Mg-Gd-Zn-Zr合金中的LPSO结构和时效相

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

为了研究Mg-Gd-Zn-Zr合金中的长周期堆垛有序(LPSO)结构的形成及其演化,对铸态、固溶态和时效态Mg_(96.32)Gd_(2.50)Zn_(1.00)Zr_(0.18)合金的显微组织进行了观察.OM,SEM和TEM观察表明,合金铸态组织由α-Mg固溶体、晶内层片状14H-LPSO结构和晶界处树枝状α-Mg+β-(Mg,Zn)_3Gd共晶相组成;500℃/35 h固溶处理后,晶界处发生β→X的固态相变,层片状X相也具有14H-LPSO结构;再经200℃/128 h峰时效处理后,晶内析出椭球状β′相和片状β_1相与14H-LPSO结构共存.室温拉伸实验和Vickers硬度测试表明,500℃/35 h+200℃/128 h处理改善了合金的力学性能,抗拉强度为290.7 MPa,屈服强度为162.5 MPa,Vickers硬度为108.0 HV,延伸率为10.4%.力学性能的改善与晶界、晶内的14H-LPSO结构及时效相等复合强韧化作用有关.

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.