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Hydrogen‐enhanced plastic deformation during indentation for bulk metallic glass of Zr65Al7.5Ni10Cu17.5
Materials Letters 61 (2007) 1625 – 1628 www.elsevier.com/locate/matlet
Hydrogen‐enhanced plastic deformation during indentation for bulk metallic glass of Zr65Al7.5Ni10Cu17.5 G.B. Shan a , J.X. Li a,⁎, Y.Z. Yang b , L.J. Qiao a , W.Y. Chu a a
Department of Materials Physics, University of Science and Technology Beijing, Beijing 100083, PR China b Material and Energy College, Guangdong University of Technology, Guangzhou 510075, China Received 1 March 2006; accepted 22 July 2006 Available online 28 August 2006
Abstract The effect of hydrogen charging on shear bands and plastic zone during Vickers indentation for Zr65Al7.5Ni10Cu17.5 bulk metallic glass has been studied. The results showed that hardness increased gradually with charging time and reached saturation. The shear bands and the size of the plastic zone on the surface and subsurface of indentation increased evidently when charging time was less then 40 h at i = 10 mA/cm2 or 10 h at i = 100 mA/cm2, respectively. After that, the size of the plastic zone began to reduce with charging time because hydrogen blisterings began formation and growth. © 2006 Elsevier B.V. All rights reserved. Keywords: Bulk metallic glass; Hydrogen; Indentation; Plastic zone; Hardness
1. Introduction Metallic glass generally exhibits hydrogen embrittlement [1–9]. For example, an increase of flow stress [1,2], decrease of ultimate tensile strength [3,4], degradation of fracture toughness [1,5,6] and change of fracture mode from ductile to completely brittle fracture [1] were observed after hydrogen charging. Hydrogen‐induced delayed fracture could occur during charging under sustained load [7,8]. On the other hand, hydrogen blistering (or bubble) and crack appear during charging in the absence of external stress [8,9]. The degradation of fracture toughness suggested that plastic deformation at the crack tip was suppressed in the presence of hydrogen consistent with the observed increase in flow stress [1, 2]. In situ observation, however, showed that hydrogen could promote formation and growth of shear bands from notch of tip of Zr57Cu15.4Ni12.1Al10Nb5 bulk metallic glass during charging under sustained load, i.e, hydrogen‐enhanced localized plastic deformation [10].
⁎ Corresponding author. Tel.: +86 10 62334493; fax: +86 10 62332345. E-mail address: [email protected] (J.X. Li). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.07.181
For bulk metallic glasses, shear bands instead of crack appeared during indenting, therefore, indentation technique has been extensively used to study the plastic deformation [11–15]. The aim of this paper is to study the effect of hydrogen charging on localized plastic deformation around Vickers indentation for Zr65Al7.5Ni10Cu17.5 bulk metallic glass. 2. Experimental Bulk metallic glass of Zr65Al7.5Ni10Cu17.5 (nominal composition in atomic percent) was produced by arc melting the pure elements together into ingots, which were then remelted twice to ensure a homogeneous composition. Then a plate with thickness of 2 mm was made by suction casting the molten alloy into a copper mold, which was confirmed to be amorphous by X‐ray diffraction. Samples were charged in 0.5 mol/l H2SO4 + 0.25 g/l As2O3 solution with current densities of 10 mA/cm2 and 100 mA/cm2 for different times. Our previous work showed that there was no hydrogen blistering after charging at i = 10 mA/cm2 for 40 h, but hydrogen blistering appeared after charging at i = 60 mA/ cm2 for 40 h [16]. Charged samples were put into a glass tube filled with silicone oil. Based on the saturation volume of
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Fig. 1. Vickers hardness of Zr65Al7.5Ni10Cu17.5 vs charging time at i = 100 mA/ cm2, P = 9.8 N.
hydrogen evolved at room temperature V, the diffusible hydrogen concentration is C 0 (ppm) = 2 × 10 –6 V (cm 3 )/ (82.06m (g)T (K)), where m (g) is the weight of the sample and T (K) is the absolute temperature [17]. The shear bands and the plastic zone were observed using differential interference contrast (DIC) microscope. In general, the shear bands originate from the sides instead of the tips of the indent, and then the plastic zone is cut apart into four parts for each indentation. The distances from the centre and the side of the indent to the boundary of the plastic zone are defined as the size of apparent plastic zone δ and that of real plastic zone, δr, respectively. If the size of the indent becomes evidently small because of increase in hardness after charging, δr is more suitable to describe the change in plastic zone after charging. In order to measure the size of the plastic zone on the subsurface of an indentation, the polished sides of two samples were joined together using a clamp, and then indenting was
Fig. 3. The size of the plastic zone δ and δr vs time of charging at i = 100 mA/ cm2; the data are the mean of 10 indentations and the vertical line is the 95% confidence interval of the mean.
performed on the surface with the diagonal of the indent along the bounded interface. After removing the clamp, the plastic zone on the subsurface can be measured. 3. Results and discussion The average diffusible hydrogen concentration of the three samples after charging at i = 10 mA/cm2 for 168 hrs was C0 = 50 ppm; and the hydrogen concentration in traps which was measured through heating the sample to 800 °C was Ct = 497 ppm. The hardness of charged sample increases with increasing charging time, as shown in Fig. 1. The results are an average of at least 10 indentations with the error bars representing the 95% confidence interval of the mean. As a consequence, the indent in charged sample decreased with increasing charging time. The shear bands and plastic zone for the samples charged at i = 100 mA/cm2 for different times are shown in Fig. 2. Fig. 2 shows that the indent becomes gradually
Fig. 2. Shear bands and plastic zones for the samples uncharged (a), precharged at i = 100 mA/cm2 for 9.5 h (b) and 110 h (c).
G.B. Shan et al. / Materials Letters 61 (2007) 1625–1628
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Fig. 4. Shear bands and plastic zone around four indentations with P = 9.8 N on the surface of Zr65Al7.5Ni10Cu17.5 alloy uncharged (a) and precharged at i = 10 mA/cm2 for 20 h (b).
Fig. 5. Plastic zone size of δ and δr vs time of charging at i = 10 mA/cm2, the data are the mean of 10 indentations and the vertical lines the 95% confidence interval of the mean.
small with increasing charging time because of increase in hardness (Fig. 1), and the shear bands or plastic zone increases evidently after charging for 9.5 h but a few shear bands appear after charging for
110 h. Because of evident change in the indent size, δr, in which the effect of the indent size has been eliminated, is more suitable to describe the change in the plastic zone after charging. The size of the plastic zone, i.e., δ or δr, vs time of charging at i = 100 mA/cm2 is shown in Fig. 3. Fig. 3 indicates that the size of plastic zone increases evidently after charging at i = 100 mA/cm2 for 9.5 h, but begins to decrease when charging time is greater than 20 h. Our previous work indicated that a large hydrogen blistering with diameter of 0.2 mm formed in the Zr65Al7.5Ni10Cu17.5 metallic glass after charging at i = 60 mA/cm2 for 40 h, and there was no hydrogen blistering formed after charging at i = 10 mA/cm2 for 40 h, but some small hydrogen blistering formed after charging at i = 10 mA/cm2 for 168 h and larger blistering appeared after charging at i = 10 mA/cm2 for 760 h [16]. The result shows that the incubation time necessary for forming hydrogen blistering decreases with increasing the current density, and hydrogen blistering grows continuously with charging time after nucleating. The hydrogen blistering is a void with molecule hydrogen and an unsaturated trap of atomic hydrogen. As soon as hydrogen blistering forms, hydrogen concentration in matrix will decrease continuously during charging rather than increase. Because a large hydrogen blistering formed after charging at i = 60 mA/cm2 for 40 h, if i = 100 mA/cm2, a large hydrogen blistering will appear when charging time is less then 40 h, and small blistering will form at the early stage of charging, e.g. 10 to
Fig. 6. Shear bands and plastic zones on the subsurface of three indentations with P = 9.8 N in samples uncharged (a) and precharged at i = 10 mA/cm2 for 40 h (b).
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20 h. Therefore, the size of the plastic zone begins to decrease when time charged at i = 100 mA/cm2 is greater than 10 h, because of formation and growth of hydrogen blistering. Since there was no hydrogen blistering after charging at i = 10 mA/ cm2 for 40 h, charging with small current, e.g., i = 10 mA/cm2 is more suitable to study the effect of hydrogen on the plastic zone. Fig. 4 shows that the shear bands and the size of the plastic zone around the indentation increase evidently after charging at i = 10 mA/cm2 for 20 h. The plastic zone size, i.e., δ and δr, increase evidently when charging time is less than 50 h, but begin to decrease when charging time is greater than 140 h, as shown in Fig. 5. The decrease in the plastic zone size after t ≥ 140 h is believed to correspond to formation and growth of hydrogen blistering [16]. The plastic zones on the subsurface of the indentations for the samples uncharged and precharged at 10 mA/cm2 for 40 h are shown in Fig. 6. The size of the plastic zone on the subsurface of the indentation, δ⁎, is defined as the distance from the bottom of the indent to the boundary of the plastic zone, i.e. δ⁎ = AB, as shown in Fig. 6(a). Measuring for 10 indentations indicated that δ⁎ = 67 ± 3 μm for the uncharged sample and δ⁎(H) = 72 ± 3 μm for the sample precharged at 10 mA/cm2 for 40 h. Above‐mentioned results indicate that hydrogen can enhance localized plastic deformation during indentation for the bulk metallic glass if there is no hydrogen blistering. This result is consistent with the idea that hydrogen promoted formation and growth of shear bands from a notch tip of sustained load sample [10]. We don't know how to explain hydrogen embrittlement of bulk metallic glasses using hydrogen‐enhanced localized plastic deformation. It is a question that needs further investigation.
4. Summary For metallic glass, hardness increases gradually with charging time and reached saturation. The shear bands and plastic zone on the surface and subsurface of indentation increased evidently when charging time was less than 40 h at 10 mA/cm2 or 10 h at 100A/cm2. After that, the size of the
plastic zone began to reduce with increasing charging time because hydrogen blisterings began formation and growth. Acknowledgements This project was supported by National Natural Science Foundation of China (No. 50271006) and Beijing Key Laboratory of Corrosion, Erosion and Surface Technology. References [1] D. Suh, R.H. Dauskardt, Scr. Mater. 42 (2000) 233. [2] F. Spaepen, A.I. Taub, in: F.E. Luborsky (Ed.), Amorphous Metallic Alloys, Butterworth, London, 1983, p. 231. [3] J.J. Lin, T.P. Perng, Metall. Mater. Trans. 26A (1995) 197. [4] T.K. Namboodhiri, T.A. Ramesh, G. Singh, S. Seghal, Mater. Sci. Eng. 61 (1983) 23. [5] V. Schroeder, C.J. Gilbert, R.O. Ritchie, Scr. Mater. 49 (1998) 1057. [6] D. Suh, P. Asoka‐Kumar, R.H. Dauskardt, Acta Mater. 50 (2002) 537. [7] J.X. Guo, J.X. Li, L.J. Qiao, K.W. Gao, W.Y. Chu, Corros. Sci. 45 (2003) 735. [8] G.B. Shan, Y.W. Wang, W.Y. Chu, J.X. Li, X.D Hui, Corros. Sci. 47 (2005) 2731. [9] N. Eliaz, D. Eliezer, Metall. Mater. Trans. 31A (2000) 2517. [10] Y.W. Wang, W.Y. Chu, J.X. Li, X.D. Hui, Y.B. Wang, K.W. Gao, L.J. Qiao, Mater. Lett. 58 (2004) 2393. [11] Q.J. Zhou, J.Y. He, D.B. Sun, W.Y. Chu, L.J. Qiao, Scr. Mater. 54 (2006) 603. [12] D. Kramer, H. Huang, M. Kriese, J. Robach, J. Nelson, A. Wright, D. Bahr, W.W. Gerberich, Acta Mater. 47 (1999) 333. [13] R. Vaidyanathan, M. Dao, G. Ravichandran, S. Suresh, Acta Mater. 49 (2001) 3781. [14] S. Jana, U. Ramamurty, K. Chattopadhyay, Y. Kawamura, Mater. Sci. Eng., A 375–377 (2004) 1191. [15] U. Ramamurty, S. Jana, Y. Kawamura, K. Chattopadhyay, Acta Mater. 53 (2005) 705. [16] G.B. Shan, J.X. Li, B.C. Wei, L.J. Qiao, W.Y. Chu, Mater. Sci. Eng., A (submitted for publication). [17] X. Peng, Y.J. Su, K.W. Gao, L.J. Qiao, W.Y. Chu, Mater. Lett. 58 (2004) 2073.
Hydrogen‐enhanced plastic deformation during indentation for bulk metallic glass of Zr65Al7.5Ni10Cu17.5 G.B. Shan a , J.X. Li a,⁎, Y.Z. Yang b , L.J. Qiao a , W.Y. Chu a a
Department of Materials Physics, University of Science and Technology Beijing, Beijing 100083, PR China b Material and Energy College, Guangdong University of Technology, Guangzhou 510075, China Received 1 March 2006; accepted 22 July 2006 Available online 28 August 2006
Abstract The effect of hydrogen charging on shear bands and plastic zone during Vickers indentation for Zr65Al7.5Ni10Cu17.5 bulk metallic glass has been studied. The results showed that hardness increased gradually with charging time and reached saturation. The shear bands and the size of the plastic zone on the surface and subsurface of indentation increased evidently when charging time was less then 40 h at i = 10 mA/cm2 or 10 h at i = 100 mA/cm2, respectively. After that, the size of the plastic zone began to reduce with charging time because hydrogen blisterings began formation and growth. © 2006 Elsevier B.V. All rights reserved. Keywords: Bulk metallic glass; Hydrogen; Indentation; Plastic zone; Hardness
1. Introduction Metallic glass generally exhibits hydrogen embrittlement [1–9]. For example, an increase of flow stress [1,2], decrease of ultimate tensile strength [3,4], degradation of fracture toughness [1,5,6] and change of fracture mode from ductile to completely brittle fracture [1] were observed after hydrogen charging. Hydrogen‐induced delayed fracture could occur during charging under sustained load [7,8]. On the other hand, hydrogen blistering (or bubble) and crack appear during charging in the absence of external stress [8,9]. The degradation of fracture toughness suggested that plastic deformation at the crack tip was suppressed in the presence of hydrogen consistent with the observed increase in flow stress [1, 2]. In situ observation, however, showed that hydrogen could promote formation and growth of shear bands from notch of tip of Zr57Cu15.4Ni12.1Al10Nb5 bulk metallic glass during charging under sustained load, i.e, hydrogen‐enhanced localized plastic deformation [10].
⁎ Corresponding author. Tel.: +86 10 62334493; fax: +86 10 62332345. E-mail address: [email protected] (J.X. Li). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.07.181
For bulk metallic glasses, shear bands instead of crack appeared during indenting, therefore, indentation technique has been extensively used to study the plastic deformation [11–15]. The aim of this paper is to study the effect of hydrogen charging on localized plastic deformation around Vickers indentation for Zr65Al7.5Ni10Cu17.5 bulk metallic glass. 2. Experimental Bulk metallic glass of Zr65Al7.5Ni10Cu17.5 (nominal composition in atomic percent) was produced by arc melting the pure elements together into ingots, which were then remelted twice to ensure a homogeneous composition. Then a plate with thickness of 2 mm was made by suction casting the molten alloy into a copper mold, which was confirmed to be amorphous by X‐ray diffraction. Samples were charged in 0.5 mol/l H2SO4 + 0.25 g/l As2O3 solution with current densities of 10 mA/cm2 and 100 mA/cm2 for different times. Our previous work showed that there was no hydrogen blistering after charging at i = 10 mA/cm2 for 40 h, but hydrogen blistering appeared after charging at i = 60 mA/ cm2 for 40 h [16]. Charged samples were put into a glass tube filled with silicone oil. Based on the saturation volume of
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Fig. 1. Vickers hardness of Zr65Al7.5Ni10Cu17.5 vs charging time at i = 100 mA/ cm2, P = 9.8 N.
hydrogen evolved at room temperature V, the diffusible hydrogen concentration is C 0 (ppm) = 2 × 10 –6 V (cm 3 )/ (82.06m (g)T (K)), where m (g) is the weight of the sample and T (K) is the absolute temperature [17]. The shear bands and the plastic zone were observed using differential interference contrast (DIC) microscope. In general, the shear bands originate from the sides instead of the tips of the indent, and then the plastic zone is cut apart into four parts for each indentation. The distances from the centre and the side of the indent to the boundary of the plastic zone are defined as the size of apparent plastic zone δ and that of real plastic zone, δr, respectively. If the size of the indent becomes evidently small because of increase in hardness after charging, δr is more suitable to describe the change in plastic zone after charging. In order to measure the size of the plastic zone on the subsurface of an indentation, the polished sides of two samples were joined together using a clamp, and then indenting was
Fig. 3. The size of the plastic zone δ and δr vs time of charging at i = 100 mA/ cm2; the data are the mean of 10 indentations and the vertical line is the 95% confidence interval of the mean.
performed on the surface with the diagonal of the indent along the bounded interface. After removing the clamp, the plastic zone on the subsurface can be measured. 3. Results and discussion The average diffusible hydrogen concentration of the three samples after charging at i = 10 mA/cm2 for 168 hrs was C0 = 50 ppm; and the hydrogen concentration in traps which was measured through heating the sample to 800 °C was Ct = 497 ppm. The hardness of charged sample increases with increasing charging time, as shown in Fig. 1. The results are an average of at least 10 indentations with the error bars representing the 95% confidence interval of the mean. As a consequence, the indent in charged sample decreased with increasing charging time. The shear bands and plastic zone for the samples charged at i = 100 mA/cm2 for different times are shown in Fig. 2. Fig. 2 shows that the indent becomes gradually
Fig. 2. Shear bands and plastic zones for the samples uncharged (a), precharged at i = 100 mA/cm2 for 9.5 h (b) and 110 h (c).
G.B. Shan et al. / Materials Letters 61 (2007) 1625–1628
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Fig. 4. Shear bands and plastic zone around four indentations with P = 9.8 N on the surface of Zr65Al7.5Ni10Cu17.5 alloy uncharged (a) and precharged at i = 10 mA/cm2 for 20 h (b).
Fig. 5. Plastic zone size of δ and δr vs time of charging at i = 10 mA/cm2, the data are the mean of 10 indentations and the vertical lines the 95% confidence interval of the mean.
small with increasing charging time because of increase in hardness (Fig. 1), and the shear bands or plastic zone increases evidently after charging for 9.5 h but a few shear bands appear after charging for
110 h. Because of evident change in the indent size, δr, in which the effect of the indent size has been eliminated, is more suitable to describe the change in the plastic zone after charging. The size of the plastic zone, i.e., δ or δr, vs time of charging at i = 100 mA/cm2 is shown in Fig. 3. Fig. 3 indicates that the size of plastic zone increases evidently after charging at i = 100 mA/cm2 for 9.5 h, but begins to decrease when charging time is greater than 20 h. Our previous work indicated that a large hydrogen blistering with diameter of 0.2 mm formed in the Zr65Al7.5Ni10Cu17.5 metallic glass after charging at i = 60 mA/cm2 for 40 h, and there was no hydrogen blistering formed after charging at i = 10 mA/cm2 for 40 h, but some small hydrogen blistering formed after charging at i = 10 mA/cm2 for 168 h and larger blistering appeared after charging at i = 10 mA/cm2 for 760 h [16]. The result shows that the incubation time necessary for forming hydrogen blistering decreases with increasing the current density, and hydrogen blistering grows continuously with charging time after nucleating. The hydrogen blistering is a void with molecule hydrogen and an unsaturated trap of atomic hydrogen. As soon as hydrogen blistering forms, hydrogen concentration in matrix will decrease continuously during charging rather than increase. Because a large hydrogen blistering formed after charging at i = 60 mA/cm2 for 40 h, if i = 100 mA/cm2, a large hydrogen blistering will appear when charging time is less then 40 h, and small blistering will form at the early stage of charging, e.g. 10 to
Fig. 6. Shear bands and plastic zones on the subsurface of three indentations with P = 9.8 N in samples uncharged (a) and precharged at i = 10 mA/cm2 for 40 h (b).
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20 h. Therefore, the size of the plastic zone begins to decrease when time charged at i = 100 mA/cm2 is greater than 10 h, because of formation and growth of hydrogen blistering. Since there was no hydrogen blistering after charging at i = 10 mA/ cm2 for 40 h, charging with small current, e.g., i = 10 mA/cm2 is more suitable to study the effect of hydrogen on the plastic zone. Fig. 4 shows that the shear bands and the size of the plastic zone around the indentation increase evidently after charging at i = 10 mA/cm2 for 20 h. The plastic zone size, i.e., δ and δr, increase evidently when charging time is less than 50 h, but begin to decrease when charging time is greater than 140 h, as shown in Fig. 5. The decrease in the plastic zone size after t ≥ 140 h is believed to correspond to formation and growth of hydrogen blistering [16]. The plastic zones on the subsurface of the indentations for the samples uncharged and precharged at 10 mA/cm2 for 40 h are shown in Fig. 6. The size of the plastic zone on the subsurface of the indentation, δ⁎, is defined as the distance from the bottom of the indent to the boundary of the plastic zone, i.e. δ⁎ = AB, as shown in Fig. 6(a). Measuring for 10 indentations indicated that δ⁎ = 67 ± 3 μm for the uncharged sample and δ⁎(H) = 72 ± 3 μm for the sample precharged at 10 mA/cm2 for 40 h. Above‐mentioned results indicate that hydrogen can enhance localized plastic deformation during indentation for the bulk metallic glass if there is no hydrogen blistering. This result is consistent with the idea that hydrogen promoted formation and growth of shear bands from a notch tip of sustained load sample [10]. We don't know how to explain hydrogen embrittlement of bulk metallic glasses using hydrogen‐enhanced localized plastic deformation. It is a question that needs further investigation.
4. Summary For metallic glass, hardness increases gradually with charging time and reached saturation. The shear bands and plastic zone on the surface and subsurface of indentation increased evidently when charging time was less than 40 h at 10 mA/cm2 or 10 h at 100A/cm2. After that, the size of the
plastic zone began to reduce with increasing charging time because hydrogen blisterings began formation and growth. Acknowledgements This project was supported by National Natural Science Foundation of China (No. 50271006) and Beijing Key Laboratory of Corrosion, Erosion and Surface Technology. References [1] D. Suh, R.H. Dauskardt, Scr. Mater. 42 (2000) 233. [2] F. Spaepen, A.I. Taub, in: F.E. Luborsky (Ed.), Amorphous Metallic Alloys, Butterworth, London, 1983, p. 231. [3] J.J. Lin, T.P. Perng, Metall. Mater. Trans. 26A (1995) 197. [4] T.K. Namboodhiri, T.A. Ramesh, G. Singh, S. Seghal, Mater. Sci. Eng. 61 (1983) 23. [5] V. Schroeder, C.J. Gilbert, R.O. Ritchie, Scr. Mater. 49 (1998) 1057. [6] D. Suh, P. Asoka‐Kumar, R.H. Dauskardt, Acta Mater. 50 (2002) 537. [7] J.X. Guo, J.X. Li, L.J. Qiao, K.W. Gao, W.Y. Chu, Corros. Sci. 45 (2003) 735. [8] G.B. Shan, Y.W. Wang, W.Y. Chu, J.X. Li, X.D Hui, Corros. Sci. 47 (2005) 2731. [9] N. Eliaz, D. Eliezer, Metall. Mater. Trans. 31A (2000) 2517. [10] Y.W. Wang, W.Y. Chu, J.X. Li, X.D. Hui, Y.B. Wang, K.W. Gao, L.J. Qiao, Mater. Lett. 58 (2004) 2393. [11] Q.J. Zhou, J.Y. He, D.B. Sun, W.Y. Chu, L.J. Qiao, Scr. Mater. 54 (2006) 603. [12] D. Kramer, H. Huang, M. Kriese, J. Robach, J. Nelson, A. Wright, D. Bahr, W.W. Gerberich, Acta Mater. 47 (1999) 333. [13] R. Vaidyanathan, M. Dao, G. Ravichandran, S. Suresh, Acta Mater. 49 (2001) 3781. [14] S. Jana, U. Ramamurty, K. Chattopadhyay, Y. Kawamura, Mater. Sci. Eng., A 375–377 (2004) 1191. [15] U. Ramamurty, S. Jana, Y. Kawamura, K. Chattopadhyay, Acta Mater. 53 (2005) 705. [16] G.B. Shan, J.X. Li, B.C. Wei, L.J. Qiao, W.Y. Chu, Mater. Sci. Eng., A (submitted for publication). [17] X. Peng, Y.J. Su, K.W. Gao, L.J. Qiao, W.Y. Chu, Mater. Lett. 58 (2004) 2073.