Microstructural, Mechanical and Corrosion Properties of AZ91 Magnesium Alloy Processed by a Severe Plastic Deformation Method of Hydrostatic Cyclic Expansion Extrusion

This research investigates the microstructural evolution, mechanical properties, and corrosion behavior of AZ91 magnesium alloy after the process of hydrostatic cyclic expansion extrusion (HCEE) at elevated temperature. The HCEE process is able to produce long length ultrafine-grained rods by applying high hydrostatic pressure. The results showed that ultrafine-grained microstructure appears after the consecutive passes of the process, which led to the higher hardness and strength with an increased elongation to failure in the processed rods at room temperature. The ultimate tensile strength and elongation to failure of the processed rod after two cycles of the HCEE increased more than 2 and 2.6 times, respectively. Moreover, an increase in hardness was more than 100% after the second cycle of the process and its distribution was more uniform. Furthermore, the ultrafine-grained microstructure after the HCEE resulted in the movement of potentiodynamic polarization derived curves to higher values of corrosion potential (E-corr) and lower current density (I-corr), which shows the capability of the HCEE process in improving the corrosion resistance of AZ91 magnesium alloy rods. These increases in corrosion resistance were further indicated by the Nyquist diagram derived from the electrochemical impedance spectroscopy scanning and evolved hydrogen amount after 208 h of immersion in 3.5% NaCl solution. The novel HCEE process shows further its capability in producing long length ultrafine-grained rods with superior mechanical and corrosion properties rather than other severe plastic deformation techniques.