On the microstructure evolution during low cycle fatigue deformation of wrought ATI 718Plus alloy

Abstract

In the present work, strain-controlled low cycle fatigue (LCF) tests were performed at 704 °C using 0.25 Hz in cyclic frequency with tension-compression loading on the forged alloy 718Plus to investigate the fatigue performances determined by structures, and the corresponding microstructure evolutions and deformation mechanisms were analyzed through techniques of scanning electron microscope (SEM) and transmission electron microscope (TEM). The fatigue specimens from different locations (core and rim) showed similar response on cycle stress, but smaller grain size contributed to superior fatigue performance. Fatigue deformation induced the generation of η precipitates in grain interior and promoted the growth of grains and η precipitates, but brought insignificant change on the morphology of spherical γ′ phase and slightly increase in particle size and γ′/γ lattice misfit, indicating its excellent mechanical stability at 704 °C. A transition area was observed between the η precipitates and γ matrix, which was determined to be γ phase via high resolution transmission electron microscopy (HRTEM) and elements mapping techniques. Twinning and dislocation planar slip bands were confirmed to be the primary fatigue deformation modes of alloy 718Plus. Dislocations piled up and formed networks at the edges of η precipitates during the fatigue process, inducing the initiation and propagation of micro-cracks. As the cycle accumulated, the η and γ′ precipitates were sheared by gliding dislocations, resulting in weakened resistance to deformation and fatigue softening.