Hydrogen transport in nickel-base alloys (original) (raw)

1992, Metallurgical Transactions A

The electrochemical permeation technique has been used to characterize hydrogen transport and trapping in pure nickel and in alloys 600, X-750, and 718 at a temperature of 80 °C. The "effective diffusivity" of hydrogen atoms in alloy 600 is reduced by a factor of about 5 compared to pure nickel. This is attributed to both compositional changes and the presence of [(Ti, Nb)C] carbides. Aging of alloy 600, with subsequent M23C 6 carbide precipitation, does not significantly influence the measured "effective diffusivity," which is explained by the dominant effect of preexisting [(Ti, Nb)C] carbides. The "effective diffusivity" of hydrogen atoms in solution-annealed alloy X-750 is reduced by a factor of about 9 compared to that of pure nickel. This is also attributed to compositional changes and [(Ti, Nb)C] carbides. Aging of alloy X-750, which causes precipitation of y'[Ni3(A1, Ti)], reduces the "effective diffusivity" by an additional factor of 5 or more. Double aging at 885 °C/24 hours, 704 °C/20 hours following hot working yields the greatest reduction in "effective diffusivity." Analysis of permeation transients using a diffusiontrapping model indicates a binding energy associated with trapping due to the y' phase of between-31 and-37 kJ/mol. The "effective diffusivity" of hydrogen in alloy 718 is about 40 pct greater than for alloy X-750 for the same double and direct aging treatments. The average "effective diffusivities" of the double-aged and direct-aged alloy 718 are comparable, but the permeation transients for the double-aged treatment are significantly steeper. The double-aged treatment with predominantly 6 phase (orthorhombic Ni3Nb) yields a binding energy of about-30 kJ/mol. Analysis of the direct aged-treated 718, which contains predominantly 3/' phase (body-centered tetragonal Ni3Nb) gave a binding energy between-23 and-27 kJ/mol. Segregation of hydrogen atoms to the y'/matrix interface, combined with a large volume fraction of y' at grain boundaries, provides the most likely explanation for the enhanced cracking associated with the double-aging treatment in alloy X-750.