Effect of solid fraction on microstructures and mechanical properties of a Mg-Al-La-Ca alloy processed by rheocasting

Morphological evolution of microstructures and tensile properties in Mg-Al alloy containing La and Ca (Mg-6Al-3La-1Ca) processed by rheocasting with different solid fractions (fs) were investigated at this paper. An apparatus for semisolid metal alloy processing was used to melt and to obtain the rheocast material. Isothermal mechanical stirring experiments were carried out with fs = 0.30, 0.49 and 0.61 at 950 rpm for 10 min. A conventional casting (no stirring) was also obtained for comparisons. Results showed that microstructure in conventional casting is composed by α-Mg dendritic matrix and Al₁₁La₃, (Al,Mg)₂Ca and Mg₂Ca compounds. Non-dendritic and globular microstructures were observed in the rheocasting. For fs = 0.30 the acicular Al₁₁La₃ phase precipitated at globule boundaries, while for the fs = 0.49 and fs = 0.61 the acicular phase precipitated both inside and at globule boundaries. The ideal rheocasting condition was achieved fs = 0.30 due to the improvement of 76% for the ultimate tensile strength (UTS) and 80% in the strain to fracture when compared to the conventional casting. Fracture surface analysis pointed to a mixed ductile-brittle fracture mode containing dimples and quasi-cleavage facets for the lowest solid fraction (fs = 0.3), and a fully brittle fracture mode consisting exclusively of cleavage planes for the highest solid fraction (fs = 0.61).