New Process to Fabricate Magnesium Composites Using SiO 2 Glass Scraps

Transcription

1 Materials Transactions, Vol. 44, No. 12 (23) pp to 2474 Special Issue on New Systems and Processes in Recycling and High Performance Waste Treatments #23 The Japan Institute of Metals New Process to Fabricate Magnesium Composites Using Glass Scraps Katsuyoshi Kondoh 1 and Tachai Luangvaranunt 2 1 Research Center for Advanced Science and Technology, The University of Tokyo, Tokyo , Japan 2 Department of Metallurgical Engineering, Chulalongkorn University, Phyathai road, Pratumwan, Bangkok 133, Thailand To realize the lightweight effects by materials technology, a new process for fabricating high-performance magnesium composites via a solid-state reaction by using high purity glass scraps has been established. From a viewpoint of the microstructures control of the composites, the core technologies to improve the physical and mechanical properties are; a solid-state synthesis of Mg 2 Si and MgO particles by the deoxidization of glass by magnesium, and a refinement of both their dispersoids and the magnesium matrix grains by the RPW process. For example, when using the elemental AZ31 magnesium alloy and 2 mass% glass powder mixture as starting raw materials, the hot extruded composite including Mg 2 Si and MgO shows 363 MPa of the ultimate tensile strength. The addition of only 2 mass% powder also causes the remarkable improvement of the corrosion resistance because of the uniform distribution of refined Mg 2 Si not only at the particle boundary but also inside the grains. This process is quite safety and environmentally benign compared to the conventional re-melting process, because of utilizing course magnesium raw powder and no use of SF 6 toxic gas. It also shows a possibility to employ glass scraps as starting raw materials to fabricate magnesium alloys. (Received July 22, 23; Accepted October 1, 23) Keywords: magnesium composite, glass, solid-state reaction, deoxidization, Mg 2 Si, MgO, mechanical properties, corrosion resistance 1. Introduction Weight reduction is one of the important core technologies to create the safe and comfortable environment. For example, the lightweight effect of automotives reduces both the energy consumption and the air pollutions in traveling, such as CO 2, NO x and SO x. On the other hand, that of the medical equipments gratefully assists the aged or handicapped people in their movement, and contributes Barrie-free Life System. Magnesium is the lightest metals of the industrial alloys, and currently applied to the components or systems, such as housings of personal digital assist (PDA), mobile phones and PCs. Therefore, the environmentally benign process for recycling magnesium wastes, including in-house scraps, should be established. The previous works 1 4) indicated that comparing the conventional magnesium remelting process, the solid-state recycling one via hot extrusion or equal channel angular extrusion (ECAE) process shows the following merits; the reduction of energy consumption, no need of SF 6 toxic gas used in melting and the remarkably improved mechanical properties of re-produced magnesium alloys due to the refined microstructures. The cyclically repeated plastic working (RPW) process 5) is effective on solid-state recycling chips, fragments or coarse powder of the light metal wastes. It also serves the improvement of mechanical properties by refining the microstructures. For example, the refinement of magnesium matrix grains by the RPW process caused the 4 8% increase of the ultimate tensile strength compared to the conventional casted material, when employing AZ31 coarse powder or AZ91 magnesium alloy chips. 6) On the other hand, from a viewpoint in applying magnesium alloys to a lot of components, high-performance magnesium composites were developed, in using the elemental magnesium and silicon powder mixture as raw materials. The key technology is the solid-state synthesis of Mg 2 Si compounds, which were employed as reinforcements of the composites, because of their high hardness, high Young s modulus and superior corrosion resistance. In the present work, a new process to fabricate highperformance magnesium composites in solid-state was established by employing high purity glass scraps instead of silicon particles. In-situ reaction of magnesium and glass particles progresses to synthesize Mg 2 Si and MgO, and distributed in the magnesium matrix. First of all, the possibility to form these dispersoids via the deoxidization of glasses by magnesium is discussed. The effect of the glass content on the physical and mechanical properties of the magnesium composites is also evaluated. Finally, from a viewpoint of the refinement of the Mg 2 Si and MgO dispersoids, the repeated plastic working process is applied to the magnesium alloy and glass powder mixture. 2. New Recycling Process Design of Glass Wastes The mechanical properties and corrosion resistance should be improved when applying magnesium alloys to the structural components or automotive parts, which strongly require lightweight effects. Mg 2 Si bulky material, fabricated via the solid-state reaction with an exothermic heat, 7) showed high micro-hardness of 6 7 Hv and high Young s modulus of 12 GPa, in employing the elemental powder mixture of magnesium and silicon. In the result of the spray salt test (JIS Z 2731), no corrosion phenomenon of Mg 2 Si intermetallic bulk occurred after spraying 5% salt water over 1 hrs at 38 K. On the other hand, the corrosion started at 3 6 h in using the conventional 34 stainless steel, that is, Mg 2 Si has superior corrosion resistance. Accordingly, the dispersion of Mg 2 Si particles in the matrix of the magnesium alloys has a possibility to improve the above mechanical and physical properties. In the previous work, magnesium composites were fabricated via a solid-state reaction to synthesize Mg 2 Si using the elemental magnesium and silicon powder mixture. They showed higher mechanical properties; for example, the ultimate tensile strength of 271 MPa, when employing the elemental AZ31 magnesium

2 New Process to Fabricate Magnesium Composites Using Glass Scraps 2469 Input raw materials Green billet Pre-heating Hot plastic workings Mg alloy chips or coarse powder e.g. hot extrusion Wasted glass powder Compaction of elemental Mg- powder mixture Solid-state synthesis 4Mg+ Mg 2 Si+2MgO Fig. 1 Schematic illustration of new recycling process in solid-state to fabricate magnesium composite alloys from elemental mixture of glass wastes and magnesium powder. alloy and 5 mass% silicon powder mixture. The UTS is much higher than that of the hot extruded AZ31 alloy without Mg 2 Si on route of the same powder metallurgy process. The reaction process is, however, invalid to fabricate the same magnesium composite via casting process because the coarsened Mg 2 Si intermetallics during melting and solidification reduce the strength or elongation of the magnesium alloy. The materials and processing designs for fabricating magnesium composites with Mg 2 Si dispersoids are available in the case of using the powder as input raw materials instead of silicon. From a viewpoint of a free energy of oxides formation G, the Ellingham diagram 8) shows that G of magnesium at 923 K is quite lower than that of silicon. It means that silicon is easily formed after deoxidizing by magnesium at elevated temperature, and Mg 2 Si synthesis occurs by a reaction of the silicon with magnesium as explained in eq. (1). 4Mg þ! Mg 2 Si þ 2MgO ð1þ Considering that the main ingredient of the glass products is, the glass scraps, in particular with a high purity, also have a large possibility to be employed as input raw materials to form Mg 2 Si and MgO by oxidization. Figure 1 schematically illustrates a new process to fabricate the magnesium composite alloys reinforced by Mg 2 Si/MgO fine dispersoids in using glass scraps with a high purity of 99:9 99:99%, such as sputtering target materials for electron devices and optical glass fiber. They are mechanically fractured into pieces or powder via grinding or milling process. After sieving them, the glass powder is prepared as input raw materials. Concerning the magnesium alloys as another starting materials, from a viewpoint of the safety and its cost reduction, the conventional magnesium alloy coarse powder, having a mean particle size is :5 2 mm, is employed. The elemental glass and magnesium powder mixture is consolidated at room temperature. The green compact, having a relative density of 85 9%, is heated in argon or nitrogen gas atmosphere. The heating temperature is an important parameter to synthesize Mg 2 Si and MgO, and strongly depends on the particle size of input raw materials, the purity and content of glass powder, and relative density of the green compact. In the previous work, when employing the elemental Mg and crystalline powder mixture, the differential scanning calorimeter (DSC) thermogram of the compact revealed an exothermic peak due to the above reaction between raw materials. 9) That is, this remarkably exothermic heat causes the reaction to form Mg 2 Si and MgO by a self-propagating high-temperature synthesis (SHS). Accordingly, an ignition temperature (T i ), which corresponds to the starting temperature of the exothermic peak of the DSC thermogram, is employed as a suitable heating temperature to synthesize them in this study. After heating the compact, it is immediately consolidated into a full density by the hot plastic working, such as hot extrusion or forging process, to fabricate the magnesium composite alloy including in-situ formed Mg 2 Si and MgO dispersoids. From industrial and economical points of view, the exothermic heat during hot working assists the progress of the above solid-state synthesis. This process also does not require any SF 6 toxic gas used in the conventional melting process of the magnesium alloys. 3. Experimental Procedure Figure 2 shows an appearance of in-house scraps of the wrought optical glass fiber with a purity of 99.99% (a), and their small blocks or fragments by a ball milling equipment (b). Furthermore, the glass blocks are grinded into fine powder, having a mean particle size of 16.8 mm. X-ray diffraction (XRD) of glass powder shown in Fig. 2(c) indicates a broadened pattern due to its non-crystal. Another input raw material is the conventional AZ31 magnesium alloy coarse powder (Mg-3Al-1Zn/mass%).When mixing both raw materials, the glass particle content of the powder mixture is, 2, 4, 6, and 8 mass%. Each elemental powder mixture is consolidated at room temperature by cold pressing under the applied pressure of 6 MPa. The relative density of the green compact, having a diameter of 34 mm and 2 mm length, is about 86 88%. DSC thermal analysis on each green compact of 5. mg is carried out to examine T i for each compact specimen at the heating rate of.33 Ks 1, when heating from 293 K to 1 K in Ar gas with the flowing rate of 5: Nm 3 /s. After heating each green compact at T i for 3 s in nitrogen gas atmosphere, it is immediately consolidated into a full density by hot extrusion with an extruding ratio of 37. The structural evolution of the green compacts is monitored by using XRD with Cu K 1 radiation ( ¼ :1545 nm) operated at 4 kv and 2 ma. The optical microstructure observation, micro-hardness and

3 247 K. Kondoh and T. Luangvaranunt (a) Exo. glass (b) DSC (a.u.) 8% 6% 4% 2% % Temperature, T / K Fig. 3 DSC thermograms of magnesium green compacts with various content of glass. Intensity (c) Diffraction Angle, 2θ Exothermic Heat, H ex / Jg Matrix; AZ31 alloy Content of Glass (mass%) Fig. 2 Appearance of wasted glass used for optical fiber (a), refined blocks of glass wastes (b) and XRD pattern of glass powder (c). Fig. 4 Dependence of exothermic heat of Mg- glass green compacts in DSC curves on glass content. ultimate tensile strength of each magnesium composite are evaluated, compared to those in using the crystal particles. 4. Results and Discussion 4.1 Solid-state synthesis of Mg 2 Si and MgO using glass Figure 3 shows the DSC thermograms of the green compacts in employing the elemental AZ31 and glass powder mixture. Each DSC curve except for % glass distinctly reveals an exothermic peak at about 73 K. The exothermic heat gradually increases with increase in the glass content as shown in Fig. 4. The endothermic peak is also obviously detected at 923 K due to the latent heat of magnesium melting. In general, non-crystalline glasses indicate an endothermic peak at 844 K (glass transition temperature) due to the phase transformation from to. 1) The DSC curve of the green compact including glass, however, shows no endothermic peak at 844 K, which is much higher than the starting temperature of the exothermic at 73 K. In other word, no powder remains in the green compact after heated over 844 K. On the other hand, XRD analysis was carried out on the green compacts annealed at 723 K for 3 s in nitrogen gas atmosphere. Figure 5(a) indicates the peaks of Mg 2 Si and MgO of the green compacts, excepting % glass content. As shown in Fig. 5(b), the relative peak intensity ratio of Mg(11) to Mg 2 Si increases roughly in proportion to the glass content of the starting materials. Furthermore, the DSC curves of the green compacts heated at 723 K show no exothermic peak as explained in Fig. 3. Accordingly, these results indicate that glass particles completely react with magnesium alloy powder, which is based on the SHS behavior, to form Mg 2 Si and MgO by heating over T i. From a viewpoint in preventing the grain growth or microstructure coarsening, the ignition temperature, T i is desirable to be as low as possible. T i strongly depends on (a) contacting surface area between powder and magnesium matrix, (b) impurities of powder (e.g., other oxides), and (c) dislocation density of matrix. 7) Figure 6 shows the dependence of the ignition temperature in DSC curve on the crystalline powder, having various purities, compared to the glass powder. The particle size distribution

4 New Process to Fabricate Magnesium Composites Using Glass Scraps 2471 Peak Intensity Ratio, I(Mg 2 Si) / I(Mg) / % Intensity (a.u.) (a) Mg 2 Si MgO Mg glass 8% 6% 4% 2% % Diffraction Angle, 2θ (b) Glass Content (mass%) Fig. 5 XRD patterns of green compacts after heating at 723 K for 3 s (a) and peak intensity ratio dependence on glass content based on XRD patterns. of the crystalline powder is almost the same as that of glass powder. Compared to T i values in using glass powder, the crystalline one reveals a high ignition temperature of about 1 2 K. In increasing the impurities of raw powder, it also remarkably increases. That is, the impurity is more effective on the reaction rather than the difference of the crystal structure. Table 1 shows the chemical compositions of various glasses used in the industrial components. Each glass includes 6 7 mass% of, however the content of other oxides is quite different. Ellingham diagram indicates that CaO, BaO, and MgO are not deoxidized by magnesium, and exist in the magnesium matrix even in heating at 9 K. It is not clarified that these thermally stable dispersoids are effective on strengthening of the composites, or act as barriers to obstruct the reaction of with magnesium. Some other oxides such as Al 2 O 3, Fe 2 O 3,B 2 O 3,Na 2 O, SrO possibly react with magnesium powder at 773 K. Mg-Al intermetallics, such as Mg 17 Al 12, Ignition Temperature of DSC, T i / K Crystalline powder (Purity; 98.5%) Crystalline powder glass powder (Purity; 99.8%) (Purity; 99.99%) Content of Glass (mass%) Fig. 6 Effect of purity of raw materials on ignition temperature at exothermic peak in DSC curve. Mg 2 Al 3, are well known as available dispersoids to improve the mechanical properties and corrosion resistance. Included Fe and B metallic elements, which exist in the matrix after deoxidization, are harmful to magnesium alloys because they reduce the corrosion resistance. In particular, the content of Fe element is limited to.1 mass% of the magnesium alloys. Therefore, the content of each glass scrap powder, employed as starting raw materials, should be less about 7 mass%. Concerning Na 2 O, SrO, ZrO 2 and ZnO, their effects on the synthesis of Mg 2 Si and MgO or characteristics of the composites are not obvious. It is necessary to clarify the influences of each oxide on the mechanical and physical properties of magnesium alloys, when employing the wastes or scraps of the industrial glass components as raw materials to fabricate the magnesium composites with Mg 2 Si via solidstate reaction. 4.2 Fabrication of magnesium composites using glass scraps Figure 7 shows the optical microstructures of the hot extruded AZ31 alloys with Mg 2 Si and MgO dispersoids via the above solid-state reaction, when using the elemental AZ31 and 2 mass% glass powder mixture. There is no pore in the AZ31 matrix, and its relative density is 99.8%. Most of the particles are uniformly distributed in the matrix, however, some ones locally gathered at the primary particle boundaries. This is due to a quite difference of the particle size between AZ31 and glass powder. XRD analysis detects the dispersoids as Mg 2 Si and MgO. The dispersoid size is the same as that of glass raw power because of no remarkable grain growth and coarsening during heating and hot extrusion due to its solid-state synthesis. The matrix grain size of the Table 1 Chemical compositions of wasted glass materials used in industrial components. (mass%) CaO MgO BaO Al 2 O 3 Fe 2 O 3 B 2 O 3 Na 2 O SrO ZrO 2 ZnO Plates TFT-LCD Bottles TFT-LCD; Thin Film Transistor Liquid Crystal

5 2472 K. Kondoh and T. Luangvaranunt Mg 2 Si/MgO TS,YS / MPa (a) Elongation TS Y.S. % 2% 4% 6% 8% Glass Content (mass%) Elongation (%) 2 µ m Fig. 7 Optical microstructure of hot extruded AZ31-2 mass% composite alloy on route of solid-state synthesis process. Vickers Hardness, Hv Matrix; AZ31 alloy Glass Content (mass%) Fig. 8 Hardening effect of magnesium composite alloys with various content of in-situ formed Mg 2 Si/MgO dispersoids. composite alloys is 1-15 mm, and significantly small compared to the input AZ31 raw powder with a mean particle size of 12 mm. The refined texture is due to the dynamic recrystallization during hot extrusion. Figure 8 shows the micro-hardness dependence of the AZ31 composite alloys on the glass content. It proportionally increases with increase in the content, because the hardness of Mg 2 Si and MgO are 6 7 Hv and Hv, respectively, and much harder than that of the AZ31 matrix alloy. In general, the dispersoids size is one of the dominant factors on the mechanical properties of the composite materials. Concerning the influence of glass powder on the tensile properties of the composites, Fig. 9 indicates the dependence of TS, Y.S. and elongation on the glass content. A mean particle size of glass raw powder is 16.8 mm (a) and 52.5 mm (b), respectively. Elongation decreases with increase in the content, because the solid-state synthesized Mg 2 Si and MgO dispersoids, having a strong bonding with the matrix, are much more brittle than the AZ31 matrix. Both magnesium composites indicate the gradual increase of the yield stress according to the increase of content. This is because the Young s modulus of the TS,YS / MPa (b) Elongation Y.S. % 2% 4% 6% 8% Glass Content (mass%) Fig. 9 Effect of glass particle size on tensile strength and elongation of hot extruded AZ31 composite alloys with Mg 2 Si/MgO dispersoids, in employing glass powder with a mean particle size of 16.8 mm (a) and 52.5 mm (b). dispersoids is extremely higher than that of the magnesium matrix. Concerning the tensile strength, however, the effect of the glass particle size is clarified. TS gradually increases with increase in the glass content, in using fine glass powder. That is, the refined Mg 2 Si and MgO act as the effective reinforcements to improve the mechanical properties by the dispersion strengthening, when they are uniformly distributed in the matrix. The AZ31 composite with coarse dispersoids, however, shows that the tensile strength decreases when the glass powder content is over 6 mass%. This is because the coarse Mg 2 Si and MgO dispersoids in the matrix cause the stress concentration, and promote the crack propagation under the tensile stress condition. Therefore, not only the elongation but also TS decrease with increase in the glass content when employing coarse glass particles as starting raw materials. 4.3 Microstructure control by repeated plastic working The utilization of fine glass particles has a possibility to improve the mechanical properties of the magnesium composites with Mg 2 Si and MgO dispersoids. When employing ultra-fine additives, however the partial gathering of them in the matrix causes the extreme decrease of the properties. In this study, the cyclically repeated plastic working (RPW) process is applied as a microstructure control method to distribute the fine particles uniformly in the matrix. As illustrated in Fig. 1, it consists of the alternative plastic working on the starting raw powder mixture at room temperature. The lower punch and the die are fixed in the TS Elongation (%)

6 New Process to Fabricate Magnesium Composites Using Glass Scraps 2473 Feeding input materials <Repeated plastic working process> Compaction Upper punch Exo. Via Repeated Plastic Working Upper punch Input raw materials Die Lower punch DSC (a.u.) T i T i Via Conventional compaction Output green compact Withdrawal Backward Extrusion Fig. 1 Schematic illustration of cyclically repeated plastic working on refining microstructures by using high speed screw-driven press machine. press machine. The two upper punches go down into the die alternatively. The punch I and II, for the compaction of raw powder mixture and the backward extrusion of the compact, respectively, are automatically controlled by PC. The impact energy into the compact via the upper punch II is much effective on the fragmentation and refinement of raw materials. The relative density control of the compact by the punch I is also important to refine them effectively. At the same time, the backward extrusion promotes a mixing effect of the two kinds of the starting raw materials. Refined glass powder are embedded in the magnesium matrix during plastic working, that is, the partial gathering of them at the primary magnesium raw particle boundaries is completely obstructed. From a viewpoint of the high-speed compaction with large impact energy, the 1 kn screw-driven press machine (ENOMOTO 1AF-AB type) is used in this study. According to the increase with the number of cycles of the plastic working, the uniform distribution of refined particles in the AZ31 matrix occurs drastically. After a suitable cycle number (e.g., 1 2 cycles), it outputs the columnar AZ31 green compact dispersed with the refined glass particles. The compact is supplied to the pre-heating process for the solid-state synthesis of Mg 2 Si and MgO dispersoids. In order to decide the pre-heating temperature, DSC analysis on the small piece of the compact is carried out. Figure 11 shows the DSC curves of the compacts on the route of the RPW process, compared to that by the conventionally cold press, when employing the elemental AZ31 and 4 mass% glass powder mixture. The repeated plastic worked compact indicates the extremely lower ignition temperature of 643 K than that in using the conventional one, not via RPW process. When the number of cycles is 2, the difference of T i between both green compacts is about 7 K. It means that the reaction for the synthesis of Mg 2 Si and MgO occurs at lower temperature by the RPW process. The reasons of the remarkable decrease of T i on the route of the RPW process are considered as follows; (1) Increase of the contacting area between and AZ31 powder because of the increase of the specific surface area of refined glass particles. (2) Formation of the fresh surface area of AZ31 powder after mechanical breakage of its surface oxide films by the RPW process. (3) Increase of the defect density of AZ31 powder matrix Temperature, T / K Fig. 11 DSC thermograms of AZ31-4 mass% green compact on route of repeated plastic working process, compared to conventionally cold compaction. Mg 2 Si/MgO 1 µ m Fig. 12 Optical microstructure of hot extruded magnesium composite with Mg 2 Si/MgO dispersoids via RPW process in employing elemental AZ31-2 mass% powder mixture. and internal energy by a lot of plastic deformation. (4) Progress on diffusion between and magnesium by the refinement of the matrix grains. The lower pre-heating temperature is effective on the microstructure control, because the matrix grain growth and the coarsening of the synthesized compounds are obstructed under the small thermal history. That is, the refined matrix grains and fine dispersoids in the matrix cause the strengthening effect of the magnesium composite alloys. Figure 12 shows the optical microstructures of the hot extruded AZ31-2 mass% glass alloys via the RPW process with 2 cycles. The pre-heating temperature of the green compact is 653 K for 24 s in nitrogen atmosphere. There is no pore of the composite, and the extrusion is available for the densification of the green compact. The XRD analysis results reveal that glass particles are completely reacted with AZ31 magnesium alloys to form Mg 2 Si and MgO dispersoids. They are distributed even more uniformly in the matrix, compared to the microstructures by the conventional compaction shown in Fig. 7. The mean particle size of the dispersoids, measured by the image analysis, is 3.4 mm in a diameter. It means that

7 2474 K. Kondoh and T. Luangvaranunt the input raw glass powder, having a mean particle size of 16.8 mm, is effectively refined by the RPW process. The matrix grain size with about 3 8 mm is also significantly smaller than that of the hot extruded composite via the conventionally cold press. Concerning the mechanical properties of the hot extruded AZ31-2 mass% glass alloys on route of the RPW process, the ultimate tensile strength is 363 MPa, and much higher than that of the magnesium composite via the conventional compaction. The remarkably improved strength of the magnesium alloys is due to the refined matrix grain, the uniform distribution of fine Mg 2 Si and MgO particles by the RPW process, and the obstruction of microstructure coarsening by the low temperature pre-heating. The electricochemical test is carried out to evaluate the corrosion resistance of this magnesium composite alloy. Sodium chloride solution with.1 kmolm 3 concentration is used in this test. The potentiodynamic current-potential curves indicate that AZ31 composites, including Mg 2 Si and MgO dispersoids, have a current of 2: : Acm 2, which is approximately half of that of the conventional one with no dispersoid (5: Acm 2 ). The Mg 2 Si uniform dispersion not only at the primary particle boundaries, but also in the matrix is much effective to improve the corrosion potential of the conventional magnesium alloys. Additionally, it is improved with increase in the Mg 2 Si content. Accordingly, high purity glass scraps can be employed as useful raw materials to fabricate the magnesium alloys with superior physical and mechanical properties, when using the solid-state synthesis of Mg 2 Si and the refining effect by the repeated plastic working. 5. Conclusions New process for fabricating high-performance magnesium composites via a solid-state reaction by using high purity glass scraps has been established. This process is quite safety and environmentally benign compared to the conventional re-melting process, because of utilizing course magnesium raw powder and no use of SF 6 toxic gas. From a viewpoint of the microstructures control of the composites, the core technologies to improve the physical and mechanical properties are; a solid-state synthesis of Mg 2 Si and MgO particles by the deoxidization of glass by magnesium, and a refinement of both their dispersoids and the magnesium matrix grains by the RPW process. For example, when using the elemental AZ31 magnesium alloy and 2 mass% glass powder mixture as starting raw materials, the hot extruded composite including Mg 2 Si and MgO shows 363 MPa of the ultimate tensile strength. The addition of only 2 mass% glass powder also causes the remarkable improvement of the corrosion resistance because of the uniform distribution of refined Mg 2 Si not only at the particle boundary but also inside the grains. Acknowledges Authors gratefully express our thanks to Dr. R. Tsuzuki, The University of Tokyo and Mr. H. Oginuma, Graduate school, Musashi Institute of Technology, in the experiments for their assistances. This study was financially supported by the project Development of Environmentally Benign Manufacturing Process of High-Performance Magnesium Alloys from Kanagawa Academy Science and Technology (KAST). REFERENCES 1) M. Mabuchi, K. Kubota and K. Higashi: Mater. Trans. 36 (1995) ) J. W. Yeh, S. Y. Yuan and C. H. Peng: Mater. Sci. Eng. A252 (1998) ) Y. Iwahashi, J. T. Wang, Z. Horita and T. G. Langdon: Scr. Mater. 35 (1996) ) K. Nakashima, Z. Hirota, M. Nemoto and T. G. Langdon: Acta Mater. 46 (1997) ) K. Kondoh and T. Aizawa: Mater. Trans. 44 (23) (in printing). 6) K. Kondoh, T. Luangvaranunt and T. Aizawa: Mater. Trans. 43 (22) ) K. Kondoh, H. Oginuma, E. Yuasa and T. Aizawa: Mater. Trans., JIM 42 (21) ) J. F. Elliott and M. Gleise: Thermochemistry for Steelmaking 1 (196). 9) K. Kondoh, H. Oginuma and T. Aizawa: Mater. Trans. 44 (23) ) B. Reynard, F. Takir, F. Guyot G. D. Gawanmesia, R. C. Liebermann and P. Gillet: Amer. Mineral. 81 (1996) 585.