Mechanical Behavior And Formability Of Mg-Gd-Y-Zn-Zr Rolled Sheet

The Mg-RE-Zn alloy containing long period-stacking order(LPSO)phase demonstrated the superior thermal stability,and can be used as the structural material at elevated temperature.The Mg-RE-Zn alloy with low RE content contained LPSO lamella and was widely applied.However,the addition of the higher RE element contribute to the formation of the interdendritic LPSO phase besides the intragranular14H-LPSO lamella,which can effectively improve the strength at room temperature and elevated temperature.Currently,the application of the Mg-RE-Zn alloy mainly focused on the cast alloy.The Mg sheets were commonly applied in the automobile and communication electronic industry,and especially there are urgent demands for Mg-RE sheets with high strength and heat resistance in the aviation and aerospace industry.Thus,it is significant to investigate the fabrication of the Mg-RE-Zn rolled sheets,the fracture behavior and thermal formability of the rolled sheets for the wide application of the Mg-RE-Zn sheets.The microstructure evolution and mechanicl properties of the cast and homogenized Mg-13Gd-4Y-2Zn-0.6Zr alloy with different homogenization parameters were firstly invesitigated,and the deformation mechanism at different deformation parameters(400?475?,0.001?1s^-1)of the alloy with optimum homogenization parameters were also studied.The eutectic Mg₃(Gd,Y)phase dissolved and the interdendritic 18R-LPSO phase transformed into interdendritic 14H-LPSO phase during the homogenization process.At the higher temperature and lower strain rate(ln Z<40),the intragranular 14H-LPSO lamella within the coarse deformed grains were kinked and broken during the thermal compression process,and the inhibitation effects of the 14H-LPSO lamella on the dislocation slip and lattice rotation reduced,which contributed to the continuous dynamic recrystallization within the coarse deformed grains.Furthermore,the broken 14H-LPSO phase can behave as the nucleation particles and contribute to the discontinuous dynamic recrystallization through the particle-stimulated nucleation(PSN)mechanism.The continuous dynamic recrystallization within the coarse deformed grains and the discontinuous dynamic recrystallization through the particle-stimulated nucleation(PSN)mechanism were dominant at the corresponding deformation parameters range.At the lower temperature and higher strain rate(43>ln Z>40),the dislocations accumulated adjacent to the 14H-LPSO lamella,and thus the 14H-LPSO lamella can be kinked and zigzagged.The kinking of the intragranular 14H-LPSO lamella was dominant deformation mechanism and the discontinuous dynamic recrystallization at the grain boundaries also existed.Therefore,the bimodal microstructure,which the coarse deformed grains were surrounded with the finer DRX grains,formed during the thermal compression at the lower temperature and higher strain rate(43>ln Z>40).The DRX grains at grain boundaries spread into the coarse deformed grains,and the<0001>of the coarse deformed grains gradually rotate towards ND of the resulted sheets with the increase of the accumulative rolling reduction.The interdendritic 14H-LPSO phase were more effectively broken with the increase of the final rolling reduction,and the interdendritic 14H-LPSO phase along the RD can effectively strengthen the TYS along the RD.The maximum critical resolved shear stress was parallel with the RD and mainly exerted on the?-Mg matrix and coarse deformed grains,and thus the cracks rarely nucleate within the interdendritic 14H-LPSO phases,which results in the superior fracture elongation along the 45°.The increase of the final rolling temperature facilitates the formation of the basal texture,and the interdendritic 14H-LPSO phase were effectively broken and distributed along the RD with the final rolling temperature elevating,which can effectively strengthen the tensile yield strength along the RD and restrain the cracks propagation within the interdendritic block 14H-LPSO phase.The optimum tensile properties can be obtained in the resulted sheets with the92%accumulative rolling reduction,50%final rolling reduction and 450?final rolling temperature,and the tensile yield strength along RD can reach 367MPa,ultimate tensile strength along RD can reach 446MPa,and the fracture elongation along RD can reach10.30%.The resulted sheets with the optimum parameters demonstrated the superior mechanical properties at elevated temperature,the ultimate tensile strength can reach274MPa and fracture elongation can reach 43%at 300?,and the ultimate tensile strength can reach 142MPa and fracture elongation can reach 64%at 350?.The fracture behavior of the resulted sheets at room temperature and elevated temperature were studied through the 3D-XRM.The holes or microcracks nucleated,propagated and coalesced along the 30-45°to the tensile direction within the interdendritic 14H-LPSO phase,which resulted in more tear ridges in the fracture at room temperature.The fracture mechanism transformed from mixed fracture mechanism of microporous aggregation fracture and cleavage fracture to ductile fracture mechanism of microporous aggregation with the tensile temperature elevating.The microcracks mainly nucleated at the interface between the interdendritic 14H-LPSO phases and?-Mg matrix during tension at 200?,and the cracks were also inclined to nucleate at the equilibrium?phase precipitated at trigeminal boundary of?-Mg grains.The microcracks mainly nucleated at the interdendritic 14H-LPSO phase during tension at 300?and mainly propagated along the direction of maximum resolved shear stress.The formability of the resulted sheets along TD was superior to that along the RD during the uniaxial hot tension,however,the formability of the resulted sheets along the RD was superior to that along the TD under the tension-tension strain and near plane strain.The difference between the forming limit of the resulted sheets along the RD and TD under the tension-tension strain and near plane strain can be ascribed to the difference of the deformation coordination interface between the interdendritic 14H-LPSO phase and?-Mg matrix.The resulted sheets with different final rolling temperature were conducted with post aging treatment,the results indicated that the peak aging treatment of the resulted sheets with lower final rolling temperature requires shorter duration due to the more widespread dislocation accumulation and nucleation sites at lower final rolling temperature compared with the resulted sheets with higher final rolling temperature.