A Study On The Recrystallization And Toughening Mechanisms Of Wrought Magnesium-Rare Earth Alloy By High-Energy Electropulse Treatment

In this paper,based on the problem that the commercial application of wrought Mg alloys is largely restricted by their poor formability at room temperature,a commercial AZ31 and Mg-Gd rare-earth(RE)magnesium alloys were treated by“cold rolling+electropulse treatment(EPT)”process,aiming to clarify the respective effect and the combined effect of electropulse and rare earth element on the recrystallization mechanism and the mechanical properties of magnesium alloys to improve the forming ability and mechanical properties of magnesium alloys.The main content includes the following three aspects:Microstructure evolutions of the cold rolled AZ31 and ZG11 treated by EPT and conventional heat treatment(HT)show that EPT substantially accelerates the static recrystallization(SRX)rate and shortens the duration of completely SRX,EPT also makes it possible to nucleate at extension twin(ET)boundaries.And compared with AZ31,ZG11treated by EPT can get more randomly distributed texture due to the increased number of ETs and shear bands,which will act as nucleation sites for off-basal oriented nuclei with the addition of Gd element.At the same time,the grain boundary mobility is influenced by grain boundary segregation of Gd atoms,resulting in preferential growth of off-basal oriented grains during grain growth.In addition,EPT effectively makes up for the deficiency that the addition of Gd cannot completely inhibit the growth advantage of basal oriented grains and achieve significant modification of SRX texture for Mg alloys.“Cold-rolling+intermediate EPT”is a promising processing scheme to improve the mechanical properties of the AZ31.With pass reduction increasing,the shear bands become denser and distribute more uniformly in cold rolled AZ31,grain size is refined from the 5.4μm to 0.9μm and the texture changes from typical basal texture to a weakened“TD-texture”after EPT.And the fine grains with“TD-texture”result in higher strength and reduced mechanical anisotropy of AZ31.However,cold rolled AZ31 with lower pass reduction is easy to form a“sandwich”grain structure due to the introduction of sparse shear bands.EPT with different current densities and HT were carried out on cold rolled G1 alloy.As the current density increases,i.e.,when the current density is 152.5 A/mm~2,a good balance between strength and ductility is obtained:The ultimate tensile strength(UTS)and elongation-to-failure(FE)are 198.7 MPa and 20.2%,respectively,which are about 30 MPa higher than the UTS of initial G1 alloy(167.9 MPa and 20.7%)and the FE is well preserved.The FE of cold rolled G1 treated by EPT is higher than treated by HT at 440℃,mainly because the EPT is beneficial to obtain uniform grain structure,weakened texture,and the select heating effect of EPT can heal the micro crack generated during cold rolling;however,the UTS of EPT 310 and EPT 350 is larger due to grain refinement and texture enhancement,and the formation of the“TD-texture”with double peaks is related to the different effects of different temperatures on the nucleation and grain growth process during EPT.