Some of the rare earth metals (RE) are known for a long time as favorable additives in Mg alloys for improvement of their strength. There is significant difference between effects of separate rare earth metals on strength of magnesium connected with regular change of the respective phase diagrams. Regularities of the phase diagrams of separate rare earth metals with any other metal were shown in many works. However, there is a specific feature of Mg systems. It consists of existence of solubility of RE in solid Mg which changes regularly with increasing the RE atomic number. This regularity is caused by favorable differences between atomic radii of separate RE and Mg. The higher solubility corresponds to less difference between atomic radii of RE and Mg. Nevertheless, solubility of RE of the yttrium subgroup in solid Mg is significantly more than that of RE of the cerium subgroup suggesting different electron interaction between Mg and RE of different subgroups when Mg solid solutions are formed. Decrease of the RE solubility in solid Mg with lowering temperature creates a possibility of Mg supersaturated solid solution decomposition accompanied by strengthening effect. Behavior of Mg alloys with RE of different subgroups during this process are significantly different. Solubility of RE in solid Mg and Mg supersaturated solid solution decomposition are responsible for high strength properties of Mg alloys with RE. With increasing solubility of RE in solid Mg the strength properties increase, but up to the first elements of yttrium subgroup (Gd, Tb). Although the separate RE are used as alloying elements in Mg alloys for a long time there are possibilities to develop new alloys with better strength. This can be reached by alloying Mg with RE which were not used so far and by addition of RE of yttrium and cerium subgroups together in certain ratios.

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