Abstract
The microstructure, corrosion resistance, and stress corrosion cracking behavior of an as-cast and solid-solution Mg-1.8Zn-0.5Zr-1.5Gd biomagnesium alloy after extrusion deformation were studied. Results show that when the extrusion ratio was 7.7 and the extrusion temperatures were 350 and 360 °C, the as-cast and solid-solution alloys underwent complete dynamic recrystallization, and the grains were significantly refined. The precipitated phase in the extruded alloy is mainly composed of a nano-scale (Mg, Zn)₃Gd and an Mg₂Zn₁₁ phase. A small amount of undissolved and broken micron-sized (Mg, Zn)₃Gd phase particles appeared in the as-cast extruded alloy. The electrochemical corrosion produced during the stress corrosion process peeled off the (Mg, Zn)₃Gd phase particles and induced rapid dissolution of the adjacent matrix. As a result, stresses were accumulated, and stress corrosion sensitivity occurred. At the same time, some larger-sized cracks appeared in the tensile fracture of the alloy. The difference in grain size and orientation in different regions of the solid-solution extruded alloy leads corrosion fracture to be composed of parallel grooves in different directions. Solution treatment of the magnesium alloy was found to reduce the stress corrosion susceptibility of extruded magnesium alloy and make it promising as a biodegradable implanting material.

Key words
magnesium alloy, extrusion, microstructure, stress corrosion cracking

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