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Grain refinement of commercially pure zirconium by ECAP and subsequent intermediate heat treatment
Materials Science and Engineering A 449–451 (2007) 1087–1089
Grain refinement of commercially pure zirconium by ECAP and subsequent intermediate heat treatment B.S. Lee a,∗ , M.H. Kim a , S.K. Hwang a , S.I. Kwun b , S.W. Chae c a
b
School of Materials Science and Engineering, Inha University, Incheon 402-751, South Korea Department of Materials Science and Engineering, Korea University, Seoul 136-701, South Korea c Department of Mechanical Engineering, Korea University, Seoul 136-701, South Korea
Received 22 August 2005; received in revised form 12 November 2005; accepted 27 February 2006
Abstract Microstructural evolution of commercially pure zirconium (Zr702) processed by equal channel angular pressing (ECAP) and subsequent intermediate heat treatment was investigated. ECAP was carried out at room temperature, and the sample was rotated 90◦ in the same sense in each pass. Also, intermediate heat treatment of the ECAP samples processed by was performed at 773 and 873 K for their recovery and recrystallization temperatures. When the samples were annealed at recrystallization temperature of 873 K, the mean grain size of these samples decreased from ∼40 to ∼3 m by the further four passes of ECAP. However, when the samples were annealed at recovery temperature of 773 K, the mean grain size was found to be 0.2 m after the same ECAP process. A difference in microstructural evolution with annealing processes indicated that the rate of formation of new grains for ultra-fine-grained Zr702 was accelerated due to the high fraction of subgrains into the matrix of Zr702 after recovery. © 2006 Elsevier B.V. All rights reserved. Keywords: ECAP; Zr alloys; UFG materials; Recovery; Recrystallization
1. Introduction Equal channel angular pressing (ECAP) has attracted the attention of many researchers because of its effectiveness in producing bulk ultra-fine-grained (UFG) materials of grain sizes in a range of 200–500 nm [1–9]. In this technique, a sample is pressed through a die with two intersecting channels equal in cross section and deformed via a simple shear at the intersection of angular channels [2–15]. There have been many reports on the UFG materials produced by the ECAP process, which include face centered cubic (FCC) (Al alloys, Cu and Ni) and body centered cubic (BCC) (low carbon steel) metals [1–11]. Application of ECAP to hexagonal close-packed (HCP) metals such as zirconium and its alloys is of great interest, since the plastic deformability of these alloys is normally inferior to that of cubic structured metals. This inferior deformability is mainly due to the limited slip systems available only on basal or prismatic planes in a close-packed direction (a-axis direction), i.e., a-slip. Recently, it was reported that grain refinement of
∗
Corresponding author. E-mail address: [email protected] (B.S. Lee).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.02.261
commercially pure titanium by using ECAP [16], and the deformation mechanism of commercially pure titanium during the second pass changed to dislocation slip. The objective of the present study is to investigate the microstructural evolution of commercially pure zirconium (Zr702) by ECAP and subsequent intermediate heat treatment. ECAP was carried out at room temperature via route BC . Also, subsequent intermediate heat treatment of ECAP samples processed by was performed at recovery and recrystallization temperature. The microstructural observation of samples processed further by ECAP after intermediate heat treatment was conducted by using optical microscopy (OM) and transmission electron microscopy (TEM). 2. Experimental procedures Commercially pure zirconium (Zr702) with an average grain size of ∼40 m was used as a starting material. Its composition was Zr–1.35Hf–0.14O in wt.%. The specimen of 5 mm × 5 mm × 40 mm were machined and ECAP was carried out at room temperature with a content pressing speed of 0.83 mm/s. The ECAP dies was designed to yield an effective strain of ∼0.45 (Φ, 135◦ ; Ψ , 45◦ ) for each pass [15].
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B.S. Lee et al. / Materials Science and Engineering A 449–451 (2007) 1087–1089
Specimens for microstructural evolution were cut from the transverse and longitudinal section of the pressed billets. The specimen for optical microscopy was prepared by mechanical polishing, and the specimen for TEM was prepared by mechanically polishing down to ∼80 m followed by twin jet polishing with a solution of 5% perchloric acid and 95% methanol at an applied potential of 80 V and at 213 K. TEM (Philips CM200) images and the corresponding selected area diffraction (SAD) patterns (aperture sizes, 3.5 and 0.6 m) were obtained at 200 V. Samples of the recrystallization and recovery experiments were cut from Zr702 processed by ECAP in a size of 5 mm × 5 mm × 5 mm. The samples were isothermal annealed at the temperature range of 733–873 K for 1 h in a vacuum less than 10−4 mbar. Vickers hardness (HV ) test with a load of 500 g and duration time of 30 s were conducted after the annealing. 3. Results and discussion
Fig. 2. Change of micro-hardness of ECAP’ed Zr702 with annealing temperature for 1 h.
ECAP for grain-refinement of Zr702 was successfully extruded up to four passes. However, as shown in Fig. 1(a), the crack in Zr702 sample occurred during the five passes. These results indicate that further ECAP for Zr702 was not easy. The shear bands (SBs) were observed in microstructure of Zr702 after five passes, as shown in Fig. 1(b). It is now well accepted that the formation of SBs is the major factor causing crack nucleation as well as premature failure. As a dominant surface deformation feature, SBs in cyclically deformed UFG copper were first reported by Agnew and Weertman [17], and were
Fig. 1. (a) Sample and (b) TEM micrograph of the Zr702 after five passes ECAP at room temperature.
Fig. 3. TEM micrographs and SAD patterns (aperture size, 3.5 m) of the further four passes ECAP’ed Zr702 (a) after recrystallization at 873 K and (b) after recovery at 773 K.
B.S. Lee et al. / Materials Science and Engineering A 449–451 (2007) 1087–1089
1089
CP-titanium processed by ECAP with route BC were reported by Stolyarov et al. [20]. As shown Fig. 3(b), the mean grain size of the Zr702 was decreased by ECAP and subsequent intermediate heat treatment (recovery). Fig. 4 shows the TEM images and corresponding SAD pattern of Zr702 after recovery at 773 K. In Fig. 4, the subgrains with size of ∼0.1 m were found to be formed within elongated grains. These results indicated that the rate of deformation for ultra-fine-grained Zr702 was accelerated due to the high fraction of subgrains with size of ∼0.1 m in Zr702 after recovery. 4. Summary
Fig. 4. TEM micrograph and corresponding SAD pattern (aperture size, 0.6 m) of the Zr702 after recovery at 773 K.
further confirmed by Hashimoto et al. [18]. Deformation heterogeneities are observed in most of the grains. Fig. 1(b) shows the presence of a banded structure developed within a single coarse grain. SBs resulted from the fragmentation of unstable grains into regions of common orientations separated by narrow transition bands. These regions deformed uniformly but with different combinations of slip systems. The tendency of grains for banding can be related to the size and orientation effects [19]. In order to grain refinement of Zr702, subsequent intermediate heat treatment after four passes was performed at the temperature of recrystallization and recovery. During annealing, kinetics both for recovery and recrystallization were very inhomogeneous in processed by ECAP Zr702. Fig. 2 shows the measured hardness of Zr702 samples after intermediate heat treatment. The hardness values of Zr702 samples were decreased with increasing annealing temperature, and a plateau at 170 HV is attained. At 793 K, the hardness dramatically decreased was caused by recrystallization into the matrix of Zr702. These results indicate that the recovery and recrystallization temperatures of ECAP’ed Zr702 were 773 and 873 K, respectively. After intermediate heat treatment, further four passes ECAP with route BC was performed at room temperature. Fig. 3 shows the TEM micrographs and corresponding SAD patterns of further ECAP’ed Zr702 samples after intermediate heat treatment. As shown in Fig. 3(a), from the Zr702 sample submitted to four further ECAP passes after recrystallization at 873 K, high dislocation density within coarse grains was observed. And the SAD pattern, which was taken from the transverse section, showed the spot pattern, which indicated the coarse grains. However, from the Zr702 sample submitted to four further ECAP passes after recovery at 773 K, as shown in Fig. 3(b), homogeneous and equiaxed grains with mean size of 0.26 m was observed. And the SAD pattern indicated the existence of a large fraction of ultra fine grains. Equiaxed structure of ultra-fine grains
ECAP has been used to deform samples of commercial purity zirconium (Zr702) to a strain of ∼1.8 at room temperature. However, fracture was occurred during five passes. In order to continue of ECAP, intermediated heat treatment was performed after the four passes. The microstructure of the further four passes ECAP’ed Zr702 after recrystallization revealed that the mean grain size was decrease from ∼40 to ∼3 m. However, for further four passes ECAP’ed Zr702 after recovery, the minimum grain size was found to be 0.2 m without fracture occurrence. Such a difference in microstructural evolution was caused by the fraction of subgrains into the matrix of Zr702 after annealing. These resulted indicated that the rate of deformation for-ultra fine-grained Zr702 was accelerated due to the high fraction of subgrain with size of ∼0.1 m in Zr702 after recovery. References [1] R.Z. Valiev, D.A. Sailmomenko, N.K. Tsenev, P.B. Berbon, T.G. Langdon, Scripta Mater. 37 (1997) 1945. [2] Y. Iwahashi, M. Furukawa, Z. Horita, M. Nemoto, T.G. Langdon, Metall. Mater. Trans. A 29 (1998) 2245. [3] D.H. Shin, W.J. Kim, W.Y. Choo, Scripta Mater. 41 (1999) 259. [4] R.Z. Valiev, I.V. Alexandrov, Nanostruct. Mater. 12 (1999) 35. [5] R.Z. Valiev, Met. Meter. Int. 7 (2001) 413. [6] D.H. Shin, J. Kim, K.T. Park, Met. Meter. Int. 7 (2001) 431. [7] L. Lin, Z. Liu, L. Chen, T. Liu, S. Wu, Met. Meter. Int. 10 (2004) 501. [8] Y.B. Lee, D.H. Shin, W.J. Nam, Met. Meter., Inter. 10 (2004) 407. [9] K.T. Park, D.Y. Hwang, D.H. Shin, Met. Meter. Int. 8 (2002) 519. [10] P.B. Berbon, N.K. Tsenev, R.Z. Valiev, M. Furukawa, Z. Horita, M. Nemoto, T.G. Langdon, Metall. Mater. Trans. A 29 (1998) 2237. [11] S.L. Semiatin, V.M. Segal, R.L. Goetz, R.E. Goforth, K.T. Hartwig, Scpita Metall. Mater. 24 (1995) 535. [12] V.M. Segal, Mater. Sci. Eng. A 197 (1995) 157. [13] V.V. Stoyalov, T. Yuntian, T.C. Lowe, R.Z. Valiev, Mater. Sci. Eng. A 303 (2001) 82. [14] M. Furukawa, Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, Mater. Sci. Eng. A 257 (1988) 328. [15] Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, Acta Mater. 45 (1997) 4724. [16] D.H. Shin, I. Kim, J. Kim, Met. Meter. Int. 8 (2002) 513. [17] S.R. Agnew, J.R. Weertman, Mater. Sci. Eng. A 244 (1998) 145. [18] S. Hashimoto, Y. Kaneko, K. Kitagawa, A. Vinogradov, R.Z. Valiev, Mater. Sci. Forum 312–314 (1999) 593. [19] C.S. Lee, B.J. Duggan, R.E. Smallman, Acta Metall. Mater. 41 (1993) 2265. [20] V.B. Stolyarov, Y.T. Zhu, I.V. Alexandrov, T.C. Lowe, R.Z. Valiev, Mater. Sci. Eng. A. 303 (2001) 82.
Grain refinement of commercially pure zirconium by ECAP and subsequent intermediate heat treatment B.S. Lee a,∗ , M.H. Kim a , S.K. Hwang a , S.I. Kwun b , S.W. Chae c a
b
School of Materials Science and Engineering, Inha University, Incheon 402-751, South Korea Department of Materials Science and Engineering, Korea University, Seoul 136-701, South Korea c Department of Mechanical Engineering, Korea University, Seoul 136-701, South Korea
Received 22 August 2005; received in revised form 12 November 2005; accepted 27 February 2006
Abstract Microstructural evolution of commercially pure zirconium (Zr702) processed by equal channel angular pressing (ECAP) and subsequent intermediate heat treatment was investigated. ECAP was carried out at room temperature, and the sample was rotated 90◦ in the same sense in each pass. Also, intermediate heat treatment of the ECAP samples processed by was performed at 773 and 873 K for their recovery and recrystallization temperatures. When the samples were annealed at recrystallization temperature of 873 K, the mean grain size of these samples decreased from ∼40 to ∼3 m by the further four passes of ECAP. However, when the samples were annealed at recovery temperature of 773 K, the mean grain size was found to be 0.2 m after the same ECAP process. A difference in microstructural evolution with annealing processes indicated that the rate of formation of new grains for ultra-fine-grained Zr702 was accelerated due to the high fraction of subgrains into the matrix of Zr702 after recovery. © 2006 Elsevier B.V. All rights reserved. Keywords: ECAP; Zr alloys; UFG materials; Recovery; Recrystallization
1. Introduction Equal channel angular pressing (ECAP) has attracted the attention of many researchers because of its effectiveness in producing bulk ultra-fine-grained (UFG) materials of grain sizes in a range of 200–500 nm [1–9]. In this technique, a sample is pressed through a die with two intersecting channels equal in cross section and deformed via a simple shear at the intersection of angular channels [2–15]. There have been many reports on the UFG materials produced by the ECAP process, which include face centered cubic (FCC) (Al alloys, Cu and Ni) and body centered cubic (BCC) (low carbon steel) metals [1–11]. Application of ECAP to hexagonal close-packed (HCP) metals such as zirconium and its alloys is of great interest, since the plastic deformability of these alloys is normally inferior to that of cubic structured metals. This inferior deformability is mainly due to the limited slip systems available only on basal or prismatic planes in a close-packed direction (a-axis direction), i.e., a-slip. Recently, it was reported that grain refinement of
∗
Corresponding author. E-mail address: [email protected] (B.S. Lee).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.02.261
commercially pure titanium by using ECAP [16], and the deformation mechanism of commercially pure titanium during the second pass changed to dislocation slip. The objective of the present study is to investigate the microstructural evolution of commercially pure zirconium (Zr702) by ECAP and subsequent intermediate heat treatment. ECAP was carried out at room temperature via route BC . Also, subsequent intermediate heat treatment of ECAP samples processed by was performed at recovery and recrystallization temperature. The microstructural observation of samples processed further by ECAP after intermediate heat treatment was conducted by using optical microscopy (OM) and transmission electron microscopy (TEM). 2. Experimental procedures Commercially pure zirconium (Zr702) with an average grain size of ∼40 m was used as a starting material. Its composition was Zr–1.35Hf–0.14O in wt.%. The specimen of 5 mm × 5 mm × 40 mm were machined and ECAP was carried out at room temperature with a content pressing speed of 0.83 mm/s. The ECAP dies was designed to yield an effective strain of ∼0.45 (Φ, 135◦ ; Ψ , 45◦ ) for each pass [15].
1088
B.S. Lee et al. / Materials Science and Engineering A 449–451 (2007) 1087–1089
Specimens for microstructural evolution were cut from the transverse and longitudinal section of the pressed billets. The specimen for optical microscopy was prepared by mechanical polishing, and the specimen for TEM was prepared by mechanically polishing down to ∼80 m followed by twin jet polishing with a solution of 5% perchloric acid and 95% methanol at an applied potential of 80 V and at 213 K. TEM (Philips CM200) images and the corresponding selected area diffraction (SAD) patterns (aperture sizes, 3.5 and 0.6 m) were obtained at 200 V. Samples of the recrystallization and recovery experiments were cut from Zr702 processed by ECAP in a size of 5 mm × 5 mm × 5 mm. The samples were isothermal annealed at the temperature range of 733–873 K for 1 h in a vacuum less than 10−4 mbar. Vickers hardness (HV ) test with a load of 500 g and duration time of 30 s were conducted after the annealing. 3. Results and discussion
Fig. 2. Change of micro-hardness of ECAP’ed Zr702 with annealing temperature for 1 h.
ECAP for grain-refinement of Zr702 was successfully extruded up to four passes. However, as shown in Fig. 1(a), the crack in Zr702 sample occurred during the five passes. These results indicate that further ECAP for Zr702 was not easy. The shear bands (SBs) were observed in microstructure of Zr702 after five passes, as shown in Fig. 1(b). It is now well accepted that the formation of SBs is the major factor causing crack nucleation as well as premature failure. As a dominant surface deformation feature, SBs in cyclically deformed UFG copper were first reported by Agnew and Weertman [17], and were
Fig. 1. (a) Sample and (b) TEM micrograph of the Zr702 after five passes ECAP at room temperature.
Fig. 3. TEM micrographs and SAD patterns (aperture size, 3.5 m) of the further four passes ECAP’ed Zr702 (a) after recrystallization at 873 K and (b) after recovery at 773 K.
B.S. Lee et al. / Materials Science and Engineering A 449–451 (2007) 1087–1089
1089
CP-titanium processed by ECAP with route BC were reported by Stolyarov et al. [20]. As shown Fig. 3(b), the mean grain size of the Zr702 was decreased by ECAP and subsequent intermediate heat treatment (recovery). Fig. 4 shows the TEM images and corresponding SAD pattern of Zr702 after recovery at 773 K. In Fig. 4, the subgrains with size of ∼0.1 m were found to be formed within elongated grains. These results indicated that the rate of deformation for ultra-fine-grained Zr702 was accelerated due to the high fraction of subgrains with size of ∼0.1 m in Zr702 after recovery. 4. Summary
Fig. 4. TEM micrograph and corresponding SAD pattern (aperture size, 0.6 m) of the Zr702 after recovery at 773 K.
further confirmed by Hashimoto et al. [18]. Deformation heterogeneities are observed in most of the grains. Fig. 1(b) shows the presence of a banded structure developed within a single coarse grain. SBs resulted from the fragmentation of unstable grains into regions of common orientations separated by narrow transition bands. These regions deformed uniformly but with different combinations of slip systems. The tendency of grains for banding can be related to the size and orientation effects [19]. In order to grain refinement of Zr702, subsequent intermediate heat treatment after four passes was performed at the temperature of recrystallization and recovery. During annealing, kinetics both for recovery and recrystallization were very inhomogeneous in processed by ECAP Zr702. Fig. 2 shows the measured hardness of Zr702 samples after intermediate heat treatment. The hardness values of Zr702 samples were decreased with increasing annealing temperature, and a plateau at 170 HV is attained. At 793 K, the hardness dramatically decreased was caused by recrystallization into the matrix of Zr702. These results indicate that the recovery and recrystallization temperatures of ECAP’ed Zr702 were 773 and 873 K, respectively. After intermediate heat treatment, further four passes ECAP with route BC was performed at room temperature. Fig. 3 shows the TEM micrographs and corresponding SAD patterns of further ECAP’ed Zr702 samples after intermediate heat treatment. As shown in Fig. 3(a), from the Zr702 sample submitted to four further ECAP passes after recrystallization at 873 K, high dislocation density within coarse grains was observed. And the SAD pattern, which was taken from the transverse section, showed the spot pattern, which indicated the coarse grains. However, from the Zr702 sample submitted to four further ECAP passes after recovery at 773 K, as shown in Fig. 3(b), homogeneous and equiaxed grains with mean size of 0.26 m was observed. And the SAD pattern indicated the existence of a large fraction of ultra fine grains. Equiaxed structure of ultra-fine grains
ECAP has been used to deform samples of commercial purity zirconium (Zr702) to a strain of ∼1.8 at room temperature. However, fracture was occurred during five passes. In order to continue of ECAP, intermediated heat treatment was performed after the four passes. The microstructure of the further four passes ECAP’ed Zr702 after recrystallization revealed that the mean grain size was decrease from ∼40 to ∼3 m. However, for further four passes ECAP’ed Zr702 after recovery, the minimum grain size was found to be 0.2 m without fracture occurrence. Such a difference in microstructural evolution was caused by the fraction of subgrains into the matrix of Zr702 after annealing. These resulted indicated that the rate of deformation for-ultra fine-grained Zr702 was accelerated due to the high fraction of subgrain with size of ∼0.1 m in Zr702 after recovery. References [1] R.Z. Valiev, D.A. Sailmomenko, N.K. Tsenev, P.B. Berbon, T.G. Langdon, Scripta Mater. 37 (1997) 1945. [2] Y. Iwahashi, M. Furukawa, Z. Horita, M. Nemoto, T.G. Langdon, Metall. Mater. Trans. A 29 (1998) 2245. [3] D.H. Shin, W.J. Kim, W.Y. Choo, Scripta Mater. 41 (1999) 259. [4] R.Z. Valiev, I.V. Alexandrov, Nanostruct. Mater. 12 (1999) 35. [5] R.Z. Valiev, Met. Meter. Int. 7 (2001) 413. [6] D.H. Shin, J. Kim, K.T. Park, Met. Meter. Int. 7 (2001) 431. [7] L. Lin, Z. Liu, L. Chen, T. Liu, S. Wu, Met. Meter. Int. 10 (2004) 501. [8] Y.B. Lee, D.H. Shin, W.J. Nam, Met. Meter., Inter. 10 (2004) 407. [9] K.T. Park, D.Y. Hwang, D.H. Shin, Met. Meter. Int. 8 (2002) 519. [10] P.B. Berbon, N.K. Tsenev, R.Z. Valiev, M. Furukawa, Z. Horita, M. Nemoto, T.G. Langdon, Metall. Mater. Trans. A 29 (1998) 2237. [11] S.L. Semiatin, V.M. Segal, R.L. Goetz, R.E. Goforth, K.T. Hartwig, Scpita Metall. Mater. 24 (1995) 535. [12] V.M. Segal, Mater. Sci. Eng. A 197 (1995) 157. [13] V.V. Stoyalov, T. Yuntian, T.C. Lowe, R.Z. Valiev, Mater. Sci. Eng. A 303 (2001) 82. [14] M. Furukawa, Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, Mater. Sci. Eng. A 257 (1988) 328. [15] Y. Iwahashi, Z. Horita, M. Nemoto, T.G. Langdon, Acta Mater. 45 (1997) 4724. [16] D.H. Shin, I. Kim, J. Kim, Met. Meter. Int. 8 (2002) 513. [17] S.R. Agnew, J.R. Weertman, Mater. Sci. Eng. A 244 (1998) 145. [18] S. Hashimoto, Y. Kaneko, K. Kitagawa, A. Vinogradov, R.Z. Valiev, Mater. Sci. Forum 312–314 (1999) 593. [19] C.S. Lee, B.J. Duggan, R.E. Smallman, Acta Metall. Mater. 41 (1993) 2265. [20] V.B. Stolyarov, Y.T. Zhu, I.V. Alexandrov, T.C. Lowe, R.Z. Valiev, Mater. Sci. Eng. A. 303 (2001) 82.