Phase compositions and microstructure evolutions of three Mg–Y–Zn cast alloys during isothermal annealing at 773 K have been systematically investigated to clarify the formation behavior of 14 H long period stacking ordered(LPSO) structure from α-Mg grains.The annealed microstructure characteristics indicate that the 18 R phase is thermal stable in Mg86Y8Zn6 alloy where 18 R serves as matrix,and 14 H lamellar phase only forms within tiny α-Mg slices(less than 1% for volume fraction).The α-Mg grains in Mg88Y8Zn4 and Mg89Y8Zn3 alloys exhibit cellular shape,and 14 H phase forms and develops into lamellar shape in these cellular grains after annealing.The results suggest that the presence of α-Mg grains is a requirement for the generation of 14 H phase.The nucleation and growth rates of 14 H lamellas are accelerated in α-Mg grains with higher concentrations of stacking faults and solute atoms.Moreover,the 14 H lamellas are parallel to adjacent 18 R plates in Mg86Y8Zn6 alloy,but the 14 H phase precipitated in cellularα-Mg grains of Mg88Y8Zn4 and Mg89Y8Zn3 alloys exhibits random orientation relationship with surrounding 18 R phase,indicating that the orientation relationship between 14 H and 18 R phases depends on the relationship between α-Mg grains and 18 R phase.
Microstructures and precipitation behaviours of Mg_(94)Y_4Zn_2(at.%) extruded alloy during solution treatment and ageing processes were investigated.Three major phases were observed in the as-cast Mg_(94)Zn_2Y_4 alloy:a-Mg,block shaped 18R long period stacking ordered(LPSO) phase and M_(94)Y_5 cuboid particles.After homogenization and extrusion,the block shaped LPSO phase changed into plate-like shape aligned along the direction of extrusion.During solution treatment,a small fraction of LPSO phase was transformed from 18R structure to14H type.The nano-scale β' phase with its close-packed planes being perpendicular to the direction of both a-Mg and LPSO structure was precipitated at ageing stage.The coexistence of β' and LPSO phase contributes to the strengthening of the alloy,with microhardness for the matrix and LPSO structures reaching 145.8 and155.0 HV,respectively.
