Nousha Kheradmand

Novel methods for micromechanical examination of hydrogen and grain boundary effects on dislocations

Most of what is known about the local interaction of dislocations with grain boundaries and hydrogen is based on transmission electron microscopy studies, which suffer from the distinct disadvantage that only extremely thin samples can be used. Recently, micropillar compression testing has become a popular means by which investigation of the size effect is conducted. This method, in combination with orientation imaging techniques, is used here to study the interaction of dislocations with a pre-selected grain boundary during the deformation of a bicrystalline specimen. Furthermore, by utilizing a custom built electrochemical cell, the micropillar compression testing can be extended to study in situ examination of micropillars charged with hydrogen. The effects of hydrogen and grain boundary on the deformation process in this small, but still bulk-like volume are presented, and our initial results reveal the value of this new technique for investigations of hydrogen embrittlement and grain boundary strengthening.

Investigation of the role of grain boundary on the mechanical properties of metals

Compression testing of micropillars was used to investigate the gain boundary effect on the strength of metals which is especially interesting in ultra fine grained and nanocrystalline metals. Single and bicrystal micropillars of different sizes and crystallographic orientations were fabricated using a focused ion beam system and the compression test was performed with a nanoindenter. A reduction of the pillar size as well as the introduction of a grain boundary results in an increase in the yield strength. The results show that the size and the orientation of different adjoining crystals in bicrystalline pillars have an obvious effect on dislocation nucleation and multiplication.

Hydrogen enhanced cracking studies on Fe–3wt%Si single and bi-crystal microcantilevers

Hydrogen (H) enhanced cracking was studied in Fe–3wt%Si by means of in situ electrochemical microcantilever bending test. It was clearly shown that the presence of H causes hydrogen embrittlement (HE) by triggering crack initiation and propagation at the notch where stress concentration is existing. Additionally, the effect of carbon content and the presence of a grain boundary (GB) in the cantilever were studied. It was shown that in the presence of H the effect of carbon atom on pinning the dislocations is reduced. On the other hand, the presence of a GB, while the chemical composition of material kept constant, will promote the HE. Crack initiation and propagation occur in the presence of H, while the notch blunting was observed for both single and bi-crystalline beams bent in air. Post-mortem analysis of the crack propagation path showed that a transition from transgranular fracture to intragranular fracture mechanism is highly dependent on the position of the stress concentration relative to the GB. This article is part of the themed issue ‘The challenges of hydrogen and metals’.