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Intermetallics 27 (2012) 6e13

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Intermetallics

journal homepage: www.elsevier .com/locate/ intermet

Synergistic effect of Y and Nb on the high temperature oxidation resistance of highNb containing TiAl alloys

L.L. Xiang a, L.L. Zhao a,b, Y.L. Wang a, L.Q. Zhang a, J.P. Lin a,*

a State Key Laboratory for Advanced Metals and Materials, University of Science & Technology of Beijing, Beijing 100083, Chinab State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China

a r t i c l e i n f o

Article history:Available online 20 February 2012

Keywords:A. Titanium aluminides, based on TiAlB. MicroalloyingB. Oxidation

* Corresponding author.E-mail address: [emailprotected] (J.P. Lin).

0966-9795/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.intermet.2012.01.015

a b s t r a c t

The synergistic effect of Y and Nb addition on the long time isothermal and cyclic oxidation behaviors ofTie45Ale(6e9)Nb alloys containing 0.1e0.4 at.% Y was investigated in air at 900 �C. The results showedthat the high temperature oxidation resistance was improved with an increase of Nb content. However,even 9 at.% Nb content was not sufficient to form the perfect Al2O3 layer. Scale spallation still occurredunder hundreds of cyclic exposure. Y addition remarkably enhanced the long time oxidation resistance ofhigh Nb containing TiAl alloys, and only a proper amount of Y addition could improve the long timeoxidation resistance under both isothermal and cyclic oxidation. In this work, it was concluded that theprinciple of Y addition was choosing the minimum amount on the premise of meeting enough spallationresistance. Moreover, the optimum amount of Y changed regularly with the variation of Nb content. Thereasonable addition of Y decreased from 0.4 at.% to 0.2 at.% with Nb content increasing from 6 at.% to9 at.%. Based on synergistic effect of Y and Nb, the best oxidation resistance of alloys was achieved.

1. Instruction

TiAl intermetallic compounds are a promising class of structuralmaterials used in aerospace, automotive and gas turbine industriesdue to their low density, high specific strength, good oxidationresistance and creep resistance [1,2]. Recently, GE have announcedthat gamma TiAl low pressure turbine blades were used on its GEnxengine, which powered the Boeing 787 and Boeing 747-8 aircrafts.This was the first large-scale use of this material on a commercialjet engine, which became a milestone of application of TiAl alloys.However, the conventional TiAl alloys can not be used at highertemperature (about 700 �C). The high Nb containing TiAl alloysdeveloped by Chen et al were the first example of the TiAl alloysapplication at high temperature (up to 900 �C) [1e9]. This potentialapplication makes the high Nb containing TiAl alloys a tendency totake place of Ni based superalloys [3e9].

However, the poor anti-spalling capacity of scale under longtime oxidation limits the further application of high Nb containingTiAl alloys. How to improve the long time oxidation resistance forthis kind of TiAl alloys is still a critical issue for the application. Agreat deal of research work has been undertaken to improve the

oxidation behavior of these alloys by alloying and surface treat-ment, of which alloying is the basic and most important one.Studies have shown that high Nb containing TiAl alloys witha proper amount of Y performed outstanding long-term oxidationresistance because of the strengthened spallation resistance ofscale [9e15]. In our previous work, we found that 0.3 at.% Yaddition could enhance the adherence of oxide scale onTie45Ale8Nb alloys while guarantee low oxidation rate [11]. AndTie45Ale7Nb alloy with 0.4 at.% Y possessed the most excellentlong-term high temperature oxidation resistance [13]. Suggestedby these figures, the optimum content of Y probably has a regularchange along with the variation of Nb content. Y and Nb wouldhave some synergistic effect. But until now, no research on thesynergistic effect of Y and Nb has been reported. Therefore, theaim of this paper is to continue discussing the effect of Y on highNb containing TiAl alloys and more importantly, to explore thesynergy of Y and Nb on improving the long-term oxidationresistance of alloys.

In present work, the range of Nb content was further expandedto 6 at.% and 9 at.%. Different content of Y (0e0.4 at.%) was addedinto two series of Tie45Ale6Nb and Tie45Ale9Nb alloys, respec-tively. The optimum addition of Y content of each series wasobtained by long-term isothermal oxidation experiments and cyclicoxidation experiments. Then the general law of optimum content ofY changing along with variation of Nb was studied. The principle of

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L.L. Xiang et al. / Intermetallics 27 (2012) 6e13 7

Y and Nb addition in high Nb containing TiAl alloys could providea theoretical support and experimental base for the compositiondesign of alloys.

2. Experimental

The alloys with nominal composition of Tie45Ale6Nbe(0,0.1,0.2,0.3,0.4)Y and Tie45Ale9Nbe(0,0.1,0.2,0.3,0.4)Y (at.%)were prepared by tungsten-arc melting in an argon atmosphere(KWII model). To minimize compositional segregation the ingotsweremelted four times thenunderwent a heat treatment for being offully lamellar microstructure (FL). The 6 at.% Nb containing alloyswere kept at 1320 �C for 12 h (the 9 at.% Nb containing alloys at1345 �C) in the high temperature furnace (KSY-8-18A model) andthen moved to 900 �C for 30 min in another furnace followed by aircooling. For oxidation testing, the standard specimens with approx-imate sizes of 10 � 10 � 1 mm3 were obtained by electro-dischargemachine. Subsequently, their surfaces were ground by SiC abrasivepaper up to No.1200 followed by ultrasonic cleaning for 15 min inacetone. Lastly, the initialweight and the sizes of each specimenwererecorded after drying out for calculating the total surface area andmass gain later.

In this paper, oxidation tests include isothermal oxidation testsand cyclic oxidation tests at 900 �C. Isothermal oxidation tests wereperformed in a box furnace in static laboratory air for 100 h. Cyclicoxidation testswere carried out in a cyclic oxidation furnacewith anauto moving beam (YHL 1.5-12 model). One cycle was composed ofoxidation for 1 h at 900 �C and 12 min at ambient temperature. Thespecimenswere inserted into and removed from the hot zone of thefurnace within a few seconds to guarantee a rapid heating andcooling. Theweight of each specimenwas recorded every 20 cycles.

The original microstructure of each specimen was observed byoptical microscope (OM). The surface and cross-sectional micro-structure of scale after oxidation were analyzed by the model ofZEISS SUPRA55 three-dimensional field emission scanning electronmicroscopy (SEM). The energy dispersive spectroscopy (EDS)equipped in SEM was used to characterize the cross-sectionalelement distribution of scale. The carriers with cross-sectionspecimens were sprayed by carbon on the surface to improve theconductibility.

Fig. 1. Optical micrographs showing microstructures of Tie45Ale6Nb and Tie45Ale9Tie45Ale6Nbe0.2Y. (c) Tie45Ale6Nbe0.4Y. (d) Tie45Ale9Nb. (e) Tie45Ale9Nbe0.2Y. (f) T

3. Results

3.1. Microstructure observation

Optical microstructures of the alloys before oxidation are shownin Fig. 1 from which equiaxed fully lamellar microstructures areobserved. The grain size gradually decreases as Y content increasesfrom 0 to 0.4 at.% either on Tie45Ale6Nb alloys or Tie45Ale9Nballoys. The reason is that Y-enriched phase is easy to segregatealong crystal grain boundaries.

It was reported that the TiAl alloys with fine fully lamellarmicrostructure possessed the best high temperature performance[16]. The high Nb containing TiAl alloys without Y only obtainedcoarse fully lamellar microstructure. Y addition could not onlyimprove the high temperature oxidation of alloys but also enhancethe mechanical properties through refining the fully lamellarmicrostructure.

3.2. Isothermal and cyclic oxidation kinetics

The mass gains of Tie45Ale6Nb and Tie45Ale9Nb alloys with0e0.4 at.% Y content after isothermal oxidation at 900 �C for 100 hare shown in Fig. 2. On Tie45Ale6Nb series, there are different levelof scale spallations on the alloys with Y content less than 0.3 at.%. Y-free alloy displays the most serious mass loss which could beimproved by addition of Y. When Y content is up to 0.3 at.%, thescale shows a good anti-spallation ability. Obviously, Y-added alloyshas lower mass gains than Y-free alloy and the alloy with 0.3 at.% Yshows the lowest. Therefore, on Tie45Ale6Nb series, 0.3 at.%Y-added alloy has the best oxidation resistance under 100 hisothermal oxidation. On Tie45Ale9Nb series, oxide scale spall-ations aren’t observed after 100 h oxidation. The mass gains ofY-added alloys (0.4 at.% Y-added alloy excluded) are much less thanthose of Y-free alloy, and the mass gains of 0.2 at.% Y-added alloy isthe lowest. Similar to 6 at.% Nb alloys, the oxidation growth of0.4 at.% Y-added alloy is faster than those of other four alloys. So0.2 at.% is the optimum content of Y on Tie45Ale9Nb series underisothermal oxidation at 900 �C for 100 h.

Comparing the results of Tie45Ale6Nb and Tie45Ale9Nballoys, the mass gains of 9 at.% Nb alloys are less than those of

Nb alloys with different Y addition after heat treatment. (a) Tie45Ale6Nb. (b)ie45Ale9Nbe0.4Y.

0.0 0.1 0.2 0.3 0.4

0.0

0.4

0.8

1.2

1.6

ss g

ain

g/cm

4.0

4.5

5.0

5.5

ain

g/c

Oxidation cycles

Ti45Al9Nb0Y Ti45Al9Nb0.1Y Ti45Al9Nb0.2Y Ti45Al9Nb0.3Y Ti45Al9Nb0.4Y

under long-term cyclic oxidation. One cycle was composed of 1 h at 900 �C and 12 min

Fig. 4. SEM micrographs of the surface morphologies of Tie45Ale6Nb and Tie45Ale9Nb alloys with different Y addition after long-term cyclic oxidation (aec: 460 cycles; def:1000 cycles). (a) Tie45Ale6Nb. (b) Tie45Ale6Nbe0.2Y. (c) Tie45Ale6Nbe0.4Y. (d) Tie45Ale9Nb (e) Tie45Ale9Nbe0.2Y (f) Tie45Ale9Nbe0.4Y Note: the insets are themorphologies where out-layer have spalled off in (a), (d) and the lower magnification micrograph of surface in (f).

L.L. Xiang et al. / Intermetallics 27 (2012) 6e13 9

Tie45Ale6Nb alloys, the oxide particles grew on Tie45Ale9Nballoys are denser and finer. The EDS analysis result listed inTable 1 shows that Ti oxides decreasewhile Al oxides increase by anincrease of Nb or Y addition. (The actual content of Al onTie45Ale6Nbe0Y and Tie45Ale9Nbe0Y is lower than that fromthe EDS results, because the inner Al-enriched layers were exposedas some outer layers of scale spalled off shown in the insets ofFig. 4(a) and (d), and Ti are opposite to Al). The XRD diffractionspectra of oxide scale are shown in Fig. 5. Results illustrate there aretwo types of Al2O3 formed on Tie45Ale9Nbe0.4Y surface, whichare a-Al2O3 and q-Al2O3. Combined with the typical details of scaleshown in Fig. 6 and the EDS analysis shown in Table 2, it could beconcluded that the coarse crystalline particles formed on surfaceare rutile TiO2, and the denser refined particles are protective Al2O3shown in Fig. 4. In addition, the existence of Ti4O7 demonstrates

Table 1The EDS analysis of oxide film of alloys after long-term cyclic oxidation.

Element Line Tie45Ale6Nb Tie45Ale6Nbe0.4Y

atom% atom% error atom% atom% e

O K 68.15 �2.90 69.41 �3.22Al K 4.44 �0.17 7.89 �0.24Ti K 27.41 �0.39 22.70 �0.41

that TiO2 is a non-metal deficiency n-type oxide and cause oxygenvacancy in alloys. The TiNb2O7 peak detected in Fig. 5 shows that Nbfunction as a dopant element with a higher valence than Ti. OnY-added alloys surface, the oxide particles are in spherical-likeshape shown in Fig. 4(b), (c), (e) and (f). The results of typicaldetails of spherical-shaped particles displayed in Fig. 6 reveal thatrutile TiO2 crystalline (marked ‘A’) is surrounded by dense and fineAl2O3 particles (marked ‘B’). In our previous study, L. L. Zhao et alfound that there is Y-rich phase in the spherical-shaped particles[11]. With more addition of Y, the amount of spherical-shapedparticles increase and the oxides are refined, as shown in Fig. 4.

The results indicate that the Y and Nb have synergy on pre-venting the oxidation of Ti while enhancing the selective oxidationof Al, but the oxidation resistance of alloys is not always improvedwith an increase of Y. Studying the influence of Y content on the

Tie45Ale9Nb Tie45Ale9Nbe0.4Y

rror atom% atom% error atom% atom% error

67.38 �2.60 66.69 �2.8610.96 �0.24 11.19 �0.2621.66 �0.34 22.12 �0.36

Fig. 6. SEM micrograph of the spherical-like shape on the surface ofTie45Ale9Nbe0.4Y alloy after cyclic oxidation for 1000 cycles.

L.L. Xiang et al. / Intermetallics 27 (2012) 6e1310

scale adhesion to substrate and clarifying the role of inter-oxidationinduced by Y require the scale cross-sectional observation andanalysis which will be discussed as follows.

3.3.2. Cross-section analysis of oxide scalesThe cross-sectional morphologies of oxide scale after long-term

cyclic oxidation at 900 �C are presented in Fig. 7. The Y-free alloysreveal the flat interface between scale and substrate while it isconvex-shaped and extend to the substrate on the Y-added alloys.However, the coarse-grained Y2O3 and severe internal oxidation areobserved on excessive Y-added alloys, and the roots-like internaloxides extend the matrix deeply as shown in Fig. 7 (c) and (f). Thecross-sectional elemental scanning and composition analysis ofTie45Ale9Nbe0.4Y are exhibited in Fig. 8 and Table 3. The whiteparticles surrounded by serious internal oxides in the oxide scaleare Y oxides marked as ‘A’ in Fig. 8. Fig. 9 shows the scale cross-sectional elements distribution of Tie45Ale6Nbe0.2Y. Trace ofNb and Ti in the same trend indirectly proves the theory that Nbatoms present in the normal position of Ti, and the layer-structureof scale is clearly visible but disappears when adding more contentof Y as observed in Fig. 7.

According to the observation and analysis of the cross-sectionof scale after cyclic oxidation at 900 �C, the conclusion can bedrawn that Y plays an important role in the formation ofmorphology of interface between matrix and scale as well asdetermining whether the serious internal oxidation occurs or notby variation of content.

4. Discussion

As regards the oxidation resistance of alloys, oxidation growthrate and oxide scale spallation resistance should be considered. Theformer is related to the properties of oxide scale, and the degree ofinternal oxidation. The latter is mainly influenced by themorphology and characteristics of interface between substrate andscales. Addition of alloying elements can affect one or more thefactors above. The generated effects of different alloying elementsaddition would be opposite to or synergistic with each other.

There are several types of mechanisms of Nb lowering theoxidation growth rate given by previous researchers. Overall, thebasic theory is that Nb element is beneficial to the formation ofprotective Al2O3 while prevent the growth of non-protective TiO2.

20 30 40 50 60 70 80

θ -Al2O3

2 (degree)

Ti45Al9Nb0.4Y

CPS

rutile TiO2

α -Al2O3

Ti4O7

TiNb2O7

Fig. 5. XRD diffraction spectra of the oxide scale on Tie45Ale9Nbe0.4Y alloy afterlong-term cyclic oxidation.

In this paper, one of the important mechanisms will be dis-cussed as follows. It has been found that Nb substituted for Ti inTiO2 as a cation with a valence of 5 to form (Ti,Nb)O2 in the inneroxide layers [4]. TiO2 is the non-metal deficiency n-type oxide andis often expressed as TiO2-x. The reaction is:

TiO2 ¼ TiO2�x þ x=2O2ðgÞ (1)

So the deficiency reaction is given by:

OO ¼ V��O þ 2e0 þ 1=2O2ðgÞ (2)

where OO denotes an O anion on a normal site, V��O is an oxygen

vacancy, e0 is an electron. The V��O is regarded as a major defect.

When a dopant with a higher valence than that of Ti, it is expectedto decrease the oxygen vacancy concentration due to the electro-neutrality in the oxide. The reaction for the doping effect of Nb canbe written as:

Nb2O5 þ V��O ¼ 2Nb�Ti þ 5OO (3)

where the Nb. Ti denotes Nb5þ cation on a normal Ti-site. Thesubstitution of two Nb5þ cations can reduce one V��

O . As a result,oxygen diffusion via a vacancy mechanism in the (Ti, Nb) O2 issuppressed following a dynamical law. The marker experimentsshowed that the TiO2 growth is governed by inward oxygen diffu-sion [17]. Thus, Nb addition can suppress the TiO2 growth in theinner oxide layer.

Unfortunately, when alloys are served under long-term cyclicoxidation or long-term oxidation, the weak adhesion betweenscale and substrate and internal stress increasing with the thick-ening of scale will lead to the spallation of scale and alloy failurefinally. The present work demonstrated that the Y element couldimprove the adhesion of scale greatly. Especially, the excellentoxidation resistance of alloys would be obtained by addition ofa proper amount of Y.

Table 2The EDS analysis of the selected regions of the oxide scale surface after cyclicoxidation shown in Fig. 6.

Element Line ‘A’ ‘B’

atom% atom% error atom% atom% error

O K 76.43 �2.28 69.37 �1.15Al K 0.70 �0.06 27.26 �0.25Ti K 22.87 �0.29 3.37 �0.09

Fig. 7. SEM micrographs of the cross-sectional morphologies of Tie45Ale6Nb and Tie45Ale9Nb alloys with different Y addition after long-term cyclic oxidation (aec: 460 cycles;def: 1000 cycles). (a) Tie45Ale6Nb. (b) Tie45Ale6Nbe0.2Y. (c) Tie45Ale6Nbe0.4Y. (d) Tie45Ale9Nb (e) Tie45Ale9Nbe0.2Y (f) Tie45Ale9Nbe0.4Y.

L.L. Xiang et al. / Intermetallics 27 (2012) 6e13 11

The solid solubility of Y element in g and a2 is less than 0.1 at. %,so Y is easy to segregatewhen the addition ismore than 0.1 at. %. Ourpreviouswork has demonstrated that there are two exists of Y, mostof them segregate along the grains boundaries while some fineparticles are uniformly dispersedwithin the grains, results revealedthe phases along the grain boundaries are YAl2 and the fine particles

Fig. 8. The higher magnification of cross-sectional morphology and Y ele

within the grains are Y2O3 [11]. Y is an active element with a strongaffinity to oxygen, and is prone to reactingwith oxygen to formY2O3during the melting processing of experimental alloys, which couldreduce the amount of dissolved oxygen. According to the Wagnertheory, the reduction of dissolved oxygen in alloy contributes tostrengthening the external oxidation of Al [18]. In the process of

ment distribution maps in cross-section of Tie45Ale9Nbe0.4Y alloy.

Table 3EDS analysis of the selected region of the oxide scale surface after cyclic oxidationmarked as ‘A’ shown in Fig. 8.

Element Line Weight% Weight% Error Atom% Atom% Error

O K 24.22 �0.38 60.41 �0.95Al K 4.39 �0.04 6.50 �0.06Ti K 2.75 �0.05 2.29 �0.05Y L 68.84 �0.25 30.80 �0.11Total 100.00 100.00

L.L. Xiang et al. / Intermetallics 27 (2012) 6e1312

oxidation, the YAl2 phase accumulated along the grain boundaries isapt to oxidize because of the activity of Yaccording to this equation:4YAl2 þ 9O2 / 2Y2O3 þ 4Al2O3, and the reaction product Y2O3particles are surrounded by Al2O3 particles which are in spherical-like shape due to the large grain size of Y2O3.As a result, Y addi-tion could promote the formation of protective Al2O3 outer layer

Fig. 9. Element distribution maps of the cross-section of Tie

during oxidation processes. In addition, when the Y located at theinterface betweenmetal and scale preferentially oxidized, therewillbe convex shape nail-liked oxides formed, which extend to thematrix and play a role in pinning the scale (Fig. 7). Moreover, theconvex shape increases the contact area of the matrix and scale,the microstructure is refined because of the Y segregation alonggrains boundaries devoting to improving the plasticity of scale.With those reasons mentioned above, the adhesion of scale tomatrix is enhanced under long-term cyclic oxidation or long-termoxidation. Therefore, the Y-added alloys possess the anti-spallingcapacity of scale significantly better than Y-free alloys.

However, higher levels of Y would be harm to the oxidationresistance of alloys. Excessive Yaddition can cause the considerablesegregation of YAl2 along the grain boundaries before oxidation.When these YAl2 phase preferential oxidized due to the strongaffinity of Y to oxygen, the coarse Y2O3 will produce closed networkstructure which would act as the inward diffusion channel of

45Ale6Nbe0.2Y alloy after long-term cyclic oxidation.

6 7 8 9 10

0.0

0.1

0.2

0.3

0.4

0.5

e o

ptim

nten

t o

f Y

t.%

)

Nb content ( at.%)

Fig. 10. The optimum content of Y in different high Nb containing alloys.

L.L. Xiang et al. / Intermetallics 27 (2012) 6e13 13

oxygen and result in the deterioration of internal oxidation. Theinternal oxidation rapidly extends to substrate through thenetwork structure from the nail-like oxides, the details are shownin Fig. 7(c) and (d). According to the above analysis, only anappropriate of Y addition can improve efficiently the long-termoxidation resistance of high Nb containing TiAl.

On the Tie45Ale6Nb alloys, 0.3 at.% Y-add alloy performed thebest oxidation resistance under the 100 h isothermal oxidation at900 �C. However, when served in the long-term cyclic oxidation at900 �C, 0.3 at.% Y-added alloy still showed scale spallation afterhundreds of cycles and 0.4 at.% Y-added alloy displayed a betteroxidation resistance. As a result, 0.4 at.% is the optimal Y content toensure the alloy adequate anti-spalling capacity in Tie45Ale6Nballoys. On the Tie45Ale9Nb alloys, the 0.2 at.% Y-added alloyalways displayed themost excellent oxidation resistance bothunderthe 100 h isothermal oxidation and long-term cyclic oxidation at900 �C. Our previous work [12] reported that the Tie45Ale7Nbadding 0.4 at.% Y alloy had the excellent high temperature oxida-tion resistance, while the alloy Tie45Ale8Nb possessed the bestoxidation resistance with content of 0.3 at.% Y under long timeoxidation [11]. According to those data, the changing trend ofoptimum content of Y with an increase of Nb is shown in Fig. 10. Itcan be concluded that the optimum content of Y is in a reducingtendency as the Nb content increases. Although the Tie45Ale6Nballoy adding 0.4 at.% Y shows the better long-term oxidation resis-tance, slightly deterioration of internal oxidation still occurs. Thisdeficiency can be improved by adding more amount of Nb or otheralloying method. Result also demonstrates that the best oxidationresistance needs the synergy of Nb and Y, it is impossible to improveoxidation resistance of alloys greatly only by increasing Y content.However, the complex reaction mechanism between Y and Nb stillneeds further research and will be the next study focus.

5. Conclusions

(1) The high temperature oxidation resistance of high Nb con-taining TiAl alloys is enhanced with an increase of Nb content.

But the anti-spalling ability of the oxide scale is still poor underlong-term oxidation, and this deficiency can be improved bythe addition of Y element.

(2) When Nb content is fixed, the oxide scale spallation resistanceof alloys is strengthened as Y content increases. However, onlyadding the optimum content of Y would ensure alloys with themost excellent oxidation resistance.

(3) The best performance of oxidation resistance of alloys needsthe synergistic effect of Y and Nb. The optimum Y content hasa decreasing tendency as Nb content increases. The principle ofY addition is choosing the minimum amount on the premise ofmeeting enough spallation resistance on the high Nb contain-ing TiAl alloys.

Acknowledgment

This work was supported by the National Basic ResearchProgram of China (973 Program) under contract No.2011CB605500 and the National Natural Science Foundation ofChina (No.50871127).

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References