Evolution Of Lpso Structure And Dynamic Precipitation Behavior In Deformed Mg-RE-Zn Alloys

Owing to outstanding mechanical properties,the Mg-RE-Zn alloys with long-period stacking ordered（LPSO）structures have become an important research subject for developing the high performance magnesium alloys.Deformation kink in the LPSO structure is one of the main deformation modes and determines the final mechanical properties.In addition,the dynamic precipitation behaviors of dynamic recrystallization（DRX）and nano-sized precipitated phase,as two important deformation modes in these Mg alloys,can increase the comprehensive mechanical properties effectively.Hence,it is necessary to study the three deformation modes systematically.Li alloying and solid-solution treatment are conducted for Mg-Gd-Zn and Mg-Y-Zn alloys before hot extrusion.The results indicate that Li alloying has a serious influence onα-Mg matrix,β-（Mg,Zn）₃Gd phase,（Mg,Zn）₂₄Y₅ phase,18R and 14H.Specifically,Li addition can refine the grain size of theα-Mg matrix effectively and accelerate the formation ofβ-（Mg,Zn）₃Gd and（Mg,Zn）₂₄Y₅phases significantly.However,only little Li（<1 at.%）addition can facilitate the precipitation of 18R but exceeded Li（>1 at.%）addition can inhibit the formation of 18R seriously.Though the 14H are precipitated fromα-Mg matrix in the Mg-Gd-Zn and Mg-Y-Zn alloys,the transformation fromβ-（Mg,Zn）₃Gd phase to 14H is limited owing to the addition of Li.Li addition make the netβ-（Mg,Zn）₃Gd phase change to particle.Moreover,the formation behavior of 14H is changed from the direct transformation via 18R to the precipitation from supersaturatedα-Mg matrix due to Li addition.The yield stress and elongation of the deformed Mg-Gd-Zn-Li and Mg-Y-Zn-Li alloys can reach to 300 MPa and 15%,which is 2 and 4.5 times larger than that of corresponding as-cast alloys.The main strength mechanism include the refined eutectic phases,deformation kink in LPSO,fine DRX grains and refined LPSO andγ’in DRX grains.Moreover,the fluctuations inα-Mg/18R interface are formed due to the incompatibilities withα-Mg matrix,which induces the nucleation and growth of discontinuous DRX grains.14H can inhibit the development of DRX grains at the beginning of the hot extrusion,but the deformation kink in 14H can stimulate the nucleation of continuous DRX grains.In addition,partial 14H are dissolved under the extrusion pressure,which leads to abundant Y and Zn elements are enriched inα-Mg matrix.As a results,plentiful fine LPSO structures andγ’phases are precipitated in DRX grains.A new high performance Mg₈₉Y₄Zn₂Li₅ alloy is prepared by adjusting the extrusion ratio and speed.The ultimate compress tensile,yield stress and elongation reach to 632 MPa,410 MPa and 20.2%,which is due to the fine 18R and DRX grains,refined 14H in DRX grains,large linear density of high-angle grain boundaries and weaken texture.In this work,the Li alloying,deformation of LPSO structures and dynamic precipitation behavior of Mg-Gd-Zn and Mg-Y-Zn alloys are systematically explored.The results reveal that the deformation kink and refining of LPSO phases,fine DRX grains as well as refined LPSO phases in the DRX grains can effectively hinder the dislocation movement,thus improving the strength and ductility.