Role of void nucleation at primary-γ'/γ interface on strain softening of nickel base superalloy 720Li

High-temperature tensile deformation behavior of polycrystalline nickel based superalloy 720Li used for manufacturing aero engine disks was studied by conducting uniaxial tensile tests in the temperature range of 25–800 °C and subsequent transmission and scanning electron microscopic examinations to decipher strain hardening/softening behavior. The alloy is strengthened by different sizes of γ'-precipitates named as primary, secondary, and tertiary-γ'. The sizes of primary γ' is in the range of 0.5–3 µm and most of them are incoherent with γ-matrix as confirmed by the TEM study. From engineering stress-strain curves, it was observed that specimens tested in the temperature range 25–650 °C exhibited a significant amount of strain hardening, whereas specimens tested at 720 and 800 °C softened after the onset of plastic yielding. From extensive SEM examinations, it is concluded that void nucleation at primary-γ'/γ interface at 720 and 800 °C was the main reason for strain softening after the onset of plastic yielding. Nucleation of voids at primary-γ'/γ interface is ascribed to (i) γ' size, which loses coherency with the matrix after reaching a critical value (rcrit); and (ii) difference in Schmid factor between γ' precipitate and γ matrix, which varies with test temperature. From the misfit data, the rcrit values were calculated at 25 and 800 °C and are found to be in the range of 1250–1500 nm and 150–200 nm, respectively. From the electron backscatter diffraction (EBSD) analysis, it is confirmed that at 720 and 800 °C when the difference in Schmid factors between γ' precipitate and γ-matrix is ≥ 0.1, the probability of formation of the void is high.