Microstructure Regulation And Plastic Deformation Behavior Of Ultrahigh Strength Mg-Y-Ni Alloy Containing LPSO Phase

Ultra-high strength magnesium（Mg）alloys containing long period stacking ordered（LPSO）phase have received widespread attention for their excellent mechanical properties.However,its high strength relies on high rare-earth additions,resulting in high costs and limited applications.In order to develop ultra-high strength,low rare-earth content wrought LPSO-type Mg alloys,the relationship between the microstructure evolution and mechanical properties of wrought Mg-Y-Ni alloys were studied through composition design and thermal deformation process regulation in this paper.Ultra-high strength Mg-Y-Ni alloys with tensile yield strength greater than450 MPa were prepared by varying the Y/Ni atomic ratio and the contents of Y and Ni,as well as the extrusion and equal channel angular processing（ECAP）processes to regulate the volume fraction of LPSO andγ′phases,and the recrystallized（DRXed）and DRXed grain size.The tensile and compressive plastic deformation behaviors of Mg-Y-Ni alloys with different volume fractions of LPSO phase were studied by in-situ synchrotron radiation and microstructure evolution.The micro-yield and macro-yield behavior,tension-compression asymmetry and strain hardening behavior were analyzed,which provided theoretical guidance for the research and development of ultra-high strength LPSO-type Mg alloys.With the decrease of Y/Ni atomic ratio,the main second phase of as-cast Mg-Y-Ni alloys changes from LPSO phase,LPSO+γ’to LPSO+γ’+Mg₂Ni.Different second phases have a significant effect on the recrystallization behavior during extrusion.LPSO phase and Mg₂Ni phase promotes recrystallization,while the plate-shapedγ’phase inhibits recrystallization.After extrusion,the Mg₉₆Y₂Ni₂（at.%）alloy containing block-shaped LPSO phase and plate-shapedγ’phase has finer DRXed grains and lower DRXed proportion,exhibiting the tensile yield strength of 465 MPa,ultimate tensile strength of 510 MPa and elongation to failure of 7.2%.In Mg-Y-Ni alloys containing block-shaped LPSO phase and plate-shapedγ’phase,with the increase of volume fraction of LPSO phase,the tensile yield strength of the alloys increases while the elongation to failure decreases.The as-extruded Mg₉₄Y₃Ni₃（at.%）alloy with～75%volume fraction of LPSO phase achieves a tensile yield strength of502 MPa,an ultimate tensile strength of 542 MPa and an elongation to failure of 4.5%.The extrusion temperature significantly affects the microstructure and mechanical properties of Mg-Y-Ni alloys.The microstructure of the Mg₉₆Y₂Ni₂（at.%）alloy extruded at 390℃consists of DRXed grains with the size of 0.6μm,non-recrystallized（non-DRXed）grains with high dislocation density,LPSO phase andγ′phase,which exhibits tensile yield strength of 505 MPa,ultimate tensile strength of511 MPa,and elongation to failure of 3.5%.After extrusion at 450℃,the DRXed grains were coarsened to 2.1μm,the DRXed proportion increased,and the dislocation density in the non-DRXed grains decreased,resulting in a decrease in tensile yield strength and an increase in elongation to failure.After 8 passes of ECAP at 400℃,a fully recrystallized microstructure with grain size of 3.2μm was formed in the alloy extruded at 450℃,and the LPSO phase was partially fragmented,resulting in the tensile yield strength decreased to 320 MPa and the elongation to failure increased to18%.After further 4 passes of ECAP at 300℃,a fully recrystallized microstructure with ultrafine grains of～0.55μm was formed in the alloy,the fragmentation degree of LPSO phase was increased,leading to that the tensile yield strength was increased from 320 MPa to 450 MPa,and elongation to failure was decreased from 18%to 8%.Synchrotron radiation in-situ compression studies and deformation trace analysis of the as-cast and as-extruded Mg₉₂Y₅Ni₃ alloy show that the plastic deformation modes activated in LPSO phase are basal slip,prismatic slip and kinking in order.The micro-yield of Mg₉₂Y₅Ni₃ LPSO alloy is controlled by basal slip.The extrusion deformation leads to the refinement of LPSO phase and the increase of dislocation density,so that the micro-yield strength of the as-extruded alloy is significantly higher than that of the as-cast alloy,reaching 253 MPa.The basal texture formed in the as-extruded alloy inhibits basal slip,resulting in the macroscopic yield of the as-extruded alloy being controlled by the prismatic slip,and the macroscopic yield strength of the alloy is significantly improved to 535 MPa.When the as-extruded alloy is subjected to compressive stress parallel to the extrusion direction,the kinking activates after macroscopic yielding.The formation of kinking increases the dislocation density,resulting in high strain hardening rates.Synchrotron radiation in-situ tensile and compression studies of the as-extruded Mg-Y-Ni alloy containing LPSO phase parallel to the extrusion direction show that the basal slip within the DRXed grains during tension leads to micro-yielding of the alloy.During compression,tensile twinning is activated in the non-DRXed grains,resulting in micro-yield.As the volume fraction of LPSO phase increases,the volume fraction of non-DRXed grains decreases,and the compressive micro-yield mechanism changes from tensile twinning within non-DRXed grains to basal slip in DRXed grains.At macroscopic tensile yielding,only the basal slip within DRXed grains is activated.Both basal slip within DRXed grains and tensile twinning within non-DRXed grains are activated at macroscopic compressive yielding.The LPSO phase is not plastically deformed but shares more stress through load transfer at macroscopic yielding.Therefore,increasing the volume fraction of LPSO phase can simultaneously improve the tensile and compressive yield strength.As the compressive yield strength of the LPSO phase is greater than the tensile yield strength,the tensile-compression asymmetry of the alloys is significantly improved as the volume fraction of LPSO phase increases and the volume fraction of the non-DRXed grains decreases.The Mg₉₆Y₂Ni₂（at.%）alloy with～50%volume fraction of LPSO phase has good tension-compression symmetry.When the volume fraction of LPSO phase is higher than 50%,reverse tension-compression asymmetry of the alloy is more obvious.As the volume fraction of LPSO phase increases,the compressive strain hardening behavior of the alloys changes from being controlled by tensile twinning to being controlled by kinking of LPSO phase.When the alloy is compressively deformed,a large number of tensile twinning and kinking are activated,resulting in a significantly higher compressive strain hardening rate than the tensile strain hardening rate.