Correlation between primary and secondary microstructure and properties of AlMg5-SiCP composites
* Katarzyna N. Braszczyńska, ** Andrzej Zyska, * Andrzej Bochenek * Politechnika Częstochowska, Instytut Inżynierii Materiałowej, al. Armii Krajowej 19, 42-200 Częstochowa ** Politechnika Częstochowska, Katedra Odlewnictwa, al. Armii Krajowej 19, 42-200 Częstochowa
Annals 2 No. 4, 2002 pages 157-162
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abstract The results of microstructure and mechanical properties investigations of AlMg5-20wt%SiCp composite have been presented. The particles have been artificially covered with SiO2 layer by oxtidation to protect SiC against reactions with molten aluminium alloy. The composite has been obtained by casting method involving mechanical mixing of liquid metal with the introduced particles and subsequent casting in metal moulds. The microstucture of the cast composite is characterised by an uniform distribution of SiC particles within the entire volume of matrix alloy (Fig. 2a). The strong and mostly consistent interfaces between SiC and matrix bave been originated as a result of reaction between the SiO2 layer and AlMg5 alloy. TEM examinations allowed to determine the occuring of MgAl2O4 spinel at the components interfaces (Fig. 4). The above reactions also caused the negative result of the introduction of the silicon to the matrix alloy and formation of a large area of pseudoeutectic of α+Mg2Si (Figs 2b and 3). The performed strudies on the mechanical properties have made it possible to determine the increase of 40% in the hardness and yield strength of composite as compared with unreinforced matrix alloy. The tensile strength of composite was on the level of the value obtained for matrix alloy. The composite cracking process preceeded mostly through silicon carbide particles, as a result of high strength bond between components. Hovewer, the analyses of fracture surfaces revealed also a brittle fracture process through the area of α+Mg2Si eutectices. Those processes are documented by scanning micrographs taken for two mutually corresponding fracture surfaces (Figs 5, 6). The performed heat treatment process of the composite enables to change the shape of the eutectices and obtain the decrease in the volume fraction of α+Mg2Si areas due to two processes: precipitates hardenning and spheroidisation of the Mg2Si intermetallic phase. The changes in the mictrostructre of composite due to heat treatment is shown in Figure 7. For this reason the tensile strength of composite increased about 20%. The fractographical examinations of fracture surfaces revealed a transition from the brittle fracture of the as-cast composite to the predominantly plastic fracture of composite after heat treatment (Fig. 8). Neverthelees, analyses of fracture surfaces of composite indicated that interfaces between components remained intact after heat treatment process. The cracking process preceeded also through the silicon carbide particles in the same way as in the as-cast material (Fig. 9).