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

In the present work, strain-controlled low cycle fatigue (LCF) tests were performed at 704 degrees C using 0.25 Hz in cyclic frequency with tension-compression loading on the forged alloy 718Plus to investigate the fatigue per-formances 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 eta precipitates in grain interior and promoted the growth of grains and eta precipitates, but brought insignificant change on the morphology of spherical gamma ' phase and slightly increase in particle size and gamma '/gamma lattice misfit, indicating its excellent mechanical stability at 704 degrees C. A transition area was observed between the eta precipitates and gamma matrix, which was determined to be gamma 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 eta precipitates during the fatigue process, inducing the initiation and propagation of micro-cracks. As the cycle accumulated, the eta and gamma ' precipitates were sheared by gliding dislocations, resulting in weakened resistance to deformation and fatigue softening.