Refinement Of The Icosahedral Quasicrystalline Phase And Its Effect On Mechanical Properties Of Mg-Zn-Y Alloys

Icosahedral quasicrystal phases possess attractive mechanical and physical properties due to their peculiar lattice structure.More importantly,nano-quasicrystals reinforced Mg alloys possess attractive comprehensive performances,and will also have wide application in the future.The present study was aimed at quasicrystal strengthened Mg-6.52Zn-0.95Y alloy manufactured by our laboratory.Firstly,hot compression deformation behavior of the extruded Mg-6.52Zn-0.95Y alloy was investigated at temperatures ranging from 300 ? to 400?.Meanwhile,a constitutive equation was developed to describe the flow characteristics.Secondly,in order to obtain ultra-high strength of I-phase strengthened Mg-Zn-Y alloy with low alloy content,semi-solid process and severe plastic deformation were used to refine grain size and fragment the I-phase.The constitutive equation of as-extruded Mg-6.52Zn-0.95Y alloy at temperatures ranging from 300 to 400 ? and strain rates ranging from 0.01s-1 to 1s--was ?=2.319×108[sinh(0.017?)]3.991exp(-115.041/RT)?For the extruded Mg-6.52Zn-0.95Y alloy,the processing map revealed a large region corresponding to a feasible processing window apart from the domain at 325?370?/0.32?1s-1.Based on the processing map and microstructures,the optimal hot working conditionals were determined to be temperature of 300-325 ? and strain rates of 0.1?1s-1.The sample of Mg-6.52Zn-0.95Y alloy after extrusion and secondary extrusion showed homogeneous microstructure with the elongation to failure of 24.7%,which was 75.2%larger than the initial extruded sample.The sample after semi-solid process and secondary extrusion showed excellent tensile properties with ultimate tensile strength of 375MPa and 0.2%proof strength of 328MPa,with 21.0%and 53.3%increment of UTS and YS,respectively.However,the small strain and insufficient fluidity introduced by double extrusion caused unsatisfactory fragment of(a-Mg + I-phase)eutectic.The agglomerated distributed I-phase could easily lead to stress concentration and limit the further strengthen of the alloy.Fine grains(2.9?m)with broken I-phase particles of 50-500nm were obtained of the semisolid Mg-6.52Zn-0.95Y alloy after 2 passes of ECAP at 300? followed by 2 passes of ECAP at 200?.The alloy exhibited best tensile properties with ultimate tensile strength of 385 ± 18MPa,0.2%proof strength of 290 ± 3MPa and elongation to failure of 22.0 ± 1.6%,with 24.2%,35.5%and 56.0%increment of UTS,YS and EL,respectively.The(a-Mg + I-phase)eutectic was fragmented and distributed in bands along the extrusion direction of ECAP.The strengthening effects of nanoscale I-phase particles were mainly attributed to recrystallization related to particle stimulated nucleation(PSN),as the precipitation strengthening effect was no more than 15%.