Hydrogen trapping in thoria-dispersed nickel
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I.
INTRODUCTION
THE important contribution of hydrogen trapping
at defects in hydrogen embrittlement of metals is well known. A generally recognized feature of most existing mechanisms of hydrogen embrittlement is that enough hydrogen must exist at a particular site for crack initiation. Inclusions such as oxides, carbides, or sulfides are strong hydrogen traps (tl and act as crack initiation sites in iron and steel. 12~ According to the trap theory of hydrogen embrittlement, [2m a crack will initiate at a defect as soon as the amount of trapped hydrogen reaches the critical concentration value. The critical concentration depends on many factors such as the nature of the trapping site, its shape, its location in the lattice, the state of stress at the site, etc. In view of this concept, if a microstructure presents a uniform distribution of small, strong trap sites like inclusions, the distribution of hydrogen will also be uniform and fine. This will make the control of hydrogen embrittlement possible. Thompson and Wilcox [41 investigated the deformation and fracture of thoria-dispersed nickel affected by hydrogen. The results showed that thoria-dispersed nickel was much less severely embrittied by hydrogen than was pure nickel. They suggested that the thoria particles trapped the hydrogen, removing it from the lattice of the nickel and thus reducing the embrittling effect of the hydrogen. Quantitative evaluation of the trapping capability of the ThO2-matrix interface for hydrogen would seem to be important in developing an understanding of the embrittlement mechanism. Robertson investigated the trapping of hydrogen in nickel-2 vol pet thoria by permeation
techniques and reported that the thoria particles strongly trap hydrogen. 151He assumed that the effect of cold-rolling the material was attributed only to increased trap-site density. Thus, the trapping character of dislocations as well as voids formed at the particle-matrix interface during cold-working was evaluated as the same as that of the particle-matrix interface. However, the nature of these traps appear to be different from the fact that they have a different trap-activation energy, t6.Tj Therefore, there is still considerable uncertainty regarding trapping in the nickel-thoria system. Trapping behavior in nickel and austenitic steels with higher intrinsic lattice solubility cannot be easily discerned by permeation measurements. This is solved by utilizing the hydrogen thermal desorption technique, which enables us to separate the trapping effect of each defect. Therefore, in this work, the trapping effect of the nickel-ThO2 interface was measured using a hydrogen thermal desorption method.
II.
THEORETICAL BACKGROUND
Determination of Hydrogen Trap-Binding Energy When equilibrium between hydrogen atoms in trapping sites and those in interstitial sites of the normal lattice is achieved at temperatures during gaseous hydrogen charging, the concentration of trapped hydrogen, Cr, is given by Eq. [1]. [8"9A~
Nr NL exp Cr =
[11 1 + - - exp
SUNG-MAN L
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