Four-point bend (4PB) tests of notched specimens loaded at various loading rates, for low alloy steel with different grain sizes, were done, and the microscopic observation and finite-element method (FEM) calculations were carried out. It was found that for the coarse-grained (CG) microstructure, an appreciable drop in notch toughness with a loading rate of around 60 mm/min appeared, and further increasing the loading rate leads to a slight additional decrease in notch toughness. For the finegrained (FG) microstructure, the effect of loading rate was not apparent. The change in toughness resulted from a change of the critical event controlling the cleavage fracture with increasing loading rate. For the CG microstructure with a lower cleavage-fracture stress ( f ), with an increasing loading rate, the critical event of cleavage fracture can be changed from the propagation of a pearlite colony-sized crack or a ferrite grain-sized crack, through the mixed critical events of crack propagation and crack nucleation, then to crack nucleation. This change deteriorates the toughness. For the FG microstructure with a higher cleavage-fracture stress, the critical event of cleavage fracture is the crack propagation and does not change in the loading-rate range from 120 to 500 mm/min. The measured f values do not change with loading rate, as long as the critical event of cleavage fracture does not change. The higher notch toughness of the FG microstructure arises from its higher f and the critical plastic strain ( pc ) for initiating a crack nucleus, and the fracture behavior of this FG steel is not sensitive to loading rate in the range of this work.
The nucleation process of hydrogen blister in metals was investigated through experiments and the mechanism was discussed. Small hydrogen blister in charged Ni-P amorphous coating and steel was studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The thermodynamics and kinetics of hydrogen and vacancies in metals are analyzed. Further, an approach of the nucleation mechanism of hydrogen blister is proposed as follows. Atomic hydrogen can induce superabundant vacancies in metals. The superabundant vacancies and hydrogen aggregate into a hydrogen-vacancy cluster (small cavity). The hydrogen atoms in the hydrogen-vacancy cluster become hydrogen molecules that can stabilize the cluster. And the hydrogen blister nucleates. The pressure in the small cavity increases as the hydrogen atoms enter the cavity. The cluster, that is, the hydrogen blister nucleus, grows through vacancies diffusing into it under the action of cluster-hydrogen binding energy and hydrogen pressure. When the blister nucleus grows to a critical size C cr cracks will initiate from the wall of the cavity due to the internal hydrogen pressure.
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