Effect of microstructure and heat treatment of hypereutectic Al-Si alloys obtained by Rapid Solidification

Eutectic AA-4032 alloy is widely used in internal combustion engine pistons due to its good combination of properties at intermediate temperatures. An important improvement in the properties could be obtained by increasing the Si fraction in Rapidly Solidified (SR) alloys and subsequently compacted, thus avoiding the formation of large Si plates characteristic of hypereutectic alloys. However, although there are several studies on its mechanical behavior, the study of its behavior against corrosion is still incomplete. In this work, two hypereutectic alloys were studied, with 20%wt and 30%wt of Si, obtained by RS and compaction by Hot Extrusion, in comparison with the eutectic AA-4032 alloy (12%m of Si). Subsequently, Solubilization heat treatments (1h-530 deg C) and artificial aging (170 deg C-10hs) were carried out. Microstructural characterization was carried out using X-ray Diffraction and Scanning Electron Microscopy (SEM) and Energy Dispersive X-Ray Spectroscopy (EDX). The behavior against corrosion was analyzed by potentiodynamic polarization tests in 0.1M and 1M NaCl solution at room temperature and neutral pH. The surfaces of the tested specimens were morphologically characterized by SEM and EDX. The influence of the Si content in the alloys and its distribution in the microstructure on the shape of the polarization curves and their characteristic parameters is discussed. Microstructurally, it was observed that the SR process prevented the formation of the primary Si plates, while the heat treatments promoted, among other things, the spheroidization of intermetallic particles. On the other hand, the corrosion tests showed that: the pitting potential of the alloys turns out to be independent of the Si content, the solubilized samples presented the lowest passivation current and all cases the preferential dissolution of aluminum occurs. (author)