Modification of aluminium-silicon alloys by rare-earth additions

Abstract

Cast aluminium-silicon (Al-Si) alloys are used extensively in various industries due to their advantageous properties such as high strength-to-weight ratio, good corrosion resistance and high fluidity which allows for defect-free complex castings. Under normal casting conditions the microstructure is composed of silicon needles in an aluminium matrix. These provide propagation planes for defects and therefore deteriorate the mechanical properties. By adding certain elements, usually strontium (Sr), the Si needles change to fibres, however this is also known to increase porosity in castings. The mechanism that causes the change from needles to fibres has been extensively debated and a number of theories can be found in the literature, revolving around both the nucleation and growth stages of eutectic Si. In this thesis high purity materials were used to prepare hypoeutectic unmodified and Sr-modified Al-Si alloys to which cerium (Ce) or yttrium (Y) were added and differences between these alloys in the solidification progression and microstructure were investigated. The addition of 1% Ce or Y to unmodified Al-Si produced a partially modified eutectic Si, whilst full modification was retained when these were added to Sr-modified Al-Si. These additions also resulted in a significant decrease in the eutectic growth temperatures and in the formation of Al2Si2Ce or Al2Si2Y intermetallic phases. It is suggested that similar to the Al2Si2Sr in Sr-modified Al-Si these intermetallic phases nucleate on aluminium phosphide (AlP) and thus do not allow for the nucleation of eutectic silicon on this phase. Three dimensional atom probe tomography (3D APT) of Y-partially-modified Al-Si showed a preferential segregation of yttrium within the eutectic Si. By means of optical microscopy and high resolution x-ray computed tomography (XCT), it was also demonstrated that the Sr modification significantly increases the porosity in cast Al-Si alloys which is reduced following the rare-earth additions