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A repetitive thermomechanical process to produce nano-crystalline in a metastable austenitic steel
Scripta Materialia 52 (2005) 1311–1315 www.actamat-journals.com
A repetitive thermomechanical process to produce nano-crystalline in a metastable austenitic steel Yunqing Ma, Jae-Eun Jin, Young-Kook Lee
*
Department of Metallurgical Engineering, Yonsei University, Shinchon-dong 134, Seodaemun-gu, Seoul 120-749, Korea Received 27 December 2004; received in revised form 7 February 2005; accepted 21 February 2005 Available online 21 March 2005
Abstract Nano-crystalline grains of about 100 nm were obtained in a metastable austenitic steel by a repetitive thermomechanical process consisting of conventional cold rolling and annealing. The nano-grained austenite was transformed on annealing from the straininduced martensite, which had formed during cold rolling. The nano-structured austenitic steel exhibits not only high strength (above 1 GPa) but also good elongation (above 30%). 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Nano-crystalline; Reverse transformation; Metastable austenite; Grain refinement
1. Introduction In recent years severe plastic deformation (SPD) processes, such as high pressure torsion (HPT), equal channel angular pressing (ECAP), and accumulative roll-bonding [1–4], have been developed to produce bulk nano-crystalline materials. The SPD processes have successfully refined the coarse grains of pure metals and alloys to the grain size of a few tens to a few hundreds of nanometers. The nano-grained materials revealed very high strengths as expected. However, their elongation, especially uniform elongation, was dramatically decreased to only a few per cent because of the low strain hardening in nano-sized grains [5,6]. In addition, the present SPD processes still need some improvements in terms of materials dimensions, mass production, sample preparation, etc. Some researchers have tried to fabricate nano-crystallines of 200–500 nm size by using a reverse transformation from strain-induced a 0 -martensite to c-austenite *
Corresponding author. Tel.: +82 2 2123 2831; fax: +82 2 312 5375. E-mail address: [email protected] (Y.-K. Lee).
in metastable austenitic stainless steels [7,8]. The process is characterized by heavy cold rolling to induce the c to a 0 -martensitic transformation, followed by annealing for the reverse transformation of strain-induced a 0 to c. The nano-crystalline exhibits both high strength and good uniform elongation, resulting from enhanced work hardening ability through the straininduced martensitic transformation during tensile tests [7,8]. As the process is carried out only by conventional cold rolling and annealing, it seems more appropriate for large-sized sheets, and has more potential for actual applications, as compared with other SPD techniques. It is wondered, however, whether a repetitive application of the thermomechanical process, as schematically shown in Fig. 1, can reduce the grain size below 200 nm, and whether the chemical composition can be extended beyond that of stainless steel, which is normally defined as a steel containing Cr at more than 12 wt%. The mechanical properties of the resultant nano-crystalline are also of great interest. The present study has been performed to address these issues.
1359-6462/$ - see front matter 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2005.02.018
1312
Y. Ma et al. / Scripta Materialia 52 (2005) 1311–1315
Temperature
Solution Treatment
First cycle
Second cycle
Reverse Transformation Af As
Reverse Transformation
Cold Rolling
Cold Rolling
Tensile tests were performed at ambient temperature using an Instron 1127 machine at a crosshead speed of 2 mm/min. The tensile direction was parallel to the rolling direction. The size of the gauge part of the tensile specimen after the repetitive thermomechanical treatment was 6 mm wide, 1.0 mm thick, and 15 mm long according to the relationship of L0 = 5.65 · A1/2 [9], where A is the cross-sectional area and L0 is the length of the gauge part. The gauge length of the first reverse transformed specimen was 20 mm because the specimen thickness was about 2.2 mm then.
Time
The microstructural changes investigated by X-ray diffraction at each stage of the repetitive thermomechanical process are shown in Fig. 2. The microstructure of the solution-treated specimen consists of a mixture of c-austenite (face-centered cubic), a 0 -martensite (bodycentered cubic), and a little e-martensite (hexagonal close-packed), respectively. This means that the martensitic transformation start temperature (Ms) of the solution-treated specimen is higher than the room temperature. After the first cold rolling, as shown by the X-ray pattern (B) in Fig. 2, the microstructure changed to fully a 0 -martensite due to the heavy cold rolling. After the first annealing, the a 0 -martensite reversely transformed to c-austenite, and no peaks of martensite could be monitored from the pattern (C) of Fig. 2. This indicates that Ms temperature has dropped to below room
α '(110)
(E)
γ (111)
(D)
(C)
50
60
80
90
γ (222)
α '(200)
70
γ (311)
40
α '(211)
30
γ (220)
(A)
ε (10 2)
γ (200)
(B)
ε(10 1)
An Fe-0.1%C-10%Cr-5%Ni-8%Mn alloy in nominal composition was prepared using a high frequency vacuum induction furnace. The actual chemical composition of the alloy is listed in Table 1. The Cr content of the alloy is less than 12%, which is usually regarded as a minimum Cr concentration for stainless steels. The Ni content is half of the previous one [7,8] and replaced by Mn to reduce the material cost. The ingot was homogenized at 1200 C for 12 h under a protective atmosphere, hot-rolled to the plates of 10 mm thick, and followed by solution treatment at 1200 C for 30 min. Using the plates, the repetitive thermomechanical processes were performed, as shown in Fig. 1. The reverse transformation start and finish temperatures (As and Af) were determined from dilatational curves measured at the rate of 10 C/s during the heatup of the cold-rolled sheets. The annealing for the reverse transformation was carried out at a temperature of Af+10 C for 10 min using a salt bath. The reduction in thickness of the first cold rolling was about 75% and the annealing temperature was 640 C. These two parameters were 50% and 630 C, respectively, for the second cycle. The microstructures were observed using a transmission electron microscope (TEM). Thin foils for TEM investigation were jet-polished in a solution of 10% perchloric acid +90% ethanol at 40 C, and observed in a JEM 2000 EX operating at 160 KV. The phase components were identified by using X-ray diffraction with Cu Ka radiation.
3.1. Microstructure
ε(10 0)
2. Experimental
3. Results and discussion
Intensity
Fig. 1. A schematic illustration of a repetitive thermomechanical process to obtain the nano-crystalline in a metastable austenitic steel. As and Af are the reverse transformation start and finish temperatures, respectively.
100
2theta (deg)
Table 1 Chemical composition of the steel used (wt%) C
Ni
Cr
Mn
Fe
0.078
4.92
9.94
7.72
Balance
Fig. 2. Microstructural changes during the repetitive thermomechanical process: (A) after solution treatment; (B) after the first cold rolling; (C) after the first annealing; (D) after the second cold rolling; and (E) after the second annealing.
Y. Ma et al. / Scripta Materialia 52 (2005) 1311–1315
temperature after the first cycle due to the increased austenite stability caused by grain refinement. After the second cold rolling (X-ray pattern (D) in Fig. 2), the sample is mostly a 0 -martensite with a small amount
1313
of retained c-austenite. After the second annealing, the specimen revealed single c-austenite phase again. The X-ray diffraction patterns of two cold rolled specimens show peak broadening due to the strong internal stress.
Fig. 3. TEM microstructures and corresponding SAED patterns during the repetitive thermomechanical process: (A) after the first cold rolling; (B) SAED pattern of (A); (C) after the first annealing; (D) SAED pattern of (C); (E) after the second cold rolling; (F) SAED pattern of (E); (G) after the second annealing; and (H) SAED pattern of (G).
Y. Ma et al. / Scripta Materialia 52 (2005) 1311–1315
3.2. Mechanical properties Fig. 4 shows the change in hardness during the repetitive thermomechanical process. Solution treatment state shows the lowest hardness of HRC 14.1, and this value dramatically increases to HRC 51.6 after the first
70 60
51.6
Hardness (HRC)
Fig. 3 shows the TEM microstructures of the specimens taken at each stage of the repetitive thermomechanical process. The selected area electron diffraction (SAED) pattern was also taken from the center of the bright field image using an aperture with a diameter of 1 lm. After the first cold rolling, the TEM micrograph (Fig. 3(A)) shows typical lath martensite with a high density of dislocations. The width of the martensitic lath is 200–300 nm. The SAED pattern (Fig. 3(B)) shows a complete ring of a 0 -martensite, which is consistent with the result of X-ray diffraction. After the first annealing (Fig. 3(C) and (D)), the microstructure consisted of nearly equiaxed c-grains with the average size of about 300 nm and the grain boundaries are distinct because c-austenite grains newly nucleated and grew in the strain-induced a 0 -martensite during the reverse transformation. The microstructure is clearly different from those fabricated by other SPD techniques. For example, the grain boundaries in nano-crystallines made by HPT and ECAP are generally curved or wavy, and poorly defined [1,10]. The residual internal stress in the reversed austenite is certainly less than that in the specimens fabricated by other SPD techniques. After the second cold rolling, consistent with the results of X-ray diffraction, the SAED pattern (Fig. 3(F)) shows a mixture of the strain-induced a 0 -martensite and retained c austenite, because of the lack of thickness reduction (50%) during the second cold rolling and the increased austenite stability. The martensite laths (Fig. 3(E)) seem much finer than those of the first cold rolled sample (Fig. 3(A)) because of the finer austenite grain size before cold rolling. After the second annealing, the grain size is successfully reduced further, as shown in Fig. 3(G). However, the homogeneity of the grain sizes seems to be decreased as compared with that of the grains after the first annealing (Fig. 3(C)). Some grains are even smaller than 100 nm while others are a little larger than 200 nm. The SAED pattern also reveals fully c phase, which is the same as Fig. 3(D), and relatively more uniform diffraction rings, indicative of finer grain size. Considering overall microstructural changes during the repetitive thermomechanical process, there is an obvious relationship between the width of the martensite lath and the austenite grain size. The smaller the initial c austenite grain size before cold rolling, the narrower the width of martensite lath, and the finer are the reversed c austenite grains after annealing.
50.0
50 40
32.4
30.4 30 20
14.1
10 0
A
C
B
D
E
Fig. 4. The Rockwell hardness (C scale) of the samples taken at different stages during the repetitive thermomechanical process: (A) after solution treatment; (B) after the first cold rolling; (C) after the first annealing; (D) after the second cold rolling; and (E) after the second annealing.
1200 1000
Stress (MPa)
1314
(C)
800
(B) 600
(A) 400 200 0 0
5
10
15
20
25
30
35
40
45
50
Strain (%) Fig. 5. The engineering tensile stress–strain curves of the samples taken at different stages during the repetitive thermomechanical process: (A) after solution treatment; (B) after the first annealing; and (C) after the second annealing.
cold rolling, due to the strain-induced a 0 -martensite. The first annealed specimen has a hardness of HRC 30.4, which is much higher than that of the solution treated one because of the fine austenite grains. After the second cold rolling, the hardness is HRC 50.0. This value is a little lower than that of the first cold rolled sample, probably due to the existence of some amount of retained c austenite, which is confirmed by the previous X-ray and TEM results. The finer grains, obtained after the second annealing, possess higher hardness (HRC 32.4) than that of the first annealed specimen. The engineering stresss–strain curves of the nanocrystalline steels are shown together with that of the
Y. Ma et al. / Scripta Materialia 52 (2005) 1311–1315 Table 2 Tensile properties of the specimens measured at different stages during the thermomechanical treatment Stage
Yield strength (MPa)
Tensile strength (MPa)
Uniform elongation (%)
Total elongation (%)
After solution treatment After the first annealing After the second annealing
120
1041
36.5
38.9
708
1070
32.6
36.0
779
1102
28.0
32.0
solution treated sample in Fig. 5 and the major tensile properties of the steels are listed in Table 2. The solution treated sample shows low yield strength (120 MPa) and large elongation (38.9%). After the first annealing, the yield strength greatly increased to 708 MPa, which is nearly six times that of the solution treated one, without a great loss of the elongation (36.0%). The second annealing increased both yield and tensile strengths further to 779 and 1102 MPa, respectively, probably due to the finer grains. The elongation slightly decreased (32.0%), but it is still much greater than those of the nano-crystalline materials reported before [5,6]. Previous research has shown that it is normal for nano-crystalline materials to show much higher strength than their coarse-grained counterparts, as expected from the well-known Hall–Petch relationship. But the elongation of the most nano-crystalline materials is greatly decreased due to little work hardening ability, which greatly reduces the uniform elongation prior to necking under uniaxial tension [11,12]. In the case of the metastable austenitic steel studied here, the great work hardening ability is obtained through the strain-induced martensitic transformation during tensile tests, leading to a good uniform elongation as well as high strength [8].
4. Conclusions A metastable austensitic steel, containing less Cr and Ni, and higher Mn contents than those of typical austen-
1315
itic stainless steels, repeatedly underwent a cold rolling and annealing process. During the repetitive thermomechanical process, c austenite decomposed to a 0 -martensite on cold rolling and its reverse transformation to austenite occurred during annealing just above the austenite finish temperature. The repetitive thermomechanical treatment generated nano-crystalline austenite grains of about 100 nm, possessing an excellent combination of high strength and good elongation. This study reveals the possibilities for the new thermomechanical process and extended alloy chemistry for fabricating the bulk nano-crystalline material with high strength and good elongation. This technique also seems to have great advantages for practical applications, especially for large-sized sheets, as compared with previous SPD processes.
Acknowledgement This work was financially supported by the Ministry of Commerce, Industry and Energy of Korea. The authors acknowledge the support.
References [1] Valiev RZ, Islamgaliev RK, Alexandrov IV. Prog Mater Sci 2000;45:103. [2] Zhu YT, Jiang HG, Huang JY, Lowe TC. Metall Mater Trans 2001;32A:1559. [3] Saito Y, Tsuji N, Utsunomiya H, Sakai T, Hong RG. Scripta Mater 1998;39:1221. [4] Saito Y, Utsunomiya H, Tsuji N, Sakai T. Acta Mater 1999;47:579. [5] Jia D, Wang YM, Ramesh KT, Ma E, Zhu YT, Valiev RZ. Appl Phys Lett 2001;79:611. [6] Wang YM, Ma E. Mater Sci Eng 2004;A375–377:46. [7] Tomimura K, Takaki S, Tokunaga Y. ISIJ Int 1991;31:1431. [8] Kwon O, Lee YK. J Korean Inst Metals Mater 1994;32:958. [9] Dieter GE. Mechanical metallurgy. New York: McGraw-Hill; 1986. p. 293. [10] ChuvilÕdeev VN, Kopylov VI, Zeiger W. Ann Chim Sci Mat 2002;27:55. [11] Valiev RZ. Mater Sci Eng 1997;A234–236:59. [12] Ma E. Scripta Mater 2003;49:663.
A repetitive thermomechanical process to produce nano-crystalline in a metastable austenitic steel Yunqing Ma, Jae-Eun Jin, Young-Kook Lee
*
Department of Metallurgical Engineering, Yonsei University, Shinchon-dong 134, Seodaemun-gu, Seoul 120-749, Korea Received 27 December 2004; received in revised form 7 February 2005; accepted 21 February 2005 Available online 21 March 2005
Abstract Nano-crystalline grains of about 100 nm were obtained in a metastable austenitic steel by a repetitive thermomechanical process consisting of conventional cold rolling and annealing. The nano-grained austenite was transformed on annealing from the straininduced martensite, which had formed during cold rolling. The nano-structured austenitic steel exhibits not only high strength (above 1 GPa) but also good elongation (above 30%). 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Nano-crystalline; Reverse transformation; Metastable austenite; Grain refinement
1. Introduction In recent years severe plastic deformation (SPD) processes, such as high pressure torsion (HPT), equal channel angular pressing (ECAP), and accumulative roll-bonding [1–4], have been developed to produce bulk nano-crystalline materials. The SPD processes have successfully refined the coarse grains of pure metals and alloys to the grain size of a few tens to a few hundreds of nanometers. The nano-grained materials revealed very high strengths as expected. However, their elongation, especially uniform elongation, was dramatically decreased to only a few per cent because of the low strain hardening in nano-sized grains [5,6]. In addition, the present SPD processes still need some improvements in terms of materials dimensions, mass production, sample preparation, etc. Some researchers have tried to fabricate nano-crystallines of 200–500 nm size by using a reverse transformation from strain-induced a 0 -martensite to c-austenite *
Corresponding author. Tel.: +82 2 2123 2831; fax: +82 2 312 5375. E-mail address: [email protected] (Y.-K. Lee).
in metastable austenitic stainless steels [7,8]. The process is characterized by heavy cold rolling to induce the c to a 0 -martensitic transformation, followed by annealing for the reverse transformation of strain-induced a 0 to c. The nano-crystalline exhibits both high strength and good uniform elongation, resulting from enhanced work hardening ability through the straininduced martensitic transformation during tensile tests [7,8]. As the process is carried out only by conventional cold rolling and annealing, it seems more appropriate for large-sized sheets, and has more potential for actual applications, as compared with other SPD techniques. It is wondered, however, whether a repetitive application of the thermomechanical process, as schematically shown in Fig. 1, can reduce the grain size below 200 nm, and whether the chemical composition can be extended beyond that of stainless steel, which is normally defined as a steel containing Cr at more than 12 wt%. The mechanical properties of the resultant nano-crystalline are also of great interest. The present study has been performed to address these issues.
1359-6462/$ - see front matter 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2005.02.018
1312
Y. Ma et al. / Scripta Materialia 52 (2005) 1311–1315
Temperature
Solution Treatment
First cycle
Second cycle
Reverse Transformation Af As
Reverse Transformation
Cold Rolling
Cold Rolling
Tensile tests were performed at ambient temperature using an Instron 1127 machine at a crosshead speed of 2 mm/min. The tensile direction was parallel to the rolling direction. The size of the gauge part of the tensile specimen after the repetitive thermomechanical treatment was 6 mm wide, 1.0 mm thick, and 15 mm long according to the relationship of L0 = 5.65 · A1/2 [9], where A is the cross-sectional area and L0 is the length of the gauge part. The gauge length of the first reverse transformed specimen was 20 mm because the specimen thickness was about 2.2 mm then.
Time
The microstructural changes investigated by X-ray diffraction at each stage of the repetitive thermomechanical process are shown in Fig. 2. The microstructure of the solution-treated specimen consists of a mixture of c-austenite (face-centered cubic), a 0 -martensite (bodycentered cubic), and a little e-martensite (hexagonal close-packed), respectively. This means that the martensitic transformation start temperature (Ms) of the solution-treated specimen is higher than the room temperature. After the first cold rolling, as shown by the X-ray pattern (B) in Fig. 2, the microstructure changed to fully a 0 -martensite due to the heavy cold rolling. After the first annealing, the a 0 -martensite reversely transformed to c-austenite, and no peaks of martensite could be monitored from the pattern (C) of Fig. 2. This indicates that Ms temperature has dropped to below room
α '(110)
(E)
γ (111)
(D)
(C)
50
60
80
90
γ (222)
α '(200)
70
γ (311)
40
α '(211)
30
γ (220)
(A)
ε (10 2)
γ (200)
(B)
ε(10 1)
An Fe-0.1%C-10%Cr-5%Ni-8%Mn alloy in nominal composition was prepared using a high frequency vacuum induction furnace. The actual chemical composition of the alloy is listed in Table 1. The Cr content of the alloy is less than 12%, which is usually regarded as a minimum Cr concentration for stainless steels. The Ni content is half of the previous one [7,8] and replaced by Mn to reduce the material cost. The ingot was homogenized at 1200 C for 12 h under a protective atmosphere, hot-rolled to the plates of 10 mm thick, and followed by solution treatment at 1200 C for 30 min. Using the plates, the repetitive thermomechanical processes were performed, as shown in Fig. 1. The reverse transformation start and finish temperatures (As and Af) were determined from dilatational curves measured at the rate of 10 C/s during the heatup of the cold-rolled sheets. The annealing for the reverse transformation was carried out at a temperature of Af+10 C for 10 min using a salt bath. The reduction in thickness of the first cold rolling was about 75% and the annealing temperature was 640 C. These two parameters were 50% and 630 C, respectively, for the second cycle. The microstructures were observed using a transmission electron microscope (TEM). Thin foils for TEM investigation were jet-polished in a solution of 10% perchloric acid +90% ethanol at 40 C, and observed in a JEM 2000 EX operating at 160 KV. The phase components were identified by using X-ray diffraction with Cu Ka radiation.
3.1. Microstructure
ε(10 0)
2. Experimental
3. Results and discussion
Intensity
Fig. 1. A schematic illustration of a repetitive thermomechanical process to obtain the nano-crystalline in a metastable austenitic steel. As and Af are the reverse transformation start and finish temperatures, respectively.
100
2theta (deg)
Table 1 Chemical composition of the steel used (wt%) C
Ni
Cr
Mn
Fe
0.078
4.92
9.94
7.72
Balance
Fig. 2. Microstructural changes during the repetitive thermomechanical process: (A) after solution treatment; (B) after the first cold rolling; (C) after the first annealing; (D) after the second cold rolling; and (E) after the second annealing.
Y. Ma et al. / Scripta Materialia 52 (2005) 1311–1315
temperature after the first cycle due to the increased austenite stability caused by grain refinement. After the second cold rolling (X-ray pattern (D) in Fig. 2), the sample is mostly a 0 -martensite with a small amount
1313
of retained c-austenite. After the second annealing, the specimen revealed single c-austenite phase again. The X-ray diffraction patterns of two cold rolled specimens show peak broadening due to the strong internal stress.
Fig. 3. TEM microstructures and corresponding SAED patterns during the repetitive thermomechanical process: (A) after the first cold rolling; (B) SAED pattern of (A); (C) after the first annealing; (D) SAED pattern of (C); (E) after the second cold rolling; (F) SAED pattern of (E); (G) after the second annealing; and (H) SAED pattern of (G).
Y. Ma et al. / Scripta Materialia 52 (2005) 1311–1315
3.2. Mechanical properties Fig. 4 shows the change in hardness during the repetitive thermomechanical process. Solution treatment state shows the lowest hardness of HRC 14.1, and this value dramatically increases to HRC 51.6 after the first
70 60
51.6
Hardness (HRC)
Fig. 3 shows the TEM microstructures of the specimens taken at each stage of the repetitive thermomechanical process. The selected area electron diffraction (SAED) pattern was also taken from the center of the bright field image using an aperture with a diameter of 1 lm. After the first cold rolling, the TEM micrograph (Fig. 3(A)) shows typical lath martensite with a high density of dislocations. The width of the martensitic lath is 200–300 nm. The SAED pattern (Fig. 3(B)) shows a complete ring of a 0 -martensite, which is consistent with the result of X-ray diffraction. After the first annealing (Fig. 3(C) and (D)), the microstructure consisted of nearly equiaxed c-grains with the average size of about 300 nm and the grain boundaries are distinct because c-austenite grains newly nucleated and grew in the strain-induced a 0 -martensite during the reverse transformation. The microstructure is clearly different from those fabricated by other SPD techniques. For example, the grain boundaries in nano-crystallines made by HPT and ECAP are generally curved or wavy, and poorly defined [1,10]. The residual internal stress in the reversed austenite is certainly less than that in the specimens fabricated by other SPD techniques. After the second cold rolling, consistent with the results of X-ray diffraction, the SAED pattern (Fig. 3(F)) shows a mixture of the strain-induced a 0 -martensite and retained c austenite, because of the lack of thickness reduction (50%) during the second cold rolling and the increased austenite stability. The martensite laths (Fig. 3(E)) seem much finer than those of the first cold rolled sample (Fig. 3(A)) because of the finer austenite grain size before cold rolling. After the second annealing, the grain size is successfully reduced further, as shown in Fig. 3(G). However, the homogeneity of the grain sizes seems to be decreased as compared with that of the grains after the first annealing (Fig. 3(C)). Some grains are even smaller than 100 nm while others are a little larger than 200 nm. The SAED pattern also reveals fully c phase, which is the same as Fig. 3(D), and relatively more uniform diffraction rings, indicative of finer grain size. Considering overall microstructural changes during the repetitive thermomechanical process, there is an obvious relationship between the width of the martensite lath and the austenite grain size. The smaller the initial c austenite grain size before cold rolling, the narrower the width of martensite lath, and the finer are the reversed c austenite grains after annealing.
50.0
50 40
32.4
30.4 30 20
14.1
10 0
A
C
B
D
E
Fig. 4. The Rockwell hardness (C scale) of the samples taken at different stages during the repetitive thermomechanical process: (A) after solution treatment; (B) after the first cold rolling; (C) after the first annealing; (D) after the second cold rolling; and (E) after the second annealing.
1200 1000
Stress (MPa)
1314
(C)
800
(B) 600
(A) 400 200 0 0
5
10
15
20
25
30
35
40
45
50
Strain (%) Fig. 5. The engineering tensile stress–strain curves of the samples taken at different stages during the repetitive thermomechanical process: (A) after solution treatment; (B) after the first annealing; and (C) after the second annealing.
cold rolling, due to the strain-induced a 0 -martensite. The first annealed specimen has a hardness of HRC 30.4, which is much higher than that of the solution treated one because of the fine austenite grains. After the second cold rolling, the hardness is HRC 50.0. This value is a little lower than that of the first cold rolled sample, probably due to the existence of some amount of retained c austenite, which is confirmed by the previous X-ray and TEM results. The finer grains, obtained after the second annealing, possess higher hardness (HRC 32.4) than that of the first annealed specimen. The engineering stresss–strain curves of the nanocrystalline steels are shown together with that of the
Y. Ma et al. / Scripta Materialia 52 (2005) 1311–1315 Table 2 Tensile properties of the specimens measured at different stages during the thermomechanical treatment Stage
Yield strength (MPa)
Tensile strength (MPa)
Uniform elongation (%)
Total elongation (%)
After solution treatment After the first annealing After the second annealing
120
1041
36.5
38.9
708
1070
32.6
36.0
779
1102
28.0
32.0
solution treated sample in Fig. 5 and the major tensile properties of the steels are listed in Table 2. The solution treated sample shows low yield strength (120 MPa) and large elongation (38.9%). After the first annealing, the yield strength greatly increased to 708 MPa, which is nearly six times that of the solution treated one, without a great loss of the elongation (36.0%). The second annealing increased both yield and tensile strengths further to 779 and 1102 MPa, respectively, probably due to the finer grains. The elongation slightly decreased (32.0%), but it is still much greater than those of the nano-crystalline materials reported before [5,6]. Previous research has shown that it is normal for nano-crystalline materials to show much higher strength than their coarse-grained counterparts, as expected from the well-known Hall–Petch relationship. But the elongation of the most nano-crystalline materials is greatly decreased due to little work hardening ability, which greatly reduces the uniform elongation prior to necking under uniaxial tension [11,12]. In the case of the metastable austenitic steel studied here, the great work hardening ability is obtained through the strain-induced martensitic transformation during tensile tests, leading to a good uniform elongation as well as high strength [8].
4. Conclusions A metastable austensitic steel, containing less Cr and Ni, and higher Mn contents than those of typical austen-
1315
itic stainless steels, repeatedly underwent a cold rolling and annealing process. During the repetitive thermomechanical process, c austenite decomposed to a 0 -martensite on cold rolling and its reverse transformation to austenite occurred during annealing just above the austenite finish temperature. The repetitive thermomechanical treatment generated nano-crystalline austenite grains of about 100 nm, possessing an excellent combination of high strength and good elongation. This study reveals the possibilities for the new thermomechanical process and extended alloy chemistry for fabricating the bulk nano-crystalline material with high strength and good elongation. This technique also seems to have great advantages for practical applications, especially for large-sized sheets, as compared with previous SPD processes.
Acknowledgement This work was financially supported by the Ministry of Commerce, Industry and Energy of Korea. The authors acknowledge the support.
References [1] Valiev RZ, Islamgaliev RK, Alexandrov IV. Prog Mater Sci 2000;45:103. [2] Zhu YT, Jiang HG, Huang JY, Lowe TC. Metall Mater Trans 2001;32A:1559. [3] Saito Y, Tsuji N, Utsunomiya H, Sakai T, Hong RG. Scripta Mater 1998;39:1221. [4] Saito Y, Utsunomiya H, Tsuji N, Sakai T. Acta Mater 1999;47:579. [5] Jia D, Wang YM, Ramesh KT, Ma E, Zhu YT, Valiev RZ. Appl Phys Lett 2001;79:611. [6] Wang YM, Ma E. Mater Sci Eng 2004;A375–377:46. [7] Tomimura K, Takaki S, Tokunaga Y. ISIJ Int 1991;31:1431. [8] Kwon O, Lee YK. J Korean Inst Metals Mater 1994;32:958. [9] Dieter GE. Mechanical metallurgy. New York: McGraw-Hill; 1986. p. 293. [10] ChuvilÕdeev VN, Kopylov VI, Zeiger W. Ann Chim Sci Mat 2002;27:55. [11] Valiev RZ. Mater Sci Eng 1997;A234–236:59. [12] Ma E. Scripta Mater 2003;49:663.