Abstract
Thermal stability of Al-Si alloys and their composites is an important property for their application in engine components. In this work the thermal expansion and the physical Coefficient of Thermal Expansion (CTE) of AlSi1.1, -1.7, -7, -12 alloys and AlSi7/ Al₂O₃/20s short fibre composite material are presented. The higher the Si content the more decreases the expansion of the alloys. Random planar alumina fibres reduce the in-plane thermal expansion, but the transverse expansion is significantly increased with respect to the matrix. The precipitation kinetics of Si is studied by means of Differential Scanning Calorimetry (DSC). Precipitation peaks correlate with an increase in the CTE of the alloys and the composite.
Al-Si alloys with 7 wt.% and higher content of Si present a percolating Si-network in the as-cast condition. The disintegration of this network during solutionizing results in a decrease of the hardness of the alloy. On the other hand, the hardness of the composite is increased during solution treatment via the formation of Si bridges between the short fibres, which strengthen the Si/ Al₂O₃-network. The reinforcing networks’ architecture is revealed by deep etching. It causes reversible viscous flow of the Al-matrix at temperatures > 300°C.

Key words
thermal expansion, Si precipitation, differential scanning calorimetry, hardness, spheroidization, Al-Si alloys, metal matrix composites

Full text (1337 KB)