- Email: [email protected]
Formation of nanocrystalline structure in carbon steels by ball drop and particle impact techniques
Materials Science and Engineering A 375–377 (2004) 899–904
Formation of nanocrystalline structure in carbon steels by ball drop and particle impact techniques M. Umemoto∗ , K. Todaka, K. Tsuchiya Department of Production Systems Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan
Abstract The formation of nanocrystalline structure on the surface of bulk steel samples was studied using ball drop and particle impact techniques. Nanocrystalline layers with several microns thick were successfully fabricated by both methods. The microstructural observations, hardness measurements and annealing experiments suggest that the nanocrystalline layers have similar characteristics with those produced by ball milling (BM). The nanocrystalline regions have sharp boundaries with work-hardened neighboring regions and no intermediate regions were observed. Pre-strain and low deformation temperature was found to enhance the formation of nanocrystalline regions. Nanocrystalline layers with large shear deformation were sometimes observed. From the measured shear strain, the equivalent true strain necessary to produce nanocrystalline region was estimated to be about 3. © 2003 Elsevier B.V. All rights reserved. Keywords: Nanocrystallization; Plastic deformation; Ball mill; Steel; Work-hardening
1. Introduction Nanocrystallization by severe plastic deformation in steels has been a subject of many researches [1–5] in the last decade. A popular method to produce nanocrystalline structure is ball milling (BM). From our previous BM experiments on steels [6–11], it was found that the nanocrystalline regions produced by BM have the following characteristics: (1) homogeneous structure with sharp boundaries between the work-hardened regions as shown in Fig. 1; (2) ultrafine grains of less than 100 nm with almost no dislocations; (3) extremely high hardness (8–14 GPa); (4) include no cementite phase; and (5) no recrystallization and slow grain growth by annealing. Although BM is a powerful method to produce nanocrystalline materials, it is not suitable to study the nanocrystallization mechanism since the deformation mode is extremely complicated and contamination is hard to be avoided. To study nanocrystallization by severe plastic deformation, a method which produces simple deformation on specimens without contamination is desired. The purpose of the present study is to demonstrate new severe plastic deformation methods to produce nanocrystal∗ Corresponding author. Tel.: +81-532-44-6709; fax: +81-532-44-6690. E-mail address: [email protected] (M. Umemoto).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.198
lization in steels, a ball drop [12,13] and a particle impact methods. The nanocrystalline regions formed on the surface of steel plates by these methods were compared with those in ball milled powder. From the observed shear bands the amount of strain necessary to produce nanocrystalline regions was estimated. The intermediate stage between work-hardened and nanocrystalline regions is discussed taking into account the observed sharp boundaries between them.
2. Experimental procedures The material used in this study was eutectoid carbon steels of Fe–0.80C (Fe–0.80C–0.20Si–1.33Mn in wt.%) and Fe–0.89C (Fe–0.89C–0.25Si–0.50Mn in wt.%) with either pearlite or spheroidite structure. The pearlite structure was obtained by the patenting treatment. Specimens were austenitized at 1223 K for 1.8 ks followed by an isothermal transformation to pearlite at 873 K for 0.3 ks in a lead bath. The spheroidite structure was produced by a process of martensite tempering, in which specimens were austenitized at 1173 K for 3.6 ks and then quenched into water to obtain martensite, and then tempered at 983 K for 79.2 ks. To study the effect of pre-strain on the formation of nanocrystalline structure, specimens were rolled to various reductions by
900
M. Umemoto et al. / Materials Science and Engineering A 375–377 (2004) 899–904
Fig. 1. SEM micrographs of 360 ks ball milled Fe–0.89C powder showing the boundary between the nanocrystalline region (left-hand side) and work-hardened region (right-hand side). (a) Pearlite and (b) spheroidite structures.
multipass rolling (with 10 or 20% reduction per pass). Annealing of nanocrystallized specimens were carried out at 873 K for 3.6 ks by sealing in a quartz tube under a pure Ar protective atmosphere. Specimens were characterized by SEM, TEM and Vickers microhardness tester (load of 0.98 N for 10 s). Specimens for SEM observations were etched by 5% Nital. In a ball drop experiment (Fig. 2(a)), a weight with a ball attached on its bottom was dropped from a height of 1 or 2 m onto a bulk specimen with flat surface. The ball of 6 mm in diameter, the weight of either 4 or 5 kg and the specimens with 15 mm in diameter and 2–4 mm in thickness were used. When the ball was dropped more than two times, the specimen was systematically shifted by 2 mm step for each drop test. All tests were carried out in air at either room temperature or liquid nitrogen temperature. The details of the ball drop test were described in our previous paper [12]. A particle impact experiment (Fig. 2(b)) was done by a high-pressure light gas gun which can accelerate particles in a desired speed. Helium was used as particle carrier gas. The bore was 4.2 mm in inner diameter and the length of 4 m to provide sufficient distance for acceleration. Bearing steel (Fe–1Cr–1.5Cr) ball with 4 mm in diameter was chosen as
Fig. 2. Apparatus of ball drop and particle impact experiments. (a) Ball drop and (b) particle impact.
projectile accelerated to a speed of 120 m/s. The number of impacts studied was from 1 to 200. Specimens of 30 mm × 30 mm × 3 mm were mounted at the end of bore. All the experiments were done at room temperature in air. When microhardness is converted to grain size, the following Hall-Petch-type relationship was used. This equation was reported to hold down to 50 nm in various steels [14]. HV (GPa) = 0.363 + 1.90 d −0.5 (d, in m)
(1)
3. Results 3.1. Ball drop test The nanocrystalline regions formed by the ball drop technique usually appears at surface of specimens [13]. A typical nanocrystalline region formed in a pearlitic sample by a ball drop test (eight times of ball drops with a weight of 4 kg from a height of 1 m) is shown in Fig. 3. A layer (dark regions in SEM and bright regions in optical microscopy) about 20 m thick is seen near the bottom surface of a impact crater (indicated by an arrow). TEM samples were prepared parallel to the specimen surface. Fig. 4 shows a dark field image and electron diffraction pattern of a sample with pearlite structure after ball dropping (25 times, 4 kg, 1 m). The dark field image of Fig. 4 (taken from the (110) ring of bcc ferrite) shows that the ferrite grain size is of the order of 100 nm. All the diffraction rings correspond to bcc ferrite, and rings corresponding to cementite are hardly detected. This indicates that cementite is mostly dissolved into the ferrite. The diffraction pattern taken from the area of φ 2 m shows nearly continuous rings, indicating the random orientations of the ferrite grains. Fig. 5 is a SEM micrograph of a cross-section of a pearlitic specimen after eight times of ball drops (5 kg, 1 m) at liquid nitrogen temperature. In the nanocrystalline layer at the
M. Umemoto et al. / Materials Science and Engineering A 375–377 (2004) 899–904
901
Fig. 3. Nanocrystalline layer formed by ball drop test (eight times, 4 kg, 1 m) in Fe–0.89C pearlite.
surface of the specimen the lamellar structure of pearlite is invisible, indicating that cementite lamellae are completely dissolved. Under this layer, deformed pearlite structure is clearly observed. The microhardness of nanocrystalline layer is 11.0 GPa which corresponds to ferrite grain size of about 30 nm. This hardness is almost twice as high as the value for the adjacent work-hardened region (5.9 GPa). It was noted that the observed microstructures produced by a ball drop test were similar to those observed in ball milled powders. The number of ball drops necessary to produce nanocrystalline layer depends on the composition, microstructure and temperature of specimens and ball drop conditions (weight and height). The number of drops is less for harder sample and higher energy drop conditions (higher weight and height). Low processing temperature also reduces the number of drops. This suggests that nanocrystallization by ball drop is purely due to severe plastic deformation and not concerned with thermally produced martensite as a consequence
Fig. 5. Nanocrystalline layer formed by ball drop test (eight times, 5 kg, 1 m) in Fe–0.80C pearlitic at liquid nitrogen temperature.
Fig. 4. Dark field image and electron diffraction pattern of Fe–0.89C pearlite steel after ball drop test (25 times, 4 kg, 1 m).
902
M. Umemoto et al. / Materials Science and Engineering A 375–377 (2004) 899–904
Fig. 6. Shear deformation produced by a ball drop (one time, 5 kg, 1 m) in the pre-strained (80% cold rolling) Fe–0.80C pearlite.
of adiabatic deformation. Pre-strain of specimens also reduces the number of ball drops. In the case of the pearlitic specimen it was possible to produce the nanocrystalline region by one time of ball drop after cold rolling of 80% [13]. Shear bands with a large shear strain were often observed in the nanocrystalline layers produced by a ball drop test as shown in Fig. 6. The specimen was pre-strained to 80% by cold rolling and ball dropped one time (5 kg, 1 m). Using the stripes morphology crossing the nanocrystalline layer, the amount of shear strain in the nanocrystalline layer was estimated to be 8.1. Adding the pre-strain of 80% rolling (1.9 in true strain) and the observed shear strain of 8.1 (1.2
in true strain), the total true strain necessary to produce nanocrystalline layer is estimated to be 3.1. Fig. 7 shows a specimen with spheroidite structure after a ball drop test (eight times, 5 kg, 1 m) at liquid nitrogen temperature. Curved thin bands similar to those observed in the ball milled powders are seen parallel to the specimen surface. This suggests that the formation process of nanocrystalline regions by a ball drop test is similar to that in ball milling [6]. The annealing experiment of ball dropped specimens showed the similar results with those observed in the ball milled samples [6,10,11]. After annealing, the
Fig. 7. Nanocrystalline region formed in Fe-0.80C spheroidite by a ball drop (eight times, 5 kg, 1 m) at liquid nitrogen temperature.
M. Umemoto et al. / Materials Science and Engineering A 375–377 (2004) 899–904
903
Fig. 8. Cross-sectional SEM micrographs after various numbers of particle impacts on the pre-strained (82%, cold rolling) pearlitic specimens: (a) 1 time, (b) 8 times and (c) 200 times.
microstructures of prior work-hardened region and prior nanocrystalline region are still quite different and there is a sharp boundary between them. In the work-hardened region, recrystallization and grain growth of the ferrite took place. In contrast, a much finer microstructure is seen in the nanocrystalline region, where the fine cementite particles are re-precipitated. 3.2. Particle impact experiment The development of nanocrystalline layers with the number of particle impacts was studied using pre-strained (82% cold rolling) pearlitic specimens. Fig. 8 shows the crosssectional SEM micrographs after various numbers of particle impacts. After one time of particle impact (Fig. 8(a)), heavily deformed layers with reduced lamellar spacing were formed. The structure is similar to that observed in heavily cold rolled pearlitic samples. After eight times of particle impacts (Fig. 8(b)), featureless shear bands were formed. After 200 times of particle impacts (Fig. 8(c)), large area of nanocrystalline regions were formed. High hardness of 10.4 GPa (corresponds to grain size of 36 nm) similar to that observed in the ball milled or ball dropped specimens was obtained in the nanocrystalline regions. Shear bands were also often observed in the particle impacted specimens. The shear bands observed are much thinner (less than 5 m) than those obtained in a ball drop test.
4. Discussion In the present study, formation of nanocrystalline regions was observed by ball drop and particle impact experiments. The produced nanocrystalline regions have sharp boundaries with the work-hardened regions similar to those produced by ball milling. There are two possible explanations for the observed sharp boundaries. One is to assume that the degree of deformation is a gradual function of distance. In this case, the nanocrystallization by heavy deformation occurs by a drastic transition from work-hardened to nanocrystalline state and a critical degree of deformation or a critical dislocation density may exist corresponding to this transition. Another is to assume that the degree of deformation also changes
sharply at the boundaries between nanocrystalline and workhardened regions. In this case, localized heavy deformation like shear bands are considered to be responsible for the formation of nanocrystalline regions. At present, the second explanation sounds more reasonable since shear bands are observed in most of the specimens. In any case, it is important to realize that the regions with intermediate properties between the nanocrystalline and work-hardened regions were not observed in the specimens prepared by either ball milling, ball drop or particle impact experiments. The microstructural observations, hardness measurements and annealing experiments suggest that there are no intermediate stages between the nanocrystallization and work-hardened states. It seems that the deformation conditions suitable for the formation of nanocrystalline region prevent the formation of intermediate state, or intermediate states are unstable and change easily to nanocrystalline state by further deformation. The deformation conditions and/or a degree of deformation to produce the intermediate state seem extremely limited.
5. Summary Nanocrystalline regions can be successively fabricated in various carbon steel plates by means of ball drop and particle impact techniques. The formation of nanocrystalline regions was confirmed by TEM observations, microhardness measurements and annealing experiments. The nanocrystalline regions formed were similar to those observed in the ball milled powders. The nanocrystalline regions showed hardness higher than 10 GPa. Sharp boundaries between the nanocrystalline and work-hardened regions were observed. Nanocrystalline layers with large shear deformation were sometimes observed in ball dropped and particle impacted samples although they were not observed in ball milled powders. From the measured shear deformation, the necessary true strain to produce nanocrystalline region was estimated to be about three for the ball drop and about four for the particle impact experiments. The observed shear bands are thicker in ball dropped samples than particle impact samples. By annealing, recrystallization did not take place and slow grain growth was observed in the nanocrystalline
904
M. Umemoto et al. / Materials Science and Engineering A 375–377 (2004) 899–904
regions. When the specimen contains cementite, it dissolves completely when the matrix is nanocrystallized. Acknowledgements This study is financially supported in part by the Grant-in-Aid by the Japan Society for the Promotion of Science. References [1] J.S.C. Jang, C.C. Koch, Scripta Metall. 24 (1990) 1599. [2] H.J. Fecht, E. Hellstern, Z. Fu, W.L. Johnson, Met. Trans. A 21 (1990) 2333. [3] A.V. Korznikov, Yu.V. Ivanisenko, D.V. Laptionok, I.M. Safarov, V.P. Pilyugin, R.Z. Valiv, Nanostruct. Mater. 4 (1994) 159.
[4] N.R. Tao, M.L. Sui, J. Ku, L. Ku, Nanostruct. Mater. 11 (1999) 433. [5] D.H. Shin, B.C. Kim, Y.S. Kim, K.T. Park, Acta Mater. 48 (2000) 2247. [6] Y. Xu, Z.G. Liu, M. Umemoto, K. Tsuchiya, Metall. Mater. Trans. A 33 (2002) 2195. [7] Y. Xu, M. Umemoto, K. Tsuchiya, Mater. Trans. 43 (2002) 2205. [8] M. Umemoto, Z.G. Liu, K. Masuyama, X.J. Hao, K. Tsuchiya, Scripta Mater. 44 (2001) 1741. [9] Z.G. Liu, X.J. Hao, K. Masuyama, K. Tsuchiya, M. Umemoto, S.M. Hao, Scripta Mater. 44 (2001) 1775. [10] M. Umemoto, Z.G. Liu, X.J. Hao, K. Masuyama, K. Tsuchiya, Mater. Sci. Forum 360–362 (2001) 167. [11] J. Yin, M. Umemoto, Z.G. Liu, K. Tsuchiya, ISIJ Int. 41 (2001) 1389. [12] M. Umemoto, B. Huang, K. Tsuchiya, N. Suzuki, Scripta Mater. 46 (2002) 383. [13] M. Umemoto, X.J. Hao, T. Yasuda, K. Tsuchiya, Mater. Trans. 43 (2002), in press. [14] H. Hidaka, T. Suzuki, Y. Kimura, S. Takaki, Mater. Sci. Forum 304–306 (1999) 115.
Formation of nanocrystalline structure in carbon steels by ball drop and particle impact techniques M. Umemoto∗ , K. Todaka, K. Tsuchiya Department of Production Systems Engineering, Toyohashi University of Technology, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan
Abstract The formation of nanocrystalline structure on the surface of bulk steel samples was studied using ball drop and particle impact techniques. Nanocrystalline layers with several microns thick were successfully fabricated by both methods. The microstructural observations, hardness measurements and annealing experiments suggest that the nanocrystalline layers have similar characteristics with those produced by ball milling (BM). The nanocrystalline regions have sharp boundaries with work-hardened neighboring regions and no intermediate regions were observed. Pre-strain and low deformation temperature was found to enhance the formation of nanocrystalline regions. Nanocrystalline layers with large shear deformation were sometimes observed. From the measured shear strain, the equivalent true strain necessary to produce nanocrystalline region was estimated to be about 3. © 2003 Elsevier B.V. All rights reserved. Keywords: Nanocrystallization; Plastic deformation; Ball mill; Steel; Work-hardening
1. Introduction Nanocrystallization by severe plastic deformation in steels has been a subject of many researches [1–5] in the last decade. A popular method to produce nanocrystalline structure is ball milling (BM). From our previous BM experiments on steels [6–11], it was found that the nanocrystalline regions produced by BM have the following characteristics: (1) homogeneous structure with sharp boundaries between the work-hardened regions as shown in Fig. 1; (2) ultrafine grains of less than 100 nm with almost no dislocations; (3) extremely high hardness (8–14 GPa); (4) include no cementite phase; and (5) no recrystallization and slow grain growth by annealing. Although BM is a powerful method to produce nanocrystalline materials, it is not suitable to study the nanocrystallization mechanism since the deformation mode is extremely complicated and contamination is hard to be avoided. To study nanocrystallization by severe plastic deformation, a method which produces simple deformation on specimens without contamination is desired. The purpose of the present study is to demonstrate new severe plastic deformation methods to produce nanocrystal∗ Corresponding author. Tel.: +81-532-44-6709; fax: +81-532-44-6690. E-mail address: [email protected] (M. Umemoto).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.198
lization in steels, a ball drop [12,13] and a particle impact methods. The nanocrystalline regions formed on the surface of steel plates by these methods were compared with those in ball milled powder. From the observed shear bands the amount of strain necessary to produce nanocrystalline regions was estimated. The intermediate stage between work-hardened and nanocrystalline regions is discussed taking into account the observed sharp boundaries between them.
2. Experimental procedures The material used in this study was eutectoid carbon steels of Fe–0.80C (Fe–0.80C–0.20Si–1.33Mn in wt.%) and Fe–0.89C (Fe–0.89C–0.25Si–0.50Mn in wt.%) with either pearlite or spheroidite structure. The pearlite structure was obtained by the patenting treatment. Specimens were austenitized at 1223 K for 1.8 ks followed by an isothermal transformation to pearlite at 873 K for 0.3 ks in a lead bath. The spheroidite structure was produced by a process of martensite tempering, in which specimens were austenitized at 1173 K for 3.6 ks and then quenched into water to obtain martensite, and then tempered at 983 K for 79.2 ks. To study the effect of pre-strain on the formation of nanocrystalline structure, specimens were rolled to various reductions by
900
M. Umemoto et al. / Materials Science and Engineering A 375–377 (2004) 899–904
Fig. 1. SEM micrographs of 360 ks ball milled Fe–0.89C powder showing the boundary between the nanocrystalline region (left-hand side) and work-hardened region (right-hand side). (a) Pearlite and (b) spheroidite structures.
multipass rolling (with 10 or 20% reduction per pass). Annealing of nanocrystallized specimens were carried out at 873 K for 3.6 ks by sealing in a quartz tube under a pure Ar protective atmosphere. Specimens were characterized by SEM, TEM and Vickers microhardness tester (load of 0.98 N for 10 s). Specimens for SEM observations were etched by 5% Nital. In a ball drop experiment (Fig. 2(a)), a weight with a ball attached on its bottom was dropped from a height of 1 or 2 m onto a bulk specimen with flat surface. The ball of 6 mm in diameter, the weight of either 4 or 5 kg and the specimens with 15 mm in diameter and 2–4 mm in thickness were used. When the ball was dropped more than two times, the specimen was systematically shifted by 2 mm step for each drop test. All tests were carried out in air at either room temperature or liquid nitrogen temperature. The details of the ball drop test were described in our previous paper [12]. A particle impact experiment (Fig. 2(b)) was done by a high-pressure light gas gun which can accelerate particles in a desired speed. Helium was used as particle carrier gas. The bore was 4.2 mm in inner diameter and the length of 4 m to provide sufficient distance for acceleration. Bearing steel (Fe–1Cr–1.5Cr) ball with 4 mm in diameter was chosen as
Fig. 2. Apparatus of ball drop and particle impact experiments. (a) Ball drop and (b) particle impact.
projectile accelerated to a speed of 120 m/s. The number of impacts studied was from 1 to 200. Specimens of 30 mm × 30 mm × 3 mm were mounted at the end of bore. All the experiments were done at room temperature in air. When microhardness is converted to grain size, the following Hall-Petch-type relationship was used. This equation was reported to hold down to 50 nm in various steels [14]. HV (GPa) = 0.363 + 1.90 d −0.5 (d, in m)
(1)
3. Results 3.1. Ball drop test The nanocrystalline regions formed by the ball drop technique usually appears at surface of specimens [13]. A typical nanocrystalline region formed in a pearlitic sample by a ball drop test (eight times of ball drops with a weight of 4 kg from a height of 1 m) is shown in Fig. 3. A layer (dark regions in SEM and bright regions in optical microscopy) about 20 m thick is seen near the bottom surface of a impact crater (indicated by an arrow). TEM samples were prepared parallel to the specimen surface. Fig. 4 shows a dark field image and electron diffraction pattern of a sample with pearlite structure after ball dropping (25 times, 4 kg, 1 m). The dark field image of Fig. 4 (taken from the (110) ring of bcc ferrite) shows that the ferrite grain size is of the order of 100 nm. All the diffraction rings correspond to bcc ferrite, and rings corresponding to cementite are hardly detected. This indicates that cementite is mostly dissolved into the ferrite. The diffraction pattern taken from the area of φ 2 m shows nearly continuous rings, indicating the random orientations of the ferrite grains. Fig. 5 is a SEM micrograph of a cross-section of a pearlitic specimen after eight times of ball drops (5 kg, 1 m) at liquid nitrogen temperature. In the nanocrystalline layer at the
M. Umemoto et al. / Materials Science and Engineering A 375–377 (2004) 899–904
901
Fig. 3. Nanocrystalline layer formed by ball drop test (eight times, 4 kg, 1 m) in Fe–0.89C pearlite.
surface of the specimen the lamellar structure of pearlite is invisible, indicating that cementite lamellae are completely dissolved. Under this layer, deformed pearlite structure is clearly observed. The microhardness of nanocrystalline layer is 11.0 GPa which corresponds to ferrite grain size of about 30 nm. This hardness is almost twice as high as the value for the adjacent work-hardened region (5.9 GPa). It was noted that the observed microstructures produced by a ball drop test were similar to those observed in ball milled powders. The number of ball drops necessary to produce nanocrystalline layer depends on the composition, microstructure and temperature of specimens and ball drop conditions (weight and height). The number of drops is less for harder sample and higher energy drop conditions (higher weight and height). Low processing temperature also reduces the number of drops. This suggests that nanocrystallization by ball drop is purely due to severe plastic deformation and not concerned with thermally produced martensite as a consequence
Fig. 5. Nanocrystalline layer formed by ball drop test (eight times, 5 kg, 1 m) in Fe–0.80C pearlitic at liquid nitrogen temperature.
Fig. 4. Dark field image and electron diffraction pattern of Fe–0.89C pearlite steel after ball drop test (25 times, 4 kg, 1 m).
902
M. Umemoto et al. / Materials Science and Engineering A 375–377 (2004) 899–904
Fig. 6. Shear deformation produced by a ball drop (one time, 5 kg, 1 m) in the pre-strained (80% cold rolling) Fe–0.80C pearlite.
of adiabatic deformation. Pre-strain of specimens also reduces the number of ball drops. In the case of the pearlitic specimen it was possible to produce the nanocrystalline region by one time of ball drop after cold rolling of 80% [13]. Shear bands with a large shear strain were often observed in the nanocrystalline layers produced by a ball drop test as shown in Fig. 6. The specimen was pre-strained to 80% by cold rolling and ball dropped one time (5 kg, 1 m). Using the stripes morphology crossing the nanocrystalline layer, the amount of shear strain in the nanocrystalline layer was estimated to be 8.1. Adding the pre-strain of 80% rolling (1.9 in true strain) and the observed shear strain of 8.1 (1.2
in true strain), the total true strain necessary to produce nanocrystalline layer is estimated to be 3.1. Fig. 7 shows a specimen with spheroidite structure after a ball drop test (eight times, 5 kg, 1 m) at liquid nitrogen temperature. Curved thin bands similar to those observed in the ball milled powders are seen parallel to the specimen surface. This suggests that the formation process of nanocrystalline regions by a ball drop test is similar to that in ball milling [6]. The annealing experiment of ball dropped specimens showed the similar results with those observed in the ball milled samples [6,10,11]. After annealing, the
Fig. 7. Nanocrystalline region formed in Fe-0.80C spheroidite by a ball drop (eight times, 5 kg, 1 m) at liquid nitrogen temperature.
M. Umemoto et al. / Materials Science and Engineering A 375–377 (2004) 899–904
903
Fig. 8. Cross-sectional SEM micrographs after various numbers of particle impacts on the pre-strained (82%, cold rolling) pearlitic specimens: (a) 1 time, (b) 8 times and (c) 200 times.
microstructures of prior work-hardened region and prior nanocrystalline region are still quite different and there is a sharp boundary between them. In the work-hardened region, recrystallization and grain growth of the ferrite took place. In contrast, a much finer microstructure is seen in the nanocrystalline region, where the fine cementite particles are re-precipitated. 3.2. Particle impact experiment The development of nanocrystalline layers with the number of particle impacts was studied using pre-strained (82% cold rolling) pearlitic specimens. Fig. 8 shows the crosssectional SEM micrographs after various numbers of particle impacts. After one time of particle impact (Fig. 8(a)), heavily deformed layers with reduced lamellar spacing were formed. The structure is similar to that observed in heavily cold rolled pearlitic samples. After eight times of particle impacts (Fig. 8(b)), featureless shear bands were formed. After 200 times of particle impacts (Fig. 8(c)), large area of nanocrystalline regions were formed. High hardness of 10.4 GPa (corresponds to grain size of 36 nm) similar to that observed in the ball milled or ball dropped specimens was obtained in the nanocrystalline regions. Shear bands were also often observed in the particle impacted specimens. The shear bands observed are much thinner (less than 5 m) than those obtained in a ball drop test.
4. Discussion In the present study, formation of nanocrystalline regions was observed by ball drop and particle impact experiments. The produced nanocrystalline regions have sharp boundaries with the work-hardened regions similar to those produced by ball milling. There are two possible explanations for the observed sharp boundaries. One is to assume that the degree of deformation is a gradual function of distance. In this case, the nanocrystallization by heavy deformation occurs by a drastic transition from work-hardened to nanocrystalline state and a critical degree of deformation or a critical dislocation density may exist corresponding to this transition. Another is to assume that the degree of deformation also changes
sharply at the boundaries between nanocrystalline and workhardened regions. In this case, localized heavy deformation like shear bands are considered to be responsible for the formation of nanocrystalline regions. At present, the second explanation sounds more reasonable since shear bands are observed in most of the specimens. In any case, it is important to realize that the regions with intermediate properties between the nanocrystalline and work-hardened regions were not observed in the specimens prepared by either ball milling, ball drop or particle impact experiments. The microstructural observations, hardness measurements and annealing experiments suggest that there are no intermediate stages between the nanocrystallization and work-hardened states. It seems that the deformation conditions suitable for the formation of nanocrystalline region prevent the formation of intermediate state, or intermediate states are unstable and change easily to nanocrystalline state by further deformation. The deformation conditions and/or a degree of deformation to produce the intermediate state seem extremely limited.
5. Summary Nanocrystalline regions can be successively fabricated in various carbon steel plates by means of ball drop and particle impact techniques. The formation of nanocrystalline regions was confirmed by TEM observations, microhardness measurements and annealing experiments. The nanocrystalline regions formed were similar to those observed in the ball milled powders. The nanocrystalline regions showed hardness higher than 10 GPa. Sharp boundaries between the nanocrystalline and work-hardened regions were observed. Nanocrystalline layers with large shear deformation were sometimes observed in ball dropped and particle impacted samples although they were not observed in ball milled powders. From the measured shear deformation, the necessary true strain to produce nanocrystalline region was estimated to be about three for the ball drop and about four for the particle impact experiments. The observed shear bands are thicker in ball dropped samples than particle impact samples. By annealing, recrystallization did not take place and slow grain growth was observed in the nanocrystalline
904
M. Umemoto et al. / Materials Science and Engineering A 375–377 (2004) 899–904
regions. When the specimen contains cementite, it dissolves completely when the matrix is nanocrystallized. Acknowledgements This study is financially supported in part by the Grant-in-Aid by the Japan Society for the Promotion of Science. References [1] J.S.C. Jang, C.C. Koch, Scripta Metall. 24 (1990) 1599. [2] H.J. Fecht, E. Hellstern, Z. Fu, W.L. Johnson, Met. Trans. A 21 (1990) 2333. [3] A.V. Korznikov, Yu.V. Ivanisenko, D.V. Laptionok, I.M. Safarov, V.P. Pilyugin, R.Z. Valiv, Nanostruct. Mater. 4 (1994) 159.
[4] N.R. Tao, M.L. Sui, J. Ku, L. Ku, Nanostruct. Mater. 11 (1999) 433. [5] D.H. Shin, B.C. Kim, Y.S. Kim, K.T. Park, Acta Mater. 48 (2000) 2247. [6] Y. Xu, Z.G. Liu, M. Umemoto, K. Tsuchiya, Metall. Mater. Trans. A 33 (2002) 2195. [7] Y. Xu, M. Umemoto, K. Tsuchiya, Mater. Trans. 43 (2002) 2205. [8] M. Umemoto, Z.G. Liu, K. Masuyama, X.J. Hao, K. Tsuchiya, Scripta Mater. 44 (2001) 1741. [9] Z.G. Liu, X.J. Hao, K. Masuyama, K. Tsuchiya, M. Umemoto, S.M. Hao, Scripta Mater. 44 (2001) 1775. [10] M. Umemoto, Z.G. Liu, X.J. Hao, K. Masuyama, K. Tsuchiya, Mater. Sci. Forum 360–362 (2001) 167. [11] J. Yin, M. Umemoto, Z.G. Liu, K. Tsuchiya, ISIJ Int. 41 (2001) 1389. [12] M. Umemoto, B. Huang, K. Tsuchiya, N. Suzuki, Scripta Mater. 46 (2002) 383. [13] M. Umemoto, X.J. Hao, T. Yasuda, K. Tsuchiya, Mater. Trans. 43 (2002), in press. [14] H. Hidaka, T. Suzuki, Y. Kimura, S. Takaki, Mater. Sci. Forum 304–306 (1999) 115.