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In situ TEM observation of crack healing in α-Fe
Scripta mater. 44 (2001) 1055–1059 www.elsevier.com/locate/scriptamat
IN SITU TEM OBSERVATION OF CRACK HEALING IN ␣-Fe K.W. Gao, L.J. Qiao and W.Y. Chu Department of Materials Physics, University of Science and Technology Beijing, Beijing, 100083, People’s Republic of China (Received June 9, 2000) (Accepted in revised form December 20, 2000) Keywords: Iron; TEM; High temperature; Healing Introduction Crack healing is now considered to show promise in the recovery of the mechanical properties of the cracked materials. Thermal or mechanical treatment provides the driving force to minimize the surface energy resulting in the healing of a crack. Cracks in lithium fluoride [1], sapphire [2], ceramics [3–9], plexiglass [10] and ice [11] could heal through atom diffusion during heating. However, the crack healing is not always successful during heating. Thompson et al. [12] found that the cracks in Al2O3⫹SiC closed during annealing, whereas the cracks in Al2O3 propagated, which was induced by the grain-localized frictional traction generated by thermal expansion anisotropy relax rapidly. Till now, most observations of crack healing were carried out in ceramic materials [3–9], and three mechanisms have been proposed for crack healing in ceramics: diffusion thermal healing, adhesion from intermolecular forces and reaction products from chemical reactions at the crack tip [13]. Only a few works mentioned crack healing in metals or alloys [14 –16]. It was believed that the crack healing process in metals was mainly controlled by the diffusion and migration of metal atoms, and consisted of two steps, crack filling and grain growth [16]. Recrystallization of alloys could induce the crack to seal off by moving grain boundaries. In this present study, in-situ TEM observation of crack healing in single crystal iron during heating was carried out. The healing process of a crack at nanometer scale is presented, followed by a brief discussion on the crack healing in this experiment. Experimental The material used is single crystal iron of 99.99% purity. After mechanical polishing to a thickness of about 100m, the electropolishing was carried out in a solution of 10% H2SO4 ⫹ 90% methanol until a hole was created. In-situ TEM observations were performed using a Philip H-800 transmission electron microscope. In order to generate a precrack, sometimes the specimen was first tensioned by the loading stage in TEM. When the applied load was high enough, a main crack initiated along the edge of the hole as shown in Figure 1(a). Increasing the applied load continuously, some microcracks might initiate near the main crack, like microcrack C in Figure 2(a), and microcrack OP in Figure 3(a). Then the pre-deformed specimen was unloaded and transferred to a heating stage. During the unloading and transferring, the morphology of those precracks almost did not change and the precracks did not close. 1359-6462/01/$–see front matter. © 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(01)00671-6
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Figure 1. The healing process of the tip of a main crack along the edge of a hole during heating.
Figure 2. The healing process of a microcrack during heating.
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Figure 3. The healing process of a long and narrow microcrack during heating.
The pre-deformed specimen was heated in the TEM, with a heating rate of 10°C/min in most of the cases. The sample tilting remained constant during heating. Results and Discussion A typical original crack AB along the edge of the hole is shown in Figure 1(a). As the temperature increased from room temperature (Fig. 1(a)) through 773K (Fig. 1(b)) and 973K (Fig. 1(c)) to 1160K (Fig. 1(d)), the morphology of the specimen changed considerably due to an increase in thermal vibration of atoms, and the fringes in the thin area became invisible. After cooling down to room temperature (Fig. 1(e)), the specimen recovered part of the initial contrast. There was a microcrack (ab) ahead of the tip of the main crack AB, as shown in Figure 1(a). During heating, the microcrack (ab) started healing at 973K (Fig. 1(c)) and completely healed at 1160K (Fig. 1(d)). When the temperature was decreased down to room temperature, the microcrack(ab) did not appear again, as shown in Figure 1(e). There was a microcrack C near the main crack AB, as shown in Figure 2(a). The microcrack C did not change during heating from 300K to 973K. When the temperature increased from 973K to 1000K, however, the size of the microcrack C decreased gradually from 250nm to 160nm, as shown in Figure 2(b). Increasing the temperature to1023K, the size of the microcrack decreased further to 70nm, as shown in Figure 2(c). Keeping about 100 seconds at the temperature of 1023K, the microcrack C totally healed, as shown in Figure 2(d). The healing process of a long and narrow microcrack is shown in Figure 3. A microcrack OP was beside the main crack MN, and the tip of the main crack MN was marked by a small black arrow. Increasing the temperature to 863K, the microcrack OP started shrinking from each side of the microcrack along the length orientation, and at the same time, the main crack MN propagated(Fig. 3(b)).
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TABLE 1. The Crack Length 2a and the Rate of Crack Healing at Different Temperatures T, K
973
1000
1023
1023
2a, nm t, s da/dt, nm/s
250 0 —
160 162 0.28
70 300 0.33
0 400 0.35
At 973K, the microcrack OP had completely healed, and the main crack MN propagated further, as shown in Figure 3(c). In ceramics, a crack could heal completely when the specimen was heated to a given temperature and kept at that temperature for a long time [1,7]. Even a surface crack in ceramics could also heal after annealing at high temperature for a long time without contamination. But in the present study, when temperature increases, the single crystal Fe is oxidized gradually, and the oxide film impedes the healing process. Only when the heating rate is fast enough so that the rate of crack healing is higher than the oxidization rate of the crack surface, the crack can completely heal. Based on Figure 2, the size of the crack as a function of time during healing is given in Table 1. The rates of crack healing at different temperatures are also listed in Table 1. The oxidization rate of iron at 973K is less than 0.01nm/s, which is far smaller than the rate of crack healing [17]. The average rate of crack healing in Table 1 is about da/dt ⫽ 0.32nm/s. According to literature data, the self-diffusion coefficient of ␣-Fe is D ⫽ 2.0exp(-28950/T) cm2/s [18] or D ⫽ 118exp(-35800/ T)cm2/s [19]. The average diffusion coefficient between 973K and 1023K is D ⫽ 3.8 ⫻ 10⫺13 cm2/s, and the total diffusion time t ⫽ 400s. The distance of self-diffusion of ␣-Fe atoms is ⫻ ⫽ (Dt)1/2 ⫽ 123nm, which is basically consistent with the radius decrease of the crack C. Therefore, the healing of the crack in ␣-Fe can be due to self-diffusion of ␣-Fe atoms during heating. Conclusion In situ observations in TEM show that a crack in single crystal Fe can heal completely upon heating, and under a low oxygen partial pressure. Acknowledgment The present work was supported by a special Fund for the Major State Basic Research Projects (No.G19990650) and by the National Natural Science Foundation of China (No.19891180, 59871010). References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Z. Y. Wang, Y. Z. Li, M. P. Harmer, and Y. T. Chou, J. Am. Ceram. Soc. 75, 1596 (1992). J. Ro¨del and A. M. Glaeser, J. Am. Ceram. Soc. 73, 592 (1990). T. K. Gupta, J. Am. Ceram. Soc. 58, 143 (1975). C. Q. Ru, Acta Mater. 47, 4683 (1999). A. G. Evans and E. A. Charles, Acta Mater. 25, 919 (1977). M. Mitomo, T. Nishimura, and M. Tsutsumi, J. Mater. Sci. Lett. 15, 1976 (1996). B. A. Wilson and E. D. Case, J. Mater. Sci. 32, 3163 (1997). M. K. C. Holden and V. D. Frechette, J. Am. Ceram. Soc. 72, 2189 (1989). R. L. Lethman, R. E. Hill JR, and G. H. Sigel JR, J. Am. Ceram. Soc., 72, 474 (1989). J. D. Powers, etc., J. Am. Ceram. Soc. 71, 2225 (1993).
Vol. 44, No. 7
11. 12. 13. 14. 15. 16. 17. 18. 19.
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S. C. Colbeck, Acta Metall. 34, 89 (1986). A. M. Thompson, H. M. Chan, M. P. Harmer, and R. F. Cook, J. Am. Ceram. Soc. 75, 567 (1995). E. D. Case, J. R. Smyth, and O. Hunter, Jr., in Fracture Mechanics of Ceramics, Vol. 5, p. 507, Plenum Press, New York (1983). J. Foct, N. Akdut, and G. Gottstein, Scripta Metal. Mater. 27, 1033 (1992). R. U. Vaidya, A. K. Zurek, A. Wolfenden, and M. W. Cantu, J. Mater. Eng. Perf. 6, 46 (1997). J. T. Han, G. Zhao, and Q. X. Cao, Sci. China, 27E, 23 (1997). P. L. Fan and L. O. Brockway, J. Electrochem. Soc., 121, 1534 (1974). W. D. Callister, Materials Science and Engineering, p. 75, John Wiley & Sons, New York (1985). P. G. Shewmon, Diffusion in Solids, p. 80, McGraw-Hill, New York (1963).
IN SITU TEM OBSERVATION OF CRACK HEALING IN ␣-Fe K.W. Gao, L.J. Qiao and W.Y. Chu Department of Materials Physics, University of Science and Technology Beijing, Beijing, 100083, People’s Republic of China (Received June 9, 2000) (Accepted in revised form December 20, 2000) Keywords: Iron; TEM; High temperature; Healing Introduction Crack healing is now considered to show promise in the recovery of the mechanical properties of the cracked materials. Thermal or mechanical treatment provides the driving force to minimize the surface energy resulting in the healing of a crack. Cracks in lithium fluoride [1], sapphire [2], ceramics [3–9], plexiglass [10] and ice [11] could heal through atom diffusion during heating. However, the crack healing is not always successful during heating. Thompson et al. [12] found that the cracks in Al2O3⫹SiC closed during annealing, whereas the cracks in Al2O3 propagated, which was induced by the grain-localized frictional traction generated by thermal expansion anisotropy relax rapidly. Till now, most observations of crack healing were carried out in ceramic materials [3–9], and three mechanisms have been proposed for crack healing in ceramics: diffusion thermal healing, adhesion from intermolecular forces and reaction products from chemical reactions at the crack tip [13]. Only a few works mentioned crack healing in metals or alloys [14 –16]. It was believed that the crack healing process in metals was mainly controlled by the diffusion and migration of metal atoms, and consisted of two steps, crack filling and grain growth [16]. Recrystallization of alloys could induce the crack to seal off by moving grain boundaries. In this present study, in-situ TEM observation of crack healing in single crystal iron during heating was carried out. The healing process of a crack at nanometer scale is presented, followed by a brief discussion on the crack healing in this experiment. Experimental The material used is single crystal iron of 99.99% purity. After mechanical polishing to a thickness of about 100m, the electropolishing was carried out in a solution of 10% H2SO4 ⫹ 90% methanol until a hole was created. In-situ TEM observations were performed using a Philip H-800 transmission electron microscope. In order to generate a precrack, sometimes the specimen was first tensioned by the loading stage in TEM. When the applied load was high enough, a main crack initiated along the edge of the hole as shown in Figure 1(a). Increasing the applied load continuously, some microcracks might initiate near the main crack, like microcrack C in Figure 2(a), and microcrack OP in Figure 3(a). Then the pre-deformed specimen was unloaded and transferred to a heating stage. During the unloading and transferring, the morphology of those precracks almost did not change and the precracks did not close. 1359-6462/01/$–see front matter. © 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(01)00671-6
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Figure 1. The healing process of the tip of a main crack along the edge of a hole during heating.
Figure 2. The healing process of a microcrack during heating.
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Figure 3. The healing process of a long and narrow microcrack during heating.
The pre-deformed specimen was heated in the TEM, with a heating rate of 10°C/min in most of the cases. The sample tilting remained constant during heating. Results and Discussion A typical original crack AB along the edge of the hole is shown in Figure 1(a). As the temperature increased from room temperature (Fig. 1(a)) through 773K (Fig. 1(b)) and 973K (Fig. 1(c)) to 1160K (Fig. 1(d)), the morphology of the specimen changed considerably due to an increase in thermal vibration of atoms, and the fringes in the thin area became invisible. After cooling down to room temperature (Fig. 1(e)), the specimen recovered part of the initial contrast. There was a microcrack (ab) ahead of the tip of the main crack AB, as shown in Figure 1(a). During heating, the microcrack (ab) started healing at 973K (Fig. 1(c)) and completely healed at 1160K (Fig. 1(d)). When the temperature was decreased down to room temperature, the microcrack(ab) did not appear again, as shown in Figure 1(e). There was a microcrack C near the main crack AB, as shown in Figure 2(a). The microcrack C did not change during heating from 300K to 973K. When the temperature increased from 973K to 1000K, however, the size of the microcrack C decreased gradually from 250nm to 160nm, as shown in Figure 2(b). Increasing the temperature to1023K, the size of the microcrack decreased further to 70nm, as shown in Figure 2(c). Keeping about 100 seconds at the temperature of 1023K, the microcrack C totally healed, as shown in Figure 2(d). The healing process of a long and narrow microcrack is shown in Figure 3. A microcrack OP was beside the main crack MN, and the tip of the main crack MN was marked by a small black arrow. Increasing the temperature to 863K, the microcrack OP started shrinking from each side of the microcrack along the length orientation, and at the same time, the main crack MN propagated(Fig. 3(b)).
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TABLE 1. The Crack Length 2a and the Rate of Crack Healing at Different Temperatures T, K
973
1000
1023
1023
2a, nm t, s da/dt, nm/s
250 0 —
160 162 0.28
70 300 0.33
0 400 0.35
At 973K, the microcrack OP had completely healed, and the main crack MN propagated further, as shown in Figure 3(c). In ceramics, a crack could heal completely when the specimen was heated to a given temperature and kept at that temperature for a long time [1,7]. Even a surface crack in ceramics could also heal after annealing at high temperature for a long time without contamination. But in the present study, when temperature increases, the single crystal Fe is oxidized gradually, and the oxide film impedes the healing process. Only when the heating rate is fast enough so that the rate of crack healing is higher than the oxidization rate of the crack surface, the crack can completely heal. Based on Figure 2, the size of the crack as a function of time during healing is given in Table 1. The rates of crack healing at different temperatures are also listed in Table 1. The oxidization rate of iron at 973K is less than 0.01nm/s, which is far smaller than the rate of crack healing [17]. The average rate of crack healing in Table 1 is about da/dt ⫽ 0.32nm/s. According to literature data, the self-diffusion coefficient of ␣-Fe is D ⫽ 2.0exp(-28950/T) cm2/s [18] or D ⫽ 118exp(-35800/ T)cm2/s [19]. The average diffusion coefficient between 973K and 1023K is D ⫽ 3.8 ⫻ 10⫺13 cm2/s, and the total diffusion time t ⫽ 400s. The distance of self-diffusion of ␣-Fe atoms is ⫻ ⫽ (Dt)1/2 ⫽ 123nm, which is basically consistent with the radius decrease of the crack C. Therefore, the healing of the crack in ␣-Fe can be due to self-diffusion of ␣-Fe atoms during heating. Conclusion In situ observations in TEM show that a crack in single crystal Fe can heal completely upon heating, and under a low oxygen partial pressure. Acknowledgment The present work was supported by a special Fund for the Major State Basic Research Projects (No.G19990650) and by the National Natural Science Foundation of China (No.19891180, 59871010). References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Z. Y. Wang, Y. Z. Li, M. P. Harmer, and Y. T. Chou, J. Am. Ceram. Soc. 75, 1596 (1992). J. Ro¨del and A. M. Glaeser, J. Am. Ceram. Soc. 73, 592 (1990). T. K. Gupta, J. Am. Ceram. Soc. 58, 143 (1975). C. Q. Ru, Acta Mater. 47, 4683 (1999). A. G. Evans and E. A. Charles, Acta Mater. 25, 919 (1977). M. Mitomo, T. Nishimura, and M. Tsutsumi, J. Mater. Sci. Lett. 15, 1976 (1996). B. A. Wilson and E. D. Case, J. Mater. Sci. 32, 3163 (1997). M. K. C. Holden and V. D. Frechette, J. Am. Ceram. Soc. 72, 2189 (1989). R. L. Lethman, R. E. Hill JR, and G. H. Sigel JR, J. Am. Ceram. Soc., 72, 474 (1989). J. D. Powers, etc., J. Am. Ceram. Soc. 71, 2225 (1993).
Vol. 44, No. 7
11. 12. 13. 14. 15. 16. 17. 18. 19.
CRACK HEALING IN ␣-Fe
1059
S. C. Colbeck, Acta Metall. 34, 89 (1986). A. M. Thompson, H. M. Chan, M. P. Harmer, and R. F. Cook, J. Am. Ceram. Soc. 75, 567 (1995). E. D. Case, J. R. Smyth, and O. Hunter, Jr., in Fracture Mechanics of Ceramics, Vol. 5, p. 507, Plenum Press, New York (1983). J. Foct, N. Akdut, and G. Gottstein, Scripta Metal. Mater. 27, 1033 (1992). R. U. Vaidya, A. K. Zurek, A. Wolfenden, and M. W. Cantu, J. Mater. Eng. Perf. 6, 46 (1997). J. T. Han, G. Zhao, and Q. X. Cao, Sci. China, 27E, 23 (1997). P. L. Fan and L. O. Brockway, J. Electrochem. Soc., 121, 1534 (1974). W. D. Callister, Materials Science and Engineering, p. 75, John Wiley & Sons, New York (1985). P. G. Shewmon, Diffusion in Solids, p. 80, McGraw-Hill, New York (1963).