| RE-EX-CT-ZK60 and RE-EX-RS-ZK60 alloys with compositions of Mg-5.47%Zn-0.25%Zr-0.12Mn, and RE-EX-RS-66 with composition of Mg-6.0%Zn-1.0%Y-0.6%Ce-0.6%Zr were prepared by a combination of processes of rapid solidification (RS), reciprocating extrusion (RE) and extrusion (EX). The microstructures and phase compositions of the alloys were studied by optical microscope, scanning electronic microscope, transmission electronic microscope, X-ray diffraction and differential thermal analysis. Mechanical properties at room temperature as well as superplastic behavior at elevated temperatures were tested.The results show that the cooling rates of the RS ribbons are between 7.32×105 and 1.65×106°C/s, between 2.03×106 and 6.59×106°C/s, and between 1.03×107 and 4.12×107°C/s respectively, when the wheel speed are 18m/s, 27m/s and 35m/s. The width and thickness of the ribbons get smaller by increasing of the wheel speed (solidification rate). Compared with the as-cast microstructure, the microstructures of RS-ZK60 and RS-66 ribbons are much finer and the grain sizes are smaller than 10μm. RS-ZK60 and RS-66 ribbons mainly consist of supersaturated a-Mg solid solution and dispersed strengthening particles of secondary phases. The primary strengthening phases of RS-ZK60 ribbon are rectangular MgZn and acerose MgZn2. RS-66 ribbon mainly consists of α, W and Mg17Ce2, and its structural stability temperature range is between 300 and 400°C.The ultimate tensile strength, yield strength and elongation of EX-RS-66 alloy extruded at 298°C with grain size of 0.7μm are 506MPa, 503MPa and 14% respectively, No extrusion texture is found. However, ultimate tensile strength and yield strength reduce, elongation increases and new extrusion texture forms when extruded at temperatures above 298°C. However, it's worth noting that EX-RS-66 alloy by forward extrusion has defects between chips, which could lead to the instability of properties of the alloy.The grain size of RE-EX-RS-ZK60 alloy extruded by the mould of Φ14mm/Φ50mm is . The ultimate tensile strength, yield strength and elongation of RE-EX-RS-ZK60 alloy extruded by 4 passes are 307MPa, 194MPa and 14.67% respectively, and the comprehensive mechanical properties got the best point. Yield strength and elongation of RE-EX-RS-ZK60 alloy extruded by the mould of Φ6.4mm/Φ20mm with grain size of ～2.5μm increase with the increasing of extrusion passes, and they were 252.7MPa and 8% respectively when extruding 6 passes. The grain sizes of RE-EX-RS-66 alloy RE by 2 and 12 passes are ～0.7μm and ～1.3μm respectively. The ultimate tensile strength and yield strength get maximum of 340MPa and 336MPa respectively by 2 passes. The maximum elongation is 27% by 4 passes. Twice yield phenomena were observed on the engineering stress-strain curve due to the characters of the unique microstructure of the alloy.Both of high-strain-rate superplasticity (HSRS) and low-temperature superplasticity (LTS) of RE-EX-CT-ZK60 and RE-EX-RS-66 alloys was achieved. An excellent superplasticity with the maximum elongation to failure of 270% were obtained at 523K and the strain rate at 3.3×10-4s-1 for RE-2-EX-CT-ZK60 alloy and at 3.3×10-3s-1 for RE-2-EX-RS-66 alloy. The grain boundary sliding (GBS) is the dominant deformation mechanism in superplastic deformation when the temperature was 523K and strain rate (?) at 3.3×10-4s-1, however, the intragranular sliding (IGS) is dominant deformation mechanism of superplastic deformation when temperature was under 523K and ε >3.3×10-4s-1. During the superplastic deformation whose dominant deformation mechanism is GBS, there also have some accommodation mechanism such as dislocation creep controlled by grain boundary diffusion, atoms diffusion of liquid phase, grains rotation and cavities movement and diffusion which assisted the completion of the whole superplastic deformation.
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