Phase Structures And Phase Equilibria Of Long-Period Stacking Ordered Phases And Their Effects On Mechanical Properties Of The Mg-TM-Y(TM=Ni,Zn) Alloys

Due to unique structure and atomic arrangement,the study of Long-period Stacking Ordered(LPSO)phase strengthening magnesium alloys has been a hot spot.In this paper,by means of SEM,DSC,TEM and EPMA,the LPSO phase structures and phase equilibria of Mg-Ni-Y alloys in the as-cast state,at 400?and 500? were studied.At the same time,the mechanical properties of the as-extruded LPSO strengthening Mg-Zn-Y alloys were studied.The main results are as follows:(1)There are two types of LPSO phase in Mg-Ni-Y alloys:18R and 10H.The composition of 18R is Mg89.3Ni4.5Y6.2(at.%).The composition of 10H is Mg85.3Ni 6.2Y8.3(at.%).10H disappears after the heat treatment at 500?.(2)At 400?,there is a narrow(Mg)+LPSO 18R equilibrium region in the Mg-Ni-Y phase diagram.There are four three-phase equilibrium regions:(Mg)+LPSO18R+Mg24Y5,LPSO18R+Mg24Y5+Mg2Y,LPSO18R+LPSO10H+Mg2Y and(Mg)+LPSO18R+Mg2Ni.According to these results,the isothermal section at 4O0?has been established.At 500?,there is a narrow(Mg)+LPSO18R two-phase equilibrium region in the Mg-Ni-Y phase diagram.There are two three-phase equilibrium regions:(Mg)+LPSO18R+Mg24Y5 and(Mg)+LPSO18R+L.According to the results,the isothermal section at 500? has been established.(3)The amount of LPSO phase increases from 18%,23%,26%to 30%in the alloys of Mg98Zn0.7Y1.2,Mg97.8Zn0.8Y1.4,Mg97.5Zn0.9Y1.6 and Mg97.2Zn1.1Y1.7(at.%).In the as-cast Mg-Zn-Y alloys,LPSO phase distributes in the grain boundary.In the as-extruded state,LPSO phase distributes along extrusion direction.After extrusion,the(Mg)grains are equiaxed and the average grain size is between 6-13?m.With the increase of LPSO phase,the grain size decreases obviously,but the strength and elongation increases.When the amount of LPSO phase reaches 26%,the alloys get a maximum tensile strength and elongation.When the amount of LPSO reaches to 30%,the strength and elongation decrease obviously.