In the context of models of hydrogen-assisted cracking, such as the adsorption-induced dislocation emission model, chemisorbed hydrogen and subsurface hydrogen in the first few atomic layers are believed to influence mechanical and metallurgical behavior. The lattice concentration of hydrogen at 20 °C and 1 atm. pressure is estimated to be approximately 5.1 × 10−4 ppm by mass. By assuming a square root relationship between solubility and the partial pressure of hydrogen, the concentration can be calculated for other pressures. The diffusivity of hydrogen in a material is influenced by trap occupancy, which refers to the percentage of traps occupied by hydrogen. Trap occupancy depends on factors such as trap binding energy, density of trap sites, and lattice hydrogen content. Hydrogen embrittlement occurs when stress is applied, typically during plastic deformation. Dislocations play a crucial role in plastic deformation, and the impact of hydrogen on mechanical properties is mainly attributed to its interactions with dislocations. Hydrogen increases the density of conduction electrons near hydrogen atoms and reduces the shear modulus, the stress required for dislocation source activation, the line tension of dislocations, and the distance between dislocations in pileups. The interaction between hydrogen and dislocations is heavily influenced by the dislocation structures. It was discovered that the hydrogen absorption capacity significantly increased with plastic straining, particularly when hydrogen was present during straining. The role of grain boundaries in hydrogen embrittlement must be considered, taking into account various factors, including the cohesive strength of boundaries, modifications caused by segregated impurities or precipitates, and the concentration of plastic deformation in adjacent regions. Different mechanisms have been proposed to explain hydrogen embrittlement, and their effectiveness depends on factors such as the material, hydrogen charging conditions, and loading. A direct connection was proposed between the fracture surfaces induced by hydrogen and the microstructural state, representing a significant shift in understanding hydrogen embrittlement mechanisms. In models of brittle fracture, the influence of hydrogen is taken into account through various parameters in the Griffith condition. Experimental evidence has shown that plasticity plays a role in hydrogen embrittlement (HE), even in materials that exhibit brittle-like fracture modes. The hydrogen-enhanced localized plasticity (HELP) has been theoretically explained by considering the shielding effect of hydrogen atmosphere on elastic interactions between dislocations and other stress centers. Dilatational fields around hydrogen atoms create shear stress on dislocations, reducing their force interactions. The crack formation by the precipitation of molecular hydrogen is feasible in case of high hydrogen fugacity, but the key issue is the function of hydrogen in triggering the unstable crack extension. Hydrogen deteriorates the fatigue behavior of metals by attacking their microstructure in different ways. For better clarity about hydrogen-based degradation of metals, it is vital to identify the microstructural configurations that promote fatigue crack initiation (FCI) in the presence of hydrogen. Intergranular regions in polycrystalline metals are more prone to hydrogen attack due to the prevailing atomic structure, elastic anisotropy, and plastic inhomogeneities causing increased hydrogen accumulation in this region.

Hydrogen Embrittlement: Deformation Mechanisms

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发表日期：

2025年

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Pasquale Cavaliere

摘要

In the context of models of hydrogen-assisted cracking, such as the adsorption-induced dislocation emission model, chemisorbed hydrogen and subsurface hydrogen in the first few atomic layers are believed to influence mechanical and metallurgical behavior. The lattice concentration of hydrogen at 20 °C and 1 atm. pressure is estimated to be approximately 5.1 × 10−4 ppm by mass. By assuming a square root relationship between solubility and the partial pressure of hydrogen, the concentration can be calculated for other pressures. The diffusivity of hydrogen in a material is influenced by trap occupancy, which refers to the percentage of traps occupied by hydrogen. Trap occupancy depends on factors such as trap binding energy, density of trap sites, and lattice hydrogen content. Hydrogen embrittlement occurs when stress is applied, typically during plastic deformation. Dislocations play a crucial role in plastic deformation, and the impact of hydrogen on mechanical properties is mainly attributed to its interactions with dislocations. Hydrogen increases the density of conduction electrons near hydrogen atoms and reduces the shear modulus, the stress required for dislocation source activation, the line tension of dislocations, and the distance between dislocations in pileups. The interaction between hydrogen and dislocations is heavily influenced by the dislocation structures. It was discovered that the hydrogen absorption capacity significantly increased with plastic straining, particularly when hydrogen was present during straining. The role of grain boundaries in hydrogen embrittlement must be considered, taking into account various factors, including the cohesive strength of boundaries, modifications caused by segregated impurities or precipitates, and the concentration of plastic deformation in adjacent regions. Different mechanisms have been proposed to explain hydrogen embrittlement, and their effectiveness depends on factors such as the material, hydrogen charging conditions, and loading. A direct connection was proposed between the fracture surfaces induced by hydrogen and the microstructural state, representing a significant shift in understanding hydrogen embrittlement mechanisms. In models of brittle fracture, the influence of hydrogen is taken into account through various parameters in the Griffith condition. Experimental evidence has shown that plasticity plays a role in hydrogen embrittlement (HE), even in materials that exhibit brittle-like fracture modes. The hydrogen-enhanced localized plasticity (HELP) has been theoretically explained by considering the shielding effect of hydrogen atmosphere on elastic interactions between dislocations and other stress centers. Dilatational fields around hydrogen atoms create shear stress on dislocations, reducing their force interactions. The crack formation by the precipitation of molecular hydrogen is feasible in case of high hydrogen fugacity, but the key issue is the function of hydrogen in triggering the unstable crack extension. Hydrogen deteriorates the fatigue behavior of metals by attacking their microstructure in different ways. For better clarity about hydrogen-based degradation of metals, it is vital to identify the microstructural configurations that promote fatigue crack initiation (FCI) in the presence of hydrogen. Intergranular regions in polycrystalline metals are more prone to hydrogen attack due to the prevailing atomic structure, elastic anisotropy, and plastic inhomogeneities causing increased hydrogen accumulation in this region.