Plastic behaviour of FeC dilute alloy single crystals electron-irradiated at 77K

Scripta METALLURGICA et MATERIALIA

PLASTIC

BEHAVIOUR

Vol. 25, pp. 9S9-964, 1991 Printed in the U.S.A.

OF Fe-C DILUTE ALLOY

SINGLE

CRYSTALS

Pergamon Press plc All rights reserved

ELECTRON-IRRADIATED

AT 77K

K. Makii*, Y. Aono** and E. Kuramoto*** * I n t e r d i s c i p l i n a r y G r a d u a t e School of Engineering Sciences, Kyushu University, Kasuga 816, Japan **Hitachi R e s e a r c h Laboratory, Hitachi Ltd., H i t a c h i 319-12, Japan ***Research Institute for A p p l i e d Mechanics, Kyushu University, Kasuga 816, Japan

(Received (Revised

January February

i.

29, 21,

1991) 1991)

Introduction

Many investigations of the irradiation effect on the mechanical properties of steels have been performed and it is recognized that the interaction between a dislocation and radiation-induced defects under stress is the most fundamental problem to be studied for understanding macroscopic effects (1,2). The experimental studies performed for high-purity Fe single crystals have revealed that low temperature irradiation caused softening, i.e., decrease of the yield stress, which is characteristic behaviour in bcc metals (3 - 6). This may be due to the enhancement of kink pair formation (KPF) on a screw dislocation line by r a d i a t i o n - i n d u c e d Frenkel pairs, especially, selfinterstitial atoms (SIAs) which have much larger strain fields around them than vacancies and are stable below near i00 K because of their high m i g r a t i o n energy. Since Sato and Meshii (4,7) first discovered this phenomenon and discussed the mechanism, many discussions and computer simulations have also been performed for this process (7 - I0). In the present study, it has been investigated using Fe-C dilute alloy single crystals (5 48appmC) how such a small amount of carbon atoms influences the irradiation softening behaviour observed in high purity Fe single crystals. There is great interest on the interaction between screw d i s l o c a t i o n s and co-existing but two different interstitials, i.e., self interstitial atoms (SIAs) as a structural defect and carbon atoms as a chemical defect. It is well known that an interstitial impurity such as carbon or nitrogen itself strongly changes the plastic behaviour of bcc metals as well as SIAs. Moreover, some complexes of impurity atoms and SIAs are expected to be formed and affect the plasticity. 2. Experimental High-purity iron rods were prepared by floating zone refining in an induction heating furnace filled with flowing hydrogen gas which was forced to go through a heated p a l l a d i u m thin film for purification. As a startng material, pure iron ATOMIRON 5N (Showa Denko Co.) was used. These purified iron rods were drawn into thin wires and then single c r y s t a l l i z a t i o n was performed by a so-called strain a n n e a l i n g method. Purity was g u a r a n t e e d by such a high value of RRR H as 6000 (RRR H denotes residual r e s i s t i v i t y ratio in a longitudinal m a g n e t i c field). Carbon doping into specimens was m a d e in an a t m o s p h e r e of mixed gas of hydrogen and methane at 1023 K for one hour. Carbon content was controlled by changing the ratio of these two gases. Specimens were then q u e n c h e d into water to keep doped carbon atoms in solution. The concentration of doped carbon was determined by the electrical resistivity measurement. Specimens of five different carbon contents, i.e., 5, 12, 18, 33 and 48 appm were obtained. The size of specimens is 0.25 mm @ x 20 mm and the gauge length is I0 mm. The axial o r i e n t a t i o n of the specimen ~s X = O°The electron irradiation was p e r f o r m e d by using KURRI-LINAC (28 MeV, 6 x I0 e/cm , 77 K). The c o n c e n t r a t i o n of induced F r e n k e l - p a i r s was 120 ppm in maximum. The tensile test was performe~ in _~he wide temperature range between 4.2 K and ~ 3 K. The nominal strain rate was 1.7 x i0 sec In addition to the yield stress measurement, activation parameters such as strain rate sensitivity and activation area were o b t a i n e d from the so-called stress r e l a x a t i o n test.

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3. Results 3.1

Unirradiated

Fe-C

dilute

alloy

single

crystals

Figure. 1 shows the t e m p e r a t u r e d e p e n d e n c e of y i e l d stress of u n i r r a d i a t e d Fe-C dilute alloy single crystals (5, 12, 18, 33 and 48 appmC). The dashed lines denote that of pure Fe specimens. It is seen that carbon doping causes two m a j o r changes to the result for pure Fe. One is a shift of the t e m p e r a t u r e where a hump (inflection point on the y i e l d stress - temperature curve) appears. In pure Fe the hump is seen around 220 K, but it m o v e s to a lower t e m p e r a t u r e side with carbon doping, and is observed near 150 K in Fe-48appmC alloy specimens. The other is stronger temperature dependence of y i e l d stress w h i c h is o b s e r v e d at low t e m p e r a t u r e s in the higher carbon content alloys. As a result, s o l u t i o n h a r d e n i n g appears in the low temperature region, and solution s o f t e n i n g is o b s e r v e d around 220 K. Figure. 2 shows the carbon c o n c e n t r a t i o n d e p e n d e n c e of yield stress of u n i r r a d i a t e d Fe-C alloy single crystals, where data for the F e - 1 5 O a p p m C alloy are r e f e r r e d from the previous paper (Ii). It is seen that the y i e l d stress is very sensitive to carbon content at the very low carbon content region. C o r r e s p o n d i n g to Fig. I remarkable solution h a r d e n i n g and solution softening are observed in the lower and the higher temperature region, r e s p e c t i v e l y . 3.2

Electron-irradiated

Fe-C dilute

alloy

single

crystals

Figure. 3 shows the c a r b o n c o n c e n t r a t i o n d e p e n d e n c e of yield stress of the electron-irradiated and unirradiated Fe-C alloy single crystals m e a s u r e d at 7? K. It is clearly seen that the electron-irradiation induced the significant r e d u c t i o n of the yield stress, i.e., irradiation softening. The amount of this softening increases with r a d l a t l o n - i n d u c e d Frenkel pairs, at least until 120 ppm, but rapidly decreases with i n c r e a s i n g doped carbon content. Figures. 4a) and 4b) show the stress - strain curves o b t a i n e d at 77 K of irradiated (and unirradiated for the sake of comparison) Fe-SappmC and Fe-48appmC alloy single crystals, respectively, where changes of the activation a r e a w i t h i n c r e a s i n g shear strain are also plotted. The stress - strain curve of the irradiated Fe-5appmC is similar to that of the irradiated high purity Fe which was reported in the previous paper (5,6). But the shear strain where the irradiation softening disappears is different, namely, about 20~ in the Fe-5appmC alloy and more than 205 in high purity Fe. The same tendency is o b s e r v e d for the strain where the activation area comes back to the value of the u n i r r a d i a t e d one. A quite different feature was observed in the Fe-48appmC alloy, namely, transition from s o f t e n i n g to h a r d e n i n g appeared just after the yielding ended and the a c t i v a t i o n area began to increase at about i0~ shear strain. Figure. 5 shows the temperature d e p e n d e n c e of y i e l d stress of irradiated Fe-C alloy single crystals (5, 12 and 48 appmC). In the case of the F e - 5 a p p m C alloy the t e m p e r a t u r e dependence is very close to that of the irradiated high purity Fe r e p o r t e d in the previous paper (5,6,9). The t e m p e r a t u r e d e p e n d e n c e curve could be divided into two regions, namely higher and lower temperature regions, at 20 K similar to high purity re. No s o f t e n i n g was o b s e r v e d and the slope of the dependence curve was not so steep in the lower t e m p e r a t u r e region. On the contrary, in the higher temperature region, significant softening was o b s e r v e d and the slope was much steeper. This transition t e m p e r a t u r e seems to shift to a higher t e m p e r a t u r e side as increasing carbon content. 4. D i s c u s s i o n It has been well known that the plastic d e f o r m a t i o n of bcc metals is g o v e r n e d by motion of a screw dislocation o v e r c o m i n g a Peierls potential. This m o t i o n consists of two important parts, namely, kink pair f o r m a t i o n (KPF) and kink m i g r a t i o n (KM). In a lower temperature region, e.g., regimes II (140K < T < 250K) and III ( T < 140K) for pure iron, KPF is the most important rate c o n t r o l l i n g process, but in a higher t e m p e r a t u r e region, e.g., regime I ( T > 250K) KM mainly controls the d e f o r m a t i o n process. When point defects exist in the matrix, the i n t e r a c t i o n between a m o v i n g screw dislocation and point defects causes some change in the d e f o r m a t i o n m e c h a n i s m . This i n t e r a c t i o n can be divided into two parts, namely, d i s l o c a t i o n core interaction (direct i n t e r a c t i o n caused by point defects which exist in the core) and elastic interaction (long range i n t e r a c t i o n caused by point defects which exist outside the core). KFF is usually c o n s i d e r e d to be e n h a n c e d by the core interaction.

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Especially, in the case of s e l f - i n t e r s t i t i a l atom (SIA), we p r e s e n t e d the d e t a i l e d m e c h a n i s m of kink formation by a b s o r b i n g SIA into the core on the basis of the c o m p u t e r s i m u l a t i o n (8-I0). On the other hand, KM is u s u a l l y s u p p r e s s e d by the i n t e r a c t i o n with point defects, which, however, depends on the thermal s t a b i l i t y of point defects. For example, in a high t e m p e r a t u r e r e g i o n point defects can easily reorientate and m i g r a t e ; then they are no l o n g e r rigid pinning points. We try to u n d e r s t a n d e x p e r i m e n t a l results) rather, based on the core i n t e r a c t i o n taking this thermal stability of point d e f e c t s (SIA and carbon atom in this case) into account. 4.1Unirradiated

Fe-C dilute alloy

single

crystals

H a r d e n i n g and s o f t e n i n g due to carbon d o p i n g o b s e r v e d in Figs. I and 2 must be i n t e r p r e t e d from the v i e w p o i n t of the i n t e r a c t i o n b e t w e e n a screw d i s l o c a t i o n and point d e f e c t s as m e n t i o n e d above. Since the data in these figures c o r r e s p o n d to t e m p e r a t u r e r e g i m e s II and III, KPF is a main rate c o n t r o l l i n g process. Doped carbon atoms are c o n s i d e r e d to e n h a n c e KPF, but the effect on KM is not So simple. Since c a r b o n atoms have quite large strain fields around them and have high thermal stability, namely, high migration energy 0 . 9 eV, KM is s u p p r e s s e d by carbon atoms in a lower temperature region, which r e s u l t s in h a r d e n i n g . On the o t h e r hand, if in a higher temperature region carbon atoms b e c o m e able to r e o r i e n t a t e or m i g r a t e under an applied stress and the stress field of a d i s l o c a t i o n itself, then they may not h i n d e r the kink m i g r a t i o n . In this case softening is e x p e c t e d to appear, which can explain the r e s u l t s in the higher t e m p e r a t u r e side in Figs. I and 2. C o e x i s t e n c e of h a r d e n i n g and s o f t e n i n g on the stress - strain curve results in the shift of the hump to the lower t e m p e r a t u r e side as d e s c r i b e d in s e c t i o n 3. In the case of s u b s t i t u t i o n a l alloys of iron, e.g., Fe-Ni alloy, this shift of the hump was o b s e r v e d by Aono et al (12) and was understood by the shape change of the P e i e r l s p o t e n t i a l itself due to the existence of substitutional alloying elements (13). But, in the present case of dilute carbon alloys) this interpretation cannot be a p p l i e d b e c a u s e c a r b o n atoms are at the i n t e r s t i t i a l sites and the carbon c o n c e n t r a t i o n is too small to change the o v e r a l l P e i e r l s potential. Hence the present shift of the hump must be a result of the i n t e r a c t i o n b e t w e e n a screw d i s l o c a t i o n and carbon atoms mentioned above. 4.2 E l e c t r o n - l r r a d i a t e d

Fe-C d i l u t e

alloy

single crystals

E l e c t r o n i r r a d i a t i o n induced s e l f - i n t e r s t i t i a l atoms (SIA) and v a c a n c i e s into the specimens. Since v a c a n c i e s have m u c h s m a l l e r strain fields around them than SIAm, they can be ignored. Then, motion of d i s l o c a t i o n s in the i r r a d i a t e d s p e c i m e n s is m a i n l y a f f e c t e d by isolated SIAs and carbon atoms in the matrix. SIA can enhance kink pair f o r m a t i o n (KPF) through its large strain field and d e c o m p o s i t i o n into three kinks in a screw d i s l o c a t i o n core d e s c r i b e d in the p r e v i o u s paper (8-iO). Moreover, SIA does not prevent kinks from m i g r a t i n g freely, since it has low thermal stability; namely, m i g r a t i o n energy 0 . 3 eV is too low to make SIA a fixed pinning point. This can explain the large irradiation s o f t e n i n g o b s e r v e d in pure Fe in Fig. 3. On the other hand, a carbon atom is c o n s i d e r e d to be a m u c h stronger p i n n i n g point and to h i n d e r kink m i g r a t i o n (KM). This can explain the tendency for rapid d i s a p p e a r a n c e of i r r a d i a t i o n s o f t e n i n g with i n c r e a s i n g carbon content as shown in Fig. 3. Namely, it can be r e c o g n i z e d that the rate c o n t r o l l i n g process is changed from KPF to ~M. A n o t h e r factor we must c o n s i d e r is the n u m b e r ratio b e t w e e n SIAs and carbon atoms {SIA/C); in other words, the n u m b e r ratio b e t w e e n kinks and c a r b o n atoms, because SIAs form kinks on a screw d i s l o c a t i o n llne when they are a b s o r b e d into the core as m e n t i o n e d above. When this ratio is large, which c o r r e s p o n d s to a low carbon content region, a c c u m u l a t e d kinks , namely, a super kink interacts with a c a r b o n atom under an a p p l i e d stress. This means that the stress required to o v e r c o m e the carbon b a r r i e r can be r e d u c e d c o m p a r e d with the case of an isolated kink, and then the i r r a d i a t i o n s o f t e n i n g is not so s u p p r e s s e d in this region. But, in the case of a low ratio, namely, in the high c a r b o n content region, an isolated kink must interact with a carbon atom, which results in a s i g n i f i c a n t r e d u c t i o n of the i r r a d i a t i o n softening. R e c o v e r y of i r r a d i a t i o n s o f t e n i n g with i n c r e a s i n g s t r a i n o b s e r v e d in Figs. 4a) and 4b) can be interpreted as the result of strain induced r e c o v e r y of SIAs, i.e., reduction of SIAs through absorption into screw d i s l o c a t i o n cores under s t r e s s or stress induced r e c o m b i n a t i o n of SIAs and vacancies o c c u r r i n g near d i s l o c a t i o n lines. A b s o r b e d SIAs are thought to convert three kinks and move away along d i s l o c a t i o n lines to the ends of the s e g m e n t s . In the case of the F e - S a p p m C alloy specimen the s i t u a t i o n is s i m i l a r to that in the high p u r i t y Fe specimen, but recovery proceeds slightly faster because of s u p p r e s s i o n of kink m i g r a t i o n due to carbon atoms. In the case of the

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Fe-48appmC alloy specimen i r r a d i a t i o n s o f t e n i n g rapidly d i s a p p e a r e d and hardening appeared. This is not so easy to understand, but there may be two ways of explaining. One is a rather heterogeneous d e f o r m a t i o n mode w h i c h forms a c o m p l i c a t e d d i s l o c a t i o n structure, namely, high work hardening rate. The other is generation of a more resistant barrier such as a complex of SIA and carbon atom (I-C complex), which might be formed through the stress induced m i g r a t i o n of SIA near d i s l o c a t i o n lines. Thermal stability of the I-C complex is c o n s i d e r e d to be almost the same as that of d i - i n t e r s t i t i a l atom I . A~ for the temperature d e p e n d e n c e of the yield stress of the irradiated specimens shown in Fig. 5, a very important feature must be recognized. Namely, the t e m p e r a t u r e dependence can be divided into two regions; in the lower t e m p e r a t u r e region no s o f t e n i n g is observed, but in the higher temperature region softening is observed. In the previous paper (6) we described that in the irradiated high purity Fe specimens stress induced recovery took place at low temperatures except below 20K. This is also the t e m p e r a t u r e where the above two regions are divided. A similar situation is present in the Fe-5appmC alloy as shown in Fig. 5. Slight hardening is seen below 20 K and softening appears above it. This t r a n s i t i o n t e m p e r a t u r e is higher in the irradiated Fe-48appmC alloy specimen as shown in Fig. 5- There are two p o s s i b i l i t i e s to explain this shift. One is that suppression of irradiation softening due to carbon atoms brings the softening region to a higher temperature side. The other is the formation of I-C complexes which can hinder kink migration, again resulting in the increase of the t r a n s i t i o n temperature. 5- Summary i) T e n s i l e tests for Fe-C dilute alloy single crystals have first revealed that a systematic change of the yield stress - temperature curve occurs, namely, the hump shifts to a lower temperature side. The interpretation of this result must be made by c o n s i d e r i n g effects of carbon atoms on both enhancement of kink pair formation and s u p p r e s s i o n of kink migration. 2) Carbon doping rapidly suppresses the irradiation s o f t e n i n g at 77K because carbon atoms act as obstacles for kink migration. The effect of a c c u m u l a t e d kinks on the overcoming process must also be c o n s i d e r e d when the number ratio between kink and carbon is large. 3) Disappearance of irradiation softening during d e f o r m a t i o n can be understood mainly by the absorption of self-interstltial atoms into screw d i s l o c a t i o n lines, namely, strain induced recovery of SIAs. In the case of a higher carbon content alloy, i.e., Fe-48appmC, transition from softening to hardening was observed with increasing strain. 4) The temperature above which the irradiation softening becomes obvious depends on carbon content and shifts to a higher t e m p e r a t u r e side with increasing carbon content. 5) It is newly suggested that in the interaction between a screw d i s l o c a t i o n line and various point defects (SIA, carbon atom, complex of them and others) their thermal stability is a very important factor in appearance of softening or hardening. Acknowledgement The authors would like to express their cordial thanks to Prof. H. Y o s h i d a in KURRI for many advices and help to the electron irradiation and Dr. H. Abe and Mr. Y. U e d a for the electrical r e s i s t i v i t y measurement of the irradiated specimens. References i.

J. Diehl and G.P. Seidel,

Radiation Damage in Reactor Materials,

Vol.l,

IAEA, V i e n n a (1969)

pl~. 2. 3. 4. 5. 6. 7. 8. 9.

K. Kitajlma, Trans. Japan Inst. Met., 9 Suppl. (1968) 182. K. Kitajima, H. Abe, S. T a k a m u r a and S. Okuda, F u n d a m e n t a l Aspects of Radiation Damage in Metals, USERDA (1975) P977. A. Sato and M. Meshli, S c r i p t a Met., 8 (1974) 851. Y. Aono, E. Kuramoto, H. Abe and K. Kitajlma, Y a m a d a Conf. IX on Dislocations in Solids, Univ. of Tokyo Press, Tokyo (1984) p207. Y. Aono, E. Kuramoto, D, Brunner and J. Diehl, Mat. Sci. Forum, V o i . 1 5 - 1 8 (1987) p801. A. Sato and M. Meshil, A c t a Met., 21 (1973) 753. Y. Aono, E. Kuramoto and T. Tsutsuml, Yamada Conf. IX on Dislocations in Solids, Univ. of Tokyo Press, T o k y o (1984) p203. K. Makll, T. Tsutsuml, Y. Aono and E. Kuramoto, Proc. 8th Int. Conf. on S t r e n g t h of Metals and

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Alloys, ed. P.O. Kettunen, T. K. Lepisto and M. E. Lehtonen, Pergamon Press (1988) p179. K. Makii, T. Tsutsumi, Y. Aono and E. Kuramoto, Mat. Trans. Japan Inst. Met., ~ (1989) 505. E. Kuramoto, Y. Aono and K. Kitajima, Scripta Met., 13 ( I ~ 9 ) 1039. Y. Aono, K. Kitajima and E. Kuramoto, Scprita Met., 15 (1981) 275. E. Kuramoto, Y. Aono and K. Kitajima, Mechanical Properties of BCC Metals, ed. M. Meshii, Met. Soc. AIME, (1981) p27.

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Fig. 2

Fig. I (left-up) Temperature dependence of the yield stress of Fe-C dilute alloy single crystals (5, 12, 18, 33, 48 appmC) f

oo ~ o s ~ B ~ 200

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Fig. 2 (up) Carbon concentration dependence of the yield stress of Fe-C alloy single crystals at various temperatures

8

2BMeV eleci'ron irradiated Fe-C aUoys

[Ta :77K]

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0 Pig.

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Fig. 3 (left) Carbon concentration dependence of the yield stress of the unirradiated and irradiated Fe-C alloy single crystals at 77 K

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28MeV electron irradiated Fe-Sal~pmC aLtoy~$

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28MeV electron irradiated Fe-/.8al~omC alloys

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Fig. 4a) (left-up) Strain dependence of the flow stress and the activation area of the unirradiated and irradiated Fe5 appmC alloy single crystals at 77 K

'

Fe-SappmC 12appm[ ~8appmC

Fig. 4b) (up) Strain dependence of the flow stress and the activation area of unirradiated and irradiated Fe-48appmC alloy single crystals at 77 K

I";200 \ ~\

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Fig. 5

Fig. 5 (left) Temperature dependence of the yield stress of the irradiated Fe-C alloy single crystals (5, 12, 48 appmC) (dashed lines are those for unirradiated ones)

4