Precipitation of the ordered icosahedral quasicrystal in Mg-6wt%Zn-1wt%Y alloy has been characterized bytransmission electron microscopy observations. The lamellar-type icosahedral qusicrystal phases (I-phase) with the face-centeredicosahedral (FCI) structure are observed in alloy after solution treatment at 550oC. In the alloy annealed at 400oC, polygon-shaped I-phases are observed in the α-Mg matrix. The interfaces of the I-phase with the matrix are facetted and the facets areon five-fold and two- fold plane of the I-phase. The orientation relationship of the I-phase with the matrix is determined tobe [I5]I//[001]Mg, (2f)I//()Mg and [I2]I//[311]Mg, (5f)I//()Mg. The icosahedral grains are occasionally found to be twinnedwith one of the five-fold axis as the twin axis. The twin boundaries appear to be fairly straight and perpendicular to the five-fold twin axis. The icosahedral twin can be expressed as a rotation of 63.4o or 116.6o around two fold zone axis.

The long-period stacking order (LPSO) structures and stacking faults (SFs) in rapidly solidified powder metallurgy (RS P/M) Mg97Zn1Y2 alloy were investigated by high resolution transmission electron microscopy (HRTEM) observations. The 18R-type LPSO structure with a stacking sequence of ACBCBCBACACACBABAB and a period of 4.86 nm was observed in the as-extruded RS P/M Mg97Zn1Y2 alloy. After annealing at 773 K for 5 hr, the 18R-type LPSO structure was transformed to the 14H-type LPSO structure with a stacking sequence of ABABABACBCBCBC and a period of 3.64 nm. The 24R-type LPSO structure containing 24 atomic layers of ABABABABCACACACABCBCBCBC with period of 6.18 nm coexists with the 14H-type LPSO structure in the same grains. The LPSO structures contain intrinsic Type II SFs such as BCB/CABA and ABA/CBCB stacking sequences of a closely packed plane.

The precipitation behavior in Mg-6 wt%Zn-1 wt%Y alloy annealed at different temperatures of 200˚C and 400˚C has been characterized by high resolution transmission electron microscope. When the alloy is annealed at 200˚C for 6 hr, the plate- and the rod-shaped β2' phases are precipitated in the matrix. The orientation relationship of plate-shaped precipitates with the matrix exhibits a [11ar20]β2 || [10ar10]Mg, (0001)β2' || (0001)Mg. While the rod-shaped precipitates have two kinds of the orientation relationships with the matrix, i.e. [11ar20]β2'||[0001] Mg, (0001)β2'||(11ar20) Mg and [11ar20]β2'||[0001] Mg, (ar1106)β2'||(10ar10) Mg. With increasing annealing time at 200˚C the β1' phases are also precipitated in the matrix and the orientation relationship exhibits a [010]β1' || [0001]Mg, (603)β1' || (01ar10)Mg between the β1' precipitate and the matrix. The icosahedral phases are precipitated in the alloy annealed at 400˚C and exhibit a [I2]I || [0001]Mg relationship with the matrix.

Using Spark Plasma Sintering process (SPS), consolidation behavior of gas atomized alloys were investigated via examining the microstructure and evaluating the mechanical properties. In the atomized ahoy powders, fine particles were homogeneously distributed in the matrix. The phase distribution was maintained even after SPS at 723 K, although particles were newly precipitated by consolidating at 748 K. The density of the consolidated bulk Mg-Zn-Y alloy was . The ultimate tensile strength (UTS) and elongation were varied with the consolidation temperature.

Mg-4.3Zn-0.7Y (at%) alloy powders were prepared using an industrial scale gas atomizer, followed by warm extrusion. The powders were almost spherical in shape. The microstructure of atomized powders and those extruded bars was examined using Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscope (EDS) and X-ray Diffractometer (XRD). The grain size of the powders was coarsen as the initial powder size increased. After the extrusion, the grain size became fine due to the severe plastic deformation during the extrusion with the ratio of 10:1. Both the ultimate strength and elongation were enhanced with the decrease of initial particle size.

alloy powders were prepared using an industrial scale gas atomizer, followed by warm extrusion. The powders were almost spherical in shape. The microstructure of powders as atomized and bars as extruded was examined as a function of initial powder size distribution using Scanning Electron Microscope (SEM), Energy Dispersive X-ray Spectroscope (EDS) and X-ray Diffractometer (XRD). The grain sizes were decreased with extruding as well as decreasing the initial powder sizes. Both the ultimate strength and elongation were enhanced as the initial powder sizes were decreased.

The microstructure and mechanical properties of the alloy prepared by spark plasma sintering of gas atomized powders have been investigated. After consolidation, precipitates were observed to form in the solid solution matrix of the alloy. These precipitates consisted of and phases. The density of the consolidated bulk Mg-Zn-Y alloy was . The ultimate tensile strength and elongation were dependent on the consolidation temperature, which were in the ranges of 280 to 293 MPa and 8.5 to 20.8 %, respectively.