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Oxidation behavior of Ti3Al alloyed with niobium and silicon
ScaiptaMc~ca
etMaterialie.Vol. 33, No. 2, pp. 213-217.1995 ~~~Itellae$GrGrarlmnlm
Pergamon
09%716x/95 $9.50 + .oo
0956-716X(95)001506
OXIDATION BEHAVIOR OF T&Al ALLOYED WITH NIOBIUM AND SILICON Guohua Qiu, Jiansheng Wu, Lanting Zhang and Dongliang Lin(T.L.Lii) Department of Materials Science Shanghai Jiao Tong University Shanghai 200030, P.R.China (Received August 30,1994) (Revised January 13,1995) htroduction
T&Al has the potential for aerospace applications due to its superior strength to weight ratio compared to supemlloys [l]. S-stabii elements such as molybdenum, niobium, tantalum, tungsten and vanadium have been added to improve its poor room temperature ductility. Another drawback of T&Al is its lack of high temperature oxidation resistance. Early work[2] showed that the addition of Nb can increase the high temperature oxidation msiatanm of T&Al alloy. Recent work[3,4] revealed the beneficial e.@ectof Si to the high temperature mechanical and oxidation properties. The combined addition of Nb and Si is even better for oxidation resistance of TiAl and T&Al alloys[5], however, the mechanism of improving~h@~ theeftixtofNbandSiisnotclearlyuude&xd. In this paper, the oxidation kinetics of T&Al alloys with a Nb and/or Si addition is investigated and the oxidation me&a&m is discussed. Fmerimental
The alloy buttons were am-melted from pure element metals and drop cast into 0 20x 100 mm ingots. The ~ti~ofthealloysislistedinTable1.Couponsmeasuring10~10~3mmwerecut~omtheingotsby spark erosion aud polished to a 600 grade emery paper finish. The coupons were oxidized in open air at high temperature for dBerent times and then weighed. The oxide scale on the coupons after 100 hours of exposure in hot air was analyzed by scanning electron microscopy(SEM), electron probe analysis(EPMA) and X-ray di.&ction(XRD).
Redts and Discussion The microstructure of Alloy 400, with 11 at% Nb addition, shows the tie basket-weave feature which is stated in many references[6]. Alloys 305 and 405, with the addition of 5 at?? Si, give another phase Ti,Si,, which forms a fine eutectic microsuucture[7]. Specific wei&t change vs time curves of different alloys at 800°C and 900°C, respectively, are shown in Fig. 1. The stoichiometric T&Al (alloy 300) gains weight extremely rapidly. It gains weight of 8.45 mg/cn? 213
214
Vol. 33, No. 2
OXIDATIONOF T&Al
TABLE 1 Nominal Composition of the Alloys
60.0
405
24.0
5.0
11.0
at 800°C and 108.5 mg/cm* at 900°C after 100 hours of exposure. The oxide scale spalls at the early stage of oxidation so that it shows ahnost a linear weight change vs time relationship. The element Si greatly reduces the weight change of T&Al alloy. Its oxide scale does not spall at the end of the 100 hours oxidation procedum at 8OO”C,however, the oxide scale spalls after 50 hours in air at 900°C. The addition of 11 at% Nb yields a better e&ct to the oxidation msi&mce of T&Al than the addition of 5 at’%Si. Alloy 400 gains weight more slowly than alloy 305 at both 800°C and 900°C. Similar to alloy 305, its oxide scale does not spall at 8OO”C,but only spalls a&r 90 hours in the 900°C hot air. The alloy with a combined addition of Nb and Si shows the best oxidation resistance at these two temperatures. Alloy 300 gains weight of 2.59 mgkm* at 800°C and 9.19 mg/cm* at 900°C after 100 hours of exposure while alloy 400 gains weight of 0.88 mg/cm* and3.86mgkm* atthese w respectively. The combined addition of Si and Nb further reduces the specific weight change to 0.45 mg/cm* and 9.19 mg/cm* at 800°C and 900%. Furthermore, the oxide scale of alloy 405 does not spall even atter 100 hours of exposure at 900°C. All alloys except the stoichiometric Ti&l fit the parabolic relation quite well at both temperatures. Figure 2 is the SEM photograph and the EPMA results, showing the cross section of the oxide scale of the T&Alalloy. In the stoichiomettic T&Al(Fig. 2-a), the outer layer is Al rich while the inner layer is Al depleted. Contmry to the distribution ofAl, Ti is rich in the inner part but poor in the outer layer. It is suggested that the outer layer is the mixture of A120sand Ti02 while the inner layer is mostly composed of TiO, with many pores.
(4
(b) 4.0
10.0
800“ C
‘li@
2 <
C 7.5
E
0
20
/
I
0.0 40 EXPOSURE
60
80
100
7
0
20
TIME (h) Figure 1. Oxidation kinetics ofthe T&Al alloys.
40
I
60
EXPOSURE TIME (h)
I 80
I
1 M)
Vol. 33, No. 2
OXIDATION OF T&Al
(4
(4
Figure 2. Scanning Electron Micrograph and EPMA line profiles of (a) T&Al, (5) Ti,AL5at%Si at 800% and (c) T&l-l and(d) Ti,Al-Sat?&-1
215
lat%Nb at 9OOoCatIer 100 hours expwe.
lat%Nb
216
Vol. 33, No. 2
OXIDATIONOF T&Al
The addition of 5at% Si (Fig.2-b) does not seem to change the oxide scale of T&Al very much, except the quantity of pores in the TiO, is mdnced. There is no strong indication of the existence of a SiO, layer although weak Si richness appears in EPMA In alloy 400 (fig-c), the distribution of Ti and Al is similar to the stoichiomettic ahoy 300, except at the interface of the oxide scale and the base material there is an Al richness which mggests the f&on of T&l. An important feature of alloy 400 being quite diflbrent from that of alloy 300 is that a new phase with N and Ti richness, probably TiN, is observed on the base metal side at the interface of oxide scale and base alloy. No strong difference between the oxide scale of alloy 400 and alloy 405 (Fig.3-d) is observed. X-ray diEaction analysis con&ms the above rest&s. The outer oxidation layer of stoichiometric T&Al is the mixture of TiOz and Al,O,, while the inner layer is TiO, only. The addition of element Si does not alter the outer layer, which is still composed of TiO, and Al,O,, and the inner layer changes into a mixture of TiO, and trace amount of SiO,. When 11at% Nb is added to T&Al,the inner layer, in addition to TiO,, contains TiN and TiAl .The combined addition of Si and Nb changes the outer layer into a mixture of TiO,, Al,O, and SiO,, while the inner layer, TiN and TiAl. The addition of Nb and/or Si can improve the oxidation resistance of TiAl and T&Al, but up to now the mechanism is not well known. Maki et al.[5] suggested that 1 at % Nb can accelerate the formation of a contimrous protective Al,O, layer which did not form on the alloying free TiAl. In our work, however, even the addition of 11 at% Nb or the combined addition of Nb and Si does not form a continuous GO3 layer on T&Al.In fact, in the case of the Nb addition, Al richness, in the form of TiAl, is found on the base metal side at the interface between oxide scale and base alloy. However, in the case of stoichiometric T&Al, TiAl is not observed along the interface between oxide scale and base alloy. This means that the oxide scale contains less Al than that of the stoichiometric T&Al. Wallace et al.[8] studied the oxidation process of super-a2. They found that in this Nb added T&Alalloy a second parabolic stage existed with a reduced oxidation rate probably due to the formation of TiAl. M Kabbaj et al.[9] revealed that TiN coating on TiAl, can improve its oxidation resistance, which is better than that of TiAl. Taniguchi et al. [ lo] also indicated that silicon nitride can improve the oxidation resistance of TiAl. The above infbrmation suggests that TiN could be a better oxidation resistance barrier than TiAl. Our high temperature in situ X-ray difI?action study of the oxidation process cmrently under investigationsuggests that the beneficial effect of Nb or Si is not the promotion of A&O, layer, but probably the formation of TiN and the reduction of porosity in the oxide scale[ 111. Conclusion_
The addition of 1 lat % Nb or 5at % Si can improve the oxidation resistance of TiAl,. The effect of the combined addition of Nb and Si is even better. No continuous protective A&O, layer was formed atIer the addition of 11at% Nb, 5at% Si or the combined addition of Nb and Si. The beneficial effect of the addition of Nb, Si or the combination of them is probably due to the formation of TiN and the reduction of porosity in the oxide scale. Acknowlee
This work is sponsored by the National Natural Science Foundation of China and Corrosion Science Laboratory, Academia Sinica. References 1. 2.
Y-W.Kim and F.H.Froea, in High-Temperature .‘ihmhides and -cs. H.A.Lipsitt, D.Shechtmm and RE.Sa Meiall. Trans. 1 lA(1980).
Warrend& Pa,TMS, 1990,485 1369
Vol. 33, No. 2
3. 4. 5. 6. 7. 8. 9. 10. 11.
OXIDATION OF T&Al
J.S.Wu,P.AEkavea,RWagw,ChHdigandJ.Seeger,inHighTeanpedm~~cAlloys~&.C.T.Liuet al, MRS Symp. Proc. Vol.133(1989), 761 MS.EoSouniD.QqP~&IMaaandRWagea;ProceediagofletSymposiumcm~cCompouada Japan(l991), KM&i, M.&i& MSayaahi, T.Shimim and S.bobe, hf&ai& Sckce. & En@ecr& A153(1992), 591 P. H.Frwi, C.Stmyawayaua and D.Eliezer, Journal ofkteriab Science ad IXngbz& vo1.27(1992), 5113 J.S.Wu,G.H.QiumdLT.Z.hmgSuiptah4etdl.Mater.vol.30(1994),213 T.AWallace, RK.Clark, KE.Wicdemm aad S.N.solllraran, Gxidation ofMetals, voL137(1992), 111 M. Kabbaj, AGderie ad M.C!ai&t, Journal ofless Camon Metds,voLlOS(l985), 1 S.Tan@chi, T.Shibata, T.Yanada, XLiu and SZ.ou, ISIJ Intmational, Vol. 33(1993), 869 G.H.Qiu, J.S.Wu,L.T.ZhangandT.L.L.in,MRSPaUMeetiq, 1994,Hoston,U.S.A
217
525
etMaterialie.Vol. 33, No. 2, pp. 213-217.1995 ~~~Itellae$GrGrarlmnlm
Pergamon
09%716x/95 $9.50 + .oo
0956-716X(95)001506
OXIDATION BEHAVIOR OF T&Al ALLOYED WITH NIOBIUM AND SILICON Guohua Qiu, Jiansheng Wu, Lanting Zhang and Dongliang Lin(T.L.Lii) Department of Materials Science Shanghai Jiao Tong University Shanghai 200030, P.R.China (Received August 30,1994) (Revised January 13,1995) htroduction
T&Al has the potential for aerospace applications due to its superior strength to weight ratio compared to supemlloys [l]. S-stabii elements such as molybdenum, niobium, tantalum, tungsten and vanadium have been added to improve its poor room temperature ductility. Another drawback of T&Al is its lack of high temperature oxidation resistance. Early work[2] showed that the addition of Nb can increase the high temperature oxidation msiatanm of T&Al alloy. Recent work[3,4] revealed the beneficial e.@ectof Si to the high temperature mechanical and oxidation properties. The combined addition of Nb and Si is even better for oxidation resistance of TiAl and T&Al alloys[5], however, the mechanism of improving~h@~ theeftixtofNbandSiisnotclearlyuude&xd. In this paper, the oxidation kinetics of T&Al alloys with a Nb and/or Si addition is investigated and the oxidation me&a&m is discussed. Fmerimental
The alloy buttons were am-melted from pure element metals and drop cast into 0 20x 100 mm ingots. The ~ti~ofthealloysislistedinTable1.Couponsmeasuring10~10~3mmwerecut~omtheingotsby spark erosion aud polished to a 600 grade emery paper finish. The coupons were oxidized in open air at high temperature for dBerent times and then weighed. The oxide scale on the coupons after 100 hours of exposure in hot air was analyzed by scanning electron microscopy(SEM), electron probe analysis(EPMA) and X-ray di.&ction(XRD).
Redts and Discussion The microstructure of Alloy 400, with 11 at% Nb addition, shows the tie basket-weave feature which is stated in many references[6]. Alloys 305 and 405, with the addition of 5 at?? Si, give another phase Ti,Si,, which forms a fine eutectic microsuucture[7]. Specific wei&t change vs time curves of different alloys at 800°C and 900°C, respectively, are shown in Fig. 1. The stoichiometric T&Al (alloy 300) gains weight extremely rapidly. It gains weight of 8.45 mg/cn? 213
214
Vol. 33, No. 2
OXIDATIONOF T&Al
TABLE 1 Nominal Composition of the Alloys
60.0
405
24.0
5.0
11.0
at 800°C and 108.5 mg/cm* at 900°C after 100 hours of exposure. The oxide scale spalls at the early stage of oxidation so that it shows ahnost a linear weight change vs time relationship. The element Si greatly reduces the weight change of T&Al alloy. Its oxide scale does not spall at the end of the 100 hours oxidation procedum at 8OO”C,however, the oxide scale spalls after 50 hours in air at 900°C. The addition of 11 at% Nb yields a better e&ct to the oxidation msi&mce of T&Al than the addition of 5 at’%Si. Alloy 400 gains weight more slowly than alloy 305 at both 800°C and 900°C. Similar to alloy 305, its oxide scale does not spall at 8OO”C,but only spalls a&r 90 hours in the 900°C hot air. The alloy with a combined addition of Nb and Si shows the best oxidation resistance at these two temperatures. Alloy 300 gains weight of 2.59 mgkm* at 800°C and 9.19 mg/cm* at 900°C after 100 hours of exposure while alloy 400 gains weight of 0.88 mg/cm* and3.86mgkm* atthese w respectively. The combined addition of Si and Nb further reduces the specific weight change to 0.45 mg/cm* and 9.19 mg/cm* at 800°C and 900%. Furthermore, the oxide scale of alloy 405 does not spall even atter 100 hours of exposure at 900°C. All alloys except the stoichiometric Ti&l fit the parabolic relation quite well at both temperatures. Figure 2 is the SEM photograph and the EPMA results, showing the cross section of the oxide scale of the T&Alalloy. In the stoichiomettic T&Al(Fig. 2-a), the outer layer is Al rich while the inner layer is Al depleted. Contmry to the distribution ofAl, Ti is rich in the inner part but poor in the outer layer. It is suggested that the outer layer is the mixture of A120sand Ti02 while the inner layer is mostly composed of TiO, with many pores.
(4
(b) 4.0
10.0
800“ C
‘li@
2 <
C 7.5
E
0
20
/
I
0.0 40 EXPOSURE
60
80
100
7
0
20
TIME (h) Figure 1. Oxidation kinetics ofthe T&Al alloys.
40
I
60
EXPOSURE TIME (h)
I 80
I
1 M)
Vol. 33, No. 2
OXIDATION OF T&Al
(4
(4
Figure 2. Scanning Electron Micrograph and EPMA line profiles of (a) T&Al, (5) Ti,AL5at%Si at 800% and (c) T&l-l and(d) Ti,Al-Sat?&-1
215
lat%Nb at 9OOoCatIer 100 hours expwe.
lat%Nb
216
Vol. 33, No. 2
OXIDATIONOF T&Al
The addition of 5at% Si (Fig.2-b) does not seem to change the oxide scale of T&Al very much, except the quantity of pores in the TiO, is mdnced. There is no strong indication of the existence of a SiO, layer although weak Si richness appears in EPMA In alloy 400 (fig-c), the distribution of Ti and Al is similar to the stoichiomettic ahoy 300, except at the interface of the oxide scale and the base material there is an Al richness which mggests the f&on of T&l. An important feature of alloy 400 being quite diflbrent from that of alloy 300 is that a new phase with N and Ti richness, probably TiN, is observed on the base metal side at the interface of oxide scale and base alloy. No strong difference between the oxide scale of alloy 400 and alloy 405 (Fig.3-d) is observed. X-ray diEaction analysis con&ms the above rest&s. The outer oxidation layer of stoichiometric T&Al is the mixture of TiOz and Al,O,, while the inner layer is TiO, only. The addition of element Si does not alter the outer layer, which is still composed of TiO, and Al,O,, and the inner layer changes into a mixture of TiO, and trace amount of SiO,. When 11at% Nb is added to T&Al,the inner layer, in addition to TiO,, contains TiN and TiAl .The combined addition of Si and Nb changes the outer layer into a mixture of TiO,, Al,O, and SiO,, while the inner layer, TiN and TiAl. The addition of Nb and/or Si can improve the oxidation resistance of TiAl and T&Al, but up to now the mechanism is not well known. Maki et al.[5] suggested that 1 at % Nb can accelerate the formation of a contimrous protective Al,O, layer which did not form on the alloying free TiAl. In our work, however, even the addition of 11 at% Nb or the combined addition of Nb and Si does not form a continuous GO3 layer on T&Al.In fact, in the case of the Nb addition, Al richness, in the form of TiAl, is found on the base metal side at the interface between oxide scale and base alloy. However, in the case of stoichiometric T&Al, TiAl is not observed along the interface between oxide scale and base alloy. This means that the oxide scale contains less Al than that of the stoichiometric T&Al. Wallace et al.[8] studied the oxidation process of super-a2. They found that in this Nb added T&Alalloy a second parabolic stage existed with a reduced oxidation rate probably due to the formation of TiAl. M Kabbaj et al.[9] revealed that TiN coating on TiAl, can improve its oxidation resistance, which is better than that of TiAl. Taniguchi et al. [ lo] also indicated that silicon nitride can improve the oxidation resistance of TiAl. The above infbrmation suggests that TiN could be a better oxidation resistance barrier than TiAl. Our high temperature in situ X-ray difI?action study of the oxidation process cmrently under investigationsuggests that the beneficial effect of Nb or Si is not the promotion of A&O, layer, but probably the formation of TiN and the reduction of porosity in the oxide scale[ 111. Conclusion_
The addition of 1 lat % Nb or 5at % Si can improve the oxidation resistance of TiAl,. The effect of the combined addition of Nb and Si is even better. No continuous protective A&O, layer was formed atIer the addition of 11at% Nb, 5at% Si or the combined addition of Nb and Si. The beneficial effect of the addition of Nb, Si or the combination of them is probably due to the formation of TiN and the reduction of porosity in the oxide scale. Acknowlee
This work is sponsored by the National Natural Science Foundation of China and Corrosion Science Laboratory, Academia Sinica. References 1. 2.
Y-W.Kim and F.H.Froea, in High-Temperature .‘ihmhides and -cs. H.A.Lipsitt, D.Shechtmm and RE.Sa Meiall. Trans. 1 lA(1980).
Warrend& Pa,TMS, 1990,485 1369
Vol. 33, No. 2
3. 4. 5. 6. 7. 8. 9. 10. 11.
OXIDATION OF T&Al
J.S.Wu,P.AEkavea,RWagw,ChHdigandJ.Seeger,inHighTeanpedm~~cAlloys~&.C.T.Liuet al, MRS Symp. Proc. Vol.133(1989), 761 MS.EoSouniD.QqP~&IMaaandRWagea;ProceediagofletSymposiumcm~cCompouada Japan(l991), KM&i, M.&i& MSayaahi, T.Shimim and S.bobe, hf&ai& Sckce. & En@ecr& A153(1992), 591 P. H.Frwi, C.Stmyawayaua and D.Eliezer, Journal ofkteriab Science ad IXngbz& vo1.27(1992), 5113 J.S.Wu,G.H.QiumdLT.Z.hmgSuiptah4etdl.Mater.vol.30(1994),213 T.AWallace, RK.Clark, KE.Wicdemm aad S.N.solllraran, Gxidation ofMetals, voL137(1992), 111 M. Kabbaj, AGderie ad M.C!ai&t, Journal ofless Camon Metds,voLlOS(l985), 1 S.Tan@chi, T.Shibata, T.Yanada, XLiu and SZ.ou, ISIJ Intmational, Vol. 33(1993), 869 G.H.Qiu, J.S.Wu,L.T.ZhangandT.L.L.in,MRSPaUMeetiq, 1994,Hoston,U.S.A
217
525