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Novel laser additive-manufactured Mo-based composite with enhanced mechanical and oxidation properties
Journal of Alloys and Compounds xxx (xxxx) xxx
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Novel laser additive-manufactured Mo-based composite with enhanced mechanical and oxidation properties Weiwei Zhou a, Keiko Kikuchi a, Naoyuki Nomura a, *, Kyosuke Yoshimi b, Akira Kawasaki a a b
Department of Materials Processing, Graduate School of Engineering, Tohoku University, Sendai, Miyagi, 980-8579, Japan Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai, Miyagi, 980-8579, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 August 2019 Received in revised form 30 October 2019 Accepted 10 November 2019 Available online xxx
A novel Mo-based composite with simultaneously improved mechanical and oxidation properties was fabricated by laser powder bed fusion (L-PBF), using uniform Al2O3-nanoparticle-decorated MoTiAl powders bridged by functionalized carbon nanotubes. A tight ceramic coating ~2.57 mm in thickness consisting of an a-Al2O3 matrix with dispersed TiC particles was formed on the surface of the Al2O3-CNT/ MoTiAl composite, which has been proved to effectively increase resistance to oxidation at 1173 K. Meanwhile, the nanoparticles were homogenously dispersed and tightly contacted with the matrix, giving rise to an enhanced Vickers hardness. This work shed light on designing and producing highperformance Mo-based composites for application to ultrahigh-temperature materials. © 2019 Elsevier B.V. All rights reserved.
Keywords: Laser powder bed fusion (L-PBF) Metal matrix composites (MMCs) Carbon nanotubes Oxidation resistance Mechanical property
1. Introduction Because of their high melting points, high stiffness, and low coefficient of thermal expansion, Mo-based alloys show great potential for ultrahigh-temperature applications [1,2]. However, there is still a long way to go before nickel-based superalloys can be replaced to improve energy efficiency in heat engines (e.g., gas turbines or jet engines). The unsatisfactory strength at elevated temperatures and poor oxidation resistance are the main concerns for Mo-based alloys in practical use [3,4]. Toward this end, fabricating ceramic-particle-reinforced metal matrix composites (MMCs) has been deemed to be an effective approach. Fillers can improve the mechanical strength of MMCs via impeding the dislocation motion or grain boundary sliding at high temperatures [5,6]. Moreover, better oxidation behavior is expected, since the ceramic dispersions limit the penetration rate of oxygen atoms into the matrix and are beneficial for the spallation resistance of oxide scales formed on MMCs [7,8]. For instance, Michels [8] showed an increase in the oxidation resistance of hot-rolled NieCr alloys at 1273e1473 K with the addition of Y2O3. Chen et al. [6] proved that the elevated-temperature strength and oxidation resistance of
* Corresponding author. E-mail address: [email protected] (N. Nomura).
Mo5Si3 were improved with the addition of Al2O3 by hot pressing. However, such conventional manufacturing techniques for MMC products usually include processing steps that are highly material, energy, and time consuming (e.g., hot forging or shaping) [9,10]. Because of their high melting points and low ambienttemperature ductility, it is especially challenging to manufacture Mo-based materials with complex geometries by traditional processes [5,11]. Fortunately, laser powder bed fusion (L-PBF), as a newly developed layer-by-layer additive manufacturing technique, is extremely promising for metallic parts. As compared to conventional approaches, L-PBF process can solve production difficulties induced by complexities of shape, material, and hierarchy, while also decreasing manufacturing time and total cost [12,13]. More importantly, due to its high-temperature, non-equilibrium characteristics, L-PBF is capable of fabricating MMCs with an improved mechanical property, which is caused by the formation of a mechanically graded [14] or chemically bonded interface between the filler and the matrix [15]. Presently, the L-PBF technique has been applied widely for producing Ti- [13,16e21], Fe- [22e27], Al[15,28e30], and Ni-based MMCs [14,31e34] (see Table 1). Although promising results on L-PBF of MMCs have been reported, there are still several challenges, including fabrication of uniform nanofiller/ metal mixed powders, design of high-quality MMCs with complex geometries, and realization of the multi-functionality of L-PBF parts in practical application [35,36]. In addition, less attention has been
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Please cite this article as: W. Zhou et al., Novel laser additive-manufactured Mo-based composite with enhanced mechanical and oxidation properties, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152981
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Table 1 Fabrication of Ti-, Fe-, Al-, and Ni-based MMCs by L-PBF process. Type of MMCs Authors
Starting fillers
Powder fabrication
fabricated MMCs
Maximum dimension Measured properties
Ti-based
TiB2: 2.71 mm TiC: 50 nm TiB2: 3.5e6 mm
Ball milling Ball milling Blending mixing
TiB/Ti TiC/Ti TiB/Ti
e 8 8 8 mm3 Ø50 mm 20 mm
Ball Ball Ball Ball
TiB2/TiAl TiC/Ti TiC/316 L stainless steel TiB2/S136 mould steel
10 10 10 mm3 60 mm in length Ø8mm 6 mm ~60 mm in length
Fe-based
Al-based
Ni-based
Cai et al. [16] Gu et al. [17] Attar et al. [18,19]
Li et al. [20] TiB2: 3e5 mm, He et al. [21] TiC: 45 nm AlMangour et al. [23] TiC: 50 nm or 1 mm Wen et al. [24] TiB2: 200e300 nm
milling milling milling milling
Kang et al. [25]
WC: 3e5 mm
Tumbling mixing
WC/maraging steel
~50 mm in length
Zhao et al. [26] Song et al. [27] Jue et al. [28] Gu et al. [29] Li et al. [30] Zhou et al. [15] Wang et al. [31] Rong et al. [32] Zhang et al. [33]
TiC: 40 or 800 nm SiC: 27 mm Al2O3: 9 mm TiC: 50 nm nano-TiB2 Graphene oxide:1.0 mm Graphene: 5e50 mm. WC1-x:15e45 mm, TiC:50 nm
Ball milling Tumbling mixing Ball milling Ball milling Gas atomization Hetero-agglomeration Hetero-agglomeration Ball milling Ball milling
TiC/316 L stainless steel SiC/Fe Al2O3/Al TiC/AlSiMg TiB2/AlSiMg Al4C3/Al Graphene/Inconel 718 WC1-x/Inconel 718 TiC/Inconel 718
100 10 3 mm3 ~50 mm in length e e ~60 mm in length 4 4 1.4 mm3 35 mm in length e e
Yao et al. [34]
TiC:80e150 nm
Tumbling mixing
TiC/Inconel 718
e
paid to refractory materials such as Mo-based materials [11,37]. In this work, a novel nanoceramic/Mo-based composite was fabricated by L-PBF; its feasibility for high-temperature applications was verified by microstructural characterization, hardness testing, and oxidation experiments. 2. Experimental Starting Mo-33 wt%Ti-13 wt% Al alloy powders (D50 ~12.8 mm) were prepared by a combination of arc melting, mechanical milling, and sieving [11]. High-purity Al2O3 nanopowders with an average particle size of ~200 nm (Fig. 1a) were received from Taimei Chemicals Co., Ltd., Japan. Functionalized carbon nanotubes (CNTs)
Nanohardness, wear property Microhardness, wear property Nanohardness, Compressive property Nanohardness Tensile property Microhardness, wear performance Tensile property, Microhardness Wear performance Thermal conductivity, Tensile property Tensile property, microhardness Tensile property Wear property Microhardness, wear property Tensile property, microhardness Vickers hardness Tensile property Microhardness, wear property Microhardness, Corrosion behavior Tensile property
(Hodogaya Chemical Co., Ltd., Japan) had an average diameter of 70 nm and length of 6.9 mm (Fig. 1b and c) [11]. Following our previous work [11], a 10 wt%Al2O3-0.31 wt%CNT/MoTiAl mixed powder was fabricated via the heteroagglomeration method. Due to the bridging effect of negatively charged CNTs (Fig. 1f), the surfaces of the MoTiAl powders were well covered with positively charged Al2O3 nanoparticles (Fig. 1d and e) under electrostatic selfassembly. Subsequently, an in-house-developed L-PBF machine [11,15] was utilized to prepare a Mo-based alloy or composite. A set of optimized L-PBF parameters, corresponding to the laser power of 20.6 W, scan speed of 10 mm s1, hatching distance of 100 mm, and layer thickness of 25 mm, was utilized for fabricating rectangular builds (~4 4 1.4 mm3).
Fig. 1. TEM images of (a) Al2O3 powders and (b, c) functionalized CNTs; (def) FESEM images of 10 wt%Al2O3-0.31 wt%CNT/MoTiAl mixed powders. Arrows in (c) show the surface nanodefects of functionalized CNTs.
Please cite this article as: W. Zhou et al., Novel laser additive-manufactured Mo-based composite with enhanced mechanical and oxidation properties, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152981
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To determine the oxidation resistance of L-PBF builds, several coupons with a size of 2 2 1 mm3 were prepared after removing the Ti substrate. For comparison, a MoTiAl alloy part was also fabricated by conventionally spark plasma sintering (SPS) (Dr. Sinter S511, SPS Sintex Inc.) at 1673 K for 5 min under a pressure of 50 MPa. Oxidation tests were conducted isothermally at 1173 K for 1 h in a thermogravimetric analyzer (Hitachi SDT Q600) with an air flow of 100 ml/min. Moreover, a cyclic thermal treatment (3 cycles) were performed for L-PBF builds using the same condition of the oxidation testing. The microstructure was evaluated via electron back-scattered diffraction (EBSD) (OIM ver. 6, TSL Solutions), field-emission scanning electron microscope (FESEM) (JEOL JSM-6500 F), electron probe micro-analysis (EPMA) (JEOL JXA-8530 F), transmission electron microscopy (TEM) (Hitachi HF-2000EDX), and scanning TEM (STEM) (JEOL JEM-ARM200F). A focused ion beam system (JEOL JIB-4600 F) was used for preparing all TEM or STEM specimens. The mechanical performance was evaluated using a microhardness tester (Mitutoyo MZT-500) with a peak force of 0.588 N
3
and a holding period of 30 s. 3. Results and discussion A 10 wt%Al2O3-0.31 wt%CNT/MoTiAl composite bulk was successfully fabricated after L-PBF. Unmelted metallic powders or macropores, in addition to certain cracks (Fig. 2a1) caused by high thermal stress [38], were not detected (Fig. 2a1). Further FESEM energy-dispersive spectroscope (EDS) mappings in Fig. 2a2 revealed a striking observation; elemental Mo was barely visible, while rich O element existed on the top surface. To clarify this phenomenon, we checked the longitudinal cross section of this composite, in which a continuous layer with an average thickness of 2.57 mm was formed on the surface (Fig. 2b1). As revealed by TEM-EDS mapping (Fig. 2b2) and diffraction patterns (Fig. 2b3eb4), the surface layer consists of an a-Al2O3 matrix with dispersed TiC particles. The ceramic layer was tightly coated on the composite build without microcracks (Fig. 2b1). Since the whole L-PBF process was conducted in a protective Ar atmosphere with a low oxygen
Fig. 2. Morphologies of a 10 wt%Al2O3-0.31 wt%CNT/MoTiAl composite at (a) the top surface, (b) the surface part in the longitudinal cross section, and (c) the transversal cross section. (a1) FESEM image; (a2) SEM-EDS mappings. (b1) FESEM-back-scattered electron (BSE) image; (b2) TEM-EDS mappings; (b3) and (b4) diffraction patterns taken from the red and yellow spots in (b2), respectively. (c1) FESEM image; (c2) TEM image; (c3) EDS analysis and (c4) diffraction pattern taken from the red and yellow spots in (c2), respectively; (c5) high-resolution TEM image of a MoTiAleAl2O3 interface. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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content (<0.1%), it is unlikely that Mo was oxidized and sublimated during L-PBF. Otherwise, the surface layer would mainly contain TiOx and Al2O3 phases. Thus, the disappearance of elemental Mo on the top surface in Fig. 2a2 was caused by formation of the ceramic protective layer. On the other hand, two categories of nanoparticles were uniformly distributed within the composite (Fig. 2c1e2). They were spherical Al2O3 and irregular TiC phases, respectively, as illustrated by the corresponding EDS analysis (Fig. 2c3) and diffraction pattern (Fig. 2c4). It is noted that the Al2O3 particles retained a size of ~200 nm (Fig. 2c2), which is similar to the starting size (Fig. 1a). The interface between the Al2O3 and MoTiAl matrix was free of interfacial compounds or nanocracks (Fig. 2c5). All TiC structures were monocrystalline (Fig. 2c4), showing a stripe shape (Fig. 2c1). Since the C þ Ti / TiC reaction is an exothermic, self-sustaining, and rapid process driven by a large negative enthalpy of TiC formation (184.5 kJ/mol) [39], the existence of TiC should be induced by the reaction between defective CNTs (Fig. 1c) and Ti atoms [40]. No CNTs were retained in the composite after L-PBF (Fig. 2c2). As we know, the movement and equilibrium of nanofillers within molten pools of MMCs are determined by relevant forces (e.g., buoyancy or the Marangoni force) due to the density difference and high thermal gradient [41]. It is thought that most Al2O3 or TiC particles were dispersed individually within the molten pools during L-PBF, while some tended to float, forming a thin ceramic coating on the surface of the composite build.
The influence of ceramic nanoparticles on the mechanical performance of the Mo-based composite is displayed in Fig. 3a. The Vickers hardness of the Al2O3-CNT/MoTiAl composite was 31.9% or 28.0% higher than that of the MoTiAl alloy in the transversal or longitudinal cross section, respectively. That is predominantly due to the well-dispersed nanoparticles that suppress the dislocation motion and plastic deformation of the matrix under loading [5]. Moreover, the Al2O3-CNT/MoTiAl composite underwent a morphological evolution from columnar to fine equiaxed grains, as well as a dramatic grain refinement from 12.1 to 3.4 mm (Fig. 3b and c). A similar result was reported by Martin et al. [42], that nanofillers contributed to the preparation of high-strength MMCs with equiaxed, fine-grained microstructures during L-PBF. Considering the application to heat-resistant materials, the oxidation behavior of Mo-based parts was further evaluated. Fig. 4 presents the morphology of a MoTiAl alloy or Al2O3-CNT/MoTiAl composite build in the longitudinal cross section after oxidation at 1173 K for 1 h. For the alloy build, an oxide layer 20 mm in thickness, which mainly consisted of Al and Ti oxides, was introduced on the surface (Fig. 4a and b). Since molybdenum is easily oxidized to MoO3 and sublimates after exposure to oxygen above 923 K [3], a small quantity of the Mo element was detected in the oxide layer. For the composite build, in contrast, no oxide layer formed between the ceramic coating and the composite (Fig. 4c and d). This result suggests that the ceramic coating could effectively increase the oxidation resistance by limiting the penetration and diffusion of
Fig. 3. (a) Vickers hardness of an L-PBF-processed MoTiAl alloy or Al2O3-CNT/MoTiAl composite in the transversal and longitudinal cross sections; inverse pole figures of (a) MoTiAl alloy and (b) Al2O3-CNT/MoTiAl composite in the transversal cross sections. Inset in (a) shows a Vickers indentation imprint on the MoTiAl alloy.
Please cite this article as: W. Zhou et al., Novel laser additive-manufactured Mo-based composite with enhanced mechanical and oxidation properties, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152981
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Fig. 4. Morphologies of an L-PBF-processed (a, b) MoTiAl alloy and (c, d) Al2O3-CNT/MoTiAl composite build in the longitudinal cross section near the surface part after oxidation at 1173 K for 1 h. (a) FESEM-BSE image; (b) SEM-BSE image and corresponding EDS elemental mappings; (c) FESEM-BSE image; (d) magnified FESEM-BSE image and corresponding EPMA elemental mappings taken from the white square in (c).
oxygen atoms [43]. Interestingly, the ceramic coating retained strong, tight contact with the Al2O3-CNT/MoTiAl composite at 1173 K after the cyclic oxidation treatment (Fig. 5), showing good thermodynamic and mechanical stability. In addition, as compared to the conventionally made MoTiAl alloy part by SPS (Fig. S1), the L-PBF-processed composite showed superior oxidation resistance. This work proved the promising feasibility of 3D-printed Mo-based composites in ultrahigh-temperature applications.
4. Conclusions In summary, an Al2O3-CNT/MoTiAl composite with improved mechanical and oxidation properties was fabricated successfully via L-PBF. The MoTiAl powders were covered with uniform Al2O3 nanoparticles with the assistance of functionalized CNTs during heteroagglomeration. During L-PBF processing, a thin ceramic layer consisting of an a-Al2O3 matrix with a dispersed TiC phase was tightly coated on the surface of the Al2O3-CNT/MoTiAl composite,
Fig. 5. (a) FESEM image of the L-PBF-processed Al2O3-CNT/MoTiAl composite build in the longitudinal cross section near the surface part after the cyclic oxidation treatment; (b) EDS analysis taken from the red spot in (a). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Please cite this article as: W. Zhou et al., Novel laser additive-manufactured Mo-based composite with enhanced mechanical and oxidation properties, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152981
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giving rise to dramatically enhanced oxidation resistance. Meanwhile, the improved Vickers hardness was caused by the homogeneously dispersed, tightly contacted ceramic nanoparticles in the matrix. This work demonstrated that the L-PBF process has great potential for manufacturing high-performance heat-resistant Mobased materials with tailored architectures for ultrahightemperature applications. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors would like to thank Dr. Takamichi Miyazaki and Dr. Kosei Kobayashi for the TEM observations at Tohoku University. We also appreciated beneficial discussions with Dr. Yuanyuan Lu and Mr. Ryuichi Miyata at Tohoku University. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.152981. References [1] J.H. Perepezko, The hotter the engine, the better, Science 326 (2009) 1068e1069. [2] D.M. Dimiduk, J.H. Perepezko, Mo-Si-B alloys: developing a revolutionary turbine-engine material, MRS Bull. 28 (2003) 639e645. [3] T.A. Parthasarathy, M.G. Mendiratta, D.M. Dimiduk, Oxidation mechanisms in Mo-reinforced Mo5SiB2(T-2)-Mo3Si alloys, Acta Mater. 50 (2002) 1857e1868. [4] S. Miyamoto, K. Yoshimi, S.-H. Ha, T. Kaneko, J. Nakamura, T. Sato, et al., Phase equilibria, microstructure, and high-temperature strength of TiC-added MoSi-B alloys, Metall. Mater. Trans. A 45 (2014) 1112e1123. [5] G. Liu, G.J. Zhang, F. Jiang, X.D. Ding, Y.J. Sun, J. Sun, et al., Nanostructured high-strength molybdenum alloys with unprecedented tensile ductility, Nat. Mater. 12 (2013) 344e350. [6] H. Chen, Q. Ma, X. Shao, J. Ma, C. Wang, B. Huang, Microstructure, mechanical properties and oxidation resistance of Mo5Si3eAl2O3 composite, Mater. Sci. Eng. A 592 (2014) 12e18. [7] W. Quadakkers, H. Holzbrecher, K. Briefs, H. Beske, Differences in growth mechanisms of oxide scales formed on ODS and conventional wrought alloys, Oxid Met 32 (1989) 67e88. [8] H.T. Michels, The effect of dispersed reactive metal oxides on the oxidation resistance of nickel-20 Wt pct chromium alloys, Metall Trans A 7 (1976) 379e388. [9] L.C. Zhang, H. Attar, Selective laser melting of titanium alloys and titanium matrix composites for biomedical applications: a review, Adv. Eng. Mater. 18 (2016) 463e475. [10] W. Zhou, X. Sun, K. Tsunoda, K. Kikuchi, N. Nomura, K. Yoshimi, et al., Powder fabrication and laser additive manufacturing of MoSiBTiC alloy, Intermetallics 104 (2019) 33e42. [11] W. Zhou, X. Sun, K. Kikuchi, N. Nomura, K. Yoshimi, A. Kawasaki, Carbon nanotubes as a unique agent to fabricate nanoceramic/metal composite powders for additive manufacturing, Mater. Des. 137 (2018) 276e285. [12] L. Thijs, F. Verhaeghe, T. Craeghs, J.V. Humbeeck, J.-P. Kruth, A study of the microstructural evolution during selective laser melting of Tie6Ale4V, Acta Mater. 58 (2010) 3303e3312. [13] D. Gu, Y.-C. Hagedorn, W. Meiners, G. Meng, R.J.S. Batista, K. Wissenbach, et al., Densification behavior, microstructure evolution, and wear performance of selective laser melting processed commercially pure titanium, Acta Mater. 60 (2012) 3849e3860. [14] T. Rong, D.D. Gu, Q.M. Shi, S.N. Cao, M.J. Xia, Effects of tailored gradient interface on wear properties of WC/Inconel 718 composites using selective laser melting, Surf. Coat. Technol. 307 (2016) 418e427. [15] W. Zhou, M. Dong, Z. Zhou, X. Sun, K. Kikuchi, N. Nomura, et al., In situ formation of uniformly dispersed Al4C3 nanorods during additive manufacturing of graphene oxide/Al mixed powders, Carbon 141 (2019) 67e75. [16] C. Cai, C. Radoslaw, J. Zhang, Q. Yan, S. Wen, B. Song, et al., In-situ preparation and formation of TiB/Ti-6Al-4V nanocomposite via laser additive manufacturing: microstructure evolution and tribological behavior, Powder
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Novel laser additive-manufactured Mo-based composite with enhanced mechanical and oxidation properties Weiwei Zhou a, Keiko Kikuchi a, Naoyuki Nomura a, *, Kyosuke Yoshimi b, Akira Kawasaki a a b
Department of Materials Processing, Graduate School of Engineering, Tohoku University, Sendai, Miyagi, 980-8579, Japan Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai, Miyagi, 980-8579, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 August 2019 Received in revised form 30 October 2019 Accepted 10 November 2019 Available online xxx
A novel Mo-based composite with simultaneously improved mechanical and oxidation properties was fabricated by laser powder bed fusion (L-PBF), using uniform Al2O3-nanoparticle-decorated MoTiAl powders bridged by functionalized carbon nanotubes. A tight ceramic coating ~2.57 mm in thickness consisting of an a-Al2O3 matrix with dispersed TiC particles was formed on the surface of the Al2O3-CNT/ MoTiAl composite, which has been proved to effectively increase resistance to oxidation at 1173 K. Meanwhile, the nanoparticles were homogenously dispersed and tightly contacted with the matrix, giving rise to an enhanced Vickers hardness. This work shed light on designing and producing highperformance Mo-based composites for application to ultrahigh-temperature materials. © 2019 Elsevier B.V. All rights reserved.
Keywords: Laser powder bed fusion (L-PBF) Metal matrix composites (MMCs) Carbon nanotubes Oxidation resistance Mechanical property
1. Introduction Because of their high melting points, high stiffness, and low coefficient of thermal expansion, Mo-based alloys show great potential for ultrahigh-temperature applications [1,2]. However, there is still a long way to go before nickel-based superalloys can be replaced to improve energy efficiency in heat engines (e.g., gas turbines or jet engines). The unsatisfactory strength at elevated temperatures and poor oxidation resistance are the main concerns for Mo-based alloys in practical use [3,4]. Toward this end, fabricating ceramic-particle-reinforced metal matrix composites (MMCs) has been deemed to be an effective approach. Fillers can improve the mechanical strength of MMCs via impeding the dislocation motion or grain boundary sliding at high temperatures [5,6]. Moreover, better oxidation behavior is expected, since the ceramic dispersions limit the penetration rate of oxygen atoms into the matrix and are beneficial for the spallation resistance of oxide scales formed on MMCs [7,8]. For instance, Michels [8] showed an increase in the oxidation resistance of hot-rolled NieCr alloys at 1273e1473 K with the addition of Y2O3. Chen et al. [6] proved that the elevated-temperature strength and oxidation resistance of
* Corresponding author. E-mail address: [email protected] (N. Nomura).
Mo5Si3 were improved with the addition of Al2O3 by hot pressing. However, such conventional manufacturing techniques for MMC products usually include processing steps that are highly material, energy, and time consuming (e.g., hot forging or shaping) [9,10]. Because of their high melting points and low ambienttemperature ductility, it is especially challenging to manufacture Mo-based materials with complex geometries by traditional processes [5,11]. Fortunately, laser powder bed fusion (L-PBF), as a newly developed layer-by-layer additive manufacturing technique, is extremely promising for metallic parts. As compared to conventional approaches, L-PBF process can solve production difficulties induced by complexities of shape, material, and hierarchy, while also decreasing manufacturing time and total cost [12,13]. More importantly, due to its high-temperature, non-equilibrium characteristics, L-PBF is capable of fabricating MMCs with an improved mechanical property, which is caused by the formation of a mechanically graded [14] or chemically bonded interface between the filler and the matrix [15]. Presently, the L-PBF technique has been applied widely for producing Ti- [13,16e21], Fe- [22e27], Al[15,28e30], and Ni-based MMCs [14,31e34] (see Table 1). Although promising results on L-PBF of MMCs have been reported, there are still several challenges, including fabrication of uniform nanofiller/ metal mixed powders, design of high-quality MMCs with complex geometries, and realization of the multi-functionality of L-PBF parts in practical application [35,36]. In addition, less attention has been
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Please cite this article as: W. Zhou et al., Novel laser additive-manufactured Mo-based composite with enhanced mechanical and oxidation properties, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152981
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Table 1 Fabrication of Ti-, Fe-, Al-, and Ni-based MMCs by L-PBF process. Type of MMCs Authors
Starting fillers
Powder fabrication
fabricated MMCs
Maximum dimension Measured properties
Ti-based
TiB2: 2.71 mm TiC: 50 nm TiB2: 3.5e6 mm
Ball milling Ball milling Blending mixing
TiB/Ti TiC/Ti TiB/Ti
e 8 8 8 mm3 Ø50 mm 20 mm
Ball Ball Ball Ball
TiB2/TiAl TiC/Ti TiC/316 L stainless steel TiB2/S136 mould steel
10 10 10 mm3 60 mm in length Ø8mm 6 mm ~60 mm in length
Fe-based
Al-based
Ni-based
Cai et al. [16] Gu et al. [17] Attar et al. [18,19]
Li et al. [20] TiB2: 3e5 mm, He et al. [21] TiC: 45 nm AlMangour et al. [23] TiC: 50 nm or 1 mm Wen et al. [24] TiB2: 200e300 nm
milling milling milling milling
Kang et al. [25]
WC: 3e5 mm
Tumbling mixing
WC/maraging steel
~50 mm in length
Zhao et al. [26] Song et al. [27] Jue et al. [28] Gu et al. [29] Li et al. [30] Zhou et al. [15] Wang et al. [31] Rong et al. [32] Zhang et al. [33]
TiC: 40 or 800 nm SiC: 27 mm Al2O3: 9 mm TiC: 50 nm nano-TiB2 Graphene oxide:1.0 mm Graphene: 5e50 mm. WC1-x:15e45 mm, TiC:50 nm
Ball milling Tumbling mixing Ball milling Ball milling Gas atomization Hetero-agglomeration Hetero-agglomeration Ball milling Ball milling
TiC/316 L stainless steel SiC/Fe Al2O3/Al TiC/AlSiMg TiB2/AlSiMg Al4C3/Al Graphene/Inconel 718 WC1-x/Inconel 718 TiC/Inconel 718
100 10 3 mm3 ~50 mm in length e e ~60 mm in length 4 4 1.4 mm3 35 mm in length e e
Yao et al. [34]
TiC:80e150 nm
Tumbling mixing
TiC/Inconel 718
e
paid to refractory materials such as Mo-based materials [11,37]. In this work, a novel nanoceramic/Mo-based composite was fabricated by L-PBF; its feasibility for high-temperature applications was verified by microstructural characterization, hardness testing, and oxidation experiments. 2. Experimental Starting Mo-33 wt%Ti-13 wt% Al alloy powders (D50 ~12.8 mm) were prepared by a combination of arc melting, mechanical milling, and sieving [11]. High-purity Al2O3 nanopowders with an average particle size of ~200 nm (Fig. 1a) were received from Taimei Chemicals Co., Ltd., Japan. Functionalized carbon nanotubes (CNTs)
Nanohardness, wear property Microhardness, wear property Nanohardness, Compressive property Nanohardness Tensile property Microhardness, wear performance Tensile property, Microhardness Wear performance Thermal conductivity, Tensile property Tensile property, microhardness Tensile property Wear property Microhardness, wear property Tensile property, microhardness Vickers hardness Tensile property Microhardness, wear property Microhardness, Corrosion behavior Tensile property
(Hodogaya Chemical Co., Ltd., Japan) had an average diameter of 70 nm and length of 6.9 mm (Fig. 1b and c) [11]. Following our previous work [11], a 10 wt%Al2O3-0.31 wt%CNT/MoTiAl mixed powder was fabricated via the heteroagglomeration method. Due to the bridging effect of negatively charged CNTs (Fig. 1f), the surfaces of the MoTiAl powders were well covered with positively charged Al2O3 nanoparticles (Fig. 1d and e) under electrostatic selfassembly. Subsequently, an in-house-developed L-PBF machine [11,15] was utilized to prepare a Mo-based alloy or composite. A set of optimized L-PBF parameters, corresponding to the laser power of 20.6 W, scan speed of 10 mm s1, hatching distance of 100 mm, and layer thickness of 25 mm, was utilized for fabricating rectangular builds (~4 4 1.4 mm3).
Fig. 1. TEM images of (a) Al2O3 powders and (b, c) functionalized CNTs; (def) FESEM images of 10 wt%Al2O3-0.31 wt%CNT/MoTiAl mixed powders. Arrows in (c) show the surface nanodefects of functionalized CNTs.
Please cite this article as: W. Zhou et al., Novel laser additive-manufactured Mo-based composite with enhanced mechanical and oxidation properties, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152981
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To determine the oxidation resistance of L-PBF builds, several coupons with a size of 2 2 1 mm3 were prepared after removing the Ti substrate. For comparison, a MoTiAl alloy part was also fabricated by conventionally spark plasma sintering (SPS) (Dr. Sinter S511, SPS Sintex Inc.) at 1673 K for 5 min under a pressure of 50 MPa. Oxidation tests were conducted isothermally at 1173 K for 1 h in a thermogravimetric analyzer (Hitachi SDT Q600) with an air flow of 100 ml/min. Moreover, a cyclic thermal treatment (3 cycles) were performed for L-PBF builds using the same condition of the oxidation testing. The microstructure was evaluated via electron back-scattered diffraction (EBSD) (OIM ver. 6, TSL Solutions), field-emission scanning electron microscope (FESEM) (JEOL JSM-6500 F), electron probe micro-analysis (EPMA) (JEOL JXA-8530 F), transmission electron microscopy (TEM) (Hitachi HF-2000EDX), and scanning TEM (STEM) (JEOL JEM-ARM200F). A focused ion beam system (JEOL JIB-4600 F) was used for preparing all TEM or STEM specimens. The mechanical performance was evaluated using a microhardness tester (Mitutoyo MZT-500) with a peak force of 0.588 N
3
and a holding period of 30 s. 3. Results and discussion A 10 wt%Al2O3-0.31 wt%CNT/MoTiAl composite bulk was successfully fabricated after L-PBF. Unmelted metallic powders or macropores, in addition to certain cracks (Fig. 2a1) caused by high thermal stress [38], were not detected (Fig. 2a1). Further FESEM energy-dispersive spectroscope (EDS) mappings in Fig. 2a2 revealed a striking observation; elemental Mo was barely visible, while rich O element existed on the top surface. To clarify this phenomenon, we checked the longitudinal cross section of this composite, in which a continuous layer with an average thickness of 2.57 mm was formed on the surface (Fig. 2b1). As revealed by TEM-EDS mapping (Fig. 2b2) and diffraction patterns (Fig. 2b3eb4), the surface layer consists of an a-Al2O3 matrix with dispersed TiC particles. The ceramic layer was tightly coated on the composite build without microcracks (Fig. 2b1). Since the whole L-PBF process was conducted in a protective Ar atmosphere with a low oxygen
Fig. 2. Morphologies of a 10 wt%Al2O3-0.31 wt%CNT/MoTiAl composite at (a) the top surface, (b) the surface part in the longitudinal cross section, and (c) the transversal cross section. (a1) FESEM image; (a2) SEM-EDS mappings. (b1) FESEM-back-scattered electron (BSE) image; (b2) TEM-EDS mappings; (b3) and (b4) diffraction patterns taken from the red and yellow spots in (b2), respectively. (c1) FESEM image; (c2) TEM image; (c3) EDS analysis and (c4) diffraction pattern taken from the red and yellow spots in (c2), respectively; (c5) high-resolution TEM image of a MoTiAleAl2O3 interface. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Please cite this article as: W. Zhou et al., Novel laser additive-manufactured Mo-based composite with enhanced mechanical and oxidation properties, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152981
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content (<0.1%), it is unlikely that Mo was oxidized and sublimated during L-PBF. Otherwise, the surface layer would mainly contain TiOx and Al2O3 phases. Thus, the disappearance of elemental Mo on the top surface in Fig. 2a2 was caused by formation of the ceramic protective layer. On the other hand, two categories of nanoparticles were uniformly distributed within the composite (Fig. 2c1e2). They were spherical Al2O3 and irregular TiC phases, respectively, as illustrated by the corresponding EDS analysis (Fig. 2c3) and diffraction pattern (Fig. 2c4). It is noted that the Al2O3 particles retained a size of ~200 nm (Fig. 2c2), which is similar to the starting size (Fig. 1a). The interface between the Al2O3 and MoTiAl matrix was free of interfacial compounds or nanocracks (Fig. 2c5). All TiC structures were monocrystalline (Fig. 2c4), showing a stripe shape (Fig. 2c1). Since the C þ Ti / TiC reaction is an exothermic, self-sustaining, and rapid process driven by a large negative enthalpy of TiC formation (184.5 kJ/mol) [39], the existence of TiC should be induced by the reaction between defective CNTs (Fig. 1c) and Ti atoms [40]. No CNTs were retained in the composite after L-PBF (Fig. 2c2). As we know, the movement and equilibrium of nanofillers within molten pools of MMCs are determined by relevant forces (e.g., buoyancy or the Marangoni force) due to the density difference and high thermal gradient [41]. It is thought that most Al2O3 or TiC particles were dispersed individually within the molten pools during L-PBF, while some tended to float, forming a thin ceramic coating on the surface of the composite build.
The influence of ceramic nanoparticles on the mechanical performance of the Mo-based composite is displayed in Fig. 3a. The Vickers hardness of the Al2O3-CNT/MoTiAl composite was 31.9% or 28.0% higher than that of the MoTiAl alloy in the transversal or longitudinal cross section, respectively. That is predominantly due to the well-dispersed nanoparticles that suppress the dislocation motion and plastic deformation of the matrix under loading [5]. Moreover, the Al2O3-CNT/MoTiAl composite underwent a morphological evolution from columnar to fine equiaxed grains, as well as a dramatic grain refinement from 12.1 to 3.4 mm (Fig. 3b and c). A similar result was reported by Martin et al. [42], that nanofillers contributed to the preparation of high-strength MMCs with equiaxed, fine-grained microstructures during L-PBF. Considering the application to heat-resistant materials, the oxidation behavior of Mo-based parts was further evaluated. Fig. 4 presents the morphology of a MoTiAl alloy or Al2O3-CNT/MoTiAl composite build in the longitudinal cross section after oxidation at 1173 K for 1 h. For the alloy build, an oxide layer 20 mm in thickness, which mainly consisted of Al and Ti oxides, was introduced on the surface (Fig. 4a and b). Since molybdenum is easily oxidized to MoO3 and sublimates after exposure to oxygen above 923 K [3], a small quantity of the Mo element was detected in the oxide layer. For the composite build, in contrast, no oxide layer formed between the ceramic coating and the composite (Fig. 4c and d). This result suggests that the ceramic coating could effectively increase the oxidation resistance by limiting the penetration and diffusion of
Fig. 3. (a) Vickers hardness of an L-PBF-processed MoTiAl alloy or Al2O3-CNT/MoTiAl composite in the transversal and longitudinal cross sections; inverse pole figures of (a) MoTiAl alloy and (b) Al2O3-CNT/MoTiAl composite in the transversal cross sections. Inset in (a) shows a Vickers indentation imprint on the MoTiAl alloy.
Please cite this article as: W. Zhou et al., Novel laser additive-manufactured Mo-based composite with enhanced mechanical and oxidation properties, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152981
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Fig. 4. Morphologies of an L-PBF-processed (a, b) MoTiAl alloy and (c, d) Al2O3-CNT/MoTiAl composite build in the longitudinal cross section near the surface part after oxidation at 1173 K for 1 h. (a) FESEM-BSE image; (b) SEM-BSE image and corresponding EDS elemental mappings; (c) FESEM-BSE image; (d) magnified FESEM-BSE image and corresponding EPMA elemental mappings taken from the white square in (c).
oxygen atoms [43]. Interestingly, the ceramic coating retained strong, tight contact with the Al2O3-CNT/MoTiAl composite at 1173 K after the cyclic oxidation treatment (Fig. 5), showing good thermodynamic and mechanical stability. In addition, as compared to the conventionally made MoTiAl alloy part by SPS (Fig. S1), the L-PBF-processed composite showed superior oxidation resistance. This work proved the promising feasibility of 3D-printed Mo-based composites in ultrahigh-temperature applications.
4. Conclusions In summary, an Al2O3-CNT/MoTiAl composite with improved mechanical and oxidation properties was fabricated successfully via L-PBF. The MoTiAl powders were covered with uniform Al2O3 nanoparticles with the assistance of functionalized CNTs during heteroagglomeration. During L-PBF processing, a thin ceramic layer consisting of an a-Al2O3 matrix with a dispersed TiC phase was tightly coated on the surface of the Al2O3-CNT/MoTiAl composite,
Fig. 5. (a) FESEM image of the L-PBF-processed Al2O3-CNT/MoTiAl composite build in the longitudinal cross section near the surface part after the cyclic oxidation treatment; (b) EDS analysis taken from the red spot in (a). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Please cite this article as: W. Zhou et al., Novel laser additive-manufactured Mo-based composite with enhanced mechanical and oxidation properties, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152981
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giving rise to dramatically enhanced oxidation resistance. Meanwhile, the improved Vickers hardness was caused by the homogeneously dispersed, tightly contacted ceramic nanoparticles in the matrix. This work demonstrated that the L-PBF process has great potential for manufacturing high-performance heat-resistant Mobased materials with tailored architectures for ultrahightemperature applications. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The authors would like to thank Dr. Takamichi Miyazaki and Dr. Kosei Kobayashi for the TEM observations at Tohoku University. We also appreciated beneficial discussions with Dr. Yuanyuan Lu and Mr. Ryuichi Miyata at Tohoku University. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.152981. References [1] J.H. Perepezko, The hotter the engine, the better, Science 326 (2009) 1068e1069. [2] D.M. Dimiduk, J.H. Perepezko, Mo-Si-B alloys: developing a revolutionary turbine-engine material, MRS Bull. 28 (2003) 639e645. [3] T.A. Parthasarathy, M.G. Mendiratta, D.M. Dimiduk, Oxidation mechanisms in Mo-reinforced Mo5SiB2(T-2)-Mo3Si alloys, Acta Mater. 50 (2002) 1857e1868. [4] S. Miyamoto, K. Yoshimi, S.-H. Ha, T. Kaneko, J. Nakamura, T. Sato, et al., Phase equilibria, microstructure, and high-temperature strength of TiC-added MoSi-B alloys, Metall. Mater. Trans. A 45 (2014) 1112e1123. [5] G. Liu, G.J. Zhang, F. Jiang, X.D. Ding, Y.J. Sun, J. Sun, et al., Nanostructured high-strength molybdenum alloys with unprecedented tensile ductility, Nat. Mater. 12 (2013) 344e350. [6] H. Chen, Q. Ma, X. Shao, J. Ma, C. Wang, B. Huang, Microstructure, mechanical properties and oxidation resistance of Mo5Si3eAl2O3 composite, Mater. Sci. Eng. A 592 (2014) 12e18. [7] W. Quadakkers, H. Holzbrecher, K. Briefs, H. Beske, Differences in growth mechanisms of oxide scales formed on ODS and conventional wrought alloys, Oxid Met 32 (1989) 67e88. [8] H.T. Michels, The effect of dispersed reactive metal oxides on the oxidation resistance of nickel-20 Wt pct chromium alloys, Metall Trans A 7 (1976) 379e388. [9] L.C. Zhang, H. Attar, Selective laser melting of titanium alloys and titanium matrix composites for biomedical applications: a review, Adv. Eng. Mater. 18 (2016) 463e475. [10] W. Zhou, X. Sun, K. Tsunoda, K. Kikuchi, N. Nomura, K. Yoshimi, et al., Powder fabrication and laser additive manufacturing of MoSiBTiC alloy, Intermetallics 104 (2019) 33e42. [11] W. Zhou, X. Sun, K. Kikuchi, N. Nomura, K. Yoshimi, A. Kawasaki, Carbon nanotubes as a unique agent to fabricate nanoceramic/metal composite powders for additive manufacturing, Mater. Des. 137 (2018) 276e285. [12] L. Thijs, F. Verhaeghe, T. Craeghs, J.V. Humbeeck, J.-P. Kruth, A study of the microstructural evolution during selective laser melting of Tie6Ale4V, Acta Mater. 58 (2010) 3303e3312. [13] D. Gu, Y.-C. Hagedorn, W. Meiners, G. Meng, R.J.S. Batista, K. Wissenbach, et al., Densification behavior, microstructure evolution, and wear performance of selective laser melting processed commercially pure titanium, Acta Mater. 60 (2012) 3849e3860. [14] T. Rong, D.D. Gu, Q.M. Shi, S.N. Cao, M.J. Xia, Effects of tailored gradient interface on wear properties of WC/Inconel 718 composites using selective laser melting, Surf. Coat. Technol. 307 (2016) 418e427. [15] W. Zhou, M. Dong, Z. Zhou, X. Sun, K. Kikuchi, N. Nomura, et al., In situ formation of uniformly dispersed Al4C3 nanorods during additive manufacturing of graphene oxide/Al mixed powders, Carbon 141 (2019) 67e75. [16] C. Cai, C. Radoslaw, J. Zhang, Q. Yan, S. Wen, B. Song, et al., In-situ preparation and formation of TiB/Ti-6Al-4V nanocomposite via laser additive manufacturing: microstructure evolution and tribological behavior, Powder
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Please cite this article as: W. Zhou et al., Novel laser additive-manufactured Mo-based composite with enhanced mechanical and oxidation properties, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152981