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

Abstract

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 x 10 -3 for the copper with a refined grain structure and 1.2 x 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 x 10 -3 . In-situ micro fatigue bending couples the cyclic evolution of bulk mechanical properties measurements with realtime electron microscopy analysis techniques of damage and failure mechanisms, which renders it a powerful method for developing novel fatigue resistant materials.

Revealing the local fatigue behavior of bimodal copper laminates by micropillar fatigue tests

“…This behavior can be attributed to the presence of phosphorus atoms inside the sample, which pin the grain boundaries and effectively prevent softening via grain coarsening at these stress levels. At higher stress amplitudes (> 250 MPa), the samples transition to cyclic softening, which likely results from cyclic deformation induced grain coarsening, as often observed in fatigued ultrafine grained copper specimens [33,34]. Above 300 MPa, cyclic softening weakens the sample to such an extent that it suddenly collapses during testing, which translates into a spike in plastic strain ratio, see Figure 5a.…”

Gaining sight after being blind: A tribute to Jing Zhu

“…A question to be addressed is: what happens during cyclic deformation when down-sizing the specimen to a couple of hundred nanometers? Recently, several studies have reported on the cyclic compression of crystalline nanopillars but not so many on metallic glassy pillars inside TEM [57][58][59] .…”

Recent Advances on In Situ SEM Mechanical and Electrical Characterization of Low-Dimensional Nanomaterials

“…Allison et al [ 48 ] performed in situ SEM microcantilever beam experiments on bioinspired nanocomposites and the deformation mechanism was similar to nacre. Howard et al [ 49 ] even studied the cyclic deformation of metal microbeam under in situ SEM bending test and found that dislocation pileup within these microbeams occurs exactly as it would in a macroscopic fatigue specimen. The in situ single point bending test can also be used in the bending of nanoscale materials; for example, Vlassov et al [ 50 ] measured Young's modulus and yield point of the Ag nanowires and even observed their plastically deformation before fracture.…”