Hydrogen-Assisted Cracking Investigated by In Situ Electrochemical Micro-Cantilever Bending Test
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Most high strength and high performance alloys experience a premature rupture and loss in the ductility when exposed to hydrogen. The interaction of hydrogen with dislocations and other microstructural defects such as different type of interfaces, boundaries, vacancies and precipitates with different composition, size and distribution in the alloys plays a crucial role in the degradation phenomenon which is called hydrogen embrittlement. Deeper understanding of the involved mechanisms necessitates studying the hydrogen interaction with the microstructure and crystal defects at their respective length scale, i.e., the nano- and micro-scales. However, investigation of hydrogen embrittlement phenomenon in small scales imposes some experimental complications. As an example, in situ hydrogen charging is mandatory versus ex situ hydrogen charging. For most materials, the hydrogen outgassing during ex situ testing results in hydrogen concentration gradient or even complete hydrogen depletion within a short time. Accordingly, the main objective of the thesis research was to establish in situ electrochemical micro-cantilevers bending test method, which would enable studying the hydrogen-assisted cracking phenomenon for the pre-notched micro- cantilevers under in situ electrochemical hydrogen charging. To reduce the complexities toward understanding the hydrogen effect on crack propagation, Fe-3wt.% Si alloy (a simple ferritic steel) was chosen as the testing material. However, the major challenge during small-scale testing combined with the electrochemical charging of the sample is to preserve the surface roughness and the integrity of the micron-sized samples at the nanometer scale. In this regard, a glycerol-based solution as a suitable replacement for the current aqueousbased solutions for hydrogen charging of steels is introduced in this research. The developed solution facilitates studying the hydrogen embrittlement in small-scale while keeping the chemical and topography alteration on the sample surface in a minimum level. The glycerol-based solution used for in situ electrochemical nano-indentation test under different chemical potentials. It is shown that a higher hydrogen chemical potential results in a lower pop-in load, which is an indication of an easier dislocation nucleation below the loading tip. In situ electrochemical micro-cantilevers bending tests were done on pre-notched single crystal beams. While notch blunting occurred for the cantilevers bent in air, crack initiation and propagation were observed for the beams bent in the presence of hydrogen. Additionally, the effect of carbon content alternation and a grain boundary addition to the micro-cantilever were studied. It is shown that the effect of carbon atom on pinning the dislocations reduces in the presence of hydrogen. Postmortem analyses of the crack propagation path for the bi-crystal cantilevers tested in the presence of hydrogen concluded that the transition from transgranular fracture to intragranular fracture is highly dependent on the location of the stress concentration relative to the grain boundary in the tested material.