High-temperature oxidation performance and its mechanism of TiC/Inconel 625composites prepared by laser metal deposition additive manufacturingChen HongDongdong Gu, Donghua Dai, and Sainan CaoMoritz AlkhayatQingbo JiaAndres Gasser and AndreasWeisheitIngomar KelbassaMinlin ZhongReinhart Poprawe

Citation: J. Laser Appl. 27, S17005 (2015); doi: 10.2351/1.4898647View online: http://dx.doi.org/10.2351/1.4898647View Table of Contents: http://lia.scitation.org/toc/jla/27/S1Published by the Laser Institute of America

High-temperature oxidation performance and its mechanism of TiC/Inconel625 composites prepared by laser metal deposition additive manufacturing

Chen HongChair for Laser Technology LLT, RWTH Aachen, Steinbachstraße 15, D-52074 Aachen, Germany

Dongdong Gu,a) Donghua Dai, and Sainan CaoCollege of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics,Yudao Street 29, 210016 Nanjing, People’s Republic of China and Institute of Additive Manufacturing(3D Printing), Nanjing University of Aeronautics and Astronautics, Yudao Street 29, Nanjing 210016,People’s Republic of China

Moritz AlkhayatChair for Laser Technology LLT, RWTH Aachen, Steinbachstraße 15, D-52074 Aachen, Germany

Qingbo JiaCollege of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics,Yudao Street 29, 210016 Nanjing, People’s Republic of China and Institute of Additive Manufacturing(3D Printing), Nanjing University of Aeronautics and Astronautics, Yudao Street 29, Nanjing 210016,People’s Republic of China

Andres Gasser and Andreas WeisheitFraunhofer Institute for Laser Technology ILT, Steinbachstraße 15, D-52074 Aachen, Germany

Ingomar KelbassaChair for Laser Technology LLT, RWTH Aachen, Steinbachstraße 15, D-52074 Aachen, Germany andFraunhofer Institute for Laser Technology ILT, Steinbachstraße 15, D-52074 Aachen, Germany

Minlin ZhongSchool of Materials Science and Engineering, Tsinghua University, 100084 Beijing,People’s Republic of China

Reinhart PopraweChair for Laser Technology LLT, RWTH Aachen, Steinbachstraße 15, D-52074 Aachen, Germany andFraunhofer Institute for Laser Technology ILT, Steinbachstraße 15, D-52074 Aachen, Germany

(Received 13 February 2014; accepted for publication 7 October 2014; published 9 December 2014)

The laser metal deposition (LMD) additive manufacturing process was applied to produce TiC/Inconel

625 composite parts. The high-temperature oxidation performance of the LMD-processed parts and

the underlying physical/chemical mechanisms were systematically studied. The incorporation of the

TiC reinforcement in the Inconel 625 improved the oxidation resistance of the LMD-processed parts,

and the improvement function became more significant with increasing the TiC addition from 2.5 wt.

% to 5.0 wt. %. The mass gain after 100 h oxidation at 800 �C decreased from 1.4130 mg/cm2 for the

LMD-processed Inconel 625 to 0.3233 mg/cm2 for the LMD-processed Inconel 625/5.0 wt. % TiC

composites. The oxidized surface of the LMD-processed Inconel 625 parts was mainly consisted of

Cr2O3. For the LMD-processed TiC/Inconel 625 composites, the oxidized surface was composed of

Cr2O3 and TiO2. The incorporation of the TiC reinforcing particles favored the inherent grain refine-

ment in the LMD-processed composites and, therefore, the composite parts possessed the sound sur-

face integrity after oxidation compared with the Inconel 625 parts under the same oxidation

conditions. The LMD-processed TiC/Inconel 625 composites exhibited the excellent oxidation resist-

ance under the oxidation temperature of 800 �C. A further increase in the oxidation temperature to

1000 �C caused the severe oxidation attack on the composites, due to the unfavorable further oxidation

of Cr2O3 to CrO3 at the elevated treatment temperatures. VC 2014 Laser Institute of America.

[http://dx.doi.org/10.2351/1.4898647]

Key words: additive manufacturing, laser metal deposition (LMD), metal matrix composites,

oxidation

I. INTRODUCTION

Inconel 625 is a solid-solution or/and precipitation

strengthened nickel-based superalloy, exhibiting the good

combination of the superior mechanical properties and the

a)Author to whom correspondence should be addressed; electronic mail:

dongdonggu@nuaa.edu.cn

1938-1387/2015/27(S1)/S17005/11/$28.00 VC 2014 Laser Institute of AmericaS17005-1

JOURNAL OF LASER APPLICATIONS LASER ADDITIVE MANUFACTURING FEBRUARY 2015

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 prop-

erties, 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 compo-

nents.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 pro-

duce metal matrix composites (MMCs) is regarded as a prom-

ising 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 function-

ally gradient MMCs. Microhardness and wear resistance tests

showed a significant improvement with increased TiC con-

tent.6 Nurminen et al applied the laser cladding to produce the

Inconel 625 MMCs coatings reinforced with 50 vol. % chro-

mium 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 investiga-

tion 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

625þ 5 wt. % TiC powder mixture. The hardness and modu-

lus 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 par-

ticles 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 oxida-

tion 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 mod-

ern industries. It is now well recognized that the poor oxida-

tion 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 demon-

strated the outstanding feasibility to a broad range of applica-

tions 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 incre-

mental manufacturing. Laser metal deposition (LMD) is a

typical AM process, exhibiting the unique capability of con-

solidating powders or wire feedstock in a layer-by-layer way

to form the three-dimensional parts with an almost unchal-

lenged freedom of design.13–15 In the case of LMD, it creates

the dense metal parts directly from the user-defined configu-

rations, 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 pro-

ductions including the surface coatings, the near net shaping

parts, the rebuilt, and repaired components in complex geo-

metries.16–18 On the other hand, during laser process, a high-

power laser beam can be focused to a power density up to

1010–1012 W/cm2 and can rapidly heat a metal surface layer

to a temperature up to 105 K, which then offers high heating/

cooling rates (106–107 K/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 per-

formance 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 per-

formed 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 oxida-

tion behaviors and mechanisms were systematically eluci-

dated, 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 improve-

ment of high-temperature oxidation performance of LMD-

processed NMMCs.

II. EXPERIMENTAL PROCEDURES

A. Powder preparation

The as-used powders were the gas atomized, spherical

Inconel 625 powder with the particle size distribution of

S17005-2 J. Laser Appl., Vol. 27, No. S1, February 2015 Hong et al.

15–45 lm and the irregular-shaped TiC powder (99.5% pu-

rity) with the particle size distribution of 4–7 lm. The chemi-

cal 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.

B. LMD process

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 parame-

ters 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

LED ¼ P

p D=2ð Þ2 � v; (1)

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 compara-

tive testing, the Inconel 625 alloy samples were also depos-

ited using the same LMD processing conditions.

C. Investigation of oxidation performance

The relative density of the LMD-processed NMMCs1

and NMMCs2 parts was determined based on the

Archimedes’ principle. The LMD-processed parts were fur-

ther cut in half by wire-cutting electrical discharge machining

to obtain cross sections. The obtained specimens in a rectan-

gular 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 ca-

pable of weighting to a precision of 0.1 mg. The weight gains

were measured using the following equation:

DW=S ¼ ðWt �W0Þ=S0; (2)

where DW=S represents for mass gain per unit area (mg/cm2),

Wt is the weight before oxidation, W0 is the weight after oxi-

dation, and S0 is the surface area before oxidation.

D. Characterization of microstructures andcompositions

Samples for metallographic observations were ground,

polished, and electrolytic etched with 5% oxalic acid accord-

ing to the standard procedures. Phase identification of

oxidized products was determined by a D8 Advance X-ray

diffractometer (XRD) with Cu Ka 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 oxi-

dized 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 Ka, spot size 500 lm, 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

III. RESULTS AND DISCUSSION

A. Microstructures of LMD-processed parts

Figure 1 shows the etched cross sections of the deposited

layers in LMD-processed Inconel 625 based parts with vari-

ous 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 metallurgi-

cally 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 colum-

nar dendrite structure with a considerably refined crystalline

size was obtained for LMD-processed pure Inconel 625 part

TABLE I. Chemical compositions of Inconel 625 powder (In weight per-

cent, wt. %).

Cr Fe Ni Nb Mo Al Ti C

22.65 2.9 Balance 3.53 8.73 0.16 0.2 0.01

J. Laser Appl., Vol. 27, No. S1, February 2015 Hong et al. S17005-3

(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

106 K/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 previ-

ously solidified materials during the laser multilayer clad-

ding process. This positive temperature gradient from the top

FIG. 1. OM images showing cross sections of the deposited layers in the LMD-processed Inconel 625 based parts using different materials combinations: (a)

Pure Inconel 625 alloy; (b) Inconel 625/2.5 wt. % TiC (termed as NMMCs1); and (c) Inconel 625/5.0 wt. % TiC (termed as NMMCs2).

FIG. 2. SEM micrographs showing characteristic microstructures on cross sections of the LMD-processed parts using: (a) Pure Inconel 625 alloy; (b) Inconel

625/2.5 wt. % TiC (NMMCs1); and (c) Inconel 625/5.0 wt. % TiC (NMMCs2).

S17005-4 J. Laser Appl., Vol. 27, No. S1, February 2015 Hong et al.

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 dis-

persed homogeneously at the grain boundaries of the den-

drite 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 suffi-

ciently 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 par-

ticles 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.

B. Oxidation kinetics of LMD-processed parts

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 ox-

idation 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 oxi-

dation. 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 �Cfor 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 mecha-

nism.24 The mass gain of LMD-processed parts during the

isothermal-oxidation process follows the parabolic relation-

ship that can be expressed by25

DW=S ¼ ðKptÞ1=2; (3)

where Kp and t are rate constant and oxidation time, respec-

tively. 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

Kp ¼ A exp�Q

RT

� �; (4)

FIG. 3. Isothermal-oxidation kinetics of the LMD-processed Inconel 625,

Inconel 625/2.5 wt. % TiC (NMMCs1), and Inconel 625/5.0 wt. % TiC

(NMMCs2) parts.

FIG. 4. Plots of mass gain versus exposure time for the LMD-processed

Inconel 625/5.0 wt. % TiC (NMMCs2) parts at different oxidation

temperatures.

J. Laser Appl., Vol. 27, No. S1, February 2015 Hong et al. S17005-5

where Q is the effective activation energy for oxidation, A is

the constant for a given material, T is the absolute tempera-

ture, and R is the universal gas constant. Figure 5 shows the

variation of lnKp with the reciprocal of the absolute tempera-

ture (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 acti-

vation 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 consti-

tutional phases, chemical compositions, and micro-structural

features of the oxidation products are studied and presented

in the following sections C and D.

C. Phases and compositions identification andchemical thermodynamic analysis

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 c (Ni-Cr) ma-

trix and Cr2O3 phases were captured by the X-ray in all con-

ditions. A small amount of TiO2 was detected from XRD

results within the oxidation layer of the composite parts.

Interestingly, the 2h locations of c 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 c 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 c 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 c matrix decreased, while the

Cr2O3 peaks experienced an opposite trend. The clear reduc-

tion (or even absence) of the TiO2 peaks were also observed

in samples oxidized at 1000 �C. The variation concerning the

peak intensity of c 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.

FIG. 5. Arrhenius plot of lnKp with 1=T during oxidation of the LMD-

processed Inconel 625/5.0 wt. % TiC (NMMCs2) parts in the range of

600–1000 �C.

FIG. 6. XRD characterization of oxidation layers of the LMD-processed

Inconel 625, Inconel 625/2.5 wt. % TiC (NMMCs1) and Inconel 625/5.0 wt.

% TiC (NMMCs2) parts after oxidation for 100 h at 800 �C.

FIG. 7. XRD spectra of the LMD-processed Inconel 625/5.0 wt. % TiC

(NMMCs2) parts without the oxidation tests and with 100 h oxidation at

temperature range of 600–1000 �C.

S17005-6 J. Laser Appl., Vol. 27, No. S1, February 2015 Hong et al.

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 spec-

tra 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 signifi-

cant 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 sensi-

tivity factors (F) and the intensity (I) of a photoelectron

FIG. 8. XPS wide energy range survey (a) and high-resolution XPS spectra of Cr2p (b), Ti2p (c), Fe2p (d), O1s (e), and C1s (f) scans in the LMD-processed

Inconel 625/5.0 wt. % TiC (NMMCs2) part after 100 h oxidation at 800 �C.

J. Laser Appl., Vol. 27, No. S1, February 2015 Hong et al. S17005-7

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

2Cr sð Þ þ 3

2O2 gð Þ ! Cr2O3 sð Þ; (5)

TiðsÞ þ O2ðgÞ ! TiO2ðsÞ; (6)

1

2TiC sð Þ þ O2 gð Þ !

1

2TiO2 sð Þ þ 1

2CO2 gð Þ: (7)

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

DGhT1ðKJ=molÞ ¼ �753:12þ 0:1826TðKÞ; (8)

DGhT2ðKJ=molÞ ¼ �944:75þ 0:1854TðKÞ; (9)

DGhT3ðKJ=molÞ ¼ �577:464þ 0:08502TðKÞ: (10)

Apparently, these equations suggest that the above oxidation

reactions generally initiate thermodynamically, since the cor-

responding 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 reac-

tion (7) is likely to take place thermodynamically at the first

stage of oxidation.

D. Surface morphologies of oxidized samples

The characteristic surface features of the LMD-

processed pure Inconel 625 parts and TiC/Inconel 625 com-

posite 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 microstruc-

tures 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 for-

mation 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 oxida-

tion 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 magnifica-

tion (Figs.9(c) and 9(d)). For the LMD-processed NMMCs2

part, the sample presented the compact, flat, and homogene-

ous 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 condi-

tions, 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 analy-

ses 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 ele-

ment 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 experi-

enced the dramatic changes by increasing the subjected tem-

perature 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 signifi-

cantly, 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 contrib-

uted 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 ox-

idation of the LMD-processed NMMCs2 parts at 1000 �C.

E. Oxidation mechanism of LMD-processed Inconel625 based composite parts

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 correspond-

ing 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

S17005-8 J. Laser Appl., Vol. 27, No. S1, February 2015 Hong et al.

underlying oxidation mechanisms are established and can be

described as follows.

During the initial stage of the oxidation process, the pri-

mary 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 preferen-

tially 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 ele-

ment 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 oxida-

tion 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 previ-

ously, 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

TABLE II. EDX analyses showing chemical compositions of oxidation

layers of the LMD-processed parts oxidized at 800 �C for 100 h.

Sample Cr O Fe Ni Ti Mo Nb

Pure Inconel 625 29.76 59.38 2.48 6.89 0.23 0.79 0.47

Inconel 625/2.5 wt. %

TiC (NMMCs1)

27.60 60.40 0.77 0.95 9.86 0.33 0.08

Inconel 625/5.0 wt. %

TiC (NMMCs2)

29.31 61.23 0.55 1.93 6.26 0.51 0.22

FIG. 9. SEM images showing typical surface microstructures of the LMD-processed Inconel 625 based parts after oxidation for 100 h at 800 �C: (a) Pure

Inconel 625; (c) Inconel 625/2.5 wt. % TiC (NMMCs1); (e) Inconel 625/5.0 wt. % TiC (NMMCs2). (b), (d), and (f) are local magnification of (a), (c), and (e),

respectively.

J. Laser Appl., Vol. 27, No. S1, February 2015 Hong et al. S17005-9

sites of the formed oxides by providing high surface areas to

oxygen and reduce the internuclear distance, limiting the lat-

eral 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 signifi-

cant 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 resist-

ance under the subjected temperature of 800 �C. However,

the NMMCs2 part experienced severe oxidation attack on

increasing the subjected temperature to 1000 �C. The gener-

ated gas oxides from the further oxidation of Cr2O3 and its

subsequent vaporization cause the formation of porous oxi-

dation 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 sig-

nificant researches efforts are still needed to focus on the oxi-

dation 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.

IV. CONCLUSIONS

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 oxi-

dized layers on the surface were composed of Cr2O3 and

TiO2.

(3) The incorporation of TiC reinforcing particles had the in-

hibitory 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

FIG. 10. SEM images showing characteristic surface morphologies of the LMD-processed Inconel 625/5.0 wt. % TiC (NMMCs2) parts oxidized at (a) 600 �Cand (c) 1000 �C for 100 h. (b) and (d) are local magnification of (a) and (c), respectively. The oxidized surface treated at 800 �C for NMMCs2 is featured in

Figs. 9(e) and 9(f).

S17005-10 J. Laser Appl., Vol. 27, No. S1, February 2015 Hong et al.

compared with the pure Inconel 625 part under the same

oxidation conditions.

(4) The LMD-processed Inconel 625/5.0 wt. % TiC compo-

sites 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 compo-

sites, due to the unfavorable further oxidation of Cr2O3

to CrO3 at elevated treatment temperatures.

ACKNOWLEDGMENTS

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.

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