Deformation Behavior And Microstructure Evolution Of Mg-Gd-Y-Zn-Zr Alloy By Rotating Reverse Extrusion

With the rapid development of the aerospace and military industries,shell components,as one of the main components of major national defense equipment,are increasing in demand for their integration and lightweightness.In order to obtain higher comprehensive mechanical properties,people have conducted a lot of researches on the molding process of shell components.As a new type of large plastic deformation(SPD)process,Rotary Back Extrusion(RBE)generates large shear stress through one deformation,thereby achieving grain refinement and improving mechanical properties.It is used in the preparation of high-performance shells.The component has great potential for development.In this paper,the microstructure and mechanical properties of Mg-9Gd-3.5Y-2Zn-0.35Zr alloy under different heat treatment conditions and RBE deformation parameters(temperature and number of turns)were characterized by OM,SEM,TEM and Vickers hardness tester.The optimal heat treatment system was determined,and the influence of deformation parameters on the microstructure and mechanical properties of the alloy was studied.The experimental data and theoretical support will be provided for the preparation of high performance shell parts.The main conclusions are as follows:The as-cast Mg-Gd-Y-Zn-Zr alloy mainly contains Mg₅(Gd,Y,Zn)and LPSO phases distributed in a network at the grain boundaries.Due to the uneven size of the second phase,the mechanical properties of the as-cast alloy are poor.After homogenization treatment at520?×15 h,the fracture mechanism of the alloy changes from brittleness to plasticity,and the mechanical properties are greatly improved.Its tensile strength reaches 213.5 MPa and yield strength reaches 154.5 MPa.The RBE process can effectively refine the alloy grains and improve the uniformity of the structure.As the number of rotations increases or the deformation temperature decreases,the average grain size of the alloy gradually decreases.The DRX score is mainly affected by the deformation temperature and the number of rotations.With the increase of the number of rotations and the deformation temperature,the uniformity of the tissue gradually improves,and the number of rotations has a greater impact on DRX.At the same temperature,as the number of rotations increases,the cumulative strain increases,the intergranular bulk LPSO phase fragmentation and dispersion increased.When deforming at 450?,a large number of grain phases are precipitated between the crystals.With the increase of the deformation temperature and the number of rotations,the precipitation of the phases between the crystals decreases.During the RBE process,the alloy structure is mainly evolved through continuous dynamic recrystallization(CDRX)and discontinuous dynamic recrystallization(DDRX)mechanisms.The dominant mechanism of microstructure change is different due to the different strain variables at different locations of alloy samples.When the DRX score is greater than 90%,the grain refinement is dominated by the DDRX mechanism.As the deformation temperature and the number of rotations increase,the texture strength of the base surface gradually weakens.The shear strain introduced by the RBE deformation causes the base surface texture to be deflected to varying degrees.RBE has different effects on the microhardness of the inner and outer walls of the alloy sample.The hardness value gradually decreases from the inside to the outside.And as the number of rotations and deformation temperature increase,the hardness value at the same position decreases.