Ultrahigh strength Mg-Al-Ca-Mn extrusion alloys with various aluminum contents

In this work, Mg-(5, 6, 7) Al-3Ca-0.3Mn (wt.%) alloys were fabricated by permanent mould direct chill casting followed by indirect extrusion, to achieve ultrahigh strength. The microstructure and mechanical properties of these alloys were systematically investigated. While C36-(Mg, Al)2Ca was identified to be the major eutectic phase in the as-cast alloys, and a bimodal microstructure composed of fine dynamically recrystallized (DRXed) grains and coarse unDRXed grains was observed in the as-extruded alloys. The as-extruded Mg-5Al-3Ca-0.3Mn (wt.%) alloy exhibits a yield strength of 420 MPa and ultimate tensile strength of 451 MPa with an elongation to failure of 4.1%. The ultrahigh strength is mainly due to dense homogeneously dispersed Mg₂Ca nano-precipitates, grain refinement and strong basal texture. Further Al addition substantially decreases the number density of Mg₂Ca precipitates, causing a decrease in strength. Fracture observations indicate that the low ductility of the alloys is likely caused by non-uniform distribution of the fragmented eutectic particles.