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ZK60 magnesium alloy matrix composite
Journal of Materials Processing Technology 187–188 (2007) 429–432
Surface treatment of Al18B4O33 whisker and development of Al18B4O33/ZK60 magnesium alloy matrix composite G. Sasaki a,∗ , W.G. Wang a , Y. Hasegawa a , Y.B. Choi a , N. Fuyama b , K. Matsugi a , O. Yanagisawa a a
Department of Mechanical Systems Engineering, Hiroshima University, Kagamiyama 1-4-1, Higashi-Hiroshima 739-8527, Japan b Western Hiroshima Prefecture Industrial Research Institute, Agaminami 2-10-1, Kure 737-0004, Japan
Abstract In order to improve high temperature properties and good aging behavior of ZK60 (Mg–6%Zn–0.6%Zr) magnesium alloy moreover, Al18 B4 O33 whisker reinforced composites were developed. Al18 B4 O33 whisker reacts with ZK60 alloy thermodynamically. In order to suppress the interfacial reaction between Al18 B4 O33 and molten state ZK60 matrix, as-received whiskers were surface treated chemically. Whisker surface in preform was coated in vacuum by pure magnesium vapor, and then heat-treated at atmosphere. By the chemical reaction of whisker, MgAl2 O4 layer with 10–20 nm in thickness was produced on the whisker and covered with whisker completely. Then, Al18 B4 O33 /ZK60 magnesium alloy matrix composite was fabricated by squeeze casting process. As the preform with surface treated whisker is stable for molten ZK60 alloy, the interfacial reaction did not occur during fabrication process and heat treatment after fabrication. The composites have good aging behavior because of no reaction in the interface. Although the precipitation tended to occur around interface preferentially during aging, but the obvious age hardening behavior were observed. © 2006 Elsevier B.V. All rights reserved. Keywords: Magnesium composite; Surface treatment; Aging; Hardness; Aluminum borate whisker; Microstructure
1. Introduction Recently, magnesium alloy has been grate-watched for lightweight structural materials, but has low mechanical properties at ambient and high temperature. Especially, the degradation of mechanical properties at high temperature is fatal defect. The combing magnesium alloy with reinforcement is one of the good improvement for solving these problems. In actual, magnesium alloy matrix composites have been developed for improving the mechanical properties of monolithic magnesium alloy at ambient and high temperatures. As these composites have good specific strength and specific rigidity, the application such as the parts for transportation vehicles are expected. On the other hand, as the mechanical properties of many magnesium alloys are enhanced by aging behavior, the mechanical properties of its composites will be improved. In actual, the hardness of Al18 B4 O33 reinforced AZ91D (Mg–9%Al–1%Zn) and ZK60 (Mg–6%Zn–0.6%Zr) magnesium alloy matrix composites fabricated by compo-casting at ambient temperature was improved by aging behavior [1,2]. The interface between Al18 B4 O33 and ∗
Corresponding author. Tel.: +81 82 424 7545; fax: +81 82 422 7193. E-mail address: [email protected] (G. Sasaki).
0924-0136/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2006.11.076
AZ91D matrix in as-cast composites was very stable for the chemical reaction because of the formation of protection film with thin MgAl2 O4 layer, which was covered with the whisker completely with 10–20 nm in thickness. On the other hand, the stability of interfacial reaction for ZK60 matrix composites is relatively less stable than that of AZ91D matrix composites [3]. Whisker has damages because of interfacial reaction at fabrication temperature. Fig. 1 shows the extracted whisker from Al18 B4 O33 /ZK60 magnesium alloy composites fabricated by stir casting. The whisker damaged completely with heavy irregularity. As stir casting process keeps at high temperature for long time, the reaction seems to proceed heavily compared to squeeze casting process. This reaction leads to degrade the mechanical properties of the composites. In this study, the surface improvement to Al18 B4 O33 whisker was attempted in order to inhibit the interfacial reaction, and the mechanical properties of the composites fabricated by using its surface treated whisker was estimated. 2. Experimental procedure Matrix used is ZK60 magnesium alloy, which is conventional alloy for high temperure use and its composition is Mg–6%Zn–0.6%Zr. Furthermore, ZK60 magnesium alloy has good aging behavior with two steps. Reinforcement used is
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G. Sasaki et al. / Journal of Materials Processing Technology 187–188 (2007) 429–432 tion of whisker in preform is about 20%. Pure magnesium (99.9% in purity) was evaporated on the whisker in preform at 650 ◦ C for 1 h in vacuum furnace with 10 Pa. Then, the magnesium coated preform was heat-treated at 800 ◦ C for 30 min in atmosphere. Magnesium on whisker react with oxygen in air to produce MgAl2 O4 thin layer obeying as following chemical reaction formula: 4Al18 B4 O33 + 33Mg → 33MgAl2 O4 + 16B + 6Al
(1)
Then, surface treated whisker was observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Composites were fabricated by squeeze casting process under the conditions of applied pressure, 100 MPa, temperature of molten alloy 700 ◦ C, and preheating temperature of preform, 750 ◦ C. The composite was solution heat treated at 500 ◦ C for 2 h in argon gas, and then water-quenched. Aging was treated at 130 ◦ C. In order to investigate the stability for chemical reaction at interface between whisker and matrix, the extracted whisker was observed by SEM.
3. Results and discussion Fig. 1. Extracted whiskers from the composite fabricated by stir casting process, which whisker was not surface treated before squeeze casting. Al18 B4 O33 whisker (type M-20, Shikoku Chemicals Co.) formed with 0.5–1 m in diameter and 10–20 m in length, which has good strength and rigidity, and has high cost performance. At first, Al18 B4 O33 whisker preform was fabricated by burning at 1100 ◦ C for 4 h with polyvinyl alcohol as binder. Volume frac-
Fig. 2 shows the microstructure around the interface between the whisker and matrix in as-cast composite, where whisker in preform was not surface treated. Composition of element around the interface was estimated by SEM with EDS analysis. Some interfacial reaction products containing aluminum, magnesium,
Fig. 2. Interfacial reaction between Al18 B4 O33 whisker and ZK60 matrix in composite.
Fig. 3. SEM images of: (a) as-received whiskers and (b) surface treated whiskers.
G. Sasaki et al. / Journal of Materials Processing Technology 187–188 (2007) 429–432
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Fig. 4. TEM image of surface treated whisker.
oxygen, zinc and zirconium were observed as shown in EDS analysis of points 3 and 4 in Fig. 2, which seems to have polycrystalline structure. As the reaction speed was relatively slow at this fabrication temperature, the whisker damage was few. On the other hand, as the precipitation consists of magnesium and zinc, it seems aging behavior suppresses because of low amount of precipitations. In actual, the composites fabricated by stir casting shows low aging behavior [3]. Fig. 3 shows the microstructure of as-received and MgAl2 O4 coated whiskers in preform. Surface on as-received whisker is very smooth and flat, which reveal (1 0 0) and (1 1 0) plains without atomic steps and other defects. By MgAl2 O4 coating, the whisker surface changed to have some irregularity in overall. Fig. 4 shows the surface structure of MgAl2 O4 coated whisker observed by high-resolution TEM. Coating layer covered with the whiskers completely with 10–20 nm in thickness, and grew on whisker surface epitaxially. (2 0 2) Plain in Al18 B4 O33 was parallel to (2 2 2) in MgAl2 O4 . Thickness of MgAl2 O4 layer is relatively homogeneous. Convex on surface treated whisker is caused by MgO, which is observed in upper-right on MgAl2 O4 layer in Fig. 4.
Fig. 6. Age hardening behavior of monolithic alloy and composite.
Then, the composites were fabricated using the preform with surface treated whisker. Fig. 5 shows the extracted whisker from as-cast composite and age treated composite till the time it reaches peak hardness. There is no change between extracted whisker from as-cast composite and surface treated whisker before casting shown in Fig. 3(b) except the shortening of whisker length. As high pressure of molten alloy in squeeze casting lead to the fracture of whisker, the shortening of whisker length is usually observed. Furthermore, no change was observed on the configuration of extracted whiskers from as-cast composite and age treated composite. It shows the surface treated whisker is very stable for the reaction between the matrix and the whisker in age treatment. Fig. 6 shows the age hardening behavior of the monolithic alloy and the composite as a function of time. Age hardening curves of composite with 20 vol% surface treated whisker fabricated by squeeze casting and with 5 vol% no surface treated whisker fabricated by stir casting are shown in Fig. 6. Monolithic
Fig. 5. SEM images of extracted whisker from: (a) as-cast composite and (b) age treated composites till time to reach the peak hardness.
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Fig. 7. Relative Intensity of (2 0 1) plane in MgZn2 against (1 0 1) in Mg for ZK60 alloy and Al18 B4 O33 whisker/ZK60 alloy composites in XRD pattern.
alloy and composite using surface treated whisker shows high hardness at 78 h and 44 h, respectively, whose hardness are 85 Hv and 182 Hv. Increment of hardness for the composite is 45 Hv, which is higher than that of monolithic alloy. Consequently, the composite has good age hardening behavior. On the other hand, the increment of hardness for composite without surface treatment of whisker is 15 Hv and it is very low compared to that of monolithic alloy and composite with surface treated whisker. The reach time to peak hardness of the composite is shorter than that of monolithic alloy, and hardness of composites after peak age decreased as increasing time, however monolithic alloy has no decrement. Fig. 7 shows relative intensity of (2 0 1) plane in MgZn2 against (1 0 1) in Mg for monolithic alloy and composite in XRD pattern. MgZn2 was dissolved completely for monolithic alloy by solution heat treatment. On the other hand, the peak of MgZn2 existed in composites. Even by increasing cooling rate after solution heat treatment or decreasing the thickness of specimen, the peak of MgZn2 did not disappear, which is caused by the low thermal conductivity. With increasing aging time, the relative intensity of (2 0 1) plane in MgZn2 increased. Even after time to reach peak hardness, the precipitation of MgZn2 increased continuously. Especially, the relative intensity for the composites increased heavily compared with that of monolithic alloy. Upper limit of relative intensity is estimated about 10% by thermo-dynamics. The relative intensity for composite exceeds the limit and the reason is still unknown. But the relative intensity for monolithic alloy are about 4% at 200 h and is not reached to limit of relative intensity. For monolithic alloy, it seems to need more long time for reaching equilibrium state because of low driving force. For composite, as there are many driving force for reaching the equilibrium state such as the whisker/matrix interface energy caused by the strain field, it seems the relative intensity reach for short time. As MgZn2 grains did not dissolve in composites by solution heat treatment, the sizes of precipitations of the composites are larger than that of monolithic alloy. Furthermore, the number of precipitations in composite is fewer than that of monolithic alloy. Fig. 8 shows the microstructure around the interface between whisker and matrix in peak aged composite. MgZn2 layers are covered with the whisker. It shows the precipitation proceed at the interface preferentially. As the interface has strain field caused by the difference of thermal expansion, many MgZn2 precipitate
Fig. 8. TEM image of interfacial precipitate in composite age for 44 h.
at the interface. With increasing annealing time, the precipitation increased gradually. But the hardness of the composite with surface treated whisker decreased after showing peak hardness. MgZn2 precipitates at interface and in matrix. With increasing annealing time, growth of MgZn2 at interface rarely affect to the improvement of hardness. On the other hand, it seems large sized MgZn2 precipitations in matrix grow, whereas small sized precipitations are annihilated. As the number of precipitations decreased as increasing annealing time, the effect of hardening caused by Orowan mechanism decreased. 4. Conclusion In order to suppress the interfacial reaction between Al18 B4 O33 whisker and ZK60 matrix in composite in fabrication process and heat treatment, MgAl2 O4 layer was coated on the whisker by vapor deposition of pure magnesium in vacuum and then burned at atmosphere. Consequently, MgAl2 O4 layer has grown 10–20 nm in thickness on whisker surface and covered with whisker completely. By using the surface treated Al18 B4 O33 whisker preform, 20 vol% Al18 B4 O33 /ZK60 magnesium alloy composite was fabricated by squeeze casting process. MgAl2 O4 layer was stable for interfacial reaction during squeeze casting process and aging treatment because of no damage for the whisker. After solution heat treatment and then water-cooling, MgZn2 precipitations already existed because of low thermal conductivity of the composites. With increasing aging time, the amount and length of precipitation were increased till the time to reach the peak hardness. Furthermore MgZn2 was precipitated around the interface preferentially. But the composite has good aging behavior. References [1] W.G. Wang, K. Matsugi, O. Yanagisawa, G. Sasaki, Aging behaviors of Al18 B4 O33 whisker reinforced magnesium alloy composites, in: Proceedings of the 5th China-Japan-US Joint Conference on Composites, 2002, pp. 208–213. [2] W.G. Wang, K. Matsugi, O. Yanagisawa, G. Sasaki, Relationship between aging behaviors and interfacial reaction of casting Al18 B4 O33 whisker reinforced magnesium alloy composites, in: Proceedings of JSME/ASME International Conference on Materials and Processing, vol. 2, 2002, pp. 352–356. [3] G. Sasaki, M. Yoshida, O. Yanagisawa, N. Fuyama, T. Fujii, Mechanical properties and microstructure of/magnesium alloy composites prepared by compo-casting, Mater. Sci. Forum 419–422 (2003) 777–782.
Surface treatment of Al18B4O33 whisker and development of Al18B4O33/ZK60 magnesium alloy matrix composite G. Sasaki a,∗ , W.G. Wang a , Y. Hasegawa a , Y.B. Choi a , N. Fuyama b , K. Matsugi a , O. Yanagisawa a a
Department of Mechanical Systems Engineering, Hiroshima University, Kagamiyama 1-4-1, Higashi-Hiroshima 739-8527, Japan b Western Hiroshima Prefecture Industrial Research Institute, Agaminami 2-10-1, Kure 737-0004, Japan
Abstract In order to improve high temperature properties and good aging behavior of ZK60 (Mg–6%Zn–0.6%Zr) magnesium alloy moreover, Al18 B4 O33 whisker reinforced composites were developed. Al18 B4 O33 whisker reacts with ZK60 alloy thermodynamically. In order to suppress the interfacial reaction between Al18 B4 O33 and molten state ZK60 matrix, as-received whiskers were surface treated chemically. Whisker surface in preform was coated in vacuum by pure magnesium vapor, and then heat-treated at atmosphere. By the chemical reaction of whisker, MgAl2 O4 layer with 10–20 nm in thickness was produced on the whisker and covered with whisker completely. Then, Al18 B4 O33 /ZK60 magnesium alloy matrix composite was fabricated by squeeze casting process. As the preform with surface treated whisker is stable for molten ZK60 alloy, the interfacial reaction did not occur during fabrication process and heat treatment after fabrication. The composites have good aging behavior because of no reaction in the interface. Although the precipitation tended to occur around interface preferentially during aging, but the obvious age hardening behavior were observed. © 2006 Elsevier B.V. All rights reserved. Keywords: Magnesium composite; Surface treatment; Aging; Hardness; Aluminum borate whisker; Microstructure
1. Introduction Recently, magnesium alloy has been grate-watched for lightweight structural materials, but has low mechanical properties at ambient and high temperature. Especially, the degradation of mechanical properties at high temperature is fatal defect. The combing magnesium alloy with reinforcement is one of the good improvement for solving these problems. In actual, magnesium alloy matrix composites have been developed for improving the mechanical properties of monolithic magnesium alloy at ambient and high temperatures. As these composites have good specific strength and specific rigidity, the application such as the parts for transportation vehicles are expected. On the other hand, as the mechanical properties of many magnesium alloys are enhanced by aging behavior, the mechanical properties of its composites will be improved. In actual, the hardness of Al18 B4 O33 reinforced AZ91D (Mg–9%Al–1%Zn) and ZK60 (Mg–6%Zn–0.6%Zr) magnesium alloy matrix composites fabricated by compo-casting at ambient temperature was improved by aging behavior [1,2]. The interface between Al18 B4 O33 and ∗
Corresponding author. Tel.: +81 82 424 7545; fax: +81 82 422 7193. E-mail address: [email protected] (G. Sasaki).
0924-0136/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2006.11.076
AZ91D matrix in as-cast composites was very stable for the chemical reaction because of the formation of protection film with thin MgAl2 O4 layer, which was covered with the whisker completely with 10–20 nm in thickness. On the other hand, the stability of interfacial reaction for ZK60 matrix composites is relatively less stable than that of AZ91D matrix composites [3]. Whisker has damages because of interfacial reaction at fabrication temperature. Fig. 1 shows the extracted whisker from Al18 B4 O33 /ZK60 magnesium alloy composites fabricated by stir casting. The whisker damaged completely with heavy irregularity. As stir casting process keeps at high temperature for long time, the reaction seems to proceed heavily compared to squeeze casting process. This reaction leads to degrade the mechanical properties of the composites. In this study, the surface improvement to Al18 B4 O33 whisker was attempted in order to inhibit the interfacial reaction, and the mechanical properties of the composites fabricated by using its surface treated whisker was estimated. 2. Experimental procedure Matrix used is ZK60 magnesium alloy, which is conventional alloy for high temperure use and its composition is Mg–6%Zn–0.6%Zr. Furthermore, ZK60 magnesium alloy has good aging behavior with two steps. Reinforcement used is
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G. Sasaki et al. / Journal of Materials Processing Technology 187–188 (2007) 429–432 tion of whisker in preform is about 20%. Pure magnesium (99.9% in purity) was evaporated on the whisker in preform at 650 ◦ C for 1 h in vacuum furnace with 10 Pa. Then, the magnesium coated preform was heat-treated at 800 ◦ C for 30 min in atmosphere. Magnesium on whisker react with oxygen in air to produce MgAl2 O4 thin layer obeying as following chemical reaction formula: 4Al18 B4 O33 + 33Mg → 33MgAl2 O4 + 16B + 6Al
(1)
Then, surface treated whisker was observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Composites were fabricated by squeeze casting process under the conditions of applied pressure, 100 MPa, temperature of molten alloy 700 ◦ C, and preheating temperature of preform, 750 ◦ C. The composite was solution heat treated at 500 ◦ C for 2 h in argon gas, and then water-quenched. Aging was treated at 130 ◦ C. In order to investigate the stability for chemical reaction at interface between whisker and matrix, the extracted whisker was observed by SEM.
3. Results and discussion Fig. 1. Extracted whiskers from the composite fabricated by stir casting process, which whisker was not surface treated before squeeze casting. Al18 B4 O33 whisker (type M-20, Shikoku Chemicals Co.) formed with 0.5–1 m in diameter and 10–20 m in length, which has good strength and rigidity, and has high cost performance. At first, Al18 B4 O33 whisker preform was fabricated by burning at 1100 ◦ C for 4 h with polyvinyl alcohol as binder. Volume frac-
Fig. 2 shows the microstructure around the interface between the whisker and matrix in as-cast composite, where whisker in preform was not surface treated. Composition of element around the interface was estimated by SEM with EDS analysis. Some interfacial reaction products containing aluminum, magnesium,
Fig. 2. Interfacial reaction between Al18 B4 O33 whisker and ZK60 matrix in composite.
Fig. 3. SEM images of: (a) as-received whiskers and (b) surface treated whiskers.
G. Sasaki et al. / Journal of Materials Processing Technology 187–188 (2007) 429–432
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Fig. 4. TEM image of surface treated whisker.
oxygen, zinc and zirconium were observed as shown in EDS analysis of points 3 and 4 in Fig. 2, which seems to have polycrystalline structure. As the reaction speed was relatively slow at this fabrication temperature, the whisker damage was few. On the other hand, as the precipitation consists of magnesium and zinc, it seems aging behavior suppresses because of low amount of precipitations. In actual, the composites fabricated by stir casting shows low aging behavior [3]. Fig. 3 shows the microstructure of as-received and MgAl2 O4 coated whiskers in preform. Surface on as-received whisker is very smooth and flat, which reveal (1 0 0) and (1 1 0) plains without atomic steps and other defects. By MgAl2 O4 coating, the whisker surface changed to have some irregularity in overall. Fig. 4 shows the surface structure of MgAl2 O4 coated whisker observed by high-resolution TEM. Coating layer covered with the whiskers completely with 10–20 nm in thickness, and grew on whisker surface epitaxially. (2 0 2) Plain in Al18 B4 O33 was parallel to (2 2 2) in MgAl2 O4 . Thickness of MgAl2 O4 layer is relatively homogeneous. Convex on surface treated whisker is caused by MgO, which is observed in upper-right on MgAl2 O4 layer in Fig. 4.
Fig. 6. Age hardening behavior of monolithic alloy and composite.
Then, the composites were fabricated using the preform with surface treated whisker. Fig. 5 shows the extracted whisker from as-cast composite and age treated composite till the time it reaches peak hardness. There is no change between extracted whisker from as-cast composite and surface treated whisker before casting shown in Fig. 3(b) except the shortening of whisker length. As high pressure of molten alloy in squeeze casting lead to the fracture of whisker, the shortening of whisker length is usually observed. Furthermore, no change was observed on the configuration of extracted whiskers from as-cast composite and age treated composite. It shows the surface treated whisker is very stable for the reaction between the matrix and the whisker in age treatment. Fig. 6 shows the age hardening behavior of the monolithic alloy and the composite as a function of time. Age hardening curves of composite with 20 vol% surface treated whisker fabricated by squeeze casting and with 5 vol% no surface treated whisker fabricated by stir casting are shown in Fig. 6. Monolithic
Fig. 5. SEM images of extracted whisker from: (a) as-cast composite and (b) age treated composites till time to reach the peak hardness.
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Fig. 7. Relative Intensity of (2 0 1) plane in MgZn2 against (1 0 1) in Mg for ZK60 alloy and Al18 B4 O33 whisker/ZK60 alloy composites in XRD pattern.
alloy and composite using surface treated whisker shows high hardness at 78 h and 44 h, respectively, whose hardness are 85 Hv and 182 Hv. Increment of hardness for the composite is 45 Hv, which is higher than that of monolithic alloy. Consequently, the composite has good age hardening behavior. On the other hand, the increment of hardness for composite without surface treatment of whisker is 15 Hv and it is very low compared to that of monolithic alloy and composite with surface treated whisker. The reach time to peak hardness of the composite is shorter than that of monolithic alloy, and hardness of composites after peak age decreased as increasing time, however monolithic alloy has no decrement. Fig. 7 shows relative intensity of (2 0 1) plane in MgZn2 against (1 0 1) in Mg for monolithic alloy and composite in XRD pattern. MgZn2 was dissolved completely for monolithic alloy by solution heat treatment. On the other hand, the peak of MgZn2 existed in composites. Even by increasing cooling rate after solution heat treatment or decreasing the thickness of specimen, the peak of MgZn2 did not disappear, which is caused by the low thermal conductivity. With increasing aging time, the relative intensity of (2 0 1) plane in MgZn2 increased. Even after time to reach peak hardness, the precipitation of MgZn2 increased continuously. Especially, the relative intensity for the composites increased heavily compared with that of monolithic alloy. Upper limit of relative intensity is estimated about 10% by thermo-dynamics. The relative intensity for composite exceeds the limit and the reason is still unknown. But the relative intensity for monolithic alloy are about 4% at 200 h and is not reached to limit of relative intensity. For monolithic alloy, it seems to need more long time for reaching equilibrium state because of low driving force. For composite, as there are many driving force for reaching the equilibrium state such as the whisker/matrix interface energy caused by the strain field, it seems the relative intensity reach for short time. As MgZn2 grains did not dissolve in composites by solution heat treatment, the sizes of precipitations of the composites are larger than that of monolithic alloy. Furthermore, the number of precipitations in composite is fewer than that of monolithic alloy. Fig. 8 shows the microstructure around the interface between whisker and matrix in peak aged composite. MgZn2 layers are covered with the whisker. It shows the precipitation proceed at the interface preferentially. As the interface has strain field caused by the difference of thermal expansion, many MgZn2 precipitate
Fig. 8. TEM image of interfacial precipitate in composite age for 44 h.
at the interface. With increasing annealing time, the precipitation increased gradually. But the hardness of the composite with surface treated whisker decreased after showing peak hardness. MgZn2 precipitates at interface and in matrix. With increasing annealing time, growth of MgZn2 at interface rarely affect to the improvement of hardness. On the other hand, it seems large sized MgZn2 precipitations in matrix grow, whereas small sized precipitations are annihilated. As the number of precipitations decreased as increasing annealing time, the effect of hardening caused by Orowan mechanism decreased. 4. Conclusion In order to suppress the interfacial reaction between Al18 B4 O33 whisker and ZK60 matrix in composite in fabrication process and heat treatment, MgAl2 O4 layer was coated on the whisker by vapor deposition of pure magnesium in vacuum and then burned at atmosphere. Consequently, MgAl2 O4 layer has grown 10–20 nm in thickness on whisker surface and covered with whisker completely. By using the surface treated Al18 B4 O33 whisker preform, 20 vol% Al18 B4 O33 /ZK60 magnesium alloy composite was fabricated by squeeze casting process. MgAl2 O4 layer was stable for interfacial reaction during squeeze casting process and aging treatment because of no damage for the whisker. After solution heat treatment and then water-cooling, MgZn2 precipitations already existed because of low thermal conductivity of the composites. With increasing aging time, the amount and length of precipitation were increased till the time to reach the peak hardness. Furthermore MgZn2 was precipitated around the interface preferentially. But the composite has good aging behavior. References [1] W.G. Wang, K. Matsugi, O. Yanagisawa, G. Sasaki, Aging behaviors of Al18 B4 O33 whisker reinforced magnesium alloy composites, in: Proceedings of the 5th China-Japan-US Joint Conference on Composites, 2002, pp. 208–213. [2] W.G. Wang, K. Matsugi, O. Yanagisawa, G. Sasaki, Relationship between aging behaviors and interfacial reaction of casting Al18 B4 O33 whisker reinforced magnesium alloy composites, in: Proceedings of JSME/ASME International Conference on Materials and Processing, vol. 2, 2002, pp. 352–356. [3] G. Sasaki, M. Yoshida, O. Yanagisawa, N. Fuyama, T. Fujii, Mechanical properties and microstructure of/magnesium alloy composites prepared by compo-casting, Mater. Sci. Forum 419–422 (2003) 777–782.