K. Sukumaran et al., THE EFFECTS OF MAGNESIUM ADDITIONS ON THE STRUCTURE AND PROPERTIES OFAL-7 SI-10 SICP COMPOSITES, Journal of Materials Science, 30(6), 1995, pp. 1469-1472
The additions of magnesium to an aluminium alloy matrix, which contain
s insufficient magnesium, was found to be essential during the synthes
is of composites by the stir-casting technique. Magnesium promotes int
erfacial wetting between the dispersoid surface and the matrix. Disper
sion of SiCp in Al-7 Si-0.3 Mg (356) alloy matrix without agglomeratio
n and rejection was not possible. Hence, the addition of up to 3 wt %
Mg was made to the alloy matrix during the dispersion of 10 wt % SiCp
(34 mu m), and the microstructure and mechanical properties of the com
posites were investigated with a view to optimize the magnesium conten
t. With a magnesium content less than 1 wt % in the matrix, the SiCp p
articles were essentially in agglomerated form. The highest UTS of 280
-300 MPa was obtained with 1 wt % Mg content and SiCp was uniformly di
stributed in the matrix. A higher magnesium content (> 1.0 wt %) did n
ot further improve the uniformity in the dispersion of SiCp but the ul
timate tensile strength properties deteriorated. This decrease in stre
ngth was attributed to the observed coarseness of the Mg2Si phase, the
precipitation of Mg5Al8 phase and the presence of a higher amount of
porosity in the composites in the heat-treated condition. The aspect r
atio (length/width) of precipitates changed from 1-3 for 1% Mg to 3-9
for 3.2% Mg in the matrix. Corresponding values for per cent porosity
were 2% and 6%, respectively.