Multimodal Microstructure And Deformation Behavior Of Mg-Gd-Y-(Zn)-Zr Alloys

Magnesium alloys possess great potential in the fields of automobile,aerospace and electronic information due to the advantages of low density,high specific strength,good anti-damping performance and easy to recycle.However,low absolute strength and poor plasticity are the main blocks for Mg alloy to be used widely.In this paper,Mg-Gd-Y-(Zn)-Zr alloys with multimodal microstructures were fabricated by adjusting the composition of alloys and extrusion parameters.The influence of the multimodal microstructure on the plastic deformation and fracture behavior of the alloys was analyzed,in order to investigate the strengthening and toughening mechanisms of the Mg-Gd-Y-(Zn)-Zr alloys.The homogenized Mg-Gd-Y-Zr alloy with a typical bimodal microstructure was extruded under varied extrusion ratio.Large deformation grains and fine recrystallized grains are alternately distributed in the alloy which forms a heterogeneous layered structure.As the extrusion ratio increases,the fraction of recrystallized grains and the aspect ratio of the deformed grains increases gradually,however,the grain size of recrystallized grains remains substantially unchanged.A variety of strengthening mechanisms work together,the strength of the alloy increases first and then decreases as the extrusion ratio increases,and the elongation increases monotonically.The relationship among extrusion process,microstructure and mechanical properties was investigated.The strain distribution and evolution process of alloys were revealed by digital image correlation technology(DIC).The deformed grains with strong basal texture mainly deform through cylindrical slip,which is easy to yield strain concentration during plastic deformation.However,the main deformation mechanism of fine recrystallized grains with random orientation is dominated by basal slip.Therefore,the grains deformed more uniformly.The deformed grains and recrystallized grains are alternately distributed,which can effectively alleviate strain localization and improve the ability to deform homogeneously.The fracture behavior of the alloy was studied by tomography(CT).Additionally,the heterogeneous layered structure with the deformed grains and recrystallized grains alleviated the micro-crack propagation to some extent and prevent the premature fracture of the alloy.Long-period ordered stack(LPSO)phase are able to form in Mg-Gd-Y-Zn-Zr alloy through the addition of Zn.Therefore,the extruded Mg-Gd-Y-Zn-Zr alloy forms the multimodal microstructure composed of a bulk LPSO phase,fine recrystallized grains and deformed grains with a large amount of lamellar LPSO phase/?? phase precipitated therein.The LPSO phase/?? phase effectively inhibits dynamic recrystallization so that it reduces the amount of dynamic precipitates and refines the recrystallized grain size,which effectively improves the strength of the alloy and maintains a high elongation,showing excellent overall mechanical properties.Strain concentration occurs at the interface between the LPSO phase and the magnesium matrix easily,however,the surrounding recrystallized grain region can effectively release the stress.In addition,the elastic modulus of the LPSO phase is relatively high and the transverse strain between the deformed grains is difficult to coordinate,which results in the initiation of microcracks at the interface.The region with recrystallized grains hinders crack propagation effectively and improves the elongation of the alloy.Obvious age hardening appears in the extruded Mg-Gd-Y-(Zn)-Zr alloy at 200 °C.In the peak aging state,a large amount of metastable ?? phase precipitates in the alloy,which significantly increases the strength of the alloy while the elongation decreases slightly.The ?? phase effectively hinders the dislocation slip on the basal plane,and the ability of the recrystallized grain region to coordinate deformation is reduced,resulting in strain localization.The layered structure composed of deformed grains and recrystallized grains relieves strain localization effectively and improves the ductility of aged alloys.