High-temperature oxidation performance and its mechanism of TiC/Inconel 625 composites prepared by laser metal deposition additive manufacturing Open Access

Inconel 625 is a solid-solution or/and precipitation strengthened nickel-based superalloy, exhibiting the good combination of the superior mechanical properties and the good workability in the highly aggressive environments at the elevated temperatures.^1,2 Inconel 625 has the merits of the improved balance of the tensile, fatigue, and creep properties, favoring its wide use in the aerospace, automotive, and nuclear industries.³ Moreover, Inconel 625 is featured by the good resistibility to the harsh working conditions, e.g., hot corrosion and severe oxidation environments, which makes it an attractive candidate as hot-end structure components.⁴ Among the above essential properties of Inconel 625, the high-temperature oxidation resistance has become more and more important, since the poor oxidation performance of any thermo-resistance component may result in dealloying, surface spalling, or even ultimately failure.⁵ Furthermore, the development of higher temperature resistant and more reliable Inconel 625 parts needs to be accelerated to meet the demanding requirements of the modern industry.

Typically, the incorporation of the hard and temperature resistant ceramic particles within the Inconel matrix to produce metal matrix composites (MMCs) is regarded as a promising method to improve the mechanical performance of Inconel alloys.^6–9 In Wilson and Shin's work, the titanium carbide (TiC) reinforcement particles were embedded in Inconel 690 with laser direct deposition to build the functionally gradient MMCs. Microhardness and wear resistance tests showed a significant improvement with increased TiC content.⁶ Nurminen et al applied the laser cladding to produce the Inconel 625 MMCs coatings reinforced with 50 vol. % chromium carbide (CrC). The MMCs offered the sound abrasion resistance and most of the original carbides were dissolved and reformed in the matrix.⁷ Liu et al reported an investigation of the effects of laser surface treatment on the corrosion and wear performance of Inconel 625 based WC HVOF (high-velocity oxy-fuel) sprayed MMCs coatings. The results indicated that the significant improvement of corrosion and wear resistance was achieved after laser treatment as a result of the elimination of the discrete splat-structure and porosity, and also the reduction of compositional gradient between the WC and the matrix due to the formation of interfacial phases.⁸ Jiang et al developed the nano-TiC particle reinforced Inconel 625 composite coatings by laser cladding of Inconel 625 + 5 wt. % TiC powder mixture. The hardness and modulus of the nano-particle reinforced MMCs increased by 10.33% and 12.39%, respectively, as compared to the laser cladded Inconel 625 substrate.⁹ MMCs have accordingly attracted extensive attentions and are considered technically superior because of their high specific modulus, high specific strength, and high strength at elevated temperatures. However, the limited densification rate and inhomogeneous microstructures induced by the segregation of reinforcing particles have restricted the applications of the conventionally processed MMCs. Researchers have always been pursuing the better fabrication methods to improve the performance of MMCs, e.g., using the advanced laser processing technology. A review of the existing literature reveals that the carbide (e.g., TiC, WC, and CrC) reinforcement in Inconel alloys has been studied mainly for hardness and wear resistance. Besides these properties of Inconel alloys, the high-temperature oxidation resistance becomes more and more important, since the development of the more reliable Inconel components applied in the higher temperatures is in increasing demand in the modern industries. It is now well recognized that the poor oxidation resistance of any thermo-resistance component may cause a potential risk to its service reliability, which further results in the severe degradation of its service life.¹⁰ Nevertheless, to the best of authors' knowledge, there are still no comprehensive previous studies focusing on the inherent relationship of the oxidation performance, constitution phases, and microstructures of laser processed Inconel based MMCs reinforced by carbide particles.

Laser-based additive manufacturing (AM), as the rapidly developing advanced processing technology, has demonstrated the outstanding feasibility to a broad range of applications in both industrial and engineering fields.^11,12 Unlike the conventional material removal methods, the AM technology was based on a totally opposite principle of material incremental manufacturing. Laser metal deposition (LMD) is a typical AM process, exhibiting the unique capability of consolidating powders or wire feedstock in a layer-by-layer way to form the three-dimensional parts with an almost unchallenged freedom of design.^13–15 In the case of LMD, it creates the dense metal parts directly from the user-defined configurations, using a computer-controlled handling machine coupled with a laser energy source. Due to its flexibility in materials and shapes, LMD can be applied to obtain the sound material integrity and the dimensional accuracy productions including the surface coatings, the near net shaping parts, the rebuilt, and repaired components in complex geometries.^16–18 On the other hand, during laser process, a high-power laser beam can be focused to a power density up to 10¹⁰–10¹²W/cm² and can rapidly heat a metal surface layer to a temperature up to 10⁵K, which then offers high heating/cooling rates (10⁶–10⁷K/s) for the development of fine grained phases/microstructures with novel properties.^19,20 Therefore, to date, the high melting point alloys, such as Ti-based Ti-6Al-4 V, Fe-based stainless, Ni-based superalloys, and its corresponding metal matrix composites in higher performance have been successfully prepared by LMD.¹² 

In the present study, the TiC particle reinforced nickel-based metal matrix composites (NMMCs) were prepared by LMD. The isothermal-oxidation investigations were performed on the LMD-processed Inconel 625 based parts. The oxidation kinetic plots of weight gain per unit surface area as a function of time were established. Characterizations of the oxidation products and the morphologies of the oxidation scale were carried out. Based on the experimental results and theoretical analyses, the underlying high-temperature oxidation behaviors and mechanisms were systematically elucidated, which were applicable and/or transferable to other laser-based AM technologies. The present work proves to be useful to promote a substantial understanding and improvement of high-temperature oxidation performance of LMD-processed NMMCs.

The as-used powders were the gas atomized, spherical Inconel 625 powder with the particle size distribution of 15–45 μm and the irregular-shaped TiC powder (99.5% purity) with the particle size distribution of 4–7 μm. The chemical compositions of Inconel 625 powder are listed in Table I. The Inconel 625 and TiC components, according to the weight ratios of 97.5: 2.5 and 95.0: 5.0, were homogeneously mixed in a planetary mill to prepare two kinds of NMMCs, using a ball-to-powder weight ratio of 10:1, a rotation speed of the main disk of 200 rpm and a milling time of 10 h. The corresponding powder systems were termed as NMMCs1 and NMMCs2, respectively.

The LMD processing system consisted of a Nd:YAG laser source with a maximum output power of 3 kW and a focused spot diameter of 0.6 mm, a powder feed system, a five-axis CNC machine, and a standard optics equipped with a coaxial powder nozzle. The commonly used C45 carbon steel was taken as the substrate material, considering the experimental facility. The oxidation behavior of the LMD-processed TiC/Inconel 625 composites was studied mainly through the microstructural characterization of the upper surfaces of the deposited samples. Therefore, the elemental contamination from the carbon steel substrate material was negligible. The as-prepared TiC/Inconel 625 powder (NMMCs1 and NMMCs2) was injected into the melted pool through the nozzle with Argon as carrier gas, using a powder feeding rate of 2.4 g/min. Through a series of preliminary experiments, the laser power was optimized at 600 W and the scan speed was set at 500 mm/min. Three main parameters were involved in LMD process, i.e., spot diameter (D), laser power (P), and scan speed (v). The “laser energy density” of 255 J/mm³, which was defined by ¹² 

was used to estimate the laser energy input to the track being deposited. Ten coherently welded tracks were cladded for each layer and four layers were deposited on the substrate to produce the desired three-dimensional parts. For comparative testing, the Inconel 625 alloy samples were also deposited using the same LMD processing conditions.

The relative density of the LMD-processed NMMCs1 and NMMCs2 parts was determined based on the Archimedes' principle. The LMD-processed parts were further cut in half by wire-cutting electrical discharge machining to obtain cross sections. The obtained specimens in a rectangular contour were ground with the SiC abrasive paper. The as-prepared specimens were ultrasonically rinsed with ethanol and then dried in desiccator for high-temperature oxidation tests. Prior to oxidation tests, the laboratory muffle furnaces were preheated up to the corresponding service temperatures. The alumina crucibles were heated repeatedly until there were no mass fluctuations. Afterward, the crucibles with specimens inside were subjected to oxidation environments and weighted precisely at each predetermined time. The weight changes of the specimens were measured using an electronic balance capable of weighting to a precision of 0.1 mg. The weight gains were measured using the following equation:

where $ΔW/S$ represents for mass gain per unit area (mg/cm²), $Wt$ is the weight before oxidation, $W0$ is the weight after oxidation, and $S0$ is the surface area before oxidation.

Samples for metallographic observations were ground, polished, and electrolytic etched with 5% oxalic acid according to the standard procedures. Phase identification of oxidized products was determined by a D8 Advance X-ray diffractometer (XRD) with Cu Kα radiation at 40 kV and 40 mA, using a continuous scan mode. The microstructures on the cross sections of LMD-processed parts and on the oxidized surface of the samples were characterized by an Olympus PMG3 optical microscope (OM) and a Hitachi S-4800 scanning electron microscopy (SEM), fitted with an EDAX Genesis energy dispersive X-ray spectrometer (EDX) for the determination of chemical compositions. The X-ray photoelectron spectra of samples were determined by a Thermo ESCALAB 250 X-ray photoelectron spectroscopy (XPS). The acquisition parameters were as follows: Source type Al Kα, spot size 500 μm, pass energy 30.0 eV, and energy step size 0.050 eV. The identification of peaks was performed by reference to the standard XPS database.²¹ 

Figure 1 shows the etched cross sections of the deposited layers in LMD-processed Inconel 625 based parts with various materials combinations. Regardless of the contents of the TiC reinforcement added, the density of the Inconel 625 based parts after LMD process was generally high, free of any apparent pores or cracks. The quantitative measurement of the density of the LMD-processed samples using the Achimedes principle revealed that all the processed samples were nearly fully dense with the relative density approaching 100%. The LMD-processed parts consisted of metallurgically bonded layers, showing clear, stable, and continuous configurations of the solidified molten pool (Fig. 1).

The characteristic LMD-processed microstructures of the pure Inconel 625 and the corresponding NMMCs1 and NMMCs2 composite parts are illustrated in Fig. 2. A columnar dendrite structure with a considerably refined crystalline size was obtained for LMD-processed pure Inconel 625 part (Fig. 2(a)). Based on the results of Zhong and Liu¹⁹ and Boccalini and Goldenstein,²² the high cooling rate within the high-energy laser induced molten pool could reach above 10⁶K/s, facilitating the formation of fine crystalline grain structures. On the other hand, most of the heat within the molten pool was dissipated through the substrate or previously solidified materials during the laser multilayer cladding process. This positive temperature gradient from the top to bottom provided the thermodynamic possibilities for the formation of typical columnar dendrite phase morphology. For the LMD-processed NMMCs1 and NMMCs2 composite parts, the TiC reinforcement particles were found to be dispersed homogeneously at the grain boundaries of the dendrite matrix (Figs. 2(b) and 2(c)). Interestingly, the columnar dendrites became finer on increasing the amount of particle additions from 2.5 wt. % to 5.0 wt. % TiC, which were sufficiently proved by the significantly decreased distance of adjacent primary dendrites at the same magnification. The inherent grain refinement mechanism was believed to be caused by the inhibitory effect of the incorporated TiC particles on the growth of the Inconel matrix. The inhibitory effect of the incorporated reinforcing particles on the crystal growth of the matrix was testified in our previous work on laser processing of the WC particle reinforced Cu matrix composites,²³ and this phenomenon/mechanism was general for the melting/solidification process of particle reinforced MMCs.

The respective kinetic curves of isothermal-oxidation (i.e., mass gain per unit area as a function of time) for LMD-processed Inconel 625 and the corresponding NMMCs1 and NMMCs2 composites at 800 °C are plotted in Fig. 3. The oxidation kinetic behaviors of all oxidized parts revealed that the mass gain increased gradually as the exposure time extended. There was an initial decrease in oxidation rate that then seemed to stabilize at a constant rate after 20–25 h oxidation. Meanwhile, the mass gain per unit area decreased with the increment of the TiC reinforcement contents. The mass gain of pure Inconel 625 at 800 °C for 100 h was 1.4130 mg/cm², whereas the mass gain of LMD-processed NMMcs1 and NMMCs2 composites were only 0.5475 and 0.3233 mg/cm², respectively. It was accordingly reasonable to conclude that the incorporation of TiC reinforcement in Inconel 625 matrix improved the oxidation resistance of LMD-processed parts and the improvement effect was more significant with increasing the TiC content in the present materials system.

Figure 4 depicts the oxidation kinetic curves of LMD-processed Inconel 625/5.0 wt. % TiC (NMMCs2) parts with respect to the subjected temperatures at the range of 600–1000 °C. As revealed from the figure, the mass gain of LMD-processed NMMCs2 parts increased significantly on increasing the subjected temperature above 800 °C. The mass gain of the LMD-processed NMMCs2 part at 1000 °C for 100 h was measured to be 4.1352 mg/cm², whereas the mass gain of the part at 600 °C for 100 h was 0.2524 mg/cm², which was only 6% of the former.

The mass gain data shown in Figs. 3 and 4 indicate that the oxidation behavior well follows the parabolic rate law in the present study. This parabolic behavior existed between the mass gain and the oxidation time suggests a diffusion process as the rate-limiting step in the oxidation mechanism.²⁴ The mass gain of LMD-processed parts during the isothermal-oxidation process follows the parabolic relationship that can be expressed by²⁵ 

where $Kp$ and $t$ are rate constant and oxidation time, respectively. By use of the least square analysis of the oxidation kinetics, the parabolic rate constants of LMD-processed pure 625, NMMCs1, and NMMCs2 parts are determined to be 19.97 × 10⁻², 2.99 × 10⁻², and 1.1 × 10⁻² mg² cm⁻⁴ h⁻¹. The calculated values of $Kp$ reveal that the parabolic rate constant decreased greatly because of the incorporation of TiC reinforcing particles, which further confirms that the LMD-processed TiC/Inconel 625 composites possess better oxidation resistance than the pure Inconel 625 parts.

Normally, the rate constant $Kp$ follows an Arrhenius relation as follows:²⁵ 

where Q is the effective activation energy for oxidation, A is the constant for a given material, T is the absolute temperature, and R is the universal gas constant. Figure 5 shows the variation of $lnKp$ with the reciprocal of the absolute temperature (⁠$1/T$⁠). Based on Eq. (4), the slope of the best-fit line in Fig. 5 can be used to determine the value of $−Q/R$⁠. The activation energy for the oxidation of the LMD-processed NMMCs2 parts in the range of 600–1000 °C was accordingly calculated roughly to be ∼129 kJ/mol, which contributes to the formation of the different oxidation products. The constitutional phases, chemical compositions, and micro-structural features of the oxidation products are studied and presented in the following sections C and D.

Figure 6 depicts the typical XRD patterns of the LMD-processed pure Inconel 625 and composite parts in different reinforcement contents after oxidation for 100 h at 800 °C. The strong diffraction peaks corresponding to γ (Ni-Cr) matrix and Cr₂O₃ phases were captured by the X-ray in all conditions. A small amount of TiO₂ was detected from XRD results within the oxidation layer of the composite parts. Interestingly, the 2θ locations of γ and Cr₂O₃ phases in LMD-processed composite parts generally shifted to higher angles. A significant shift in XRD peaks of a certain phase means that there is a change in its lattice parameter, most probably due to the incorporation of elements in solution.²⁶ In the present study, it is very likely that some small-sized TiC reinforcing particles dissolve in the liquid, resulting in the incorporation of Ti and/or C in the Ni-Cr γ solution.

Figure 7 shows the XRD spectra of the LMD-processed NMMCs2 parts without the oxidation tests and with 100 h oxidation at temperature range of 600–1000 °C. Diffraction peaks corresponding to γ and Cr₂O₃ phases (major phases) and TiO₂ phase in a small amount were detected in all the oxidized samples. The TiC diffraction peaks, which appeared in the initial samples without the oxidation tests, disappeared completely in the samples after the oxidation treatment. As the oxidation temperature increased to 1000 °C, the peak intensity of γ matrix decreased, while the Cr₂O₃ peaks experienced an opposite trend. The clear reduction (or even absence) of the TiO₂ peaks were also observed in samples oxidized at 1000 °C. The variation concerning the peak intensity of γ matrix and Cr₂O₃ phase indicated that the thickness of oxidation layer on the surface of samples, mainly consisting of the Cr₂O₃, increased significantly on increasing the subjected temperatures to 1000 °C.

Figure 8(a) depicts the wide energy range survey of the LMD-processed NMMCs2 sample experienced 100 h oxidation at 800 °C, in which the XPS peaks of Cr, Ti, Fe, O, and C were detected. Based on this survey, data were further acquired for the Cr2p (595.1–571.3 eV), Ti2p (468.8–453.0 eV), Fe2p (739.2–705.8 eV), O1s (536.0–525.9 eV), and C1s (294.8–280.4 eV) regions, as revealed in Figs. 8(b)–8(f), respectively. It showed that the Cr2p spectra consisted of three peaks at 586.30 eV, 576.80 eV, and 575.90 eV, which were identified as Cr 2p1/2 Cr₂O₃, Cr 2p3/2 Cr₂O₃, and Cr 2p3/2 Cr₂O₃ (Fig. 8(b)). The detected peaks in the Ti2p spectra located at 464.19 eV and 458.00 eV, which corresponded to C1s Ti 2p1/2 TiO₂, and Ti 2p3/2 TiO₂ (Fig. 8(c)). Meanwhile, there was no significant XPS peak in the Fe2p scan spectra (Fig. 8(d)). Thus, it was reasonable to conclude that the oxidized surface of the LMD-processed NMMCs2 part after 100 h oxidation at 800 °C was mainly composed of Cr₂O₃ and TiO₂, which was in accordance with the XRD results (Figs. 6 and 7). The atomic percentage of the elements concerned was determined based on the experimentally determined sensitivity factors (F) and the intensity (I) of a photoelectron peak which was taken as the integrated area under the peak following the subtraction of a linear background.²⁷ The quantification of compositions based on XPS method showed that the atomic fractions of the detected Cr, Ti, Fe, O, and C elements were 11.55 at. %, 7.25 at. %, 0.68 at. %, 43.7 at. %, and 36.82 at. %, respectively.

The above experimental results regarding the high-temperature oxidation behaviors of the LMD-processed pure Inconel 625 and the corresponding composites reveal that the oxidation layer is composed of protective Cr₂O₃ and TiO₂. As the LMD-processed NMMCs samples are subjected to high-temperature environments, the following reactions will occur on basis of the Ellingham-Richardson principle, leading to oxide formation

The respective values of standard Gibbs energies changes as a function of temperature (T) for the above reactions can be determined from the following equations:^28,29

Apparently, these equations suggest that the above oxidation reactions generally initiate thermodynamically, since the corresponding Gibbs free energy values are negative in the whole research temperature range (600–1000 °C). Moreover, as can be found from Eqs. (9) and (10), the Gibbs free energy for reaction (7) is higher that of (6), indicating that the reaction (7) is likely to take place thermodynamically at the first stage of oxidation.

The characteristic surface features of the LMD-processed pure Inconel 625 parts and TiC/Inconel 625 composite parts after 100 h oxidation at 800 °C are shown in Figs. 9(a), 9(c), and 9(e), respectively. The corresponding SEM micrographs obtained using a higher magnification were also included to accurately reflect the microstructural features of the oxidized surfaces, as revealed in Figs. 9(b), 9(d), and 9(f), respectively. The surface of the oxidized pure Inconel 625 parts presented the inhomogeneous microstructures characterized by the cracks and locally raised areas (Fig. 9(a)). High-magnification micrograph revealed that the “mismatch behavior” within the oxidation film, i.e., the formation of residual microcracks and resultant imperfection of the oxidation film, was regarded as the primary reason for the relatively roughness surface (Fig. 9(b)). Meanwhile, the granular oxides with large-sized particles embedded were detected along the edges of the mismatched areas, indicating that the sample experienced severe oxidation attack in this circumstance. Differently, the considerably compact oxidation film was formed on the oxidized surface of the LMD-processed NMMCs1 part, although a few large-sized oxides in a faceted structure were observed at a higher magnification (Figs.9(c) and 9(d)). For the LMD-processed NMMCs2 part, the sample presented the compact, flat, and homogeneous oxidized surface (Fig. 9(e)). The significantly refined granular oxides in a uniform size distribution were observed on the present oxidized surface (Fig. 9(f)). The composite parts possessed the sound surface integrity compared with the pure Inconel 625 part under the same oxidation conditions, which contributed to the fact that the LMD-processed composites have the finer-grained microstructures due to the incorporation of TiC reinforcing particles. The EDX analyses of the chemical compositions of the oxidation layers are summarized in Table II. The well-developed crystals formed on the oxidized surfaces of LMD-processed pure Inconel 625 parts were mainly consisted of Cr and O elements. For the LMD-processed TiC/Inconel 625 composites, the Ti element was also detected in the oxidized layers, besides the presence of Cr and O elements. EDX results were in good agreement with the former XRD and XPS analyses, which demonstrated that the oxidized layers of the composites were consisted of Cr₂O₃ and TiO₂.

Figure 10 illustrates the typical surface morphologies of NMMCs2 parts oxidized at the temperature range of 600–1000 °C. The obtained surface morphologies experienced the dramatic changes by increasing the subjected temperature environments. It was observed that some dispersed spherical oxides started to grow at the oxidized temperature of 600 °C (Fig. 10(a)). The deep microcracks were found on the surface of the spherical oxides at a magnified state, which might attribute to the complex stresses developed during the oxidation process (Fig. 10(b)). EDX results indicated that the spherical oxides were composed mainly Cr and O elements as a function of selective external oxidation.³⁰ The oxidized surface became porous as the oxidized temperature increased from 800 °C to 1000 °C (Fig. 9(e) versus Fig. 10(c)). Meanwhile, the oxidation particles size increased significantly, as shown in higher magnifications (Fig. 9(f) versus Fig. 10(d)). Based on the theory of Kumar et al.,³¹ it was pointed out that Cr₂O₃ might be oxidized into CrO₃ gas at higher temperatures. The volatilization of the generated gas during the high-temperature oxidation attack process contributed to the formation of the porous structures on the oxidized sample surface. Therefore, a further oxidation of Cr₂O₃ was regarded as a significant factor responsible for the severe oxidation of the LMD-processed NMMCs2 parts at 1000 °C.

From the above experimental results and theoretical analyses, it is verified that the oxidation behavior of the LMD-processed pure Inconel 625 parts and the corresponding composite parts is a diffusion-controlled process, i.e., the process is controlled by the inward penetration of oxygen and outward diffusion of oxides forming elements. The underlying oxidation mechanisms are established and can be described as follows.

During the initial stage of the oxidation process, the primary mechanism of oxidation is the chemical adsorption of the oxygen occurred between the sample surface and the ambient atmosphere.³² The oxides tend to nucleate preferentially along the grain boundaries in the surface layer that provide the favorable sites for heterogeneous nucleation.³³ The complete formation and covering of the oxidation film on the oxidized surface, which is typically consisted of Cr₂O₃ and TiO₂, as revealed in Fig. 8, is realized by means of the growth of oxides nuclei and the subsequent conjunction to each other. As the oxidation time prolongs to the stage that a compact and continuous oxidation film is formed on the sample surface, the further oxidation process and weight gain behavior are mainly controlled by the element diffusion through grain boundaries.^32,33 Actually, the protective oxide scales, which are composed mainly of very small oxide grains, favor the plastic deformation and creep of the scales. Therefore, the thermal stress produced during the weighting process can effectively release through the deformation of the scales, keeping the integrity of the oxidation film.^29,34

It is believed that the density of grain boundaries played a significant role in the nucleation of oxide formations; in other words, the grain refinement of the initially untreated samples can improve the oxidation resistance at elevated temperatures.³⁵ For the oxidation of composites parts, the reason for the encouraging results is that the incorporation of TiC particles can form dense and compact oxidation film that increases the oxidation resistance.²⁹ As elucidated previously, the incorporated TiC particles play an inhibitory effect on the grain growth of the Inconel matrix, which decreases the columnar grain size greatly. The TiC particles dispersed at grain boundaries can also act as heterogeneous nucleation sites of the formed oxides by providing high surface areas to oxygen and reduce the internuclear distance, limiting the lateral growth of oxides and resulting in the grain refinement. Henceforth, the increased nucleation sites for oxides can accelerate the formation process of an integrated oxidation film, which protects the base alloy from further oxidation. On the other hand, the uniformly distributed TiC ceramic particles can act as the oxygen diffusion barrier during the high-temperature oxidation process,³⁶ which plays a significant role in decreasing the high-temperature oxidation attack of Inconel matrix, thereby favoring the practical engineering application of the LMD-processed NMMCs parts at elevated temperatures.

The oxidation behaviors of the LMD-processed NMMCs2 parts oxidized at the temperature range of 600–1000 °C reveal that the composite parts exhibited excellent oxidation resistance under the subjected temperature of 800 °C. However, the NMMCs2 part experienced severe oxidation attack on increasing the subjected temperature to 1000 °C. The generated gas oxides from the further oxidation of Cr₂O₃ and its subsequent vaporization cause the formation of porous oxidation scale on the sample surface. Such a porous oxidation structure induces poor oxidation resistance, which is regarded as the primary factor for the decrease of oxidation resistance at the temperature of 1000 °C. Therefore, the significant researches efforts are still needed to focus on the oxidation behaviors of the LMD-processed NMMCs parts to obtain a protective oxide layer to decrease the diffusion rate of oxygen and the transformation rate from Cr₂O₃ to CrO₃ at the higher temperatures.

The high-temperature oxidation behavior of the LMD-processed TiC/Inconel 625 composites was systematically studied and the main conclusions were summarized as follows:

1. The incorporation of TiC reinforcement in Inconel 625 matrix improved the oxidation resistance of the LMD-processed parts, and the improvement function was more significant with increasing the TiC content from 2.5 wt. % to 5.0 wt. % in the composite system. The mass gain after 100 h oxidation at 800 °C decreased from 1.4130 mg/cm² for the LMD-processed Inconel 625–0.3233 mg/cm² for the LMD-processed Inconel 625/5.0 wt. % TiC composites.

2. The oxidized surface of the LMD-processed pure Inconel 625 parts was mainly consisted of Cr₂O₃. For the LMD-processed TiC/Inconel 625 composites, the oxidized layers on the surface were composed of Cr₂O₃ and TiO₂.

3. The incorporation of TiC reinforcing particles had the inhibitory effect on the grain growth of the Inconel matrix, leading to an inherent grain refinement in the LMD-processed composites. The composite parts accordingly possessed the sound surface integrity after oxidation compared with the pure Inconel 625 part under the same oxidation conditions.

4. The LMD-processed Inconel 625/5.0 wt. % TiC composites exhibited the excellent oxidation resistance under the oxidation temperature of 800 °C. A further increase in the oxidation temperature to 1000 °C caused the severe oxidation attack on the LMD-processed composites, due to the unfavorable further oxidation of Cr₂O₃ to CrO₃ at elevated treatment temperatures.

The authors appreciate the financial support from the Sino-German Centre (No. GZ712), the National Natural Science Foundation of China (Nos. 51322509 and 51104090), the Outstanding Youth Foundation of Jiangsu Province of China (No. BK20130035), the Program for New Century Excellent Talents in University (No. NCET-13-0854), the Science and Technology Support Program (The Industrial Part), Jiangsu Provincial Department of Science and Technology of China (No. BE2014009-2), the Program for Distinguished Talents of Six Domains in Jiangsu Province of China (No. 2013-XCL-028), the Fundamental Research Funds for the Central Universities (No. NE2013103), and the Qing Lan Project, Jiangsu Provincial Department of Education of China.