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Comparison of shape memory behavior and properties of iron-based shape memory alloys containing samarium addition
Materials Science and Engineering A 457 (2007) 169–172
Comparison of shape memory behavior and properties of iron-based shape memory alloys containing samarium addition R.A. Shakoor, F. Ahmad Khalid ∗ Faculty of Metallurgy and Materials Engineering, GIK Institute of Engineering Sciences and Technology, Topi, NWFP, Pakistan Received 17 October 2006; received in revised form 2 December 2006; accepted 13 December 2006
Abstract The effect of a samarium addition on the shape memory behavior of Fe–14Mn–3Si–10Cr–5Ni (wt%) alloys has been studied. It is found that the addition of samarium resulted in 27% increase in the shape memory effect. It was noticed that the addition of samarium had also improved the mechanical properties, increased c/a ratios and inhibited the formation of ␣´ which is considered detrimental for the shape memory behavior. The results of alloys with and without samarium additions are compared to elucidate the beneficial response of samarium. © 2006 Elsevier B.V. All rights reserved. Keywords: Shape memory alloys; Shape memory effect (SME); Strain; Training cycle
1. Introduction Shape memory alloys are used in an array of applications ranging from biomedical, automobile and aerospace engineering. Iron-based shape memory alloys are considered as potential candidates for different applications owing to their low cost, good workability, good machinability and good weldability [1–5]. However, the applications of these alloys have been limited so far primarily because of the fact that the shape memory effect (SME) is not comparable to that observed in Ni–Ti-based alloys. Nevertheless, the alloys produced by special methods such as single crystals and thin foil specimens have exhibited considerable improvements in the shape memory behavior. The shape recovery in these alloys which is based on the thermomechanical treatment, may lead to increase in the production cost and difficulties involved in fabrication of complex shapes. There have been several investigations carried out to consider improvement in the shape memory effect by involving training cycles, solid solution strengthening, centrifugal casting, thermomechanical treatments, precipitation hardening and high strain rate deformation [6–8]. Recently it has been found that the precipitation of VN can improved the shape memory behavior owing to their fine dispersion in the austenitic matrix. This
∗
Corresponding author. Tel.: +92 938 271858; fax: +92 938 271880. E-mail address: [email protected] (F.A. Khalid).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.12.048
can eventually increase the strength and also prevent permanent deformation caused by the slip [6]. Furthermore, it has been reported [9–12] that the shape recovery strain and shape recovery stress can be improved by the precipitation of NbC. These precipitates are the potential sites for nucleation of martensite plates and increase the strength of austenite, thus effectively preventing the slip deformation when subjected to the external force for producing a shape change of the specimen. This would be another factor considered to improve SME in the NbC containing alloys. Although the NbC precipitates are potential nucleation sites for stress-induced martensite formation, but they may also become obstacles for martensite growth, which will then generate back stress acting on the martensite plate tip. This back stress helps the reverse movement of the Shockley partial dislocations at the tip of the martensite plate, resulting in a good shape recovery. Solid solution strengthening is also another suitable choice for improvement of shape memory effect in Fe–Mn–Si shape memory alloys. It is reported [13] that rare earth additions may reduce the stacking fault energy, increase the amount of thermal or stress-induced martensite, lower the TN temperature of austenite and strengthen the austenite. Consequently, the addition of rare earth element in the Fe–Mn–Si alloy system has attracted considerable attention owing to the improved shape memory behavior. This may reduce the proof stress, increase the strain-hardening exponent by solid solution strengthening and raise the Ms and As temperatures. Similar influence of the
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Table 1 Chemical compositions of the alloys Alloy
% Mn
% Si
% Cr
% Ni
% Sm
% Fe
% TEa
1 2
13.94 13.29
2.73 2.82
9.89 9.67
5.20 5.27
– 0.64
68.12 68.28
0.12 0.03
a
Trace elements (Al,Ti,Cu).
rare-earth (RE) additions in single and combinations on the shape memory effect of iron-based alloy is described elsewhere [14–17]. The effect of samarium addition on the shape memory of behavior of Fe–Mn–Si alloys has not been reported previously. This work reports the behavior of shape memory alloys containing samarium which have shown an increase in the SME. A comparison of the properties is presented to highlight the role of samarium addition which contributed to the evolution of the final microstructure.
Fig. 1. Effect of samarium additions on the transformation temperatures of alloy samples (1 and 2).
2. Experimental The chemical compositions of the two alloys with and without Sm addition are presented in Table 1. They were prepared in an arc-melting furnace using commercial purity elements in an argon atmosphere. The arc-melted button samples were homogenized at 1100 ◦ C for 3 h in an argon atmosphere. After homogenizing the samples were hot rolled at 1100 ◦ C to 0.50–0.70 mm thick strips. The strip samples were annealed at 900 ◦ C for 10 min followed by furnace cooling. 25 mm × 3 mm × 0.55 mm size specimens were wire cut by an electrical discharge machine. The solution treatment was carried out by austenitizing at 1000 ◦ C for 5 min in an argon atmosphere followed by water quenching. The specimens were strained by 5% at room temperature using an Instron (30 kN) tensile testing machine at a strain rate of 0.08 × 10−6 s−1 . The specimens were recovered at 600 ◦ C for 20 min in the presence of argon. The specimens were subjected to six training cycles during the thermomechanical treatments i.e. alloy specimens of both the alloys were repeatedly strained to 5% at room temperature and then were recovered at 600 ◦ C for six times in alternate manner. After every training cycle i.e. straining and recovery treatment, the shape memory effect was measured by analysis of their stress strain curves, according to the procedure described in [17]. The transformation temperatures, Ms , As , Af and TN of both alloys in solution treated condition were determined by differential scanning calorimetry (DSC). The volume fraction of (hcp) martensite and lattice parameters were determined by a Philips X-ray diffractometer (model PW3710) using Cu K␣ radiation. A procedure similar to that described in [18] was adopted to compare the relative peak intensities of the phases identified in the alloys samples. Optical microscopy was performed on the samples using an Olympus microscope. 3. Results and discussion The differential scanning calorimetry (DSC) results of both the alloys are presented in Fig. 1. It can be seen that the addition
Fig. 2. Comparison of mechanical behavior of alloy samples (1 and 2).
of samarium to the alloy system has depressed the Ms temperature while it increased the As and Af temperatures. The addition of samarium also appeared to increase the thermal hysteresis [T(Af − Ms )]. These parameters are found in agreement with the properties and phases determined in the alloys. The mechanical properties measured on the solutionized alloy samples with and without samarium addition are presented in Fig. 2. A considerable improvement in the strength of alloy 2 containing samarium can be explained by an increase in the strength of the austenite. The comparison of calculated values of lattice constants and c/a ratios of the alloy samples with and without samarium are presented in Table 2. The values determined for the alloy samples are found consistent with the previous work [19–21]. An increase in the c/a ratios in the samarium containing alloy samples is clearly evident. Fig. 3 illustrates a comparison of shape memory effect of the alloy samples after inducing training cycles and strained up to 5%. The shape memory behavior of Table 2 Comparison of lattice constants and c/a ratio Alloy
a␥ (nm)
a (nm)
c (nm)
c /a
1 2
0.3557 0.3562
0.25123 0.2497
0.406 0.42558
1.6182 1.704
R.A. Shakoor, F.A. Khalid / Materials Science and Engineering A 457 (2007) 169–172
171
Fig. 5. XRD peaks showing presence of austenite (␥), epsilon () and alpha (␣) ´ martensite phases in alloy 1 (without Sm) in both annealed and 5% strained conditions. Fig. 3. Comparison of shape memory behavior of alloy samples (1 and 2) strained at 5%.
the alloys when strained to 5% showed that alloy 1 reached to its maximum value (i.e. 65.82%) after two cycles and then declined with an increase in the number of cycles and attained a minimum value (52.24%) after six training cycles. In contrast alloy 2 exhibited that the shape memory effect increased to maximum value (79.32%) after six cycles of training. It is evident that no significant change was observed after third training cycle. This indicates that the samarium addition has contributed to improvement of 27% in the shape memory effect. This can be attributed to an increase in the strength of the austenitic matrix, the high
Fig. 4. The optical micrographs showing austenite (␥) and epsilon () martensite in the alloys in the annealed condition: (a) alloy 1 and (b) alloy 2.
c/a ratios observed and the shift in the Ms temperature during training. The solid solution strengthening by samarium and rise in the Ms temperature during training may also have increased the driving force for the ␥ to transformation thus resulting in the higher amount of stress induced martensite and good shape memory effect. Similar observations have been reported in previous work [7–9,14,20,22,23]. On the contrary the drop in the shape memory effect of alloy sample 1 can be explained by formation of ␣´ martensite which is considered to be detrimental for the shape memory behavior of the alloys [24,25]. Fig. 4 shows examples of optical micrographs revealing the austenitic (␥) microstructure in the alloy samples (1 and 2) in the annealed condition. The microstructure was found identical and the presence of epsilon () martensite can also be noted in the alloy samples. Figs. 5 and 6 clearly indicate that the presence of high volume fraction of epsilon martensite () in alloy samples 1 and 2 at 5% strain. Furthermore, the presence of ␣´ martensite was identified only in alloy sample 1 which was strained at 5%. It is likely that the ␣´ martensite may have nucleated in association with the epsilon () martensite owing to the accumulation of large degree of residual stress as compared to alloy sample 2. It is reported that ␣´ martensite may restrict the reversion of () martensite and transform directly into austenite [24–27]. The findings are consistent with the thermomechanical behav-
Fig. 6. XRD peaks showing presence of austenite (␥), and epsilon () martensite phases in alloy 2 (with Sm) in both annealed and 5% strained conditions.
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R.A. Shakoor, F.A. Khalid / Materials Science and Engineering A 457 (2007) 169–172
ior observed and explain the reason for the decline in the shape memory effect of alloy sample 1 at higher strains. 4. Conclusions It has been found that the addition of samarium to the Fe–Mn–Si–Cr–Ni alloy system has produced an increase of 27% in the shape memory effect. In comparison this appears to be the consequence of strengthening of austenite, higher c/a ratios, and circumventing the formation of ␣´ martensite thus eventually contributing toward better shape memory behavior. References [1] Y.H. Wen, M. Yan, N. Li, Scripta Mater. 50 (2004) 835. [2] N. Bergeon, S. Kajiwara, T. Kikuchi, Acta Mater. 48 (2000) 4053. [3] Q. Meng, Y. Rong, S. Chen, T.Y. Hsu (Xu Zuyao), Mater. Lett. 50 (2001) 328. [4] H.C. Lin, K.M. Lin, Y.S. Chen, Surf. Coat. Technol. 194 (2005) 68. [5] Y.H. Wen, M. Yan, N. Li, Mater. Lett. 58 (2004) 899. [6] S. Farjami, K. Hiraga, H. Kubo, JIM Mater. Trans. 3 (2004) 930. [7] S. Farjami, K. Hiraga, H. Kubo, Acta Mater. 53 (2005) 419. [8] H. Kubo, S. Farjami, K. Nakamura, T. Maruyama, Mater. Sci. Eng. A 378 (2004) 343. [9] S. Kajiwara, D. Liu, T. Kikuchi, N. Shinya, Scripta Mater. 44 (2001) 2809.
[10] S. Kajiwara, D.Z. Liu, T. Kikuchi, N. Shinya, J. Phys. IV 11 (2001) 199. [11] A. Baruj, T. Kikuchi, S. Kajiwara, N. Shinya, Mater. Sci. Forum 394/395 (2002) 403. [12] A. Baruj, T. Kikuchi, S. Kajiwara, N. Shinya, Mater. Trans. 3 (2002) 585. [13] W. Zhou, B. Jiang, X. Qi, T.Y. Hsu (Xu Zuyao), Scripta Mater. 10 (1998) 1483. [14] X. Huang, Y. Lei, B. Huang, S. Chen, T.Y. Hsu, Mater. Lett. 57 (2003) 2787. [15] N. Si, Z. Jia, L. Qi, Univ. Sci. Technol. Beijing 11 (2004) 75. [16] C. Zhao, G. Liang, C. Li, Z. Jin, Scripta Mater. 7 (1998) 163. [17] J. Zhang, S. Chen, L. Li, T.Y. Hsu (Xu Zuyao), Mater. Charact. 40 (1998) 37. [18] B.D. Cullity, S.R. Stock, Elements of X-ray Diffraction, 3rd ed., Prentice Hall, NJ, 2001. [19] T.Y. Hsu, Xu Zuyao, Mater. Sci. Eng. A 273 (1999) 494. [20] L.J. Rong, D.H. Ping, Y.Y. Li, C.X. Shi, Scripta Metall. Mater. 12 (1995) 1905. [21] A. Ariapour, I. Yakubtsov, D.D. Perovic, Mater. Sci. Eng. A 262 (1999) 39. [22] Y. Watanabe, Y. Mori, A. Sato, J. Mater. Sci. 28 (1993) 1509–1514. [23] Y.H. Wen, M. Yan, N. Li, Scripta Mater. 50 (2004) 441–444. [24] W.Y. Jang, Q. Gu, J.V. Humbeek, L. Delaey, Mater. Charact. 346 (1995) 67. [25] G.J. Aruda, V.T.L. Buono, M.S. Andrade, Mater. Sci. Eng. A 273–275 (1999) 528. [26] J. Otubo, P.R. Mei, S. Koshimizu, A.H. Shinohara, C.K. Suzuki, Mater. Sci. Eng. A 273–275 (1999) 533. [27] A. Baruj, T. Kikuchi, S. Kajiwara, N. Shinya, J. Phys. IV 112 (2003) 373.
Comparison of shape memory behavior and properties of iron-based shape memory alloys containing samarium addition R.A. Shakoor, F. Ahmad Khalid ∗ Faculty of Metallurgy and Materials Engineering, GIK Institute of Engineering Sciences and Technology, Topi, NWFP, Pakistan Received 17 October 2006; received in revised form 2 December 2006; accepted 13 December 2006
Abstract The effect of a samarium addition on the shape memory behavior of Fe–14Mn–3Si–10Cr–5Ni (wt%) alloys has been studied. It is found that the addition of samarium resulted in 27% increase in the shape memory effect. It was noticed that the addition of samarium had also improved the mechanical properties, increased c/a ratios and inhibited the formation of ␣´ which is considered detrimental for the shape memory behavior. The results of alloys with and without samarium additions are compared to elucidate the beneficial response of samarium. © 2006 Elsevier B.V. All rights reserved. Keywords: Shape memory alloys; Shape memory effect (SME); Strain; Training cycle
1. Introduction Shape memory alloys are used in an array of applications ranging from biomedical, automobile and aerospace engineering. Iron-based shape memory alloys are considered as potential candidates for different applications owing to their low cost, good workability, good machinability and good weldability [1–5]. However, the applications of these alloys have been limited so far primarily because of the fact that the shape memory effect (SME) is not comparable to that observed in Ni–Ti-based alloys. Nevertheless, the alloys produced by special methods such as single crystals and thin foil specimens have exhibited considerable improvements in the shape memory behavior. The shape recovery in these alloys which is based on the thermomechanical treatment, may lead to increase in the production cost and difficulties involved in fabrication of complex shapes. There have been several investigations carried out to consider improvement in the shape memory effect by involving training cycles, solid solution strengthening, centrifugal casting, thermomechanical treatments, precipitation hardening and high strain rate deformation [6–8]. Recently it has been found that the precipitation of VN can improved the shape memory behavior owing to their fine dispersion in the austenitic matrix. This
∗
Corresponding author. Tel.: +92 938 271858; fax: +92 938 271880. E-mail address: [email protected] (F.A. Khalid).
0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.12.048
can eventually increase the strength and also prevent permanent deformation caused by the slip [6]. Furthermore, it has been reported [9–12] that the shape recovery strain and shape recovery stress can be improved by the precipitation of NbC. These precipitates are the potential sites for nucleation of martensite plates and increase the strength of austenite, thus effectively preventing the slip deformation when subjected to the external force for producing a shape change of the specimen. This would be another factor considered to improve SME in the NbC containing alloys. Although the NbC precipitates are potential nucleation sites for stress-induced martensite formation, but they may also become obstacles for martensite growth, which will then generate back stress acting on the martensite plate tip. This back stress helps the reverse movement of the Shockley partial dislocations at the tip of the martensite plate, resulting in a good shape recovery. Solid solution strengthening is also another suitable choice for improvement of shape memory effect in Fe–Mn–Si shape memory alloys. It is reported [13] that rare earth additions may reduce the stacking fault energy, increase the amount of thermal or stress-induced martensite, lower the TN temperature of austenite and strengthen the austenite. Consequently, the addition of rare earth element in the Fe–Mn–Si alloy system has attracted considerable attention owing to the improved shape memory behavior. This may reduce the proof stress, increase the strain-hardening exponent by solid solution strengthening and raise the Ms and As temperatures. Similar influence of the
170
R.A. Shakoor, F.A. Khalid / Materials Science and Engineering A 457 (2007) 169–172
Table 1 Chemical compositions of the alloys Alloy
% Mn
% Si
% Cr
% Ni
% Sm
% Fe
% TEa
1 2
13.94 13.29
2.73 2.82
9.89 9.67
5.20 5.27
– 0.64
68.12 68.28
0.12 0.03
a
Trace elements (Al,Ti,Cu).
rare-earth (RE) additions in single and combinations on the shape memory effect of iron-based alloy is described elsewhere [14–17]. The effect of samarium addition on the shape memory of behavior of Fe–Mn–Si alloys has not been reported previously. This work reports the behavior of shape memory alloys containing samarium which have shown an increase in the SME. A comparison of the properties is presented to highlight the role of samarium addition which contributed to the evolution of the final microstructure.
Fig. 1. Effect of samarium additions on the transformation temperatures of alloy samples (1 and 2).
2. Experimental The chemical compositions of the two alloys with and without Sm addition are presented in Table 1. They were prepared in an arc-melting furnace using commercial purity elements in an argon atmosphere. The arc-melted button samples were homogenized at 1100 ◦ C for 3 h in an argon atmosphere. After homogenizing the samples were hot rolled at 1100 ◦ C to 0.50–0.70 mm thick strips. The strip samples were annealed at 900 ◦ C for 10 min followed by furnace cooling. 25 mm × 3 mm × 0.55 mm size specimens were wire cut by an electrical discharge machine. The solution treatment was carried out by austenitizing at 1000 ◦ C for 5 min in an argon atmosphere followed by water quenching. The specimens were strained by 5% at room temperature using an Instron (30 kN) tensile testing machine at a strain rate of 0.08 × 10−6 s−1 . The specimens were recovered at 600 ◦ C for 20 min in the presence of argon. The specimens were subjected to six training cycles during the thermomechanical treatments i.e. alloy specimens of both the alloys were repeatedly strained to 5% at room temperature and then were recovered at 600 ◦ C for six times in alternate manner. After every training cycle i.e. straining and recovery treatment, the shape memory effect was measured by analysis of their stress strain curves, according to the procedure described in [17]. The transformation temperatures, Ms , As , Af and TN of both alloys in solution treated condition were determined by differential scanning calorimetry (DSC). The volume fraction of (hcp) martensite and lattice parameters were determined by a Philips X-ray diffractometer (model PW3710) using Cu K␣ radiation. A procedure similar to that described in [18] was adopted to compare the relative peak intensities of the phases identified in the alloys samples. Optical microscopy was performed on the samples using an Olympus microscope. 3. Results and discussion The differential scanning calorimetry (DSC) results of both the alloys are presented in Fig. 1. It can be seen that the addition
Fig. 2. Comparison of mechanical behavior of alloy samples (1 and 2).
of samarium to the alloy system has depressed the Ms temperature while it increased the As and Af temperatures. The addition of samarium also appeared to increase the thermal hysteresis [T(Af − Ms )]. These parameters are found in agreement with the properties and phases determined in the alloys. The mechanical properties measured on the solutionized alloy samples with and without samarium addition are presented in Fig. 2. A considerable improvement in the strength of alloy 2 containing samarium can be explained by an increase in the strength of the austenite. The comparison of calculated values of lattice constants and c/a ratios of the alloy samples with and without samarium are presented in Table 2. The values determined for the alloy samples are found consistent with the previous work [19–21]. An increase in the c/a ratios in the samarium containing alloy samples is clearly evident. Fig. 3 illustrates a comparison of shape memory effect of the alloy samples after inducing training cycles and strained up to 5%. The shape memory behavior of Table 2 Comparison of lattice constants and c/a ratio Alloy
a␥ (nm)
a (nm)
c (nm)
c /a
1 2
0.3557 0.3562
0.25123 0.2497
0.406 0.42558
1.6182 1.704
R.A. Shakoor, F.A. Khalid / Materials Science and Engineering A 457 (2007) 169–172
171
Fig. 5. XRD peaks showing presence of austenite (␥), epsilon () and alpha (␣) ´ martensite phases in alloy 1 (without Sm) in both annealed and 5% strained conditions. Fig. 3. Comparison of shape memory behavior of alloy samples (1 and 2) strained at 5%.
the alloys when strained to 5% showed that alloy 1 reached to its maximum value (i.e. 65.82%) after two cycles and then declined with an increase in the number of cycles and attained a minimum value (52.24%) after six training cycles. In contrast alloy 2 exhibited that the shape memory effect increased to maximum value (79.32%) after six cycles of training. It is evident that no significant change was observed after third training cycle. This indicates that the samarium addition has contributed to improvement of 27% in the shape memory effect. This can be attributed to an increase in the strength of the austenitic matrix, the high
Fig. 4. The optical micrographs showing austenite (␥) and epsilon () martensite in the alloys in the annealed condition: (a) alloy 1 and (b) alloy 2.
c/a ratios observed and the shift in the Ms temperature during training. The solid solution strengthening by samarium and rise in the Ms temperature during training may also have increased the driving force for the ␥ to transformation thus resulting in the higher amount of stress induced martensite and good shape memory effect. Similar observations have been reported in previous work [7–9,14,20,22,23]. On the contrary the drop in the shape memory effect of alloy sample 1 can be explained by formation of ␣´ martensite which is considered to be detrimental for the shape memory behavior of the alloys [24,25]. Fig. 4 shows examples of optical micrographs revealing the austenitic (␥) microstructure in the alloy samples (1 and 2) in the annealed condition. The microstructure was found identical and the presence of epsilon () martensite can also be noted in the alloy samples. Figs. 5 and 6 clearly indicate that the presence of high volume fraction of epsilon martensite () in alloy samples 1 and 2 at 5% strain. Furthermore, the presence of ␣´ martensite was identified only in alloy sample 1 which was strained at 5%. It is likely that the ␣´ martensite may have nucleated in association with the epsilon () martensite owing to the accumulation of large degree of residual stress as compared to alloy sample 2. It is reported that ␣´ martensite may restrict the reversion of () martensite and transform directly into austenite [24–27]. The findings are consistent with the thermomechanical behav-
Fig. 6. XRD peaks showing presence of austenite (␥), and epsilon () martensite phases in alloy 2 (with Sm) in both annealed and 5% strained conditions.
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R.A. Shakoor, F.A. Khalid / Materials Science and Engineering A 457 (2007) 169–172
ior observed and explain the reason for the decline in the shape memory effect of alloy sample 1 at higher strains. 4. Conclusions It has been found that the addition of samarium to the Fe–Mn–Si–Cr–Ni alloy system has produced an increase of 27% in the shape memory effect. In comparison this appears to be the consequence of strengthening of austenite, higher c/a ratios, and circumventing the formation of ␣´ martensite thus eventually contributing toward better shape memory behavior. References [1] Y.H. Wen, M. Yan, N. Li, Scripta Mater. 50 (2004) 835. [2] N. Bergeon, S. Kajiwara, T. Kikuchi, Acta Mater. 48 (2000) 4053. [3] Q. Meng, Y. Rong, S. Chen, T.Y. Hsu (Xu Zuyao), Mater. Lett. 50 (2001) 328. [4] H.C. Lin, K.M. Lin, Y.S. Chen, Surf. Coat. Technol. 194 (2005) 68. [5] Y.H. Wen, M. Yan, N. Li, Mater. Lett. 58 (2004) 899. [6] S. Farjami, K. Hiraga, H. Kubo, JIM Mater. Trans. 3 (2004) 930. [7] S. Farjami, K. Hiraga, H. Kubo, Acta Mater. 53 (2005) 419. [8] H. Kubo, S. Farjami, K. Nakamura, T. Maruyama, Mater. Sci. Eng. A 378 (2004) 343. [9] S. Kajiwara, D. Liu, T. Kikuchi, N. Shinya, Scripta Mater. 44 (2001) 2809.
[10] S. Kajiwara, D.Z. Liu, T. Kikuchi, N. Shinya, J. Phys. IV 11 (2001) 199. [11] A. Baruj, T. Kikuchi, S. Kajiwara, N. Shinya, Mater. Sci. Forum 394/395 (2002) 403. [12] A. Baruj, T. Kikuchi, S. Kajiwara, N. Shinya, Mater. Trans. 3 (2002) 585. [13] W. Zhou, B. Jiang, X. Qi, T.Y. Hsu (Xu Zuyao), Scripta Mater. 10 (1998) 1483. [14] X. Huang, Y. Lei, B. Huang, S. Chen, T.Y. Hsu, Mater. Lett. 57 (2003) 2787. [15] N. Si, Z. Jia, L. Qi, Univ. Sci. Technol. Beijing 11 (2004) 75. [16] C. Zhao, G. Liang, C. Li, Z. Jin, Scripta Mater. 7 (1998) 163. [17] J. Zhang, S. Chen, L. Li, T.Y. Hsu (Xu Zuyao), Mater. Charact. 40 (1998) 37. [18] B.D. Cullity, S.R. Stock, Elements of X-ray Diffraction, 3rd ed., Prentice Hall, NJ, 2001. [19] T.Y. Hsu, Xu Zuyao, Mater. Sci. Eng. A 273 (1999) 494. [20] L.J. Rong, D.H. Ping, Y.Y. Li, C.X. Shi, Scripta Metall. Mater. 12 (1995) 1905. [21] A. Ariapour, I. Yakubtsov, D.D. Perovic, Mater. Sci. Eng. A 262 (1999) 39. [22] Y. Watanabe, Y. Mori, A. Sato, J. Mater. Sci. 28 (1993) 1509–1514. [23] Y.H. Wen, M. Yan, N. Li, Scripta Mater. 50 (2004) 441–444. [24] W.Y. Jang, Q. Gu, J.V. Humbeek, L. Delaey, Mater. Charact. 346 (1995) 67. [25] G.J. Aruda, V.T.L. Buono, M.S. Andrade, Mater. Sci. Eng. A 273–275 (1999) 528. [26] J. Otubo, P.R. Mei, S. Koshimizu, A.H. Shinohara, C.K. Suzuki, Mater. Sci. Eng. A 273–275 (1999) 533. [27] A. Baruj, T. Kikuchi, S. Kajiwara, N. Shinya, J. Phys. IV 112 (2003) 373.