An Electron Microscopy Investigation on as-Cast Az91D Alloy Modified with Nitrogen Addition

An attempt has been made to modify the microstructure of the most widely used magnesium alloy, AZ91D, via introducing nitrogen into the alloy in molten state. The microstructures of the samples before and after nitrogen addition were compared by using scanning and trans mission electron microscopy examinations. It was observed that despite the near-zero solubility of nitrogen in magnesium the microstructure of the small ingots cast after purging the molted alloy with nitrogen showed considerable modifications as compared to the control sample cast before purging operation. Nitrogen addition appeared to lead to the elimination of the classical lamellar morphology from the microstructure. Furthermore, the divorced eutectic structure commonly observed in AZ91 was greatly promoted in the modified microstructure leading to the formation of much more α (a solid solution of Mg and Al) particles in grainboundary coarse β (Al₁₇Mg₁₂) as compared to the unmodified structure. Such modifications may be expected to bring about considerable changes in mechanical behavior, or create anopportunity to assess the individual effect of the lamellar structure on corrosion andmechanical behavior of AZ91 alloy.