STUDY OF THE EFFECT OF MICROSTRUCTURE ON HYDROGEN EMBRITTLEMENT SUSCEPTIBILITY IN A Fe-Ni-Co BASED ALLOY BY POSITRON ANNIHILATION TECHNIQUE

Positron annihilation,TEM,SEM and high purity high pressure hot charging of hydrogen have been used to investigate the effect of microstructure,changed by varying of Ti content, on the hydrogen performance of the Fe-Ni-Co based alloys.It was found that the interface between the strengthening phase γ′with large size and the matrix was a kind of relative strong defect, which could accelerate the local accumulation of hydrogen. As a result, when the size of intragranular γ′particles is large, very fine dimples can be observed on the fracture surface of hydrogen charged specimens, and hydrogen-induced intergranular fracture can occur when large γ′particles precipitate at grain or twin boundaries.However,if the size of γ′ is small. the interface of γ-γ′is very weak defect and no intergranular fracture occurs even the γ′ particles precipitate at grain or twin boundaries. The interface between the stable phase η,with hop structure, and the matrix was also found to be strong defect. When the η phase precipitates at grain boundaries, hydrogen-induced intergranular fracture could happen, and when η phase formed within grains, hydrogen induced second cracks could be found on the fracture surface. In the present work, the alloy with medium Ti content is relatively good at resisting hydrogen embrittlement because the size and distribution of γ′are homogeneous and no stable phase is formed,while the hydrogen performance of the alloys with higher or lower content of Ti becomes bad lust because the defect density and intensity in these alloys are high.