Research Of Microstructure,Mechanical Properties And Fracture Toughness Of Mg-15Gd-1Zn-Zr Wrought Magnesium Alloy

Environment-friendly magnesium?Mg?alloys with high specific strength,high elastic modulus and good machine processing property are widely applied in automobile,electrical and aeronautics and space industries.However,the yield strength?YS?and fracture toughness(K_IC)of the Mg alloys are still too low to be structure material.Based on a number of reports on the high-strength Mg alloys,we designed a Mg-15Gd-1Zn-0.3Zr?wt.%??GZ151K?alloy produced by semi-continuous casting.The treatments including solution treatment,differential thermal extrusion?DT-Ext?,cold rolling and ageing treatment were applied to produce the GZ151K alloy under different conditions.Optical microscopy?OM?,X-ray diffraction?XRD?,scanning electron microscopy?SEM?and transmission electron microscope?TEM?were used to observe the microstructure evolution of the alloy under as-cast,solution-treated,peak-aged,as-extruded,extruded-aged and extruded-rolled-aged conditions.Vickers hardness test,room temperature tensile test and fracture toughness test were applied to test the mechanical properties and fracture mechanism of the alloy under different conditions.After that,we calculated the phase ratio,grain size,tensile properties and fracture toughness to build a relationship between the above factors,which are significant for the development of Mg alloys with both high strength and high ductility.The results reveal that the solution-treated GZ151K alloy consists?-Mg matrix,eutectic?Mg,Zn?₃Gd phase along grain boundaries,Gd-rich phase,?-Zr particles and Zn-Zr compounds.The biomodal structure forms during the dynamic recrystallization,which consists of coarse un-dynamic recrystallized?unDRXed?grains containing long period stacking ordered?LPSO?structure and fine dynamic recrystallized?DRXed?grains.With increasing extrusion temperature or the ageing treatment following cold rolling,the degree of recrystallization increases.When the bar extruded with a billet temperature of 500?and a mold temperature of 370?was cold rolling and aged at 200?for 16 h,the microstructure was nearly fully recrystallized.According to the analysis of the fracture surface,the main facture mechanism of the alloy is the combination of cleavage and intergranular fractures.The remained eutectic?Mg,Zn?₃Gd phase along the grain boundaries are the main crack initiations.According to the room-temperature and fracture toughness test,the GZ151K alloy in 1-T10 state?500?×12h solution treatment+500?×320?DT-Ext+5.5%Cold-Rolling+200?×16h aging?exhibit the highest UTS of 488MPa and YS of 430MPa,El.of 6%,respectively.However,it has an low KIC of 10.52 MPa?m^1/2.In 3-AE state?500?×12h solution treatment+500?×420?DT-Ext?,The GZ151K alloy with the highest K_ICC of 21.24MPa?m^1/2 exhibits the highest UTS of 488MPa and YS of 430MPa,El.of 6%.Above results indicate that only increasing strength is not beneficial for the fracture toughness when the ductility is sacrificed.The cold rolling and following ageing treatments after the extrusion results in finer grains,stronger hard texture,denser precipitates and considerably increased the yield stress.However,the treatments also decrease the alloy's ductility and the fracture toughness also decreases as a result.