LPSO STRUCTURE AND AGING PHASES IN Mg--Gd--Zn--Zr ALLOY

Abstract

At present,long period stacking ordered(LPSO) structures in Mg-RE-Zn(RE=Y, Dy,Ho,Er,Gd,Tb,Tm) alloys have been focused on.According to some reports,Mg-Gd-Zn alloys are classified as typeⅡ,i.e.,there are no LPSO structures in as-cast alloys,but LPSO structures appear after heat treatment.To further study the formation of LPSO structure in Mg-Gd-Zn(Zr) alloys,a Mg_(96.32)Gd_(2.50)Zn_(1.00)Zr_(0.18) alloy was prepared by ingot metallurgy(I/M) in this work.Based on OM,SEM and TEM observations,the as-cast microstructure of Mg-Gd-Zn-Zr alloy consists ofα-Mg solid solution,lamellar 14H-type LPSO structure withinα-Mg grains and dendritic eutectic structure at grain boundaries,in which the eutectic phase is theβ-phase((Mg,Zn)_3Gd)),thus,Mg-Gd-Zn(Zr) alloys should be attributed to typeⅠ,i.e.,there are LPSO structures in as-cast alloys.During solid solution treatment at 500℃for 35 h,a solid transformation,dendriticβ-phase with fcc structure→lamellar X-phase with 14H structure,was observed at grain boundaries.During subsequent peak-aging treatment at 200℃for 128 h,ellipsoidalβ' and rhombusβ_1 phases precipitated withinα-Mg grains. Tensile tests at room temperature and Vickers hardness tests show that the alloy solution-treated and aging-treated has higher mechanical properties,i.e.,σ_b=290.7 MPa,σ_s=162.5 MPa,δ=10.35%and 108.0 HV.The improvement of mechanical properties are attributed to the composite strengthening- and-toughening effect of the 14H-type LPSO structures,and theβ' andβ_1 aging phases.