Structure of gravity cast AK7-SiC composites
Marian Mitko, Szczepan Tomczyński Politechnika Częstochowska, Katedra Odlewnictwa, al. Armii Krajowej 19, 42-200 Częstochowa
Annals 2 No. 4, 2002 pages 220-222
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abstract The work presents results of investigation concerning the structure of aluminium-silicon cast alloy matrix composite reinforced with SiC particles of dimensions 63÷100 μm. The composite has been prepared by mechanical mixing of the liquid alloy and the SiC particles. The protective argon atmosphere has been maintained over the melt surface, and during the mixing process the AlMg30 master alloy (1 wt. %) has been added on the melt surface. The mixture has been gravity cast into metal molds. Optical microscopy and fracture surface examination by means of scanning microscopy have confirmed the uniform distribution of the SiC particles in the composite matrix and revealed that the metal matrix closely adheres the particles (Fig. 1a). Examination of fractures reveals the existence of strong bonding between the particles and the matrix. Cracks which frequently go across the SiC particles, and not along the matrix-SiC interface, are clear evidence of this (Fig. 1b). Diffractometry has shown that the α (Al) and β (Si) phases as well as silicon carbide (SiC) are present in the composite, i.e. the same phases as in the basic components: AK7 alloy and added particles (Fig. 2). Surface X-ray microanalysis reveals distinct boundaries between SiC particles and the matrix. The Al-rich α phase grains are clearly visible, as well as (α + β ) eutectic where the Si content is higher (Fig. 3). There can be also distinguished microregions enriched with Mn, Fe and Mg, characteristic for the used cast alloy (Fig. 3 and 4). Linear microanalysis performed across the SiC particle-metal matrix interface reveals the increased Mg content in the matrix layer several micrometers wide and adherent to the SiC particles (Fig. 4). The increased concentration of Mg in the bonding zone confirm the advantageous influence of applying magnesium master alloy on the melt surface during mixing the components. This allows for good wettability of particles by the liquid metal matrix and enables easy distribution of SiC particles in the liquid alloy while mixing.