Temperature-dependent deformation mechanisms and microstructural degradation of a polycrystalline nickel-based superalloy

Abstract

The mechanical behavior of cast nickel-based superalloy Inconel 718 was investigated via hot tensile experiments at 650 °C, 800 °C, and 950 °C to understand the thermal instability of its metallurgic microstructure. Test specimens, which were machined from investment casting, were subjected to a standard heat treatment. Post-tensile testing, microstructure characterization, and fractography observations were conducted with a scanning electron microscope (SEM) and a transmission electron microscope (TEM). The resulting data revealed that the deterioration of toughness and ductility occurred at temperatures above 650 °C. Numerous shear bands and lattice mismatch suggested that plastic deformation through slip movement dominated the ductile fracture mode of the nickel-based superalloy at 650 °C. In contrast, surficial microporosity and grain boundary weakening caused early brittle rupture, before the material yielded, at 800 °C. Significant strain accumulation within grains contributed to plastic flow in the direction of the applied stress load, resulting in intergranular and transgranular fracture at 950 °C. Microstructural evidence displayed the interface debonding between the carbide and the matrix at 950 °C, indicating the temperature limitation of the carbide pileup.