Lattice Defects Underneath Hydrogen-induced Intergranular Fracture Surface of Ni-Cr Alloy Evaluated by Low- energy Positron Beam

To investigate mechanism of hydrogen-induced intergranular fractures, low energy positron beams with different energies were applied to hydrogen-induced intergranular fracture surfaces of 80Ni-20Cr alloy, and depth distributions of the lattice defects were evaluated by Doppler broadening spectroscopy and positron annihilation lifetime one. At least at depths between 0.1 mu m and 1 mu m from the fracture surface, a large number of the lattice defects were homogeneously distributed. Both the dislocation density and mono-vacancy-equivalent vacancy-type defect one were around ten-times as large as inside the grains. On the other hand, the absolute value of the monovacancy-equivalent vacancy-type defect density was about 9 appm, and obviously not large enough to cause strength reduction and fractures. It was suggested that stress concentration and disordered structures formation at and on the grain boundaries due to hydrogen-enhanced dislocation nucleation, and reduction in the grain boundary cohesive energy due to the trapped hydrogen atoms jointly cause the intergranular fractures.