Effect of microstructures on the room temperature fracture toughness of NiAl–32Cr–6Mo hypereutectic alloy directionally solidified at different withdrawal rates

The effect of microstructures on the room temperature fracture toughness of NiAl–32Cr–6Mo (at%) hypereutectic alloy was investigated. The solidification microstructure changed from planar eutectic to cellular eutectic and dendritic eutectic with increasing withdrawal rate. The fracture toughness of alloy with planar eutectic microstructure solidified at 10 μm s−1 was 23.74 MPa m1/2, and then it dropped to 15.35 MPa m1/2 when the alloy solidified at rate of 15 μm s−1. But higher fracture toughness of 22.92 MPa m1/2 was obtained when the alloy solidified at 25 μm s−1 and had a perfect cellular microstructure. In this case, the production efficiency can be markedly improved. The fracture surfaces of the NiAl–32Cr–6Mo hypereutectic alloy showed quasi-cleavage fracture mode, some cleavage steps and tearing ridges were observed. The alloys with cellular and dendritic eutectic microstructures exhibited transcellular fracture morphologies; the conically shaped ridges or valleys were observed in each eutectic cell. Owing to the perfect cellular microstructure, the bonding strength of the cell boundary was high; the eutectic cells could largely deform consistently and offer higher resistance of the crack propagation.