Journal of Shanghai University ( English Edition ), 2005, 9(6 ) : 544 - 549 Article El): 1007-6417(2005)06-0544-06

Effect of Ordering on Embrittlement of Ni4Mo Alloy in Hydrogen Gas CHENG Xiao-ying (:~ ~/~,:~:), L / H u i - g a i ( * $ ~J~) Institute of Materials, ~/tanohai University, ~aanghai 200072, P . R . China Abstract The fracture behavior of disordered and ordered Ni4Mo alloy was investigated by tensile tests in hydrogen gas or durm~ hydrogen charging. The results show that the ductility of the disordered alloy decreased slightly with the hydrogen pressure increasivg, while that of the ordered alloy decreased rapidly with the hydrogen pressure increasiv~g. However, the ductility of both disordered and ordered alloys reduced similarly seriously with the charging current density increasing. Therefore, the mechanism of order-induced embrittlemerit of Ni4Mo alloy in hydrogen gas is supposed to be that atomic order accelerates the kinetics of the catalytic reaction for the dissociation of molecular I-~ into atomic H. Key words Ni4Mo alloy, hydrogen embrittlement, order, disorder, hydrogen gas, hydrogen charging.

1 Introduction Nickel-based alloys containing 20 - 30 wt % Mo is known to be essential for corrosion resistance in reduciv~g media E~] , especially for resistance to HC1 at all temperatures and concentrations t2J . However, upon

regate extensively at grain boundaries which lowers the grain-boundary cohesion. The two points of view are mainly focused on how the atomic hydrogen induces embrittlement. However, our previous study~3"8'~J has ~_ggested that atomic ordering m a y accelerate the kine~cs of the catalytic reaction for the dissociation of

6 0 0 - 800 °C, these alloys suffer an almost complete

molecular hydrogen into atomic hydrogen on the surLace of transition met~l.q. This point of view is mainly

loss of r o o m temperature ductility as a result of longrange ordering (LRO) to Ni4 Mo. It is considered that

focused on how m a n y hydrogen atoms can be produced in ~ or hydrogen gas on the surface of tensile

the loss of ductility mainly results from the environmental embrittlement induced by moisture in air, especially by hydrogen gast3.41. There are three kinds of

specimen, which can affect the ductile behavior of

exposure to elevated temperatures in the range of

alloys. In present study, we will conduct two main experiments to clarify the mechanism of LRO-induced embrittlement of N4 Mo alloy in hydrogen g a s - - e l e c -

point view to explain the effect of atomic ordering on the environmental embrittlement. Camus, et al. t6~ ,

trolytic loading and gas pressure loading. The electro-

Takasugi and Hanada Es~ suggested that the LRO intensi-

lytic loading eliminates the dissociation step from mo-

fies planar slip and restricts at the grain boundaries, hence promotes hydrogen atoms to segregate at grain

lecular hydrogen to atomic

boundaries, and enhances the susceptibility of the alloy to hydrogen embrittlement. Nishimura and Liu t7~

hydrogen (H~--~2H) which

often hinders uptake from the gaseous phase, in addition, our previous results show that the ordered Ni4 Mo alloy exhibits severe hydrogen gas-induced environ-

demonstrated that the atomic-size cavities at grain

mental embrittlement regardless of ordering time [3] .

boundaries of strongly ordered alloys serve as hydrogen trapping sites, and hence hydrogen atoms seg-

Therefore, the initial ordering is selected for c ~ the key effect of ordering on the environmental embrittlement in this paper.

Received Jun. 2, 2004; Revised May 16, 2005 Project st~pported by National Natural Science Foundatron of China(Grant No.59895157), and Science Foundation of Shanghai Municipal Commission of Science and Technology (Grant No.

o2zE14o33) CHENG Xiao-ying, Ph. D., Asso. Prof., E-mail: chevgxy @ staff. shu. edu. cn

2 Experimental Procedure A 2.5 kg ingot of Ni4 Mo ( Ni-28wt 96 Mo) alloy w a s melted in an induction furnace under vacuum usin~ high purity of Ni and Mo and then cast into a 40 m m diameter ingot. The ingot w a s sliced into four sheets

Vol.9 No.6 Dec.2005

CHENGX Y,

et al. :

Effect of Ordering on Embrittlement of Ni4Mo Alloy in Hydrogen Gas

with a thickness o f about 10 m m and each sheet was hot rolled at 1 260 °C to about 2 nun thickness, and then cold rolled to about 1.2 ram. Tensile specimens with a gage section of 16 x 3 x 1 mm s were electric-discharge machined from the sheets and sealed in an evacuated quartz tube. The specimens were disordered by heating at 1 066 °(3 for 30 min, and then the capsule was broken under water. For ordering treatment, some disorder treated specimens were sealed in an evacuated quartz tube again and annealed at 650 ~C for 0.5 h, followed by air cooling. The ordered structure was characterized by selected area elecU'on diffraction patterns (SAD) in a H-800 Wansmission electron microscopy ( T E M ) . Before being observed in TEM, thin discs of disordered and ordered specimens were cut from the sheets with electric-discharge machine. These discs were polished by fine abrasive paper to about 0.1 nun and then electrochemically thinned in a mixture of one part of perchloric acid and three parts of alcohol at temperature of 243 K and potential of 20 V. Tensile tests were performed on a MTS testing machine equipped with a vacuum chamber. For the gas pressure loading, the chamber was evacuated twice to 2 x 10 -2 Pa and backfilled with the hydrogen gas to different pressures. For the electrolytic loading, the tensile tests were conducted simultaneously cathodic charging at various current densities in a solution o f 1N I~SO 4 containing 0. 05 g/1 NRAsO3 as a hydrogen

545

1 ( a ) ) , which was consistent with the results in Ref. [ 1 0 - 12 ]. It is illustrated that the disordered alloy is not completely disordered, and the process of ordering at 650 °C for 0 . 5 h is an initial step in the disorder to order process. ~20

I I

0~0~ 2~

220 ~'2°° 1/4(2i0)

(a) Disordered, showingSRO (white arrow) spots only

1/4(240) i/5(240) C Fundam~tal spOtS o Short-rangeorder spots • Long-range order spots

(b) Ordered, showingLRO (b~ck arrow) and SRO (wm~) spots Fig. 1 [ 001 ] electron diffraction patterns f~om a Ni4Mo alloy The results of the tensile tests performed in vacuum, air and 0.1 MPa H~ were summarized in Fig. 2 (a) and ( b ) . When tested in vacuum and air, the disordered

recom-bination poison. All specimens were tested at an initial strain rate of 2 x 10-3 s-~ at laboratory temperature of 25 °C and humidity of 60 % . Ductility was obtained by compariv~ the specimen length before and after tensile tests and assuming that all swain occurred in the gage section. The fracture surfaces were examined in a scanning electron microscope (SEM).

and ordered alloys displayed similar el~gineeriv~ strain curves and the ductilities of disordered and ordered alloys tested in air remained nearly the same value as tested in vacuum. It is indicated that both disordered and ordered alloys exhibit no or a little moisOxe-induced environmental embrittlement as tensile tested in air. When the alloy was tensile tested in 0.1 MPa I-Is,

3 Results

the ductility reduced by 28 96 for the disordered alloy and 8996 for the ordered alloy, indicating that the I~-

No ordered peak pattern of Ni4Mo tetragonal type structure was observed in the X-ray diffraction patterns of Ni4 Mo alloy ordered at 650 °C for 0.5 h (it is denoted by ordered alloy latter). However, it was seen on the diffraction patterns in TEM using a < 001 > zone axis that the ordered alloy exhibited the pattern of both love-range order (LRO) and short-range order (SRO) spots ( see Fig. 1 ( b ) ) , while the disordered alloy only exhibited the pattern of SRO spots (see Fig.

induced environmental embrittlement is more severe for ordered alloy than that for disordered alloy. Fig.3 illustrated typical tensile fracture surfaces. As expected, in all specimens except the ordered state tested in 0 . 1 M P a I ~ , fracture occmTed Wansgranularly (dimple-type rupture) as shown in F i g . 3 ( a ) (disordered alloy tested in vacuum). The ordered alloy tested in 0.1 MPa H~ exhibited brittle intergranular fracOn~ ( ~ g . 3 ( b ) ) . This is consistent with the results of tensile tests.

546

Journal o f Shanghai University 1 200 1

1.Invacuum 2.1nak

ooo 80o

"~

600 400 200

o

iO

1

i

I

20 30

i

I

40

50

i

I

60 70

8'0

Strain(%) (a) Disordered Ni4Mo alloy tested in various environments 1200 1.In vacuum ~~._.,~.~ 2 . I ..n a i r I0(0) ..~.m . u.x~ / ~ _ _ _ _ ~ 1

decreased slightly with hydrogen pressure increasing, while the ductility of ordered alloy dropped rapidly with hydrogen pressure increasing. The fracture surface of the disordered alloy tested in different pressures of hydrogen gas mainly exhibited ductile dimpled l~nsgranular fracture as the alloy tested in vacuum (see Fig. 3 (a) ). However, the fracture surface of the ordered alloy exhibited a mixed mode of fracture with ductile dimpled transgranular fracture in the center and brittle inter-gralmlar fracture along the perimeter (see Fig. 5). The percentages of intergranulax and 1xansgraDular failure varied in amount and were relative to the pressure of hydrogen gas. When tested in 0.1 MPa I-I~, the fracture surface exhibited a completely intergranular fracture path (see Fig.3(b) ). 1200 1.Vacuum 2.1.8xl04 Pa H2 IO00 4 . yH3 1 6Pa 2,. x 1~0, 4 , ,

m 400 200

~" I

0

1

I

20 30

I

I

40

50

I

60 70

*~ 43 2

soo

I

80

"~ 6oo

Strain(%)

4oo

(b) Ordered Ni4Mo alloy tested in various environments Fig.2 Engineering stress-strain curves

200 I

0

10 20 30 40 50 Strain(%)

60

70

80

I

I

I

(a) Disordered 1 200

f

1000

2

400 (a) Disordered Ni4Mo alloy tested in vacuum

200 I

0

lo

I

I

20 30 40 50 60 70 80 Strain(%)

1. Vacuum; 2. 6x 10a Pall2; 3. 8x 10a PaI4-2; 4. 1.4x 104 Pall2; 5. 4.8x 104 Pall2; 6. 0.1 MPaH2 (b)Ordered

Fig.4

Engineering stress-sWain curves for Ni4Mo alloy tensile tested in hydrogen gas with different pressure

(b) Ordered Ni4Mo alloy t~---~u~clin 0.1MPa H2 Fig.3

Fracture surface morphologies of Ni4Mo alloy

When the specimen was simultaneously hydrogen charged at different current densities during tensile testing, the ductility and the ultimate tensile strength reduced s'Lmilarly seriously in both disordered and or-

Fig. 4 showed the engineering stress-strain curves of Ni4Mo alloy tensile tested in hydrogen gas with

cated that the atomic hydrogen atoms were forced to

different pressures. The ductility of disordered alloy

get into the Ni4 Mo alloy during cathodic chaxging and

dered conditions (see Fig.6 ( a ) and ( b ) ) .

It is indi-

Vol.9

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C H E N GX Y, et o2. :

Effect of Ordering on Embrittlement of Ni4Mo Alloy in Hydrogen Gas

it c a n severely e m b r i t t l e the alloy regardless o f the a t o m i c LRO. The typical fractographs of the s p e c i m e n s

4

547

Discussion

tensile t e s t e d a n d s i m u l t a n e o u s l y charged h y d r o g e n at

Many s t u d i e s have revealed that h y d r o g e n embrittle-

different c u r r e n t densities w e r e s h o w n i n Fig. 7. It ex-

merit of m e t a l s is c a u s e d b y h y d r o g e n a t o m s . T h e r e

h i n t e d ductile t r a n s g r a n u l a r failure i n the c e n t e r a n d

1 200

brittle i n t e r g r a n u l a r failure a l o n g the p e r i m e t e r . The

1000

p e r c e n t a g e s of i n t e r g r a n u l a r a n d t r a n s g r a n u l a r failure e m b r i t t l e m e n t resulting from h y d r o g e n charging,

2;6

1

800

varied i n a m o u n t a n d w e r e relative to the degree of

600

i.

2

400

e . , the charging c u r r e n t d e n s i t i e s . f/3

0 -200 -10

i

i

i

i

i

i

i

i

i

0

10

20

30

40

50

60

70

80

strain(%) 1. Vacuum; 2. 1 mA/cmz ; 3. 5 mA/cm2; 4. 15 mA/cm2; 5. 30 mA/cmz ; 6. 50 mA/cm2 (a) Disordered I 200 lO00

1

(a) 6 x I08 Pa 112 ~"

80O

2

~6oo r~

400

3

'

30

40

2OO 0

10

20

50

60

' 70

8'o

Strain(%)

Fracture surface morphologies of ordered N4 Mo alloy

1. Vacuum; 2. 1 inA/cm2; 3. 5 mA/cm2; 4. 15 mA/(nn2;6.50 mA/craz (b) Ordered Engineering stress-strain curves for disordered and ordered Ni4 Mo alloy tensile tested and simultaneously

tensile tested in hydrogen gas with different pressure

hydrogen-clinked at different current densities

20.0kV

Fig. 6

(b) 1.4x 104 Pa H2

~ng.s

(

..... :i

i

P (a) 1 mA/cm 2

(b) 5 mA/cm 2

Fig.7 Fracture surface morphologies of the ordered alloy tested and simultaneously hydrogen charged

(c) 15 m A / c m 2 at

different current densities

548

Journal of ~ a n g h a i University

exists a critical concentration of hydrogen causing embrittlement. To achieve the critical concentration of hydrogen at a crack nucleation site, the following processes should be passed through: ( 1 ) adsorption of water vapor a n d / o r hydrogen gas ; (2) decomposition of water vapor ( Eq. ( 1 ) ) a n d / o r hydrogen gas ( Eq. (2)) ; (3) absorption of hydrogen; (4) transportation of hydrogen and ( 5 ) transfer of hydrogen to grain boundaries. I-I20 + Metal(Al, Mo)--~Metal - O + 2H Ni

ties can be shown in Fig. 8. It is clear that the embrittlement factor I~ both for the ordered and disordered Ni4Mo alloy has the same dependence on the charging current densities. This means that the embrittlement induced by the cathodic charging both for ordered and disordered alloy depends only on the number of hydrogen atoms entering the material, while not depends on the LRO. 1.0

(1) 0.8

~2H

(2) 0.6

In these steps, steps (2) and (4) have been considered as rate con~'olling steps of the embrittlement[L~] . Therefore, in study of LRO-induced embrittlement, the difference in steps ( 2) and ( 4 ) between disordered and ordered alloys is important. To avoid the dissodafion step ( 2 ) , the electrolytic loading is firstly considered because hydrogen atoms are generated during the electrolytic loading. According to Faraday' s Law, the atomic hydrogen on the electrode is (3)

n = FAit,

where n is the mole of atomic hydrogen on the electrode, i is charging current density, t is hydrogen charging time; F is the Faraday' s constant 96 490 C/tool, A is the area doping hydrogen. With the concenlra~on of atomic hydrogen on the surface of elec~ode increasing, the atomic hydrogen can be combined into gas hydrogen as Eq. (4) except the atomic hydrogen that has penetrated into the electrode. H + H--~I~

(4)

Therefore, the atomic hydrogen entering materials began to increase proportionally with the charging current density and event~lally reached saturation, independent of current density. The hydrogen embrittlement induced by electrolytic loading can be evaluated by an embrittlement factor IH : I. = ( 8 ~

where 8 , ~

.~

0.4

"r--e-- Disobey - ~ - Order I

I

I

I

I

10

20

30

40

50

Current density (mA/cm2) Fig.$ Embrittlementfactor versus ~ t

Secondly, the hydrogen embrittlement induced by gaseous hydrogen is also evaluated by an embrittlement factor Ix :

X~ = ( 8 ~

- 8% ) / 8 ~

where 8 ~

x 10096,

(5)

the elongation tested in hydrogen gas. Based on the stress-stain curves shown in Fig. 4, the H~-induced embrittlement factor as a function of hydrogen pressure may be shown in Fig.9. It indicates that the embrittlement factor for ordered material increases more rapidly with the hydrogen pressure increasing than that for disordered material. It is well known that the gaseous H~-induced environmental embrittiement of intermetallics is due to the catalytic reaction on the sur1.0 ~ 0.8

•

0.6

f ~

,.o 0.2 !

Disorder

4 - Ord~

~ l . ~ vacutml __..~..- S

is the elongation tested in vacuum, ~n is

the elongation tested simultaneously hydrogen charging. Based on the data shown in Fig. 6, the embrittlemerit factor IH as a function of charging current densi-

(6)

is the elongation tested in vacuum, 8% is

~ 0.4

- 8 . , ) / 8 , ~ , ~ x 10096,

density

2

4

i

I

1

6

8

]0

Hydrogenpressure (x 104pa) Embrittlement factor versus hydrogenpressure

Vol.9

No.6

Dec.2005

CHENGXY, eta].:

Effect of Ordering on Embritflement of Ni4Mo Alloy in Hydrogen Gas

f a c e o f t r a n s i t i o n a l e l e m e n t s for d e c o m p o s i t i o n o f m o lecular I~

into a t o m i c h y d r o g e n .

The a b o v e r e s u l t

[3 ]

i m p l i e s t h a t n o o r a little e n v i r o n m e n t a l e m b r i t t l e m e n t in g a s e o u s I ~ for d i s o r d e r e d Ni4 Mo a l l o y is c o n t r i b u t e d to n o o r l e s s h y d r o g e n a t o m s p r o d u c e d o n t h e s u r f a c e of material.

In

other

words,

the

ordered

Ni4Mo

r e v e a l e d v e r y brittle f r a c t u r e in g a s e o u s h y d r o g e n is due to more atomic hydrogen produced on the surface

a//., 2002, 46:465-470. Wright J L, Zhu J H. Environmental embrittlement of an ordered Ni-Mo alloy[ J ] . Scr/pta Mater., 1998, 38:253 - 257. E 5 ] Camus G M, Stoloff N S, Duquette D J. The effect of order on hydrogen embrittlement of Nis Fe [ J J. Acta Met[6]

a//., 1989, 37:1 497 - 1 501. Takssugi T, Hanada S. The influence of constitute elements and atomic orderi~4g on hydrogen embrittle of Nis Fe

[7]

polycrystals [ J ] . lntermetaUics, 1994, 2:225-232. Nishnmura C, Liu C T. Effect of ordered state on environmental embrittlement in (Co,Fe)3V [ J ] . Scripta Met-

[8 ]

a//., 1996,35: 1 4 4 1 - 1 4 4 7 . Cheng X Y, Wan X J. The influence of a~omic ordering on the hydrogen embrittlement of ( Co,Fe)3V polycryst~

5 Conclusion ture-induced environmental embrittlement, but has a c o n s i d e r a b l e effect o n H~-induced e n v i r o n m e n t a l e m b r i t t l e m e n t . W h e n t e n s i l e tested in h y d r o g e n g a s w i t h d i f f e r e n t p r e s s u r e s , t h e ductility o f t h e d i s o r d e r e d a l l o y d e c r e a s e d slightly w i t h

the

hydrogen

pressure

in-

creasing, while that of the ordered alloy decreased

[9 ]

rapidly with the hydrogen pressure increasing. (2) When hydrogen was simultaneously charged at v a r i o u s c u r r e n t d e n s i t i e s during t e n s i l e t e s t i n g ,

the

ductilities o f t h e d i s o r d e r e d a n d o r d e r e d a l l o y s r e d u c e d

[ 1O]

s i m i l a r l y s e r i o u s l y w i t h t h e charging c u r r e n t d e n s i t y increasing.

[ 11 ]

(3) The mechanism of atomic LRO-induced embritt l e m e n t in h y d r o g e n g a s is s u p p o s e d t o b e t h a t a t o m i c LRO a c c e l e r a t e s t h e k i n e t i c s o f t h e c a t a l y t i c r e a c t i o n for t h e d i s s o c i a t i o n o f m o l e c u l a r h y d r o g e n into a t o m i c

[12]

hydrogen.

References [ 1]

[2]

Galen H F. Corrosion and Corrosion Protection Handbook [ Z]. Schweitzer P A, Dekker M, eds., New York, 1983, 55. Friend W Z. Corrosion of Nickel-Based Alloy [ M ].

W'fley, New York, 1980, 248. Chepg X Y, Wan X J. The effect of ordering on the environmental embrittlement of Ni~Mo alloy [ J]. Svripta Met-

[4 ]

of the material.

( 1 ) T h e a t o m i c LRO d o e s n o t i n f l u e n c e t h e m o i s -

549

[13]

[J]. Scr/pta MetaU., 2001, 44:325 - 329. Cheng X Y, Wan X J. Effect of atomic ordering on environmental embrittlement of ( Co, Fe )3 V alloy in gaseous hydrogen [ J ]. Trans. Nonferrous Met. Soc. China, 2002, 12:786- 791. Brooks C R, Cao S. The development of the ordered structure from cold worked disordered alpha in Ni4Mo [J]. Philosophical Magaz/ne A, 1992, 65:327-353. Cao S, Brooks C R, Allard L. In situ ~ o n electron microscopy study of ordering in a splat-cooled Ni20at. %Mo aUoy [J]. Mater/a/s Character/zat/~n, 1995, 34:87 - 95. Brooks C R, SpruiellJE, StansburyEE. Physicalmetallurgy of nickel-molybdenum alloys [ J ]. International Metals Rev/ews, 1984, 29:210 - 248. KimuraA, IzumiH, MisawaT, etal. Criticalpresstmes of water vapor and hydrogen gas causing intergranular brittle fracture of C% Ti at room temperature [ J ] . Mater. Trans. J/M, 1994, 35:879-887. (Editor CHEN Ai-ping )