The benefits of Sr additions to Al–Si alloys to modify the eutectic are often impaired by the development of porosity, sometimes to the degree that benefits are negated. Experimental reports are reviewed in this paper, suggesting an explanation in terms of the oxide population in the melt. The unmodified silicon particles are nucleated by AlP, which has in turn nucleated on oxide bifilms. The oxide bifilms, which are essentially cracks, are straightened by the crystalline growth of Si particles, leading to increased crack size and consequently reduced mechanical properties. The addition of Sr improves properties by suppressing the formation of Si on bifilms and thereby preventing the straightening of the pre-existing cracks. Si is now forced to precipitate at a lower temperature as a coral-like eutectic. Unfortunately, the bifilms are now freed (the primary Si particles no longer exist to grow around and sequester the bifilms), remaining in suspension in the liquid metal, allowing them to act to block interdendritic flow and aid the initiation of the formation of pores, countering the benefits of the improved structure.
The yield strength-elongation relationship in cast Al-7 pct Si-Mg castings has been investigated by analyzing 18 datasets from the literature on premium quality castings. The data representing the elongation for a given yield strength have been found to fall within an envelope, the top limit of which represents the ductility potential of Al-7 pct Si-Mg alloy castings. Analysis of maximum elongation data indicated that secondary arm spacing has no effect on elongation. It has also been determined that the quality index of Drouzy et al. does not provide a particularly accurate representation of the trends in maximum ductility data. A new quality index based on those proposed previously by Ca´ceres and Din et al. is proposed. Moreover, using the fracture toughness equation by Hahn and Rosenfield, intrinsic plane strain fracture toughness and intrinsic elongation values have been found to be correlated, indicating the internal consistency of the approach taken in this study. The implications of these results are discussed in the article.
A comparison was made between the tensile properties of an Al-5 pct Si-3 pct Cu-1 pct Fe-0.3 pct Mg (319-type) alloy in the as-cast condition. The alloy was either unmodified or modified with Mn or Sr. The additions of 0.6 wt pct Mn, or 0.9 wt pct Mn, or 300 ppm Sr were made. All modifying additions increased the average and decreased the scatter in tensile strength, and increased elongation, with the Sr addition being most effective among the three. Fractographic examinations showed that the b phase, the polyhedral a phase, and porosity appear to be the main defects responsible for failure. The distribution of tensile strength has been quantitatively assessed using both two-and three-parameter Weibull distribution functions. In the twoparameter Weibull analysis, the high Weibull modulus (46) achieved by Sr modification is due to its binary effect on eutectic silicon and b platelets. Reservations about potentially misleading implications of the two-parameter approach are explored. The three-parameter Weibull analysis provides new information. In particular, minimum values of strength below which the material is extremely unlikely to fail are found. These values again show the clear benefit of Sr addition. The results are explained by the effects of modifying agents on the microstructure.
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