Influence_of_oxygen_on_solidification_be

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Influence of oxygen on solidification behaviour of
cast TiAl-based alloys
J. Zollinger a, J. Lapin b, D. Daloz a,*, H. Combeau a
a Ecole Nationale Supérieure des Mines de Nancy, LSG2M, Parc de Saurupt, CS14234, F-54042 Nancy, Cedex, France
b Institute of Materials and Machine Mechanics, Slovak Academy of Sciences, Racianska 75, 830 02 Bratislava, Slovak Republic
Received 27 October 2006; received in revised form 2 April 2007; accepted 10 April 2007
Available online 27 June 2007
Abstract
The influence of oxygen on solidification behaviour of cast TiAl-based alloys containing 40e48 at.% of Al was studied. Twelve alloys with
fixed Ti:Al ratios ranging from 1.08 to 1.5 and oxygen content of 0.1, 0.8 and 1.5 at.% were prepared by induction melting and casting in a cold
crucible under protective atmosphere. Increasing oxygen content affects significantly the macrostructure of the as-cast ingots, increases volume
fraction of the a phase formed during peritectic solidification and leads to a change of the b primary solidification phase to the a phase in ternary
Tie44.2Ale1.4O, Tie47.3Ale0.9O and Tie47.2Ale1.5O (at.%) alloys. In the case of the alloy with the a primary solidification phase, the
partition coefficients achieve values of k
s=l
Al=a ¼ 0:9 and k
s=l
O=a ¼ 1:29. In alloys containing 1.5 at.% of oxygen, it has been observed that the
b/a solidification phase transition is shifted to a lower aluminium contents when compared to that of binary systems. Oxygen extends the stability
of single a phase region.
� 2007 Elsevier Ltd. All rights reserved.
Keywords: A. Titanium aluminides, based on TiAl; B. Phase transformations; C. Casting; D. Microstructure
1. Introduction
Since the last two decades, TiAl-based alloys have attracted
attention as potential candidates for high-temperature structural
applications in the aerospace and automotive industries. Due to
low density, high specific strength, high Young’s modulus and
oxidation resistance at high temperatures, these materials repre-
sent a good alternative to nickel-based superalloys [1,2]. How-
ever, their metallurgical preparation including melting and
casting is still constrained by several limitations. One of them
results from their high reactivity with a large number of
elements and especially with oxygen. Despite of the use of vac-
uum or protective atmosphere processes (vacuum arc remelting,
plasma arc remelting, electron beam melting, etc.), contamina-
tion by oxygen cannot be fully avoided. Hence, industrial alloys
must be considered at least as ternary TieAleO systems with
an oxygen content ranging usually from 300 to 500 wt. ppm
[3e5]. It should be mentioned that interactions between
titanium aluminides and interstitial elements such as C, O and
N were widely studied to understand the effect of these ele-
ments on solid state phase transformations, microstructure evo-
lution and mechanical properties [6e13]. Some works were
devoted to the interactions between mould materials and melts
with the aim to improve hardness by oxides and to elucidate the
effect of ceramic particles on mechanical properties of TiAl-
based alloys [14e17]. The ternary TieAleO system has also
been studied from the point of view of oxidation behaviour of
Ti-based alloys or with the aim to develop new refractory oxide
materials [18e21]. However, to our knowledge, no data exists
on the effect of oxygen on solidification behaviour of TiAl-
based alloys in the available literature. Solidification path and
the heterogeneities ensuing from it have a great influence on
grain size and subsequent heat treatments of these alloys
[22,23].
* Corresponding author. Tel.: þ333 83 58 42 05; fax: þ333 83 58 40 56.
E-mail address: daloz@mines.inpl-nancy.fr (D. Daloz).
0966-9795/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.intermet.2007.04.002
Intermetallics 15 (2007) 1343e1350
www.elsevier.com/locate/intermet
mailto:daloz@mines.inpl-nancy.fr
http://www.elsevier.com/locate/intermet
The aim of the present paper is to study the influence of
oxygen on the solidification behaviour and microstructure evo-
lution of cast TiAl-based alloys containing 40e48 at.% of Al.
While industrial applications require maximum oxygen level
of about 1000 wt. ppm, in this study, higher level of oxygen
has been added (3400 up to 6400 wt. ppm, i.e. 0.8 at.% and
1.5 at.%) to allow reliable quantitative analysis of O microse-
gregation via wavelength dispersive spectrometry (WDS) or
electron probe microanalysis technique (EPMA). Thus, for
the purpose of this work, 12 intermetallic TiAl-based alloys
with oxygen content ranging from 0.1 to 1.5 at.% were
selected and prepared by induction melting in a cold crucible.
In addition, the effect of oxygen induced change of primary
solidification phase on microsegregation behaviour of alumin-
ium and oxygen is evaluated and discussed.
2. Experimental procedure
Fig. 1 shows part of the most recent binary TieAl phase
diagram according to Ref. [24] with the marked compositions
of selected binary alloys. These four binary systems with Ti:Al
ratio ranging from 1.08 to 1.5, which solidify with the b pri-
mary solidification phase (Ti-based solid solution with a cubic
crystal structure), were alloyed with 0.8 and 1.5 at.% of oxy-
gen keeping the Ti:Al ratio constant. Ingots with a weight of
20 g and targeted nominal chemical compositions given in
Table 1 were prepared from commercial purity titanium
(99.96%, 300 at. ppm O) and aluminium (99.99%). Oxygen
contents of 0.8 and 1.5 at.% were achieved by the addition
of titanium oxide powder (TiO2 with a purity of 99.99%)
into the binary systems. Each charge was weighed with a pre-
cision of 0.001 g then melted under 1.1� 105 Pa flowing
helium atmosphere in an electromagnetic levitation furnace
(allowing no contact of liquid metal with any solid wall). Be-
fore melting, the furnace was ‘‘cleaned’’ with an alternation of
vacuum and partial pressure of helium to desorb gases from
the copper crucible walls and other surfaces of the furnace.
For each experiment, the melt was stabilized for 20 min at
a temperature of about 100 �C above the liquidus temperature
determined from the binary phase diagram. Temperature of the
melt was controlled with an optical monochromatic pyrometer
calibrated with a black-body furnace. For the measurements,
the emissivity of TiAl alloys in the molten state was evaluated
to be 0.36 from HageneRubens law, with the electrical resis-
tivity of liquid TiAl-based alloys taken from Ref. [25]. After
switching off the induction current, the alloy dropped at the
bottom of water cooled copper crucible where it solidified.
Due to the direct chill cooling, such process results in high
solidification rates, which are significantly higher than those
leading to ‘‘equilibrium microstructures’’. However, this was
the only technique available to the authors to avoid melt con-
tamination by oxygen due to the contact with crucible walls.
Fig. 2 shows the typical macrograph of heart-shaped ingot
obtained by this method and the resulting columnar/equiaxed
macrostructure.
Table 1 summarizes the results of the chemical composi-
tions of the as-cast ingots measured by gas chromatographic
separation using LECOTC-436 N/O for the lowest oxygen con-
tent alloys (0.1 at.%) and by EPMA measurements for higher
oxygen content alloys (in the latter case, the indicated value
corresponds to the average of 400 measurements located regu-
larly all over the ingot section). In the following text, all the
compositions cited will refer to the measured one (Table 1)
and expressed in at.%.
Microstructural analysis was performed in the equiaxed
zone by light optical microscopy (OM) in bright field contrast
or differential interferential contrast (DIC) and backscattered
secondary electron microscopy (BSE). OM samples were pre-
pared using standard metallographic techniques (grinding and
polishing) and etchedin a solution prepared from 18 g NaOH
and 180 ml H2O, which was heated up to 80
�C and then 20 ml
Fig. 1. Part of binary TieAl phase diagram according to Ref. [24] with marked
chemical composition of studied alloys (x).
Table 1
Nominal and measured chemical compositions of TiAl-based alloys (in at.%)
Nominal Ti:Al ratio Nominal Measured
Ti Al O Ti Al O
1.50 60.0 40.0 0.0 60.5 39.4 0.1
59.5 39.7 0.8 60.0 39.4 0.6
59.1 39.4 1.5 59.8 39.0 1.2
1.32 57.0 43.0 0.0 57.0 42.9 0.1
56.5 42.7 0.8 56.9 42.3 0.8
56.1 42.4 1.5 56.4 42.1 1.6
1.22 55.0 45.0 0.0 55.3 44.6 0.1
54.6 44.6 0.8 55.1 44.1 0.8
54.2 44.3 1.5 54.4 44.2 1.4
1.08 52.0 48.0 0.0 52.4 47.5 0.1
51.5 47.7 0.8 51.8 47.3 0.9
51.2 47.3 1.5 51.3 47.2 1.5
1344 J. Zollinger et al. / Intermetallics 15 (2007) 1343e1350
H2O2 was added. Modified Kroll’s etchant consisting of
134 ml H2O, 46 ml H2O2, 14 ml HF and 6 ml HNO3 was
used to reveal microstructure of the alloys with more than
45 at.% Al. Samples for BSE observations were polished
with colloidal silica with 30% H2O2. Preparation of samples
for EPMA followed the procedure described elsewhere [26].
In addition, to minimize surface contamination, final polishing
was performed with neutral alumina (particle size: 0.02 mm)
then the samples were plasma coated by carbon and placed
in the microprobe analysis chamber (P¼ 5.10�5 Pa). In all
the samples, phases were identified with X-ray diffraction
(XRD) using a SIEMENS D500 diffractometer (wavelength,
lCoKa1¼ 0.1788965 nm). Quantitative metallographic analysis
concerning volume fraction of coexisting phases was per-
formed by a computerized image analyser.
The EPMA investigations were performed with a Cameca
SX50 electron microprobe with three wavelength dispersive
detectors. Spectra, which were obtained at an accelerating
voltage of 15 kV for titanium and aluminium and at 10 kV
for oxygen, were compared to those of the pure elements.
PAP matrix correction procedure was used and counting
time for each elements was adjusted not to exceed a relative
error of 0.5 at.% for Ti and Al, and 0.15 at.% for oxygen. Par-
ticular attention was paid to light element analysis parameters
[27,28]. For this purpose, the apparatus was calibrated using
standards of pure titanium (99.99%), pure aluminium
(99.999%) and TiO2 (99.999%). EPMA of microsegregation
behaviour of alloying elements was performed using a random
sampling approach of 625 points on 0.5� 0.5 mm2 square grid
located in the central part of the equiaxed zone, as illustrated
by the square in Fig. 2. The analysed points have been sorted
with the weighted interval rank sorting (WIRS) method taking
into account the contribution of all analysed elements for sort-
ing and uncertainty associated with the measurements [29].
3. Results
3.1. Effect of oxygen on microstructure
3.1.1. Systems with Ti:Al ratio of 1.5
The alloys Tie39.4Ale0.1O, Tie39.4Ale0.6O and Tie
39.0Ale1.2O with the b solidification phase show a type of
microstructure resulting from the b/ a solid state transfor-
mation according to the binary phase diagram shown in
Fig. 1. The needle-like a phase grows from the b grain bound-
aries and leads to a microstructure similar to the basket weave
microstructure observed in titanium alloys (Fig. 3a). XRD
analysis showed that the a phase formed during transformation
of the b phase is not stable and fully transforms to the ordered
a2 phase during cooling to room temperature. Increase of
oxygen content does not affect the overall resulting room tem-
perature microstructure, except in the case of Tie39Ale1.2O
alloy, where 2% volume fraction (2 vol.%) of lamellar
a2(Ti3Al)/g(TiAl) microstructure is observed in the interden-
dritic region due to microsegregation of alloying elements.
3.1.2. Systems with Ti:Al ratio of 1.32
The Tie42.9Ale0.1O alloy entirely solidifies within the
b phase and shows the same microstructure as the Tie
39.4Ale0.1O described in Section 3.1.1. While increasing the
oxygen content, one observes a decrease in the volume fraction
of the primary b cubic dendrite and an increase of the a2/g
lamellar structure in the interdendritic region (from 20% to
60% for the Tie42.3Ale0.8O alloy, and Tie42.1Ale1.6O
alloy, respectively), indicating the occurrence of peritectic
transformation and final solidification within a phase that sub-
sequently transform into a2/g lamellar structure during cooling
(Fig. 3b).
3.1.3. Systems with Ti:Al ratio of 1.22
In the alloys Tie44.6Ale0.1O and Tie44.1e0.8O the
microstructure is composed of cores of single a2 phase that
appear in white contrast, surrounded by fully lamellar a2/g
regions (grey contrast) and single g phase (black contrast)
(Fig. 3c). Such microstructure corresponds to solidification
starting with the primary b phase, followed by a first peritectic
transformation, for which a phase nucleates around b dendrites
and grows both within the liquid and by covering the b den-
drites. Then, due to microsegregation of alloying elements,
remaining interdendritic liquid transforms through the same
mechanism to form g phase.
When increasing the oxygen addition (Tie44.2Ale1.4O al-
loy), the primary solidification phase changes from the b to a,
as indicated by the six-fold symmetry of dendrite shown in
Fig. 3d. Due to microsegregation, peritectic transformation
and subsequent growing of g phases occur in the interdendritic
region until complete solidification. During cooling, the a den-
drites transform to fully lamellar a2/g microstructure and the
interdendritic region remains single g phase.
Fig. 2. Typical macrograph of as-cast ingot (here Tie47.2Ale1.5O), showing
columnar and equiaxed zones. White rectangle shows where microstructural
and microsegregation analyses have been done.
1345J. Zollinger et al. / Intermetallics 15 (2007) 1343e1350
3.1.4. Systems with Ti:Al ratio of 1.08
Hyperperitectic Tie47.5Ale0.1O alloy shows four-fold sym-
metry dendrites, which indicate that the bwas the primary solidifi-
cation phase. However, contrary to the Tie44.6Ale0.1O alloy
presented in Fig. 3c, during peritectic transformation the b primary
phase is totallycoveredwith thegrowingaphase, andsinglegphase
is formed during the second peritectic transformation with the
remaining interdendritic liquid. During cooling, the a phase within
the dendrites transforms to a2/g lamellar microstructure and a new
phase precipitates from the interdendritic monolithic g (probably
TiAl2 but the attempt to identify this new phase by XRD was not
successful due to its low volume fraction in the microstructure).
Fig. 3. Optical micrographs showing microstructures of the as-cast alloys: (a) Tie39.4Ale0.1O; (b) Tie42.1Ale1.6O. Differential interferential contrast (DIC)
was used to reveal a2 needles. (c) Tie44.6Ale0.1O; (d) Tie44.2Ale1.4O; (e) Tie47.3Ale0.9O; (f) Tie47.2Ale1.5O.
1346 J. Zollinger et al. / Intermetallics 15 (2007) 1343e1350
Increase of the oxygen content in Tie47.3Ale0.9O alloy
changes the b primary solidification phase to a (Fig. 3e). In
this alloy, the six-fold symmetry of the dendrites is not well
defined and frequently, five-fold symmetry of the a dendrites
is observed in the microstructure similar to that reported for
oxygen contaminated Tie46Ale2We0.5Si (at.%) alloy direc-
tionally solidified in alumina crucibles [29]. As in the previous
case, the interdendritic g phase is formed during peritectic
reaction. During cooling, the a dendrites transform to a2/g
lamellar microstructure and g phase transforms similarly as
described for the binary system. Further increase of the oxy-
gen content in Tie47.2Ale1.5O alloy leads to solidification
with the a primary solidification phase. In this case, the typical
six-fold symmetry of the a dendrites is very well developed, as
seen in Fig. 3f. The interdendritic region solidifies as the g
phase due to peritectic aþ L/ g transformation. As revealed
by BSE and XRD observations, after cooling, the microstruc-ture within the dendrites is fully lamellar composed of a2 and
g phases and single g phase remains in the interdendritic
region.
3.2. Effect of primary solidification phase on
microsegregation behaviour
Effect of oxygen induced transformation of primary solidi-
fication phase on microsegregation behaviour was studied in
two alloys with nominal compositions Tie39Ale1.2O and
Tie47.2Ale1.5O (at.%) with the primary solidification phases
b and a, respectively. Fig. 4 shows correlations between Ti
solvent and solutes of Al and O, corresponding to the 625 an-
alysed points localized in the core of the equiaxed region. Due
to some macrosegregation, the local content of oxygen (Tie
42Ale1O and Tie46Ale0.9O) is somehow lower than the
nominal composition. For both alloys, positive and negative
slopes for oxygen and aluminium evolution indicate partition
coefficients (i.e. ratio between the concentration of O (respec-
tively Al) in the solid and concentration of O (respectively Al)
in the liquid) k
s=l
O > 1 and k
s=l
Al < 1. As seen in Fig. 4a, for both
alloys, the aluminium content decreases linearly with
increasing titanium content. However, segregation amplitude
for Al is higher for the alloy with the a primary solidification
phase. Fig. 4b shows that the oxygen content linearly increases
with increasing titanium content. As in the previous case, seg-
regation amplitude for oxygen is higher for the alloy with the
a primary solidification phase.
Fig. 5 shows evolution of Al and O contents with the cumu-
lative fraction of analysed points after sorting using WIRS
method [29]. For Tie39Ale1.2O (at.%) alloy, the curves
show smooth development with increasing cumulative frac-
tion, as shown in Fig. 5a. In the case of Tie47.2Ale1.5O
alloy, the cumulative curves show some discontinuities at the
cumulative solid volume fraction fS of 0.94, as seen in
Fig. 5b. These discontinuities for Al and O can be explained
by the presence of the g phase (about 7 vol.%), which is
formed due to segregation of alloying elements in the inter-
dendritic region and affects the measurements.
Fig. 5 also shows the distribution of aluminium calculated
from Scheil relation in the form [32]:
C�S ¼
k
s=l
i=4C0
ð1� fSÞ
1�k
s=l
i=4
ð1Þ
where C�S is the composition of the solid at the solid/liquid
interface, k
s=l
i=4 is the partition coefficient between solid and liq-
uid of the element i when solidifying within the 4 phase, C0 is
the average composition and fS is the solid fraction. In these
calculations, the partition coefficient for Al has been taken
equal to that measured in binary system. For the alloy with
a primary solidification phase, Scheil distribution for oxygen
was calculated taken into account the partition coefficient
for O determined according to the relationship:
k
s=l
i=4 ¼
CS
CL
ð2Þ
where CS and CL are the element concentrations in solid and
liquid, respectively. Assuming that: (i) there is no peritectic
transformation in the alloy, (ii) there is no diffusion in the
solid phase and that CLz C0 at the beginning of solidification
Fig. 4. Soluteesolvent correlation relationships for Tie39Ale1.2O and Tie47.2Ale1.5O (at.%) alloys with the b and a primary solidification phases, respectively:
(a) OeTi; (b) AleTi. The evaluated alloys are indicated in the figures.
1347J. Zollinger et al. / Intermetallics 15 (2007) 1343e1350
(C0 is the nominal composition of the alloy), k
s=l
i=4 can be es-
timated as the ratio between the composition of the first solid
to the nominal composition of the alloy. Due to some scatter
of measured values, the partition coefficients for oxygen k
s=l
O
were determined as a ratio of the average concentration cal-
culated from the data for the first 0.05 of cumulative fraction
of solid to the average concentration. Table 2 summarizes ex-
perimentally determined and calculated values from commer-
cial database [31] of partition coefficients for oxygen (k
s=l
O ),
aluminium (k
s=l
Al ) and corresponding segregation amplitudes
(Cmax� Cmin) for both studied alloys.
4. Discussion
4.1. Effect of oxygen on primary solidification phase and
associated microstructures
The primary solidification phase affects significantly the
grain size of as-cast TiAl-based alloys [22]. Increase of the
oxygen content can lead to a change of primary solidification
phase and affects the columnar to equiaxed transition. As illus-
trated schematically for the studied alloys in Fig. 6, due to the
increase of the oxygen content the peritectic transformation
appears in alloys containing less aluminium compared to
binary alloys. This transformation appears also in the Tie
42.3Ale0.8O (at.%) alloy, whereas the peritectic transforma-
tion starts at about 44.5 at.% of Al in the binary systems.
When increasing the oxygen content, the primary solidifica-
tion phase transition from the b phase to the a phase is shifted
to lower aluminium contents. Increase of the oxygen content
leads to a change of the b primary solidification phase to the
a phase in Tie44.2Ale1.4O, Tie47.3Ale0.9O and Tie
47.2Ale1.5O (at.%) alloys, whereas the b phase remains the
primary solidification phase in all basic binary systems. The
change of the primary solidification phase can have two ori-
gins in peritectic alloys: (i) kinetic and (ii) thermodynamic.
Assuming kinetic aspects of solidification, the a phase can
be the primary solidification phase at high cooling rates and
thus correspondingly at high undercooling. Such assumption
requires that also the solidification of the studied binary alloys
should be affected by such kinetic effect resulting in a change
of the primary solidification phase at high growth rates applied
during casting. However, all binary systems showed the b pri-
mary solidification phase. This indicates that the kinetic
effects alone had no significant influence on solidification be-
haviour of the alloys containing 0.8 or 1.5 at.% of O. Hence,
the change of the primary solidification phase should be linked
to thermodynamic equilibrium considerations. Since ternary
TieAleO phase diagrams have not been defined at tempera-
tures close to liquidesolid transition yet, some assumptions
can be only made on the basis of binary TieO system. Assum-
ing the domains in binary phase diagram shown in Fig. 1 lead-
ing to formation of b or a primary solidification phase and
taking into account the same regions in binary TieO phase
diagram shown in Fig. 7 [33], a wide expansion of the stability
of a phase domain towards higher titanium content with
increasing oxygen content can be identified. We suppose that
the increasing oxygen content will expand also the a phase
Fig. 5. Dependence of content of aluminium and oxygen on cumulative frac-
tion of solid: (a) Tie39Ale1.2O (at.%) alloy; (b) Tie47.2Ale1.5O (at.%) al-
loy. The evaluated elements and fits according to Scheil equation are indicated
in the figures.
Table 2
Partition coefficients for aluminium k
s=l
Al , oxygen k
s=l
O and microsegregation amplitude Cmax�Cmin measured in Tie39Ale1.2O (b) and Tie47.2Ale1.5O (a)
(at.%) alloys
Primary
solidification
phase
Aluminium Oxygen
k
s=l
Al Cmax�Cmin k
s=l
O Cmax�Cmin
This work Ref. [30] This work Ref. [30]
b 0.94 0.88 6.88 1.08 0.60 0.24
a 0.90 0.88 11.28 1.29 0.51 0.81
1348 J. Zollinger et al. / Intermetallics 15 (2007) 1343e1350
domain of TieAleO alloys studied in this work as the same
crystallographic phases are involved. Expansion of the a phase
domain will lead to a change of primary solidification phase as
shown in Fig. 3d and f.
4.2. Microsegregation behaviour
4.2.1. b Solidifying alloy
The oxygen addition leads to the appearance of 2% volume
fraction of lamellar structure, i.e. solidification ended within
single a phase. This may be interpreted to either a decrease
of the aluminium partition coefficient, a decrease of its diffu-
sion coefficient or a stabilization of the a phase. This last point
may be predominantfrom the microstructures observation (see
Section 4.1). When comparing the experimental measurement
with Scheil model for aluminium, one observes a very low
correlation that indicates a strong back-diffusion during solid-
ification, which is consistent with the open BCC structure.
The observation that the oxygen content decreases during
solidification may indicate a partition coefficient slightly
higher than one. The b primary solidification phase in Tie
39Ale1.2O (at.%) alloy transforms to the a phase during cool-
ing. This solid phase transformation can affect the measured
partition coefficient k
s=l
O . Assuming binary TieO phase dia-
gram show in Fig. 7 and mechanisms of transformation of
the b phase to a, which start in aluminium rich regions and
continues towards middle parts of dendrites, one clearly see
that the forming a phase is enriched by oxygen. In order to
achieve partition coefficient k
s=l
O=b > 1, which was observed
from the measurement in the transformed a2 phase, preferen-
tial segregation of oxygen into the b dendrites during solidifi-
cation is required. b to a solid phase transformation probably
decreases the segregation amplitude of the original distribution
curve.
4.2.2. a Solidifying alloy
In this case (Fig. 5b), the good agreement between experimen-
tallymeasured oxygen redistribution curves and the Scheilmodel
can be explained by the low diffusion of oxygen in the solid phase
and the high applied solidification rates (of the order of mm s�1)
during solidification. As reported recently by Blanter et al. [34],
the diffusion of interstitial elements such as oxygen in metals
with hcp type of crystal structure is reduced by a stress induced
at the lattice level. Hence, back diffusion of oxygen in the alloy
with the a primary solidification phase is very limited and cannot
affect significantly the microsegregation behaviour.
Application of the Scheil model for redistribution of
aluminium results in calculated curves, which are close to mea-
sured values up to a cumulative fraction of solid of about 0.8 as
seen in Fig. 5b. However, the calculated values for aluminium
concentration are slightly lower than those measured experi-
mentally. This discrepancy can be explained taking into account
Fig. 6. Schematic picture showing the effect of increasing aluminium and oxygen contents on solidification behaviour of the studied alloys. The dashed lines define
the regions for the b and a solidification phases.
Fig. 7. Calculated binary TieO phase diagram and experimental data up to
0.62 mole fraction of oxygen [34].
1349J. Zollinger et al. / Intermetallics 15 (2007) 1343e1350
the high solidification rate which leads to a limited diffusion of
aluminium in the liquid [35]. The solute pile up in the liquid at
the solid/liquid interface induces a higher concentration in sol-
ute in the solid which forms. Very poor approximation at cumu-
lative fractions higher than 0.8 can be related to peritectic
transformation, to a non-constant partition coefficient and/or
to finite liquid volume in the system, where Scheil model pre-
dicts an infinite concentration of alloying element.
The partition coefficient of k
s=l
O=a ¼ 0:9 for Tie47.2Ale
1.5O (at.%) alloy with the a primary solidification phase cor-
responds well to the literature data [31]. To the best of our
knowledge, no literature data have been published yet to com-
pare the measured partition coefficients for oxygen k
s=l
O=a of
1.29 in the alloys with the a primary solidification phase
with similar oxygen contents.
Thecalculatedvalues for oxygenpartition coefficients listed in
Table 2 were obtained for significantly higher oxygen contents
than that of 0.4 at.% recommended by the database [31]. This
work clearly shows an opposite segregation behaviour of oxygen
during solidification of the studied alloyswith thea andbprimary
solidification phases than that assumed by database calculations.
Such different oxygen behaviourmeasured experimentally in this
work can affect significantly the current numerical modelling of
various casting processes for TiAl-based alloys.
5. Conclusions
The investigation of the influence of oxygen on solidification
behaviour of cast TiAl-based alloys suggests the following
conclusions:
1. Increase of the oxygen content increases volume fraction of
the a phase formed during peritectic solidification and leads
to a change of the b primary solidification phase to the
a phase in ternary Tie44.2Ale1.4O, Tie47.3Ale0.9O
and Tie47.2Ale1.5O (at.%) alloys. The b phase remains
the primary solidification phase in other studied binary
and ternary TieAleO systems.
2. Strong back-diffusion occurs in b solidifying alloy. In such
alloy, the partition coefficients for aluminium and oxygen
are determined to be inferior and superior to 1, respectively.
3. For the oxygen containing alloys with the a primary solid-
ification phase, very limited back-diffusion is observed.
The partition coefficients for aluminium and oxygen are
found to be k
s=l
Al=a ¼ 0:9 and k
s=l
O=a ¼ 1:29, respectively.
4. In oxygen containing alloys, the peritectic transformation
is observed in alloys at lower aluminium contents when
compared to the binary systems. Oxygen extends the
stability of single a phase region.
Acknowledgements
This work was financially supported by EU Integrated
Project IMPRESS ‘‘Intermetallic Materials Processing in
Relation to Earth and Space Solidification’’ under the contract
No. NMP3-CT-2004-500635.
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1350 J. Zollinger et al. / Intermetallics 15 (2007) 1343e1350
	Influence of oxygen on solidification behaviour of cast TiAl-based alloys
	Introduction
	Experimental procedure
	Results
	Effect of oxygen on microstructure
	Systems with Ti:Al ratio of 1.5
	Systems with Ti:Al ratio of 1.32
	Systems with Ti:Al ratio of 1.22
	Systems with Ti:Al ratio of 1.08
	Effect of primary solidification phase on microsegregation behaviour
	Discussion
	Effect of oxygen on primary solidification phase and associated microstructures
	Microsegregation behaviour
	beta Solidifying alloy
	alpha Solidifying alloy
	Conclusions
	Acknowledgements
	References