Abstract
Hydrogen effect on fatigue performance at relatively high values of stress intensity factor range, ΔK, of pure BCC iron has been studied with a combination of various electron microscopy techniques. Hydrogen-assisted fatigue crack growth rate is manifested by a change of fracture features at the fracture surface from ductile transgranular in air to quasi-cleavage in hydrogen gas. Grain reference orientation deviation (GROD) analysis has shown a dramatic suppression of plastic deformation around the crack wake in samples fatigued in hydrogen. These results were verified by preparing site-specific specimens from different fracture features by using Focused Ion Beam (FIB) technique and observing them with Transmission Electron Microscope (TEM). The FIB lamella taken from the sample fatigued in air was decorated with dislocation cell structure indicating high amount of plasticity, while the lamella taken from the quasi-cleavage surface of the sample fatigued in hydrogen revealed a distribution of dislocation tangles which corresponds to smaller plastic strain amplitude involved at the point of fracture. These results show that a combination of critical hydrogen concentration and critical stress during fatigue crack growth at high ΔK values triggers cleavage-like fracture due to reduction of cohesive force between matrix atoms.
Highlights
Hydrogen has the potential to become one of the major energy carriers, but to become a feasible fuel alternative it has to close the gap against the traditional energy sources in terms of cost
The crack growth curve is linear in logarithmic scale for air and nitrogen throughout the tested ΔK range, but in the case of hydrogen gas, there is an obvious distinction between Stage I and Stage II fatigue crack growth (FCG)
FCG acceleration enhancement was controlled by the coverage of QC fracture, and as it reaches 100% there is no further enhancement by increasing hydrogen pressures
Summary
Hydrogen has the potential to become one of the major energy carriers, but to become a feasible fuel alternative it has to close the gap against the traditional energy sources in terms of cost. According to fracture surface examinations the hydrogen-assisted cracking in high-strength steels manifests as a brittle intergranular fracture [4,5], while low or medium-strength steels subjected to hydrogenassisted fatigue crack growth (HAFCG) show transgranular quasi-cleavage (QC) fracture accompanied by brittle-like striations on the fracture surfaces [6,7] These observations resulted in several proposed models for explanation of the QC fracture formation. Mechanism [6,8,9], which are based on the most common theories for HE, such as hydrogen-enhanced decohesion (HEDE) [10,11], hydrogen-enhanced localized plasticity (HELP) [12,13], hydrogen-enhanced stabilization of lattice defects [8,14], or the combination of them These models still lack direct and critical supporting evidence, mostly due to the fact that most of the experiments have been performed on materials with complex microstructures and several alloying elements. The combination of these results implies that hydrogeninduced cleavage cracking, resulting in a significant reduction of plastic deformation, was the main contributor to the HAFCG in stage II regime
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