Microstructural characterization, formation mechanism and fracture behavior of the needle δ phase in Fe-Ni-Cr type superalloys with high Nb content

Microstructural characterization, formation mechanism and fracture behavior of the needle δ phase in Fe-Ni-Cr type superalloys with high Nb content (GH4169, equivalent to Inconel 718) have been quantitatively investigated in this research. The typical microstructures of δ phases with the stick, mixed and needle shapes obviously present in Inconel 718 after the isothermal upsetting at the temperature of 980-1060 °C with the initial strain rate of 10^{- 3}-10^{- 1} s^{- 1}. It is found that the shape of the δ phase has a great effect on the mechanical properties of the alloy, viz., the stick δ phase behaves good plasticity and the needle δ phase has good strength. In addition, the needle δ phase can be used to control the grain size as it can prevent grain growth. The combined effect of the localized necking and microvoid coalescence leads to the final ductile fracture of the GH4169 components with the needle δ phase. Both dislocation motion and atom diffusion are the root-cause for the needle δ phase to be firstly separated at grain boundary and then at sub-boundary. The formation mechanism of the needle δ phase is the new finding in this research. Furthermore, it is the primary mechanism for controlling the needle δ phase in Fe-Ni-Cr type superalloys with high Nb content.