Formation process of unique microstructure in rapidly solidified Mg97Zn1Y2 alloy

The formation process of 6H-phase in rapidly solidified powder metallurgy (RS P/M) Mg97Zn1Y2 alloy has been investigated by transmission electron microscopy (TEM) observations. The initial microstructure of RS powder consists of equiaxed grains about 1 μm in average diameter which is surrounded by Y and Zn rich amorphous grain boundaries. The 6H-phase was observed at grain boundaries in the RS ribbon prepared at circumferential speed of 42 m/s. By lowering the circumferential speed, the 6H-phase at grain boundary in the RS ribbon changes to amorphous phase. The amount of Y and Zn segregation at the grain boundary in RS Mg97Zn1Y2 alloy increases with decreasing the cooling rate. In other words, the amorphization of grain boundary phase in RS alloy was promoted by the enrichment of Y and Zn. The growth of 6H-phase starts from grain boundary in the RS ribbon annealed at 573 K for 1.2 ks. It can be concluded that the 6H-phase in RS P/M Mg97Zn1Y2 alloy crystallizes in the Y and Zn rich amorphous phase at grain boundary and then grows into grain interior during extrusion. In order to clarify the stability of 6H-phase with heat treatment, microstructure changes in RS ribbon are also described.