aLo

at P

re deoxidized via a simple method called hydrogen treatment (HT),

greatly decreased after HT. Activated hydrogen atoms dissociated at high temperatures,

and the hydrogen molecules in the melting chamber seemed to affect the deoxidation

reactions, which are represented by the following equations: O 2HH2O and

and poor fracture resistance at room temperatures [2]. A

[3,4]. The extensive glide of ordinary 1/2 dislocations

occurs in the case of high-purity alloys and these alloys

exhibit some tensile ductility. However, the activity of these

dislocations is very much reduced with increasing oxygen

content, which may be due to pinning along the dislocation

lines by oxygen [3]. And TieAl alloys with high oxygen

oxygen during the melting process. Their primary materials

application [6]. A common method for deoxidation of TieAl

alloys is the calciumealuminum (CaeAl) method [7]. Unfor-

tunately, it was found that some Ca remained in the alloys

post-deoxidation. Thus, researchers focused their efforts on

discovering a deoxidation method that did not leave

unwanted elements in the alloy.

* Corresponding author. Tel.: 86 451 86417395; fax: 86 451 86415776.

Avai lab le at www.sc iencedi rect .com

w.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 9 2 1 4e9 2 1 7E-mail address: suyq@hit.edu.cn (Y. Su).detailed study was carried out to study the deformation

behaviors of TieAl-based alloys containing different levels of

oxygen at room temperature, which revealed that oxygen

had negative affection on the room ductility of TieAl alloys

also contain some oxygen, thus further increasing the

oxygen content. However, oxygen has harmful influences on

the mechanical properties of TieAl alloys as described

above, so it is necessary to deoxidize them prior to theirTitaniumealuminum (TieAl)-based intermetallics are

candidate materials for replacing nickel-based superalloys in

some gas turbine engine applications, such as low-pressure

or power turbine blades [1]. The critical barrier for

commercial applications of TieAl alloys is their low ductility

room temperature. The ductility of TieAl alloys becomes

higher with decreasing oxygen content. So for better appli-

cations of TieAl alloy, it is expected that the oxygen content

is as low as possible. TieAl alloys are melted mostly in

vacuum melting apparatuses, examples of which include

plasma-arc melt furnaces [5]. These alloys easily absorb25 May 2010

Accepted 26 May 2010

Available online 14 July 2010

Keywords:

Hydrogen treatment

Intermetallics

Deoxidation

Free energy

1. Introduction0360-3199/$ e see front matter 2010 Profedoi:10.1016/j.ijhydene.2010.05.115OH2H2O. Based on a comparison of the changes in Gibbs free energy, the hydrogenatoms were found to play a major role in deoxidation.

2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

content deform by superdislocations and are totally brittle atReceived 23 March 2010

Received in revised formwhich involves deoxidation with hydrogen in a melting process. Because of the increase in

the partial pressure of hydrogen and the melting duration, the oxygen content of the alloysArticle history: Alloys of Tie47Al wea r t i c l e i n f o a b s t r a c tTechnical Communication

Deoxidation of TieAl intermet

Yanqing Su*, Xinwang Liu, Liangshun Luo,

National Key Laboratory of Science and Technology for Precision He

Harbin 150001, PR China

journa l homepage : wwssor T. Nejat Veziroglu. Pllics via hydrogen treatment

ng Zhao, Jingjie Guo, Hengzhi Fu

rocessing of Metals, Harbin Institute of Technology,

e lsev ie r . com/ loca te /heublished by Elsevier Ltd. All rights reserved.

In this study, hydrogen treatment (HT) is used to deoxidize

TieAl alloys. In the HT process, TieAl alloys are melted in the

presence of hydrogen. Deoxidation occurs via a series of

reactions of hydrogen and oxygen. In this way, any hydrogen

existing in the samples can be easily removed by vacuum

annealing prior to their application. Possibly the remnant

hydrogen can be utilized as a temporary element to improve

the processing of TieAl alloys, including sintering, compact-

ing, machining, and hot working before dehydrogenation

[8e10]. This process does not retain other phases or elements.

The process can also be completed in the same amount of

time that themelting process takes. Thus, HT is more likely to

be an efficient method for deoxidation of TieAl alloys than

other approaches.

diagram of this method is shown in Fig. 1. During deoxidation,

TieAl ingots were melted under a gaseous mixture of

hydrogen. Some diatomic hydrogen could be excited into

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 9 2 1 4e9 2 1 7 9215hydrogen and argon in the furnace. The hydrogen analyzer

then detected both the volume fraction of hydrogen and its

pressure in the melting chamber, and the partial pressure of

hydrogen in the system could be controlled by the hydrogenThe aim of the present work is to validate the effects of the

HTmethod on the deoxidation of Tie47Al alloys (all expressed

in terms of at.% in this paper). The deoxidation mechanism of

hydrogen is also discussed.

2. Experimental

TieAl binary alloys were selected for this study. In order to

compare the deoxidation effects of HT more clearly, pre-cast

ingots containing specific amounts of oxygen were first

prepared. To better understand the deoxidation of TieAl alloys

by HT, oxygen in the form of TiO2 powder was added to the

system prior to melting. The TieAl alloy ingots were prepared

using a water-cooled copper crucible in a non-consumable

electrode arc furnace under an atmosphere of pure argon. The

Ti sponge and high-purity Al (>99.99%) were used for the

preparation of TieAl alloys. About 25e30 g of alloy buttons

were melted 4e5 times in order to improve chemical homo-

geneity.Thenominal compositionof theTieAlalloy isTie47Al.

Deoxidation was performed with hydrogen treatment

consisting of a non-consumable tungsten electrode arc

melting furnace and a hydrogen analyzer. A schematicFig. 1 e Schematic diagram of the hydrogen charging

system.ionized hydrogen by electrons emitted from the cathode. The

hydrogen atoms or ions moved with the arc and reached the

melt surface. Some of the hydrogen that reached the alloy

melt surface dissociated into monatomic hydrogen and

diffused into the melt. This process could be described by the

equation:

H2 (g)H (atom) in liquid metal, DG0 (J)44,780 3.38T(1)analyzer. Different partial pressures of hydrogen and different

melting durations were studied for their effects on deoxida-

tion via HT. The temperature of the alloy melt was measured

by an infrared thermoscope. The oxygen content was

obtained from chemical analysis. Dehydrogenation was per-

formed by vacuum annealing at 1023 K for 2 h. During this

process, the pressure of the furnace chamber was kept at

104 Pa. After that, the hydrogen content of the alloys wasdetermined by chemical analysis.

3. Result and discussion

Fig. 2(a) shows the changes in the oxygen content of the

Tie47Al alloys after HT with different partial pressures of

hydrogen. Two sets of ingots with different levels of starting

oxygen content were conducted by HT, which is marked by A

and B respectively. The oxygen content of the second set with

high starting oxygen content after HT was divided by 8 for

clearer comparison, which is shown by B. The melting time

was set at 240 s. During the course of the experiment, the

oxygen content of the alloys decreased rapidly with an

increase in the partial pressure of hydrogen. When the partial

pressure of hydrogen reached 10 kPa, however, the oxygen

content remained steady and no longer decreased. Thus, we

propose that a hydrogen partial pressure of 10 kPa can remove

a significant amount of oxygen via HT. This finding also allows

us to conclude that HT is an effective method for deoxidizing

TieAl alloys.

Changes in the oxygen content of the Tie47Al alloys with

respect to increasing melting time are shown in Fig. 2(b). In

this set of experiments, hydrogen partial pressures of 10 and

20 kPa were used. The oxygen content in the alloys clearly

decreased with an increase in the melting duration. Initially,

the oxygen content decreased rapidly until the melting dura-

tion reached 360 s, after which the oxygen content no longer

decreased. Thus, we conclude that a melting duration of 360 s

is sufficient for deoxidation of TieAl alloys via HT. Increasing

the partial hydrogen pressure from 10 kPa to 20 kPa resulted in

minor effects on the deoxidation process. When the melting

duration exceeded 360 s, the deoxidation effects for both

pressure conditions were nearly the same.

In order to discuss the deoxidation mechanism of HT, it is

necessary to first understand the behavior of hydrogen,

particularly those at the surface of the arc zone and the

molten pool. The temperatures of the arc and melt were high

enough for diatomic hydrogen to dissociate into monatomicSome hydrogen atoms could also combine to form mole-

cules on the melt surface and escape into the melting

s with variations in (a) partial pressure of hydrogen, and

m their original ones divided by 8.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 9 2 1 4e9 2 1 79216chamber. With the above reactions, more and more hydrogen

could diffuse into the TieAl melt. When the alloy melt was

saturated with hydrogen, a dynamic equilibrium between the

alloy melt and the melting chamber was achieved. The

process throughwhich this dynamic equilibrium is attained is

described by Sieverts [11].

There are always activated hydrogen atoms on the melt

surface [12], and deoxidation reactions are considered to

take place on the melt surface. Deoxidation by hydrogen

includes two reactions with both hydrogen atoms and

molecules on the melt surface. When the oxygen atoms in

the alloy melt move to the melt surface, they can combine

with the hydrogen atoms that come from the alloy melt or

simply dissociate on the surface and form water molecules.

Oxygen can also react with the hydrogen molecules that are

near the melt surface to generate water [13]. This water

stream then flows with the mixture of argon and hydrogen.

Fig. 2 e Changes in the oxygen content of the Tie47Al alloy

(b) melting duration. The values of B in (a) were modified froIt is reported that hydrogen can also remove oxygen from

Zr, Nb and Ta metals based on the same reactions [14].

These processes can be expressed in the following

equations:

O (in melt) 2HH2O, DG0 (J)476,778 110.16T (2)

O (in melt)H2H2O, DG0 (J)24,476 10.96T (3)

In addition to deoxidation from the melt, there is also

another reaction that might also benefit deoxidation. Some

residual air containing oxygen might exist in the melting

chamber, even though the molecular pump attempted to

somewhat evacuate this space. As such, hydrogen could

also react with the oxygen near the arc zone or melt surface

where the temperature was higher than the ignition point

of hydrogen. This process is described by the following

equation:

O2H2H2O, DG0 (J)247,500 55.86T (4)The schematic of the deoxidation reactions is shown in

Fig. 3, in which the reactions involved in deoxidation are

marked. Fig. 4 shows the dependence of the Gibbs free

energy (DG0) on the temperature for the deoxidation by

hydrogen. The Gibbs free energy for Eq. (4) had the most

negative value, indicating that this reaction was the easiest

to initiate. For this study, however, there was limited

residual air in the chamber and the reactions of Eq. (4) were

very weak. In this study, reaction (4) play a minimal role in

deoxidation. Deoxidation was believed to have been achieved

mainly by reactions between the hydrogen and the oxygen

atoms. The deoxidation processes by H2 and H were both

feasible, since the values of DG0 for both were negative.

However, the process initiated by H2 was estimated to be

much more difficult than the process initiated by H because

the values of DG0 for H2 were much higher and closer to zero

than the latter. Thus, deoxidation is likely to be controlled byEq. (2). Most of the oxygen in the system was considered to

have been removed by hydrogen atoms. When the temper-

ature increased, the free energy of Eq. (2) increased rapidly

Fig. 3 e Schematic diagram of the deoxidation reactions.

and that of Eq. (3) decreased very slowly. As such, deoxida-

Acknowledgements

The authors would like to thank the National Natural Science

Foundation of China (50975060) and the Foundation of State

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 5 ( 2 0 1 0 ) 9 2 1 4e9 2 1 7 9217tion became more difficult as the temperature increased. To

obtain better deoxidation results, the superheat should be

kept as low as possible.

After the HT process, the hydrogen which dissolved into

the TieAl alloy melt remained in the alloy during the

solidification process and formed hydrides, which might

induce brittleness in TieAl alloys [15]. This left-over

hydrogen could be easily removed by vacuum annealing

[16,17]. After dehydrogenation, chemical analysis was per-

formed to determine the left-over hydrogen content, which

indicated that the hydrogen was reduced below 50 weight

ppm by 2 h of vacuum annealing. This confirmed that

hydrogen treatment is a feasible method for deoxidation of

TieAl alloys.Fig. 4 e Dependence of the Gibbs free energy on

temperature for the deoxidation by hydrogen.4. Conclusions

Alloys of Tie47Al were subjected to hydrogen treatment to

determine the deoxidation effects. The method showed good

deoxidation activity on TieAl alloys, as evidenced by the

decrease in oxygen content of the alloys when the partial

pressure of hydrogen and the melting duration were

increased. A partial pressure of 10 kPa and a melting dura-

tion of 360 s were the optimized effective parameters for

deoxidation of these alloys. The dissociated hydrogen atoms

and molecules might both play roles in the deoxidation

reactions. Of the two, however, hydrogen atoms were

considered to play a bigger role, as its Gibbs free energy was

found to be much lower (more negative) than that for the

hydrogen molecules.

[17] Froes FH, Senkov ON, Qazi JI. Hydrogen as a temporary

alloying element in titanium alloys: thermohydrogenprocessing. Int Mater Rev 2004;49:227e45.Key Laboratory of Advanced Welding Production Technology

of China for their financial support.

r e f e r e n c e s

[1] SaariH,Beddoes J,SeoDY,ZhaoL.Developmentofdirectionallysolidified g-TiAl structures. Intermetallics 2005;13:937e43.

[2] Hu D. Effect of boron addition on tensile ductility in lamellarTiAl alloys. Intermetallics 2002;10:851e8.

[3] Morris MA. Dislocation mobility, ductility and anomalousstrengthening of two-phase TiAl alloys: effects of oxygen andcomposition. Intermetallics 1996;4:417e26.

[4] Wu Y, Hwang SK. Microstructural refinement andimprovement of mechanical properties and oxidationresistance in EPM TiAl-based intermetallics with yttriumaddition. Acta Mater 2002;50:1479e93.

[5] Sugilal G. Experimental study of natural convection in a glasspool inside a cold crucible induction melter. Int J Therm Sci2008;47:918e25.

[6] Noda T. Application of cast gamma TiAl for automobiles.Intermetallics 1998;6:709e13.

[7] Okabe TH, Oishi T, Ono K. Deoxidation of titanium aluminideby CaeAl alloy under controlled aluminum activity. MetallMater Trans B 1992;23:583e90.

[8] Sun ZG, Zhou WL, Hou HL. Strengthening of Tie6Ale4Valloys by thermohydrogen processing. Int J Hydrogen Energy2009;34:1971e6.

[9] Murzinova MA, Salishchev GA, Afonichev DD. Formation ofnanocrystalline structure in two-phase titanium alloy bycombination of thermohydrogen processing with hotworking. Int J Hydrogen Energy 2002;27:775e82.

[10] Liu HJ, Zhou L, Liu P, Liu QW. Microstructural evolution andhydride precipitation mechanism in the hydrogenatedTie6Ale4V alloy. Int J Hydrogen Energy 2009;34:9596e602.

[11] Sieverts A. Absorption of gases by metals. Zeitschrift furMetallkunde 1929;21:37e46.

[12] Su YQ, Wang L, Luo LS, Jiang XH, Guo JJ, Fu HZ. Deoxidationof titanium alloy using hydrogen. Int J Hydrogen Energy 2009;34:8958e63.

[13] Mimura K, Komukai T, Isshiki M. Purification of chromiumby hydrogen plasma-arc zone melting. Mater Sci Eng A 2005;403:11e6.

[14] Elanski D, Limb JW, Mimura K, Isshiki M. Impurity removalfrom Zr, Nb and Ta metals by hydrogen plasma arc meltingand thermodynamic estimation of hydride formation. JAlloys Compd 2006;413:251e8.

[15] ChuWY,ThompsonAW.Effectofmicrostructureandhydrideson fracture of TiAl. Scripta Metall Mater 1991;25:2133e8.

[16] Senkov ON, Froes FH. Thermohydrogen processing oftitanium alloys. Int J Hydrogen Energy 1999;24:565e76.

Deoxidation of Ti-Al intermetallics via hydrogen treatmentIntroductionExperimentalResult and discussionConclusionsAcknowledgementsReferences