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The literature on grain refinement of magnesium alloys is reviewed with regard to two broad groups of alloys: alloys that contain aluminum and alloys that do not contain aluminum. The alloys that are free of aluminum are generally very well refined by Zr master alloys. On the other hand, the understanding of grain refinement in aluminum bearing alloys is poor and in many cases confusing probably due to the interaction between impurity elements and aluminum in affecting the potency of nucleant particles. A grain refinement model that was developed for aluminum alloys is presented, which takes into account both alloy chemistry and nucleant particle potency. This model is applied to experimental data for a range of magnesium alloys. It is shown that by using this analytical approach, new information on the refinement of magnesium alloys is obtained as well as providing a method of characterizing the effectiveness of new refiners. The new information revealed by the model has identified new directions for further research. Future research needs to focus on gaining a better understanding of the detailed mechanisms by which refinement occurs and gathering data to improve our ability to predict grain refinement for particular combinations of alloy and impurity chemistry and nucleant particles.

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Despite the extensive literature on grain refinement, there is not a consensus on the mechanism of grain refinement in aluminum alloys. Recently, there has been a shift in understanding of the grainrefinement paradigm from purely being concerned with the nucleation event, called here the "nucleant paradigm," to also being concerned with the effect of solute elements, or, the "solute paradigm," on the final grain structure. This article is divided into two parts. In Part I, the literature underpinning both paradigms is explained, and the validity of the paradigm shift toward the solute paradigm as a more complete understanding of grain refinement is presented. Part II experimentally confirms the validity of the solute paradigm and details a mechanism which explains the need for both effective nucleants and a solute of a good segregating power in order to obtain grain refinement.

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