Scripta METALLURGICA Vol, 29, pp. 1029-1033, 1993 Pergamon Press Ltd. et MATERIALIA Printed in the U.S.A. All rights reserved

ANALYSIS OF HYDROGEN EVOLVED FROM FRACTURE SURFACES OF A TiAI ALLOY

Tadaaki Hino*, Hiroshi Okada* and Motohiro Kanno**

* Graduate School, Materials Science Course, Division of Engineering, The University of Tokyo, Tokyo 113, Japan.

** Department of Materials Science, Faculty of Engineering, The University of Tokyo, Tokyo 113, Japan.

(Received July 14, 1993)

Introduction

Intermetall ic compounds are known to have some very at t ract ive properties but to date their drawbacks have prevented their practical use. One drawback is their low duc t i l i ty at ambient temperature, and extensive attempts have been made to overcome this by, for instance, chemical modification and microstructural control(i-5). For example, i t has been reported that the duct i l i ty of a Ti-48at%A1 alloy is increased by the addition of a small amount of manganese: the elongation of the ternary alloy with lat%Mn is estimated to be over 3% through a bending test, while the value of a binary alloy is below 1%(6). The duct i l i ty of various binary inter- metallic compounds is also known to be enhanced by the addition of a third element. Such com- pounds, however, show lower duc t i l i ty when tested in hydrogen gas or in air with humidity, a phenomenon called environmental embrittlement(?-13). Environmental embrittlement has been sug- gested to be associated with hydrogen atoms which penetrate into a specimen from the environ- ment, and the hydrogen atoms are thought to be formed by the reduction of water vapor and/or the decomposition of hydrogen molecules.

I f hydrogen atoms from test ing atmospheres do indeed embrittle the alloys, why does impu- r i ty hydrogen, which has already been introduced into the alloys during processing, have no influence on their low duct i l i ty? Li t t le or no attention has been given to the relat ion between the low duc t i l i ty of alloys and impurity hydrogen, although almost al l alloys contain some quan- t i ty of this hydrogen. If the impurity hydrogen in the alloys correlates with fracture of the alloys, hydrogen atoms are expected to evolve from the fracture surfaces. To check this line of reasoning, gases evolved from specimens of a TiA1 alloy were analyzed using a newly developed experimental system(14).

Experimental Procedure

A Ti-48at%A1 alloy was prepared by skull melting and precision casting in vacuum of 13Pa. The alloy ingots, which were nearly net-shaped to tensile specimens, were ho t - i sos ta t i ca l ly pressed (HIP-ed) at llO0°C for 4h in argon with a gas pressure of 2xlOsPa. Round tensi le speci- mens having a diameter of 6 mm and a gauge length of 15 mm were f inal ly prepared by means of machining. The content of impurity hydrogen was analyzed to be ?6 mass ppm. Tensile tes t s and gas analyses were performed simultaneously using the experimental system schematically shown in Fig. 1. This system consists of a tensile-compression fatigue testing machine, a quadrupole mass spectrometer, and an ul tra high vacuum specimen-chamber which is evacuated with a tandem turbo molecular pump and a non-evaporation getter pump.

The inside of the chamber is kept at a pressure of about lO-~Pa during tensile testing. The analysis of gases is made with the quadrupole mass spectrometer and main residual gases in

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1030 HYDROGEN EVOLVED FROM FRACTURE Vol. 29, No. 8

the chamber before testing were found to be hydrogen and water vapor. Tensile tes ts of the specimen were performed at ambient temperature in vacuum of 1. OxlO-~Pa at an in i t i a l s t ra in rate of 5.6xlO-'s -1. Analysis can be made of ten kinds of species having di f ferent m/e values al l through the testing, where [] is the mass and e is the valence of species.

Results and Discussion

Figure 2 is an optical []icrograph of the specimen, and shows that the structure consists of gamma and lamellar grains of (7+a2) . The average sizes of lamellar and gamma grains were estimated to be about lO0~m and 50~m, respectively. A fracture surface of the specimen tested in vacuum of 1. OxlO-VPa is shown in Fig. 3. The surface appearance is a mixed mode of transgran- ular fracture of ga[]ma grains and interface separation of lamellar transformed grains.

The result of gas analysis during a tensile tes t is shown in Fig. 4. Hydrogen gas (m/e=2) and methane (m/e=16) can been seen at the moment of fracture ' , although these gases are rarely detected during deformation before fracture. Other kinds of gases having m/e values of 28 and 44 are also evolved at the moment of f racture . Nearly the sa[]e resu l t s were obtained from another specimen. Thus, i t is clear that the major gas evolved from the fracture surface is hydrogen.

From the pressure change of the specimen cha[]ber occurring at the moment of fracture of the specimen, i t was possible to estimate the a[]ount of hydrogen gas evolved from the specimen. The amount was calculated to be nearly equal to that of hydrogen dissolved within the region lnm below from each fracture surface, i f the impurity hydrogen is dissolved unifor[]ly in the speci- men. Of course, there is a poss ib i l i ty that hydrogen ato[]s come to fracture surface from a more distant part of the specimen.

Methane gas is considered to be synthesized at the fracture surfaces between hydrogen and carbon ato[]s at the moment of fracture. Fresh surfaces, i .e . , fracture surfaces of the TiA1 alloy formed at the mo[]ent of fracture may act as a catalyst in this synthesis. Other gases might be formed also at the fracture surfaces.

I t is noteworthy that hydrogen evolution is found for the f i r s t time from the TiM speci- mens at the moment of fracture. This result suggests that hydrogen is concerned with the forma- tion of the fracture surface. In this case, the poss ibi l i ty that hydrogen comes fro[] the envi- ronment can be neglected, since the tensi le tes ts and analyses were performed in the vacuum. I t is therefore possibile that low duc t i l i t y is brought about by impurity hydrogen. To confirm this idea, specimens must be investigated which have a hydrogen content low enough not to be detected at the moment of fracture. In any event, the impurity hydrogen should be taken into account when the low duc t i l i ty of a TiM alloy is discussed from now on.

Sum[]ary

The analysis of gases which were evolved from specimens of a Ti-48at%A1 alloy during ten- s i le tes t ing was performed using a newly developed experimental system which consists of a ten- sile-compression fatigue tes t ing machine, a quadrupole mass spectro[]eter, and an ultra high vacuum specimen-chamber tha t was evacuated with a tandem turbo molecular pump and a non- evaporation getter pump. I t was found that impurity hydrogen is evolved from fracture surfaces at the moment of fracture. Therefore, impurity hydrogen of the TiA1 alloy is believed to be closely related to the low duc t i l i t y of this alloy.

* A preliminary tes t using specimens of an AI-Cu alloy has revealed that detected hydrogen is def ini te ly evolved not from inner walls of the specimen cha[]ber but fro[] the speci[]en i t s e l f : a large amount of hydrogen is detected from the a l loy melted and cast in air, whereas only a sl ight amount is found from the alloy melted and cast in vacuu[].

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Acknowledgm@nts

This work was performed using the equipment newly developed under a Grant-in-Aid for Developmental Scientific Research from the Ministry of Education, Science and Culture of Japan. One of the authors (H. Okada) would like to thank Iketani Science and Technology Foundation (Tokyo, Japan) for partial f inancial support. The authors are grateful to Dr.T. Yamada and Mr.M. Suga of NKK Corporation (Tokyo, Japan) for providing them with the TiM alloy specimens.

References

1. K. 2. N. 3. J. 4. S. 5. N. 6. K.

5, 7. Y. 8. C. 9. E.

10. T. ll. E. 12. C.T. Liu, and E.P. George, Scripta Metall., 24, 1285(1090). 13. T. Takasugi and O. Izumi, Scripta Metall., 19, 903(1985). 14. M. Kanno, H. Okada and &Itoh, J. Japan Inst. Metals., 12,

Aoki and O. Izumi, J. Japan Inst. Metals, 43, 1190(1979). Masahashi, T. Takasugi and O. Izumi, Metall. Trans., 19A, 353(1988). A. Horton, C.T. Liu and L.M. Santella, Metall. Trans., 18A, 1265(1987). C. Huang and E.L. Hall, Metall. Trans., 22A, 427(1991). Masahashi, W. Takasugi and O. Izumi, Metall. Trans., 19A, 345(1988). Hashimoto, H. Doi, K. Kasahara, T. Tsujimoto and T. Suzuki, J. Japan Inst. Metals,

530(1990). Liu, T. Takasugi, O. Izumi and W. Yamada, Acta Metall., 37, 507(1989). T. Liu and Y.W. Kim, Scripta Metall., 27, 500(1992). P. George, C.T. Liu and D.P. Pope, Scripta Metall., 28, 857(1993). Wakasugi and O. Izumi, Acta Metall., 34, 607(1986). P. George, C.T. Liu and D.P. Pope, 8cripta Metall., 2% 365(1992).

1501(1992).

Load Cell

F--"

Ultra High ; Vacuum Chamber

L ~ '1 GetterPump

E O:o::U0O, ~ Spectrometer

~'--'-" L.__.Jx. Turbo Molecu[er

L ] ~ Pump

I ] Rotary Pump

FI&I Schematic Diagram of an Ultra High Vacuum Testing Machine Equipped with a Quadrupole Mass Spectrometer.

1032 HYDROGEN EVOLVED FROM FRACTURE Vol. 29, No. 8

FIG. 2 Optical Microstructureof Ti-48at%Al.

FIG. 3 SEM Fractograph of Ti-48at%Al.

Vol. 29, No. 8 HYDROGEN EVOLVED FROM FRACTURE 1033

~{- I rile = 2 (H~)-

l , . l / e : 1~, (C)-

-7 -8 --9 -10 -II

-7 --8

,~, I{ i.,}.hL,{}.li,{ {,,L {, ,, ,,{I ,i,,,l,il~ ,~,.l,,i ~d1,,, m,,. I,

w/e= 14 (Cl{2)J

~, ,,+ ,.,,, ,{llli l, L,., ,1~, Ji,~L,.~+,.U,k~,, ..{ l,.{ildd{,,+ ,,~,,.,l,J, l

z/e: 15 (OH3)- [ ,,,{,i~,Li, u. ul dld,.=,lI,,.k,tL.,,,.,,I ,,.,{m,i,llh,.,l~,,i~J,,lu,=l,ll

f , / e = 16 (OH,)- ,== -11

' - (H 0) ,-- - =/e = 18 2 0

= -7 9,8 (N +CO) ° -8 l / e = 2 -

-9 l i l l l l i l i~ .... ~..,.~.~.i .=~ , , , , , , . , . - . i~ . . . . . . l i l l -10 T,~m~?ni.,~.r,~,,..r.nr,..r,.,e,,r,,.m,,.,,,,~ ~- -II

l /e : 31 (0~)- ,, ,i,, ih.l/,,llu{ I~ ,,l ,,, i,,~,i,,.L,u ,, ~i 1 ,kii,il,.ilhiJi

~t i/e = lO (iir):

f i /e : 44 (CO~) i ] i i l l i l l i l i i l i l i l d i t l u "d " l t ~ i l l , itlll=,li}= ,~l~lt~i~i,lita

Displaomat ~o,z,,,

-7 -8 -9 -lO -11

-7 -8 -9

-10 -11

-7 -8 -9

-I0 -II

-7 -8 -9

-10 -11

-7 -8 -9 -10 -11

FIG. 4 Correla t ion between the Load-Displacement Curve and Gas Evolution Behavior of a r iAl Alloy Tested in a Vacuum of l, OxlO-TPa.