Microstructure evolution and room temperature fracture toughness of directionally solidified NiAl-31Cr3Mo-0.2Si near-eutectic alloy at different withdrawal rates

The microstructure evolution and room temperature fracture toughness of directionally solidified NiAl-31Cr3Mo-0.2Si (at%) near-eutectic alloy at different withdrawal rates were investigated. The results showed that NiAl dendrites were eliminated and fully eutectic structure was achieved by applying appropriate withdrawal rates. The microstructure evolved from planar eutectic to primary NiAl dendrites + cellular eutectic and then to dendritic eutectic with the increasing withdrawal rate. Correspondingly, the interlamellar spacing and cell size decreased gradually. The relationship between eutectic interlamellar spacing with withdrawal rate can be summarized as λ=4.74V−0.38. The fracture toughness of alloys with fully eutectic structure was much higher than that of alloys with NiAl dendrites. It increased up to 2.5 times. The high fracture toughness may be attributed to some toughening mechanisms, such as crack bridging, crack deflection, interface debonding and microcrack linkage. The existence of primary dendrites would seriously deteriorate the fracture toughness.