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Plastic deformation and martensitic transformation in an iron-base alloy
Scripta METALLURGICA
Vol. i0, pp. 353-358, 1976 Printed in the United States
Pergamon Press, Inc.
PLASTIC DEFORMATION AND MARTENSITIC TRANSFORMATION
IN AN IRON-BASE ALLOY
T. Suzuki, H. Kojima*, K. Suzuki, T. Hashimoto, S. Koike* and M. Ichihara Institute for Solid State Physics, University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo, Japan (Received February 15, 1976)
Gunter and Reed(l) and Tamura et al.(2) have found that polycrystalline fcc Fe-Ni and Fe-Cr-Ni alloys deform plastically in three stages by external stress in a certain low temperature region, while such a behavior is hardly seen in Fe-Mn alloys. Studying the AISI 304 series of alloys, Gunter and Reed stated that the plastic strain of the first stage was due to the stress-induced hexagonal martensites(e'). Such an explanation is impossible in the case of FeNi alloys, however, since no e'-martensites are formed. The conclusion, therefore, seems to be natural that the appearance of the three stages in the stressstrain curve is due to the formation of bcc martensites(e') during plastic deformation. Although it has been suggested that the variation with strain of the rate of the formation of e'-martensites causes the three stages, it is questionable. In the present note, we will report by a precise examination of the formation process of ~'-martensites in Fe-Cr-Ni alloys that there is an important interrelation between the formation of u'-martensites and the development of slip bands, which seems to have been missed so far. That is, the deformationinduced ~'-martensites do not behave as simple hardening entities but as peculiar demons, who prefer only the pass of the dislocations of a certain slip system closely related to their birth, to induce the three stages of plastic deformation. A commercial SUS 304 Fe-Cr-Ni alloy of a hot-rolled sheet of 1 mm thickness was supplied by Nippon Metal Industry Corp. Ltd. Chemical analyses were provided by the producer as 9.14%Ni, 18.38%Cr, 0.91%Mn, 0.59%Si, 0.027%P, 0.016%S, and 0.05 %C. The material was machined to form tensile specimens, the dimension of which was 30xSxl mm except for the grip area. The specimen axes were taken parallel to the rolling direction. Two groups of specimens were annealed separately for three minutes and for one hour at I050°C in air, respectively, and then rapidly cooled by pulling out o£ the furnace. The grain size was about 50~m and 250um, respectively. Specimen surfaces were electrolytically polished by using two solutions, firstly 6 parts phosphoric acid and 4 parts sulfuric acid at 0°C and secondly nitric acid and methylalcohol of 1:2 in volume ratio at -40°C. Tensile tests were conducted with an Instron-type machine at a cross-head speed of 0.2mm/ min in parallel with magnetic measurements between liquid nitrogen temperature and 150°C. Magnetic measurements were made with AC current of SOHz to produce an external magnetic field of iSSOe. The magnetic induction induced in a search * On leave from Department of Physics, Shinjuku-ku, Tokyo.
Science University of Tokyo, Kagurazaka,
3S3
354
DEFORMATION AND TRANSFORMATION
IN IRON ALLOY
Vol. I0, No. 4
coil wound with a least gap around a tensile specimen was recorded as well as the tensile load as a function of cross-head displacement. Transmission-electronmicroscopic observations were made on these specimens deformed to various amounts of strain and in situ observations were also made on thin specimens pulled inside the electronmicroscope(HU-I/500 kV). First of all, in the present experiment, a particular caution was paid to measure as precisely as possible the initial part of load vs. elongation curve. An example of the curve obtained at -196°C is shown in Fig. 1 as well as the magnetization due to the formation of ferromagnetic ~'-martensites as a function of plastic strain. The proportional limit oe' is found to be the stress at which ¢'-martensites are first induced. It is so for all the specimens deformed below -30°C. As shown in Fig. 2, ~E' decreases with lowering temperature, while the stress a e, at which e'-martensite is first induced, detected by magnetic measurement, increases with lowering temperature. According to electronmicroscopic observations, these a' are always formed at the junction of two intersecting e'bands as seen in Fig. 3. Slip dislocations can be seen inside or along these ¢'-bands at the stage of deformation where e' start to be formed, although this is not shown here. Fig. 4 is an example of in situ observations made at -40°C as increasing an external stress from (a) to {b). Ca) is a photograph taken just before the formation of e' and {b) is just after the formation of e' at the intersection of two c'-bands, each of which is parallel to {III} plane. These ~' run along the intersection line parallel to to form a long thin plate, the face of which is parallel to one of the two bands. Kelly and Nutting(3), Venables(4), LagneborgCS), and Stone and ThomasC6) report results from investigations of e'-martensites formed by deformation that are in agreement with the
/)
I0( 4~1 eo
4
~
0
IO
DO
I0
-,I
~
I
b~
!,o ,
0
2
4
6
$
I0
12
lO
Tensile Strain ~" (%) Tmlmm~m ( K )
Fig. 1 Stress-strain and magnetization{Ma,)-strain curves at -196°C. Grain size = SOpm.
Fig. 2 Proportional limit (o~,) and yield stress of slip {ay and a0.2) plotted against temperature, ae' is the stress at which ~'-martensites are first formed.
Vol. I0, No. 4
DEFORMATION AND TRANSFORMATION
IN IRON ALLOY
355
present. Stone and Thomas studied deformation-induced martensites in Fe-Cr-Ni single crystals. According to the present in situ observations, a long thin plate of e'-martensite is formed just outside one of the intersecting two bands, which transforms gradually to a slip band by destroying the e'-martensites. Going back again to Fig. I, it is noticed that the formation of u'-martensites decreases gradually the hardening rate to a level of the first stage of deformation. Yield stress av of slip is defined customarily by the intersection of the two extrapolations f?om the elastic deformation and the first stage of deformation of the stress-strain curve as shown in Fig. 1 and it is plotted against temperature in Fig. 2. It will be natural that Sv thus defined is slightly higher than ~e' corresponding to the initiation of development of slip bands. By the way, ~0 2 plotted above -50°C is the proof stress corresponding to 0.2% plastic strain produced by pure slip. Fig. 2 shows a jump of yield stress for slip
[Ol
')\. ilO
/\,// lOi
(111)y//(0001) e'/l(llO)c~' 0
eA
(a)
(b)
(c)
F i g . 3 E l e c t r o n m i c r o g r a p h s of s ' - m a r t e n s i t e s formed a t the j u n c t i o n o f two i n t e r s e c t i n g d e f o r m a t io n bands i n a specimen deformed by 6.4% i n t e n s i o n at -44°C with an e l e c t r o n - d i f f r a c t i o n p a t t e r n [111]~. The t r a c e s o f (111) and ( l i l ) p l a n e s , and th e p r o j e c t i o n o f t e n s i l e a x i s ar e i n d i c a t e d . Ca) b r i g h t - f i e l d , (b) and (c) d a r k - f i e l d micrographs taken with the spots b and c, t w i n - r e l a t e d , d e s i g n a t e d in the d i f f r a c t i o n p a t t e r n , r e s p e c t i v e l y .
356
DEFORMATION AND TRANSFORMATION IN IRON ALLOY
(a)
Vol. I0, No. 4
(b)
Fig. 4 In-situ observation of the formation of ~'-martensite at the junction of two intersecting bands which involve a number of slip dislocations as well as e'-discs. (a) before the formation and (b) just after the formation of u'-martensite.
at about -28°C, which is conceived as due to the presence of e'-bands to be intersected below this temperature, and it also shows a minimum in av vs. temperature curve at about -60°C. The occurence of this minimum must be uhderstood similarly as the variation of ~a, with temperature. If the external stress influences directly the formation of ~'-martensite as first proposed by Patel and Cohen(?), it is expected that ~_,~ continues.. to decrease with decreasing temperature even below -60°C as a~, does, whlch is contrary to Fig.2, however. Together with a fact that u'-martensites are formed at the junction and just outside one of the intersecting bands, it leads us to conclude that not external stress but internal stress due to the dislocations piled up at the intersection induces the formation of e'. The stress concentration in front of the piled up dislocations promoted by the e'-martensite thus formed will lead to the propagation of dislocations of the primary slip system through the grain concerned. The increase of ~u, with decreasing temperature is then understood as due to an increase of stress needed for slip dislocations to cross over the intersecting e'-bands. A most important conclusion from the present work is that the deformationinduced ~'-martensites do not behave as simple hardening entities but as strange demons, who promote a certain slip directly related to their birth. It is possible because the deformation due to ~' is accomodated to that due to the slip dislocations which contributed to their birth, while it is generally ill accomodated to that due to the dislocations of other slip systems. In the first stage of deformation, therefore, induced slip is the one of the primary.slip system in each grain, and a plastic constraint imposed to grain boundaries is relieved by the deformation due to the further development of secondary c'-bands nonparallel to the primary e'-bands. In the second stage of deformation, however, u'-martensites become to be formed along every active slip plane at all the junctions. Gunter and Reed report that e'-martensites tend to disappear in this stage. A higher rate of work-hardening caused by these u'-martensites is naturally associated with the second stage of deformation.
Vol. i0, No. 4
DEFORMATION AND TRANSPORMATION
IN IRON ALLOY
3S7
Table 1 K-S variants of a'-martensites and statistics of occurence of two twinrelated variants in the first stare of deformation at -44°C. K-S RELATION VARIANT
DIRECTION
(Y)/I(~')
[X]#[~']
TWIN-RELATED ORDER OF OBSERVED VARIANT SCHMID FACTOR FREQUENCY (296 ex.) (slip)
-i
BC [10i] [ i l i ]
1-2
-2
CB [i01] [11i]
i-i
DB [ 0 i l ]
[11i]
1-4
61%
BD [01i]
[11i]
I-3
39%
-3 1
PLANE
-4
(111) (011)
-s
CD [i10] [ i l i ]
1-6
-6
DC [ii0] [11i]
l-S
B
Thompson's tetrahedron
In connection with the above statement that the internal stress due to piled-up dislocations induces a'-martensites, it is noticeable that two twinrelated variants of a', related to the primary slip, are mostly formed in the first stage of deformation, although the component of shear in the primary slip plane produced by either of them is unpreferable to tensile stress but preferable to compression stress applied. A similar finding was recently reported by Higo et al.(8) in the study of deformation of Fe-Cr-Ni single crystals. They concluded that the two variants were always formed in pair as twins to annihilate a longrange shear deformation. This may give an explanation of the appearance of unpreferable a'-variants. We are not satisfied with this explanation, however. As shown in Fig. 5, we examined this by dark field electronmicroscopy, results of which are summarized in Table i. It is found that most a'-variants distribute separately and randomly, not to form twins except for 8 to 13% of the whole as seen in Fig. 5, and the variant of pceferable shear is higher in density than the variant of unprefereble shear, which was concluded by measuring each population in a few grains. Since the nature of the internal stress concerned depends upon the nature of the piled-up dislocations and also upon the location around them, generally both tensile and compressive stress fields are generated in the vicinity of the junction of two intersecting bands and, therefore, it is not strange that the two variants, twin-related to each other, are formed independently of each other. Finally, the present authors wish to thank Mr. T. Monma, Nippon Metal Industry Corp. Ltd., for supplying specimen materials used in this study.
358
DEFORMATION AND TRANSFORMATION IN IRON ALLOY
Vol. 10, No. 4
References 1,
2.
C. J. Gunter and R. P. Reed, Trans. ASM 55, 399 (1962). I. Tamura, T. Maki and H. Hato, Trans. Is~TJ 10, 163 (1970); I. Tamura and T. Maki, Proc. Symposium on Toward Improved Duct-~lity and Toughness (Climax Mo. Develop. Co. Japan Ltd.}, p.183 (1971). P. H. Kelly and J. Nutting, J. Iron St. Inst. 197, 199 (1961}. J. A. Venables, Phil. Mag 7, 55 (1962). R. Lagneborg, Act. Met. 1½,--825 (1964). G. Stone and G. Thomas, M-et. Trans. 5, 2095 (1974). J. R. Patel and M. Cohen, Act. Met. T, 531 (1955). Y. Higo, F. Lecroisey and T. Mori, AFt. Met. 2__22,313 (1974).
Vol. i0, pp. 353-358, 1976 Printed in the United States
Pergamon Press, Inc.
PLASTIC DEFORMATION AND MARTENSITIC TRANSFORMATION
IN AN IRON-BASE ALLOY
T. Suzuki, H. Kojima*, K. Suzuki, T. Hashimoto, S. Koike* and M. Ichihara Institute for Solid State Physics, University of Tokyo, 7-22-1 Roppongi, Minato-ku, Tokyo, Japan (Received February 15, 1976)
Gunter and Reed(l) and Tamura et al.(2) have found that polycrystalline fcc Fe-Ni and Fe-Cr-Ni alloys deform plastically in three stages by external stress in a certain low temperature region, while such a behavior is hardly seen in Fe-Mn alloys. Studying the AISI 304 series of alloys, Gunter and Reed stated that the plastic strain of the first stage was due to the stress-induced hexagonal martensites(e'). Such an explanation is impossible in the case of FeNi alloys, however, since no e'-martensites are formed. The conclusion, therefore, seems to be natural that the appearance of the three stages in the stressstrain curve is due to the formation of bcc martensites(e') during plastic deformation. Although it has been suggested that the variation with strain of the rate of the formation of e'-martensites causes the three stages, it is questionable. In the present note, we will report by a precise examination of the formation process of ~'-martensites in Fe-Cr-Ni alloys that there is an important interrelation between the formation of u'-martensites and the development of slip bands, which seems to have been missed so far. That is, the deformationinduced ~'-martensites do not behave as simple hardening entities but as peculiar demons, who prefer only the pass of the dislocations of a certain slip system closely related to their birth, to induce the three stages of plastic deformation. A commercial SUS 304 Fe-Cr-Ni alloy of a hot-rolled sheet of 1 mm thickness was supplied by Nippon Metal Industry Corp. Ltd. Chemical analyses were provided by the producer as 9.14%Ni, 18.38%Cr, 0.91%Mn, 0.59%Si, 0.027%P, 0.016%S, and 0.05 %C. The material was machined to form tensile specimens, the dimension of which was 30xSxl mm except for the grip area. The specimen axes were taken parallel to the rolling direction. Two groups of specimens were annealed separately for three minutes and for one hour at I050°C in air, respectively, and then rapidly cooled by pulling out o£ the furnace. The grain size was about 50~m and 250um, respectively. Specimen surfaces were electrolytically polished by using two solutions, firstly 6 parts phosphoric acid and 4 parts sulfuric acid at 0°C and secondly nitric acid and methylalcohol of 1:2 in volume ratio at -40°C. Tensile tests were conducted with an Instron-type machine at a cross-head speed of 0.2mm/ min in parallel with magnetic measurements between liquid nitrogen temperature and 150°C. Magnetic measurements were made with AC current of SOHz to produce an external magnetic field of iSSOe. The magnetic induction induced in a search * On leave from Department of Physics, Shinjuku-ku, Tokyo.
Science University of Tokyo, Kagurazaka,
3S3
354
DEFORMATION AND TRANSFORMATION
IN IRON ALLOY
Vol. I0, No. 4
coil wound with a least gap around a tensile specimen was recorded as well as the tensile load as a function of cross-head displacement. Transmission-electronmicroscopic observations were made on these specimens deformed to various amounts of strain and in situ observations were also made on thin specimens pulled inside the electronmicroscope(HU-I/500 kV). First of all, in the present experiment, a particular caution was paid to measure as precisely as possible the initial part of load vs. elongation curve. An example of the curve obtained at -196°C is shown in Fig. 1 as well as the magnetization due to the formation of ferromagnetic ~'-martensites as a function of plastic strain. The proportional limit oe' is found to be the stress at which ¢'-martensites are first induced. It is so for all the specimens deformed below -30°C. As shown in Fig. 2, ~E' decreases with lowering temperature, while the stress a e, at which e'-martensite is first induced, detected by magnetic measurement, increases with lowering temperature. According to electronmicroscopic observations, these a' are always formed at the junction of two intersecting e'bands as seen in Fig. 3. Slip dislocations can be seen inside or along these ¢'-bands at the stage of deformation where e' start to be formed, although this is not shown here. Fig. 4 is an example of in situ observations made at -40°C as increasing an external stress from (a) to {b). Ca) is a photograph taken just before the formation of e' and {b) is just after the formation of e' at the intersection of two c'-bands, each of which is parallel to {III} plane. These ~' run along the intersection line parallel to
/)
I0( 4~1 eo
4
~
0
IO
DO
I0
-,I
~
I
b~
!,o ,
0
2
4
6
$
I0
12
lO
Tensile Strain ~" (%) Tmlmm~m ( K )
Fig. 1 Stress-strain and magnetization{Ma,)-strain curves at -196°C. Grain size = SOpm.
Fig. 2 Proportional limit (o~,) and yield stress of slip {ay and a0.2) plotted against temperature, ae' is the stress at which ~'-martensites are first formed.
Vol. I0, No. 4
DEFORMATION AND TRANSFORMATION
IN IRON ALLOY
355
present. Stone and Thomas studied deformation-induced martensites in Fe-Cr-Ni single crystals. According to the present in situ observations, a long thin plate of e'-martensite is formed just outside one of the intersecting two bands, which transforms gradually to a slip band by destroying the e'-martensites. Going back again to Fig. I, it is noticed that the formation of u'-martensites decreases gradually the hardening rate to a level of the first stage of deformation. Yield stress av of slip is defined customarily by the intersection of the two extrapolations f?om the elastic deformation and the first stage of deformation of the stress-strain curve as shown in Fig. 1 and it is plotted against temperature in Fig. 2. It will be natural that Sv thus defined is slightly higher than ~e' corresponding to the initiation of development of slip bands. By the way, ~0 2 plotted above -50°C is the proof stress corresponding to 0.2% plastic strain produced by pure slip. Fig. 2 shows a jump of yield stress for slip
[Ol
')\. ilO
/\,// lOi
(111)y//(0001) e'/l(llO)c~' 0
eA
(a)
(b)
(c)
F i g . 3 E l e c t r o n m i c r o g r a p h s of s ' - m a r t e n s i t e s formed a t the j u n c t i o n o f two i n t e r s e c t i n g d e f o r m a t io n bands i n a specimen deformed by 6.4% i n t e n s i o n at -44°C with an e l e c t r o n - d i f f r a c t i o n p a t t e r n [111]~. The t r a c e s o f (111) and ( l i l ) p l a n e s , and th e p r o j e c t i o n o f t e n s i l e a x i s ar e i n d i c a t e d . Ca) b r i g h t - f i e l d , (b) and (c) d a r k - f i e l d micrographs taken with the spots b and c, t w i n - r e l a t e d , d e s i g n a t e d in the d i f f r a c t i o n p a t t e r n , r e s p e c t i v e l y .
356
DEFORMATION AND TRANSFORMATION IN IRON ALLOY
(a)
Vol. I0, No. 4
(b)
Fig. 4 In-situ observation of the formation of ~'-martensite at the junction of two intersecting bands which involve a number of slip dislocations as well as e'-discs. (a) before the formation and (b) just after the formation of u'-martensite.
at about -28°C, which is conceived as due to the presence of e'-bands to be intersected below this temperature, and it also shows a minimum in av vs. temperature curve at about -60°C. The occurence of this minimum must be uhderstood similarly as the variation of ~a, with temperature. If the external stress influences directly the formation of ~'-martensite as first proposed by Patel and Cohen(?), it is expected that ~_,~ continues.. to decrease with decreasing temperature even below -60°C as a~, does, whlch is contrary to Fig.2, however. Together with a fact that u'-martensites are formed at the junction and just outside one of the intersecting bands, it leads us to conclude that not external stress but internal stress due to the dislocations piled up at the intersection induces the formation of e'. The stress concentration in front of the piled up dislocations promoted by the e'-martensite thus formed will lead to the propagation of dislocations of the primary slip system through the grain concerned. The increase of ~u, with decreasing temperature is then understood as due to an increase of stress needed for slip dislocations to cross over the intersecting e'-bands. A most important conclusion from the present work is that the deformationinduced ~'-martensites do not behave as simple hardening entities but as strange demons, who promote a certain slip directly related to their birth. It is possible because the deformation due to ~' is accomodated to that due to the slip dislocations which contributed to their birth, while it is generally ill accomodated to that due to the dislocations of other slip systems. In the first stage of deformation, therefore, induced slip is the one of the primary.slip system in each grain, and a plastic constraint imposed to grain boundaries is relieved by the deformation due to the further development of secondary c'-bands nonparallel to the primary e'-bands. In the second stage of deformation, however, u'-martensites become to be formed along every active slip plane at all the junctions. Gunter and Reed report that e'-martensites tend to disappear in this stage. A higher rate of work-hardening caused by these u'-martensites is naturally associated with the second stage of deformation.
Vol. i0, No. 4
DEFORMATION AND TRANSPORMATION
IN IRON ALLOY
3S7
Table 1 K-S variants of a'-martensites and statistics of occurence of two twinrelated variants in the first stare of deformation at -44°C. K-S RELATION VARIANT
DIRECTION
(Y)/I(~')
[X]#[~']
TWIN-RELATED ORDER OF OBSERVED VARIANT SCHMID FACTOR FREQUENCY (296 ex.) (slip)
-i
BC [10i] [ i l i ]
1-2
-2
CB [i01] [11i]
i-i
DB [ 0 i l ]
[11i]
1-4
61%
BD [01i]
[11i]
I-3
39%
-3 1
PLANE
-4
(111) (011)
-s
CD [i10] [ i l i ]
1-6
-6
DC [ii0] [11i]
l-S
B
Thompson's tetrahedron
In connection with the above statement that the internal stress due to piled-up dislocations induces a'-martensites, it is noticeable that two twinrelated variants of a', related to the primary slip, are mostly formed in the first stage of deformation, although the component of shear in the primary slip plane produced by either of them is unpreferable to tensile stress but preferable to compression stress applied. A similar finding was recently reported by Higo et al.(8) in the study of deformation of Fe-Cr-Ni single crystals. They concluded that the two variants were always formed in pair as twins to annihilate a longrange shear deformation. This may give an explanation of the appearance of unpreferable a'-variants. We are not satisfied with this explanation, however. As shown in Fig. 5, we examined this by dark field electronmicroscopy, results of which are summarized in Table i. It is found that most a'-variants distribute separately and randomly, not to form twins except for 8 to 13% of the whole as seen in Fig. 5, and the variant of pceferable shear is higher in density than the variant of unprefereble shear, which was concluded by measuring each population in a few grains. Since the nature of the internal stress concerned depends upon the nature of the piled-up dislocations and also upon the location around them, generally both tensile and compressive stress fields are generated in the vicinity of the junction of two intersecting bands and, therefore, it is not strange that the two variants, twin-related to each other, are formed independently of each other. Finally, the present authors wish to thank Mr. T. Monma, Nippon Metal Industry Corp. Ltd., for supplying specimen materials used in this study.
358
DEFORMATION AND TRANSFORMATION IN IRON ALLOY
Vol. 10, No. 4
References 1,
2.
C. J. Gunter and R. P. Reed, Trans. ASM 55, 399 (1962). I. Tamura, T. Maki and H. Hato, Trans. Is~TJ 10, 163 (1970); I. Tamura and T. Maki, Proc. Symposium on Toward Improved Duct-~lity and Toughness (Climax Mo. Develop. Co. Japan Ltd.}, p.183 (1971). P. H. Kelly and J. Nutting, J. Iron St. Inst. 197, 199 (1961}. J. A. Venables, Phil. Mag 7, 55 (1962). R. Lagneborg, Act. Met. 1½,--825 (1964). G. Stone and G. Thomas, M-et. Trans. 5, 2095 (1974). J. R. Patel and M. Cohen, Act. Met. T, 531 (1955). Y. Higo, F. Lecroisey and T. Mori, AFt. Met. 2__22,313 (1974).