| Effect of Si and Ca additions on the microstructural refinement of AM60 based Mg alloys were investigated. The microstructural characterization was performed by utilizing optical microscope, energy-dispersive spectrometer(EDS), scanning electron microscope(SEM), transmission electron microscope(TEM), differential scanning calorymeter(DSC) and Image analyser. As-cast microstructure of the alloys investigated consisted of Mg_(17)Al_(12). Mg₂Si and α-Mg matrix. Area fraction of Mg_(17)Al_(12) precipitate increased with Al content, the total amount being kept in the range of 4-7%. Continuous or discontinuous precipitation of Mg_(17)Al_(12), was dependant on the amount of Al; i.e., no continuous-type precipitation in alloys containing<2wt.% Al and no discontinuoustype precipitation in alloys containing <4wt.% Al. Two types of Mg₂Si were observed; i.e., chinese script type in alloys containing < 2wt.% Si and polygon type in alloys containing >2wt.% Si. The morphology of Mg₂Si was also depentent on the Al amount; e.g. only chinese script type was observed in alloys containing less than 2wt.% of A1 even though enough Si amount was present for polygon type Mg₂Si. The total amount of Mg₂Si in as-cast alloys was approximately 7%, and kept constant even after heat treatment. The grain size of Si containing alloys was refined by the presence of two types of Mg₂Si: e.g. 237㎛ in AM60 and 125㎛ in AM60-1Si-0.2Ca after aging. Addition of Ca refined the distribution of Mg₂Si by providing heterogeneous nucleation sites for Mg₂Si. |
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