Formation of Al-Y oxide during processing of γ-TiAl

Article abstract of DOI:10.1016/S0925-8388(01)02028-X

While processing Y₂O₃ dispersed γ-TiAl, Y₂O₃ particles which dissolved during hot isostatic pressing (HIP'ing) were found to precipitate during the heat treatment in the form of a mixed Al-Y oxide. To understand the chemical reaction that occurs between Y₂O₃ and γ-TiAl during the heat treatment cycle, a powder mixture comprising of γ-TiAl and 10 wt.% Y₂O₃ was mechanically alloyed (MA'd) for 8 h and the milled powder was subjected to differential thermal analysis (DTA) at 1150 °C prior to analyzing it using X-ray diffraction technique. The present study clearly demonstrates that aluminum in the combined form either as γ-TiAl or Al₂O₃ reacts in a similar manner with Y₂O₃ when milled and heat treated at 1150 °C. In either case there is formation of Al₂Y₄O₉ (2Y₂O₃.Al₂O₃).

Full text of DOI:10.1016/S0925-8388(01)02028-X

Journal of Alloys and Compounds 340 (2002) 231–235
L
www.elsevier.com/locate/jallcom
Formation of Al–Y oxide during processing of g-TiAl
a
a ,
a
b
*
P.B. Trivedi , S.N. Patankar , F.H. (Sam) Froes , E.G. Baburaj
a
Institute for Materials and Advanced Processes, Mines Building, University of Idaho, Moscow, ID 83844-3026, USA
b
Department of Mechanical Engineering, University of Houston, Houston, TX 77204, USA
Received 3 December 2001; accepted 14 December 2001
Abstract
While processing Y O dispersed g-TiAl, Y O particles which dissolved during hot isostatic pressing (HIP’ing) were found to
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3
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3
precipitate during the heat treatment in the form of a mixed Al–Y oxide. To understand the chemical reaction that occurs between Y O
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3
and g-TiAl during the heat treatment cycle, a powder mixture comprising of g-TiAl and 10 wt.% Y O was mechanically alloyed (MA’d)
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3
for 8 h and the milled powder was subjected to differential thermal analysis (DTA) at 1150 8C prior to analyzing it using X-ray diffraction
technique. The present study clearly demonstrates that aluminum in the combined form either as g-TiAl or Al O reacts in a similar
2
3
manner with Y O when milled and heat treated at 1150 8C. In either case there is formation of Al Y O (2Y O .Al O ).
 2002
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Elsevier Science B .V . All rights reserved.
Keywords: High temperature alloys; Composite materials; Mechanical alloying; X-ray diffraction
1
. Introduction
when the MA’d TiAl12 wt.% Y O powder mixture was
2 3
hot isostatically pressed (HIP’d) and heat treated at
1150 8C. The objective of this paper is to discuss the
formation of a mixed Al–Y oxide during the processing of
Y O dispersed g-TiAl.
Conventional heat resistant steels, superalloys and TiAl
alloys are promising materials for high temperature appli-
cations because of their low density, high modulus and
high temperature capability. However, widespread accept-
ance of TiAl alloys as structural material is limited because
of their room temperature brittleness that arises from the
localized atomic bonding and associated ordering [1,2].
The most effective way of overcoming the room tempera-
ture brittleness is to obtain TiAl in nanocrystalline form
using mechanical alloying (MA’ing) [3]. Similar to oxide
dispersion strengthened (ODS) superalloys processed via
MA’ing, the addition of non-shearable second phase
particles, especially Y O can potentially improve the
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3
2. Experimental details
Gas atomized powder of Ti–48Al–2Cr–2Nb, with a
particle size of 2325 mesh and analytical grade Y O
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3
powder were used as the starting materials. The charge
comprising of 10 g of Ti–48Al–2Cr–2Nb powder and 0.2
g of Y O powder was milled using a Spex 8000 ball mill,
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consisting of a hardened steel vial and steel balls as the
grinding media. Milling was carried out, dry, for 8 h under
an argon atmosphere to minimize atmospheric contamina-
tion. The charge to ball ratio was 1:10. Hot isostatic
pressing (HIP’ing) of the MA’d powder mixture was
carried out at 795 8C using a compaction pressure of 50
MPa. HIP’d compact was then heat treated at 1150 8C for
3 h, with a heating rate of 20 8C/min using the vacuum
furnace. Structural transformation was monitored using the
Philips X-ray diffractometer (XRD) with Cu Ka radiation.
A Perkin Elmer differential thermal analyzer (DTA-7) was
used to study the phase transformation that occurs during
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3
strength and creep properties of g-TiAl [4,5]. Yttria
dispersion in superalloys is not stable and often transforms
into mixed Al–Y oxides [6]. According to the mechanism
proposed by Naka et al., in Y O dispersed titanium, there
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3
is a dissolution and precipitation of small Y O particles
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3
by the migrating grain boundaries [7]. In our present study
dealing with processing of Y O dispersed g-TiAl, dis-
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persoids of mixed Y–Al oxide were found to precipitate
*
Corresponding author.
E-mail address: pata1630@uidaho.edu (S.N. Patankar).
0
925-8388/02/$ – see front matter
 2002 Elsevier Science B .V . All rights reserved.
PII: S0925-8388(01)02028-X

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P.B. Trivedi et al. / Journal of Alloys and Compounds 340 (2002) 231–235
exposure of as-blended as well as milled powder to high
temperature. Thin foils for transmission electron micro-
scopy (TEM) analysis were prepared by a conventional
electro-thinning technique. The effect of heat treatments on
the dispersion of oxide phase and grain growth rate was
studied using a JEOL JEM 2010 TEM operating at 200 kV.
phase is composed of Al Y O . Transmission electron
2 4 9
microscopy study performed on the as-HIP’d compact
failed to show the presence of any dispersed phase.
Absence of dispersed phase in the HIP’d sample could be
attributed to the dissolution of the dispersed phase in the
matrix during HIP’ing, and the dispersed Al Y O ob-
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served in Fig. 1 is probably generated during the heat
treatment of the Y O dispersed g-TiAl at 1150 8C for 3 h.
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3
. Results and discussion
3
.2. MA g-TiAl110 wt.% Y O
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3
3
.1. HIP’d g-TiAl12 wt.% Y O
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3
To understand the chemical reaction that occurs between
Fig. 1a is the bright field image of the HIP’d compact of
Y O and g-TiAl during HIP’ing and heat treatment cycle,
2 3
g-TiAl containing 2 wt.% Y O . This image was taken
powder mixture comprising of g-TiAl and 10 wt.% Y O
2 3
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3
after the sample was heat treated at 1150 8C for 3 h. The
prominent feature of the image is the distribution of fine
dispersoids in nanocrystalline g-TiAl matrix. Some dis-
persoids appear along the grain boundaries. The analysis of
the selected area diffraction pattern (SADP) of the dis-
persed phase shown in Fig. 1b, revealed that the dispersed
was MA’d for 8 h and the milled powder was subjected to
differential thermal analysis (DTA) at 1150 8C. The heat-
ing rate and heating cycle during the DTA was adjusted so
as to simulate the heating conditions experienced by the
Y O dispersed g-TiAl during the HIP’ing and the sub-
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sequent heat treatment. A higher quantity of Y O was
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used to make sure that the amount of mixed oxide phase
formed during DTA is sufficient enough to be within the
detection limit of the XRD technique. The DTA curve
shown in Fig. 2 indicates how the powder mixture
consisting of g-TiAl and 10 wt.% Y O would behave
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when exposed to temperatures similar to those used during
the heat treatment of HIP’d g-TiAl dispersed with 2 wt.%
Y O . The various exothermic and endothermic peaks in
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3
the DTA curve represents the different phase transforma-
tions that occur when milled powder mixture is exposed to
elevated temperatures during the DTA cycle. The TEM
study done on the as-HIP’d Y O dispersed g-TiAl (HIP’d
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3
at 795 8C) failed to show the presence of any oxide
particles. Therefore, the distinct exothermic peak observed
in the DTA curve (Fig. 2) at 750 8C probably represents
dissolution of Y O in g-TiAl. This dissolved Y O
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eventually precipitates in the form of mixed Al–Y oxide as
indicated by inflection point at 900 8C in the DTA curve,
Fig. 2. This intriguing phenomenon involving dissolution
of Y O in g-TiAl during the HIP’ing stage is similar to
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the one reported by Naka et al. [7].
Fig. 1. TEM photomicrograph of g-TiAl12 wt.% Y O 8 h MA’d,
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HIP’d and heat treated at 1150 8C for 3 h. (a) BF showing distribution of
Al Y O oxide particles and (b) SADP corresponding to Al Y O oxide
Fig. 2. DTA heating curve for MA (TiAl110 wt.% Y O ) powder
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particles.
mixture.

P.B. Trivedi et al. / Journal of Alloys and Compounds 340 (2002) 231–235
233
As reported by Wang et al., in the present study also
oxygen is probably introduced into the powder mixture by
the oxygen impurity (,1%) that is present in argon gas
(used during DTA) and/or due to atmospheric contamina-
tion during MA’ing [4]. Extremely large surface area of
the g-TiAl fraction of the milled powder mixture make it
easily prone to oxidation leading to the formation of
aluminum oxide. The aluminum oxide formed would
eventually transform into Al–Y-oxide during subsequent
heat treatment of the powder mixture. Formation of the
Al–Y oxide dispersed phase ensures that the g-TiAl matrix
is devoid of free oxygen. Thus addition of Y O particles
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3
apart from generating fine Al–Y oxide dispersoids, would
also lower the oxygen content in g-TiAl matrix. Fig. 3 is
the XRD pattern of MA’d powder of g-TiAl and 10 wt.%
Y O after annealing at 1150 8C using DTA. The XRD
Fig. 4. DTA curve for MA’d (Al O 12Y O ) powder mixture.
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dissolution of Y O , which is followed by a precipitation
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pattern shows peaks that are characteristic of g-TiAl,
of mixed Al–Y oxide, as indicated by the presence of
Al Y O peaks in the XRD pattern shown in Figs. 3 and 5.
elemental Ti and Al Y O . Based on these X-ray diffrac-
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tion results, the probable reaction that occurs during the
heat treatment cycle could be represented by the following
chemical equation
(
1) MA
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3
Y O 1 2TiAl 1 3/2O
→
Al Y O 1 2Ti
(1)
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2
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(
2) 1150 8C
.3. MA Al O 12Y O
3
2
3
2
Similar observations were made when powder mixture
containing Y O and Al O (mixed in stoichiometric
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proportion of 1:2) was MA’d for 8 h and subjected to DTA
Fig. 4). In the DTA curve of MA’d Y O and Al O
(
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mixture there is a distinct exothermic peak at around
00 8C followed by an inflection point at 800 8C. These
findings suggest that during MA’ing and subsequent heat
treatment, Y O shows similar behavior in the presence of
6
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Fig. 5. X-ray diffraction pattern of MA’d (Al O 12Y O ) powder
2 3 2 3
g-TiAl and Al O . In either case there is an initial
mixture heat treated at 1150 8C.
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Fig. 3. X-ray diffraction pattern of MA (TiAl110 wt.% Y O ) powder mixture heat treated at 1150 8C.
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P.B. Trivedi et al. / Journal of Alloys and Compounds 340 (2002) 231–235
Fig. 6. X-ray diffraction pattern of MA’d (2Al 1Y O ) powder mixture.
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Fig. 8. X-ray diffraction pattern of MA’d (2Al 1Y O ) powder mixture
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heat treated at 1150 8C.
The reaction that leads to the formation of Al Y O can be
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represented by the following chemical equation
a lower form of Al–Y-oxide and yttrium aluminide (Al Y),
(
1) MA
2
2
3
Y O 1 Al O
→
Al Y O
9
(2)
Fig. 8. The presence of an exothermic peak in the DTA
curve shown in Fig. 7 probably indicates the nature of the
reaction that leads to the formation of Al Y and AlYO .
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(
2) 1150 8C
.4. MA Al12Y O
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The chemical reaction that leads to the formation of Al Y
2
It has been successfully demonstrated that aluminum in
combined state whether in the form of TiAl or alumina
Al O ), tends to form Al Y O when it is MA’d with
and AlYO could be represented by the following set of
equations
3
(
2
3
2
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MA
2
Y O 1 2Al 1 3/2O →Al Y O
(3)
(4)
Y O and the milled powder mixture is heat treated. To
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2
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confirm whether aluminum in the free form also shows
similar behavior, aluminum powder was mixed with Y O
MA
1150 8C
Y O 1 3Al →[Al Y O ] → Al Y 1 AlYO
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3
2
3
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3
intermediate
in the stoichiometric ratio of 2:1 and subjected to MA’ing.
Fig. 6 is the XRD pattern of the MA powder mixture.
The feasibility of the above reaction will strongly
depend on the availability of the oxygen. The oxygen
required for the formation of Al Y O could partially be
From the presence of distinct Al Y O in the XRD pattern
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shown in Fig. 6 it appears that elemental aluminum when
MA’d with Y O leads to the formation of Al Y O . In
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9
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3
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made available by the oxygen impurity (,1%) that is
present in the argon gas (used during DTA) and/or due to
the atmospheric contamination during the MA’ing. Such
an interference of oxygen has been reported by Wang et al.
[4]. Susceptibility of the ultrafine MA g-TiAl to oxidation
would lead to formation of increased amount of Al O .
the presence of TiAl/Al O however, Al Y O could be
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formed only when TiAl/Al O was MA alloyed with Y O
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and the MA powder mixture was heat treated at 1150 8C.
Fig. 7 shows the DTA curve of the MA Al12Y O
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powder mixture. The characteristic feature of the present
DTA curve is the exothermic peak observed at 900 8C.
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The Al O formed would eventually transform into
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When the MA Al12Y O was heat-treated at 1150 8C,
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3
Al Y O as per the following set of reactions
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Al Y O formed during MA’ing disintegrated into AlYO ,
2
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9
3
2
2
TiAl 1 3/2O → Al O 1 2Ti
2 2 3
Y O 1 Al O → Al Y O
9
]
2
3
2
3
2
4
2
TiAl 1 2Y O → Al Y O 1 2Ti
2 3 2 4 9
The other probable source of oxygen required for the
formation of Al Y O could be the interstitial oxygen [2].
2
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In Ti–Y alloys, ultra fine yttrium oxide particles are known
to form through the oxidation of yttrium by the interstitial
oxygen [8]. This elimination of interstitial oxygen atoms
from titanium matrix has been recorded at temperatures
below 300 8C.
It has also been reported that yttria forms very fine
dispersion of Y TiO in oxide dispersion strengthened
2
5
ferritic steel containing Ti [9]. In most yttria dispersion
strengthened alloys, four types of yttrium–aluminum-ox-
Fig. 7. DTA curve for MA’d (2Al1Y O ) powder mixture.
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P.B. Trivedi et al. / Journal of Alloys and Compounds 340 (2002) 231–235
235
ides have been identified [6]. These include, (1) yttrium–
aluminum garnet, YAG (3Y O .5Al O ), (2) yttrium–
pattern of the powder mixture containing g-TiAl and 10
wt.% Y O , taken after exposure to 1150 8C during
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3
2
3
2
3
aluminum perovskite YAP (Y O .Al O ), (3) YAP9
thermal analysis revealed that the reaction between g-TiAl
2
3
2
3
(
Y O .Al O ), and (4) yttrium–aluminum monoclinic
and Y O leads to the formation of elemental titanium
2
3
2
3
2
3
YAM (Y O .Al O ). In the present study however, a
along with Y–Al mixed oxide, Al Y O (2Y O .Al O ).
2
3
2
3
2 4 9 2 3 2 3
lesser known mixed oxide Al Y O (2Y O .Al O ) was
The present study clearly demonstrates that aluminum in
2
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9
2
3
2
3
precipitated in g-TiAl matrix. As per the reaction repre-
the combined form either as g-TiAl or Al O reacts in a
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3
sented by Eq. (1), addition of 1 g of Y O would result in
similar manner when MA with Y O and the milled
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3
2 3
reduction of 1.2 g of TiAl to elemental titanium. Thus use
powder mixture is heat treated at 1150 8C. In either case
of Y O as dispersoid in g-TiAl results in transformation
there is formation of Al Y O (2Y O .Al O ). Because of
2
3
2
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9
2
3
2
3
of some amount of g-TiAl into elemental titanium. This
partial transformation of g-TiAl caused by addition of
Y O is likely to alter the properties of the resulting
the very fine grain size of the g-TiAl matrix and dispersion
of nanocrystalline precipitated mixed oxide phase along
g-TiAl grain boundaries, Y O dispersed g-TiAl following
2
3
2
3
material.
However, the ultra fine Al Y O dispersed along the
heat treatment at 1150 8C is likely to be strong and creep
resistant.
2
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9
grain boundaries in g-TiAl matrix as seen in Fig. 1 could
mean that this mixed oxide is available for causing the
restriction of grain boundary movement at elevated tem-
perature, thereby making the material creep resistant. The
heat treatment is likely to impart room temperature
strength to Y O dispersed g-TiAl due to the fine grain
References
[1] W. Guo, A. Iosanna, M. Magini, S. Martelli, F. Padella, J. Mater.
Sci. 29 (1994) 2436–2444.
2
3
[2] C.C. Koch, Mater. Sci. Technol. 15 (1991) 194–198.
size of g-TiAl matrix and dispersion of non-shearable
second phase particles in the form of Al Y O .
[3] B.S. Murty, S. Ranganathan, Int. Mater. Rev. 43 (1998) 101–139.
2
4
9
[4] J.S.C. Wang, S.G. Donnelly, P. Godavarti, C.C. Koch, Int. J. Powder
Metals 24 (1988) 315–325.
[
[
5] M. Mujahid, J.W. Martin, Mater. Sci. Technol. 10 (1994) 703–710.
6] M. Raghavan, J.W. Seeds, R. Petkovic-luton, Metall. Trans. 13A
4
. Conclusions
Heat treatment of g-TiAl dispersed with 2 wt.% Y O at
(
1982) 953–957.
[
7] S. Naka, H. Otor, E. Bouchaud, T. Khan, Scripta Metall. 23 (1989)
2
3
501–505.
1
150 8C for 3 h resulted in fine dispersoids of Al Y O in
2 4 9
[8] C. Perrier, S. Naka, L.P. Kubin, Scripta Metall. 23 (1989) 477–482.
[9] M. Fujiwara, T. Nishida, Kobelco Technol. Rev. (1990) 43–46.
an ultra fine grained g-TiAl matrix. X-ray diffraction