Microstructural characterization, formation mechanism and fracture behavior of the needle 6 phase in Fe-Ni-Cr type superalloys with high Nb content

Microstructural characterization, formation mechanism and fracture behavior of the needle delta 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 delta phases with the stick, mixed and needle shapes obviously present in Inconel 718 after the isothermal upsetting at the temperature of 980-1060 degrees C with the initial strain rate of 10(-3)-10(-1) s(-1). It is found that the shape of the delta phase has a great effect on the mechanical properties of the alloy, viz., the stick delta phase behaves good plasticity and the needle delta phase has good strength. In addition, the needle delta 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 delta phase. Both dislocation motion and atom diffusion are the root-cause for the needle delta phase to be firstly separated at grain boundary and then at sub-boundary. The formation mechanism of the needle delta phase is the new finding in this research. Furthermore, it is the primary mechanism for controlling the needle delta phase in Fe-Ni-Cr type superalloys with high Nb content. (C) 2015 Elsevier Inc. All rights reserved.