Electrochemical hydrogen permeation in wrought and electron beam melted Ti-6Al-4V alloys

Ti-6Al-4V is one of the most widely used titanium alloys. However, it is susceptible to hydrogen embrittlement, e.g. due to hydride formation. Hydrogen permeability through Ti-based alloys is affected by the rapid formation of hydride or oxide surface layers as well as by trapping in the bulk alloy. Here, we analyze and compare the electrochemical hydrogen permeation (EHP) through wrought and electron beam melted (EBM) Ti-6Al-4V alloys. Hydrogen diffusion coefficients are calculated from the permeation current transients. The effective diffusion coefficient of hydrogen in the wrought alloy is found to be an order of magnitude higher than in the AM'ed alloy. The different microstructures of the wrought and EBM alloys are found to play an important role in hydrogen distribution and phase transformation in the alloy. In the EBM alloy, hydrogen is distributed more uniformly and, consequently, more pronounce phase transformations occur, eventually leading to brittle cracking. In the wrought alloy, on the other hand, hydrogen is distributed unevenly in the bulk of the membrane, forming hydride clusters, most probably at α/β interphase boundaries. These findings suggest that the EBM Ti-6Al-4V alloy is more susceptible to HE than its wrought counterpart.