Microstructure Modification On Second Phases And Strength-ductility Optimization In Wrought Mg-Al-Ca-Mn Alloys

In the present work,the microstructure modification of the primary phases and dynamic precipitates were realized by composition design in Mg-Al-Ca-Mn alloys.Ultra-high tensile yield strength of over 400 MPa was achieved by conventional casting and extrusion process.The effect of primary phases and dynamic precipitates on mechanical properties was systematically investigated.The ductility of the as-extruded Mg-Al-Ca-Mn was significantly improved by homogenization treatment.The strengthening and fracture mechanism of as-extruded Mg-Al-Ca-Mn alloys were clarified.The category and morphology of primary Laves phases depend on Ca/Al ratio.Both thermodynamic calculation and experimental results show that Ca/Al ratio affects the solidification path and decides the category of Laves phases in the as-cast Mg-Al-Ca-Mn alloys.Among the Mg-Al-Ca-Mn alloys with a total Ca and Al content of 6 wt.%,the primary Laves phase varies from coarse devoiced eutectic C36-(Mg,Al)₂Ca phase to fine lamellar eutectic C14-Mg₂Ca phase with the Ca/Al mass ratio increasing from 0.25 to 1.30.These primary Laves phases was fragmented into particles after extrusion,forming strips along the extrusion direction.The as-extruded Mg-Al-Ca-Mn alloys show a bimodal grain structure with fine equiaxed dynamic recrystallized grains near the strips and coarse elongated unrecrystallized grains away from the strips.The size of C14 phase particles is much smaller than that of C36 phase,which is more effective on grain boundary pinning.The precipitates in the as-extruded Mg-Al-Ca-Mn alloys with a higher Ca/Al ratio exhibit a more refined structure.With an increasing Ca/Al ratio,the yield strength of Mg-Al-Ca-Mn alloys increase significantly while the ductility decrease dramatically.The as-extruded Mg-2.7Al-3.5Ca wt.%alloy exhibits an ultra-high yield strength of 420 MPa and an ultimate strength of 438 MPa with an elongation to failure of 2.5%.The predominate strengthening mechanism is grain boundary strengthening and precipitate strengthening.Metastable C14 phase is the main strengthening dynamic precipitate in the as-extruded Mg-Al-Ca-Mn alloys.The size and distribution of Mg₂Ca precipitate is affected by Mn and Al content.The trace addition of 0.1 wt.%Mn into Mg-5.5Al-3Ca wt.%alloy can greatly refine the microstructure of C14 precipitate,thus increase the strength of the as-extruded alloy.Further increase of Mn content has little effect on the microstructure of C14 precipitate.In the Mg-(5-7)Al-3Ca-0.3Mn wt.%alloys,the increasing of Al content leads to the decrease in the number density of C14precipitates,which causes the decrease of yield strength in the as-extruded alloys.The Mg-5.1Al-3.1Ca-0.3Mn wt.%alloy exhibits an ultra-high yield strength of 420MPa and an ultimate strength of 451 MPa,with an elongation to failure of 4.1%.The low ductility of Mg-Al-Ca-Mn alloys is ascribed to the brittle primary Laves phases which act as microcrack initiators.The ductility of the as-extruded alloy can be improved by modifying the morphology and distribution of primary Laves phase particles.After homogenized at 400 ? and 500 ?,the lamellar eutectic C14transforms into a spherical morphology.The size and the interspacing of the C14particles increase while the fraction of C14 phase decreases with increasing homogenization temperature and time.The 500 ?/20 h homogenized and as-extruded Mg-3.0Al-2.7Ca-0.4Mn wt.%alloy exhibits a yield strength of 322 MPa and an elongation to failure of 10.2%.The ductility of the as-extruded alloy is significantly improved by homogenization.Based on the facture behavior of the as-extruded Mg-Al-Ca-Mn alloys,a ductility model for alloys containing cracking forming particles was developed,by which the relationship between the microstructure of the C14 particles in the as-extruded Mg-3.0Al-2.7Ca-0.4Mn wt.%alloy and the ductility is well quantitatively described.This model is as well applicable to other wrought Mg alloys.Finally,it is noteworthy that the present work reveals the effect of alloying composition on the microstructure of second phases in as-extruded Mg-Al-Ca-Mn alloys.Fruthermore,the strengthening and fracture mechanisms of the as-extruded alloys are revealed.Therefore,this work is promising to provide theoretical direction for phase modification and mechanical property improvement in wrought Mg alloys.