Fabrication of Short Alumina Fiber and In-situ Mg 2 Si Particle- Reinforced Magnesium Alloy Hybrid Composite and Its Strength Properties

Transcription

1 Fabrication of Short Alumina Fiber and In-situ Mg 2 Si Particle- Reinforced Magnesium Alloy Hybrid Composite and Its Strength Properties K. Asano* and H. Yoneda* * Kinki University, Japan Abstract Magnesium alloy matrix composites reinforced with short alumina fibers and in-situ formed Mg 2 Si particles were fabricated in a permanent mold by the infiltration with the molten magnesium alloy into the preforms consisting of fibers having Si particles attached to their surfaces. P or CaF 2 particles were used as the refiners of the Mg 2 Si particles. Fine Mg 2 Si particles of approximately 5μm grain size were formed due to the rapid solidification in the permanent mold, regardless of the introduction of the refiners. The tensile strengths of the composites that ranged from 293K to 623K were investigated. The strength of the alumina fiber-reinforced composite, in which the Mg 2 Si particles were homogeneously dispersed in the matrix, was higher than that of the conventional fiber-reinforced composite. Key words composite, alumina fiber, magnesium, in-situ Mg 2 Si, high temperature strength 120/1

2 Introduction To improve the high-temperature strength of magnesium alloys, their reinforcement with heat-resistant ceramics fibers has been presented. We have fabricated an alumina fiber-reinforced AZ91D magnesium alloy composite, and revealed that the strength of the composite at 523K was 160MPa, which was superior to that of the unreinforced alloy [1]. To develop a lightweight composite which has an increased high-temperature strength available for high-performance pistons, we proposed the dispersion of heat-resistant particles in the matrix. The homogeneous dispersion of the particles leads to an improvement of the heat-resistance of the matrix, or the improvement of the stress transmission between fiber and matrix by preventing the fiber-to-fiber contact. We fabricated the magnesium alloy matrix composites reinforced with the short alumina fibers and in-situ formed Mg 2 Si, which is hard, and has a low density and high-melting point [2], by the infiltration of the preform consisting of short alumina fibers and pure Si particles with the molten magnesium alloy, and investigated the conditions to finely disperse the Mg 2 Si [3]. As a result, it was found that the rapid solidification after the infiltration effectively dispersed the fine Mg 2 Si particles, and that the introduction of P or CaF 2 particles as the refiner of Mg 2 Si was also effective [4]. We have deduced that the composite with finer Mg 2 Si particles could be obtained by squeeze casting, because the infiltration in the permanent mold leads to rapid solidification. In the present study, magnesium alloy matrix composites reinforced with the short alumina fibers and the Mg 2 Si particles were fabricated by squeeze casting. The microstructure and tensile strength of the composites ranging from 293K to 623K were investigated and compared with that of the conventional fiber-reinforced composite. Furthermore, the effect of the P or CaF 2 particles as the refiner was investigated. Experimental The AZ91D magnesium alloy (Mg-9.2mass%Al-0.7mass%Zn alloy, hereinafter called AZ91D) was used as the matrix. Short alumina fibers (Saffil, from ICI, diameter: 3.5 μ m, length: 200 μ m, tensile strength: 1000MPa, hardness: 700HV) were used as the reinforcement. Pure Si particles (99.9mass%Si) were used as the starting material to form the Mg 2 Si. Si particles with an average size of 5μm and 50μm were used to investigate the effect of size on the morphology of Mg 2 Si. P or CaF 2 particles with an average size of 5μm was used as the refiner. The preforms were fabricated as follows. The fibers, Si particles and refiner were dispersed and agitated in an aqueous medium containing an organic binder (polyvinyl alcohol) and an inorganic binder (Al 2 O 3 sol), and then the fibers having Si particles and refiner attached to their surfaces were dewatered to fabricate a cylindrical preform of 55mm diameter and 30mm height, followed by drying and sintering. The fiber volume fraction of 120/2

3 the composite was set at 18%, because the high-temperature strength of the short alumina fiber-reinforced AZ91D composite became a maximum at 18% [1]. The fiber volume fraction of the preform was set at 15%, because the preform contraction during the infiltration occurs which leads to a 3% increase in the fraction of the composite. The volume fraction of Si particle was set at 3.2%, and that of refiner was set at 0.3%. Fig.1 is the scanning electron micrographs of the preforms, showing that the Si particles and refiner (arrows in the figure) were attached to the fibers and held among the fibers. The composites were fabricated by squeeze casting. The fabrication parameters are as follows: melt and preform temperature, 1003K; mold temperature, 673K; applied pressure, 40MPa. Optical microscopy and a Vickers hardness test (98N,15s) of the composites were performed on the planar section. The tensile test specimens were machined parallel to the planar direction, with a gage length of 10mm, a cross sectional width of 6mm and a thickness of 3mm. The measured tensile strength was ranged from room temperature (293K) to 623K and the fracture surfaces were observed by scanning electron microscopy. Results and Discussion Fig.2 shows the macrostructure of the vertical section of a specimen fabricated with the 5μm Si particle-attached preform. In the figure, the dark part is the composite part and the light part is the unreinforced part. It can be seen that the melt infiltration was completely accomplished with no observable defects. The fiber volume fraction estimated from the height of the composite part (approximately 25mm) was 18%, which was the prescribed value. The macrostructures of the composite fabricated with the 50μm Si particle-attached preform (hereinafter called composite 50Si) and the composites with the refiners were almost same as shown in Fig.2. Fig.3 shows the change in hardness (HV) with the distance from the upper surface of the composite fabricated with the 5 μ m Si particle-attached preform (hereinafter called composite 5Si). Every region has the same hardness value of 170HV. The hardness of composite 50Si and the composites with refiners was also 170HV in every region of each composite, indicating that the homogeneous composites can be obtained by the present process. Fig.4 shows the microstructures of the composites. Alumina fibers, which appear black in the microstructures, were oriented as random configurations. Fine granular phases are also observed in the matrices of the composites. Examination of the composite by EPMA and XRD revealed that the granular phase was Mg 2 Si. Fine Mg 2 Si particles were homogeneously dispersed in the matrix of the composite 5Si (Fig.4(a)). Fine Mg 2 Si particles were also formed in the composite 50Si (Fig.4(d)), and this observation suggests that Si dissolves in the melt and the Mg 2 Si particles are rapidly formed [2] in the present experiment. However, 120/3

4 agglomerated Mg 2 Si particles were observed in the composite 50Si (Fig.4(d)). A similar tendency was observed when P or CaF 2 particles were introduced (Fig.4(b)(c)(e)(f)). During the infiltration, the average size of the Mg 2 Si particle was approximately 5μm regardless of the Si particle size or the introduction of the refiners. The average cooling rate between the melt temperature and the eutectic point of α-mg and Mg 2 Si in the Mg-Si system (912K) [5] was measured at the center of the preform. It was 36K/s and we think it is high enough to refine the Mg 2 Si particles without the introduction of P or CaF 2 particles. Since residual Si was not observed in every composite, it can be stated that the Si particles in the preform changed to the Mg 2 Si particles during infiltration and were then dispersed in the matrix. Tensile tests were performed with the composites without the refiners, because no difference between the microstructure of the composite with and without refiners was observed. The tensile strength of the composites was compared with that of the 18% alumina fiber-reinforced composite without Mg 2 Si particles (hereinafter called fiber-reinforced composite) obtained in the previous study [1]. The tensile strength of the matrix (AZ91D) and the composites is plotted versus the temperature in Fig. 5. Since the tensile strength of the fiber-reinforced composite was higher than that of AZ91D at every temperature, reinforcement with the fibers is effective for improving the strength. The tensile strength of the composite 5Si was further higher than that of the fiber-reinforced composite. The strength at 523K was approximately 180MPa, which is higher than that of the heat-resistant aluminum alloy. The strength at 623K was 160MPa, which is higher by 75MPa (approximately 90%) compared to that of AZ91D. On the other hand, the strength of the composite 50Si was lower than that of the fiber-reinforced composite at every temperature, although higher than that of AZ91D. Subsequently, fractography was used for examining the effect of the dispersion of the Mg 2 Si on the strength. SEM micrographs of the fracture surfaces of the composites after tensile testing at 293K are shown in Fig. 6. For the fiber-reinforced composite, many fibers longitudinally oriented in the stress direction fractured without fiber-pullout, and the fiber surfaces transversely oriented in the stress direction were only slightly seen (Fig.6(a)). These results suggest that the interfacial bond between fibers and matrix was strong. However, several bunches of a few fibers were seen (Fig.6(a) circle). On the other hand, there was almost no bunching on the fracture surface of the composite 5Si (Fig.6(b)). Several cleavage planes were observed on the fracture surface of the composite 50Si (Fig.6(c) circle), suggesting that brittle fracture occurred in these areas. It would appear that these areas correspond to the agglomerated areas of the Mg 2 Si particles shown in Fig.4(d). Fig.7 shows the fracture surfaces of the composites after tensile testing at 623K, indicating that the variation in the fracture surface morphology due to the Mg 2 Si particles was similar to that at 293K. 120/4

5 From these results, the strength mechanism by the dispersion of the Mg 2 Si particles can be discussed. The reason for the improvement in the composite strength by the homogeneous dispersion of the fine Mg 2 Si particles is considered as follows: (1) Mg 2 Si particles reduce the grain size of the matrix ( α -Mg) and prevent the grain growth even at hightemperature [6], and (2) a homogeneous dispersion of the fine Mg 2 Si particles reduces the fiber-to-fiber contact and the stress concentration at the contact point, and thus the stress transmission between the fiber and the matrix becomes easy. As shown in Fig.6(b) and Fig.7(b), the fiber bunches were not observed on the fracture surface of the composite 5Si, suggesting that the Mg 2 Si particles dispersed in the matrix reduced the fiber contact. Previous researches have demonstrated that reinforcement with a large fiber fraction reduced the strengthening effect of the fiber due to the increase in the fiber contact points [7] and increased the smooth fiber surfaces on the fracture surface [8]. However, as we demonstrated in a previous study on the strength properties of the continuous alumina fiber-reinforced aluminum alloy composite, the strength was improved by the particle dispersion because the particles prevented the fiber-to-fiber contact [9]. Also in the present study, the dispersion of the fine Mg 2 Si particles would reduce the fiber-to-fiber contact and the stress concentration at the contact point, and thus the stress transmission between the fiber and the matrix becomes easier. The result that the tensile strength of the composite 5Si was higher than that of the fiberreinforced composite can be explained by the dispersion effect. However, if the agglomerated Mg 2 Si particles exist, cracks would first generate in the agglomerated area when the stress was applied because the deformability of this area is poor. A previous study demonstrates that the cracks easily join and thus the strength of the composite decreases when the agglomerated area of the brittle reinforcement is large or these areas extensively exist [10]. A decrease in the strength due to the presence of the coarse brittle phase was shown for the fiber- or whisker-reinforced aluminum alloy composites [11]. In addition, as shown in Fig.4(d), the Mg 2 Si particles were heterogeneously dispersed in the composite 50Si. In this situation, a fiber-to-fiber contact might occur in the area without the Mg 2 Si particles. The result that the tensile strength of the composite 50Si was lower than that of the fiber-reinforced composite can be explained by these effects. Conclusions 1. By infiltration in the permanent mold, fine Mg 2 Si particles having the average size of 5 μm were formed regardless of the introduction of the refiners, and dispersed in the matrix of the composite. 2. Finer Si particles as the starting material are favorable for homogeneously dispersing the in-situ formed Mg 2 Si particles in the present process. 3. Ranging from 293K to 623K, the tensile strength of the alumina fiberreinforced composite, in which the fine Mg 2 Si particles were 120/5

6 homogeneously dispersed in the matrix, was higher than that of the conventional fiber-reinforced composite or AZ91D. The tensile strength of the composite with the homogeneously dispersed Mg 2 Si particles was approximately 180MPa at 523K, which is higher than that of the heat-resistant aluminum alloy at the same temperature. This high strength is a consequence of the reduction in the fiber-tofiber contact and the improvement of the stress transmission between the fiber and the matrix. References 1. K. Asano and H. Yoneda, Strength of AZ91D Magnesium Alloy Composite Reinforced with Alumina Short Fiber Using Alumina Binder, J. JFS, 74, No.11, Nov 2002, pp G. H. Li, H. S. Gill and R. A. Varin, Magnesium Silicide Intermetallic Alloys, Metall.Trans. 24A, Nov 2003, pp K. Asano and H. Yoneda, Refinement of In-Situ Formed Mg 2 Si Particles in Fiber-Reinforced Magnesium Alloy, 66th World Foundry Congress, Istanbul, Turkey, 6-9 Sept, 2004, pp , pub Toksad: The Foundrymen s Association of Turkey, K. Asano and H. Yoneda, Effect of P and CaF 2 on Refinement of In- Situ Mg 2 Si in Fiber-Reinforced Magnesium Alloy, Report of JFS Meeting, 144, May 2004, p T. B. Massalski, Binary Alloy Phase Diagrams, ASM International, 1990, p Y. Kojima, Handbook of Advanced Magnesium Technology, Kallos Publishing co., ltd., Japan, 2000, p H. Akbulut, M. Durman and F. Yilmaz, A Comparison of As-cast and Heat Treated Properties in Short Fiber Reinforced Al-Si Piston Alloys, Scripta Mater., 36, No.7,1997, pp T. Shinkawa, H. Kageyama, S. Kamado and Y. Kojima, Structures and Mechanical Properties of Hybrid Mg-Zn-Ca Alloy Composites Reinforced with δ -Al 2 O 3 Short Fiber and 9Al 2 O 3 2B 2 O 3 Whisker, J.Jpn.Inst.Light Met., 46, No.12, Dec 1996, pp K. Asano and H. Yoneda, Effects of Particle-Dispersion on the Strength of an Alumina Fiber-Reinforced Aluminum Alloy Matrix Composite, Mater. Trans., 68, No.6, Jun 2003, pp H. Morimoto, H. Iwamura, K. Ohuchi and Y. Ashida, Fabrication of Thin Wall Tubes of SiC Whisker Reinforced 6061 Aluminum Alloy Composite by Extrusion and Their Mechanical Properties, J.Jpn.Inst.Light Met.,45, No.2, Feb 1995, pp C. M. Friend, I. Horsfall and C. L. Burrows, The Effect of Particulate: Fiber Ratio on the Properties of Short-Fiber/Particulate Hybrid MMC Produced by Preform Infiltration, J.Mater.Sci., 26,1991, pp /6

7 Figures (a) With 5μm Si (b) With 50μm Si (c) With 50μm Si and P P Si 25μm 40μm 40μm Fig.1 SEM micrographs of the preforms. Unreinforced part Composite Hardness, HV Distance from upper surface, mm 5 10mm Fig.2 Macrostructure of a vertical section of a specimen (composite 5Si, without refiner). Fig.3 Hardness distribution of the composite 5Si (without refiner). Without refiner P-added CaF 2 -added (a) (b) (c) 5Si (d) (e) (f) 50Si 20μm Fig.4 Microstructure of parallel section of the composites. 120/7

8 Tensile strength, MPa 300 Composite 5Si Composite 50Si Fiber-rei nforced AZ91D Temperature, K 650 Fig.5 Effect of temperature on the tensile strength of AZ91D and the composites. (a) Fiber-reinforced composite (b) Composite 5Si (c) Composite 50Si 20μm Fig.6 SEM micrographs of fracture surfaces of the composites (without refiner) after tensile testing at 293 K. (a) Fiber-reinforced composite (b) Composite 5Si (c) Composite 50Si 20μm Fig.7 SEM micrographs of fracture surfaces of the composites (without refiner) after tensile testing at 623 K. asano@mech.kindai.ac.jp Previous Paper Next Paper 120/8 Back to Programme