During in situ deformation in an electron microscope, dislocation behavior in various conditions has been observed near the crack tip of the metals, such as Fe-3% Si, pure nickle, commercially pure iron, Al-Fe alloy, etc. Resurts show: (1) A DFZ is present in front of the crack tip in all observed metals, and the dislocations in the plastic zone are in the form of an inverse pile-up. (2) Boundary impedes the emission of dislocation from the crack tip, but the solute hydrogen can promote the dislocation emission and make the DFZ short. (3) Antishilding dislocations are lined in the flank of the crack, and accelerate the brittle propogation of crack if the specimen is prestrained. (4) If the specimen is charged with hydrogen after predeformation, the DFZ is present ahead of the crack tip, which demonstrates the theoretical model suggested by us that hydrogen may make the CIST decreased and DFZ short. (5) Hydrogen may gather on some selected planes, which can promote the nucleation of microcracks. (6) Moire' patterns in the DFZ are observed in high magnification, which shows domain structure may be existed. (7) In some metals of good plasticity, dislocation cell structures are formed in the plastic zone, and microcracks are easily nucleated in the centre of the dislocation cells. Theoretical calculation indicates: the solute hydrogen makes the CIst decreased due to the reduction of the friction stress of lattice by hydrogen; Emitted from the crack tip, dislocation carries a fraction of hydrogen atoms there and is exerted by an additional attractive force, which makes the DFZ short. Theoretical analyses can explain the experimental results well.
修改评论