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Modulated structures of Fe–10Mn–2Cr–1.5C alloy
Materials and Design 23 (2002) 717–720
Modulated structures of Fe–10Mn–2Cr–1.5C alloy Li Hea,b,*, Zhihao Jina, Jinde Lu, Jun Tangb a
b
Institute of Material Science and Engineering, Xi ‘Jiaotong University Xi’ Shaanxi, PR China Department of Metallurgical Engineering, Guizhou University of Technology, Guiyang, Guizhou, PR China Received 2 May 2002; accepted 7 August 2002
Abstract In Fe–10Mn–2Cr–1.5C alloy the superlattice diffraction spots and satellite reflections have been observed by transmission electron microscopy, these results show that the ordering structure and modulated structure have taken place in this alloy. X-ray diffraction proved that austenitic steel in this alloy is more stable than in traditional austenitic manganese steel. Based on this investigation, we consider that the C–Mn ordering clusters were existing in austenitic manganese steel and the chromium could strengthen this effect by linking the weaker C–Mn couples together. These structures may play an important role in the work hardening of austenitic manganese steel. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Austenitic manganese steel; Structure; Modulated structure; Work hardening
1. Introduction The high work hardening of austenitic manganese steel may be one of the most important properties for this material, but until now its mechanism remains uncertain. It was first thought that the hardening appeared with strain-induced martensite transformation of g to a or ´ w1,2x, but detailed investigation has shown that austenitic manganese steel of Hadfield composition is very stable during plastic strain w3x. Roberts attributed the strain hardening to deformation twinning w4x, while Adler considered the anomalous hardening of the Hadfield steel as a result of a higher hardness of pseudotwin phase w5x. Their models, however, failed to explain the flow stress at low and high temperatures w6x. Dastur thought that the dynamic strain aging (DSA) was the principal cause of work hardening in Hadfield steel w6x, but the ordering C–Mn couples, which were mentioned in Dastur’s theory, were short of clear structure image. In the present study, the superlattice diffraction spots and satellite reflections have been observed by transmis*Corresponding author. E-mail address: [email protected] (L. He).
sion electron microscopy in austenitic manganese steel, which contained 2%(mass) chromium. This may imply that the C–Mn short-range ordering structures are indeed existing in traditional austenitic manganese steel. When chromium is contained in this material, it can increase this effect and enlarge the ordering structures and therefore, transmission electron microscopy can detect the effect. 2. Experimental procedures The alloy for the present study was melted in an induction furnace. The chemical composition is 10.02% Mn; 1.58% C; 2.75% Cr; 0.51% Ni, 0.04% Re. The specimens were solution treated in a salt bath at 1373 K for 2 h and water-quenched, and then tempered at different temperatures, respectively. The structures of this alloy were characterized by Xray diffractometer with Cu-Ka radiation at 35 KV and 30 mA. The transmission electron microscopy specimens were sparks eroded in 0.15=10=10 mm3, and then electrothinned in a dual-jet electropolisherythinner, and foils were examined in transmission electron microscopy at 200 KV.
0261-3069/02/$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 1 - 3 0 6 9 Ž 0 2 . 0 0 0 7 2 - 9
L. He et al. / Materials and Design 23 (2002) 717–720
718
Fig. 1. Electron diffraction pattern, (a) showing w112x zone; (b) showing w111x zone.
3. Results and discussion Fig. 1 shows a transmission electron diffraction pattern in w112x and w111x zone. In this pattern the {110} and {021} superlattice reflections are marked with ‘x’. The diffuse superlattice reflections indicate that ordering structure took place in the present alloy. This is the result of chromium enlarging the ordering structures, which are contained in this alloy. Previous studies w7,9x, have shown that carbon atoms are located at the body center site of face-center cubic (f.c.c.) cells of austenitic manganese steel, and manganese atoms tend to occupy the face-center sites of these f.c.c. cells. The carbon and manganese atoms linked together, formed the ordering C–Mn couple, which plays an important role in the work hardening of austenitic manganese steel w6–11x. On average, there is approximately one manganese atom in two f.c.c. cells and one carbon atom in three f.c.c. cells for present composition. In ordering structure cluster, considering the composition fluctuation, we suggested there is one manganese atom and one carbon atom in an f.c.c. cell (Fig. 2). Because of this, maintaining a higher carbon content is important for the present alloy. In this model, atom distribution in a f.c.c. cell is as follows:
Fe 000; Mn or Cr C
0
11 ; 22
11 0 22
1 1 0 2 2
111 222
The structure factor Shkl of {110} and {021} are as follows, respectively: Shkl8fjexpi2pŽxjhqyjkqzjI. j
S110sfFeyfMnqfCf2.2 S021sfFeyfMnyfCf1.2 Here f Fe; f Mn; f C are atomic scattering factors of Fe; Mn; C, respectively, and (sinuyl)=10y8f0.5. The diffraction strength is in proportion to ZShklZ2. When these ordering structure clusters are big enough, diffuse superlattice reflections should be observed. For traditional Hadfield steel, the attractive force between manganese and carbon atoms is weak, the size of ordering structure is so small that diffuse superlattice reflection is too diffuse to be seen. In the present study, the attractive force between chromium and carbon is rela-
Fig. 2. Model of ordering structure cluster: (a) C–Mn couple, (b) Cr strengthening C–Mn(Cr) cluster.
L. He et al. / Materials and Design 23 (2002) 717–720
ls
Fig. 3. The bright field image of modulate structure in present alloy.
tively strong, then the chromium atoms become the center linking the weaker Mn–C couple nearby and forming bigger ordering clusters, and the diffuse superlattice reflections of {110} and {021} appeared in transmission electron diffraction pattern. On the other hand, ZS021Z2 y ZS110Z2f0.3 so the reflection of {110} is more easily observed. Fig. 1b is the transmission electron diffraction pattern in w111x zone, from here we can see the superlattice reflection spots of {110} crystal face also. Also from Fig. 1, we can see the satellite reflections around the 220 and 222 fundamental diffraction spot, which is characteristic of modulated structure diffraction. Fig. 3 is the bright field image of this modulated structure. The wavelength of modulated structure is approximately 4 nm, which is determined from the spacing between the satellites and the main spot of 222 using the Daniel–Lipson equation.
719
ha0tanu DuŽh2qk2ql2.
Where l is the average modulation wavelength, a0 is the lattice parameter of austenite, and u is the Bragg angle. Fig. 4 is the X-ray diffraction pattern. We can see that the austenite of traditional Hadfield steel is beginning to decompose when it is tempered at 673 K for 4 h, and at 793 K (4 h), the a-phase has become the major phase in this steel. In the present alloy, the austenite is still the major phase when tempered at 823 K for 4 h. These results indicate that in austenitic manganese steel chromium can strengthen the C–Mn couples and reduce the activity of carbon, so the austenite is more stable. In fact, Dastur and Leslie w6x have suggested the ordering Mn–C couples locking the dislocations, and this is a major factor contributing to rapid work hardening of Hadfield steel. But mentioned said above, the size of the ordering structure in Hadfield steel is so small that the ordering structure and modulated structures cannot be detected by transmission electron microscopy. In Fe–Mn–Al–C alloy (containing approx. 20% Mn, 8% Al and 1% C), some authors w7,12x have observed the ordering structure and modulated structures. In these Al atoms occupied the corner of f.c.c. cells, and made manganese atoms tend to occupy the face center of f.c.c. cells strongly, and as a result, the size of the ordering structure and modulated structures increased and can be observed by transmission electron microscopy. In the present alloy, the ordering clusters had been enlarged and strengthened by chromium atoms.
Fig. 4. Partial X-ray diffraction patterns: (a1–a4) traditional Hadfield steels quenched at 1050 8C and non-tempered (a1) , tempered at 400 8C for 4 h (a2), 520 8C for 4 h (a3) and 550 8C for 4 h (a4); (b1–b4) present alloys quenched at 1050 8C and non-tempered (b1), tempered at 400 8C for 4 h (b2), 520 8C for 4 h (b3) and 550 8C for 4 h (b4).
720
L. He et al. / Materials and Design 23 (2002) 717–720
Adjacent to these clusters alloy atoms depleted zones are formed. These effects caused the composition fluctuation tendency to increase and the activity of carbon in austenite to decrease, promoting the modulated structures formed. All of these impelled the ordering structure and modulated structures to be observable. To improve the wear resistance of austenitic manganese steel, Dastur et al. w6x have recommended reducing the activity of carbon by the addition of another substitutional solute. Our previous studies w13,14x have shown that the gouging wear resistance of the present alloy was higher than traditional austenitic manganese steel obviously. So we considered that chromium could strengthen the ordering structure and modulated structures in the austenitic manganese steel. This structure caused an important influence on work hardening behavior of austenitic manganese steel. 4. Conclusions In this study, the superlattice spots and satellite reflections have been observed in Fe–10Mn–2Cr–1.5C alloy with transmission electron microscopy. It is shown that the ordering structure and modulated structure have taken place in this alloy. This is because the C–Mn ordering clusters in austenitic manganese steel have been strengthened and enlarged by chromium. X-ray diffraction has proved that the austenitic in this alloy is more stable than traditional Hadfield steel. Therefore, we consider that the C–Mn ordering clusters exist in austenitic manganese steel, and chromium can promote
their growth. This structure may be a major factor contributing to work hardening of austenitic manganese steel. Acknowledgments The authors are grateful to Mr Yaguo Li for his assistance in TEM operation, and also to Mr Xiaolin Chen, Jian Fu, Ms Yujiao Wu and Ms Jun Tang for their assistance in specimen preparation. References w1x Collette G, Crussard C, Kohn A, Plateau J, Weisz M. Rev. Metall. 1957;54:433. w2x Cahn RW. The Encyclopedia of Ignorance. NY: Pergamon Press, 1977. p. 140. w3x Doepken CH, White CH, Honeycombe RW. J. Iron Steel Inst. 1962;200:457. w4x Roberts WN. Trans. TMS-AIME 1964;230:372. w5x Adler PH, Olson GB, Owen WS. Metall. Trans. A 1986;17A:1725. w6x Dastur YN, Leslie WC. Metall. Trans. A 1981;12A:749. w7x Choo WK, Kim JH, Yoon JC. Acta Mater. 1997;45:4877. w8x Ruifu Z, Yupeng L, Shitong L, Zhiqiang C, Shiqing W. Sci. China (E) (in Chinese) 1997;27:193. w9x Li H, Zhihao J, et al. Iron Steel (in Chinese) 2000;35:48. w10x Tjong SC, Zhu SM. Mater. Trans. JIM 1997;38’:112. w11x Shun TS, Wan CM, Byrne JG. Scripta Metall. 1991;25:1769. w12x Han KH, Yoon JC, Choo WK. Scripta Metall. 1986;20:33. w13x Li H, Zhihao J, Jinde L. Mater. Mech. Eng. (in Chinese) 2000;24:22. w14x Li H, Zhihao J, Jinde L, et al. Trans. Met. Heat Treatment (in Chinese) 2000;21:51.
Modulated structures of Fe–10Mn–2Cr–1.5C alloy Li Hea,b,*, Zhihao Jina, Jinde Lu, Jun Tangb a
b
Institute of Material Science and Engineering, Xi ‘Jiaotong University Xi’ Shaanxi, PR China Department of Metallurgical Engineering, Guizhou University of Technology, Guiyang, Guizhou, PR China Received 2 May 2002; accepted 7 August 2002
Abstract In Fe–10Mn–2Cr–1.5C alloy the superlattice diffraction spots and satellite reflections have been observed by transmission electron microscopy, these results show that the ordering structure and modulated structure have taken place in this alloy. X-ray diffraction proved that austenitic steel in this alloy is more stable than in traditional austenitic manganese steel. Based on this investigation, we consider that the C–Mn ordering clusters were existing in austenitic manganese steel and the chromium could strengthen this effect by linking the weaker C–Mn couples together. These structures may play an important role in the work hardening of austenitic manganese steel. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Austenitic manganese steel; Structure; Modulated structure; Work hardening
1. Introduction The high work hardening of austenitic manganese steel may be one of the most important properties for this material, but until now its mechanism remains uncertain. It was first thought that the hardening appeared with strain-induced martensite transformation of g to a or ´ w1,2x, but detailed investigation has shown that austenitic manganese steel of Hadfield composition is very stable during plastic strain w3x. Roberts attributed the strain hardening to deformation twinning w4x, while Adler considered the anomalous hardening of the Hadfield steel as a result of a higher hardness of pseudotwin phase w5x. Their models, however, failed to explain the flow stress at low and high temperatures w6x. Dastur thought that the dynamic strain aging (DSA) was the principal cause of work hardening in Hadfield steel w6x, but the ordering C–Mn couples, which were mentioned in Dastur’s theory, were short of clear structure image. In the present study, the superlattice diffraction spots and satellite reflections have been observed by transmis*Corresponding author. E-mail address: [email protected] (L. He).
sion electron microscopy in austenitic manganese steel, which contained 2%(mass) chromium. This may imply that the C–Mn short-range ordering structures are indeed existing in traditional austenitic manganese steel. When chromium is contained in this material, it can increase this effect and enlarge the ordering structures and therefore, transmission electron microscopy can detect the effect. 2. Experimental procedures The alloy for the present study was melted in an induction furnace. The chemical composition is 10.02% Mn; 1.58% C; 2.75% Cr; 0.51% Ni, 0.04% Re. The specimens were solution treated in a salt bath at 1373 K for 2 h and water-quenched, and then tempered at different temperatures, respectively. The structures of this alloy were characterized by Xray diffractometer with Cu-Ka radiation at 35 KV and 30 mA. The transmission electron microscopy specimens were sparks eroded in 0.15=10=10 mm3, and then electrothinned in a dual-jet electropolisherythinner, and foils were examined in transmission electron microscopy at 200 KV.
0261-3069/02/$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 1 - 3 0 6 9 Ž 0 2 . 0 0 0 7 2 - 9
L. He et al. / Materials and Design 23 (2002) 717–720
718
Fig. 1. Electron diffraction pattern, (a) showing w112x zone; (b) showing w111x zone.
3. Results and discussion Fig. 1 shows a transmission electron diffraction pattern in w112x and w111x zone. In this pattern the {110} and {021} superlattice reflections are marked with ‘x’. The diffuse superlattice reflections indicate that ordering structure took place in the present alloy. This is the result of chromium enlarging the ordering structures, which are contained in this alloy. Previous studies w7,9x, have shown that carbon atoms are located at the body center site of face-center cubic (f.c.c.) cells of austenitic manganese steel, and manganese atoms tend to occupy the face-center sites of these f.c.c. cells. The carbon and manganese atoms linked together, formed the ordering C–Mn couple, which plays an important role in the work hardening of austenitic manganese steel w6–11x. On average, there is approximately one manganese atom in two f.c.c. cells and one carbon atom in three f.c.c. cells for present composition. In ordering structure cluster, considering the composition fluctuation, we suggested there is one manganese atom and one carbon atom in an f.c.c. cell (Fig. 2). Because of this, maintaining a higher carbon content is important for the present alloy. In this model, atom distribution in a f.c.c. cell is as follows:
Fe 000; Mn or Cr C
0
11 ; 22
11 0 22
1 1 0 2 2
111 222
The structure factor Shkl of {110} and {021} are as follows, respectively: Shkl8fjexpi2pŽxjhqyjkqzjI. j
S110sfFeyfMnqfCf2.2 S021sfFeyfMnyfCf1.2 Here f Fe; f Mn; f C are atomic scattering factors of Fe; Mn; C, respectively, and (sinuyl)=10y8f0.5. The diffraction strength is in proportion to ZShklZ2. When these ordering structure clusters are big enough, diffuse superlattice reflections should be observed. For traditional Hadfield steel, the attractive force between manganese and carbon atoms is weak, the size of ordering structure is so small that diffuse superlattice reflection is too diffuse to be seen. In the present study, the attractive force between chromium and carbon is rela-
Fig. 2. Model of ordering structure cluster: (a) C–Mn couple, (b) Cr strengthening C–Mn(Cr) cluster.
L. He et al. / Materials and Design 23 (2002) 717–720
ls
Fig. 3. The bright field image of modulate structure in present alloy.
tively strong, then the chromium atoms become the center linking the weaker Mn–C couple nearby and forming bigger ordering clusters, and the diffuse superlattice reflections of {110} and {021} appeared in transmission electron diffraction pattern. On the other hand, ZS021Z2 y ZS110Z2f0.3 so the reflection of {110} is more easily observed. Fig. 1b is the transmission electron diffraction pattern in w111x zone, from here we can see the superlattice reflection spots of {110} crystal face also. Also from Fig. 1, we can see the satellite reflections around the 220 and 222 fundamental diffraction spot, which is characteristic of modulated structure diffraction. Fig. 3 is the bright field image of this modulated structure. The wavelength of modulated structure is approximately 4 nm, which is determined from the spacing between the satellites and the main spot of 222 using the Daniel–Lipson equation.
719
ha0tanu DuŽh2qk2ql2.
Where l is the average modulation wavelength, a0 is the lattice parameter of austenite, and u is the Bragg angle. Fig. 4 is the X-ray diffraction pattern. We can see that the austenite of traditional Hadfield steel is beginning to decompose when it is tempered at 673 K for 4 h, and at 793 K (4 h), the a-phase has become the major phase in this steel. In the present alloy, the austenite is still the major phase when tempered at 823 K for 4 h. These results indicate that in austenitic manganese steel chromium can strengthen the C–Mn couples and reduce the activity of carbon, so the austenite is more stable. In fact, Dastur and Leslie w6x have suggested the ordering Mn–C couples locking the dislocations, and this is a major factor contributing to rapid work hardening of Hadfield steel. But mentioned said above, the size of the ordering structure in Hadfield steel is so small that the ordering structure and modulated structures cannot be detected by transmission electron microscopy. In Fe–Mn–Al–C alloy (containing approx. 20% Mn, 8% Al and 1% C), some authors w7,12x have observed the ordering structure and modulated structures. In these Al atoms occupied the corner of f.c.c. cells, and made manganese atoms tend to occupy the face center of f.c.c. cells strongly, and as a result, the size of the ordering structure and modulated structures increased and can be observed by transmission electron microscopy. In the present alloy, the ordering clusters had been enlarged and strengthened by chromium atoms.
Fig. 4. Partial X-ray diffraction patterns: (a1–a4) traditional Hadfield steels quenched at 1050 8C and non-tempered (a1) , tempered at 400 8C for 4 h (a2), 520 8C for 4 h (a3) and 550 8C for 4 h (a4); (b1–b4) present alloys quenched at 1050 8C and non-tempered (b1), tempered at 400 8C for 4 h (b2), 520 8C for 4 h (b3) and 550 8C for 4 h (b4).
720
L. He et al. / Materials and Design 23 (2002) 717–720
Adjacent to these clusters alloy atoms depleted zones are formed. These effects caused the composition fluctuation tendency to increase and the activity of carbon in austenite to decrease, promoting the modulated structures formed. All of these impelled the ordering structure and modulated structures to be observable. To improve the wear resistance of austenitic manganese steel, Dastur et al. w6x have recommended reducing the activity of carbon by the addition of another substitutional solute. Our previous studies w13,14x have shown that the gouging wear resistance of the present alloy was higher than traditional austenitic manganese steel obviously. So we considered that chromium could strengthen the ordering structure and modulated structures in the austenitic manganese steel. This structure caused an important influence on work hardening behavior of austenitic manganese steel. 4. Conclusions In this study, the superlattice spots and satellite reflections have been observed in Fe–10Mn–2Cr–1.5C alloy with transmission electron microscopy. It is shown that the ordering structure and modulated structure have taken place in this alloy. This is because the C–Mn ordering clusters in austenitic manganese steel have been strengthened and enlarged by chromium. X-ray diffraction has proved that the austenitic in this alloy is more stable than traditional Hadfield steel. Therefore, we consider that the C–Mn ordering clusters exist in austenitic manganese steel, and chromium can promote
their growth. This structure may be a major factor contributing to work hardening of austenitic manganese steel. Acknowledgments The authors are grateful to Mr Yaguo Li for his assistance in TEM operation, and also to Mr Xiaolin Chen, Jian Fu, Ms Yujiao Wu and Ms Jun Tang for their assistance in specimen preparation. References w1x Collette G, Crussard C, Kohn A, Plateau J, Weisz M. Rev. Metall. 1957;54:433. w2x Cahn RW. The Encyclopedia of Ignorance. NY: Pergamon Press, 1977. p. 140. w3x Doepken CH, White CH, Honeycombe RW. J. Iron Steel Inst. 1962;200:457. w4x Roberts WN. Trans. TMS-AIME 1964;230:372. w5x Adler PH, Olson GB, Owen WS. Metall. Trans. A 1986;17A:1725. w6x Dastur YN, Leslie WC. Metall. Trans. A 1981;12A:749. w7x Choo WK, Kim JH, Yoon JC. Acta Mater. 1997;45:4877. w8x Ruifu Z, Yupeng L, Shitong L, Zhiqiang C, Shiqing W. Sci. China (E) (in Chinese) 1997;27:193. w9x Li H, Zhihao J, et al. Iron Steel (in Chinese) 2000;35:48. w10x Tjong SC, Zhu SM. Mater. Trans. JIM 1997;38’:112. w11x Shun TS, Wan CM, Byrne JG. Scripta Metall. 1991;25:1769. w12x Han KH, Yoon JC, Choo WK. Scripta Metall. 1986;20:33. w13x Li H, Zhihao J, Jinde L. Mater. Mech. Eng. (in Chinese) 2000;24:22. w14x Li H, Zhihao J, Jinde L, et al. Trans. Met. Heat Treatment (in Chinese) 2000;21:51.