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Transition from hydrogen induced cleavage fracture to hydrogen induced intergranular fracture in Fe3Al
Scripta METALLURGICA et M A T E R I A L I A
Vol.
27, pp. 1295-1299, 1992 P r i n t e d in the U.S.A.
TRANSITION FROM HYDROGEN INDUCED I N T E R G R A N U L A R F R A C T U R E IN Fe~AI
Dept.
J.P. of M a t e r i a l s
Beijing,
100083,
CLEAVAGE
Lin, W.Y. Chu, D.Z. Physics, U n i v e r s i t y
People's
Republic
P e r g a m o n Press Ltd. All rights r e s e r v e d
FRACTURE
TO HYDROGEN
Zhang and C.M. H s i a o of S c i e n c e and T e c h n o l o g y
INDUCED
Beijing,
of C h i n a
(Received July 16, 1992) (Revised S e p t e m b e r 9, 1992) Introduction Iron a l u m i n i d e s b a s e d on Fe:AI offer m a n y a d v a n t a g e s for s t r u c t u r a l uses at elevated temperatures because of their sufficient high strength, excellent oxidation
resistance
and
relatively
low m a t e r i a l
density
(I-3).
However,
the
low
room t e m p e r a t u r e e m b r i t t l e m e n t r e s t r i c t their use. Recently,
in air and the e n v i r o n m e n t a l s e n s i t i v i t y s e v e r e l y Liu et al. (i) r e p o r t e d that the tests in pure
o x y g e n of
ductility
Fe~AI
gave
tensile ductility
a tensile
of 12.4% w h i l e only
of 11.7% and
the tests
3.2% and 2.1% were
in v a c u u m
gave a
a c h i e v e d in air and w a t e r
vapor, r e s p e c t i v e l y . They c o n s i d e r e d that e m b r i t t l e R e n t involves the r e a c t i o n of w a t e r v a p o r w i t h a l u m i n i u m atoms and the release of e m b r i t t l i n g atomic hydrogen, There
have
been
some
investigations
of
hydrogen embrittlement
and h y d r o g e n
i n d u c e d d e l a y e d c r a c k i n g of o r d e r e d alloy Fe~AI (4,5) complete. The f r a c t u r e s u r f a c e s of the h y d r o g e n induced
w h i c h were not n e a r l y d e l a y e d c r a c k i n g of high
strength
room
steels,
in
which
ductile
fracture
occurs
at
temperature,
are
d e p e n d e n t upon the K[ and h y d r o g e n c o n t e n t (6,7). This work will i n v e s t i g a t e w h e t h e r the f r a c t u r e s u r f a c e s of h y d r o g e n induced d e l a y e d fracture of o r d e r e d alloy Fe~AI , in w h i c h c l e a v a g e f r a c t u r e (CF) occurs at room d e p e n d e n t u p o n the KI and the h y d r o g e n c o n t e n t introduced.
Experimental The a l l o y iron
(S
=
was p r e p a r e d
0.0017,
P
=
by v a c u u m
0.0026,
C
=
are
Procedure
induction melting
0,01,
temperature,
Si
<
0.005,
using
all
in
commercially wt%)
and
pure
aluminium
(99%). The ingot was h o m o g e n i z e d at 1050 °C for 12 h. The alloy Fe3A1 had f o l l o w i n g c o m p o s i t i o n (in wt%): A1 = 14.86, C = 0.01, P < 0.005, S = 0.03, O = 0.001, H = 0.06 ppm, b a l a n c e Fe. The ingot was hot formed to 1 6 x 4 0 x 2 0 0 RR at 800 - 900 aC and then g i v e n a d u p l e x heat t r e a t m e n t for 1 h at 850 °C for r e c r y s t a l l i z a t i o n and 5 days at 500 °C for DO: ordering. The m o d i f i e d w e d g i n g o p e n i n g loading (WOL) s p e c i m e n s had a t h i c k n e s s of I0 mR, a w i d t h of 26 mR and a h e i g h t of 32 mm. The n o t c h was made by e l e c t r i c spark c u t t i n g using a Mo wire, and the radius of notch
1295 0 9 5 6 - 7 1 6 X / 9 2 $5.00 + .00 C o p y r i g h t (c) 1992 P e r g a m o n Press
Ltd.
1296
FRACTURE
IN Fe3AI
Vol.
27, No.
root was 0.06 mm. A f t e r loading with a screw to c r a c k i n g the specimens were added to the solution. The m o d i f i e d WOL specimens were d y n a m i c a l l y c h a r g e d at various current d e n s i t i e s of 0.2, 2 and 10 m A / c ~ using a s o l u t i o n of 1 N NaOH. The fracture surfaces were e x a m i n e d by scanning e l e c t r o n microscopy.
Results
and D i s c u s s i o n
The test s p e c i m e n s were cracked by stress c o r r o s i o n in water. The K I d e c r e a s e s with the c r a c k growth and KI$cc is the ~ at the points where stress c o r r o s i o n crack (SCC) g r o w t h stopped. It has been c o n f i r m e d that the m e c h a n i s m of SCC in water is due to h y d r o g e n induced c r a c k i n g in ordered alloy Fe3AI (5). The fracture surfaces of the SCC in water of o r d e r e d alloy Fe3AI are shown in Fig. 1. C l e a v a g e fracture (CF) o c c u r r e d and the fracture surfaces were i n d e p e n d e n t upon K x at the crack tip. The specimens still e x h i b i t e d t r a n s g r a n u l a r CF, indepe n d e n t upon K I , a l t h o u g h the a d d i t i o n of a h y d r o g e n r e c o m b i n a t i o n the s o l u t i o n (0.05 g per liter CS2) d e c r e a s e s the Kisc¢ (5).
w h i c h was p o i s o n in
W h e n the loading s p e c i m e n s were d y n a m i c a l l y charged, a change in the fracture modes with K[ took place, as shown in Fig. 2. When a current d e n s i t y of c hargi n g (i) was lower (i = 0.2 mA/cm2), the CF p r o c e e d e d at high ~ (K I = 34.5 MPamln), as shown in Fig. 2a. The fracture modes were about 90% cleavage and 10% i n t e r g r a n u l a r and only the individual facets of grain b o u n d a r i e s were cracked (Fig, 2b) at i n t e r m e d i a t e K z (K x = 24.7 MPamln). When the K I was close to the Kz~ (K I = 16.5 MPamln), the p e r c e n t a g e of i n t e r g r a n u l a r fracture (IGF) s u f f i c i e n t l y increased and all facets of grain b o u n d a r i e s were c r a c k e d (Fig. 2c). Some d e f o r m a t i o n traces can be seen along the g r a i n boundaries, w h i c h shows that d i s l o c a t i o n m o t i o n o c c u r r e d during the grain b o u n d a r y c r a c k i n g (Fig. 2d). The p e r c e n t a g e of IGF increased as i rose to 2 m A / c m 2 , as shown in Fig. 3. The CF formed at higher K I (Fig. 3a). About 50% IGF formed at i n t e r m e d i a t e ~ (K l = 24.7 MPamln), as shown in Fig. 3b. About 80% IGF formed at low K I (K I = 16.5 MPamln), as shown in Fig. 3c. The p e r c e n t a g e of IGF s u f f i c i e n t l y increased as the i rose to 10 m A / c ~ , as shown in Fig. 4. The CF o c c u r r e d at K I of 34.5 MPam In (Fig. 4a) and about 90% IGF o c c u r r e d at K~ of 24.7 MPam In (Fig. 4b) and about 100% IGF o c c u r r e d at K x of 16.5 MPam In (Fig. 4c). The change of the fracture s u r f a c e s with the ~ and current d e n s i t y m e n t i o n e d above is s u m m a r i z e d in Table I. It is c o n c l u d e d that increasing the current d e n s i t y and d e c r e a s i n g K! f a c i l i t a t e the o c c u r r e n c e of IGF. The h y d r o g e n e n t e r e d Fe3AI from the crack tip to matrix, causing IGF. The CF o c c u r r e d when the p o i s o n was added to water, w h i c h indicates that the IGF did not p r e f e r e n t i a l l y occur b e c a u s e the h y d r o g e n content was not enough to form a large d i f f e r e n c e b e t w e e n the g r a i n b o u n d a r y and g r a i n interior. But this still means that some g r a i n b o u n d a r y facets with a large Schmid factor c r a c k e d by cleavage. There was no d i f f e r e n c e in the fracture surfaces , i.e., only the cleavage fracture was observed. As the h y d r o g e n content rises to a critical value of Cm0, sufficient s e g r e g a t i o n occurs, so that a d i f f e r e n c e in the h y d r o g e n content b e t w e e n grain b o u n d a r y and g r a i n i n t e r i o r also forms, w h i c h p r o d u c e s the c r a c k i n g of the grain
i0
Vol.
27, No.
i0
Table
FRACTURE
Effect
1
of the
IN Fe3AI
1297
i and the K I on the fracture K, (MPam In
Condition
surfaces
)
34.5
24.7
16.5
H20
CF
CF
CF
H20 + CS 2
CF
CF
CF
IN NaOH + 0.2 m A / c ~
CF
CF + 10% IGF
CF + 40% IGF
IN NaOH + 2 mAlc~
CF
CF + 50% IGF
CF + 80% IGF
CF + 90% IGF
IGF
IN
NaOH
+
10 m A / c ~
CF - cleavage
boundaries
I
CF fracture
with
a lower
IGF
Schmid
-
intergranular
fracture
factor.
K, value has an effect on the h y d r o g e n distribution. With increasing K I the d i s l o c a t i o n density at a crack tip increases. The increase in the d i s l o c a t i o n d e n s i t y increases the c a p a c i t y of the grain interior to trap h y d r o g e n (8), so the d i f f e r e n c e in the h y d r o g e n content between grain b o u n d a r y and grain interior is small, which can facilitate the o c c u r r e n c e of t r a n s g r a n u l a r cracking. With d e c r e a s i n g K! the d i s l o c a t i o n d e n s i t y at a crack tip decreases, so that the h y d r o g e n trapsdecrease and h y d r o g e n p r e d o m i n a n t l y segregates to grain boundaries which in turn can facilitate the o c c u r r e n c e of IGF.
Conclusion The
fracture
displayed
cleavage,
surfaces
of
which was
D e c r e a s i n g the K I and the o c c u r r e n c e of IGF.
the
hydrogen
independent
increasing
induced
delayed
cracking
in
water
upon K,
the current
density
of charging
facilitate
References
[i] [2] [3] [4] [5]
C.T. Liu, C.G. M e K a m e y and E.H. Lee, Scripta Metall., 24, 285 (1990). C.T. Liu, E.H. Lee and C.G. McKamey, Scripta Metall., 23, 875 (1989). C.T. Liu and E.P. George, Scripta Metall., 24 (1990). A. C a s t a g n a and N.S. Stoloff, S c r i p t a Metall., 26, 673 (1992). D.Z. Zhang, D.L. She, G.W. Du, F.W. Zhu and C.M. Hsiao, Scripta Metall., press.
in
1298
[6] [7] [8]
FRACTURE
IN Fe3AI
Vol.
W.Y. Chu, C.M. Hsiao and W.X. Li, Metall. Trans., 15A, 2007 (1984}. W.Y. Chu, C.M. Hsiao and S.Q. Li, Corrosion, 37, 320 (1981). J. Kameda, Res Mechanica, 26, 215 (1989).
A
Fig.
Fig. 2
B
1
Fracture
surfaces of SCC in water of ordered alloy F % A I
A
8
C
D
Change of the fracture surfaces during the dynamic charging at low current d e n s i t y (i = 0.2 mA/cm z) in FesAI with the K I (a) K! = 34.5 MPam In (b) K! = 24.7 MPam In (c) and (d) Z I = 16.5 MPam I/z
27, No.
i0
Vol.
27, No.
i0
FRACTURE
IN Fe3AI
A
1299
S
C
Fig.
3
Change of the fracture surfaces during the dynamic charging at intermediate current density ( i = 2 m A / c ~ ) i n Fe3A1 w i t h t h e ( a ) ~ = 3 4 . 5 MPa~/2 (b} KI = 2 4 . 7 MPam~/2 ( c ) ~ = 1 6 . 5 MPam1/~
A
S
C
Fig.
4
Change
of
the
fracture
surfaces
during
the
dynamic
charging
high current density (i = i0 m A / c ~ } in Fe3AI with the ( a ) ~ = 34.5 MPa~/z ( b ) ~ = 24.7 MPa~/z ( c ) ~ = 16.5 MPamt/2
at
Vol.
27, pp. 1295-1299, 1992 P r i n t e d in the U.S.A.
TRANSITION FROM HYDROGEN INDUCED I N T E R G R A N U L A R F R A C T U R E IN Fe~AI
Dept.
J.P. of M a t e r i a l s
Beijing,
100083,
CLEAVAGE
Lin, W.Y. Chu, D.Z. Physics, U n i v e r s i t y
People's
Republic
P e r g a m o n Press Ltd. All rights r e s e r v e d
FRACTURE
TO HYDROGEN
Zhang and C.M. H s i a o of S c i e n c e and T e c h n o l o g y
INDUCED
Beijing,
of C h i n a
(Received July 16, 1992) (Revised S e p t e m b e r 9, 1992) Introduction Iron a l u m i n i d e s b a s e d on Fe:AI offer m a n y a d v a n t a g e s for s t r u c t u r a l uses at elevated temperatures because of their sufficient high strength, excellent oxidation
resistance
and
relatively
low m a t e r i a l
density
(I-3).
However,
the
low
room t e m p e r a t u r e e m b r i t t l e m e n t r e s t r i c t their use. Recently,
in air and the e n v i r o n m e n t a l s e n s i t i v i t y s e v e r e l y Liu et al. (i) r e p o r t e d that the tests in pure
o x y g e n of
ductility
Fe~AI
gave
tensile ductility
a tensile
of 12.4% w h i l e only
of 11.7% and
the tests
3.2% and 2.1% were
in v a c u u m
gave a
a c h i e v e d in air and w a t e r
vapor, r e s p e c t i v e l y . They c o n s i d e r e d that e m b r i t t l e R e n t involves the r e a c t i o n of w a t e r v a p o r w i t h a l u m i n i u m atoms and the release of e m b r i t t l i n g atomic hydrogen, There
have
been
some
investigations
of
hydrogen embrittlement
and h y d r o g e n
i n d u c e d d e l a y e d c r a c k i n g of o r d e r e d alloy Fe~AI (4,5) complete. The f r a c t u r e s u r f a c e s of the h y d r o g e n induced
w h i c h were not n e a r l y d e l a y e d c r a c k i n g of high
strength
room
steels,
in
which
ductile
fracture
occurs
at
temperature,
are
d e p e n d e n t upon the K[ and h y d r o g e n c o n t e n t (6,7). This work will i n v e s t i g a t e w h e t h e r the f r a c t u r e s u r f a c e s of h y d r o g e n induced d e l a y e d fracture of o r d e r e d alloy Fe~AI , in w h i c h c l e a v a g e f r a c t u r e (CF) occurs at room d e p e n d e n t u p o n the KI and the h y d r o g e n c o n t e n t introduced.
Experimental The a l l o y iron
(S
=
was p r e p a r e d
0.0017,
P
=
by v a c u u m
0.0026,
C
=
are
Procedure
induction melting
0,01,
temperature,
Si
<
0.005,
using
all
in
commercially wt%)
and
pure
aluminium
(99%). The ingot was h o m o g e n i z e d at 1050 °C for 12 h. The alloy Fe3A1 had f o l l o w i n g c o m p o s i t i o n (in wt%): A1 = 14.86, C = 0.01, P < 0.005, S = 0.03, O = 0.001, H = 0.06 ppm, b a l a n c e Fe. The ingot was hot formed to 1 6 x 4 0 x 2 0 0 RR at 800 - 900 aC and then g i v e n a d u p l e x heat t r e a t m e n t for 1 h at 850 °C for r e c r y s t a l l i z a t i o n and 5 days at 500 °C for DO: ordering. The m o d i f i e d w e d g i n g o p e n i n g loading (WOL) s p e c i m e n s had a t h i c k n e s s of I0 mR, a w i d t h of 26 mR and a h e i g h t of 32 mm. The n o t c h was made by e l e c t r i c spark c u t t i n g using a Mo wire, and the radius of notch
1295 0 9 5 6 - 7 1 6 X / 9 2 $5.00 + .00 C o p y r i g h t (c) 1992 P e r g a m o n Press
Ltd.
1296
FRACTURE
IN Fe3AI
Vol.
27, No.
root was 0.06 mm. A f t e r loading with a screw to c r a c k i n g the specimens were added to the solution. The m o d i f i e d WOL specimens were d y n a m i c a l l y c h a r g e d at various current d e n s i t i e s of 0.2, 2 and 10 m A / c ~ using a s o l u t i o n of 1 N NaOH. The fracture surfaces were e x a m i n e d by scanning e l e c t r o n microscopy.
Results
and D i s c u s s i o n
The test s p e c i m e n s were cracked by stress c o r r o s i o n in water. The K I d e c r e a s e s with the c r a c k growth and KI$cc is the ~ at the points where stress c o r r o s i o n crack (SCC) g r o w t h stopped. It has been c o n f i r m e d that the m e c h a n i s m of SCC in water is due to h y d r o g e n induced c r a c k i n g in ordered alloy Fe3AI (5). The fracture surfaces of the SCC in water of o r d e r e d alloy Fe3AI are shown in Fig. 1. C l e a v a g e fracture (CF) o c c u r r e d and the fracture surfaces were i n d e p e n d e n t upon K x at the crack tip. The specimens still e x h i b i t e d t r a n s g r a n u l a r CF, indepe n d e n t upon K I , a l t h o u g h the a d d i t i o n of a h y d r o g e n r e c o m b i n a t i o n the s o l u t i o n (0.05 g per liter CS2) d e c r e a s e s the Kisc¢ (5).
w h i c h was p o i s o n in
W h e n the loading s p e c i m e n s were d y n a m i c a l l y charged, a change in the fracture modes with K[ took place, as shown in Fig. 2. When a current d e n s i t y of c hargi n g (i) was lower (i = 0.2 mA/cm2), the CF p r o c e e d e d at high ~ (K I = 34.5 MPamln), as shown in Fig. 2a. The fracture modes were about 90% cleavage and 10% i n t e r g r a n u l a r and only the individual facets of grain b o u n d a r i e s were cracked (Fig, 2b) at i n t e r m e d i a t e K z (K x = 24.7 MPamln). When the K I was close to the Kz~ (K I = 16.5 MPamln), the p e r c e n t a g e of i n t e r g r a n u l a r fracture (IGF) s u f f i c i e n t l y increased and all facets of grain b o u n d a r i e s were c r a c k e d (Fig. 2c). Some d e f o r m a t i o n traces can be seen along the g r a i n boundaries, w h i c h shows that d i s l o c a t i o n m o t i o n o c c u r r e d during the grain b o u n d a r y c r a c k i n g (Fig. 2d). The p e r c e n t a g e of IGF increased as i rose to 2 m A / c m 2 , as shown in Fig. 3. The CF formed at higher K I (Fig. 3a). About 50% IGF formed at i n t e r m e d i a t e ~ (K l = 24.7 MPamln), as shown in Fig. 3b. About 80% IGF formed at low K I (K I = 16.5 MPamln), as shown in Fig. 3c. The p e r c e n t a g e of IGF s u f f i c i e n t l y increased as the i rose to 10 m A / c ~ , as shown in Fig. 4. The CF o c c u r r e d at K I of 34.5 MPam In (Fig. 4a) and about 90% IGF o c c u r r e d at K~ of 24.7 MPam In (Fig. 4b) and about 100% IGF o c c u r r e d at K x of 16.5 MPam In (Fig. 4c). The change of the fracture s u r f a c e s with the ~ and current d e n s i t y m e n t i o n e d above is s u m m a r i z e d in Table I. It is c o n c l u d e d that increasing the current d e n s i t y and d e c r e a s i n g K! f a c i l i t a t e the o c c u r r e n c e of IGF. The h y d r o g e n e n t e r e d Fe3AI from the crack tip to matrix, causing IGF. The CF o c c u r r e d when the p o i s o n was added to water, w h i c h indicates that the IGF did not p r e f e r e n t i a l l y occur b e c a u s e the h y d r o g e n content was not enough to form a large d i f f e r e n c e b e t w e e n the g r a i n b o u n d a r y and g r a i n interior. But this still means that some g r a i n b o u n d a r y facets with a large Schmid factor c r a c k e d by cleavage. There was no d i f f e r e n c e in the fracture surfaces , i.e., only the cleavage fracture was observed. As the h y d r o g e n content rises to a critical value of Cm0, sufficient s e g r e g a t i o n occurs, so that a d i f f e r e n c e in the h y d r o g e n content b e t w e e n grain b o u n d a r y and g r a i n i n t e r i o r also forms, w h i c h p r o d u c e s the c r a c k i n g of the grain
i0
Vol.
27, No.
i0
Table
FRACTURE
Effect
1
of the
IN Fe3AI
1297
i and the K I on the fracture K, (MPam In
Condition
surfaces
)
34.5
24.7
16.5
H20
CF
CF
CF
H20 + CS 2
CF
CF
CF
IN NaOH + 0.2 m A / c ~
CF
CF + 10% IGF
CF + 40% IGF
IN NaOH + 2 mAlc~
CF
CF + 50% IGF
CF + 80% IGF
CF + 90% IGF
IGF
IN
NaOH
+
10 m A / c ~
CF - cleavage
boundaries
I
CF fracture
with
a lower
IGF
Schmid
-
intergranular
fracture
factor.
K, value has an effect on the h y d r o g e n distribution. With increasing K I the d i s l o c a t i o n density at a crack tip increases. The increase in the d i s l o c a t i o n d e n s i t y increases the c a p a c i t y of the grain interior to trap h y d r o g e n (8), so the d i f f e r e n c e in the h y d r o g e n content between grain b o u n d a r y and grain interior is small, which can facilitate the o c c u r r e n c e of t r a n s g r a n u l a r cracking. With d e c r e a s i n g K! the d i s l o c a t i o n d e n s i t y at a crack tip decreases, so that the h y d r o g e n trapsdecrease and h y d r o g e n p r e d o m i n a n t l y segregates to grain boundaries which in turn can facilitate the o c c u r r e n c e of IGF.
Conclusion The
fracture
displayed
cleavage,
surfaces
of
which was
D e c r e a s i n g the K I and the o c c u r r e n c e of IGF.
the
hydrogen
independent
increasing
induced
delayed
cracking
in
water
upon K,
the current
density
of charging
facilitate
References
[i] [2] [3] [4] [5]
C.T. Liu, C.G. M e K a m e y and E.H. Lee, Scripta Metall., 24, 285 (1990). C.T. Liu, E.H. Lee and C.G. McKamey, Scripta Metall., 23, 875 (1989). C.T. Liu and E.P. George, Scripta Metall., 24 (1990). A. C a s t a g n a and N.S. Stoloff, S c r i p t a Metall., 26, 673 (1992). D.Z. Zhang, D.L. She, G.W. Du, F.W. Zhu and C.M. Hsiao, Scripta Metall., press.
in
1298
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FRACTURE
IN Fe3AI
Vol.
W.Y. Chu, C.M. Hsiao and W.X. Li, Metall. Trans., 15A, 2007 (1984}. W.Y. Chu, C.M. Hsiao and S.Q. Li, Corrosion, 37, 320 (1981). J. Kameda, Res Mechanica, 26, 215 (1989).
A
Fig.
Fig. 2
B
1
Fracture
surfaces of SCC in water of ordered alloy F % A I
A
8
C
D
Change of the fracture surfaces during the dynamic charging at low current d e n s i t y (i = 0.2 mA/cm z) in FesAI with the K I (a) K! = 34.5 MPam In (b) K! = 24.7 MPam In (c) and (d) Z I = 16.5 MPam I/z
27, No.
i0
Vol.
27, No.
i0
FRACTURE
IN Fe3AI
A
1299
S
C
Fig.
3
Change of the fracture surfaces during the dynamic charging at intermediate current density ( i = 2 m A / c ~ ) i n Fe3A1 w i t h t h e ( a ) ~ = 3 4 . 5 MPa~/2 (b} KI = 2 4 . 7 MPam~/2 ( c ) ~ = 1 6 . 5 MPam1/~
A
S
C
Fig.
4
Change
of
the
fracture
surfaces
during
the
dynamic
charging
high current density (i = i0 m A / c ~ } in Fe3AI with the ( a ) ~ = 34.5 MPa~/z ( b ) ~ = 24.7 MPa~/z ( c ) ~ = 16.5 MPamt/2
at