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In this study, the synergistic effects of Y and Nd on the grain refinement of the as-cast Mg-Y-Nd-Al alloy was investigated by varying Y and Nd contents while fixing their total content (Y + Nd) at 7%. When the contents of Y and Nd were 3% and 4%, respectively, the refinement was most effective and the smallest grain size of 49±5 μm was achieved. The grain refinement process was primarily controlled by the heterogeneous nucleation and the growth restriction factor (Q value). Y could only form Al2RE phase with Al, while both Al2RE and Al11RE3 were formed between Nb and Al. The Al2RE phase acted as nucleation particles of α-Mg and facilitated heterogeneous nucleation; however, Al11RE3 was an unfavorable phase for heterogeneous nucleation. When the Y content was less than 3%, the quantity of Al2RE phase increased when more Y was available, which resulted in more active nucleation particles and better refinement. When the Y content exceeded 3%, the Q value (growth limiting factor) of the alloy decreased, while the quantity of Al2RE phase remained unchanged. This would lead to a decreased number of active nucleation particles and a less effective grain refinement.

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In order to explore the microstructure evolution of an Mg-RE alloy refined by Al during solution treatment, an Mg-3Y-4Nd-2Al alloy was treated at 545 °C for different time periods. Phase evolution of the alloy was investigated. After solution treatment, the Mg-RE eutectic phase in the Mg-3Y-4Nd-2Al alloy dissolves, the granular Al2RE phase does not change, the acicular Al11RE3 phase breaks into the short rod-like Al2RE phase, and the lamellar Al2RE phase precipitates in the grains. With the extension of solution time, the precipitated phase of the lamellar Al2RE increased at first and then decreased, and its orientation relationship with the matrix is <112>Al2RE//<21¯1¯0>Mg and {111}Al2RE//{0002}Mg. The undissolved granular Al2RE phase can improve the thermal stability of the alloy grain by pinning the grain boundary, and the grain size did not change after solution treatment. Solution treatment significantly improved the plasticity of the alloy. After 48 h of solution treatment, the elongation increased to 17.5% from 8.5% in the as-cast state.