Study On Homogeneous Fabrication Of Mg-Gd-Y-Zn-Mn Alloy With High-strength And High-toughness And Its Plastic Deformation And Mechanisms Of Strengthening And Toughening

The high-strength and high-toughness Mg-RE-Zn alloys with long period stacking ordered(LPSO)phase are a promising lightweight structural material which have attracted considerable attention in the major equipment in key areas such as aerospace,rail transit and national defense.In order to further expand their application,the topic of the present work is focused on the Mg-Gd-Y-Zn-Mn alloy.A large-scale Mg-Gd-Y-Zn-Mn alloy ingot was prepared through semi-continuous casting.Mg-Gd-Y-Zn-Mn alloy bars with ultrahigh-strength and high-toughness were fabricated by using different heat treatments before extrusion,extrusion and ageing processes.Weakly anisotropic Mg-Gd-Y-Zn-Mn alloy sheets with high-strength and high-toughness were prepared by large-strain high-efficiency rolling process via introducing hard-plate rolling process.By means of finite element simulation and optimization of extrusion parameters,a large cross-section solid profile and a complex cross-section hollow profile with high-strength and high-toughness were successfully fabricated.Optical microscope,differential scanning calorimeter instrument,X-ray diffraction,scanning electron microscopy and transmission electron microscopy were utilized to investigate the microstructure evolution and phase transformation in Mg-Gd-Y-Zn-Mn alloy during different heat treatments/deformation processes.The strengthening and toughening mechanisms of Mg-Gd-Y-Zn-Mn alloy were revealed.The technical prototypes of homogeneous fabrication of high-strength and high-toughness Mg-Gd-Y-Zn-Mn alloy profiles were obtained,resulting in providing theoretical guidance for the fabricating and processing of high-strength and high-toughness magnesium which are served as key structural components.Large-scale Mg-Gd-Y-Zn-Mn alloy ingot with a diameter of 315 mm and length of2410 mm was prepared through semi-continuous casting.Chemical composition,microstructure and mechanical properties at different locations of the samples with as-cast,T4 and T6 heat-treated states,respectively,were investigated.No obvious macro segregation has been detected in the high-quality alloy ingot.The main eutectic structures at all different locations are composed of?-Mg,Mg₃RE-type,Mg₅RE-type and LPSO phases.At the edge of ingot,the unusual casting twins including{10(?)2}extension twins and{10(?)1}compression twins were observed due to the intensive internal stress.In T4 heat-treated alloy,the micro segregation was eliminated.The remained phases were?-Mg and LPSO phase.Combined with the remarkable age-hardening response,T6 samples exhibits improved mechanical properties at ambient temperature,which derives from the dense prismatic?'precipitates and profuse basal?'precipitates.The effects of pre-ageing treatment on the microstructure and mechanical properties of the Mg-Gd-Y-Zn-Mn alloy were investigated.Microstructural analysis indicated that the ageing treatment before extrusion led to the formation of dense prismatic?'and basal?'precipitates in heat treated alloys.The presence of these precipitates and their solution obstructed the dynamic recrystallization process during hot extrusion.The lamellar long-period stacking ordered(LPSO)phases restrained the recrystallization through forming the kink band and releasing the stress concentration,and the fine?-Mg₅(Gd,Y)particles suppressed the recrystallization by the particle pinning effect.The block-shaped LPSO phases and coarse?-Mg₅(Gd,Y)particles promoted the recrystallization following the particle stimulated nucleation(PSN)mechanism.The combined effects led to the formation of the bimodal microstructure,which shows fine recrystallized grains with random grain orientation and deformed grains with strong fiber texture.Ageing at 200°C,all the samples show excellent age hardening response,which is result from the co-precipitation of prismatic??and basal??phases generating an approximate enclosed microstructure.Both the critical resolved shear stresses(CRSS)of basal slip and non-basal slip are improved.The bimodal microstructure with lower recrystallization fraction provides the alloy higher strength and lower ductility.With solid-solution and pre-ageing treatments,the as-extruded alloy shows the highest mechanical properties with ultimate tensile strength(UTS)of555 MPa,tensile yield strength(TYS)of 488 MPa and elongation to failure(EL)of5.8%.Compared with the isothermal hot extrusion process,the large difference temperature extrusion process with billet temperature of 520°C and die temperature of390°C can effectively improve the extrusion rate of Mg-Gd-Y-Zn-Mn alloy without losing the surface quality and mechanical properties of the alloy.The alloy processed by large difference temperature extrusion show fully recrystallization structure with average grain size of?7.4?m and weak texture which consists of<10(?)0>_Mg//ED fiber texture component and<0001>_Mg//ED abnormal texture component.After aged at200°C,the peak-aged sample processed by large difference temperature extrusion exhibit UTS of 488 MPa,TYS of 344 MPa and EL of 9.7%.The different strengthening mechanisms in the alloy were analyzed quantitatively,and the results showed that the strengthening effects of solid solution strengthening,grain boundary strengthening,precipitation strengthening and other strengthening effects including texture strengthening,fiber strengthening and dispersion strengthening contributed78MPa,60MPa,78MPa and 82MPa respectively to the strength of peak-aged alloy.After extruded with different extrusion ratios(6 and 15.3)and cooling methods(air cooling and water cooling),the recrystallization volume fractions of E128N,E80N and E80Q alloys were 79.1%,96.9%and 84.9%,respectively.The three alloys show two-component texture with different intensity.With the increase of recrystallization degree,the intensity of<0001>Mg//ED abnormal texture component increases gradually,while the<10(?)0>_Mg//ED fiber texture gradually weakened into a<10(?)0>_Mg//ED and<11(?)0>_Mg//ED bipolar texture component,and finally into a(0001)_Mg//ED basal texture component.The E80N alloy with the highest degree of recrystallization exhibits the highest texture intensity which shows a<0001>_Mg//ED abnormal texture.With the increase of recrystallization degree,the grains with<0001>_Mg//ED orientation grow in priority,engulf and consume the grains with other orientations,and finally lead to the formation of abnormal texture.The E128N alloy prepared under extrusion ratio of 6 and air cooling shows the best integrated mechanical properties,while the E80N alloy prepared under extrusion ratio of 15.3 and air cooling shows the best isotropic in mechanical properties.The key factor affecting the anisotropy of extruded alloys is not the texture characteristics of?-Mg grains but the strengthening mechanism of fiber texture.For the peak-aged alloy,the precipitation strengthening has a weak orientation correlation.Based on the as-extruded Mg-Gd-Y-Zn-Mn alloy,the high-strength and high-toughness alloy sheets were prepared by large-strain high-efficiency rolling process.The large strains with?87%and?91%total reduction were introduced in the hard-to-deform Mg alloy by hard-plate rolling process at 400 ^oC for merely 3 and 2rolling passes.The microstructural morphology and texture were examined,and the tensile tests were performed along rolling direction(RD)and transverse direction(TD).The large-strain rolling process brings in refined grains,homogeneous dispersive LPSO phase and?phase and weakened basal texture.The anisotropy of the strength and elongation is improved after rolling.The as-rolled sample with?91%total reduction exhibits excellent mechanical properties with UTS of 434 MPa,TYS of 318MPa and EL of 10.7%parallel to the TD.By quantitating the relation between grain orientations and the deformation modes,it is confirmed that the basal slip and prismatic slip are the dominant deformation modes at the initial stage of plastic deformation,meanwhile the pyramidal slip and extension twinning are the complementary deformation modes.Combined with the numerical simulation results of temperature field,stress field and velocity field in the extrusion process,the extrusion process was optimized and a large cross-section solid profile and a complex cross-section hollow profile with high-strength and high-toughness were successfully fabricated.The relationship between multiple field interactions and the microstructure uniformity of profiles under complex constraints was established.For the solid profile with large cross-section,the mechanical properties of the profile at different locations are uniform,with UTS range from 362 MPa to 384 MPa,TYS range from 275 MPa to 296 MPa and EL range from 11.6%to 15.4%.The optimized solid solution-ageing process can effectively improve the strength of the profile and maintain the good ductility.After solid solution-ageing process,the mechanical properties of the profile at different locations are uniform,with UTS range from461 MPa to 475 MPa,TYS range from 302 MPa to 367 MPa and EL range from8.3%to 14.2%.The strengthening and toughening mechanism is related to the co-precipitation of the prismatic?''phase,?'phase and the basic?'phase with a high number density.For the hollow profile with complex cross-section,there are some difference in mechanical properties of the extruded profile at different locations.The tensile fracture morphology at the matrix is dominated by dimples and there are a few cleavage planes and tearing edges,while the tensile fracture morphology at the weld is dominated by cleavage section.After solid solution-ageing heat treatment,the mechanical properties of the profile were significantly improved.The mechanical properties with UTS of 470 MPa,TYS of 309 MPa and EL of 7.4%were obtained in a typical location of the profile.Due to the different impurity diffusion coefficients of Gd,Y,Zn and Mn in Mg,the contents of solute elements at the extrusion weld are greatly different from that at the matrix,thus a precipitate free zone were formed at the weld.