On the microstructure and mechanical properties of Mg-Al alloys
Abstract: The microstructural features and the mechanical properties of binary Mg-Al alloys have been investigated by using a gradient solidification technique. Homogeneous and nearly defect-free samples were produced under well controlled cooling conditions, through which the microstructural characteristics as well as the relationships between microstructure and mechanical properties in Mg-AI alloys were precisely defined.It was found that the eutectic structure Mg-AI alloys becomes more and more divorced when increasing the cooling rate and by decreasing the AI concentration. A concept called divorced degree is introduced in this work to quantify the divorced level of the alloys, by which other structural parameters can be also quantified, including the eutectic fraction and, solid solution aluminium concentrations in the matrix. Concerning the relation between the microstructure and mechanical properties, it is found that yield strength and hardness are mainly dependent on the structural spacings (grain sizes and dendrite arm spacing, DAS) and solid solution aluminium content in the a-phase. For higher aluminium containing alloys (larger than 10wt% AI) the rigid eutectic networks will also contribute to the strength and hardness. All the microstructural parameters contribute to the ultimate tensile strength (UTS) as well as fracture elongation of the alloys. The premium combination was found in Mg-8AI alloy, where the highest UTS and relatively high elongation can be achieved. Mathematical modeling has been performed to relate the microstructural parameters and casting conditions, such as grain size, volume fraction of different phases, solid solute AI concentration and cooling rate, to the mechanical properties of the alloys, including yield strength, hardness and fracture elongation. The distinguishing differences in the mechanical behaviour between pressure die-cast magnesium alloy components and other casting processes were also investigated. The differences were related to mould constraint in the die during the cooling sequence.One special and severe defect called segregation band which occurs in die-cast magnesium alloy components has also been investigated. A new theory to describe the segregation band formation mechanism during die casting has been proposed in this work. The tensile stresses built up in the residual liquid due to the solidification (and cooling) shrinkage of the casting, which cause a pressure drop and viscous flow of enriched liquid inside the mushy zone, is believed to be the main reason for segregation band formation.
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