| Mg-Gd-Y-Zn-Zr alloys,containing long period stacking ordered(LPSO)phase,have attracted concentrated attention due to their excellent mechanical properties at room and elevated temperatures and have potential applications in aeronautics and space,national defense and automotive industries.However,the formation of LPSO phases consuming Gd and Y results in the poor age-hardening response and low strength.Hence,firstly,we have systematically investigated how the influence of Ag addition(0.1,0.3,0.5 and 0.7wt.%)on the microstructural and mechanical properties of Mg-6Gd-3Y-1Zn-0.4Zr(wt.%)alloy.Then,the effect of Ag addition on the morphologies,structures,compositions and transformation of precipitates of the alloys during heat treatments were comprehensively investigated.Finally,the Mg-6Gd-3Y-1Zn-0.4Zr-0.3Ag alloy was selected to investigate the deformation behaviors during uni-axial compression and multi-directional forging at elevated temperature.In addition,large-sized0.3Ag alloy bulk was prepared by multi-directional forging(MDF).The strengthening and toughening mechanisms of as-MDFed and peak-aged alloys were discussed.Results of this study possess both theoretical and practical values,and the main conclusions are summarized as follows.(1)The addition of trace Ag does not change the phase composition of the Mg-6Gd-3Y-1Zn-0.4Zr alloys in the as-cast and solution-treated conditions,while the Ag addition(≥0.3wt.%)enhances the age-hardening response and strength as the sample peak-aged at 200℃due to the coprecipitation of basalγ’’and prismaticβ’phases.The Mg-6Gd-3Y-1Zn-0.4Zr-0.3Ag alloy achieves the peak aging hardness of 111 HV after 72 h,while it(100 HV)occurs after 96 h for the 0Ag alloy.Meanwhile,the ultimate tensile strength,yield strength,and elongation to failure of the peak-aged 0.3Ag alloy are 290 MPa,218 MPa and 4.4%,respectively.By comparing the mechanical properties,cost and density of the alloys containing different Ag addition,0.3wt.%Ag is the suitable addition.(2)LPSO phases bended and fragmented as the 0.3Ag alloy axial compressed at temperature of 420℃to 480℃.With the increasing of the deformation temperature and strain,the ratio of dynamic recrystallization(DRX)is improved,more extensive fragmentation of LPSO phase is obtained.The lamellar LPSO phases improve the DRX behavior via kinks,and the interdendritic LPSO phases improve the DRX ratio via the particle-simulated nucleation mechanism.The texture is weakened by the increasing of DRX ratio.After axial compressed at 480℃for strain of 1.1,the microstructure of the 0.3Ag alloy is almost dynamic recrystallization grains.(3)The 0.3Ag alloy is multi-directional forged at 480℃.As the forging pass increased,the grain sized are refined,the LPSO phases are kinked and fragmented due to the change of deformation force directions,at the same time,the ratio of DRX is increased.In addition,dynamic precipitated particles near the grain boundaries and densely distributed short lamellar stacking faults in the grains are presented.Meanwhile,the texture is weakened with the increasing of forging passes,and the non-basal RE texture component is formed.After 18 passes of MDF processing,the ultimate tensile strength,yield strength and elongation to failure are355 MPa,276 MPa and 14.3%,respectively.The mechanical properties of the 0.3Ag alloy can be further improved after aging treatment at 200℃for the reason of the coprecipitation of basalγ’’and prismaticβ’phases,the ultimate tensile strength,yield strength and elongation to failure are 424MPa,352 MPa,and 9.8%,respectively.The grain boundary and precipitation strengthening contribute most of the strength.(4)Through MDF with strain controlled multi-passes and ageing strengthening,0.3Ag alloy bulk with the size of 460×260×155 mm~3 is prepared.Due to the uneven temperature and strain distributions in different regions of the bulk,inhomogeneous microstructures with different levels of dynamic recrystallization are formed.From the center region to the edge region,the grain size is increased,on the contrary,the DRX ratio and mechanical properties are reduced.The ultimate tensile strength and yield strength of the samples at the center region are 365 MPa and 295 MPa,respectively.After ageing treatment at 200℃,the strength of the alloy is improved by the coprecipitation of basalγ’’and prismaticβ’phases,the ultimate tensile strength and yield strength are 421 MPa and335 MPa,respectively,meanwhile,the ultimate tensile strength and yield strength of the samples testing at 200℃are 388 MPa and 317 MPa,respectively.The high stability of the LPSO phase,βphase andβ’phase leads to the excellent mechanical properties of the 0.3Ag alloy at elevate temperatures.The whole article contains 93 figures,20 tables,208 references are cited... |
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