TiAl composites

Accepted Manuscript Microstructure evolution and high-temperature oxidation behaviour of selective laser melted TiC/TiAl composites

Chenglong Ma, Dongdong Gu, Dai Donghua, Han Zhang, Hongmei Zhang, Jiankai Yang, Meng Guo, Yuexin Du, Jie Gao PII: DOI: Reference:

S0257-8972(19)30799-6 https://doi.org/10.1016/j.surfcoat.2019.07.059 SCT 24835

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

6 May 2019 13 July 2019 26 July 2019

Please cite this article as: C. Ma, D. Gu, D. Donghua, et al., Microstructure evolution and high-temperature oxidation behaviour of selective laser melted TiC/TiAl composites, Surface & Coatings Technology, https://doi.org/10.1016/j.surfcoat.2019.07.059

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ACCEPTED MANUSCRIPT Microstructure evolution and high-temperature oxidation behaviour of selective laser melted TiC/TiAl composites Chenglong Ma 1,2, Dongdong Gu 1,2, *, Dai Donghua 1,2, Han Zhang 1,2, Hongmei Zhang 1,2, Jiankai Yang 1,2, Meng Guo 1,2, Yuexin Du 1,2, Jie Gao 1,2

College of Materials Science and Technology, Nanjing University of Aeronautics

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Jiangsu Provincial Engineering Laboratory for Laser Additive Manufacturing of

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and Astronautics, Nanjing 210016

High-Performance Metallic Components，Nanjing 210016

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*Corresponding author. E-mail: [email protected] (D. Gu). Tel. /Fax: +86 25

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52112626.

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ACCEPTED MANUSCRIPT Abstract: In this work, TiC ceramics particle reinforced TiAl-based composites were synthesized by selective laser melting (SLM) by using Ti, Al, and TiC multicomponent powder. The results showed that a plenty of TiC dendritic crystals formed

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during the solidifying process. It was found that TiC dendrites exhibited three

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different modes of nucleation and growth, corresponding to the dissolution-

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precipitation of fully melted fine TiC particles, the epitaxial growth along the margin of partly melted TiC particle, and the recrystallization growth based on stress-induced

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nonequilibrium melting. Subsequently, the influence of laser energy density on

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microstructure evolution and high-temperature oxidation behaviour of SLM fabricated TiC/TiAl composites were investigated. At a high laser energy density of 189 J/mm3,

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the relatively full dense SLM-fabricated part was obtained, accompanying the

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formation of a variety of TiC dendrites with the coarsening structure, which made a contribution to the elevated high-temperature oxidation resistance with a low

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oxidation kinetics constant of 1.32×10-5 mg6cm-12h-1.

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Key words: selective laser melting (SLM); TiAl-based composites; high-temperature oxidation resistance

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ACCEPTED MANUSCRIPT 1. Introduction In the last decade, TiAl alloys, as the next-generation lightweight high-temperature structural material, have been attracting great attention around all the world. Due to its inherent physical properties such as low density and high melting point, as well as its superior

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mechanical properties including higher special strength and stiffness, more excellent creep resistance by comparing with the conventional Ni-based superalloys, TiAl alloy has achieved

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wide applications in the aerospace, automotive, etc. [1-4]. Especially for the application in the

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aerospace field (such as turbine blade, engine fuel nozzles, etc.), TiAl alloy can dramatically

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enhance the thrust-weight ratio of the engine by remarkably decreasing the engine weight, which thus is regarded as the most potential engine material. Note that, at present, the highest

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working temperature of as-developed TiAl alloys is ~760 ̊C. By the addition of reinforcement particles, the working temperature of TiAl-based composites can efficiently get increased to

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800-900 ̊C, simultaneously accompanying the elevated high-temperature properties including high-temperature strength, creep properties and high-temperature oxidation resistance [5-7].

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However, the lower ambient ductility and toughness of TiAl alloys and their composites severely deteriorate the formability, therefore significantly restraining the practical

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applications [4].

Selective laser melting (SLM) technology based on the powder bed can be used successfully to fabricate the thick-wall parts and complex shaped precise components, due to its tiny laser spot diameter and fine layer thickness, which possesses definite applications and huge development prospect in the manufacturing of prototype parts, tools and moulds with any complex shape in the aerospace and automotive fields [8-10]. Hence, the challenges such as the poor processing formability induced by the material physical properties and the 3

ACCEPTED MANUSCRIPT irregular/complex configurations of the typical aero components, can get solved efficiently as the SLM technology is developing. For instance, L. Löber et al has recently fabricated the TiAl-based dodecahedron unit cells using SLM technology at the optimized parameters, showing a relative density of around 99% and a comparable mechanical performance to

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conventional cast samples [11]. However, due to the inherent physical/chemical characters, SLM of TiAl-based materials still exists some problems, including (i) high affinity of Ti or Al

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component to oxygen element [12], (ii) melting loss and local segregation of Al component

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[13], and (iii) high cracking susceptibility [13]. Although the above existing problems, a

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complete investigation chain involving raw materials, processing, microstructure, and properties of SLM-fabricated TiAl-based parts is necessary. Gu et al fabricated in-situ

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TiC/Ti(Al) nanocomposite powder by mechanical alloying of Ti-Al-graphite elemental powder mixture, which was smoothly applied in the subsequent SLM process [14]. Y. Shi et al

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investigated the influence of laser energy density on the grain orientation, grain structure and phase transformation of Ti-Al alloy [15]. They found higher laser energy density tended to

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induce coarsening grains with weak texture and increased high-angle grain boundaries. Recently, they further studied the compressive properties of SLM-fabricated TiAl/reduced

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graphene oxide (RGO) composites, exhibiting the superior compressive strength of 1546.88 MPa and strain of 5.34 % [16]. However, limited investigations on high-temperature properties (one of the most crucial properties) of SLM-fabricated TiAl-based parts have been reported. Specially, for SLM-fabricated TiAl-based composites, the underlying hightemperature oxidation mechanism as well as the role of reinforcement particles are still not very clear. In this work, TiAl-based composite part with high weight percent of TiC reinforcement 4

ACCEPTED MANUSCRIPT particles was achieved, by SLM using the mixing powder containing Ti powder, Al powder and TiC ceramic particles. The effect of laser process parameters on the densification rate, phase and microstructure of SLM-fabricated TiC/TiAl composites was investigated. Based on morphology features of TiC phase, the non-equilibrium melting mechanism of TiC particle

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was proposed. Subsequently, we further characterized the high-temperature oxidation

oxidation dynamics features and oxidation mechanisms.

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2. Experimental details

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behaviour of SLM-fabricated TiC/TiAl composites, in order to disclose the corresponding

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The spherical Titanium powder with a mean particle size of 25 µm and Aluminium powder with an average particle size of 30 µm, as well as the irregular-shaped micron-sized

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TiC particles were used as the raw materials in this study (Fig. 1a). Before the SLM consolidation, the multi-component powders containing Titanium powder, Aluminium powder

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and TiC particle were homogenously mixed by the high-energy Pulverisette 6 planetary mono-mill (Fritsch GmbH, Germany) using a ball-to-powder weight ratio of 2:1, a ration

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speed of the main disc of 200 r/min, and a milling time of 4 h. The atomic rate of Titanium powder and Aluminium powder was 1:1 and the weight percent of TiC particle was 20 wt.%.

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The powder size distribution of the mixed powder was measured by a laser diffraction particle size analyser (BT-9300H, BETTER, China), showing an average particle size of 28.17 µm (Fig. 1b). The most concentrated in the interval of 32.41-36.08 µm. It should be pointed out that narrow particle size distribution could guarantee a good fluidity of powder. And the fact was that the measured angle of repose for the freely stacked mixed powder showed a relatively low value of 33° (Fig. 1c), which verified the above point. Besides, the spreading quality of the mixed powder was also identified as good during the subsequent SLM process. 5

ACCEPTED MANUSCRIPT The SLM processing was performed by the SLM-150 equipment developed by Nanjing University of Aeronautics and Astronautics. The SLM system mainly contains a YLR-500WC ytterbium fiber laser with a power of ~500 W and a spot size of 70 μm (IPG Laser GmbH, Germany), an automatic powder spreading device, an inert argon gas circulatory

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protection system, and a computer system for process control. In this study, the powder layer thickness d of 50 μm, a simple linear raster scan pattern was used, and a hatch spacing h of 70

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μm were set. The laser power P of 55, 66, and 88 W and the scanning speed v containing 100

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and 200 mm/s were applied. To comprehensively evaluate the influence of laser processing

according to the following formula [17]: P vhd

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 

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parameters on the microstructure and properties, laser volume energy density η was used

(1)

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Hence, different laser volume energy densities were given, namely 189 J/mm3, 157 J/mm3,

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and 126 J/mm3. The interaction of laser with the mixed powder and the final as-fabricated specimens were shown in Fig. 1d and 1e.

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The Thermo Scientific™ ARL™ X'TRA X-ray diffractometer (XRD) with Cu Kα

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radiation (λ = 0.154 18 nm) using a continuous scan mode was applied to identify the phases of the SLM-fabricated TiC/TiAl composite specimens within a narrow range of 2θ = 30-50°. Specimen for metallographic examinations were prepared by the standard polishing procedures and then etched with a solution containing HF, HNO3, HCl, and distilled water with a volume ratio of 3:3:2:1 for 30s. High-resolution study of the microstructural feature of SLM-processed specimens and the corresponding chemical compositions were performed by an S-4800 field emission scanning electron microscope (FE-SEM) (Hitachi, Japan) equipped 6

ACCEPTED MANUSCRIPT with an EDAX energy dispersive x-ray spectroscope (EDX) (EDAX, Inc., USA). Then, the high-temperature oxidation behaviour was studied using isothermal temperature oxidation method. Before the oxidation test, all surfaces of the as-fabricated specimens were polished by abrasive papers, rinsed by the ultrasonic cleaning and then dried in the drying oven. The

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oxidation temperature was set to 850°C with the oxidation time of 100 h. The specimens together with the crucibles were put into the furnace chamber which was preheated up to the

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corresponding service temperature, and then were weighted precisely at each predetermined

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time. The weight gains were measured using the following equation [18]: (2)

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ΔW/S = (Wt-W0)/S0,

whereΔW/S was the mass gain per unit area (mg/cm2), Wt was the weight before oxidation,

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W0 was the weight after oxidation, and S0 was the surface area before oxidation.

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3.1 Densification behaviour

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3. Results

Fig. 2 gives the optical images of cross section of SLM-fabricated TiC/TiAl composites

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at different ηs. It was found that with the applied η decreased, the number and size of the residual pores increased apparently. When the η of 189 J/mm3 was applied, some fine sphere

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gas pores with a rough size of 10 μm were remained, simultaneously accompanying a few microcracks emerging around the pores (Fig. 2a). As the η decreased to 157 J/mm3, the residual pores became much larger and showed an irregular shape. The size of these pores was more than 50 μm in this condition (Fig. 2b). Specially, when the η further decreased to 126 J/mm3, apart from the large residual pores, more apparent microcracks could be observed, which ran through some particles and ended in the pores (Fig. 2c). The relative densities of three specimens were exhibited in Fig. 2d based on the Archimedes’ principle and the highest 7

ACCEPTED MANUSCRIPT one of 95.92% was obtained for the specimen fabricated at the η of 189 J/mm3. 3.2 Effect of laser processing parameters on phase and microstructure Fig. 3 shows the phases and chemical compositions of SLM-fabricated TiC/TiAl composites. According to the XRD pattern (Fig. 3a), TiAl matrix phase was synthesized by

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the laser-induced in-situ reaction. Note that, comparing with the TiC diffraction peak, the intensity of TiAl diffraction peak was weak obviously, especially when a high η was applied.

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Besides, there was only one diffraction peak for TiAl phase to be detected, corresponding to

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(111) crystal plane according to the PDF # 65-0428, which might indicate a relatively stronger

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texture for the matrix existed. Different from TiAl phase, TiC phase with more diffraction peaks and stronger diffraction intensity were identified, according to the PDF # 32-1383.

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Three main diffraction peaks corresponded to (111), (200) and (220) crystal planes,

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above XRD results (Fig. 3b-e).

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respectively. Furthermore, the EDS composition analysis for different phases supported the

Subsequently, the detailed microstructure features were displayed in Fig. 4. It was found

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that many TiC particles with the larger size of 10-30 μm distributed within the TiAl matrix. Meanwhile, a plenty of TiC dendritic phase formed and dispersed among residual TiC

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particles (Fig. 4a). In addition, the dendritic morphologies were also observed at the margin of some original TiC particles and within several TiC particles (Fig. 4b). Specially, as the applied η got reduced, both the content and size of TiC dendritic phase exhibited a decreasing trend (Fig. 4a, 4c and 4d). To further disclose the quantitative relationship between the η and the morphology feature of TiC dendrites, the high-magnification characterized morphology of insitu formed TiC dendrites at different ηs was given in Fig. 4. At a high η of 189 J/mm3, TiC dendrites grew sufficiently with a dendritic trunk length of ~9.5 μm and a relatively smaller 8

ACCEPTED MANUSCRIPT dendritic arm spacing (Fig. 5a and 5b). Also, the cross-sectional morphology of dendritic arms could be observed and their growth direction was perpendicular to the paper plane (Fig. 5c), from which the thickness of dendritic arms could be estimated clearly in a range of 0.16~0.6 μm. When the η decreased to 157 J/mm3, the density of dendrite phase surrounding

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TiC particles showed a remarkable diminution (Fig. 5d). In this case, the length of dendritic trunks decreased to ~7.3 μm, and the mean thickness of dendritic arms was 0.2 μm (Fig. 5e).

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Additionally, lamellar TiC fine grains were found around the dendritic phase (Fig. 5f), the

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mean thickness of which was only 80 nm, showing the typical nanostructure feature. As the η

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further decreased to 126 J/mm3, the length of dendritic trunks and the thickness of dendritic arms decreased to ~3.9 μm and ~0.1 μm, respectively (Fig. 5g and 5h). In this condition, a

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variety of lamellar TiC grains with a relative smaller thickness of ~50 nm formed (Fig. 5i).

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3.3.1 Oxidation kinetics curves

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3.3 High-temperature oxidation performance

The high-temperature oxidation kinetics curves of SLM-fabricated TiC/TiAl composites

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are demonstrated in Fig. 6a. It should be pointed out that the oxidation mass gain results were averaged from three parallel specimens. The results indicated that the oxidation kinetics

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curves for all SLM-fabricated specimens showed a similar change trend, namely accelerated increasing mode of the mass gain at the beginning period and then gradually decreased oxidation rate with the exposure time extending. In general, the mass gain during the isothermal oxidation process follows a power exponent law which can be expressed as [19]: (ΔW)n = Kt

(4)

WhereΔW is the mass gain per unit area, n is the oxidation rate exponent, t is the oxidation time, and K is isothermal oxidation rate constant. To precisely determine the oxidation 9

ACCEPTED MANUSCRIPT kinetics behaviours of SLM-fabricated TiC/TiAl composites, the corresponding square, cubic, quadruplicate, six power of and logarithmic mass gain versus the oxidation time are given, as shown in Fig. 6b-f. When the η was relatively lower (including 126 J/mm3 and 157 J/mm3), the mass gain increased irregularly with the exposure time. Besides, for the low exponent

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conditions (n=2,3), the oxidation kinetics curves at these two laser energy densities exhibited a high consistency. As the exponent n increased to 4 or 6, the remarkable difference in the

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oxidation kinetics curves emerged. When the applied η was elevated to 189 J/mm3, the six

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power of the mass gain basically kept a linear relationship with the oxidation time. Hence, it

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could be speculated that the oxidation kinetics rate exponent n was 6 for the specimen fabricated by SLM with 189 J/mm3. According to the least square method, the oxidation

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kinetics constant was 1.32×10-5 mg6cm-12h-1.

3.3.2 Oxidized surface morphologies and cross-sectional microstructures

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Subsequently, the oxidized surface morphologies of SLM-fabricated specimens at three different ηs were characterized. In consideration of no apparent difference in oxidized surface

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morphology among three specimens, Fig. 7 only gives the corresponding FE-SEM image at the η of 189 J/mm3. From the Fig. 7a, the oxidized surface was relatively rough, but free of

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any apparent peeling or pores. Two different kinds of characterized oxidation films could be identified clearly, corresponding to the dark one (marked by B) and the light one (marked by A). Specially, the dark oxidation film showed the typical island distribution within the oxidized surface. By the high-magnification observation, the light oxidation film consisted of a plenty of crystal-like oxide particles (Fig. 7b). Many stepped textures could be found on the particle surface, presenting the typical two-dimensional lamellar growth mode (Fig. 7c). As for the other film, different from the light one, the oxide particle in the dark one was much 10

ACCEPTED MANUSCRIPT smaller and showing an irregular shape (Fig. 7d and e). To determine the phase constitution of the oxidation film, the corresponding XRD characterization was performed (Fig. 7f). The oxidation film mainly contained the rutile TiO2 and α-Al2O3. Based on Xiao’s study [20] and the observed morphology feature of oxide particles in this study, it could be figured out that

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the crystal-like particle was rutile TiO2 and the fine irregular particle was α-Al2O3. Furthermore, the cross-sectional microstructure of the oxidation film is displayed in Fig. 8,

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showing the four-layer oxidation structure. The thickness of each layer was various, and the

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innermost layer (the 4th layer) was the thinnest. The corresponding EDS mapping for the

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whole oxidation film was conducted (Fig. 8b). It could be found clearly that the 1st and 3rd layers mainly contained titanium oxide, while aluminium oxide dominated the 2nd layer. Then,

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the features of the 2nd /3rd layer interface and the morphology of the 4th layer were further observed at the high magnification (Fig. 8c-e). There still existed some titanium oxides within

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the 2nd layer from Fig. 8c. Besides, the thickness of the 4th layer was about only 400 nm (Fig. 8e). The EDS mapping for the 4th layer was also conducted (Fig. 8f), showing that the 4th

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layer was Al-rich. Differently, comparing with the 2nd layer, the 4th layer possessed more consecutive and denser oxidation structure. Besides, a variety of fine TiC particles were

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observed within the matrix in the vicinity of the innermost oxidation layer. Apparent carbon distribution existed in the innermost layer, which might derive from the carbide dendritic phase. In summary, based on the above SEM, XRD and EDS results, the outside-in four oxide layers mainly corresponded to TiO2 (with a few α-Al2O3), α-Al2O3 (with a few TiO2), TiO2 and α-Al2O3, respectively. Although the oxidized surface morphology was similar for all SLM-fabricated specimens at different ηs, the cross-sectional microstructure features of the oxidation films varied 11

ACCEPTED MANUSCRIPT dramatically (Fig. 9). When the applied η was low, due to the existed considerable pores and cracks within the specimens (Fig. 9a and 9b), the intrusion of a mass of oxygen element occurred, therefore resulting in the existence of oxidation in the internal region of the specimens. As the η increased to 189 J/mm3, the oxidation behaviour got suppressed

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apparently and a relatively intact oxidation film formed due to the fewer inner pores and

4. Discussion

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4.1 Non-equilibrium melting behaviour of TiC particles

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cracks (Fig. 9c).

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According to the microstructural features of SLM-fabricated TiC/TiAl composites, formation of TiC dendrites shows three different mechanisms: (1) the epitaxial growth along

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the margin of partly melted TiC particle (Fig. 4b); (2) dissolution-precipitation of fully melted TiC particle (Fig. 4a); (3) recrystallization-precipitation within the TiC particle (Fig. 4b).

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However, the theoretical melting point of TiC reaches as high as 3413 K, which can cause the severe evaporation of Al element. But in fact, equiatomic TiAl matrix phase was in-situ

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synthesized in this study, which indicated the non-equilibrium melting behaviour of TiC particle occurred during the laser processing.

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As for the first two kinds of formation mechanisms, the melting behaviour of TiC particle can be attributed to the diffusion effect of carbon element and the small-scale effect of reinforcement particle, which have been given the detailed illumination in our previous work [21]. Here the specific description is not repeated here. With regards to the recrystallizationprecipitation mechanism, it might be closely associated with the internal stress. Y.S. Hwang et al found that the internal stress induced by laser processing could bring about the decrease of melting point of materials [22]. Taking Al alloys for example, thermodynamic melting 12

ACCEPTED MANUSCRIPT temperature for Al reduces from 933.67 K for a stress-free condition down to 898.1 K for uniaxial strain. Besides, V.I. Levitas et al also pointed out that the strain energy at the high strain rate could intensify significantly the driving force for the melting [23]. Based on our previous investigation work [4], the heating/cooling rate could reach 7~8×106 K/s during

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SLM of TiC/TiAl composites, and a ultrahigh temperature gradient of 1~8×107 K/m formed within the molten pool. Due to the existence of the ultrahigh temperature gradient and

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prominent difference in thermal properties (such as thermal expansion coefficient) between

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reinforcing particles and the matrix, a considerable internal stress of 600-800 MPa forms

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within the reinforcing particle, which therefore induces the recrystallization process within the TiC particle.

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4.2 On the morphology/content evolution of TiC dendrite Fig. 5 indicates that the laser processing parameters have a significant effect on the

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difference in the morphology/content of TiC dendrites. This kind of difference can be mainly attributed to the melting-solidification thermal history of TiC particles [24] and the activity of

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carbon atoms at the liquid/solid interface during the SLM processing [21]. According to the

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Gibbs-Thomson equation, the temperature Tt at the tip of dendrites can be expressed as [21,

Tt  TM  mCl 

RTM2 Vt  H f V0

(3)

Where TM is the melting point of TiC, m is the slope of the liquid line, Cl is the carbon concentration at the liquid/solid interface, H is the latent heat of TiC, Vt is the growth rate of the dendritic tip and V0 is the kinetics constant. It should be noted that Vt is significantly influenced by the scanning speed v, and shows a positive relationship with v. Therefore, 13

ACCEPTED MANUSCRIPT according to the microstructure evolution induced by the laser energy density and M. Das’s similar work [26], some convincing interpretations on the growth mechanism of dendrites can be given. At a relatively higher η (v = 100 mm/s), the Vt is relatively low, thus leading to an increase of Tt, which consequently guarantees the sufficient growth driving force of dendrites

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and concomitant coarsening microstructure. Then, the relatively higher Tt is beneficial to induce the accelerated increasing of carbon activity in the growing front of dendrites and to

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drive more [C] atoms to deposit in the dendritic tip. Specially, along the maximum chemistry

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potential gradient, the dendritic trunk and the primary dendritic arm occurs to become

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significantly coarsening. As the η (v = 200 mm/s) decreases, Tt apparently decreases due to an increase of Vt, thus resulting in insufficient internal energy and limited carbon activity within

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the melt. In this case, the growth of dendrites suffers from remarkable suppression. Furthermore, in views of the condition that the v is same, with the laser power p increasing,

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temperature within the molten pool increases, inducing the enhanced carbon activity and attendant increased carbon concentration at the dendritic tip, further causing the successive

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growth and coarsening of the dendrite phase. 4.3 High-temperature oxidation mechanism of SLM-fabricated TiC/TiAl composites

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Based on the above experimental results, the high-temperature oxidation mechanism of SLM-fabricated TiC/TiAl composites can get disclosed efficiently. Taking the oxidation process of the SLM-fabricated specimen at the high η for example, the corresponding oxidation mechanism schematics is exhibited in Fig. 10. At the beginning period, the dominated oxidation mechanism is the chemisorption effect induced by the interaction between the specimen surface and the surrounding high-temperature environment [27]. Some chemical reactions will occur according to the Ellingham-Richardson principle, as follows: 14

ACCEPTED MANUSCRIPT 4 2 Al ( s, l ) +O( g) → Al 2O( s) , reaction (1) 2 3 3 3 Ti ( s) +O( g) →Ti O( s) , reaction (1) 2 2

1 1 1 Ti C( s) +O( g) → Ti O( s) + CO2( g ) , reaction (3) 2 2 2 2 2

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The corresponding standard Gibbs free energy change associated with each of the above reactions can be determined based on the following equations [28, 29], as a function of

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temperature (T).

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G1( KJ / mol )  1099. 621  0. 1486T  0. 00782T l n T

G3( KJ / mol )  577. 464  0. 08502T

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G2( KJ / mol )  943. 5  0. 1791T

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It is apparently found that at the research temperature of 850°C, all Gibbs free energy changes are negative, which means that all these reactions can occur spontaneously. Besides, reaction

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(1) and (2) have the similar Gibbs free energy change that is far lower than that of reaction (3), indicating that reaction (1) and (2) are likely to take place in thermodynamic at the very

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first stage of oxidation. Hence, at the beginning period, TiO2 and α-Al2O3 can be produced almost simultaneously. In this condition, most of the specimen surface is the fresh metal, thus

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leading to a relatively higher chemistry reaction rate and a rapid increase of the mass gain. Subsequently, the oxygen atoms further intrude into the inner, and meanwhile Ti, Al atoms successively diffuse and migrate to the oxidation layer from the interior, in which case the oxidation mechanism start to be dominated by the diffusion behaviour [30]. As a result, the oxidation rate of the whole specimen declines significantly, comparing with that at the beginning period (Fig. 6a). Note that, the diffusion rate of Ti in TiO2 is much larger than that of Al in α-Al2O3 [31], more Ti atoms in the outer layer combined with the oxygen atoms, to 15

ACCEPTED MANUSCRIPT facilitate the growth of TiO2 and the formation of the outermost oxidation layer where Al atoms with the relatively slower diffusion rate were further oxidized as α-Al2O3 with the island distribution (Fig. 7a and Fig. 10b). As a result, the Al-rich 2nd oxidation layer can be understood (Fig. 8b and Fig. 10b). As the exposure time increases, oxidation of TiC

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particles/dendrites within the specimen starts to take place, and Ti-rich oxidation layer (namely the 3rd oxidation layer) forms in the inner side of the 2nd layer due to the relatively

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slower diffusion rate of Al atom. Meanwhile, the generation of CO2 during oxidation of TiC

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leads to formation of gas pores and cracks in the 2nd/3rd layer interface (Fig. 8c). These pores

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can act as the transferring channel of oxygen atoms, facilitating the formation of TiO2 layer. For another, oxidation of TiC also retards the continuous infiltration of oxygen atoms toward

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the matrix. Owing to the consumption of Ti atoms in the 3rd oxidation layer, a thin Al-rich band forms in the innermost, thus contributing to formation of Al2O3 layer (the 4th layer, Fig.

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8d-f). Nevertheless, when an inadequate laser energy input is applied to fabricate TiC/TiAl composites, a great many voids and cracks are visible, accompanying the hypogenetic TiC

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dendrites, thus triggering the occurrence of severe oxidation behaviour [32] (Fig. 9b and 9c). 4. Conclusions

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In this work, TiC ceramics reinforced TiAl-based composites were synthesized successfully by SLM by using Ti, Al, and TiC multi-component powder. The influence of laser process parameters on the densification rate, phase, microstructure and high-temperature oxidation behaviour of SLM-fabricated TiC/TiAl composites was investigated. At the high laser energy density (189 J/mm3), TiAl-based composite part exhibited an excellent hightemperature oxidation resistance with a relatively low oxidation kinetics constant (1.32×10-5 mg6cm-12h-1) and formation of a coherent, dense and thin oxidation layer. The typical 16

ACCEPTED MANUSCRIPT oxidation film contains four layers, namely TiO2 (with a few α-Al2O3), α-Al2O3 (with a few TiO2), TiO2 and α-Al2O3 from the outside.

Acknowledgements

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This work was supported by the financial support from the National Natural Science Foundation of China (No. 51735005), the National Key Research and Development Program

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(No. 2016YFB1100101 and 2018YFB1106302), and the Priority Academic Program

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Development of Jiangsu Higher Education Institutions. Chenglong Ma thanks the financial

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support from the Funding for Outstanding Doctoral Dissertation in NUAA (No. BCXJ17-05) and Postgraduate Research & Practice Innovation Program of Jiangsu Province (No.

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KYCX17-0253). Reference

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[1] J. Zhang, Y. Wu, X. Cheng, S. Zhang and H. Wang, Study of microstructure evolution and preference growth direction in a fully laminated directional micro-columnar TiAl fabricated

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using laser additive manufacturing technique. Mater. Lett., 243 (2019), pp. 62-65. [2] T. Klein, M. Schachermayer, F. Mendez-Martin, T. Schöberl, B. Rashkova, H. Clemens,

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and S. Mayer, Carbon distribution in multi-phase γ-TiAl based alloys and its influence on mechanical properties and phase formation. Acta Mater., 94 (2015), pp. 205-213. [3] V.K. Balla, M. Das, A. Mohammad, and A.M. Al-Ahmari, Additive manufacturing of γTiAl: processing, microstructure, and properties. Adv. Eng. Mater., 18 (2016), pp. 1208-1215. [4] C.L. Ma, D.D. Gu, D.H. Dai, H.M. Zhang, L. Du, and H. Zhang, Development of interfacial stress during selective laser melting of TiC reinforced TiAl composites: Influence of geometric feature of reinforcement. Mater. Des., 157 (2018), pp. 1-11. 17

ACCEPTED MANUSCRIPT [5] X.B. Liu, and R.L. Yu, Microstructure and high-temperature wear and oxidation resistance of laser clad γ/W2C/TiC composite coatings on γ-TiAl intermetallic alloy. J. Alloys Compd., 439 (2007), pp. 279-286. [6] J. Gao, Y. He, and W. Gao, Oxidation behavior of γ-TiAl based alloy with Al2O3–Y2O3

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composite coatings prepared by electrophoretic deposition. Surf. Coat. Tech., 205 (2011), pp. 4453-4458.

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[7] X.B. Liu, and H.M. Wang, Microstructure, wear and high-temperature oxidation resistance

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of laser clad Ti5Si3/γ/TiSi composite coatings on γ-TiAl intermetallic alloy. Surf. Coat. Tech.,

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200 (2006), pp. 4462-4470.

[8] D.D. Gu, C.L. Ma, M.J. Xia, D.H. Dai, and Q.M. Shi, A Multiscale Understanding of the

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Thermodynamic and Kinetic Mechanisms of Laser Additive Manufacturing. Engineering, 3(5) (2017), pp. 675-684.

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[9] D.D. Gu, M.J. Xia, and D.H. Dai, On the role of powder flow behavior in fluid thermodynamics and laser processability of Ni-based composites by selective laser melting.

CE

Int. J. Mach. Tools Manuf, 137 (2019), pp. 67-78. [10] B. Song, S. Dong, P. Coddet, H. Liao, and C. Coddet, Fabrication and microstructure

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characterization of selective laser-melted FeAl intermetallic parts. Surf. Coat. Tech., 206 (2012), pp. 4704-4709. [11] L. Löber, F.P. Schimansky, U. Kühn, F. Pyczak, and J. Eckert. Selective laser melting of a beta-solidifying TNM-B1 titanium aluminide alloy. J. Mater. Process Technol., 214 (2014), pp. 1852–1860. [12] J. Gussone, Y.-C. Hagedorn, H. Gherekhloo, G. Kasperovich, T. Merzouk and J. Hausmann, Microstructure of γ-titanium aluminide processed by selective laser melting at 18

ACCEPTED MANUSCRIPT elevated temperatures. Intermetallics, 66 (2015), pp. 133-140. [13] X. Shi, S. Ma, C. Liu, and Q. Wu. Parameter optimization for Ti-47Al-2Cr-2Nb in selective laser melting based on geometric characteristics of single scan tracks. Opt. Laser Technol., 90 (2017), pp. 71–79.

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[14] D.D. Gu, Z.Y. Wang, Y.F. Shen, Q. Li, and Y.F. Li. In-situ TiC particle reinforced Ti-Al

behavior. Appl. Surf. Sci., 255 (2009), pp. 9230-9240.

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matrix composites: Powder preparation by mechanical alloying and Selective Laser Melting

SC

[15] W. Li, J. Liu, S. Wen, Q. Wei, C. Yan, and Y. Shi, Crystal orientation, crystallographic

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texture and phase evolution in the Ti-45Al-2Cr-5Nb alloy processed by selective laser melting. Mater. Character., 113 (2016), pp. 125-133.

MA

[16] W. Li, M. Li, J. Liu, Y. Yang, and Y. Shi, Microstructure control and compressive properties of selective laser melted Ti-43.5Al-6.5Nb-2Cr-0.5B alloy: Influence of reduced

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graphene oxide (RGO) reinforcement. Mater. Sci. Eng., A, 743 (2019), pp. 217-222. [17] A. Simchi, and H. Pohl, Effects of laser sintering processing parameters on the

CE

microstructure and densification of iron powder. Mater. Sci. Eng., A, 359 (2003), pp. 119-128. [18] C. Hong, D.D. Gu, D.H. Dai, S.N. Cao, M. Alkhayat, Q.B. Jia, A. Gasser, A. Weisheit, I.

AC

Kelbassa, M.L. Zhong, and R. Poprawe, High-temperature oxidation performance and its mechanism of TiC/Inconel 625 composites prepared by laser metal deposition additive manufacturing. J. Laser Appl., 27 (2015), pp. S17005. [19] G.A. Greene, C.C. Finfrock, Oxidation of Inconel 718 in air at high temperatures. Oxid. Met., 55(5-6) (2001), pp. 505-521. [20] H. Xiao, Y. Chen, J. He, S. Zhao, and W. Chen, Cyclic oxidation of Ti3AlC2-Al2O3 /TiAl3 composite at 700-1000 ℃. Heat Treat. Met., 43(6) (2018), pp. 33-40. 19

ACCEPTED MANUSCRIPT [21] C.L. Ma, D.D. Gu, D.H. Dai, M.J. Xia, and H.Y. Chen, Thermodynamic behaviour and formation mechanism of novel titanium carbide dendritic crystals within a molten pool of selective laser melting TiC/Ti–Ni composites. CrystEngComm, 19 (2017), pp. 1089-1099. [22] Y.S. Hwang, V.I. Levitas, Internal stress-induced melting below melting temperature at

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high-rate laser heating. Appl. Phys. Lett., 104 (2014), pp. 263106. [23] K. Momeni, V.I. Levitas, A phase-field approach to nonequilibrium phase transformations

RI

in elastic solids via an intermediate phase (melt) allowing for interface stresses. Phys. Chem.

SC

Chem. Phys., 18 (2016), pp. 12183.

NU

[24] B.C. Zhang, G.J. Bi, S. Nai, C.-N. Sun, and J. Wei, Microhardness and microstructure evolution of TiB2 reinforced Inconel 625/TiB2 composite produced by selective laser melting.

MA

Opt. Laser Technol., 80 (2016), pp. 186-195.

[25] Q.Y. Pan, X. Lin, W.D. Huang, Y.H. Zhou, and G.L. Zhang, Microstructure evolution of

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Cu–Mn alloy under laser rapid solidification conditions. Mater. Res. Bull., 33 (1998), pp. 1621-1633.

CE

[26] M. Das, K. Bhattacharya, S.A. Dittrick, C. Mandal, V.K. Balla, T.S.S. Kumar, A. Bandyopadhyay, and I. Manna, In situ synthesized TiB-TiN reinforced Ti6Al4V alloy

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composite coatings: Microstructure, Tribological and In-vitro Biocompatibility. J. Mech. Behav. Biomed. Mater., 29 (2014), pp. 259-271. [27] D.S. Li, Q.X. Dai, X.N. Cheng, R.R. Wang, and Y. Huang, High-temperature oxidation resistance of austenitic stainless steel Cr18Ni11Cu3Al3MnNb. J. Iron. Steel Res. Int., 19(5) (2012), pp. 74-78. [28] Y. Qin, W. Lu, D. Zhang, J. Qin, and B. Ji. Oxidation of in situ synthesized TiC particlereinforced titanium matrix composites. Mater. Sci. Eng., A, 404 (2005), pp. 42-48. 20

ACCEPTED MANUSCRIPT [29] G. Wu, K. Dash, M.L. Galano, and K.A.Q. O’Reilly. Oxidation studies of Al alloys: Part II Al-Mg alloy. Corros. Sci., 155 (2019), pp. 97-108. [30] Q.B. Jia, D.D. Gu, Selective laser melting additive manufactured Inconel 718 superalloy parts: High-temperature oxidation property and its mechanisms. Opt. Laser Technol., 62

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(2014), pp. 161-171. [31] Y. Xin, J. Lin, and Z. Ren, Microstructure Characteristics and Oxidation Resistance

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Properties of “in-situ” TiC Particle Reinforced Coatings by Laser Surface Alloying. Rare Met.

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Mater. Eng., 34(6) (2005), pp. 899-902.

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[32] J.D. Avila, and A. Bandyopadhyay. Influence of boron nitride on reinforcement to

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improve high temperature oxidation resistance of titanium. J. Mater. Res., (2019), pp. 1-11.

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Fig. 1 Powder characterization and SLM processing experiment. a The mixed powder

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material including Ti powder, Al powder and TiC particles; b The particle size distribution of the mixed powder; c Free stacking state of the mixed powder; d The interaction of laser beam

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with the powder material; e As-fabricated TiC/TiAl composites specimens.

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Fig.2 Densification behavior of SLM-fabricated TiC/TiAl composites. a η1=189 J/mm3; b

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η2=157 J/mm3; c η3=126 J/mm3. d The detailed relative density value for three different samples.

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Fig.3 Phase identification of SLM-fabricated TiC/TiAl composites. a Cross-sectional XRD

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section η3=126 J/mm3.

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pattern with a wide range 30-80º; b-e The EDS analysis of different regions within the cross

Fig.4 SEM images of cross-sectional microstructure features of SLM-fabricated TiC/TiAl

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composites. a and b η1=189 J/mm3; c η2=157 J/mm3; d η3=126 J/mm3. b The local highmagnification SEM image of a.

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Fig.5 Morphology evolution of TiC dendrites at different laser energy density. a-c η1=189 J/mm3; d-f η2=157 J/mm3; g-i η3=126 J/mm3. Fig.6 The isothermal-oxidation kinetics curves of SLM-fabricated TiC/TiAl composites at different oxidation rate exponent n. a n=1; b n=2; c n=3; d n=4; e n=6; f the logarithmic relation. Fig.7 Surface morphology of SLM-fabricated TiC/TiAl composites after oxidation (η1=189 J/mm3). a The low-magnification morphology showing two different features; b and c The 22

ACCEPTED MANUSCRIPT high-magnification morphology of region A in a; d and e The high-magnification morphology of region A in a; f The XRD characterization of the oxidized surface. Fig.8 Cross-sectional microstructure and the corresponding EDS mapping of oxidation layer of SLM-fabricated TiC/TiAl composites (η1=189 J/mm3). a and b The low-magnification

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microstructure and element distribution showing four oxidation layer structures (as marked 14 in a); c and d The high-magnification microstructure corresponding to the region A and B in

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a, respectively; e and f The high-magnification microstructure and element distribution in

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region C in d, showing the interface structure between the matrix and the 4th oxidation layer.

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Fig.9 Cross-sectional microstructure of oxidation layer of SLM-fabricated TiC/TiAl composites at different laser energy density. a η1=189 J/mm3; b η2=157 J/mm3; c η3=126

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J/mm3.

Fig.10 The schematic of high-temperature oxidation mechanism of SLM-fabricated TiC/TiAl

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composites.

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Highlights TiC/TiAl composites were synthesized by SLM based on multi-component powder;

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Nonequilibrium melting behavior of TiC particles was further disclosed.

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Detailed high-temperature oxidation mechanism of TiC/TiAl composites was revealed.

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