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e s e a r c h tages of Mg-based MMCs with a high volume fraction of ceramic reinforcement (particularly in the submicron range). The most significant contributor to the total cost of the material is submicron boron carbide powder, which on the other hand, is also responsible for achieving the superior performance of Mg-B 4 C composites. Thus the major limitation in further commercialisation of composites with a high amount of submicron reinforcement is the extremely high price of such reinforcement. Hence, in order to improve the competitive position of Mg-B 4 C composites, making them more attractive also for automotive applications, a significant price reduction of submicron boron carbide reinforcement will be necessary. conclusions The main conclusions that may be derived from this work are as follows: Doping a B 4 C preform with 5 vol.% of titanium powder as wetting agent, Mg-B 4 C composites with approx. 30 vol.% of fine boron carbide particles (with an average particle size of 0.8 µm) and approx. 20 vol.% of coarse boron carbide particles (with an average particle size of 44 µm) were successfully prepared by spontaneous pressureless infiltration. On the other hand, the pressureless infiltration of preforms with a higher amount of fine boron carbide particles was, under the same experimental conditions, unsuccessful or, in some cases, incomplete. Moreover, preforms consisting of fine boron carbide particles with an average particle size of 0.8 µm were not successfully infiltrated even at higher temperatures (up to 900°C), a longer holding time (2 h) and a higher amount of wetting agent (10 vol.% of Ti powder) indicating that for spontaneous infiltration some volume fraction of coarse boron carbide particles should be incorporated into the preform. As was experimentally confirmed, the successful initiation of spontaneous infiltration depends on wettability enhancement by wetting agents and optimal selection of geometrical properties characterising porous performs (porosity, average particle size and particle size distribution of boron carbide powders, pore size distribution, etc.). Wetting agents in the system Mg-B 4 C, which is a non-reactive one, enhance wetting by enabling various chemical reactions between the matrix, reinforcement and wetting additives. Addition of Ti improves wetting by forming metal-like TiC which is more stable than B 4 C. On the other hand, silicon has an affinity for dissolving boron which, once dissolved, reacts with molten magnesium forming MgB 2 . In addition, both wetting agents react with molten magnesium forming intermetallics (Mg 2 Si and MgTi) and bright inclusions appear in the solidified matrix, consisting of MgO, TiO 2 , SiO 2 and TiC combined in different molar ratios. Titanium as a wetting agent also helps in stripping and inter-penetrating the oxide film on the surface of molten magnesium, enhancing the formation of an oxide-free infiltration front between the molten magnesium and ceramic reinforcement. Regarding the geometrical quantities characterising porous preforms, it was found that for preforms with an average particle size of the mixed boron carbide powders less than 15 µm and a volume fraction of fine, submicrometre particles higher than 60 vol.%, spontaneous infiltration did not take place or proceeded only in a thin surface layer of the preform. In addition, attempts to initiate spontaneous infiltration without adding wetting agents were unsuccessful. Finally, below 750°C, spontaneous infiltration was not attained under any other experimental conditions, most probably because of poor wetting and low chemical reactivity in the system necessary for enhancing wetting behaviour. However, once successfully initiated, the spontaneous pressureless infiltration of porous preforms of dimensions 20 mm diameter and 50 mm long was completed rapidly, in less that 15-20 min. The hardness (HR15T), elastic modulus, tensile strength and 0.2% tensile yield strength of composites of Mg-B 4 C composites were more than doubled compared to unalloyed magnesium. The main limitation remained ductility, which decreased on reinforcement by one order of magnitude. Improvement of hardness and tensile properties (except ductility) was Material Composition vol.% P-A P-B Mg Cost of material (EUR/m 3 ) E (GPa) Selected property (average from table ) Tensile strength (MPa) 0.2% tensile yield strength (MPa) Non alloyed magnesium 100 2000 41 216 125 Powder A (P-A) 600 Powder B (P-B) 20.000 Sample 1* 43 57 1.398 81 403 287 Sample 2 29 11 60 3.574 89 443 304 Sample 3 26 12 62 3.756 91 453 308 Sample 4 20 18 62 4.960 96 478 318 Sample 5 47 53 1.342 83 416 303 Sample 6 35 12 53 3.670 94 462 326 Sample 7 26 23 51 5.776 100 501 347 Sample 8 18 30 52 7.148 108 535 364 * from table 1 Table 6: Correlation between the cost of the material and the achieved properties in various composite samples 84 ALUMINIUM · 11/2008
e s e a r c h and Development of the Republic of Slovenia, as well as the Impol Aluminium Company from Slovenska Bistrica, Slovenia, under contract no. 1000- 05-217061. Fig.6 a, b: XRD of composite samples: (a) sample 4 (Table 1)-wetting agent silicon, (b) sample 8 (Table 1)-wetting agent titanium evidently higher in samples doped with titanium as wetting agent. The increase of the volume fraction of fine boron carbide particles in the magnesium matrix and the wetting agent added to the preform resulted in about 30% higher values of elastic modulus and tensile strength, about 20% improvement in 0.2% tensile yield strength, as well as slight improvement in hardness (about 5 %) of the composites. The main advantage of Mg-B 4 C composites with a high (50 vol.%) volume fraction of low density ceramic reinforcement is in achieving superior tensile properties (except ductility) – better than in aluminium matrix composites – while not exceeding 90% of the density of aluminium. The best tensile properties were obtained by introducing 30 vol.% of submicron boron carbide reinforcement, which also significantly increases the cost of the composite. Acknowledgement This work is supported by funding from the Public Agency for Research References 1. J. Kaneko, M. Sugamata, J. S. Kim, M. Kon, “Fabrication and Properties of Cast and Extruded SiC w /AZ91Mg Composites”, in Magnesium Alloys and their Applications, ed. K. U. Kainer, 222-228, Wiley-VCH, Weinheim, 2000 2. V. Kevorkijan, “Mg AZ80/ SiC Composite Bars Fabricated by Infiltration of Porous Ceramic Preforms”, Metall. Mater. Trans. A, Vol. 35A, 707- 715 (2004) 3. V. Laurent, P. Jarry, G. Regazzoni, D. Apelian, “Processing-Microstructure Realtionships in Compocast magnesium/SiC”, J. Mater. Sci., 27, 4447-4459 (1992) 4. A. Stalmann, W. Sebastian, H. Friedrich, S. Schumann, K. Droeder, “Properties and Processing of Magnesium Wrought Products for Automotive Applications”, Adv. Eng. Mater., Vol. 3(12), 969- 974 (2001) 5. H. W. Wagner, “Cold Extrusion of Magnesium Alloys and MMCs”, in Magnesium Alloys and their Applications, ed. B. L. Mordike, K. U. Kainer, 557- 562, Werstohf-Informationsgesellschaft, Frankfurt, 1998 6. A. Mortensen, “Melt Infiltration of Metal matrix Composites”, in Comprehensive Composite Materials, Vol. 3, eds. A. Kelly, C. Zweben, 521-555, Elsevier, Amsterdam, 2000 7. R. Asthana, “Properties of Cast Composites”, Key Engineering Materials, Vol. 151-152, 351-398 (1998) 8. A. A. Luo, “Magnesium: Current and Potential Automotive Applications”, JOM, Vol. 54(2), 42-48 (2002). Author Dr. Varuzan Kevorkijan (1957), Principal Scientist, is head of applied and industrial projects in the field of aluminium and aluminium-based composites. He is an independent researcher. ALUMINIUM · 11/2008 85
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