Fatigue softening mechanisms in UFG-EUROFER97 steel. A microstructural study

A novel Thermo-Mechanical Treatment (TMT) developed by us leads to a microstructure with Ultra-Fine Grains (UFG) resulting in improved mechanical properties compared to those of standard EUROFER97 steel, foreseen for nuclear fusion applications. Since softening was observed after High Cycle Fatigue (HCF) tests at room temperature, this work investigated the specific microstructural mechanisms. The microstructure evolution of fatigued samples has been studied by means of Electron Back-Scattered Diffraction (EBSD), X-Ray Diffraction (XRD), Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM). Softening resulted to be originated by the phenomenon of fatigue-assisted grain coarsening. HCF always leads to the increase of grain and sub-grain size together with texture change, and such modifications depend on the stress level. Under cyclic stress, boundary migration and absorption in the boundaries of dislocations produced by Frank-Read sources inside the grains lead to an excess of dislocations in the boundaries. Although the stability and mobility of High-Angle Grain Boundaries (HAGBs) and Low-Angle Grain Boundaries (LAGBs) are totally different, both the boundaries become unstable. In the final step of the process, the collapse or sliding of unstable HAGBs and LAGBs give rise to a population of grains and sub-grains of larger size. The novel TMT demonstrates improved softening behavior compared to standard EUROFER97, and the results obtained will guide future research toward further improvements.