Effect of Cr on microstructure and oxidation behavior of TiAl-based alloy with high Nb
Effect of Cr on microstructure and oxidation behavior of TiAl-based alloy with high Nb
Document code: A 2
0 1. Key Laboratory of Superlight Materials and Surface Technology, College of Materials Science and Chemical Engineering, Harbin Engineering University , Harbin 150001 , China; 2. College of Power and Energy Engineering, Harbin Engineering University , Harbin 150001 , China
1 , and Zhi-Ming Li
2 Zhu-Hang Jiang
3 He-Xin Zhang
Three novel multi-microalloying TiAl-based alloys containing high Nb were designed and fabricated. Thermogravimetric method was applied to investigate the influence of Cr on the oxidation behavior of high NbTiAl alloy at 1,073 K for 200 h in laboratory air. The 2at.% and 4at.% Cr were added into the alloy, (respectively named 2Cr and 4Cr compared to the Cr-free ternary alloy, 0Cr alloy). The alloys' microstructure and composition as well as the composition distribution of the oxidation scale were analyzed by means of Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS), and X-Ray Diffractometry (XRD). The results show that the addition of Cr decreases the grain size of the Nb-TiAl alloy and leads to a transformation from a fully lamellar structure to a nearly fully lamellar structure. When oxidized at 1,073 K for 200 h, the oxidized mass gain of the alloy increases with an increase in Cr addition amount in the first 100 h and decreases in the last 100 h. With the increase of Cr content, the oxidation surface turns compact but uneven in morphology, which may affect the oxidation resistance of the alloy by increasing the peeling off risk of the oxidation layer at friction conditions.
TiAl-based alloys; microstructure; oxidation; Cr addition
Rbeen an essential aspect in aircraft manufacturing
ecently, research on lightweight materials has
and aerospace industry. The γ-TiAl based alloys with
high content of Nb are promising candidates as a new
high temperature material due to their low density,
excellent specific strength, specific stiffness, and good
high temperature oxidation and creep resistance [
Compared with nickel-based superalloy, which has
been widely used for more than fifty years, TiAl alloy
containing high Nb is lighter in weight. The high Nb
TiAl alloy exhibits superior creep properties and high
temperature oxidation resistance compared to normal
TiAl based alloy over 973 K [
TiAl-based alloys have high mechanical strength
both at room and high temperatures because of their
ordered structure, but single γ phase TiAl-based alloys
have very low room temperature ductility. The near γ
phase TiAl based alloy shows better ductility with the
presence of a small amount of the second α2 phase. The
microstructure of the near γ phase is of α 2-γ layered
structure which is called full lamellar structure. The
room temperature ductility of the near γ phase TiAl
based alloy is closely related to the grain size and
microstructure distribution. Small grain size and good
grain boundary strength can significantly improve the
toughness of the alloy. One of the most interesting
approaches to decrease grain size is
micro-multicomponent alloying, in which a variety of alloying
elements are added in trace amounts. The near γ-phase
TiAl-based alloys have very good solid solubility for
many alloying elements, so micro-multi-component
alloying is very effective for the microstructure
optimization of the material. Alloying elements such as
Cr, B, Si and W are usually added in the TiAl alloy. It
has been found that the addition of Nb can significantly
improve the high temperature oxidation resistance of
TiAl alloy [
]. The addition of alloying elements
such as Cr, Mn and V may improve the plastic
deformation properties of TiAl-based alloys. It is also
proved that the high addition amount of Cr (<8at.%)
has a good effect on the high temperature oxidation
resistance at a temperature higher than 1,073 K , but
excess Nb and Cr addition may lead to poor ductility
at room temperature [
]. Therefore, it is necessary
to determine the effect of Cr on the microstructure
evolution and oxidation behavior of TiAl-based alloys
with high Nb.
In this study, in order to determine the effect of
Cr on microstructure and oxidation resistance of the alloys,
a low range of atom percentage of Cr (<5at.%) was added
to TiAl-based alloy with a high content of Nb (8at.%). At
the same time, 0.8% B, 0.2% Si and 0.5% W were added,
focusing on strengthening the grain boundary by solid solution
strengthening effect so as to improve the strength of the
alloy. In addition, high temperature oxidation behavior of the
three novel TiAl alloys with different percentages of Cr was
1 Experimental procedures
1.1 Specimen preparation
The three novel designed alloys are
Ti-45Al-8Nb-0.2Si-0.5W0.8B, Ti-45Al-8Nb-2Cr-0.2Si-0.5W-0.8B, and
Ti-45Al-8Nb4Cr-0.2Si-0.5W-0.8B. The raw materials used include pure Al
(≥99.9%), titanium sponge (≥99.7%), pure niobium (≥99.8%),
Cr (≥99.98%), Mo (≥99.7%), W (≥99.95%), and AlB alloy
(2.5%-3.5% mass fraction of B). Vacuum melting was applied
to cast the materials into button ingots in a non-self-consumed
tungsten electrode argon arc vacuum furnace. Before melting,
the furnace was firstly vacuumed to 0.005 Pa, then argon was
used as a protective gas and pressured in the furnace, and finally
stabilized at 0.03 Pa. The melting process was repeated four times
for material homogenization through the overturning platform.
Then, the melt was cooled down in water cooled copper crucible
into button ingots of around 40 g. The ingots were cut into small
specimens with the dimension of 10 mm × 10 mm × 1 mm after
homogenization treatment in α-phase region (1,473 K) for 24 h.
Then the specimens experienced a cyclic heat treatment, which
is a three-time heat treatment at 1,423 K for 4 h with air cooling.
The homogenization treatment can reduce the micro segregation
of the composition. The cyclic heat treatment is aimed for
obtaining a fine microscopic fully lamellar microstructure, since
a three-time heat treatment allows better transformation from
original α phase to α2-γ laminar phase. The air cooling afterwards
may further refine the grain. After the heat treatment, the ingots
were polished using mesh number 1500 SiC water proof abrasive
papers before being cleaned in acetone for 15 min using an
ultrasonic cleaning machine.
1.2 Isothermal oxidation
Isothermal oxidation experiment was carried out at 1,073 K
for 200 h under 1 atm using a chamber electric furnace. The
specimens were moved out of the furnace after a certain fixed
period of time. Their mass gain was measured by electric
balance with a resolution of 0.01 mg after cooling down to room
temperature. At least two specimens were tested at a time so as
to minimize data error.
1.3 Microstructure observation
The ingots were tested by a back-scattered scanning electron
microscope (SEM) before and after heat treatment. The
oxidation samples were also examined by SEM. For the
purpose of observing oxidation section, specimens were cut
perpendicular to the surface of oxide and were ground using
2000 grit silicon carbide paper before etching. For oxidation
surface observation, no special treatments are needed except
keeping it clean. In order to find out the structure of oxidation
scales, line scanning and EDS analysis were used for oxide layer
2 Results and discussion
Figures 1(a), (c) and (e) are ingots of 0Cr, 2Cr and 4Cr alloy in
as-cast condition, while Figs. 1(b), (d) and (f) are ingots after
It can be found from Fig. 1 that 0Cr alloy is of fully lamellar
microstructure in as-cast condition, which transformed to
near fully lamellar microstructure with smaller grain size and
thinner interlamellar spacing after heat treatment. Fully lamellar
microstructure is composed of alternating lamellae of α2-Ti3Al
and γ-TiAl. It has been found that fully lamellar structure of
γ-TiAl intermetallics is beneficial to the fracture toughness and
high-temperature strength, but the room temperature ductility of
the alloy is poor due to the inhomogeneous distribution of γ-TiAl
intermetallics in as-cast condition [
]. The microstructure after
heat treatment can surely improve the mechanical properties
of the alloy by refining grain size and improving component
homogeneity. The appearance and increase of single γ phase at
the grain boundary of the original α2-γ fully lamellar structure
comes from a transformation from high-temperature α to
highly faulted γ phase, which commonly occurs in the rapid
cooling process that suppresses diffusion. The result of the
transformation is the increase of γ phase and decrease of α2-γ
layer combination, i.e. decomposition of the mixture of α2 and γ
to single γ phase.
In general, there was a transformation from α phase to
γ phase for 2Cr and 4Cr alloys after heat treatment, which
formed a better near fully lamellar microstructure. The 2Cr
alloy showed smaller grain size and thinner layer space after
heat treatment, while 4Cr alloy barely changed in grain size.
This means that the increase of Cr content in the alloy can
effectively refine the grain size of the alloy, and that the heat
treatment process has a more obvious grain refinement effect
on the lower Cr content alloy.
A f t e r h e a t t r e a t m e n t , t h e s i n g l e - p h a s e γ p h a s e w a s
precipitated at the grain boundary of the α2-γ lamellar structure,
resulting in a better grain boundary oxidation resistance of the
alloy, because the γ-TiAl phase could produce more Al2O3 on
the oxidation layer and provide better oxidation protection than
the α2-Ti3Al phase.
The isothermal oxidation kinetics of 0Cr, 2Cr and 4Cr alloys
under laboratory air at 1,073 K are shown in Fig. 2. The mass
gain of 0Cr, 2Cr and 4Cr alloys were 0.52684, 0.63218 and
0.67203 mg•cm-2, respectively, after 200 h oxidation, which are
much lower than that of conventional TiAl binary alloy, about
1 mg•cm-2 or more at similar oxidation condition [
]. It is clear
that all three alloys have superior oxidation resistance compared
with the conventional TiAl based alloy. There are two points that
Fig. 1: Back-scattered SEM metallograph of 0Cr alloy (a and b), 2Cr alloy (c and d), 4Cr alloy (e and f)
(a, c and e are in as-cast condition, b, d and f are heat-treated)
Fig. 2: Isothermal oxidation kinetics at 1,073 K: (a) 0Cr
alloy, (b) 2Cr alloy, (c) 4Cr alloy
should be noticed in the kinetics results: firstly, the final mass
gain increases with an increase in Cr content in a quite small
range. Secondly, increase of Cr content slows down the mass
gain rate, while 0Cr alloy keeps an almost fixed rate of about
1.31×10-3 mg•(cm2•h)-1. As for 2Cr alloy, the oxidation rate
changed from 5.27×10-3mg•(cm2•h)-1 in the first 50 h to 1.05×10-3
mg•(cm2•h)-1 in the last 100 h, which is 1/5 of that in the first 50 h,
and much lower than that of 0Cr alloy.
It is clear that the oxidation scale increases along with Cr
addition in the first 100 h. In the last 100 h, the oxidation mass
gain increase of 4Cr alloy was 0.08064 mg•cm-2, which is the
least of all in this period, while 0Cr alloy was oxidized in 0.10537
mg•cm-2, which highlighted that Cr has a long term oxidation
resistance for high Nb-TiAl alloy. After the first 50 h, as the
CHINA FOUNDRY RVoels.1e5arNcoh.1&JaDnueavreylo2p01m8ent
Cr content increases, the curve changes from near straight to
convex, which may be due to the fact that the oxidized surface
formed by Cr addition became more compact and thus slowed
down the rate of oxidation.
The SEM micrographs in Fig. 3 show the morphology of the
oxide formed on those three alloys after 200 h oxidation. Figure
3(a) for 0Cr alloy shows the most compact and continuous
structure in the three alloys. Some projections are clearly
observed in Fig. 3(b), as are circled in the figure, probably
caused by the growing addition of Cr. For the 4Cr alloy in Fig.
3(c), there are many nodular oxidation structures appearing
on the oxidation surface, which seems to be a joining of the
oxidation structure in Fig. 3(b), but larger in size and more
diffuse in distribution. These oxidative structures may have a
certain relationship with the chromium oxide and directly lead
to accelerated oxidation of the alloy within the first 100 h.
The section SEM scanning results in Fig.
4 clearly shows the change in oxidation
structure from 0Cr to Cr-added alloys (2Cr
and 4Cr alloys). There are three oxidation
layers in 0Cr alloy, and the inner layer is
thicker than the other two layers. As has
been learned from the kinetics curves in
Fig. 2, the 0Cr alloy oxidizing mass gain
increased at an almost fixed rate, which
means the matrix of the ingot kept reacting
with air and the inner layer of the oxidation
scale kept increasing in width. As a result,
0Cr alloy has the largest range of oxidation
scale (4.8 μm in average) in the three alloys
(3.8 μm for 2Cr alloy and 2.9 μm for 4Cr
alloy). There are two oxidation layers in
both 2Cr and 4Cr alloys, which differ from
that of the Cr-free alloy.
The oxidation surfaces of three alloys
were tested by X-Ray Diffractometry
(XRD), as shown in Fig. 5. The oxidation
surfaces of the three alloys mainly consist of
Ti3O, TiO2, SiO2, NbO, and (Al0.9Cr0.1)2O3
(a close binder of Al 2O3 and Cr2O3), in
which the (Al0.9Cr0.1)2O3 oxide exists in
the oxides of 2Cr and 4Cr alloys.
The line scanning tests of the transverse
sections of the three alloys are shown
in Fig. 6, by which we can analyze the
constitution of the oxidation layers. There
are three layers in 0Cr alloy, the same as
being observed in Fig. 4. The inner oxide
Fig. 4: SEM scanning of oxidized section: (a) 0Cr alloy, (b) 2Cr
alloy, (c) 4Cr alloy
layer is the starting position of oxidation. The matrix reacted
with oxygen and formed an oxide mixture consisting of TiO2,
Al2O3 and a small amount of NbO. Figure 6 (a) shows that
most of the protective Al2O3 formed on the outer layer of the
0Cr alloy. The intermediate oxide layer consists of titanium
oxides, which is not a compact structure. The two separated
layers of TiO2 and Al2O3 makes it hard to prevent the oxygen
from entering the matrix. That means the oxidation structure
formed in 0Cr alloy cannot supply a continuous protection for
the matrix alloy from further oxidation. There are mainly two
oxidation layers in 2Cr alloy. The inner layer is composed of
titanic oxide, chromic oxide and their mixture. The outer layer
comprises Al2O3 and Cr2O3. As for the 4Cr alloy, the layer
composition is similar to the 2Cr alloy, but there is a larger
range and a greater volume fraction of Cr2O3 in the oxidation
layers. The change is caused by the increasing percentage of
Cr element into the alloy. Also, the tendency of the oxidation
kinetics curve can be explained by the Cr 2O3 increase. The
rapid mass gain of Cr-added alloys in the first 100 h resulted
in the incompact TiO2/Al2O3 oxidation structure forming in
the initial period of oxidation. With the Cr increasing, a large
amount of oxide Cr2O3 joins the incompact TiO2 and Al2O3
oxidation structures, forming a tight and compact structure to
prevent oxygen from reacting with the matrix. In addition, the
volume fraction and distribution range of titanic oxides in the
oxidation layers decreased with an increase in percentage of
Cr, especially in the outer layer. The outer oxidation layer is of
vital importance for the oxidation resistance of an alloy because
a compact and uniform oxidation surface layer can prevent the
hot air from entering the matrix and reduce the risk of peeling
off. The addition of 4at.% Cr has a good oxidation resistance
when the Cr oxide forms and distributes in the full range of the
oxidation layers. The oxidation surface turns rugged from 0Cr
to 4Cr alloy; this change may lead to the peeling off of the oxide
layer at friction process.
T h r e e h i g h N b - Ti A l a l l o y s c o n t a i n i n g d i ff e r e n t a t o m
percentages of Cr, i.e. 0Cr, 2Cr and 4Cr were prepared and
their microstructure evolution and antioxidant properties were
studied. The conclusions can be drawn as follows:
(1) Addition of Cr decreases the grain size of the Nb-TiAl
alloy and leads to a transformation from α 2 to γ, i.e., fully
lamellar structure to nearly fully lamellar structure.
(2) After oxidation experiments at 1,073 K for 200 h, the
oxidized mass gain of the alloy increases with an increase in Cr
addition amount in the first 100 h and decreases in the last 100 h.
(3) The formation of Cr oxide in the full range of oxidation
layer, especially in the surface layer during the first 100 h,
improves the compactness and oxidation resistance of the alloy
by joining Cr2O3 to the incompact TiO2 and Al2O3 oxidation
(4) With the increase of Cr content, the oxidation surface
turns compact but uneven in morphology which may decrease
the oxidation resistance of the alloy by increasing the peeling
off risk of oxidation layer under different friction conditions.
This work was financially supported by the Fundamental Research Funds for the Central Universities of Ministry of Education of
China (Grant Nos. HEUCFP 201731 and 201719).
 Krause D , Lerch B , Locci I E . Development and evaluation of TiAl sheet structures for hypersonic applications . Materials Science & Engineering A , 2007 , 464 ( 1 ): 330 - 342 .
 Dimiduk D M. Gamma titanium aluminide alloys-an assessment within the competition of aerospace structural materials . Materials Science & Engineering A , 1999 , 263 ( 2 ): 281 - 288 ( 8 ).
 Shida Y , Anada H. The influence of ternary element addition on the oxidation behaviour of TiAl intermetallic compound in high temperature air . Corrosion Science , 1993 (5 -8 ): 945 - 953 .
 Taniguchi S , Uesaki K , Zhu Y C , et al. Influence of niobium ion implantation on the oxidation behaviour of TiAl under thermal cycle conditions . Materials Science & Engineering A , 1998 , 249 ( 1 ): 223 - 232 .
 Liu Z C , Lin J P , Li S J , et al. Effects of Nb and Al on the microstructures and mechanical properties of high Nb containing TiAl base alloys . Intermetallics , 2002 , 10 ( 7 ): 653 - 659 .
 Yong-zhe Wang , Hong-sheng Ding , Rui-run Chen , et al. A highNb TiAl alloy with highly refined microstructure and excellent mechanical properties fabricated by electromagnetic continuous casting . China Foundry , 2016 , 13 ( 5 ): 342 - 345 .
 He X , Yu Z , Lai X. Analysis of high temperature deformation behavior of a high Nb containing TiAl based alloy . Materials Letters , 2008 , 62 ( 26 ): 4181 - 4183 .
 Bystrzanowski S , Bartels A , Clemens H , et al. Creep behaviour and related high temperature microstructural stability of Ti-46Al9Nb sheet material . Intermetallics , 2005 , 13 ( 5 ): 515 - 524 .
 Chen G , Sun Z , Zhou X . Oxidation of Intermetallic Alloys in TiAl-Nb Ternary System . Corrosion , 1992 , 48 ( 11 ): 939 - 946 .
 Lin J P , Xu X J , Wang Y L , et al. High temperature deformation behaviors of a high Nb containing TiAl alloy . Intermetallics , 2007 , 15 : 668 - 674 .
 Chen G L , Zhang L C. Deformation mechanism at large strains in a high-Nb-containing TiAl at room temperature . Materials Science & Engineering A , 2002 , 329 ( 1 ): 163 - 170 .
 K e s l e r M S , G o y e l S , R i o s O , e t a l . A s t u d y o f p h a s e transformation in a TiAlNb alloy and the effect of Cr addition . Materials Science & Engineering A , 2010 , 527 ( 12 ): 2857 - 2863 .
 Huang S C , Hall E L. The effects of Cr additions to binary TiAlbase alloys . Metallurgical & Materials Transactions A , 1991 , 22 ( 11 ): 2619 - 2627 .
 Zhao Lili , Lin Junpin , Wang Yanli , et al. Early oxidation behaviors of Ti50Al and Ti45A18Nb alloys at high temperature . Acta Metallurgica Sinica , 2008 .
 Chen H , Su Y , Luo L , et al. Influence of silicide on fracture behavior of a fully lamellar Ti-46Al-0 . 5W - 0 .5Si alloy. China Foundry , 2012 , 09 ( 2 ): 108 - 113 .
 Jian-chong Li , Rui-run Chen , Zhi-kun Ma , et al. Effect of boron on microstructure and mechanical properties of cast Ti-44Al-6Nb ingots . China Foundry , 2015 , 12 ( 1 ): 9 - 14 .
 Cheng T T , Willis M R , Jones I P. Effects of major alloying additions on the microstructure and mechanical properties of γ-TiAl. Intermetallics , 1999 , 7 ( 1 ): 89 - 99 .
 Tian W H , Nemoto M. Effect of carbon addition on the microstructures and mechanical properties of γ-TiAl alloys . Intermetallics , 1997 , 5 ( 3 ): 237 - 244 .
 Niu H Z , Chen Y Y , Xiao S L , et al. Microstructure evolution and mechanical properties of a novel beta γ-TiAl alloy . Intermetallics , 2012 , 31 : 225 - 231 .