The influence of microstructure on the cyclic deformation and damage of copper and an oxide dispersion strengthened steel studied via in-situ micro-beam bending

Service materials are often designed for strength, ductility, or toughness, but neglect the effects of cyclic time-variable loads ultimately leading to macroscopic mechanical failure. Fatigue originates as local plasticity that can first only be observed on the micro scale at defects serving as stress concentrators such as inclusions or grain boundaries. Thus, a recently developed technique to perform in-situ observation of micro scale bending fatigue experiments was applied. Micro-beams fabricated from copper, single grained and ultrafine grained (ufg), and an oxide dispersion strengthened (ODS) steel were subject to cyclic deformation and subsequent damage. The elastic stiffness, yield strength, dissipated energy, and maximum stress were measured as a function of cycle number and plastic strain amplitude. From these properties, cyclic stress-strain curves were developed. Initial pronounced monotonic hardening and an increasing Bauschinger effect were observed in all samples with increasing strain amplitude. Cyclic stability was maintained until plastic strain amplitudes reached a critical value. At this point, dramatic cyclic softening and microcracking occurred. The critical strain amplitude was found to be approximately 5.4×10⁻³ for the copper with a refined grain structure and 1.2×10⁻² for the steel specimen. Grain rotation and noticeable changes in sub-grain structure were evident in the ufg copper after a critical strain amplitude of ε_a=8.3×10⁻³. In-situ micro fatigue bending couples the cyclic evolution of bulk mechanical properties measurements with real-time electron microscopy analysis techniques of damage and failure mechanisms, which renders it a powerful method for developing novel fatigue resistant materials.