Fatigue behavior of nanostructured bainite

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Nanostructured bainite, in its first stages of its industrialization, is one promising advance steel because of its simplicity in terms of alloy design and processing, and its outstanding mechanical properties, having achieved the highest strength-toughness combinations ever recorded in bainitic steels (2.2GPa-30MPa·m1/2). Before the potentialapplication of this alloy in advance components such as bearing, gears or railway systems, the study of mechanical fatigue, i.e., material´s response to cyclic loading is essential. There are several reasons motivating a detailed study of the effect of the microstructure on the fatigue response in nanostructured bainite. First of all, monotonic properties fail to explain the fatigue performance, which is not improved by increasing the strength and the ductility. Second, a conventional approach to predict the lifetime based on S-N' curves (stress v. number of cycles) does not make distinctions between cracknucleation and propagation, which may lead to dangerous overestimates of life. Finally, and likewise, at the short crack regime, the crack growth rate is not explained by the Linear Elastic Fracture Mechanics (LEFM), i.e., the propagation speed may be higher thanthat of long cracks. It is before crack nucleation and during short crack regimes when the local microstructure controls the fatigue response. So far a global effective grain size has not been found. Morphological parameters associated to the bainite block might be controlling the intrinsic fatigue limit of these microstructures, but the question aboutthe effect of the microstructure on the fatigue life at different stress levels remains unsolved. The relationship between the crack deflection at certain grain boundaries and the crack arrest, which can contribute to enhance the fatigue life, has not been established either. The role of the second phase, the retained austenite, remains alsounknown. This project is intended to gain new knowledge on the nature of the bainitic structure, i.e., grain boundaries, mechanically induced phase transformation and defects evolution before the crack nucleation and along the crack path. The deformation and damage mechanisms will be treated in combination with the cyclic plastic hardening/softening behavior and correlated with the different stages of the fatigue crack growth. For that purpose, both advanced experimental techniques based on microscopy and diffraction and theoretical approaches will be applied and developed. It will enable the assessment of the relative importance that this evolving microstructure has on the fatigue life. As a result, an improved fatigue design in terms of safety will be possible.