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

Abstract

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 degrees C and subsequent transmission and scanning electron microscopic examinations to decipher strain hardening/softening behavior. The alloy is strengthened by different sizes of gamma-precipitates named primary, secondary, and tertiary-gamma'. The sizes of primary gamma' is in the range of 0.5-3 mu m and most of them are incoherent with gamma-matrix as confirmed by the TEM study. From engineering stress-strain curves, it was observed that specimens tested in the temperature range of 25-650 degrees C exhibited a significant amount of strain hardening, whereas specimens tested at 720 and 800 degrees C softened after the onset of plastic yielding. From extensive SEM examinations, it is concluded that void nucleation at the primary gamma/gamma interface at 720 and 800 degrees C was the main reason for strain softening after the onset of plastic yielding. Nucleation of voids at primary-gamma'/ gamma interface is ascribed to (i) gamma' size, which loses coherency with the matrix after reaching a critical value (rcrit); and (ii) difference in Schmid factor between gamma' precipitate and gamma matrix, which varies with test temperature. From the misfit data, the rcrit values were calculated at 25 and 800 degrees 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 degrees C when the difference in Schmid factors between gamma' precipitate and gamma-matrix is & 0.1, the probability of formation of the void is high.(c) 2023 Elsevier B.V. All rights reserved.