Barrier-layer formation and its control during hydrogen permeation through Ti-24AI-11Nb alloy
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INTRODUCTION
Ti-24AI-11Nb (at. pct) alloy, a modification of the intermetallic Ti3A1, has received considerable attention as a candidate material for use in hydrogen-fueled hypersonic vehicles. Structures in these vehicles are expected to be exposed to hydrogen at temperatures ranging from cryogenic to 1000 ~ and hydrogen pressures from subatmospheric to more than 10 MPa. Previous studies t~-4J have shown that this alloy has a substantial solubility for hydrogen (up to 1.9 wt pct in the temperature interval of 500 ~ to 1050 ~ and is susceptible to the formation of hydrides below 700 ~ Although the mechanisms of hydrogen interaction with Ti-24Al-llNb alloy have not been completely established, experience with pure titaniumtS,61 and other transition metals such as niobium, tantalum, and vanadiumtT,8j shows that hydrogen degrades the mechanical properties of these materials; hence, evaluation of the hydrogen compatibility of this alloy is very important. Surface oxide/nitride layers have been shown to control the absorption and transport properties of titanium and other metals, t9-~71 In general, phase-boundary reactions have been found to be the rate-limiting step in titanium alloys, t9J This is confirmed by the direct pressure dependence of hydrogen absorption behavior instead of the expected one-half power dependence, t~8] In an interesting study, ]~91it has been reported that the surface films (oxide and nitrides) inhibit the absorption process, but have no effect on the desorption behavior of a-titanium. Heubaum and BerkowitzIt61 have demonstrated that surface films can play a different role, depending on the source of hydrogen charging. They have suggested that in electrochemical measurements, surface oxide films S.N. SANKARAN, Senior Scientist, and R.K. HERRMANN, Scientist, are with Analytical Services and Materials, Inc., Hampton, VA 23666. R.A. OUTLAW, Aerospace Technologist, and R.K. CLARK, Senior Research Engineer, are with NASA Langley Research Center, Hampton, VA 23681-0001. Manuscript submitted December 9, 1992. METALLURGICAL AND MATERIALS TRANSACTIONS A
inhibit the recombination of the protonic hydrogen into molecules, enabling the surface to reach the equilibrium solute concentration. However, in gas-phase charging, surface films decrease the availability of hydrogen molecules at the film/metal interface and lower the equilibrium solute concentration. In this work, the transport characteristics of hydrogen in Ti-24Al-11Nb alloy have been studied using ultrahigh vacuum (UHV) membrane-permeation measurements. The membrane-permeation experiments were conducted at hydrogen pressures ranging from 20 to 1333 Pa and at temperatures from 500 ~ to 900 ~ This range of pressures and temperatures was chosen to represent the conditions encountered in certain regions of a typical hypersonic vehicle where alloys such as Ti-24AI-11Nb are likely to be employed. The experiments also focused on the influence of barrier-layer formation due to contaminants in the hydrogen gas on the permeation process. Procedures wer
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