavior was observed in the first 5–8 mm of height adjacent the AISI 316 plate. The tensile of the as-deposited Al0.3CoCrFeNi alloy only showed the yield strength of 194 MPa and limited work hardening. Recently, Xiang et al. [26] investigated the effects of processing parameters on microstructures and tensile properties of laser melting deposited CrMnFeCoNi HEAs, and the 1400 W sample exhibited more excellent mechanical properties in room temperature with the yield strength of 290 MPa, fracture strength of
Corresponding author. E-mail address: [email protected] (Z. Wang).
https://doi.org/10.1016/j.intermet.2019.106592 Received 4 May 2019; Received in revised form 14 August 2019; Accepted 17 August 2019 0966-9795/ © 2019 Published by Elsevier Ltd.
Intermetallics 114 (2019) 106592
K. Zhou, et al.
12.5 × 2 × 3.2 (mm3) were obtained and loaded in the direction that is parallel to the building direction. Each of the as-deposited Nbx HEAs was tested twice to confirm reproduction.
535 MPa and plastic strain of 55%. Y. Chew et al. [27] investigated microstructures and mechanical behavior of CoCrFeNiMn HEA fabricated by using laser aided additive manufacturing. Due to grain boundary strengthening, the laser aided additive manufactured CoCrFeNiMn HEA exhibited higher strength of 660 MPa and ductility of 19.8%. These primary results show the promising application of DLD on the fabrication of HEAs. However, much more HEAs systems need to be explored for better printability and mechanical properties in the DLD processing. In this study, we choose DLD to fabricate bulk CoCrFeNi HEAs with different Nb addition. The CoCrFeNi system is the most fundamental and fully investigated system [5] with a single and stable FCC structure, while Nb element is a typical precipitation hardening element in nickelbased superalloys. As Nb element was added to CoFeCrNi system, the CoCrFeNiNbx HEAs have been reported to simply solidify into FCC and Laves duplex phase structure [28,29]. In this investigation, we presented the feasibility of producing bulk CoCrFeNiNbx HEAs via DLD method and the effects of Nb addition on the evolution of phases, microstructures, mechanical properties as well as deformation behavior. Surprisingly, the CoCrFeNiNbx HEAs are much suitable for DLD and show excellent mechanical properties.
3. Results and discussion 3.1. Phase identification The XRD results of the as-deposited Nbx HEAs are shown in Fig. 2. There was only FCC phase in CoCrFeNi alloy. In Fig. 2 (a), obvious [220] texture with the abnormal peak intensity ratio formed in the CoCrFeNi alloy, showing the existence of preferred orientation. However, this peak intensity decreased sharply with the increment of Nb concentration, indicating that the preferred orientation of obtained grain crystal tends to be weakened. In addition, there were small peaks of Laves phase appearing around (111)FCC peak in the inset plot of Fig. 2 (a) as the increase of Nb content, indicating a FCC-Laves dualphase constitution. Moreover, the lattice parameters of the FCC phase were calculated via MDI jade 6.5 including all diffraction peaks, which were calculated to be 0.3562, 0.3582, 0.3591 and 0.3589 nm. In addition, the lattice parameter changing of the matrix FCC phase in asdeposited Nbx is shown in Fig. 2 (b), which exhibits the similar tendency and variation extent of the as-cast Nbx [28]. Clearly, the evolution of lattice parameters indicates that the solid solution of Nb results in a lattice dilation.
2. Experiments The experiments were performed on laser additive manufacturing system [30], which consists of a 3 kW continuous wave fiber laser with a wavelength of 1060 nm, a five-axis numerical control working table, high precision and adjustable automatic feeding device with dual powder feeders, four coaxial nozzles and an inert argon atmosphere processing chamber where the oxygen content is controlled less than 50 ppm. After a series of pre-experiments, the optimal parameters were confirmed as laser power 1.6–1.65 kW, scanning speed 7 mm s−1, laser spot diameter 3 mm and overlap rate 50%. Bi-directional deposition pattern was applied and the scanning orientation was translated 90° between each successive layer, as shown in Fig. 1 (a). Spherical CoCrFeNi powder (50–150 μm) and Nb powder (50–75 μm) were both prepared by plasma rotation electrode process, which were used as raw materials. The mixed powders with the composition of CoCrFeNiNbx (x = 0, 0.1, 0.15, 0.2) were blended uniformly in a ball milling equipment and then dried in a vacuum dryer at 120 °C for at least 2 h. AISI 316 stainless steel plates with dimensions of 140 × 60 × 10 (mm3) were used as substrate materials, which were polished by abrasive paper to remove surface oxides or contaminants and then cleaned by acetone. Deposited bulk CoCrFeNiNbx HEAs (named as Nbx hereafter) were at the dimensions about 35 × 18 × 70 (mm3). One of as-deposited bulk Nbx HEAs and the building direction (BD) as well as schematic representation of tensile specimen with respect to loading directions are shown in Fig. 1 (b). The phase constituent of the as-deposited Nbx HEAs were characterized by an X-ray diffractionmeter (X'Pert PRO), using monochromatic Cu Kα (λ = 1.54060 Å ) radiation scanning from 20° to 110° with 5°/min scanning rate. The XRD analysis plane was parallel to the building direction. The lattice parameters were calculated by XRD analysis from the obtained diffraction peaks. Microstructural investigations were performed by scanning electron microscopy (SEM SU6600) coupled with energy dispersive spectrometry (EDS) and the samples were polished and then etched in a dilute aqua regia solution with HNO3:HCl:H2O = 1:3:3. Electron backscattered diffraction (EBSD) characterizations were performed using a Tescan VEGAIILMH SEM with the HKL Technologies Channel 5 software and the EBSD samples were mechanically polished and then electro-polished in a 10% perchloric acid solution for 40 s at 30 V. All samples for XRD, SEM and EBSD tests were taken from the middle height of the as-deposited HEAs. Tensile tests were performed at room temperature by a materials testing machine (INSTRON 3382) with the strain rate of 1 × 10−3 s−1. The dog bone-shaped flat tensile specimens with gauge dimensions of
3.2. Microstructural characterization Microstructures of the as-deposited Nbx HEAs in longitudinal and transverse sections which were parallel and perpendicular to the building direction are shown in Fig. 3. Fig. 3 (a) and (e) exhibits that the as-deposited Nb0 alloy presented alternately bright and dark microstructures, showing strip-shape microstructures in longitudinal section and spot-like microstructures in transverse section respectively. Therefore, the as-deposited Nb0 alloy presented directed cellular structures. The difference in corrosion caused by the minor compositional difference resulted in alternately bright and dark microstructures, which was also confirmed in additively manufactured stainless steels [31]. With the addition of Nb element, Laves phase appeared and preferred to form in the inter-dendritic region and at grain boundary. In Nb0.1 alloy there were typical columnar microstructures with elongated grains longitudinal to the building direction and short rod-like Laves uniformly dispersed along with the grain orientation. Compared with Nb0.1 alloy, the Nb0.15 alloy had a higher volume fraction of Laves phase, which transformed from short rod-like structures to cross-linked mesh structures. In Nb0.2 alloy, obvious crosslinked network structures were obtained in both longitudinal and transverse sections. It is obvious that the increasing of Nb addition leads to the change of microstructures. The as-deposited bulk Nbx HEAs exhibited fully compact microstructures without apparent pores and microcracks, compared with the as-deposited CoCrFeMnNi HEAs [27] and the direct-laser-fabricated AlxCoCrFeNi HEAs [22,24]. Moreover, there were more uniformly dispersed and fine Laves phase in FCC matrix, compared with the counterpart as-cast Nbx HEAs [28,29]. Chemical compositions of the FCC phase and the Laves phase were analyzed by using EDS and the results are listed in Table 1, showing that the FCC phase was limited in Nb in spite of the increment of Nb addition. Combining with the XRD results, the Laves phase was identified as the (CrFe)(CoNi)Nb type with a HCP lattice structure. Moreover, backscattered electron (BSE) micrographs and corresponding EDS elemental mappings of the as-deposited Nb0.1 and Nb0.2 HEAs are presented in Fig. 4. The results shown that Co, Cr, Fe and Ni elements uniformly distributed in matrix phase, while Laves phase was enriched in Nb but poor in Cr and Fe. To further confirm the microstructure transition, Fig. 5 shows the inverse pole figure maps and pole figures of the as-deposited Nb0 and 2
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Fig. 1. (a) Laser scanning pattern and (b) the as-deposited bulk CoCrFeNiNb0.1 alloy with schematic representation of tensile specimen with respect to loading directions.
Fig. 2. (a) XRD patterns of the as-deposited CoCrFeNiNbx HEAs and (b) lattice parameter changing of the matrix FCC phase in as-deposited Nbx HEAs as the increase of Nb concentration.
phase and liquid phase. With rapid solidification, the nucleus is easily formed in the undercooled melt which induces the CET. In addition, the anisotropy of texture in the building direction of the as-deposited Nb0.2 alloy was distinctly eliminated compared with that within Nb0 alloy, due to the occurrence of CET transition, as shown in Fig. 5(c) and (d).
Nb0.2 alloys. The as-deposited Nb0 alloy exhibited large columnar crystals with significant orientation. In the as-deposited Nb0.2 alloy, microstructures were modified from large grains with columnar crystals to much finer grains with equiaxed crystals. During the solidification, the addition of Nb element not only produced the formation of Laves phase but also affected the solidification microstructures due to the constituent supercooling. In directional solidification, CET usually occurs with the nucleation in the undercooled liquid in front of the columnar dendrites [32]. The addition of Nb will increase the undercooling of the liquid because of its severe partition between the solid
3.3. Mechanical properties Room-temperature tensile properties of the as-deposited bulk Nbx HEAs were measured, as shown in Fig. 6 (a). As the Nb concentration 3
Intermetallics 114 (2019) 106592
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Fig. 3. The SEM images of the as-deposited CoCrFeNiNbx HEAs in longitudinal section (a) Nb0 (b) Nb0.1 (c) Nb0.15 (d) Nb0.2 and in transverse section (e) Nb0 (f) Nb0.1 (g) Nb0.15 (h) Nb0.2.
deposited Nbx HEAs exhibited a hybrid fracture combining dimples with raised edge. The obtained as-deposited Nbx HEAs possessed a better strengthductility combination, especially the Nb0 and Nb0.1 alloys. We investigated work-hardening behavior of the as-deposited Nbx HEAs to further understand the improvement in their mechanical properties, as shown in Fig. 6 (d). Expect the Nb0.2 HEA, the work hardening rate in the as-deposited Nbx HEAs all showed three distinctive stages, where the Stage II was the key contribution to the work hardening. The workhardening Stage II showed different trend in each alloy, which was characterized by an increasing work-hardening rate in Nb0 alloy, a stable work-hardening-rate platform in Nb0.1 alloy and a slowly decreasing work-hardening rate in Nb0.15 alloy. Compared with as-deposited CoCrFeNi alloy, the as-deposited Nbx alloys exhibited higher strengths and shown higher work-hardening rates in the work-hardening Stage II, which was mainly due to the effects of solid solution strengthening and compound reinforcement. Both the entanglement of the dislocations and Laves phase can effectively hinder the dislocation motion. Therefore, during tensile deformation the dislocations multiply and pile up rapidly and thus the corresponding work-hardening rate was increased. In addition, the unique work-hardening behavior of the as-deposited Nbx HEAs (except Nb0.2 alloy) resulted in the good strength-ductility combination. Firstly, CoCrFeNi alloy system with high inherent workhardening exponent during deformation [33] and low stacking fault [34] exhibits the higher level of strengthening and toughness that can effectively retard the localized deformation (suppressing crack formation and propagation), enhances the uniform elongation and causes an increase in work hardening rate [35]. Moreover, the relatively high cooling rate experienced during DLD process favored the production of uniform and fully compact sub-dendrite microstructures with certain
Table 1 Chemical composition of the as-deposited CoCrFeNiNbx HEAs (at.%). Alloy
Region
Co
Cr
Fe
Ni
Nb
Nb0 Nb0.1
matrix matrix precipitate matrix precipitate matrix precipitate
25.03 24.71 25.65 25.06 22.89 24.61 24.79
24.88 24.27 16.75 24.94 17.67 24.32 18.71
25.38 24.51 16.89 24.85 17.42 25.35 19.60
24.71 24.55 23.31 23.23 22.01 23.45 22.50
– 1.96 17.40 1.92 20.01 2.27 14.40
Nb0.15 Nb0.2
increased, both yield and fracture strengths increased but tensile ductility decreased. Compared with the as-cast counterparts, the as-deposited Nbx HEAs exhibited a better strength-ductility combination, showing higher strength and almost doubled elongation. The as-deposited Nb0 alloy performed extremely high elongation, attaining 92.5%. Moreover, in as-cast state, the addition of Nb element resulted in an obvious decrease in elongation of Nbx HEAs. Intriguingly, the asdeposited Nb0.1 HEA exhibited a twofold increase in strength and better ductility, compared with the as-cast Nb0 alloy. Compared with aforementioned DLD-HEA [22,24–27], the as-deposited bulk Nbx HEAs with uniform and fully compact microstructures also showed more superior tensile properties. Fig. 6 (b) shows the tensile property comparison between both as-deposited CoCrFeNiAlx (x = 0.3, 0.6) and CoCrFeNiNbx HEAs, indicating that the Nbx HEAs can be as a promising candidate system for DLD processing. The fractured morphologies after the tensile deformation of the as-deposited Nbx alloys are shown in Fig. 6 (c). Typical plastic fracture with numerous deep dimples was observed in the as-deposited Nb0 HEA. With the addition of Nb element and the formation of the ordered Laves phase, the fracture patterns of the as-
Fig. 4. The backscattered electron (BSE) micrographs and corresponding EDS element mappings of the as-deposited (a) Nb0.1 and (b) Nb0.2 HEAs. 4
Intermetallics 114 (2019) 106592
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Fig. 5. IPF maps and pole figure maps of the as-deposited (a)(c) Nb0 and (b)(d) Nb0.2 specimens obtained at the longitudinal section parallel to the building direction.
ductility, owing to optimized microstructures and the unique workhardening behavior compared to the as-cast counterparts. Among the as-deposited Nbx HEAs, the Nb0 alloy exhibited an ultra-high elongation reaching 92.5% while the Nb0.1 alloy showed not only the strength enhancement but also higher elongation compared with the as-cast Nb0 alloy. CoCrFeNiNbx HEAs can be as a promising candidate system for additive manufacturing.
orientation, showing better crack initiation resistance and suppressing the propagation of microcracks during deformation [23] and then resulting in better work-hardening behavior. Therefore, even with more Nb addition, the as-deposited Nbx HEAs showed better ductility compared with the as-cast Nbx HEAs.
4. Conclusion The bulk CoCrFeNiNbx HEAs were successfully fabricated by DLD with enhanced mechanical properties. The as-deposited Nbx HEAs exhibited uniform and fully compact microstructures due to near-rapid solidification and shown a microstructure transition from columnar structures to equiaxed structures as the increased Nb addition. The asdeposited alloys have shown the excellent combination of strength and
Conflicts of interest The authors declared that they have no conflicts of interest to this work.
Fig. 6. (a) Room temperature tensile engineering stress-strain curves of the as-deposited bulk Nbx HEAs compared with the as-cast counterparts. (b) The true ultimate tensile strength versus elongation of both as-deposited CoCrFeNiAlx and CoCrFeNiNbx HEAs. (c) Fracture surface morphologies and (d) work hardening rate of the asdeposited Nbx HEAs. 5
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Acknowledgment [18]
The work was supported by National Key R&D Program of China [grant No. 2018YFB1106000, 2016YFB1100100], National Natural Science foundation of China [grant No. 51771149] and the Fund of State Key laboratory of Solidification Processing in NWPU [grant No. 03-TS-2019].
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Intermetallics journal homepage: www.elsevier.com/locate/intermet
Direct laser deposited bulk CoCrFeNiNbx high entropy alloys ∗
T
Kexuan Zhou, Junjie Li, Lilin Wang, Haiou Yang, Zhijun Wang , Jincheng Wang State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an, 710072, China
A R T I C LE I N FO
A B S T R A C T
Keywords: High entropy alloys Direct laser deposition Microstructures Mechanical properties
Bulk CoCrFeNiNbx high entropy alloys (HEAs) were successfully fabricated by direct laser deposition. The microstructures, mechanical properties and work-hardening behavior of the as-deposited CoCrFeNiNbx HEAs were investigated. The uniform and fully compact microstructures were characterized in both longitudinal and transverse sections. Apparent pores and microcracks are absent in all samples. Moreover, a transition from columnar structure to equiaxed structure was observed with the increasing addition of Nb element. Room temperature tensile tests showed the excellent combination of strength and ductility compared with the as-cast counterparts. The CoCrFeNiNbx HEAs show better printability and excellent mechanical properties than other high entropy alloys within direct laser deposition processing.
1. Introduction The past decades have witnessed the rapid development of high entropy alloys, especially CoCrFeNi-based HEAs [1–5]. Compared with traditional alloys, HEAs consist of four or more principal elements with the concentration between 5 and 35 at.%, breaking through the conventional conceptions of alloy design [6]. Although there are multiple elements with different crystal structures, previous researches demonstrated that HEAs tend to form very simple solid solution phases, with more degrees of freedom for composition choice as predicted by Gibbs phase rule [1,7,8]. Unique mechanical and physical properties have been found in many HEAs [3,9–11]. Along with the development of HEAs, many researchers investigated different processing routes to fabricate HEAs, such as vacuum arc melting [3,12,13], spark plasma sintering [14], mechanical alloying [15,16], plasma spray deposition [17], sputter deposition [18], laser cladding [19,20] and additive manufacturing (AM) [21,22]. Among these methods, AM [23] is a rapid prototyping technology to fabricate fully dense metallic parts with complex near-net shape based on computer numerical control and laser cladding. AM has been shown as a promising method to achieve high degrees of shape complexity and industrial production. AM generates a non-equilibrium solidification process as a result of the rapid melting and cooling, and can enhance solute trapping and relieve compositional segregation. Therefore, AM can produce three-dimensional metallic parts with homogeneous and ultrafine microstructures efficiently and effectively. AM has been recognized as a promising technology to fabricate the high-entropy alloys in the very recent years. Selective laser melting
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(SLM) and electron beam melting (EBM) are both based on powder bed process but have different heat sources and working conditions, which include the chamber environments and the preheating procedure. In EBM, preheating procedure is needed to sinter the powder bed to prevent charging of the powders. Comparing metal AM technologies, SLM exhibits the highest solidification rate and EBM can be applied to high melting point alloys due to high absorption rate of electron beam by alloy powders, while direct laser deposition (DLD) based on powder feeding process possesses high deposition efficiency and near-rapid solidification. DLD is more applicable to the bulk component and reparations of damaged components. There is a great significance to explore the microstructures and mechanical properties of as-deposited HEAs for the potential application of additive manufacturing. Several basic researches have been done to explore the application of DLD in fabricating HEAs. Sistla et al. [24] fabricated thin walled AlxFeCoCrNi2-x HEA components but with poor formability due to the macrocracks. Joseph et al. [22] compared the microstructures and mechanical properties of the direct-laser-fabricated and arc-melted AlxCoCrFeNi HEAs, and further investigated the tensile and compress properties of the as-deposited Al0.3CoCrFeNi alloy [25]. Unfortunately, for all alloys cracking behavior was observed in the first 5–8 mm of height adjacent the AISI 316 plate. The tensile of the as-deposited Al0.3CoCrFeNi alloy only showed the yield strength of 194 MPa and limited work hardening. Recently, Xiang et al. [26] investigated the effects of processing parameters on microstructures and tensile properties of laser melting deposited CrMnFeCoNi HEAs, and the 1400 W sample exhibited more excellent mechanical properties in room temperature with the yield strength of 290 MPa, fracture strength of
Corresponding author. E-mail address: [email protected] (Z. Wang).
https://doi.org/10.1016/j.intermet.2019.106592 Received 4 May 2019; Received in revised form 14 August 2019; Accepted 17 August 2019 0966-9795/ © 2019 Published by Elsevier Ltd.
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12.5 × 2 × 3.2 (mm3) were obtained and loaded in the direction that is parallel to the building direction. Each of the as-deposited Nbx HEAs was tested twice to confirm reproduction.
535 MPa and plastic strain of 55%. Y. Chew et al. [27] investigated microstructures and mechanical behavior of CoCrFeNiMn HEA fabricated by using laser aided additive manufacturing. Due to grain boundary strengthening, the laser aided additive manufactured CoCrFeNiMn HEA exhibited higher strength of 660 MPa and ductility of 19.8%. These primary results show the promising application of DLD on the fabrication of HEAs. However, much more HEAs systems need to be explored for better printability and mechanical properties in the DLD processing. In this study, we choose DLD to fabricate bulk CoCrFeNi HEAs with different Nb addition. The CoCrFeNi system is the most fundamental and fully investigated system [5] with a single and stable FCC structure, while Nb element is a typical precipitation hardening element in nickelbased superalloys. As Nb element was added to CoFeCrNi system, the CoCrFeNiNbx HEAs have been reported to simply solidify into FCC and Laves duplex phase structure [28,29]. In this investigation, we presented the feasibility of producing bulk CoCrFeNiNbx HEAs via DLD method and the effects of Nb addition on the evolution of phases, microstructures, mechanical properties as well as deformation behavior. Surprisingly, the CoCrFeNiNbx HEAs are much suitable for DLD and show excellent mechanical properties.
3. Results and discussion 3.1. Phase identification The XRD results of the as-deposited Nbx HEAs are shown in Fig. 2. There was only FCC phase in CoCrFeNi alloy. In Fig. 2 (a), obvious [220] texture with the abnormal peak intensity ratio formed in the CoCrFeNi alloy, showing the existence of preferred orientation. However, this peak intensity decreased sharply with the increment of Nb concentration, indicating that the preferred orientation of obtained grain crystal tends to be weakened. In addition, there were small peaks of Laves phase appearing around (111)FCC peak in the inset plot of Fig. 2 (a) as the increase of Nb content, indicating a FCC-Laves dualphase constitution. Moreover, the lattice parameters of the FCC phase were calculated via MDI jade 6.5 including all diffraction peaks, which were calculated to be 0.3562, 0.3582, 0.3591 and 0.3589 nm. In addition, the lattice parameter changing of the matrix FCC phase in asdeposited Nbx is shown in Fig. 2 (b), which exhibits the similar tendency and variation extent of the as-cast Nbx [28]. Clearly, the evolution of lattice parameters indicates that the solid solution of Nb results in a lattice dilation.
2. Experiments The experiments were performed on laser additive manufacturing system [30], which consists of a 3 kW continuous wave fiber laser with a wavelength of 1060 nm, a five-axis numerical control working table, high precision and adjustable automatic feeding device with dual powder feeders, four coaxial nozzles and an inert argon atmosphere processing chamber where the oxygen content is controlled less than 50 ppm. After a series of pre-experiments, the optimal parameters were confirmed as laser power 1.6–1.65 kW, scanning speed 7 mm s−1, laser spot diameter 3 mm and overlap rate 50%. Bi-directional deposition pattern was applied and the scanning orientation was translated 90° between each successive layer, as shown in Fig. 1 (a). Spherical CoCrFeNi powder (50–150 μm) and Nb powder (50–75 μm) were both prepared by plasma rotation electrode process, which were used as raw materials. The mixed powders with the composition of CoCrFeNiNbx (x = 0, 0.1, 0.15, 0.2) were blended uniformly in a ball milling equipment and then dried in a vacuum dryer at 120 °C for at least 2 h. AISI 316 stainless steel plates with dimensions of 140 × 60 × 10 (mm3) were used as substrate materials, which were polished by abrasive paper to remove surface oxides or contaminants and then cleaned by acetone. Deposited bulk CoCrFeNiNbx HEAs (named as Nbx hereafter) were at the dimensions about 35 × 18 × 70 (mm3). One of as-deposited bulk Nbx HEAs and the building direction (BD) as well as schematic representation of tensile specimen with respect to loading directions are shown in Fig. 1 (b). The phase constituent of the as-deposited Nbx HEAs were characterized by an X-ray diffractionmeter (X'Pert PRO), using monochromatic Cu Kα (λ = 1.54060 Å ) radiation scanning from 20° to 110° with 5°/min scanning rate. The XRD analysis plane was parallel to the building direction. The lattice parameters were calculated by XRD analysis from the obtained diffraction peaks. Microstructural investigations were performed by scanning electron microscopy (SEM SU6600) coupled with energy dispersive spectrometry (EDS) and the samples were polished and then etched in a dilute aqua regia solution with HNO3:HCl:H2O = 1:3:3. Electron backscattered diffraction (EBSD) characterizations were performed using a Tescan VEGAIILMH SEM with the HKL Technologies Channel 5 software and the EBSD samples were mechanically polished and then electro-polished in a 10% perchloric acid solution for 40 s at 30 V. All samples for XRD, SEM and EBSD tests were taken from the middle height of the as-deposited HEAs. Tensile tests were performed at room temperature by a materials testing machine (INSTRON 3382) with the strain rate of 1 × 10−3 s−1. The dog bone-shaped flat tensile specimens with gauge dimensions of
3.2. Microstructural characterization Microstructures of the as-deposited Nbx HEAs in longitudinal and transverse sections which were parallel and perpendicular to the building direction are shown in Fig. 3. Fig. 3 (a) and (e) exhibits that the as-deposited Nb0 alloy presented alternately bright and dark microstructures, showing strip-shape microstructures in longitudinal section and spot-like microstructures in transverse section respectively. Therefore, the as-deposited Nb0 alloy presented directed cellular structures. The difference in corrosion caused by the minor compositional difference resulted in alternately bright and dark microstructures, which was also confirmed in additively manufactured stainless steels [31]. With the addition of Nb element, Laves phase appeared and preferred to form in the inter-dendritic region and at grain boundary. In Nb0.1 alloy there were typical columnar microstructures with elongated grains longitudinal to the building direction and short rod-like Laves uniformly dispersed along with the grain orientation. Compared with Nb0.1 alloy, the Nb0.15 alloy had a higher volume fraction of Laves phase, which transformed from short rod-like structures to cross-linked mesh structures. In Nb0.2 alloy, obvious crosslinked network structures were obtained in both longitudinal and transverse sections. It is obvious that the increasing of Nb addition leads to the change of microstructures. The as-deposited bulk Nbx HEAs exhibited fully compact microstructures without apparent pores and microcracks, compared with the as-deposited CoCrFeMnNi HEAs [27] and the direct-laser-fabricated AlxCoCrFeNi HEAs [22,24]. Moreover, there were more uniformly dispersed and fine Laves phase in FCC matrix, compared with the counterpart as-cast Nbx HEAs [28,29]. Chemical compositions of the FCC phase and the Laves phase were analyzed by using EDS and the results are listed in Table 1, showing that the FCC phase was limited in Nb in spite of the increment of Nb addition. Combining with the XRD results, the Laves phase was identified as the (CrFe)(CoNi)Nb type with a HCP lattice structure. Moreover, backscattered electron (BSE) micrographs and corresponding EDS elemental mappings of the as-deposited Nb0.1 and Nb0.2 HEAs are presented in Fig. 4. The results shown that Co, Cr, Fe and Ni elements uniformly distributed in matrix phase, while Laves phase was enriched in Nb but poor in Cr and Fe. To further confirm the microstructure transition, Fig. 5 shows the inverse pole figure maps and pole figures of the as-deposited Nb0 and 2
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Fig. 1. (a) Laser scanning pattern and (b) the as-deposited bulk CoCrFeNiNb0.1 alloy with schematic representation of tensile specimen with respect to loading directions.
Fig. 2. (a) XRD patterns of the as-deposited CoCrFeNiNbx HEAs and (b) lattice parameter changing of the matrix FCC phase in as-deposited Nbx HEAs as the increase of Nb concentration.
phase and liquid phase. With rapid solidification, the nucleus is easily formed in the undercooled melt which induces the CET. In addition, the anisotropy of texture in the building direction of the as-deposited Nb0.2 alloy was distinctly eliminated compared with that within Nb0 alloy, due to the occurrence of CET transition, as shown in Fig. 5(c) and (d).
Nb0.2 alloys. The as-deposited Nb0 alloy exhibited large columnar crystals with significant orientation. In the as-deposited Nb0.2 alloy, microstructures were modified from large grains with columnar crystals to much finer grains with equiaxed crystals. During the solidification, the addition of Nb element not only produced the formation of Laves phase but also affected the solidification microstructures due to the constituent supercooling. In directional solidification, CET usually occurs with the nucleation in the undercooled liquid in front of the columnar dendrites [32]. The addition of Nb will increase the undercooling of the liquid because of its severe partition between the solid
3.3. Mechanical properties Room-temperature tensile properties of the as-deposited bulk Nbx HEAs were measured, as shown in Fig. 6 (a). As the Nb concentration 3
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Fig. 3. The SEM images of the as-deposited CoCrFeNiNbx HEAs in longitudinal section (a) Nb0 (b) Nb0.1 (c) Nb0.15 (d) Nb0.2 and in transverse section (e) Nb0 (f) Nb0.1 (g) Nb0.15 (h) Nb0.2.
deposited Nbx HEAs exhibited a hybrid fracture combining dimples with raised edge. The obtained as-deposited Nbx HEAs possessed a better strengthductility combination, especially the Nb0 and Nb0.1 alloys. We investigated work-hardening behavior of the as-deposited Nbx HEAs to further understand the improvement in their mechanical properties, as shown in Fig. 6 (d). Expect the Nb0.2 HEA, the work hardening rate in the as-deposited Nbx HEAs all showed three distinctive stages, where the Stage II was the key contribution to the work hardening. The workhardening Stage II showed different trend in each alloy, which was characterized by an increasing work-hardening rate in Nb0 alloy, a stable work-hardening-rate platform in Nb0.1 alloy and a slowly decreasing work-hardening rate in Nb0.15 alloy. Compared with as-deposited CoCrFeNi alloy, the as-deposited Nbx alloys exhibited higher strengths and shown higher work-hardening rates in the work-hardening Stage II, which was mainly due to the effects of solid solution strengthening and compound reinforcement. Both the entanglement of the dislocations and Laves phase can effectively hinder the dislocation motion. Therefore, during tensile deformation the dislocations multiply and pile up rapidly and thus the corresponding work-hardening rate was increased. In addition, the unique work-hardening behavior of the as-deposited Nbx HEAs (except Nb0.2 alloy) resulted in the good strength-ductility combination. Firstly, CoCrFeNi alloy system with high inherent workhardening exponent during deformation [33] and low stacking fault [34] exhibits the higher level of strengthening and toughness that can effectively retard the localized deformation (suppressing crack formation and propagation), enhances the uniform elongation and causes an increase in work hardening rate [35]. Moreover, the relatively high cooling rate experienced during DLD process favored the production of uniform and fully compact sub-dendrite microstructures with certain
Table 1 Chemical composition of the as-deposited CoCrFeNiNbx HEAs (at.%). Alloy
Region
Co
Cr
Fe
Ni
Nb
Nb0 Nb0.1
matrix matrix precipitate matrix precipitate matrix precipitate
25.03 24.71 25.65 25.06 22.89 24.61 24.79
24.88 24.27 16.75 24.94 17.67 24.32 18.71
25.38 24.51 16.89 24.85 17.42 25.35 19.60
24.71 24.55 23.31 23.23 22.01 23.45 22.50
– 1.96 17.40 1.92 20.01 2.27 14.40
Nb0.15 Nb0.2
increased, both yield and fracture strengths increased but tensile ductility decreased. Compared with the as-cast counterparts, the as-deposited Nbx HEAs exhibited a better strength-ductility combination, showing higher strength and almost doubled elongation. The as-deposited Nb0 alloy performed extremely high elongation, attaining 92.5%. Moreover, in as-cast state, the addition of Nb element resulted in an obvious decrease in elongation of Nbx HEAs. Intriguingly, the asdeposited Nb0.1 HEA exhibited a twofold increase in strength and better ductility, compared with the as-cast Nb0 alloy. Compared with aforementioned DLD-HEA [22,24–27], the as-deposited bulk Nbx HEAs with uniform and fully compact microstructures also showed more superior tensile properties. Fig. 6 (b) shows the tensile property comparison between both as-deposited CoCrFeNiAlx (x = 0.3, 0.6) and CoCrFeNiNbx HEAs, indicating that the Nbx HEAs can be as a promising candidate system for DLD processing. The fractured morphologies after the tensile deformation of the as-deposited Nbx alloys are shown in Fig. 6 (c). Typical plastic fracture with numerous deep dimples was observed in the as-deposited Nb0 HEA. With the addition of Nb element and the formation of the ordered Laves phase, the fracture patterns of the as-
Fig. 4. The backscattered electron (BSE) micrographs and corresponding EDS element mappings of the as-deposited (a) Nb0.1 and (b) Nb0.2 HEAs. 4
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Fig. 5. IPF maps and pole figure maps of the as-deposited (a)(c) Nb0 and (b)(d) Nb0.2 specimens obtained at the longitudinal section parallel to the building direction.
ductility, owing to optimized microstructures and the unique workhardening behavior compared to the as-cast counterparts. Among the as-deposited Nbx HEAs, the Nb0 alloy exhibited an ultra-high elongation reaching 92.5% while the Nb0.1 alloy showed not only the strength enhancement but also higher elongation compared with the as-cast Nb0 alloy. CoCrFeNiNbx HEAs can be as a promising candidate system for additive manufacturing.
orientation, showing better crack initiation resistance and suppressing the propagation of microcracks during deformation [23] and then resulting in better work-hardening behavior. Therefore, even with more Nb addition, the as-deposited Nbx HEAs showed better ductility compared with the as-cast Nbx HEAs.
4. Conclusion The bulk CoCrFeNiNbx HEAs were successfully fabricated by DLD with enhanced mechanical properties. The as-deposited Nbx HEAs exhibited uniform and fully compact microstructures due to near-rapid solidification and shown a microstructure transition from columnar structures to equiaxed structures as the increased Nb addition. The asdeposited alloys have shown the excellent combination of strength and
Conflicts of interest The authors declared that they have no conflicts of interest to this work.
Fig. 6. (a) Room temperature tensile engineering stress-strain curves of the as-deposited bulk Nbx HEAs compared with the as-cast counterparts. (b) The true ultimate tensile strength versus elongation of both as-deposited CoCrFeNiAlx and CoCrFeNiNbx HEAs. (c) Fracture surface morphologies and (d) work hardening rate of the asdeposited Nbx HEAs. 5
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Acknowledgment [18]
The work was supported by National Key R&D Program of China [grant No. 2018YFB1106000, 2016YFB1100100], National Natural Science foundation of China [grant No. 51771149] and the Fund of State Key laboratory of Solidification Processing in NWPU [grant No. 03-TS-2019].
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