Surface orientation dependence of hydrogen flux in lenticular martensite of an Fe-Ni-C alloy clarified through in situ silver decoration technique

An in situ silver decoration technique was applied to investigate the effect of microstructure on hydrogen flux in an austenite/α′-martensite dual-phase Fe-32Ni-0.2C alloy. Using time-resolved hydrogen mapping, the surface orientation of the body-centered cubic lenticular martensite was found to have a significant effect on hydrogen flux. The hydrogen flux was particularly high at the near-〈0 0 1〉, while it was the lowest at the near-〈1 1 1〉 surface. This dependence of hydrogen flux on the surface orientation is attributed to hydrogen trapping at dislocations in the martensite. Particularly, 〈1 1 1〉 twinning-shear in the martensite enhances hydrogen trapping at dislocations, suppressing hydrogen diffusion in the shortest path of grains orientated to 〈1 1 1〉.