Structure and decomposition behaviour of rapidly solidified MgNdX (X Al, Si) Alloys

The microstructural characteristics and decomposition behaviour of rapidly solidified MgNdX (X Al, Si) alloys were investigated by detailed transmission electron microscopy. Rapid solidification of Mg8Nd, Mg8Nd1Al and Mg8Nd1Si produces various types of solidification microstructure depending on the alloy composition, i.e. fine precipitates of Mg₃Nd in as-melt-spun Mg8Nd, f.c.c. phase (a = 14.8 Å) at the cell boundaries in as-melt-spun Mg8Nd1Al, and supersaturated α-mg grains with fine particles at the grain boundaries in as-melt-spun Mg8Nd1Si. Annealing of as-rapidly solidified Mg8Nd and Mg8Nd1Si up to 300 °C results in precipitation and coarsening of the Mg₃Nd phase. However, no significant change occurs in Mg8Nd1Al on annealing up to 300 °C. On annealing at 400 °C, the pre-existing phase decomposes forming new phases stable at 400 °C, i.e. an f.c.c. phase (a = 5.4 A ̊) in Mg8Nd, an f.c.c. phase a = A ̊, Al₂Nd in Mg8Nd1Al, and Mg₉Nd in Mg8Nd1Si. The present study shows that of the three alloys investigated, Mg8Nd1Al can have the highest thermal stability owing to the fine, thermally stable Al₂Nd particles which appear on annealing at the higher temperature of 400 °C.