| Magnesium alloys, as "21st century green metallic structural materials", have the most promising development in material filed, for their high specific strength and specific stiffness, good damping capacity, excellent machinability and high recycling rate. The principal focus on enhancing integrated properties of magnesium alloys is to satisfy the requirements of some structural materials, such as aluminum, or even steel. Researches which aim to increase the elongation, decrease the anisotropy and improve the plastic deformation capability, simplify fabrication techniques, reduce manufacturing cost and expand the application of magnesium alloy also receive global attention.It is well known that the mechanical properties increase rapidly with refining on grain size for magnesium alloys. Centrifugal atomization, which can effectively refine grain and second-phase size, reduce segregation, and expand solid solubility of matrix, was initially employed to prepare ZK60 and AZ91 alloy powders.Combination of powder characteristics and powder extrusion simulation results, this paper proposed a high cost-effective process to promote wide application of rapid solidified/powder metallurgy (RS/PM) magnesium alloys. Finally, ultrafine grained ZK60 and AZ91 bars were successfully prepared.Based on laboratory scale, the effect of extrusion temperature on properties (such as strength, toughness and corrosion resistance, etc) were investigated. The refinement mechanism, thermal stability, oxidation particles and strengthening mechanism were also discussed in detail.Spherical-shaped ZK60 and AZ91 powders with smooth surface were prepared by centrifugal atomization technology. The average cooling rate was calculated to be 104～105K/s. These powders essentially exhibit an equiaxed grain morphology with fine intermetallic compounds along grain boundaries. Due to higher cooling rate, the grain size of RS powders is much smaller than that of conventional cast alloys; second phase particles distribute more uniformly; and quenching effect keep supersaturated solid solution state in matrix. In other ways, the native oxide layers of RS powders are composed of 2nm MgO film and 0.5nm contamination film.When exposure to air at 100～200℃for short time, the increase in oxygen content can be negligible. Therefore, powder compacts with relative density of 0.8～0.9 can readily prepared in double compression for 5-10min at low temperature, without aid of canning, vacuum degassing and other steps.The die structure and extrusion process parameters were optimized by FEM simulation. The result shows that there are similar densification behaviors to powders from deformation area and necking region of concave die. However, the former induces severe plastic deformation of powders, while the latter generates complete welding between powders. In order to keep a relatively uniform distribution and maintain high level in effective plastic strain, the entrance angle of die and the radius of transition angle are suggested to choose 60°and 2mm, respectively. Moreover, the normal friction is benefit to obtain the above function. The surface defects could be found under high extrusion speed(≥10mm/s) or higher extrusion temperature. Besides, the initial relative density of powder compacts shows less significant influence on extruded bars.The grain refinement is significant by simplified RS/PM process, and the mean grain size of ZK60 and AZ91 alloys can be refined to 0.3-2.1μm with different extrusion temperature. To ZK60 alloys, the tensile strength and elongation are 348-424MPa and 9-22%,while AZ91 alloys show higher strength (363-516MPa) due to more dispersion of second phase particles inhibiting grain movement. With strain increasing, interparticle mechanical interlock combined with mass transfer across the boundaries, develops a good adhesive bonding. The nano-oxide particles, broken from oxide layer of powder, locate at welding position, which enhances the strength of alloys further. During extrusion, the refining mechanism is dominated by continuous dynamic recrystallization (CDRX) and assisted by rotation dynamic recrystallization (RDRX). For the second phase, it is mechanical cracking refining mechanism. The thermal stability of ultrafine ZK60 alloy is closely related with the extrusion temperature. When annealing temperature lower than extrusion temperature, recovery and recrystallization happen, but no grain coarsens. In present work, the strengthening mechanisms of RS/PM ZK60 and AZ91 alloys are originated from overlapping effect of grain refinement, texture and second phase strengthening.Through immersion corrosion tests and electrochemical tests, the corrosion properties of ultra-fine grained ZK60 and AZ91 alloys were researched for the first time. During both tests in NaCl solution, the mean corrosion rate of RS/PM alloys is higher than that of cast AZ91D alloy. The reasons is attribute to the increase of boundary fraction, internal energy and defects in material after RS/PM process, all of which would result in high chemical activity throughout the surface. Moreover, because of abundant second phase particles discontinuously pinning grain boundaries in RS/PM AZ91 alloys, which introduces serious micro-cell corrosion, the worst corrosion resistance was found in RS/PM AZ91 alloys extruded at 210℃. However, as compared with the disadvantage of higher corrosion rate, the uniform corrosion is the most prominent advantage for corrosion properties of RS/PM magnesium alloys.
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