T. F. KUBATÍK: HIGH-TEMPERATURE OXIDATION OF SILICIDE-ALUMINIDE LAYER ON THE TiAl6V4 ...
257–261
HIGH-TEMPERATURE OXIDATION OF SILICIDE-ALUMINIDE
LAYER ON THE TiAl6V4 ALLOY PREPARED BY LIQUID-PHASE
SILICONIZING
VISOKOTEMPERATURNA OKSIDACIJA PLASTI
SILICID-ALUMINID, PRIPRAVLJENE S SILIKONIZIRANJEM
S TEKO^O FAZO ZLITINE TiAl6V4
Tomá{ Franti{ek Kubatík
Institute of Plasma Physics AS CR, v.v.i., Za Slovankou 1782/3, 182 00 Prague 8, Czech Republic
kubatik@ipp.cas.cz
Prejem rokopisa – received: 2015-01-02; sprejem za objavo – accepted for publication: 2015-03-26
doi:10.17222/mit.2015.002
The method of coating with silicon from the liquid phase (also called the hot-dip method) was presented by several authors who
indicated that this was an effective and inexpensive technique capable of producing Ti-Al-Si layers on titanium and
titanium-alloy substrates that are rich in ternary phases. The present study examines the effects of the preparation conditions on
the structure and properties of the layers. These layers provide excellent protection from high-temperature oxidation, even at a
temperature of 950 °C. It was proved with SEM and X-ray analyses that the original 2 ternary phase almost completely
decomposed into pure Ti5Si4 and TiSi silicides at the temperature of 950 °C. The formed layer, consisting of silicide sub-layers,
exhibited superior protective properties in high-temperature applications.
Keywords: TiAl6V4, silicides, high-temperature oxidation, liquid-phase siliconizing
Metodo prekrivanja s silicijem iz teko~e faze (v~asih imenovano hot-dip metoda) je predstavilo ve~ avtorjev, ki so potrdili, da je
to u~inkovita in poceni tehnika, s katero je mogo~e izdelati Ti-Al-Si plasti na podlagi iz titana in titanovih zlitin, ki vsebujejo
ternarne faze. Predstavljena {tudija preiskuje vpliv pogojev priprave na strukturo in lastnosti plasti. Te plasti zagotavljajo
odli~no za{~ito pred visokotemperaturno oksidacijo, celo pri temperaturi 950 °C. S SEM in z rentgensko analizo je bilo
dokazano, da se prvotna ternarna faza 2, pri temperaturi 950 °C, skoraj v celoti razgradi v ~isti Ti5Si4 in TiSi silicid. Nastala
plast, ki jo sestavlja ve~ podplasti je pokazala odli~ne za{~itne lastnosti pri visokotemperaturni uporabi.
Klju~ne besede: TiAl6V4, silicidi, visokotemperaturna oksidacija, silikoniziranje v teko~i fazi
1 INTRODUCTION
Protective layers based on silicide-aluminide phases
prepared from the liquid phase were studied by various
authors1–4 and were shown to exhibit the potential of
acting as protective layers where titanium and its alloys
are used in high-temperature applications.5–13 The prin-
ciple of the method is based on the high affinity of
silicon and aluminium to titanium. During the immersion
of titanium and its alloys in the aluminum melt com-
prising silicon, a reaction forming a Ti-Al-Si phase at the
interface takes place. The temperature of the melt and
the silicon content in the melt have effects on the kinetics
of the growth of the interface layers as well as on their
phase composition. These layers provide an excellent
barrier to the diffusion of both oxygen and nitrogen at
high temperatures. In high-temperature applications, the
layers composed of silicide-aluminide phases cannot be
compared with silicides but may offer superior mecha-
nical properties at both high and low temperatures.
The major phase present in these layers, the 2 phase,
was first described by Brukl et al.14 as Ti(AlxSi1-x)2 where
x is in the range from 0.15 to 0.3, and further charac-
terized by Schubert et al.15 with the orthorhombic
structure and a space group (Cmcm). Layers rich in this
phase exhibit an excellent oxidation resistance. During
the course of oxidation, a compact film rich in SiO2 is
formed on the surface of a layer, which substantially
mitigates oxygen diffusion. This work is focused on the
study of the changes in the phase composition during a
high-temperature oxidation of the layers formed by the 2
phase.
2 EXPERIMENTAL WORK
The substrate material used in the presented work
was the most common titanium alloy, TiAl6V4. Speci-
mens were manufactured as cylinders of 10 mm in
diameter and 6 mm in height. They were ground using
P180–1200 SiC emery papers. Then they were polished
with diamond pastes up to 1 μm. This was followed by
degreasing in acetone in an ultrasonic apparatus, rinsing
with distilled water, and drying with compressed air. An
alloy melt of AlSi20 % of mass fractions was prepared
by melting aluminum (99.5 % purity) with silicon
(99.99 % purity). To ensure the required homogeneity,
the melt was agitated for 30 min and then kept at 650 °C.
Then the oxide surface layer was raked off and the
TiAl6V4-alloy specimens were immediately introduced.
Materiali in tehnologije / Materials and technology 50 (2016) 2, 257–261 257
UDK 669.058:621.793:669.295 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(2)257(2016)
The melt was agitated again and the specimens were held
in the melt for 60 min. At the end of the process, the
samples were removed from the bath and air-cooled. The
microstructure of the cross-sections of the samples was
observed by a scanning electron microscope, EVO MA
15 (Carl Zeiss SMT, Germany, SEM).
The specimens used in the oxidation experiments
were treated in 15 % HCl to remove the residues of
solidified melt. The specimens were subjected to cyclic
oxidation in an electric-resistance furnace at the
temperatures of 850 °C and 950 °C for 144 h in air. The
results were expressed as weight gains due to oxidation
versus oxidation duration. After the oxidation the
specimens were mounted in metacrylate resin, ground
polished and studied with SEM. A phase analysis was
performed with a D8 Discover diffractometer (Bruker,
Germany); diffracted Cu-K radiation was detected with
a 1D LynxEye detector.
3 RESULTS AND DISCUSSION
3.1 Microstructure
The temperature applied in the coating process has a
significant effect on both the growth rate of the layers
and on the layer quality. From previous experiments13,
the temperature of 650 °C was chosen as the highest
temperature for this process. Up to this temperature, the
layer is formed only by Ti-Al-Si ternary phases.
The microstructure of the layers prepared by
immersion in the AlSi20 melt (with solid silicon) at the
temperature of 650 °C for 1 h (Figure 1) is compact,
non-porous and free from any cracks and fissures. The
thickness of the layer reaches approximately 55 μm. The
chemical composition across the layer is constant –
11.2 % Al (in amount fractions, x/%), 55.6 % Si, 31.9 %
Ti, and 1.3 % V. This chemical composition corresponds
to the 2 phase that occurs as the main product of the
liquid-phase siliconizing process.
According to the phase analysis, the major phase
present is TiAl0.3Si1.7 also known as the 2 phase, shown
in Figure 2. This phase was already identified by the
authors who studied the preparation of layers from
silicon-rich melts.
3.2 Oxidation resistance
The oxidation resistance of the prepared layer was
evaluated with cyclic-oxidation tests. The results of the
oxidation at 850 °C are presented in Figure 3. The plot
represents the weight gains as a function of the oxidation
duration. The weight of the scales delaminated from the
samples was added to the reported weight. The uncoated
samples of titanium and TiAl6V4 alloy were used as the
references. The weight gain of the TiAl6V4 alloy
(0.0986 g/cm2 after 144 h) is almost 64 times higher than
that of the specimen coated with a protective Ti-Al-Si
layer. It is clearly seen that the surface layer provides a
considerable protection to the alloy when exposed to the
T. F. KUBATÍK: HIGH-TEMPERATURE OXIDATION OF SILICIDE-ALUMINIDE LAYER ON THE TiAl6V4 ...
258 Materiali in tehnologije / Materials and technology 50 (2016) 2, 257–261
Figure 2: XRD pattern of a cross-section of the layer prepared in
the AlSi20 melt
Slika 2: Rentgenogram pre~nega preseka plasti, pripravljene v talini
AlSi20
Figure 1: Microstructure of the cross-section of a layer prepared in
the AlSi20 melt, at the temperature of 650 °C for 1 h
Slika 1: Mikrostruktura preseka plasti, pripravljene v talini AlSi20,
1 h pri temperaturi 650 oC
Figure 3: Cyclic-oxidation curves of coated TiAl6V4, uncoated
TiAl6V4 and Ti at 850 °C
Slika 3: Krivulje cikli~ne oksidacije TiAl6V4 s prekritjem, TiAl6V4
brez prekritja in Ti, pri 850 °C
air at high temperatures. The character of the kinetic
curve shown in Figure 3 indicates that the greatest
weight gain is reached during the first 24 h. After that,
the weight of the tested sample remains almost constant.
When taking into account that the testing temperature is
much higher than the applicability limits of titanium
alloys and the protection Ti-Al-Si layers, it can be stated
that the proposed surface treatment provides an excellent
protection.
The results of the cyclic oxidation at the temperature
of 950 °C are summarized in Figure 4. Uncoated tita-
nium exhibited an enormously high oxidation rate at the
areas where the oxide scales were peeling off from the
surface. In the case of the TiAl6V4-alloy specimens, the
weight gains were also high. The oxidation kinetic curve
does not exhibit a purely parabolic character, mainly due
to a massive delamination of the surface oxide scales.
The weight gain after 144 h of oxidation was almost 58
times higher in the case of the TiAl6V4 alloy (0.150
g/cm2) than in the case of the specimen coated with a
protective layer (0.00257 g/cm2). The high oxidation
resistance of the layers is attributed to the formation of
compact silicide sub-layers, as will be discussed bellow.
3.3 Microstructure and phase analysis of the layers
after the oxidation
After performing the oxidation experiments, the
changes in the microstructure of the layers were exa-
mined with a scanning electron microscope. The micro-
structure of the cross-section of an as-oxidized layer
after the exposure at 850 °C for 144 h is shown in Figure
5. It can be seen from the microstructure that the layer
structure was subjected to considerable changes. The ini-
tially formed layer decomposed into several sub-layers.
Chemical microanalysis results and results of the X-ray
phase analysis indicate that two new sub-layers were
formed, constituted by Ti5Si4 silicide, probably also con-
taining the TiSi phase, detected with the phase analysis.
These layers are followed by an inhomogeneous and
porous layer, composed of ternary Ti-Al-Si phases. The
SEM-EDS results revealed that the 2 phase is present,
together with the other ternary phases, more enriched in
aluminum. It is suggested that, in analogy to the struc-
ture of the layer oxidized at 950 °C, the initially present
silicide-aluminide layer decomposes yielding thermo-
dynamically stable silicides, whose subsequent growth
acts as the driving force of aluminium diffusion into the
substrate and toward the layer surface. This assumption
is also supported by the presence of the original Ti-Al-Si
phase with a higher aluminum content. This topmost
sub-layer hinders the diffusion of aluminum.
The progressive weight gain experienced during the
oxidation process indicate that in the early stages the
weight gain is minimal, probably owing to the aluminum
T. F. KUBATÍK: HIGH-TEMPERATURE OXIDATION OF SILICIDE-ALUMINIDE LAYER ON THE TiAl6V4 ...
Materiali in tehnologije / Materials and technology 50 (2016) 2, 257–261 259
Figure 6: SEM micrograph of the cross-section of the layer prepared
in the AlSi20 melt, after the oxidation at 950 °C for 144 h
Slika 6: SEM-posnetek preseka plasti, pripravljene v talini AlSi20, po
oksidaciji 144 h na temperaturi 950 °C
Figure 4: Cyclic-oxidation curves of coated TiAl6V4, uncoated
TiAl6V4 and Ti at 950 °C
Slika 4: Krivulje cikli~ne oksidacije TiAl6V4 s prekritjem, TiAl6V4
brez prekritja in Ti, pri 950 °C
Figure 5: SEM macrograph of the cross-section of the layer prepared
in the AlSi20 melt, after the oxidation at 850 °C for 144 h
Slika 5: SEM-posnetek preseka plasti, pripravljene v talini AlSi20, po
oksidaciji 144 h na temperaturi 850 °C
diffusion toward the surface and its subsequent oxida-
tion. After that, the oxidation rate is close to the experi-
mental error of the measurement. This may be related to
the fact that aluminum cannot diffuse easily within a
spongy surface structure. The cross-section of the layer
oxidized at 950 °C for 144 h is displayed in Figure 6. In
this case, there were substantial changes to the layer
structure as well. A new layer was formed, composed of
several sub-layers. With the chemical microanalysis and
X-ray analysis, it can be clearly demonstrated that the
original 2 phase decomposed yielding silicide sub-
layers.
The layer located close to the substrate is constituted
by the Ti5Si3 silicide, which is more enriched in titanium.
This layer is compact and homogeneous, with a thick-
ness of approximately 45 μm. The next layer is a thin
intermediate layer composed of nearly pure aluminum
(with TiAl determined with the phase analysis). The
layer after this one is an inhomogeneous and rather po-
rous layer of TiSi silicide with a thickness of approxi-
mately 40 μm. This layer probably also contains particles
of TiSi2 silicide that can be observed in the microphoto-
graph, but failed to be precisely detected with the SEM
microanalysis. The presence of this silicide was con-
firmed with the XRD phase analysis. This silicide layer
gradually turns into an inhomogeneous layer containing
a great number of pores, in which Al and Ti oxides as
well as residues of the original Ti-Al-Si phase were
detected by means of a semi-quantitative chemical analy-
sis. Most probably, this is related to the spongy surface
of the prepared layers, as shown in Figure 1. This also
makes oxygen diffusion easier and facilitates its oxida-
tion on the layer surface. It can be assumed that at such a
high temperature, the original silicide-aluminide phase
tends to decompose yielding thermodynamically stable
TiSi, Ti5Si4 (Ti5Si3 has a similar diffraction pattern), and
TiSi2 silicides, which were detected as the major consti-
tuents with the X-ray phase analysis.
The growth of the silicides causes aluminum to
diffuse along the layer/substrate interface and, primarily,
to diffuse toward the surface where it is subsequently
oxidized. This assumption is also supported by the pre-
sence of a thin aluminum layer between the two silicide
sub-layers, detected in the layer structure after the oxida-
tion at both temperatures (850 °C and 950 °C). It can be
assumed that the considerable resistance to high-tempe-
rature oxidation can be attributed mainly to the presence
of the sub-layer constituted by the Ti5Si4 silicide. Diffe-
rent chemical compositions of the two sub-layers can be
observed, which remain constant across the entire width
of the newly formed sub-layers. Vanadium is uniformly
distributed across the entire thickness of the layer.
The layers obtained with the oxidation at 850 °C and
at 950 °C were subjected to a phase analysis. After the
oxidation at the temperature of 850 °C (Figure 7), two
major phases were detected. They were ternary silicide-
aluminide phases having significantly higher aluminum
contents than the original 2 phase. The minor phases
included TiSi and TiAl. It was reported2 that the inter-
metallic phases of TiAl and TiSi were formed due to the
reaction of the substrate (Ti-Al alloy) with the ternary
phase of Ti-Al-Si in accordance with the following
Equation (1):
Ti-Al + (Ti-Al-Si)  TiAl + TiSi (1)
The achieved results suggest that nuclei of the TiSi
phase were formed and continued to grow within the ori-
ginal layer. Due to this process, aluminum was pushed
out and allowed to diffuse, producing aluminum-en-
riched Ti-Al-Si phases.
This assumption was confirmed by the presence of
aluminum in the substrate near the layer/substrate inter-
face, by the large quantities of aluminum found on the
layer surface and by the presence of the thin intermediate
layer at the interface between the silicide sub-layers. At
the temperature of 950 °C these changes proceeded more
rapidly and caused a nearly complete decomposition of
the original layer formed by the 2 phase. The phases
detected with the X-ray analysis at this high temperature
included TiSi, TiSi2, TiO2 and the silicide-aluminide 2
phase as the dominant phase (Figure 8).
The minor phases detected were Ti5Si4 and Al2O3.
These analyses corresponded well with the micro-
analyses performed with SEM-EDS. Due to the forma-
T. F. KUBATÍK: HIGH-TEMPERATURE OXIDATION OF SILICIDE-ALUMINIDE LAYER ON THE TiAl6V4 ...
260 Materiali in tehnologije / Materials and technology 50 (2016) 2, 257–261
Figure 8: XRD pattern of the cross-section of the layer prepared in
the AlSi20 melt, oxidized at 950 °C for 144 h
Slika 8: Rentgenogram pre~nega preseka plasti, pripravljene v talini
AlSi20, po oksidaciji 144 h na temperaturi 950 °C
Figure 7: XRD pattern of the cross-section of the layer prepared in
the AlSi20 melt, oxidized at 850 °C for 144 h
Slika 7: Rentgenogram pre~nega preseka plasti, pripravljene v talini
AlSi20, po oksidaciji 144 h na temperaturi 850 °C
tion of the thermodynamically stable TiSi and Ti5Si4
(Ti5Si3 has a similar diffraction pattern) silicides, the oxi-
dation rates were considerably lower, as can be con-
cluded from the data in Figure 5.
4 CONCLUSIONS
The liquid-phase silicon-aluminium coating process
can be adopted to produce compact and homogeneous
surface layers constituted by the ternary phase
TiAl0.3Si1.7. Such layers are prepared from the AlSi20
melt, at a temperature of 650 °C with the resulting thick-
ness of 55 μm. The layers prepared in the present study
provide outstanding protection from cyclic high-tempe-
rature oxidation, at both 850 °C and 950 °C in air. In the
early stages of the oxidation, there is a gradual increase
in the weight gain, but after about 30 h of exposure the
weight gains are almost too low to be detected on the
specimens used. The analyses of the as-oxidized layers
indicate that the original layer decomposes, yielding
compact and homogeneous sub-layers constituted solely
by the Ti5Si3 and TiSi silicides. Due to the formation of
these silicide sub-layers, a considerable oxidation resis-
tance is reached, allowing the use of titanium alloys in
high-temperature applications. The present study demon-
strates that highly resistant protective layers suitable for
high-temperature applications can be prepared using a
simple and cost-effective technique.
Acknowledgement
The research of the surface-protective layers on
TiAl6V4 is financially supported by research project VZ
MSM 2579478701.
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