The effect of the flake to fiber transition in silicon morphology on the tensile properties of Al-Si eutectic alloys

The combined and separate effects of microstructural scale and silicon phase morphology on the mechanical properties of Al-Si eutectic alloys are investigated here. The Bridgman-type gradient-zone directional solidification method is employed to produce as-cast structures characteristic of the full range of practical (i.e. casting) growth velocities, and the corresponding mechanical properties are characterized by uniaxial tension testing. The results are analyzed in light of previously reported microstructural changes associated with the flake to fiber or "quench modification" transition [1]. Both tensile strength and elongation were found to increase with solidification rate. Application of the Nan-Clarke 121 micromechanical analysis to the Al-Si composite structure, incorporating the strengthening effects of reinforcement-induced dislocations, suggests that the decreasing microstructural scale alone is sufficient to account for the increase in tensile strength with solidification rate. However, the flake to fiber transition was found to have a particular relevance with regard to the fracture behavior of the alloy, increasing tensile elongation and decreasing the overall variability of tensile properties. A maximum in elongation was observed at approximately 600 mu m/s, corresponding to the upper threshold of the flake to fiber transition associated with complete disappearance of the flake morphology and dominance of the fibrous structure. These results emphasize the importance of understanding and controlling the flake to fiber transition that occurs with increasing solidification rate in Al-Si eutectic alloys. (C) 2010 Elsevier B.V. All rights reserved.