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Reversible embrittlement in zircaloy-2
Corrosion Science, Vol. 19, pp. 141 to 145 Pergamon Press. 1979. Printed in Great Britain
SHORT
REVERSIBLE
COMMUNICATION
EMBRITTLEMENT
IN ZIRCALOY-2*
P. MAJUMDAR and J. C. SCULLY D e p a r t m e n t of Metallurgy, University of Leeds, Leeds LS2 9JT, E n g l a n d
STRESS corrosion cracking in zircaloy-2 at room temperature has been reported by several authors 1-s who have observed cracking in Hg, CHsOH/Iz, CHaOH/HCI and aqueous NaCI. In the course of an extensive study of stress corrosion cracking in the zircaloy-2/CHgOH/HCl system a reversible embrittlement attributable to absorbed hydrogen has been observed and it is the purpose of this Short Communication to describe this phenomenon which has not been previously reported. EXPERIMENTAL
METHOD
Tensile specimens of zircaloy-2 (1.5Sn, 0.14Fe, 0.1Cr, 0.05Ni, balance Zr) 5ram wide, l.Smm thick and with a gauge length of 25mm were annealed in a vacuum furnace at 800°C for I h after which they exhibited a 0.1% Proof Stress of 385 N/mm z, a U.T.S. of 450 N/ram 2 and an elongation-to-failure, eI, of c a . 32 %. Subsequently they were tested in two ways. In one type of experiment specimens were strained on an Instron Tensile Testing Machine at a crosshead speed of 16.6 ~m/s to a stress of 385 N/ram 2. Straining was stopped. A solution of CHgOH containing 0.4 vol. % of concentrated HCI (36.5 vol. %) was then added to the beaker that had been fixed around each specimen. Specimens were polarized anodically for 1200s at a current density of l0 v.A/mm ~. The solution was then removed from the beaker. Specimens were then broken either immediately or removed from the testing machine and replaced and broken after a time interval at room temperature or above, which is referred to below as the ageing time. Since the load fell slightly during polarization these are referred to as constant strain experiments. In the second type of experiment specimens were strained at several crosshead speeds within the range 0.32-83 v.m/s while subjected to one of two anodic current densities (10 and 100 v.A/mmD up to a stress of 400 N/ram 2. The solution was then removed and, as in the first type of experiment, specimens were broken at the same crosshead speed as that used in the first part of the test either immediately or after a time interval during which they were not under external stress. In these tests the total time of polarization depended upon the applied crosshead speed. Typical times were 300s with a crosshead speed of 83~ m/s and 2400s with a crosshead speed of 16 ~tm/s, both at the lower current density. These are referred to as constant crosshead speed experiments. * M a n u s c r i p t received 1 July 1978. 141
142
P. MAJUMDARand J. C. SCULLY
E X P E R I M E N T A L RESULTS
Constant straht experiments Specimens broken immediately after polarization exhibited a 3 zone fracture consisting of intergranular fracture on the outer surface, cleavage and fluting adjacent to it on the inside, and a ductile overload fracture in the centre. An example is shown in Fig. 1. After ageing, the intermediate cleavage and fluting fracture was less evident and after long ageing times at room temperature or shorter times at higher temperatures it did not occur at all. This change was accompanied by an increase in elongationto-failure, eI. All the results are shown in Fig. 2. A 2 zone fracture consisting of an intergranular and ductile overload fracture obtained after ageing for lh at 100°C is shown in Fig. 3.
Constant crosshead speed experhnents Specimens broken immediately after the end of polarization exhibited a 3 zone fracture similar to that described in Fig. 1. Upon ageing, the subsequent eI of specimens increased and the intermediate cleavage and fluting zone was reduced in extent and eventually it was not seen. The changes in elwith ageing time are shown in Fig. 4. It can be seen that the degree of embrittlement was greater in experiments at lower crosshead speeds, a result that emphasized that the embrittlement is a time-dependent phenomenon. DISCUSSION
The intergranular fracture zone is thought to arise from dissolution 1-3 and, consistent with this, it was not affected by the ageing treatments. The cleavage and fluting
35~
3o
f
IO0°C
25 Room •7-
temperature
20
15
IO"-
o
I
I
I
I
I
I
2O
4O
60
80
I00
120
Ageing
time,
h
FIG. 2. The elongation-to-failure, el, of specimens broken after 1200s at a current density of 10 v.A/mm= while under an initial stress of 385 N/mm = followed by ageing in
the unstressed condition for various ageing times and two temperatures in air.
,q
--p
3 FIG. I. A scanning electron microscope fractograph of a zircaloy-2 specimen broken after zero ageing time in the first series of experiments. Between the intergranular and ductile overload fracture a small area of cleavage is visible. (1000 x .) FtG. 3. A scanning electron microscope fractograph of a zircaloy-2 specimen broken after an ageing time of 1 h at 100°C in air irt the first series of experiments. (I000 × .)
Reversible embrittlement in zircaloy-2
145
C 25
-
C
A
20
A om 15
I I
Crosshead
i I0
speed,
I IO 0
p.mls
FIG. 4. The elongation-to-failure, el, of zircaloy-2 specimens strained to fracture as a function of crosshead speed. Specimens were polarized anodically at current densities of 10 and 100 izA/mm2 until the stress reached 400 N/ruraL At that poin~ (i) polarization was stopped, the solution removed and the crosshead movement was continued until fracture, a procedure referred to as zero ageing, or (ii) specimens were removed and aged for 24 h at room temperature in air, after which they were placed and broken in air at the same value of crosshead speed as in the first part of the experiment, or (iii) specimens were aged only for lh at 100°C but otherwise treated as in (ii). A: procedure (i), B: procedure (i i), C: procedure (iii), ~r: 10 ~A/mm 2, @: 100 v.A/mmL r e g i o n , h o w e v e r , m u s t be c a u s e d by a n a b s o r b e d species since its a p p e a r a n c e c o u l d be p r e v e n t e d by ageing, a result t h a t is similar to t h a t o b s e r v e d in 0¢-Ti alloys. 4 It is c o n c l u d e d t h a t a b s o r b e d h y d r o g e n is r e s p o n s i b l e f o r p r o m o t i n g t h e c l e a v a g e p a r t o f the f r a c t u r e . W h e t h e r this is p r e s e n t in solid s o l u t i o n o r at least p a r t l y in t h e hydrida f o r m has n o t yet b e e n e x a m i n e d .
I. 2. 3. 4.
REFERENCES K. MORI, A. TAKAMURAand T. SmMOSE, Corrosion 22, 29 0966). B. Cox, Corrosion 28, 207 (1972). K. ELAYAPERUMAL,P. K. DE and J. BALACHANDRA,Corros. Sci. 11, 579 (1971). J. C. SCULLYand T. A. ADEPOJU, Corros. Sci. 17, 789 (1977).
SHORT
REVERSIBLE
COMMUNICATION
EMBRITTLEMENT
IN ZIRCALOY-2*
P. MAJUMDAR and J. C. SCULLY D e p a r t m e n t of Metallurgy, University of Leeds, Leeds LS2 9JT, E n g l a n d
STRESS corrosion cracking in zircaloy-2 at room temperature has been reported by several authors 1-s who have observed cracking in Hg, CHsOH/Iz, CHaOH/HCI and aqueous NaCI. In the course of an extensive study of stress corrosion cracking in the zircaloy-2/CHgOH/HCl system a reversible embrittlement attributable to absorbed hydrogen has been observed and it is the purpose of this Short Communication to describe this phenomenon which has not been previously reported. EXPERIMENTAL
METHOD
Tensile specimens of zircaloy-2 (1.5Sn, 0.14Fe, 0.1Cr, 0.05Ni, balance Zr) 5ram wide, l.Smm thick and with a gauge length of 25mm were annealed in a vacuum furnace at 800°C for I h after which they exhibited a 0.1% Proof Stress of 385 N/mm z, a U.T.S. of 450 N/ram 2 and an elongation-to-failure, eI, of c a . 32 %. Subsequently they were tested in two ways. In one type of experiment specimens were strained on an Instron Tensile Testing Machine at a crosshead speed of 16.6 ~m/s to a stress of 385 N/ram 2. Straining was stopped. A solution of CHgOH containing 0.4 vol. % of concentrated HCI (36.5 vol. %) was then added to the beaker that had been fixed around each specimen. Specimens were polarized anodically for 1200s at a current density of l0 v.A/mm ~. The solution was then removed from the beaker. Specimens were then broken either immediately or removed from the testing machine and replaced and broken after a time interval at room temperature or above, which is referred to below as the ageing time. Since the load fell slightly during polarization these are referred to as constant strain experiments. In the second type of experiment specimens were strained at several crosshead speeds within the range 0.32-83 v.m/s while subjected to one of two anodic current densities (10 and 100 v.A/mmD up to a stress of 400 N/ram 2. The solution was then removed and, as in the first type of experiment, specimens were broken at the same crosshead speed as that used in the first part of the test either immediately or after a time interval during which they were not under external stress. In these tests the total time of polarization depended upon the applied crosshead speed. Typical times were 300s with a crosshead speed of 83~ m/s and 2400s with a crosshead speed of 16 ~tm/s, both at the lower current density. These are referred to as constant crosshead speed experiments. * M a n u s c r i p t received 1 July 1978. 141
142
P. MAJUMDARand J. C. SCULLY
E X P E R I M E N T A L RESULTS
Constant straht experiments Specimens broken immediately after polarization exhibited a 3 zone fracture consisting of intergranular fracture on the outer surface, cleavage and fluting adjacent to it on the inside, and a ductile overload fracture in the centre. An example is shown in Fig. 1. After ageing, the intermediate cleavage and fluting fracture was less evident and after long ageing times at room temperature or shorter times at higher temperatures it did not occur at all. This change was accompanied by an increase in elongationto-failure, eI. All the results are shown in Fig. 2. A 2 zone fracture consisting of an intergranular and ductile overload fracture obtained after ageing for lh at 100°C is shown in Fig. 3.
Constant crosshead speed experhnents Specimens broken immediately after the end of polarization exhibited a 3 zone fracture similar to that described in Fig. 1. Upon ageing, the subsequent eI of specimens increased and the intermediate cleavage and fluting zone was reduced in extent and eventually it was not seen. The changes in elwith ageing time are shown in Fig. 4. It can be seen that the degree of embrittlement was greater in experiments at lower crosshead speeds, a result that emphasized that the embrittlement is a time-dependent phenomenon. DISCUSSION
The intergranular fracture zone is thought to arise from dissolution 1-3 and, consistent with this, it was not affected by the ageing treatments. The cleavage and fluting
35~
3o
f
IO0°C
25 Room •7-
temperature
20
15
IO"-
o
I
I
I
I
I
I
2O
4O
60
80
I00
120
Ageing
time,
h
FIG. 2. The elongation-to-failure, el, of specimens broken after 1200s at a current density of 10 v.A/mm= while under an initial stress of 385 N/mm = followed by ageing in
the unstressed condition for various ageing times and two temperatures in air.
,q
--p
3 FIG. I. A scanning electron microscope fractograph of a zircaloy-2 specimen broken after zero ageing time in the first series of experiments. Between the intergranular and ductile overload fracture a small area of cleavage is visible. (1000 x .) FtG. 3. A scanning electron microscope fractograph of a zircaloy-2 specimen broken after an ageing time of 1 h at 100°C in air irt the first series of experiments. (I000 × .)
Reversible embrittlement in zircaloy-2
145
C 25
-
C
A
20
A om 15
I I
Crosshead
i I0
speed,
I IO 0
p.mls
FIG. 4. The elongation-to-failure, el, of zircaloy-2 specimens strained to fracture as a function of crosshead speed. Specimens were polarized anodically at current densities of 10 and 100 izA/mm2 until the stress reached 400 N/ruraL At that poin~ (i) polarization was stopped, the solution removed and the crosshead movement was continued until fracture, a procedure referred to as zero ageing, or (ii) specimens were removed and aged for 24 h at room temperature in air, after which they were placed and broken in air at the same value of crosshead speed as in the first part of the experiment, or (iii) specimens were aged only for lh at 100°C but otherwise treated as in (ii). A: procedure (i), B: procedure (i i), C: procedure (iii), ~r: 10 ~A/mm 2, @: 100 v.A/mmL r e g i o n , h o w e v e r , m u s t be c a u s e d by a n a b s o r b e d species since its a p p e a r a n c e c o u l d be p r e v e n t e d by ageing, a result t h a t is similar to t h a t o b s e r v e d in 0¢-Ti alloys. 4 It is c o n c l u d e d t h a t a b s o r b e d h y d r o g e n is r e s p o n s i b l e f o r p r o m o t i n g t h e c l e a v a g e p a r t o f the f r a c t u r e . W h e t h e r this is p r e s e n t in solid s o l u t i o n o r at least p a r t l y in t h e hydrida f o r m has n o t yet b e e n e x a m i n e d .
I. 2. 3. 4.
REFERENCES K. MORI, A. TAKAMURAand T. SmMOSE, Corrosion 22, 29 0966). B. Cox, Corrosion 28, 207 (1972). K. ELAYAPERUMAL,P. K. DE and J. BALACHANDRA,Corros. Sci. 11, 579 (1971). J. C. SCULLYand T. A. ADEPOJU, Corros. Sci. 17, 789 (1977).