The effect of Mg on the microstructure and mechanical behavior of Al-Si-Mg casting alloys
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I. INTRODUCTION
MAGNESIUM is added to Al-7 pct Si-Mg casting alloys to induce age hardening through precipitation of MgSi particles (in some wrought alloys, these precipitates have recently been identified as having compositions close to Mg5Si6.[1,2] The Mg content of the commercial alloys 356 and 357 (equivalent Australian designations are 601 and 603) ranges from 0.3 to 0.4 pct and from 0.45 to 0.7 pct, respectively. In general terms, it has been reported that a higher Mg content increases the yield stress while decreasing the ductility[3–8] and the fracture toughness.[9] Besides its major effect on the age-hardening potential, Mg depresses the eutectic temperature and makes the eutectic Si structure more heterogeneous.[10] It also seems to interfere with modification of the eutectic structure when using Sr additions, making the eutectic structure coarser and less uniform. The Mg content also affects the types and total volume fraction of Fe-bearing phases,[11,12] which are known to have a detrimental effect on the tensile properties,[13–16] especially in Be-free alloys.[17,18] The strain-hardening rate at low strains increases with the Mg content, increasing the rate of load shedding onto the Si particles,[19] and it has been suggested that this may lower the ductility of high-Mg unmodified alloys.[20] For a given Mg and Fe content, and ignoring possible deleterious effects of porosity,[21] the tensile ductility and strength of these alloys depend on the scale of the dendritic C.H. CACERES, Senior Lecturer, is with the CRC for Alloy and Solidification Technology (CAST), Department of Mining, Minerals and Materials Engineering, The University of Queensland, Brisbane, Australia 4072. C.J. DAVIDSON, Principal Research Scientist, and J.R. GRIFFITHS, Senior Principal Research Scientist, are with CSIRO Manufacturing Science and Technology, Kenmore, Australia 4069. Q.G. WANG, formerly Research Student, CRC for Alloy and Solidification Technology (CAST), Department of Mining, Minerals and Materials Engineering, The University of Queensland, is Research Associate, Metal Processing Institute, SPI, Worcester, MA 01609. Manuscript submitted July 30, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
structure,[4,22–25] the size and morphology of the Si particles,[23–27] and the solution treatment and aging condition.[27,28,29] Plastic deformation results in the cracking of Si particles[4,23,24,30] and Fe-rich intermetallics.[17,18] When the dendrite cell size is small, the cracking is concentrated in the eutectic along the grain boundaries so that fracture is intergranular.[24,31–33] On the other hand, when the cell size is large, the final fracture occurs by microcrack nucleation and growth along the dendrite cell boundaries.[4,24,32,33] Fracture can then be termed transgranular. The results presented in this work complement previous studies on the Al-7 pct Si-0.4 pct Mg alloy[18,19,24,25,33,34] aimed at isolating the effects of the dendrite arm spacing from those of particle size and morphology. The present experiments we
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