Surface Reaction of TiAl with Water Vapor and Oxygen

J. Mater. Sci. Technol, Vol.19 No.4, 20033350问m water vaporTIO,I|r。TO24000 50400 500Kinetic energy1 eV瘤前宓丽116廊1184Fig.1 Typical Auger electron spectra from TiAl-based alloy(a) after impact surface in vacuun, (b) and (C) afterexposure 1x10-6 Pa.s and 0.1 Pa:.s to water vapor,Fig.4 X-ray photoelectron spectra of Ti2p and Al2p fromrespectivelyintergranular fracture: as fractured (a) and (d), after2.5rexposure 1.1x10 2 Pa.g (b), 0.1 Pas(c), 1x10- 6 Pas(e), 0.1 Pas (f) to water vapor2。[(a) in oxygen0.0.5 (a) in oxygen0.4100.30.50.2，●.00.1i0:0.6 F(b) in water vapor0.25.500000.20[(b) in water vapor 90.150.100.050.0“0* 10° 102 10'10*Exposure 1 Pa-sExposure/ Pa.sFig.2 Normalized oxygen Auger electron intensity v8 waterFig.5 Normalized XPS inteneity of oxide as a function ofvapor and oxygen exposure for TiAl-based alloy atthe exposure of oxygen (a) and water vapor (b) forroom temperatureTiAl- based alloy at room temperature血oxygention value is reached after an exposure of 6x10~ 4 Pas.XPS analysis of the impact fracture surface indicates thecatesAl and Ti elements in TiA1 are in the metallic state (Fig.3(a),(d) and Fig.4(a), ()). Figures 3 and 4 show the Tizp and Al2pspectra after various exposures to water vapor and oxygen,论spectively. From the figure, it can be seen that elements AIand Ti are oxidized to form the oxide Al2O3 and TiO2 afterexposure in oxygen and water vapor. The oxidation extentof elements Al and Ti in oxygen is larger evidently than thatin water vapor under the same exposure (Figs.3 and 4). Thedkinetics of oxidation reaction in oxygen and water vapor ob-tained by XPS is shown in Fig.5. It can be seen from Fig.5前南宛800r1181181844that the phases of Al2O3 and TiOz appear simultaneouslyKInetic energy/eVafter exposure 1x10Pas to oxygen (Fig.5(a). When thealloy is exposed in water vapor, XPS results show that phaseFig.3 X-ray photoelectron spectra of Tiap and Alzp fromAl2O3 appears firstly on the surface while the Ti elermentis still in the metallic state during the expgsure of 6x 1exposure 9x10~6 Pas (b), 0.1 Pas (c), 3x10- 6 PasPas~2x10-5 Pa-s, and phase TiO2 then appears after ex-(e), 0.1 Pas (f) to oxygenposure of 2x10-5 Pas (Fig.5(b)). The result indicates thatelement Al is firstly oxidized and element Ti is also oxidizedvapor and for oxygen, respectively. The both reactions withafter certain exposure to water vapor. XPS results also showwater vapor and oxygen initiate at the same exposure aboutha中国煤化工:3 and TiO2 pbase in-For theeaction with water vapor, an appar-re-(Fig.5). Since metallic6x10. Pa-s.reactionent initial saturation is reached after an exposure of approxi-taCNMH Gatto ito oner saturation (Figs.3 andmately 1x10-4 Pas. For the reaction with oxygen, a satura-336J. Mater. Sci. Technol, Vol.19 No.4, 2003which corresponds to a rate constant he for this reaction of2.15x10* (Pa.s) 1.田H,0Comparing the results of Fig,3 with Fig.4, it is clear thatinore Al and Ti atoms in TiAl-based alloy take part in thereatoxygen than with water vapor undersame exposure condition. This means that the rate of surfacereaction of TiAl-based alloy with water vapor is lower thanothat with oxygen. This fact can be explained by the resultsof the surface reaction of Ti aloy (Ti-5 wt pct Al-2.5 wt pctSn) with water vapor and oxygen[13]. Their results show thatthe reaction of Ti alloy with water vapor produced a mono-layer of oxide，wbereas two or three layers of oxide resulted10 Efrom the reaction with oxygen. It means that the monolayerAl2O3 and TiO2 formed after exposure to water vapor can由-12 Esignifcantly reduce the rate of further reaction..0).51.55. ConclusionExposure 1 PasThe surface reaction of TiAl-based alloy with water vaporFig.6 Plot of ln(1- 0) as a function of water vapor andand oxygen is verified by AES and XPS. Both surface reac-tions initiate at a very short exposure (about 6x10-7 Pa-s).oxygen exposureIn both environments, elements Al and Ti are all oxidized.The oxide formed in the surface reactions is Al2Oz and TiO2.or two atom layers.TheofTiAlLbased alloith oxygen saturates afteran exposure of 6x10- 4 Pas. The reaction with water vapor4. DiscussionXPS results of this paper show suficiently that reactiveTiOz are very rapid and both initiate after 1x10-6 Pars ex-element Al and Ti in TiAl-based alloy can be oxidized by wa-posure. However, as the alloy is exposed in water vapor, theter vapor, leading to the formation of oxide Al2Oz and TiO2.initial exposure (about 6x10 7 Pas) for the oxidation of AlAlthough atomic bydrogen can not be monitored directly inis signifcantly lower than the initial exposure (2x10-5 Pas)this experimental condition, hydrogen atom, as a product, canfor the oxidation of Ti. This means that the susceptibilitybe inferred from reactant- water vapor and the other prod-of TiAl-based alloys to environmental embrittlement is con-uct Al2O3 and TiO2. Therefore, the work here is the directtrolled mainly by the steps of surface reaction of AI with waterexperimental confirmation of the reaction of TiAl-based alloyvapor.with water vapor, i.e,2Al+ 3H2O = Al2Ox + 6H1)AcknowledgementTi+ 2H2O= TiO2 + 4H2)This research Was sponsored by the National Natural ScienceXPS study also confirms the existence of the surface reactionFoundation of China under Contract 59771007 and a key project ofthe National Natural Science Foundation of China under Contractof Al and Ti in TiAl-based alloy with oxygen:59895157.4Al+ 3O2 = 2Al2O33)REFERENCESTi+ O2= TiO2.4)[1] C.T.Liu, E.H. Lee and C.GMcKamey: Scripta Mtall, 1989,It should be noted that the above reactions occur only23, 875.on the fresh surface, such surface can be dynamically created[2 ] C.T.Liu, C.G.McKarney and E.H.Lee: Scripta Metall. Mater,during tensile test.Suppose that the normalined AES oxygen intensity r[3 ] T.Takasugi, M.Tnkazawa and O.Izuma: J. Mater. Sci, 1990,Suppose that the !mtensutymaybe expected to be approximately proportional to the amount，25 4239.X.J.Wan and Y.Wu: Scripta Metall. Mater, 1993,of absorbed oxygen up to the saturation level. For the ini-[4] JH.2hu,tial reaction with water vapor or oxygen the following rate[5 ] EP.GeorC.TLiu and D.P.Pope: Scripta Metall. Mater.,equation is assumed to applyl3):- Sopr(2rmkT)-/25)6] X.J.Wan, J.H.Zhu and K.L.Jing: Scripta Metall. Mater, 1992,ln(1-0) =-Vo‘7 ] M.Shea, A.Castagna and N.S.Stolof: Mater. Res. Soc. Proc.,20, 473.where θ is the coverage and is defined as n/No, where n isB } E.P.George, CT.Liu and DP.Pope: Scripta Metall. Mater.,1992, 213, 609.the surface concentration of oxygen at time t and No is the1993, 28, 857.surface oxygen conicentration at saturation. So is the initial9] J.H.Zhu, S.B.Huang and XJ.Wan: Seripta Metall. Mater,sticking coeficient, p(2πmkT)~ 1/2 is the arrival rate of water1995, 82, 1399.molecules and (1- 0) is the fraction of unreacted surface. The10] W.J.Chia and Y.W.Chung: J. Vac. Sci. Technol, 1995, A13,rate constant k。is thus equal to (So/No)(2rmkT)~ 2/2.A plot of ln(1 -日) as a function of water vapor and oxy-11] Y.X.Chen, X.J.Wan and W.X.Xu: Acta Metall. Sin. (Eng.gen exposure pl is shown in Fig.6. For this plot, an So/NoLett,), 1997, 10, 363.12] Wenjue CHEN, Xiaojing WAN and Cheng JIN: J. Mater. Sci.value of 2.78x10-18 m2 is obtained for water vapor, whichcorresponds to a reaction rate constantf 1.01x 105 (Pa-s)-1.中国煤化IVei; Mate. Sei. Eng. 1984.An So/No value of 7.93x10 19 m2 is obtained for oxygen,THCNMHG