1Department of materials science and engineering, Shiraz University, Shiraz, Iran
2Department of Materials Science and Metallurgical Engineering EEBE, Universidad Politécnica de Catalunya, Barcelona, Spain
چکیده
The combined effect of grain refinement through severe plastic deformation (SPD) and alloying elements usually plays a critical role in the development of properties in Mg alloys. Among different compositions, rare earth elements (Y, Gd, and Nd) alongside Zn affect Mg properties significantly. Here, a novel dilute as extruded Mg-Zn-Gd-Y-Nd alloy was subjected to 5 passes of equal channel angular pressing (ECAP) as one of the most recognized methods to prepare bulk ultrafine-grain (UFG) materials, in a die having a channel angle of 90° following route Bc. The initial deformation temperature was 300°C decreasing with a 25°C step to 200°C up to the fifth pass. Hardness measurement, optical and scanning electron microscope as well as corrosion tests were done on initial and deformed samples. Results reveal that necklaces of fine recrystallized grains along grain boundaries of large unrecrystallized grains in as extruded samples converted to an ultrafine grained microstructure after the fifth pass of ECAP at 200°C. SEM observations, confirm the presence of very fine nanoscale dynamic recrystallization (DRX) nuclei in the context of ECAPed alloy. Additionally, hardness measurements depict a 125% increase in the hardness value from the initial state to the fifth pass of ECAP. The increase in hardness was due to dynamic recrystallization producing a higher percentage of newly formed grains as well as a higher percentage of grain boundaries. Moreover, the presence of rare earth elements improved grain refinement during ECAP and controlled dynamic recrystallization (DRX) rate. On the other hand, grain refinements lead to changes in the density and distribution of grain boundaries which influence the corrosion behavior of Mg alloys. Here, extrusion, as well as ECAP, decreased the grain size in the studied alloy which increases the corrosion rate in the case of ECAPed sample compared to the as-cast condition.
