Grain nucleation and formation in Mg-Zr alloys

Zirconium is the most potent nucleant discovered to date for magnesium. This work reports the grain nucleation and formation characteristics in zirconium-inoculated magnesium melts. It was found that nucleation of magnesium grains occurred on two types of zirconium nucleants, where the first type were small and round, suggesting that they were formed by precipitation, and the other type were larger and irregularly shaped, similar to undissolved particles. Subsequent growth of the magnesium halo structures after nucleation could occur either spherically or dendritically under similar cooling conditions but under well-inoculated conditions they grew spherically in general. Hence, achieving a high level of dissolved Zr content is not only essential to the nucleation process but also important for control of the subsequent growth morphologies of the halo structures. The nucleation process can be understood by an adsorption mechanism.