Journal of Materials Science & Technology  2019 , 35 (11): 2430-2434 https://doi.org/10.1016/j.jmst.2019.05.062

Orginal Article

Additive manufacturing of high-strength CrMnFeCoNi high-entropy alloys-based composites with WC addition

Jinfeng Lia*, Shuo Xiangb, Hengwei Luanc, Abdukadir Amarb, Xue Liua, Siyuan Lud, Yangyang Zenge, Guomin Lea*, Xiaoying Wanga, Fengsheng Qua, Chunli Jianga, Guannan Yangf*

aInstitute of Materials, China Academy of Engineering Physics, Mianyang 621907, China
bCollege of Physics and Technology, Xinjiang University, Urumqi, Xinjiang 830046, China
cSchool of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
dSchool of Mechanical Engineering and Mechanics, Ningbo University, Ningbo 315211, China
eNational key laboratory of shock wave and detonation physics, Institute of Fluid Physics, China Academy of Engineering Physics, Mianyang 621907, China
fSchool of Electromechanical Engineering, Guangdong University of Technology, Guangzhou 510006, China

Corresponding authors:   *Corresponding authors.E-mail addresses: lijinfeng305@126.com (J. Li), leguomin@126.com (G. Le),ygn@gdut.edu.cn (G. Yang).*Corresponding authors.E-mail addresses: lijinfeng305@126.com (J. Li), leguomin@126.com (G. Le),ygn@gdut.edu.cn (G. Yang).*Corresponding authors.E-mail addresses: lijinfeng305@126.com (J. Li), leguomin@126.com (G. Le),ygn@gdut.edu.cn (G. Yang).

Received: 2019-02-10

Revised:  2019-03-15

Accepted:  2019-05-26

Online:  2019-11-05

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

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Abstract

Laser melting deposition with WC addition has been developed to fabricate high-strength CrMnFeCoNi-based high-entropy alloys-based composites. By this technique, a microstructure of compact refined equiaxed grains can be achieved, and the tensile strength can be remarkably improved. The sample with 5 wt% WC addition shows a promising mechanical performance with a tensile strength of 800 MPa and an elongation of 37%. The improvement in mechanical property may be attributed to the formation of Cr23C6 reinforcement precipitates, which could promote the heterogeneous nucleation of grains and hinder the propagation of slip bands.

Keywords: High-entropy alloys ; Laser metal deposition ; Precipitates ; Microstructures ; Tensile test

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Jinfeng Li, Shuo Xiang, Hengwei Luan, Abdukadir Amar, Xue Liu, Siyuan Lu, Yangyang Zeng, Guomin Le, Xiaoying Wang, Fengsheng Qu, Chunli Jiang, Guannan Yang. Additive manufacturing of high-strength CrMnFeCoNi high-entropy alloys-based composites with WC addition[J]. Journal of Materials Science & Technology, 2019, 35(11): 2430-2434 https://doi.org/10.1016/j.jmst.2019.05.062

1. Introduction

High-entropy alloys (HEAs) are defined as alloys with equal or nearly equal quantities of five or more metal components [1]. Many HEAs can achieve excellent mechanical properties [[1], [2], [3], [4], [5]], as they prefer to form a special microstructure of face-centered cubic (FCC) or body-centered cubic (BCC) solid solution phases rather than intermetallic phases [6,7]. Among them, the CrMnFeCoNi HEA has attracted great interesting for its considerable plasticity and toughness [[8], [9], [10]]. Bernd Gludovatz and his colleagues found excellent low-temperature mechanical properties in CrMnFeCoNi HEA. With decreasing temperature from 293 K to 77 K, the yield strength, tensile strength, tensile ductility increased by $\widetilde{8}$5%, $\widetilde{7}$0% and $\widetilde{2}$5%, to 759 MPa, 1280 MPa and >70%, respectively [11,12].

With the development of additive manufacturing (AM), various HEAs have been successfully fabricated [[13], [14], [15]]. Compared to the traditional vacuum arc-melting fabrication method [8,[16], [17], [18]], AM technique provides a more powerful tool to obtain HEAs with more homogeneous composition, equiaxed grains and complex geometry [13]. However, the low yield strength due to the intrinsic properties of material is still a roadblock for the real applications of AM-fabricated CrMnFeCoNi HEAs [16,19]. To further improve the integrated mechanical properties and formability of HEAs, new fabrication and modification methods are still required.

Metal matrix composites (MMCs) are attractive materials to be employed in various applications because of their improved properties. In metal matrix composites, ceramic particulates such as borides, carbides and nitrides are added to the metal matrixes [20]. In this study, a method of WC addition has been adopted to enhance the yield strength of AM fabricated CrMnFeCoNi HEA-based composites. Remarkable grain refinement and yield strength improvement (from 300 MPa to 502 MPa) were observed at 5 wt% WC addition, with an elongation of 37%. This work provides a new method to accommodate the mechanical properties and microstructure of AM-fabricated CrMnFeCoNi HEA-based composites, and could also inspire the fabrication and mechanical studies of other HEA-based composites.

2. Experimental

CrMnFeCoNi HEA-based composite samples with different concentrations of WC addition (0 wt%, 5 wt% and 10 wt%) were synthesized by laser melting deposition (LMD). The mean sizes of the pre-alloyed spherical equiatomic CrMnFeCoNi powder and WC powder are approximately 120 μm, 10 μm, respectively, measured by a Microtrac S3500 laser particle size analyzer. The two powders were mixed according to the designed proportion and dried at a temperature of 80 °C for 2 h. Ytterbium-doped fiber laser unit with a wavelength of 1070 nm and a spot diameter of ˜1.8 mm was used as the power source. In the fabrication, the mixed powders were delivered into the laser molten pool on a 316 L steel substrate through a coaxial nozzle at an argon gas flow 15-18 L/min and a feeding rate of 7-9 g/min. The chamber was under argon gas protection, with oxygen content below 20 ppm. Thin wall samples with lengths of $\widetilde{7}$0 mm, widths of $\widetilde{3}$ mm and heights of $\widetilde{4}$0 mm were fabricated by laser multi-layer scanning under 1000 W laser powers and 500 mm/min scanning speed. The laser head rose by a certain height increment after each single laser scan, keeping a distance of 0.5 mm from the molten pool. The schematic process of LMD of high-strength CrMnFeCoNi HEA-based composites with WC addition has been summarized in Fig. 1.

Fig. 1.   Schematic process of additive manufacturing of high-strength CrMnFeCoNi HEA-based composites with WC addition.

The wall samples as shown in Fig. 1 were cut by spark erosion to inspect the microstructures in the cross-sections perpendicular to the laser scanning direction (SD), the deposition direction (DD), and the transverse direction (TD), respectively. The mechanical properties of samples have been measured by an Instron 5982 static testing machine at a strain rate of 10-3 s-1. X-ray diffractometric (XRD, D/max-RB, Rigaku, Japan) with Cu radiation, Olympus SZ61 optical microscope (OM), scanning electron microscopy (SEM, LEO1530) equipped with X-ray energy dispersive spectrometer (EDS), Zeiss Supra 35 instrument equipped with an Oxford Instruments electron backscatter diffraction (EBSD) system were used to investigate the structure, grain size distribution, fractograph and chemical composition distribution of the samples.

3. Results and discussion

Fig. 2(a) shows the XRD spectra of the as-prepared CrMnFeCoNi HEA-based composites with different WC additions. Three peaks can be observed on the three spectra, representing the face-centered cubic (FCC) single-phase solid solution crystal structure. With increasing WC addition, the peaks of M23C6 phases (M represents a solid solution element) show up, while the peaks of single-phase solid solution are still observed. Fig. 2(b) shows the DSC curves of the samples measured at a heating rate of 20 K/min. Decreased melting temperature Tm (from 1544 K to 1514 K) with WC addition (from 0 wt% to 10 wt%) can be observed, which is similar to effect of carbide addition on the melting point of steels [21,22].

Fig. 2.   (a) XRD spectra of the as-prepared CrMnFeCoNi HEA-based composites with different WC additions and (b) DSC curves of the CrMnFeCoNi HEA-based composites with different WC additions.

Fig. 3(a)-(c) shows the IPF-X EBSD maps of the samples with different concentrations of WC additions. All the three samples have formed compact equiaxed grain microstructure without any obvious pores or cracks. With the addition of WC, the average grain size significantly reduced as shown in Fig. 3(a1)-(c1). The SEM images in Fig. 3(d)-(f) reveal the existence of dendritic precipitates at micrometer-scale and cubic precipitates at ˜$\widetilde{1}$00 nm scale in the sample with 5 wt% WC. The corresponding EDS pattern of phase distribution in Fig. 3(g) shows that the Cr23C6 phase is uniformly distributed in the FCC matrix. The enlarged EDS patterns (Fig. 3(h)) indicate the accumulation of Cr and C and repulsion of Fe, Co, Mn and Ni near a precipitate. Combining the EDS and XRD (Fig. 2) results, the precipitates can be expected to be Cr23C6.

Fig. 3.   IPF-X EBSD maps of the LMD-fabricated CrMnFeCoNi HEA-based composites with (a) 0 wt%, (b) 5 wt% and (c) 10 wt% WC additions. Grain size distribution of the samples with (a1) 0 wt%, (b1) 5 wt% and (c1) 10 wt% WC additions; (d-f) SEM images and EDS pattern (g) of the sample with 5 wt% WC addition. The blue zones in (g) represent the precipitates and the red zones represent the FCC matrix. (h) Enlarged EDS patterns of different element distributions of near a precipitate.

Fig. 4(a) and (b) shows the tensile stress-strain curves of the samples with different WC additions at room temperature (RT) and 873 K, respectively. At RT, the addition of WC (5 wt% and 10 wt%) remarkably improved the yield stress σ0.2 (from 300 MPa to 502 MPa and 675 MPa) and the tensile strength σt (from 550 MPa to 776 MPa and 845 MPa) of the samples, as the plasticity decreases from 50% to 37% and 9%. At 873 K, the addition of 5 wt% WC also improved the tensile strength from 280 MPa to 405 MPa, with relatively small plasticity decrease from 57% to 45%.

Fig. 4.   Tensile curves of the LMD-fabricated CrMnFeCoNi HEA-based composites with different WC additions at (a) RT and (b) 873 K.

Fig. 5(a) and (e) shows the fractographies figure of the samples with 0 wt% and 5 wt% WC, respectively. Abundant dimple patterns can be observed on the front up of fractography as shown in Fig. 5(e) and (f), which are characteristics of ductile fracture [23]. Enlarged image of Fig. 5(f) shows the existence of precipitate in the center of each dimple in the fractograph of the sample with 5 wt% WC. As dimples usually are formed by tearing of voids [24], the precipitates probably act as nucleation points of voids, which could be a key factor controlling the plasticity of the sample. Extensive slip bands can be observed on the side surfaces of both the samples with 0 wt% and 5 wt% WC (Fig. 5(c) and (g)), which were formed during the plastic deformation. Compared to the straight slip bands on the samples with 0 wt% WC (Fig. 5(d)), the slip bands on the samples with 5 wt% WC are much smaller with the existence of precipitates (Fig. 5(h)). It suggests that the precipitates might block the dislocations and promote the hardening effect of the samples.

Fig. 5.   (a-d) Fractographies of the CrMnFeCoNi HEA-based composite without WC addition and (e-h) fractographies of the CrMnFeCoNi HEA-based composite with 5 wt%WC addition.

Though LMD-fabrication with WC addition, the mechanical property of CrMnFeCoNi HEA has been improved. The yield stress (300 MPa), tensile strength (550 MPa) and elongation (50%) of the sample with 0 wt% WC are all higher than those of the sample prepared by conventional casting method (250 MPa, 489 MPa and 42%, respectively) [14]. Though 5 wt% WC addition, a comprehensive mechanical performance of high tensile strength (776 MPa) and high elongation (37%) is achieved. Based on the EBSD, EDS and SEM results in Fig. 3, we ascribe the property improvements in the CrMnFeCoNi HEA-based composites to the change of microstructure and the formation of precipitates, achieved by the LMD method and WC addition. For the LMD method, a much higher and denser input power is introduced, which leads to higher temperature in the molten pool and higher cooling rate. This results in the formation of finer equiaxed grains and better mechanical performance. The high temperature during LMD also enables the rapid dissolution/melting of WC or other intermetallic with high melting points. XRD results (Fig. 2) indicate that the WC phases have completely dissolved during LMD, instead, precipitates of M23C6 are formed. These precipitates could act as heterogeneous nucleation points during solidification, leading to the finer grain size distributions (Fig. 3). As indicated by the SEM images (Fig. 5), the precipitates could also strengthen the matrix by hindering and deflecting the slip bands. With the combined effects of grain refinement and precipitate reinforcement, the improvement in the strength of CrMnFeCoNi HEA-based composites with WC addition can be well understood. This effect also differs from the conventional second-phase strengthening mechanism [25], in which the WC or other intermetallics plays a role in the harden phase but will not be dissolved in the matrix. The result could shed a light on the usage of WC in the fabrication of metal-based composites during LMD.

4. Conclusion

High-strength CrMnFeCoNi HEA-based composites have been successfully prepared by LMD process. Microstructures of compact equiaxed grains without any obvious pores or cracks were formed. Though WC addition, precipitates of M23C6 formed, this could promote the heterogeneous nucleation of grains and hinder the propagation of slip bands. With the combined effects of grain refinement and precipitate reinforcement, the tensile strength of the HEAs could be remarkably improved. The sample with 5 wt% WC showed a promising mechanical performance with a tensile strength of 800 MPa and an elongation of 37% at RT, and a tensile strength of 405 MPa and an elongation of 45% at 873 K. The present work provides an approach of LMD with WC addition to accommodate the microstructure and mechanical properties of CrMnFeCoNi HEA-based composites, and also insights into the fabrication and mechanical study of other HEA-based composites in AM.

Acknowledgements

This work was supported financially by the Project supported by CAEP Foundation (No. CX2019020), the Science and Technology Plan Project of Sichuan (No. 2018G20146) and the Special Fund Project of Panzhihua (No. 2017CY-G-21).


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