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:
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.
1L. Garimella, P. K. Liaw, and D. L. Klarstrom, “Fatigue behavior in
nickel-based superalloys: A literature review,” JOM 49, 67–71 (1997).2N. L. Richards and M. C. Chaturvedi, “Effect of minor elements on weld-
ability of nickel base superalloys,” Int. Mater. Rev. 45, 109–129 (2000).3V. Shankar, K. B. S. Rao, and S. L. Mannan, “Microstructure and mechan-
ical properties of Inconel 625 superalloy,” J. Nucl. Mater. 228, 222–232
(2001).4C. P. Paul, P. Ganesh, S. K. Mishra, P. Bhargava, J. Negi, and A. K. Nath,
“Investigating laser rapid manufacturing for Inconel-625 components,”
Opt. Laser Technol. 39, 800–805 (2007).5L. Zheng, M. Zhang, R. Chellali, and J. Dong, “Investigations on the
growing, cracking and spalling of oxides scales of powder metallurgy
Rene95 nickel-based superalloy,” Appl. Surf. Sci. 257, 9762–9767 (2011).6J. M. Wilson and Y. C. Shin, “Microstructure and wear properties of laser-
deposited functionally graded Inconel 690 reinforced with TiC,” Surf.
Coat Technol. 207, 517–522 (2012).7J. Nurminen, J. N€akki, and P. Vuoristo, “Microstructure and properties of
hard and wear resistant MMC coatings deposited by laser cladding,” Int. J.
Refract. Met. Hard Mater. 27, 472–478 (2009).8Z. Liu, J. Cabrero, S. Niang, and Z. Y. Al-Taha, “Improving corrosion and
wear performance of HVOF-sprayed Inconel 625 and WC-Inconel 625
coatings by high power diode laser treatments,” Surf. Coat Technol. 201,
7149–7158 (2007).9D. F. Jiang, C. Hong, M. L. Zhong, M. Alkhayat, A. Weisheit, A. Gasser,
H. J. Zhang, I. Kelbassa, and R. Poprawe, “Fabrication of nano-TiCp rein-
forced Inconel 625 composite coatings by partial dissolution of micro-
TiCp through laser cladding energy input control,” Surf. Coat Technol.
249, 125–131 (2014).10Q. B. Jia and D. D. Gu, “Selective laser melting additive manufactured
Inconel 718 superalloy parts: High-temperature oxidation property and its
mechanisms,” Opt. Laser Technol. 62, 161–171 (2014).11I. Kelbassa, T. Wohlers, and T. Caffrey, “Quo vadis, laser additive man-
ufacturing?,” J. Laser Appl. 24, 050101 (2012).12D. D. Gu, W. Meiners, K. Wissenbach, and R. Poprawe, “Laser additive
manufacturing of metallic components: Materials, processes and mecha-
nisms,” Int. Mater. Rev. 57, 133–164 (2012).
13L. Xue, Y. Li, and S. Wang, “Direct manufacturing of net-shape functional
components/test-pieces for aerospace, automotive, and other applications,”
J. Laser Appl. 23, 042004 (2011).14S. Wen and Y. C. Shin, “Comprehensive predictive modeling and paramet-
ric analysis of multitrack direct laser deposition processes,” J. Laser Appl.
23, 022003 (2011).15G. X. Zhu, D. C. Li, A. F. Zhang, G. Pi, and Y. P. Tang, “The influence of
laser and powder defocusing characteristic on the surface quality in laser
direct metal deposition.” Opt. Laser Technol. 44, 349–356 (2012).16C. Hong, D. D. Gu, D. H. Dai, A. Gasser, A. Weisheit, I. Kelbassa, M. L.
Zhong, and R. Poprawe, “Laser metal deposition of TiC/Inconel 718 com-
posites with tailored interfacial microstructures,” Opt. Laser Technol. 54,
98–109 (2013).17G. F. Sun, S. Bhattacharya, G. P. Dinda, A. Dasgupta, and J. Mazumder,
“Microstructure evolution during laser-aided direct metal deposition of
alloy tool steel,” Scr. Mater. 64, 454–457 (2011).18W. U. H. Syed and L. Li, “Effects of wire feeding direction and location in
multiple layer diode laser direct metal deposition,” Appl. Surf. Sci. 248,
518–524 (2005).19M. Zhong and W. Liu, “Laser surface cladding: The state of the art and
challenges,” Proc. Inst. Mech. Eng. Part C: J. Mech. Eng. Sci. 224,
1041–1060 (2010).20M. Das, V. K. Balla, D. Basu, S. Bose, and A. Bandyopadhyay, “Laser
processing of SiC-particle-reinforced coating on titanium,” Scr. Mater. 63,
438–441 (2010).21A. V. Naumkin, A. Kraut-Vass, S. W. Gaarenstroom, and C. J. Powell,
NIST X-ray Photoelectron Spectroscopy Database, NIST Standard
Reference Database 20, Version 4.1, http://srdata.nist.gov/xps/.22M. Boccalini and H. Goldenstein, “Solidification of high speed steels,” Int.
Mater. Rev. 46, 92–115 (2001).23D. D. Gu, Y. F. Shen, and J. Xiao, “Influence of processing parameters on
particulate dispersion in direct laser sintered WC–Cop/Cu MMCs,” Int. J.
Refract. Met. Hard Mater. 26, 411–422 (2008).24T. Iizuka and H. Kita, “Oxidation behavior and effect of oxidation on me-
chanical properties of Mo5Si3 particle-reinforced Si3N4 composites,”
Mater. Sci. Eng. A 374, 115–121 (2004).25L. J. Huang, L. Geng, Y. Fu, B. Kaveendran, and H. X. Peng, “Oxidation
behavior of in situ TiCp/Ti6Al4V composite with self-assembled network
microstructure fabricated by reaction hot pressing,” Corros. Sci. 69,
175–180 (2013).26D. D. Gu, Y. C. Hagedorn, W. Meiners, G. B. Meng, R. J. S. Batista, K.
Wissenbach, and R. Poprawe, “Densification behavior, microstructure evo-
lution, and wear performance of selective laser melting processed com-
mercially pure titanium,” Acta Mater. 60, 3849–3860 (2012).27J. F. Watts and J. Wolstenholme, An Introduction to Surface Analysis by
XPS and AES, 1st ed. (John Wiley & Sons Ltd., Chichester, 2003).28C. V. Robino, “Representation of mixed reactive gases on free energy
(Ellingham-Richardson) diagrams,” Metall. Mater. Trans. B 27, 65–69
(1996).29Y. X. Qin, W. J. Lu, D. Zhang, J. N. Qin, and B. Ji, “Oxidation of in situ
synthesized TiC particle-reinforced titanium matrix composites,” Mater.
Sci. Eng. A 404, 42–48 (2005).30C. Wagner, “Theoretical analysis of the diffusion processed determining
the oxidation rate of alloys.” J. Electrochem. Soc. 99, 369–380 (1952).31A. Kumar, M. Nasrallah, and D. L. Douglass, “The effect of yttrium and
thorium on the oxidation behavior of Ni-Cr-Al alloys,” Oxid. Met. 8,
227–263 (1974).32H. V. Atkinson, “A review of the role of short-circuit diffusion in the oxi-
dation of nickel, chromium, and nickel-chromium alloys,” Oxid. Met. 24,
177–197 (1985).33D. Caplan and G. I. Sproule, “Effect of oxide grain structure on the high-
temperature oxidation of Cr,” Oxid. Met. 9, 459–472 (1975).34F. H. Stott, G. C. Wood, and J. Stringer, “The influence of alloying ele-
ments on the development and maintenance of protective scales,” Oxid.
Met. 44, 113–145 (1995).35Z. Y. Liu, W. Gao, K. Dahm, and F. H. Wang, “The effect of coating grain
size on the selective oxidation behavior of Ni-Cr-Al alloy,” Scr. Mater. 37,
1551–1558 (1997).36T. Iizuka and H. Kita, “Oxidation mechanism of Mo5Si3 particle in Si3N4
matrix composite at 750 �C,” Mater. Sci. Eng. A 366, 10–16 (2004).
J. Laser Appl., Vol. 27, No. S1, February 2015 Hong et al. S17005-11