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

The mechanical behavior of cast nickel-based superalloy Inconel 718 was investigated via hot tensile experiments at 650 degrees C, 800 degrees C, and 950 degrees 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 degrees 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 degrees C. In contrast, surficial microporosity and grain boundary weakening caused early brittle rupture, before the material yielded, at 800 degrees 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 degrees C. Microstructural evidence displayed the interface debonding between the carbide and the matrix at 950 degrees C, indicating the temperature limitation of the carbide pileup. (C) 2018 Published by Elsevier B.V.