Microstructure and mechanical property of directionally solidified NiAl–Cr(Mo)–(Hf, Dy) alloy at different withdrawal rates

The microstructure and mechanical property of directionally solidified Ni–30.95Al–32Cr–6Mo–0.05Hf (at%)–0.1Dy (wt%) alloy were investigated by means of OM, SEM, TEM, EPMA, three-point bending tests and high-temperature tensile tests. With the withdrawal rate increasing from 6 to 120 μm/s, the microstructure changes from planar eutectic to cellular or dendritic eutectic, and both the interlamellar spacing and eutectic cell size decrease gradually. The relation between interlamellar spacing and withdrawal rate can be obtained as λ ¯ = 4.55 V − 0.40 . The corresponding room temperature fracture toughness first increases and then decreases, which should be attributed to the microstructural evolution at different withdrawal rates. Moreover, some toughness mechanisms are responsible for the improvement of fracture toughness, such as crack bridging, interface debonding and microcrack linkage. The high-temperature tensile strength is low at the planar growth rate (10 μm/s), and it increases gradually with the increase of withdrawal rate. The strengthening mechanism is also discussed.