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Al hybrid composite
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Materials Letters 62 (2008) 2670 – 2672 www.elsevier.com/locate/matlet
Preparation and characterization of Al18B4O33 + BaPbO3/Al hybrid composite G.H. Fan ⁎, L. Geng, Z.Z. Zheng, G.S. Wang, P.Q. Zheng School of Materials Science and Engineering, Harbin Institute of Technology, P.O. Box433, Harbin 150001, PR China Received 5 October 2007; accepted 5 January 2008 Available online 11 January 2008
Abstract In this paper, the aluminum borate whisker (ABOw) and BaPbO3 particles (BPOp)/Al hybrid composite with radiation protection function was fabricated by squeeze casting technique. The microstructure and properties of the hybrid composite were investigated. The test results show that BPOp and aluminum matrix have a reaction, and this reaction forms a coating on the surface of ABOw. The introduction of BPOp increases radiation protection function of the matrix evidently, at the same time, the hybrid composite has better mechanical properties compared with the aluminum matrix and ABOw/Al composite. © 2008 Elsevier B.V. All rights reserved. Keywords: Composite materials; Microstructure; Mechanical properties; Radiation protection
1. Introduction Hybrid metal matrix composites (MMCs) have attracted much more attentions due to their excellent properties compared with singularly reinforced composites [1–3]. Recently, various hybrid composites were designed to get higher stiffness [3,4], better wear resistance [5,6] and lower thermal expansion coefficient [7,8], etc. These research works were usually focused on mechanical properties and thermal physics properties of MMCs by introducing ceramics reinforcement with high modulus and low expansion coefficient. When nuclear explosion occurs at high altitude out of the atmosphere, the 70% explosion energy is released by X-ray. In this case, the semiconductor chips used in spacecraft, aircraft, especially in strategic nuclear missile, will fail by the dose enhancement [9,10] effect and soft upset [11], which are produced by X-ray. Therefore, the protection of the semiconductor devices for X-ray is very urgent and significant. However, conventional anti-radiation materials, such as polymer or ceramics matrix composites [12,13], tungsten [14], and lead, cannot meet the ⁎ Corresponding author. Tel.: +86 451 86418836; fax: +86 451 86413922. E-mail addresses: [email protected] (G.H. Fan), [email protected] (L. Geng), [email protected] (Z.Z. Zheng), [email protected] (G.S. Wang), [email protected] (P.Q. Zheng). 0167-577X/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.01.010
demands due to their disadvantages: low specific stiffness, low specific strength and prone to decompose under some certain dose of photon radiation. BPOp possess good photon absorbability because the high atomic number elements, Pb and Ba, can absorb photons radiation effectively [15]. ABOw as a reinforcement was paid more attention because of its excellent mechanical property and lower cost [1,3]. In our research, we attempted to prepare hybrid composite by introducing BPOp and ABOw to aluminum
Fig. 1. XRD pattern of as-casted (BPO + ABOw)/Al composite.
G.H. Fan et al. / Materials Letters 62 (2008) 2670–2672
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The (ABOw + BPOp)/Al composite, pure aluminum and ABOw/Al composite were cut into the plate with a size of 150 × 150 × 3 mm3 for the radiation protection measurement, and the average energy of X-ray is 250 KeV. The change of photon intensity across the plate samples was measured to characterize radiation protection property. The Archimedes technique was used to measure the density of the composite. Tensile test was performed on an Instron model 5569 tensile test machine under a cross-head speed of 0.5 mm/min. The samples of composite have been characterized using scanning electron microscope (SEM), X-ray diffraction (XRD) with CuKα (0.15406 nm) radiation and transmission electron microscope (TEM). Fig. 2. SEM micrograph of (ABOw + BPO)/Al composite.
3. Results and discussion 3.1. Microstructure characterization
matrix using squeeze casting technique and found that the composite has a good combination of mechanical property and radiation protection function. 2. Experimental The raw materials were industrial pure aluminum 1060 (≥ 99.60%), BPOp with a diameter of 0.2–0.5 µm and ABOw with a diameter of 0.5–1 µm and length of 10–30 µm. The hybrid composite was fabricated by squeeze casting technique. The total volume fraction of BPOp and ABOw is 25%, and the volume ratio of BPOp and ABOw is about 1:4. The uniform mixing between the particle and whisker is difficult due to the great difference of density between BPOp (8.20 g/cm3) and ABOw (2.93 g/cm3). Thus, Zn2+ sol was made in a mixture of BPOp and ABOw to prevent the segregation of BPOp. Proper surfactant polyvinyl alcohol-200 was used to disperse BPOp. The pressure of squeeze casting was 200 MPa. The casting temperature was 800 °C and the die temperature was about 550 °C. The aluminum and ABOw/Al composite were also fabricated under identical conditions for comparisons.
Fig. 1 shows the X-ray diffractogram of an as-casted hybrid composite. It is obvious that there exist three phases: Al, ABOw and Pb. According to this result, we may deduce that Pb was formed during the reaction between aluminum matrix and BPO. The apparent grain size of Pb was calculated using the Scherrer formula, D = 0.89λ/Bcosθ, where D is the average particle size and B is the full-width at half-maximum of the peaks. In determining the grain size of Pb, the effect of internal stress on the line broadening was neglected. The average grain size of Pb is 30.5 nm from the half-maximum width of the Pb (111), (200) diffraction peaks. The SEM image is shown in Fig. 2. It can be found that the distribution of ABOw in the composite is random and homogeneous, and there is a coating on the surface of ABOw. Furthermore, the microstructure analysis of the coating was performed by TEM observations. TEM photographs are shown in Fig. 3. Fig. 3a shows ABOw is covered by a coating with a thickness of approximately 100 nm. The energy dispersive X-ray spectroscope (EDS) shows that the coating includes several elements: Ba, Pb, Al. Many particles have a dimension of about 30 nm in this coating. Fig. 3b is the high resolution transmission electron micrograph of the particle, the distance between the parallel fringes is about 0.2859 nm, corresponding to the (111) planes of Pb. The results of HRTEM and XRD show that the particle is Pb, the reaction product of Al
Fig. 3. TEM photos of (ABOw + BPOp)/Al composite: (a) the interface between ABOw with coating and Al matrix, (b) corresponding HRTEM with clearly resolved fringes of (111) planes.
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Table 1 Comparative study of properties of composites and aluminum matrix Materials
Tensile Young's Elongation Density HVT µ/ρ strength modulus [%] [g/cm3] (cm) [cm2/g] [MPa] [GPa]
Al 64 ABOw/Al 138 ABOw + BPO/Al 224
69 107 104
25 0.48 3.5
2.705 2.752 3.029
2.53 2.61 1.65
0.1009 0.0986 0.1419
and BPOp. The existence and the size of Pb in TEM image are in well agreement with these by XRD. In addition, the other reaction phase around Pb particles in the coating has no lattice fringes while tilting sample stage, which indicates the other reactants may belong to amorphous phase. According to the above analysis, BPOp have a reaction to form a coating including nanosized Pb and possibly other amorphous phases during fabrication process.
high atomic number Z [21]. Thus, in this work, the existence of high-Z elements, Pb and Ba, plays an important role in radiation protection property of the composite; and the chemical reaction between Al and BPOp will not affect anti-radiation property.
4. Conclusions (ABOw + BPOp)/Al composite was fabricated successfully by squeezing casting technique. BPOp has a reaction with Al matrix to form a coating on ABOw. The coating may improve wettability and bonding strength between whisker and Al matrix. Therefore, (ABOw + BPOp)/Al composite has higher mechanical properties compared with ABOw/Al composite. In addition, the addition of BPO improves the radiation protection function evidently. So, the hybrid composite has a good combination of mechanical properties and radiation protection function.
3.2. Mechanical properties and radiation protection properties
Acknowledgements Detailed mechanical properties and radiation protection properties of (ABOw + BPOp)/Al, ABOw/Al and Al are shown in Table 1. According to this table, it is found that the hybrid composite is much superior to Al and ABOw/Al composite in mechanical strength, specially, specific strength and specific modulus. Because interfacial characteristics in metal matrix composites (MMCs) play a significant role in determining the mechanical properties, many researches on interface modification and its effect on the mechanical properties of ABOw/Al composite have been done [16–18]. In most cases, the interfacial wettability of ABOw and molten aluminum is poor, which results in many voids and degrades the mechanical properties [18]. W.D. Fei et al. incorporated Bi2O3 or ZnO coatings on the surface of ABOw before the fabrication of composite by mechanical method, and Bi2O3 or ZnO coatings react with molten aluminum to form alumina and Zinc and bismuth [19,20]. They found that Bi2O3 or ZnO coatings efficiently enhance the ABOw/Al wettability owing to the above interfacial reactions, and ultimate tensile strength and ductility of composite increases exactly. In our work, based on the characterization of the composite, Pb and amorphous phase were formed during squeeze casting process, and a coating with nanosized Pb covered the ABOw. This reaction is similar to Fei's work. As a result, we conclude that the improvement of mechanical property should be attributed to the improved wettability of ABOw/Al due to the presence of the reaction. If a sample is placed between a radiation beam I0 and a detector, the intensity of the beam I passing through the sample can be described by I ¼ I0 exp½ðA=qÞqv Where, (µ/ρ) is the mass attenuation coefficient (cm2/g), ρ and χ are the density and the thickness of the sample. The higher the (µ/ρ) value is, the less the energy passes through the material. In this paper, we also use HVT (half value thickness) to characterize the radiation shielding property of composites in direct. The HVT equals the thickness of a sample when the intensity of the transmission beam I was attenuated to half of the intensity of the incoming beam I0. The comparison of photon attenuation characteristics among Al, ABOw/Al, (ABOw + BPOp)/Al are given in Table 1. Compared to Al and ABOw/Al composite, the radiation protection property of the (ABOw + BPOp)/Al was increased by about 50%. The strong photon absorption probability mainly depends on the
The key program for Heilongjiang province Natural Science Foundation is acknowledged for their support (contract number: ZJC 0604). References [1] G. Li, Y. Sun, W.D. Fei, Mater. Lett. 57 (2003) 3217–3221. [2] X.N. Zhang, L. Geng, G.S. Wang, Key Eng. Mater. 249 (2003) 277–282. [3] X.X. Zhang, C.F. Deng, Y.B. Shen, D.Z. Wang, L. Geng, Mater. Lett. 61 (2007) 3504–3506. [4] J.H. Gu, X.N. Zhang, M.Y. Gu, J. Alloys Compd. 385 (2004) 104–108. [5] J. Du, Y.H. Liu, S.R. Yu, W.F. Li, Wear 257 (2004) 930–940. [6] S. Basavarajappa, G. Chandramohan, A. Mahadevan, M. Thangavelu, R. Subramanian, P. Gopalakrishnan, Wear 262 (2007) 1007–1012. [7] M.B. Mahagundappa, H.K. Shivanand, Mat. Sci. Eng. A 435–436 (2006) 745–749. [8] T.C. Wang, T.X. Fan, D. Zhang, G.D. Zhang, D.S. Xiong, Mater. Lett. 61 (2007) 1849–1854. [9] B.E. Beutlet, D.M. Fleetwood, W. Beezhold, IEEE Trans. Nucl. Sci. N-34 (1987) 1544–1550. [10] J.M. Benedetto, H.E. Boesch, T.R. Oldmam, IEEE Trans. Nucl. Sci. N-34 (1987) 1540–1543. [11] A.B. Campbell, E.A. Wolicki, IEEE Trans. Nucl. Sci. N-27 (1980) 1509–1515. [12] Q. Lin, B. Yang, J. Li, X.S. Meng, J.C. Shen, Polymer 41 (2000) 8305–8309. [13] S.S. Amritphale, A. Awneesh, C. Navin, N. Ramakrishnan, J. Eur. Ceram. Soc. 27 (2007) 1945–1951. [14] S. Kobayashi, N. Hosoda, R. Takashima, Nucl. Instrum. Meth. A 390 (1997) 426–430. [15] Y.J. Feng, X.J. Sun, L.X. Sun, Chinese J. Inorg. Chem. 18 (2002) 342–346. [16] L.M. Tham, M. Gupta, L. Cheng, Acta Mater. 49 (2001) 3243–3253. [17] J.C. Lee, J.P. Ahn, J.H. Shim, Scr. Mater. 35 (1996) 721–726. [18] Z.J. Li, W.D. Fei, H.Y. Yue, L.D. Wang, Comp. Sci. Tech. 67 (2007) 963–973. [19] Z.J. Li, L.D. Wang, W.D. Fei, Mat. Sci. Eng. A 447 (2007) 314–318. [20] H.Y. Yue, W.D. Fei, Z.J. Li, L.D. Wang, Mat. Sci. Eng. A 44 (2006) 197–201. [21] S. Naveer, J.S. Kanwar, S. Kulwant, Radiat. Meas. 41 (2006) 84–88.
Materials Letters 62 (2008) 2670 – 2672 www.elsevier.com/locate/matlet
Preparation and characterization of Al18B4O33 + BaPbO3/Al hybrid composite G.H. Fan ⁎, L. Geng, Z.Z. Zheng, G.S. Wang, P.Q. Zheng School of Materials Science and Engineering, Harbin Institute of Technology, P.O. Box433, Harbin 150001, PR China Received 5 October 2007; accepted 5 January 2008 Available online 11 January 2008
Abstract In this paper, the aluminum borate whisker (ABOw) and BaPbO3 particles (BPOp)/Al hybrid composite with radiation protection function was fabricated by squeeze casting technique. The microstructure and properties of the hybrid composite were investigated. The test results show that BPOp and aluminum matrix have a reaction, and this reaction forms a coating on the surface of ABOw. The introduction of BPOp increases radiation protection function of the matrix evidently, at the same time, the hybrid composite has better mechanical properties compared with the aluminum matrix and ABOw/Al composite. © 2008 Elsevier B.V. All rights reserved. Keywords: Composite materials; Microstructure; Mechanical properties; Radiation protection
1. Introduction Hybrid metal matrix composites (MMCs) have attracted much more attentions due to their excellent properties compared with singularly reinforced composites [1–3]. Recently, various hybrid composites were designed to get higher stiffness [3,4], better wear resistance [5,6] and lower thermal expansion coefficient [7,8], etc. These research works were usually focused on mechanical properties and thermal physics properties of MMCs by introducing ceramics reinforcement with high modulus and low expansion coefficient. When nuclear explosion occurs at high altitude out of the atmosphere, the 70% explosion energy is released by X-ray. In this case, the semiconductor chips used in spacecraft, aircraft, especially in strategic nuclear missile, will fail by the dose enhancement [9,10] effect and soft upset [11], which are produced by X-ray. Therefore, the protection of the semiconductor devices for X-ray is very urgent and significant. However, conventional anti-radiation materials, such as polymer or ceramics matrix composites [12,13], tungsten [14], and lead, cannot meet the ⁎ Corresponding author. Tel.: +86 451 86418836; fax: +86 451 86413922. E-mail addresses: [email protected] (G.H. Fan), [email protected] (L. Geng), [email protected] (Z.Z. Zheng), [email protected] (G.S. Wang), [email protected] (P.Q. Zheng). 0167-577X/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.01.010
demands due to their disadvantages: low specific stiffness, low specific strength and prone to decompose under some certain dose of photon radiation. BPOp possess good photon absorbability because the high atomic number elements, Pb and Ba, can absorb photons radiation effectively [15]. ABOw as a reinforcement was paid more attention because of its excellent mechanical property and lower cost [1,3]. In our research, we attempted to prepare hybrid composite by introducing BPOp and ABOw to aluminum
Fig. 1. XRD pattern of as-casted (BPO + ABOw)/Al composite.
G.H. Fan et al. / Materials Letters 62 (2008) 2670–2672
2671
The (ABOw + BPOp)/Al composite, pure aluminum and ABOw/Al composite were cut into the plate with a size of 150 × 150 × 3 mm3 for the radiation protection measurement, and the average energy of X-ray is 250 KeV. The change of photon intensity across the plate samples was measured to characterize radiation protection property. The Archimedes technique was used to measure the density of the composite. Tensile test was performed on an Instron model 5569 tensile test machine under a cross-head speed of 0.5 mm/min. The samples of composite have been characterized using scanning electron microscope (SEM), X-ray diffraction (XRD) with CuKα (0.15406 nm) radiation and transmission electron microscope (TEM). Fig. 2. SEM micrograph of (ABOw + BPO)/Al composite.
3. Results and discussion 3.1. Microstructure characterization
matrix using squeeze casting technique and found that the composite has a good combination of mechanical property and radiation protection function. 2. Experimental The raw materials were industrial pure aluminum 1060 (≥ 99.60%), BPOp with a diameter of 0.2–0.5 µm and ABOw with a diameter of 0.5–1 µm and length of 10–30 µm. The hybrid composite was fabricated by squeeze casting technique. The total volume fraction of BPOp and ABOw is 25%, and the volume ratio of BPOp and ABOw is about 1:4. The uniform mixing between the particle and whisker is difficult due to the great difference of density between BPOp (8.20 g/cm3) and ABOw (2.93 g/cm3). Thus, Zn2+ sol was made in a mixture of BPOp and ABOw to prevent the segregation of BPOp. Proper surfactant polyvinyl alcohol-200 was used to disperse BPOp. The pressure of squeeze casting was 200 MPa. The casting temperature was 800 °C and the die temperature was about 550 °C. The aluminum and ABOw/Al composite were also fabricated under identical conditions for comparisons.
Fig. 1 shows the X-ray diffractogram of an as-casted hybrid composite. It is obvious that there exist three phases: Al, ABOw and Pb. According to this result, we may deduce that Pb was formed during the reaction between aluminum matrix and BPO. The apparent grain size of Pb was calculated using the Scherrer formula, D = 0.89λ/Bcosθ, where D is the average particle size and B is the full-width at half-maximum of the peaks. In determining the grain size of Pb, the effect of internal stress on the line broadening was neglected. The average grain size of Pb is 30.5 nm from the half-maximum width of the Pb (111), (200) diffraction peaks. The SEM image is shown in Fig. 2. It can be found that the distribution of ABOw in the composite is random and homogeneous, and there is a coating on the surface of ABOw. Furthermore, the microstructure analysis of the coating was performed by TEM observations. TEM photographs are shown in Fig. 3. Fig. 3a shows ABOw is covered by a coating with a thickness of approximately 100 nm. The energy dispersive X-ray spectroscope (EDS) shows that the coating includes several elements: Ba, Pb, Al. Many particles have a dimension of about 30 nm in this coating. Fig. 3b is the high resolution transmission electron micrograph of the particle, the distance between the parallel fringes is about 0.2859 nm, corresponding to the (111) planes of Pb. The results of HRTEM and XRD show that the particle is Pb, the reaction product of Al
Fig. 3. TEM photos of (ABOw + BPOp)/Al composite: (a) the interface between ABOw with coating and Al matrix, (b) corresponding HRTEM with clearly resolved fringes of (111) planes.
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G.H. Fan et al. / Materials Letters 62 (2008) 2670–2672
Table 1 Comparative study of properties of composites and aluminum matrix Materials
Tensile Young's Elongation Density HVT µ/ρ strength modulus [%] [g/cm3] (cm) [cm2/g] [MPa] [GPa]
Al 64 ABOw/Al 138 ABOw + BPO/Al 224
69 107 104
25 0.48 3.5
2.705 2.752 3.029
2.53 2.61 1.65
0.1009 0.0986 0.1419
and BPOp. The existence and the size of Pb in TEM image are in well agreement with these by XRD. In addition, the other reaction phase around Pb particles in the coating has no lattice fringes while tilting sample stage, which indicates the other reactants may belong to amorphous phase. According to the above analysis, BPOp have a reaction to form a coating including nanosized Pb and possibly other amorphous phases during fabrication process.
high atomic number Z [21]. Thus, in this work, the existence of high-Z elements, Pb and Ba, plays an important role in radiation protection property of the composite; and the chemical reaction between Al and BPOp will not affect anti-radiation property.
4. Conclusions (ABOw + BPOp)/Al composite was fabricated successfully by squeezing casting technique. BPOp has a reaction with Al matrix to form a coating on ABOw. The coating may improve wettability and bonding strength between whisker and Al matrix. Therefore, (ABOw + BPOp)/Al composite has higher mechanical properties compared with ABOw/Al composite. In addition, the addition of BPO improves the radiation protection function evidently. So, the hybrid composite has a good combination of mechanical properties and radiation protection function.
3.2. Mechanical properties and radiation protection properties
Acknowledgements Detailed mechanical properties and radiation protection properties of (ABOw + BPOp)/Al, ABOw/Al and Al are shown in Table 1. According to this table, it is found that the hybrid composite is much superior to Al and ABOw/Al composite in mechanical strength, specially, specific strength and specific modulus. Because interfacial characteristics in metal matrix composites (MMCs) play a significant role in determining the mechanical properties, many researches on interface modification and its effect on the mechanical properties of ABOw/Al composite have been done [16–18]. In most cases, the interfacial wettability of ABOw and molten aluminum is poor, which results in many voids and degrades the mechanical properties [18]. W.D. Fei et al. incorporated Bi2O3 or ZnO coatings on the surface of ABOw before the fabrication of composite by mechanical method, and Bi2O3 or ZnO coatings react with molten aluminum to form alumina and Zinc and bismuth [19,20]. They found that Bi2O3 or ZnO coatings efficiently enhance the ABOw/Al wettability owing to the above interfacial reactions, and ultimate tensile strength and ductility of composite increases exactly. In our work, based on the characterization of the composite, Pb and amorphous phase were formed during squeeze casting process, and a coating with nanosized Pb covered the ABOw. This reaction is similar to Fei's work. As a result, we conclude that the improvement of mechanical property should be attributed to the improved wettability of ABOw/Al due to the presence of the reaction. If a sample is placed between a radiation beam I0 and a detector, the intensity of the beam I passing through the sample can be described by I ¼ I0 exp½ðA=qÞqv Where, (µ/ρ) is the mass attenuation coefficient (cm2/g), ρ and χ are the density and the thickness of the sample. The higher the (µ/ρ) value is, the less the energy passes through the material. In this paper, we also use HVT (half value thickness) to characterize the radiation shielding property of composites in direct. The HVT equals the thickness of a sample when the intensity of the transmission beam I was attenuated to half of the intensity of the incoming beam I0. The comparison of photon attenuation characteristics among Al, ABOw/Al, (ABOw + BPOp)/Al are given in Table 1. Compared to Al and ABOw/Al composite, the radiation protection property of the (ABOw + BPOp)/Al was increased by about 50%. The strong photon absorption probability mainly depends on the
The key program for Heilongjiang province Natural Science Foundation is acknowledged for their support (contract number: ZJC 0604). References [1] G. Li, Y. Sun, W.D. Fei, Mater. Lett. 57 (2003) 3217–3221. [2] X.N. Zhang, L. Geng, G.S. Wang, Key Eng. Mater. 249 (2003) 277–282. [3] X.X. Zhang, C.F. Deng, Y.B. Shen, D.Z. Wang, L. Geng, Mater. Lett. 61 (2007) 3504–3506. [4] J.H. Gu, X.N. Zhang, M.Y. Gu, J. Alloys Compd. 385 (2004) 104–108. [5] J. Du, Y.H. Liu, S.R. Yu, W.F. Li, Wear 257 (2004) 930–940. [6] S. Basavarajappa, G. Chandramohan, A. Mahadevan, M. Thangavelu, R. Subramanian, P. Gopalakrishnan, Wear 262 (2007) 1007–1012. [7] M.B. Mahagundappa, H.K. Shivanand, Mat. Sci. Eng. A 435–436 (2006) 745–749. [8] T.C. Wang, T.X. Fan, D. Zhang, G.D. Zhang, D.S. Xiong, Mater. Lett. 61 (2007) 1849–1854. [9] B.E. Beutlet, D.M. Fleetwood, W. Beezhold, IEEE Trans. Nucl. Sci. N-34 (1987) 1544–1550. [10] J.M. Benedetto, H.E. Boesch, T.R. Oldmam, IEEE Trans. Nucl. Sci. N-34 (1987) 1540–1543. [11] A.B. Campbell, E.A. Wolicki, IEEE Trans. Nucl. Sci. N-27 (1980) 1509–1515. [12] Q. Lin, B. Yang, J. Li, X.S. Meng, J.C. Shen, Polymer 41 (2000) 8305–8309. [13] S.S. Amritphale, A. Awneesh, C. Navin, N. Ramakrishnan, J. Eur. Ceram. Soc. 27 (2007) 1945–1951. [14] S. Kobayashi, N. Hosoda, R. Takashima, Nucl. Instrum. Meth. A 390 (1997) 426–430. [15] Y.J. Feng, X.J. Sun, L.X. Sun, Chinese J. Inorg. Chem. 18 (2002) 342–346. [16] L.M. Tham, M. Gupta, L. Cheng, Acta Mater. 49 (2001) 3243–3253. [17] J.C. Lee, J.P. Ahn, J.H. Shim, Scr. Mater. 35 (1996) 721–726. [18] Z.J. Li, W.D. Fei, H.Y. Yue, L.D. Wang, Comp. Sci. Tech. 67 (2007) 963–973. [19] Z.J. Li, L.D. Wang, W.D. Fei, Mat. Sci. Eng. A 447 (2007) 314–318. [20] H.Y. Yue, W.D. Fei, Z.J. Li, L.D. Wang, Mat. Sci. Eng. A 44 (2006) 197–201. [21] S. Naveer, J.S. Kanwar, S. Kulwant, Radiat. Meas. 41 (2006) 84–88.