The microstructure of ternary Mg 97 Zn 1 RE 2 REϭLa, Ce, Gd, Tb, Dy, Ho or Er melt-spun alloys annealed at 573 K for 1.2 ks have been investigated in order to clarify the relationship between their microstructures and hardness. The Mg 97 Zn 1 RE 2 alloys consisted of Mg grains with diameter ranging from 0.7 to 2 mm. Also, several Mg-Zn-RE ternary phases were observed inside Mg grain and at grain boundary. Long period stacking LPS structure characterized as 14H-type was formed inside some Mg grains in the Mg 97 Zn 1 RE 2 REϭGd, Tb, Dy, Ho or Er alloys, respectively. The growth of 14H-type LPS structure from another Mg-Zn-RE ternary phase at grain boundary was recognized in the Mg 97 Zn 1 RE 2 REϭGd or Er alloys, and also the electron diffraction pattern clearly indicated that the relationship between 14H-type LPS structure and 2H-Mg was 001 2H-Mg // 0014 14H-Mg , indicating the 14H-and 2H-Mg were coexisted in the identical grain with certain relationship. On the other hands, although the LPS structure was not observed in the Mg 97 Zn 1 RE 2 REϭLa or Ce alloys, a number of the Mg-Zn-RE REϭLa or Ce ternary phases with less than 50 nm dispersed in the Mg matrix, respectively. The higher hardness of the Mg 97 Zn 1 RE 2 REϭLa or Ce alloys than the Mg 97 Zn 1 RE 2 REϭGd, Tb, Dy, Ho or Er alloys originated in the formation of the numerous nano-size Mg-Zn-RE REϭLa or Ce phases dispersed interior Mg grains in the Mg 97 Zn 1 RE 2 REϭLa or Ce alloys.
Hot extrusion of machined chips of Mg 96 Zn 2 Y 2 (at%) alloy was carried out at 623 K, and microstructures and mechanical properties of the extruded Mg 96 Zn 2 Y 2 alloy were investigated. The alloy consisted of -Mg, Mg 12 ZnY, and Mg 3 Zn 3 Y 2 phases, and the Mg grains had a mean grain size of 450 nm. The Mg 12 ZnY phase was frequently observed inside the fine Mg grains. In addition, oxidation occurred around cavities remaining after extrusion. The extruded alloy exhibited a high 0.2% proof strength of 495 MPa and elongation of 3% at room temperature. The Mg grain refinement and dispersion of Mg 12 ZnY and Mg 3 Zn 3 Y 2 phases caused by hot extrusion led to high strength at room temperature. Further, the extruded alloy also exhibited superplasticity at temperatures of 623 and 723 K with initial strain rates from 2 Â 10 À1 S À1 to 2 Â 10 À3 S À1 . The maximum elongation was 450% at 723 K with an initial strain rate of 2 Â 10 À3 S À1 . Grain boundary sliding of Mg grains is the dominant deformation mechanism for the alloy at high temperature ranges.
Mg 97 Zn 1 Y 2 at% 100ϳ150 nm Mg 610 MPa s 0.2 5% d 0.67T m T m ϭ 0.1ϳ Hot extrusion has been carried out at 623 K on machined chips of Mg 96 Zn 2 Y 2 at% alloy. Microstructure and mechanical properties of the extruded Mg 96 Zn 2 Y 2 alloy have been investigated. The extruded Mg 96 Zn 2 Y 2 alloy was composed of a-Mg, Mg 12 ZnY and Mg 3 Zn 3 Y 2 phases, and the Mg grain had a mean grain size of 450 nm. The Mg 12 ZnY phase was frequently observed inside the fine Mg grains. Additionally, oxidation was occurred around cavities remained after extrusion. The extruded alloy exhibited a high 0.2% proof strength of 495 MPa and elongation of failure to 3% at room temperature. Grain refinement of Mg and dispersion of Mg 12 ZnY and Mg 3 Zn 3 Y 2 phases caused by hot extrusion lead to high strength at room temperature. Furthermore, the alloy extrusion also exhibited the superplasticity at temperatures of 623 K and 723 K with initial strain rates from 2ϫ10 Ϫ1 s Ϫ1 to 2ϫ10 Ϫ3 s Ϫ1 . And also the maximum elongation was 450% at 723 K with initial strain rate of 2ϫ10 Ϫ3 s Ϫ1 . The grain boundary sliding of Mg grains is dominant deformation mechanism for the alloy at high temperature ranges.
Mg96Zn2Y2 (at.%) extruded alloy was fabricated by hot-extrusion of the Mg96Zn2Y2 machined chip. The Mg96Zn2Y2 extruded alloy consisted of a long period stacking ordered (LPSO)-, Mg3Zn3Y2- and Mg- phases. The Mg phase with mean grain size of 450 nm was confirmed by TEM. However, the LPSO- and Mg3Zn3Y2- phases had relatively large grain size compared with Mg phase. The Mg96Zn2Y2 extruded alloy also showed superplasticity at temperatures of 623 K and 723 K with initial strain rates from 2×10−1 s−1 to 2×10−3 s−1. The maximum elongation of 450 % was achieved at 723 K with an initial strain rate of 2×10−3 s−1. From TEM observation, it is considered that grain boundary sliding of Mg grains was dominant deformation mechanism of the Mg96Zn2Y2 extruded alloy at high temperature range.
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