Deformation Mechanism and Fracture Behavior of a Coarse-Grain Ni-Co-Based Superalloy During Superplasticity

The advent of coarse-grain superplasticity has provided a pathway for novel applications in material forming. This article investigated the underlying deformation mechanisms that enabled achieving superplastic elongation exceeding 230% in a coarse-grained Ni-Co-based superalloy. The deformed microstructure and fractographic characteristics of the alloy were examined utilizing optical microscopy (OM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). The results of the analysis revealed that below 1100 °C, the process of dynamic recrystallization (DRX) occurred at a sluggish rate, resulting in low plasticity and the initiation of severe cracks. Complete DRX occurred when the deformation temperature exceeded 1100 °C, leading to a more uniformly deformed microstructure, reduced crack initiation, and enhanced ductility demonstrated by elongation to failure surpassing 230%. The augmented occurrence of the DRX facilitated prolonged plastic-forming periods, which delayed fracture propagation and promoted the deformation flow within the alloy, thereby transitioning the fracture behavior from intergranular-brittle at 1050 °C to ductile intergranular at 1140 °C. At this temperature, the deformation was predominantly governed by the discontinuous-DRX (DDRX) mechanism and grain growth, facilitated by the formation of twin boundaries.