Microstructure And Properties As Well As Creep Behavior Of Mg-Sm-Yb-Zn-Zr Alloys

Magnesium alloys,as the lightest metal structure material,are widely used in the automotive industry,aerospace,weapons and equipment and other fields.Since high strength and excellent heat resistance,Mg-RE alloys have became one of the hot topics in the field of magnesium alloy.Recently,the researchers were greatly attracted by cheap light rare earth Samarium?Sm?,and developed Mg-Sm-Zn-Zr alloys.Based on the theory that the mechanical properties of magnesium alloys can remarkably be improved via the addition of light and heavy rare earth,thus we designed Mg-Sm-Yb-Zn-Zr alloys.In present work,we mainly study the effect of Yb addition on the microstructure,properties and creep behavior of Mg-3.5Sm-0.6Zn-0.5Zr alloys.Firstly,the effect of different content of Yb addition on the microstructure and properties of the as-cast Mg-3.5Sm-0.6Zn-0.5Zr based alloys were thoroughly studied.The results indicate that Yb have a significant grain refinement effect.The dominate secondary phase is network and porous Mg₃Sm?FCC,a=0.74nm?along the triangular grain boundaries in M40alloy.The area volume of secondary phases increases with the addition of Yb.The dominate secondary phase are dendritic Mg₂₄RE₅?BCC,a=1.12nm?at the triangular grain boundaries and lath-liked Mg5RE?FCC,a=2.22nm?at the intra-grain in Yb-rich alloy.Meanwhile,the orientation relationship?OR?implied by the superimposed Mg₅RE phase and?-Mg matrix patterns is such that?200?_Mg5RE 3.03^o from?-110-2?_?-Mg,[012]_Mg5RE//[-24-23]_?-Mg.Nanoscale lath-liked Mg3Sm precipitate and precipitate were found in M40 alloy,however,nanoscale stripe-liked Mg₂₄RE₅ phase,ultra-fine needle-liked Mg₃RE phase and basal stacking fault were found in Mg-3.5Sm-2Yb-0.6Zn-0.4Zr?M42?alloy.With Yb addition,the yield strength of as-cast alloys is remarkably enhanced,and elongation is increased first and then decreased.As a result,the excellent integrated mechanical properties were obtained from the as-cast M42 alloy(?_b=234MPa,?_0.2=131MPa,?_f=18.4%),which possesses superior high temperature?less than 250^oC?tensile properties.Secondly,creep behavior of as-cast alloys was studied and creep mechanism of as-cast M40 and M42 alloy was analyzed.The results indicate that Yb addition can improve creep properties of the as-cast alloy.Based on analysis of creep activiation energy and creep stress exponent,a multiple mode of dislocation controlled creep,diffusion creep and grain boundary slip is proposed as creep deformation mechanism for M40 and M42 alloy at 200^oC²50^oC and 60¹00MPa.In addition,the formation of dynamic precipitates during creep is the primary factor for improvement of creep resistance of the as-cast alloy.Creep failure behavior was also discussed,and it is suggested that the nucleation,gather and growth of r-type holes and w-type cracks at grain boundary will lead to creep failure.Besides,solid solution treatment process is investigated,and the solid solution process are 520^oC-6h,cold water quenching and 525^oC-2h+515^oC-4h,80¹00^oC water quenching for M40,M405 alloy and M41,M42,M43 alloys,respectively.The ductility of as-cast alloys is distinctly improved,and strength is not obviously enchanced via solid solution treatment.Also,200^oC peak-aged precipitates and mechanical properties of M40 and M42 alloys were further researched.The peak-aged precipitate of M40 alloy are?'and?''phases.The?'phase with base-centred orthorhombic structure?a=0.642 nm,b=2.224 nm,c=0.521 nm?lying on{10-10}prismatic plane,and has a Burger OR with the?-Mg matrix,?001?_?'//?0001?_?,[2-10]_?'//[2-1-10]_?.The?''basal precipitates has an ordered hexagonal structure,with a=0.556 nm and c=0.444 nm.In M42 alloy,the peak-aged precipitates are mainly Compdosed of arovane-liked??'attached to the two ends of each of?''?,plate-liked?''phase,needle-liked?''and?'phase.The?''phase?DO19 structure,a=0.642nm,c=0.521nm?lying on{11-20}prismatic plane,and the OR determined between?''and the matrix phase has the form?10-10?_?''//?10-10?_?,[0001]_?''//[0001]_?.The OR between?''and the matrix phase was such that?2-1-10?_?''//?-1100?_?,[0001]_?''//[0001]_?.The?'phase has a disordered hexagonal structure?a=0.321nm,c=0.781nm?,and the OR implied by the superimposed?'phase and?-Mg matrix patterns is such that?0001?_?'//?0001?_?,[2-1-10]_?'//[2-1-10]_?.The peak-aged M42 alloy exhibits the best mechanical properties(?_b=297MPa,?_0.2=228MPa,?_f=8.1%),which excel the peak-aged WE43 alloy.Also,the peak-aged alloys display good high temperature mechanical properties,which is mainly attributed to good thermal stability?''and?'phases.Minor?0.5%?Yb addition can enhance work hardening effect of as-cast,T4 state,peak-aged alloys.The heat treatment also has a significant influence for work hardening effect of alloys.Additionally,temperature has a great influence on the work hardening behavior of as-cast and peak-aged M42 alloy.