- Email: [email protected]
Kink martensite in a duplex Fe-Mn-Al-C alloy
Pergamon
ScriptaMetallurgicaet Materialia,Vol. 31, No. 6, pp. 695-700, 1994 Copyright© 1994 ElsevierScienceLtd Printed in the USA. All rights reserved 0956-716)(/94 $6.00 + 00
KINK MARTENSITE IN A DUPLEX Fe-Mn-AI-C ALLOY W. B. Lee*, F. R. Chenl", S. K. Chen~', C. M. Wan* * Institute of Materials Science and Engineering National Tsing Hua University, Hsinchu, Taiwan, R.O.C. j" Materials Science Center National Tsing Hua University, Hsinchu, Taiwan, R.O.C.
(Received January 13, 1994) (Revised May 9, 1994) Introduction A needle phase within a BCC matrix was found in duplex Fe-Mn-A1-C alloys (1, 2). It has been identified to have the long period stacking order (LPSO) structure of 18R(51")3 transformed from the BCC parent phase (3, 4). Later, the LPSO structure was suggested to be a 18R(42)3 martensite phase(5-7). Recent reports show that the LPSO structure is close to the 18R(42)3 rather th.an the 18R(51)3 structure on the basis of high resolution TEM, diffraction patterns and computer simulation (8). The single shear phenomenological theory was used to calculate the crystallography of this BCC~lgR(42)3 martensitic transformation. The calculated crystallography is in good agreement with experiments and is similar to that of the BCC/18R (or 9R) martensite in other alloy systems (8, 9, 10). Kink martensites, as shown in Fig. 1, also were often observed in the Fe-Mn-A1-C alloy. Martensite having the kink morphology was also found in Fe-A1-C and in duplex stainless steels quenched from temperatures higher than 1200°C (11, 12). However, there has been no report on the crystallographic relationship of the kink martensites. Therefore, the purpose of this paper is to present a detailed TEM analysis and a phenomenological interpretation on the crystallography of kink martensites in the duplex Fe-Mn-A1-C alloy. Experimental Procedures The composition of the alloy is Fe-25.gwt%Mn-7.4wt%A1-0.1 lwt%C. Specimens with 100 x 10 x 2 mm 3 were cut from a cold-rolled plate. The heat treatment was an isothermal hold at 1300°C for 1 h in argon gas followed by a water-quench to room temperature. Thin foils for TEM study were obtained by twin-jet polishing in a 6% HC104 and 94% CI-I3COOtt solution operated at 15V and examined in a IEOL-200CX STEM. Results and Discussion There are 24 crystallographic variants in a transformation from the BCC parent phase to 18R martensite (13, 14). Figure 2 is a light micrograph which shows different variants of the needle-like martensite phase coexisting in BCC grains in a specimen water-quenched from 1300°C As shown in Fig. 3(a), the martensite phase contains stacking faults and its structure has been determined to be a 1gR(42)3 structure (8). The diffraction pattern of this martensite is given in Fig. 3(b). Its zone axis is [ll-0]~R. In the duplex Fe-Mn-A1-C alloy, the martensite usually forms the kink morphology as shown in Fig. 1. Figures 4(a) and (b) show the bright field image and diffraction pattern of another kink martensite. The diffraction pattern in Fig. 4(b) is actually a superimposed diffraction pattern of the two variants. Using the selected area diffraction and the dark field imaging technique, one can easily distinguish these two variants in the kink martensite. The dark field images and the selected area diffraction patterns of the two variants of the lgR martensite are depicted in Figs. 4(c) to (f). The martensites that show a white contrast in Fig. 4(c) are labeled as variant A and those in Fig. 4(e) are labeled variant B. From the diffraction pattern in Fig. 4(b), one can find that variants A and B can be regarded as twin-related martensites with the twin plane (001)~. The zone axis of the diffraction patterns for variants A and B can then be indexed as [0T0]l,~ and [010]~,R, respectively. The stacking fault planes (0 0 18)l,R of these two variants are edge-on in this orientation and the angle between them is 80L A schematic representation of the kink martensite is given in Fig. 5. 695
696
KINK MARTENSITEIN Fe-Mn-AI-C
Vol. 31, No. 6
The crystallography of the kink martensite is calculated using the CRAB theory (15). The lattice constants of the martensite are a = 0.448 rim, b = 0.259 nm, and c = 3.865 nm (8). The inhomogeneous shears for variants A and B are [101]~/(101)bcc and [10T]~J(101)~c, respectively, which are perpendicular to each other. The {110}bec plane in the parent phase becomes a close-packed 'basal' or stacking plane in martensite. The martensitic crystallography of these two variants, including habit planes, shape strain directions and orientation relationships, are summarized in Table 1. As can be seen from Table 1, the stacking plane (0 0 18)~8Rof the two martensites are 5.5° off, but in opposite directions, from the (T01)~c and (101)~ for variants A and B. The calculated orientation relationship suggests that the angle between stacking fault planes (0 0 18)~8Rof these two variants is close to 80 ° which is in good agreement with the experimental result. The angle between two shape strain directions of the two variants in the kink martensite is 10.2 °. The shape strain directions for two different variants are almost parallel. TABLE 1 Calculated Items of Variants A and B from Martensite Theory Variant A Habit Plane Shape Strain Direction
( 7.71, 6.30, 1 )bee [ 0.69, -0.72, 0.09 ]bee
(To1)bee ~ , 5.53° Orientation Relationship
( oo18
7,
Variant B ( 7.71, 6.30,'~ )bee [ 0.69, -0.72, -0.09 ]bee
(lOl)
>5.53 °
( 0018 18R,B
B
[ 111]b,c > 0 . 9 8 ° ['T10 ]18R,A
[~11 ]bee [ 1"~0] 1SR,B
0.98
The 24 variants of martensite consist of six plate groups, each of which consists of four habit plane variants which are near to and symmetrically clustered about the six {110}bcc poles of the parent phase (13, 14). Four variants of each plate group can form spear-, wedge- and fork-pair martensites (16, 17). The morphology and the diffraction patterns of the kink martensite in the Fe-Mn-AI-C alloy resemble those fork martensites in the Cu-ZnAI alloy (14). This implies that the crystallography of the kink- and fork-pair martensites are similar. It is known that the main driving force for a martensitic transformation on cooling is the chemical free energy difference between the parent and martensite, however, the nucleation and the growth of martensitic plates may also be controlled by the stress field induced from the cooling, especially, for the alloy quenched from a very high temperature. The kink morphology in Fig. 4(a) is quite similar to that in the Fe-7AI-2C as water-quenched from 1200°C (11) or in the melt-quenching duplex stainless steel (12). It is calculated that, in the Fe-Mn-AI-C alloy, if a variant A is preferably formed with a shape strain direction [0.69, -0.72, 0.09]b~, then the induced stress from a 1300°C water-quench may favor the growth of a variant B having a similar shape strain direction [0.69, -0.72, -0.09]~. After formation of the kink morphology in two variants, the total shape deformation will be lower than that of variant A alone. Conclusions A kink martensite form in a duplex Fe-Mn-AI-C alloy is found after a water-quench from 1300°C. The crystallography of the kink martensite is determined by transmission electron microscopy and is in good agreement with the calculated crystallography using the CRAB theory. The crystallography of the kink martensites is similar to that of fork-pair martensites in a Cu-Zn-AI alloy. The two variants, with perpendicular shear planes and nearly the same shape strain direction, are required to form this kink morphology. Acknowledgements The authors would like to acknowledge the National Science Council of R. O. C. for financial support under grant No. NSC81-0405-E007-531. The authors would also like to thank the Materials Science Center of National Tsing Hua University for their support.
Vol. 31, No. 6
KINKMARTENSITEIN Fe-Mn-AI-C
References 1. W. B. Lee, Master Thesis, Institute of Materials Science and Engineering, National Tsing Hua University, Taiwan, R.O.C. (1988). 2. K W. Chour, Master Thesis, Institute of Materials Science and Engineering, National Tsing Hua University, Taiwan, R.O.C. (1988). 3. K H. Hwang, W. S. Yang, T. B. Wu, C. M. Wan, and J. G. Byme, Acta Metall., 39, 825 (1991). 4. S. K. Chen, C. M. Wan, and J. G. Byrne, Scripta Metall., 24, 2139 (1990). 5. C. H. Chao and N. J. Ho, Scripta Metall., 26, 1863 (1992)~ 6. C. H. Chao and N. J. Ho, Scripta Metall., 27, 449 (1992). 7. C. H. Chao and N. J. Ho, Scripta Metall., 27, 493 (1992). 8. W. B. Lee, F. R. Chen, S. K. Chen, and C. M. Wan, to be published by Acta Metall.. 9. K. H. Hwang, S. K. Chen, W. S. Yang, T. B. Wu, C. M. Wan, and J. G. Byrne, Scripta Met,all., 24, 495 (1990). 10. K H. Hwang, C. M. Wan, and J. G. Byrne, Mater. Sci. Eng., A132, 161 (1991). 11. M. Watanabe and C. M. Wayman, Metall. Trans., 2, 2221 (1971). 12. T. Tomida, Y. Maehara, and Y. Ohmori, Mater. Trans. Jpn. Inst. Met., 30, 326 (1989). 13. M. Umemoto and C. M. Wayman, Acta Metall., 26, 1529 (1978). 14. T. Saburi and C. M. Wayman, Acta Metall., 27, 979 (1979). 15. A. G. Crocker, Journal De Phys., Colloque C4, supplement, 209 (1982). 16. T. Saburi, C. M. Wayman, K. Takata, and S. Nenno, Acta Metall., 28, 15 (1980). 17. T. A. Schroeder and C. M. Wayman. Acta Metall., 25, 1375 (1977).
Fig. 1 TEM micrograph of kink martensite.
697
698
KINK MARTENSITEIN Fe-Mn-AI-C
Vol. 31, No. 6
Fig. 2 Light micrograph of specimen water-quenched from 1300°C.
~Z
(a) fo) Fig. 3 TEM dark field image (a) and [IT0]IsRSADP ofllle martensite phase (b).
Vol. 31, No. 6
KINK MARTENSITE1N Fe-Mn-AI-C
(~)
(c)
(e)
699
(b)
(d)
(0
Fig. 4 TEM mierographs of kink martensite. (aX b) are bright field images and a superimposed SADP of martensite variants. (eXd) are dark field images and [010]IsRSADP of variant A. (eXf) are dark field images and [010] 181,,SADP of variant B.
700
K I N K M A R T E N S I T E IN Fe-Mn-AI-C
Fig. 5 Schematic representation of fig. 4(a).
Vol. 31, No. 6
ScriptaMetallurgicaet Materialia,Vol. 31, No. 6, pp. 695-700, 1994 Copyright© 1994 ElsevierScienceLtd Printed in the USA. All rights reserved 0956-716)(/94 $6.00 + 00
KINK MARTENSITE IN A DUPLEX Fe-Mn-AI-C ALLOY W. B. Lee*, F. R. Chenl", S. K. Chen~', C. M. Wan* * Institute of Materials Science and Engineering National Tsing Hua University, Hsinchu, Taiwan, R.O.C. j" Materials Science Center National Tsing Hua University, Hsinchu, Taiwan, R.O.C.
(Received January 13, 1994) (Revised May 9, 1994) Introduction A needle phase within a BCC matrix was found in duplex Fe-Mn-A1-C alloys (1, 2). It has been identified to have the long period stacking order (LPSO) structure of 18R(51")3 transformed from the BCC parent phase (3, 4). Later, the LPSO structure was suggested to be a 18R(42)3 martensite phase(5-7). Recent reports show that the LPSO structure is close to the 18R(42)3 rather th.an the 18R(51)3 structure on the basis of high resolution TEM, diffraction patterns and computer simulation (8). The single shear phenomenological theory was used to calculate the crystallography of this BCC~lgR(42)3 martensitic transformation. The calculated crystallography is in good agreement with experiments and is similar to that of the BCC/18R (or 9R) martensite in other alloy systems (8, 9, 10). Kink martensites, as shown in Fig. 1, also were often observed in the Fe-Mn-A1-C alloy. Martensite having the kink morphology was also found in Fe-A1-C and in duplex stainless steels quenched from temperatures higher than 1200°C (11, 12). However, there has been no report on the crystallographic relationship of the kink martensites. Therefore, the purpose of this paper is to present a detailed TEM analysis and a phenomenological interpretation on the crystallography of kink martensites in the duplex Fe-Mn-A1-C alloy. Experimental Procedures The composition of the alloy is Fe-25.gwt%Mn-7.4wt%A1-0.1 lwt%C. Specimens with 100 x 10 x 2 mm 3 were cut from a cold-rolled plate. The heat treatment was an isothermal hold at 1300°C for 1 h in argon gas followed by a water-quench to room temperature. Thin foils for TEM study were obtained by twin-jet polishing in a 6% HC104 and 94% CI-I3COOtt solution operated at 15V and examined in a IEOL-200CX STEM. Results and Discussion There are 24 crystallographic variants in a transformation from the BCC parent phase to 18R martensite (13, 14). Figure 2 is a light micrograph which shows different variants of the needle-like martensite phase coexisting in BCC grains in a specimen water-quenched from 1300°C As shown in Fig. 3(a), the martensite phase contains stacking faults and its structure has been determined to be a 1gR(42)3 structure (8). The diffraction pattern of this martensite is given in Fig. 3(b). Its zone axis is [ll-0]~R. In the duplex Fe-Mn-A1-C alloy, the martensite usually forms the kink morphology as shown in Fig. 1. Figures 4(a) and (b) show the bright field image and diffraction pattern of another kink martensite. The diffraction pattern in Fig. 4(b) is actually a superimposed diffraction pattern of the two variants. Using the selected area diffraction and the dark field imaging technique, one can easily distinguish these two variants in the kink martensite. The dark field images and the selected area diffraction patterns of the two variants of the lgR martensite are depicted in Figs. 4(c) to (f). The martensites that show a white contrast in Fig. 4(c) are labeled as variant A and those in Fig. 4(e) are labeled variant B. From the diffraction pattern in Fig. 4(b), one can find that variants A and B can be regarded as twin-related martensites with the twin plane (001)~. The zone axis of the diffraction patterns for variants A and B can then be indexed as [0T0]l,~ and [010]~,R, respectively. The stacking fault planes (0 0 18)l,R of these two variants are edge-on in this orientation and the angle between them is 80L A schematic representation of the kink martensite is given in Fig. 5. 695
696
KINK MARTENSITEIN Fe-Mn-AI-C
Vol. 31, No. 6
The crystallography of the kink martensite is calculated using the CRAB theory (15). The lattice constants of the martensite are a = 0.448 rim, b = 0.259 nm, and c = 3.865 nm (8). The inhomogeneous shears for variants A and B are [101]~/(101)bcc and [10T]~J(101)~c, respectively, which are perpendicular to each other. The {110}bec plane in the parent phase becomes a close-packed 'basal' or stacking plane in martensite. The martensitic crystallography of these two variants, including habit planes, shape strain directions and orientation relationships, are summarized in Table 1. As can be seen from Table 1, the stacking plane (0 0 18)~8Rof the two martensites are 5.5° off, but in opposite directions, from the (T01)~c and (101)~ for variants A and B. The calculated orientation relationship suggests that the angle between stacking fault planes (0 0 18)~8Rof these two variants is close to 80 ° which is in good agreement with the experimental result. The angle between two shape strain directions of the two variants in the kink martensite is 10.2 °. The shape strain directions for two different variants are almost parallel. TABLE 1 Calculated Items of Variants A and B from Martensite Theory Variant A Habit Plane Shape Strain Direction
( 7.71, 6.30, 1 )bee [ 0.69, -0.72, 0.09 ]bee
(To1)bee ~ , 5.53° Orientation Relationship
( oo18
7,
Variant B ( 7.71, 6.30,'~ )bee [ 0.69, -0.72, -0.09 ]bee
(lOl)
>5.53 °
( 0018 18R,B
B
[ 111]b,c > 0 . 9 8 ° ['T10 ]18R,A
[~11 ]bee [ 1"~0] 1SR,B
0.98
The 24 variants of martensite consist of six plate groups, each of which consists of four habit plane variants which are near to and symmetrically clustered about the six {110}bcc poles of the parent phase (13, 14). Four variants of each plate group can form spear-, wedge- and fork-pair martensites (16, 17). The morphology and the diffraction patterns of the kink martensite in the Fe-Mn-AI-C alloy resemble those fork martensites in the Cu-ZnAI alloy (14). This implies that the crystallography of the kink- and fork-pair martensites are similar. It is known that the main driving force for a martensitic transformation on cooling is the chemical free energy difference between the parent and martensite, however, the nucleation and the growth of martensitic plates may also be controlled by the stress field induced from the cooling, especially, for the alloy quenched from a very high temperature. The kink morphology in Fig. 4(a) is quite similar to that in the Fe-7AI-2C as water-quenched from 1200°C (11) or in the melt-quenching duplex stainless steel (12). It is calculated that, in the Fe-Mn-AI-C alloy, if a variant A is preferably formed with a shape strain direction [0.69, -0.72, 0.09]b~, then the induced stress from a 1300°C water-quench may favor the growth of a variant B having a similar shape strain direction [0.69, -0.72, -0.09]~. After formation of the kink morphology in two variants, the total shape deformation will be lower than that of variant A alone. Conclusions A kink martensite form in a duplex Fe-Mn-AI-C alloy is found after a water-quench from 1300°C. The crystallography of the kink martensite is determined by transmission electron microscopy and is in good agreement with the calculated crystallography using the CRAB theory. The crystallography of the kink martensites is similar to that of fork-pair martensites in a Cu-Zn-AI alloy. The two variants, with perpendicular shear planes and nearly the same shape strain direction, are required to form this kink morphology. Acknowledgements The authors would like to acknowledge the National Science Council of R. O. C. for financial support under grant No. NSC81-0405-E007-531. The authors would also like to thank the Materials Science Center of National Tsing Hua University for their support.
Vol. 31, No. 6
KINKMARTENSITEIN Fe-Mn-AI-C
References 1. W. B. Lee, Master Thesis, Institute of Materials Science and Engineering, National Tsing Hua University, Taiwan, R.O.C. (1988). 2. K W. Chour, Master Thesis, Institute of Materials Science and Engineering, National Tsing Hua University, Taiwan, R.O.C. (1988). 3. K H. Hwang, W. S. Yang, T. B. Wu, C. M. Wan, and J. G. Byme, Acta Metall., 39, 825 (1991). 4. S. K. Chen, C. M. Wan, and J. G. Byrne, Scripta Metall., 24, 2139 (1990). 5. C. H. Chao and N. J. Ho, Scripta Metall., 26, 1863 (1992)~ 6. C. H. Chao and N. J. Ho, Scripta Metall., 27, 449 (1992). 7. C. H. Chao and N. J. Ho, Scripta Metall., 27, 493 (1992). 8. W. B. Lee, F. R. Chen, S. K. Chen, and C. M. Wan, to be published by Acta Metall.. 9. K. H. Hwang, S. K. Chen, W. S. Yang, T. B. Wu, C. M. Wan, and J. G. Byrne, Scripta Met,all., 24, 495 (1990). 10. K H. Hwang, C. M. Wan, and J. G. Byrne, Mater. Sci. Eng., A132, 161 (1991). 11. M. Watanabe and C. M. Wayman, Metall. Trans., 2, 2221 (1971). 12. T. Tomida, Y. Maehara, and Y. Ohmori, Mater. Trans. Jpn. Inst. Met., 30, 326 (1989). 13. M. Umemoto and C. M. Wayman, Acta Metall., 26, 1529 (1978). 14. T. Saburi and C. M. Wayman, Acta Metall., 27, 979 (1979). 15. A. G. Crocker, Journal De Phys., Colloque C4, supplement, 209 (1982). 16. T. Saburi, C. M. Wayman, K. Takata, and S. Nenno, Acta Metall., 28, 15 (1980). 17. T. A. Schroeder and C. M. Wayman. Acta Metall., 25, 1375 (1977).
Fig. 1 TEM micrograph of kink martensite.
697
698
KINK MARTENSITEIN Fe-Mn-AI-C
Vol. 31, No. 6
Fig. 2 Light micrograph of specimen water-quenched from 1300°C.
~Z
(a) fo) Fig. 3 TEM dark field image (a) and [IT0]IsRSADP ofllle martensite phase (b).
Vol. 31, No. 6
KINK MARTENSITE1N Fe-Mn-AI-C
(~)
(c)
(e)
699
(b)
(d)
(0
Fig. 4 TEM mierographs of kink martensite. (aX b) are bright field images and a superimposed SADP of martensite variants. (eXd) are dark field images and [010]IsRSADP of variant A. (eXf) are dark field images and [010] 181,,SADP of variant B.
700
K I N K M A R T E N S I T E IN Fe-Mn-AI-C
Fig. 5 Schematic representation of fig. 4(a).
Vol. 31, No. 6