The structure of the zone of joint between the reinforcing particle and the metal matrix in AK64/graphite composite
Marian Mitko Politechnika Częstochowska, Katedra Odlewnictwa, al. Armii Krajowej 19, 42-200 Częstochowa
Annals 3 No. 6, 2003 pages 142-147
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abstract The paper presents the results of structural examination of AK64 alloy/graphite composite. The investigated material has been obtained by mixing AK64 cast alloy and synthetic graphite particles in the solid-liquid state and subsequent squeeze casting of the mixture. Examinations have been carried out by means of the optical and the electron microscopy, and X-ray microanalysis. The eutectic in AK64 alloy contains both globular precipitates of α phase and graphite particles (Fig. 1). Fractures of composite samples indicate that cracks propagate both across the graphite particles (Fig. 2a) and along the interface between the particle and the matrix (Fig. 2b). Diffraction pattern analysis (Fig. 3) has revealed the presence of phases and structural components which had been introduced with the matrix alloy and the reinforcing particles only. Specimens obtained by means of the thin foil technique have exhibited neither the presence of transition phases nor the lack of continuity between graphite particles and metal matrix (Fig. 4) This has been also confirmed by X-ray examinations (Figs. 5, 6, and 7). While mixing metal and graphite, the solidifying α phase coagulates and the alloy temperature drops to about 610°C. As the amount of the solidified Al-rich phase (α phase) increases, the chemical composition of the liquid phase changes and its viscosity grows. This advantageously affects the wetting of graphite by the liquid phase, which allows for distribution of graphite within the liquid phase during the mixing process. The final crystallization takes place in the metal mold under exerted pressure. The performed examinations allow for stating the adhesive type of bound between graphite particles and metal composite matrix. Key words: aluminium alloys, composites, graphit particles, structure of the composites