Effect of Dy on the microstructures of directionally solidified NiAl–Cr(Mo) hypereutectic alloy at different withdrawal rates

• The modest addition of Dy improves the efficiency of obtaining the full eutectic.
• Compared to 6 and 90 μm/s, the refined effect of Dy is more apparent at 30 μm/s.
• The addition of 0.1 wt.% Dy refines the intercellular zone and the lamellas.
• The Dy-containing phase can form when the Dy content is no less than 0.1 wt.%.

The effect of various Dy content on the microstructure of Ni–31Al–32Cr–6Mo hypereutectic alloy was studied at the withdrawal rates of 6, 30 and 90 μm/s. The results show that the solid–liquid interface morphology has an evolutionary process of planar → cellular → dendritic interface with the increasing withdrawal rate. The primary Cr(Mo) dendrites are gradually weeded out through competitive growth between the primary phase and the eutectic phase. The volume fraction of primary Cr(Mo) dendrites decreases with the modest addition of Dy (0.05 wt.%) at 6 μm/s. When the withdrawal rate increases to 30 μm/s, the appropriate addition of Dy (0.1 wt.%) refines the microstructure, such as the width of intercellular zone and the lamellar thickness in the intercellular zone. With the increase of withdrawal rate to 90 μm/s, the addition of Dy has no significant effect on the microstructure. In addition, the white Dy-containing phase can occur in the boundary of eutectic cells when the Dy content is no less than 0.1 wt.%.