Hydrogen-modified dislocation structures in a cyclically deformed ferritic-pearlitic low carbon steel

Hydrogen-assisted fatigue crack propagation in a pure BCC iron. Part I: Intergranular crack propagation at relatively low stress intensities

“…This process achieves the local stress concentration and hydrogen agglomeration around GBs, which, theoretically, is enough to satisfy the critical condition to operate atomistic decohesion [16,22,26]. Additionally, they also found that the development level of the dislocation structure underneath the IG fracture surface far exceeds the one under the absence of hydrogen at an equivalent strain level [22,26], as per the recent investigation conducted by Wang et al [14,15]. Martin et al suggested that the work hardening associated with such hydrogen-enhanced plasticity evolution has a role in assisting GB separation by rising the local stress level to exceed the ideal GB cohesive force [22].…”

“…the enhancement of plasticity under the presence of hydrogen, is consistent with the results reported by Wang et al [14], who found that the size of dislocation cells in hydrogen-charged pure nickel subjected to highpressure torsion (HPT) became smaller compared to non-charged material. More recently, they also examined the dislocation structures immediately beneath the IG fracture surface formed in the FCG test of a structural carbon steel tested in 40 MPa hydrogen gas, and reported that the dislocations showed smaller cell diameters compared to the cells formed beneath ductile striations in air [15]. Throughout the series of publications, they explained such alteration of dislocation behavior based on the framework of hydrogen-enhanced localized plasticity (HELP) hypothesis [16][17][18].…”

“…Wang et al assumed that this role of hydrogen, i.e. easing dislocation generation and mobility, can result in the finer and more evolved dislocation structures under the presence of hydrogen [14,15], which seems to be applicable also in the case of severely strained region ahead of the fatigue crack tip in pure iron. Fig.…”

“…As the cell structure evolution proceeds, more dislocations are absorbed into the cell walls and annihilate each other, which makes the cell walls thinner and results in the formation of subgrain boundaries. Moreover, both of the dislocation emission [19][20][21] and evolution [14,15,22] processes are accelerated by hydrogen. These modifications of dislocation behavior raise higher misorientation angles in GROD and KAM analyses of the sample tested in hydrogen gas (Fig.…”

Hydrogen-assisted fatigue crack growth in ferritic steels – a fractographic study

“…It draws engineers' attention since it can cause catastrophic failure of metallic structures that serve in a H-containing environment. Several different mechanisms have been developed, and the most common ones are hydrogenenhanced localized plasticity (HELP) [3][4][5][6][7][8], hydrogenenhanced decohesion (HEDE) [9][10][11][12], adsorptioninduced dislocation emission (AIDE) [13], hydrogenenhanced vacancy production [14,15], hydrogeninduced phase transformation [16][17][18] etc. These mechanisms describe the HE effect in terms of different aspects ranging from chemical bonding up to microstructure level.…”