Evolution of microstructure, phases and mechanical properties in lean as-cast Mg–Al–Ca–Mn alloys under the influence of a wide range of Ca/Al ratio

In the present investigation, the microstructure and phase evolution, and their implications on the mechanical properties for different Ca/Al ratios (within ~0.06–2.21, wt.%) in lean as-cast Mg–Al–Ca–Mn alloys are experimentally studied. Further, the evolution of the second phases and the matrix phase in different alloy compositions have been estimated through thermodynamic calculations using Scheil conditions. The primary phase in all these alloys is a solid solution of hcp Mg with the other elements like Al and Ca. Apart from the fine Al<inf>8</inf>Mn<inf>5</inf> particles present in all the compositions, the major eutectic second phase is γ-Mg<inf>17</inf>Al<inf>12</inf> in the specimen with Ca/Al ratio ~0.06, C36-(Mg,Al)<inf>2</inf>Ca in the specimen with Ca/Al ratio ~0.28 and C14–Mg<inf>2</inf>Ca in the specimens with Ca/Al ratio ~1.56 and ~2.21. The area fraction of the eutectic second phases increases from ~2.1 to ~7.2% with the Ca/Al ratio, with a concomitant increase in its network connectivity as confirmed through fractal analysis. Contrarily, the lattice parameters (both a and c) and the elastic modulus (E) of the matrix phase decrease with increasing Ca/Al ratio, due to the decreasing concentration of solute atoms in the matrix. Therefore, the specimen with the minimum Ca/Al ratio (~0.06) attains maximum strength (YS ~130 MPa, UTS ~183 MPa), primarily owing to solid solution strengthening from higher solute content and precipitation strengthening from fine γ-Mg<inf>17</inf>Al<inf>12</inf> particles. The large fraction and strong networks of hard and brittle Mg<inf>2</inf>Ca phase promote cracks during tensile deformation and deteriorates the ductility (≤1.6%) and work hardening response in the alloys with higher Ca/Al ratios (≥1.56).