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Ϫ4/s. In contrast, that of Inconel 625 with a low hydrogen diffusion rate was 9 % of the initial hydrogen content when the alloy was deformed at a strain rate of 4.2ϫ10 Ϫ6 /s. This difference in the amount of desorbed hydrogen transported by dislocations depends on the balance between the hydrogen diffusion rate and mobile dislocation velocity.KEY WORDS: hydrogen; lattice defect; dislocation; pure iron; Inconel 625; elastic deformation; plastic deformation; hydrogen desorption; hydrogen embrittlement. 115© 2010 ISIJ screw dislocations or the combination of edge dislocations with opposite characteristics located on slip planes an atomic plane distance apart.11) Since hydrogen stabilizes vacancies, 12,13) the formation of vacancies can be promoted during plastic deformation in the presence of hydrogen. Therefore, the interactions between hydrogen and dislocations are important to hydrogen-enhanced strain-induced vacancies. 14,15) Interactions between hydrogen and dislocations occur in various ways. Since the effective radius of a hydrogen atmosphere formed around a dislocation is about 0.5 nm, hydrogen is trapped from the core to the elastic field of the dislocation located at some distance from the core.16) Tritium desorption has been reported from various metals during tensile deformation, suggesting that it was transported by moving dislocations. [17][18][19][20][21] In addition, the use of a silver decoration technique revealed the deposition of Ag(CN) 2 along the slip lines on a pure aluminum surface, implying the rapid transport of hydrogen together with such dislocations. 22) Similar results were obtained using a hydrogen microprint technique: silver particles observed along the slip lines on an Al-5%Mg alloy surface presumably resulted from the transport of hydrogen to the surface by moving dislocations.23) Hydrogen desorbed from various metals during tensile deformation was detected and a hydrogen desorption peak was observed at the time of fracture. 24) Although there are a lot of reports concerning hydrogen desorption promoted by deformation, there are few detailed in situ studies on hydrogen desorption from metals during elastic/plastic deformation.In the present study, hydrogen desorption behavior attributable not to thermal energy but to mechanical energy, i.e., tensile stress, was analyzed at room temperature at which hydrogen embrittlement occurs. The desorption behavior of weakly trapped diffusive hydrogen was analyzed in situ and the effect of elastic/plastic deformation on hydrogen desorption behavior was investigated for pure iron and Inconel 625, in which a reduction of fracture strain caused by hydrogen was confirmed. ExperimentalA pure iron with a bcc lattice and a Ni-based alloy, Inconel 625, with an fcc lattice were used. The chemical compositions are shown in Table 1. Flat specimens of 1 mm in thickness, 20 mm in gage length and 2.5 mm in width were used in order to apply strain during tensile testing. Pure iron was annealed at 900°C for 0.5 h in Ar followed by furnace cooling and Inconel...

Dynamic behavior of hydrogen desorption from pure iron with a body-centered-cubic lattice and Inconel 625 with a face-centered-cubic lattice was examined during tensile deformation using a quadrupole mass spectrometer in a vacuum chamber integrated with a tensile testing machine. Hydrogen desorption from hydrogen-charged specimens was detected under various strain rates and cyclic stresses. Hydrogen desorption rarely increased under elastic deformation. In contrast, it increased rapidly at the proof stress when plastic deformation began, reached its maximum, and then decreased gradually with increasing applied strain for both pure iron and Inconel 625. This desorption behavior is closely related to hydrogen dragging by moving dislocations. The thermal desorption analysis results showed that the amount of desorbed hydrogen differed at each strain rate. This difference in the amount of desorbed hydrogen transported by dislocations depends on the balance between the hydrogen diffusion rate and mobile dislocation velocity.

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