Research On Microstructure Evolution And Mechanical Properties Of Cast Mg-Y-Nd-Al Alloys

Cast Mg-RE（Rare Earth Elements）alloys exhibit high strength and good heat resistance,offering broad application prospects in fields such as aerospace and astronautics.However,with the rapid development of aerospace and astronautics technology,the demand for higher strength,higher heat-resistant lightweight materials has grown.Traditional magnesium alloy materials with a single strengthening mechanism are no longer sufficient,and multi-mechanism composite strengthening has become an effective strategy for enhancing strength and heat resistance.In this study,we added Al element to the Mg-RE alloys,generating in-situ Al₂RE phase to refine grains.By controlling the contents of Y and Nd elements,we investigated the formation rules of the second phase during the solidification process of as-cast alloys and clarified the grain refinement mechanism and solidification microstructure formation rules.Additionally,we studied the evolution of the second phase and precipitation behavior during heat treatment,systematically exploring the effects of the second phase and precipitated phases on mechanical properties.Therefore,the composite strengthening mechanism of the alloys was revealed.The characterizations of the as-cast microstructures of Mg-x Y-（7-x）Nd-2Al（x=0,1,2,3,4,5,6,7 wt.%）alloys show that the contents of Y and Nd affect the grain sizes and secondary phases of the alloys.The mean grain size first decreases and then increases with increasing Y content.The alloy with 3%Y and 4%Nd（WEA342 alloy）has the smallest grain size of 49±4μm.The microstructure of the as-cast alloys is composed ofα-Mg matrix,granular phase inside the grain,acicular phase near the grain boundary and reticular phase at the grain boundary.When the content of Y≤3%and correspondingly Nd≥4%,the acicular phase is Al₁₁RE₃ phase.When the Y≥3%and Nd≤4%,the granular phase is Al₂RE phase.The reticular phase was primarily the Mg-RE phase.As the Y element increased（correspondingly Nd decreased）,the amount of granular phases first increased and then tended to remain constant.The amount of acicular phases gradually decreased,and the reticular Mg-RE phase first increased and then decreased.The undercooling for inducing theα-Mg nucleation first decreased and then increased.The WEA342 alloy has the smallest nucleation undercooling.Granular Al₂RE phases can act as heterogeneous nucleation particles ofα-Mg.With the variation of Y content,the number density of effective nucleation particles first increased and then decreased.The numerical density of Al₂RE phases was the highest in WEA342 alloy,which is 305±16pcs/mm².The results of tensile tests showed that as the Y content increases,the yield strength would not change significantly,but the ultimate tensile strength would first increase and then decrease,and the elongation constantly increase.The WEA342 alloy has the best mechanical properties,with yield strength,ultimate tensile strength,and elongation of 125 MPa,205 MPa,and 8.5%,respectively.Solution treatment of Mg-x Y-（7-x）Nd-2Al（x=0,1,2,3,4,5,6,7 wt.%）alloys was performed at 545℃.The reticular Mg-RE phases at grain boundaries were completely dissolved,while the granular Al₂RE phases were more stable and showed no significant changes during solution treatment.The acicular Al₁₁RE₃ phases were fused,broken and evolved into short rod-like Al₂RE phases.The needle-flake Al₂RE phases precipitated along（0002）_Mg planes within grains,with a crystallographic orientation relationship of[112]_Al2RE//[2（?）0]_α-Mg,（111）_Al2RE//（0002）_α-Mg.With the increase of Y content,the amount of granular Al₂RE phases first increased and then tended to remain constant,with sizes ranging from 2 to 12μm.The amount and size of short rod-like Al₂RE phases gradually decreased.The content of needle flake Al₂RE phases decreased gradually,and the size increased at first and then decreased.As the solution treatment time was prolonged,the granular Al₂RE phases would not change significantly,while the amount of the short rod-like Al₂RE phases increase gradually,and the quantity and size of the acicular Al₂RE phases would first increase and then decrease.The stable Al₂RE phases could hinder the movement of grain boundary,which results in a good thermal stability.Thus,the grain sizes did not change significantly after solution treatment.Compared with the as-cast alloys,the strength does not change significantly after solution treatment,while the elongation increases obviously.Still,WEA342 alloy has the best mechanical properties,with yield strength,ultimate tensile strength,and elongation of 128 MPa,206 MPa,and 17.5%,respectively.The analyses of the age-hardening curves and age-precipitation behaviors of Mg-x Y-（7-x）Nd-2Al（x=0,1,2,3,4,5,6,7 wt.%）alloys showed that with the increase of Y content,the age-strengthening effect would first increase and then decrease.Mg-x Y-（7-x）Nd-2Al alloys exhibited under-aging,peak-aging and over-aging stages at different aging temperatures.With the increase of age-temperatures,the peak strength would decrease,and the time to reach peak-aging hardness would reduce.The effect of the age-strengthening is closely related to the precipitation of nano-scale β′′,β′,and β₁ phases.Among the studied alloys,WEA342 alloy has the highest content of β′′phase during the peak age-stage and shows the best age-strengthening effect at 200℃and as the Y content increased from 0%to 3%,the driving force for dissolution（ΔGv）would gradually decrease.The WEA342 alloy has the lowestΔGv of-32.9 J/mol.With Y content increasing from 3%to 7%,theΔGv would gradually increase.With the increase of Y content,the Nd content decreased,the ultimate tensile strength of aged alloys increased at first and then decreased in the tensile test conducted at room temperature,200℃and 250℃,and the elongation increased continuously.The WEA342 alloy has the smallest grain size and the highest content of nano-scale precipitated phases during aging treatment.Still the mechanical properties are the best,with tensile strengths of 182 MPa,160 MPa,and 148 MPa,yield strengths of 285 MPa,249 MPa,and 214 MPa,and elongations of 7.4%,16.7%,and 21.8%at room temperature,200℃and 250℃,respectively.