Effect of withdrawal rate on the microstructure and room temperature mechanical properties of directionally solidified NiAl–Cr(Mo)–(Hf, Dy)–4Fe alloy

• Interlamellar spacing gradually decreases with increasing the withdrawal rate.
• Higher fracture toughness can be obtained at a moderate withdrawal rate.
• Compressive yield strength is obtained at a moderate withdrawal rate even higher.
• Microstructural refinement and the addition of Fe lead to a better property.
• Crack propagation and fracture surface demonstrates the change trend of property.

The microstructures and room temperature mechanical properties of directionally solidified NiAl–Cr(Mo)–(Hf, Dy)–4Fe alloy at different withdrawal rates were investigated using SEM, EDS, TEM, three-point bending tests and compressive tests. When the withdrawal rate changes from 6 μm/s to 240 μm/s, the solid–liquid interface undergoes the transition of planar→cellular→dendritic morphology. With increasing the withdrawal rate, the interlamellar spacing decreases by degree, and the lamellar structures at the center of eutectic dendrite are well-aligned and parallel to the growth direction, thus resulting in the improvement of fracture toughness and compressive yield strength at a moderate withdrawal rate (60 μm/s). Moreover, the observation of side surface and fracture surface can roughly reveal the changed trend of fracture toughness and compressive yield strength by identifying the zigzag and coarse degree of fracture surface or the specific morphology features. The toughening and strengthening mechanisms are also discussed.