Influence of Hot Extrusion on the Microstructure, Bio-Corrosion and SCC Resistance of a Cast Mg-2.5Gd-0.5Zr Alloy

Effects of hot extrusion on the microstructure, shear strength, bio-corrosion and stress corrosion cracking (SCC) behavior of the as-cast Mg-2.5Gd-0.5Zr alloy were studied. The 150-µm coarse-grain size of the as-cast material was refined to a fine recrystallized grain structure having a grain size of 3 µm. Compared with the as-cast condition, the extruded alloy exhibited higher strength and lower ductility after slow strain rate shear testing in the air and inside the corrosive solution. The higher strength stemmed from grain boundary strengthening, while lower ductility was explained by a more difficult dislocation movement inside the fine-grained structure of the extruded alloy. SCC tests in the corrosive medium showed decreased strength and ductility due to the deteriorating effects of the corrosive environment and also the weak adhesion of the corrosion layer to the Mg substrate. Electrochemical testing indicated that the extruded alloy possesses higher corrosion resistance. This was ascribed to the fine grains that act as obstacles against Mg matrix dissolution and enhanced corrosion layer adhesion to the substrate as well as the absence of coarse second-phase particles, which are susceptible regions to promote the galvanic corrosion mechanism.