The susceptibility to hydrogen embrittlement (HE) of martensitic steels has been examined by means of a delayed-fracture test and hydrogen thermal desorption analysis. The intensity of a desorptionrate peak around 50 ЊC to 200 ЊC increased when the specimen was preloaded and more remarkably so when it was loaded under the presence of hydrogen. The increment appeared initially at the lowtemperature region in the original peak. As hydrogen entry proceeded, the increment then appeared at the high-temperature region, while that in the low-temperature region was reduced. The alteration occurred earlier in steels tempered at lower temperatures, with a higher embrittlement susceptibility. A defect acting as the trap of the desorption in the high-temperature region was assigned to large vacancy clusters that have higher binding energies with hydrogen. Deformation-induced generation of vacancies and their clustering have been considered to be promoted by hydrogen and to play a primary role on the HE susceptibility of high-strength steel.

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Hydrogen in trapping states innocuous to environmental degradation of the mechanical properties of highstrength steels has been separated and extracted using thermal desorption analysis (TDA) and slow strain rate test (SSRT). The high-strength steel occluding only hydrogen desorbed at low temperature (peak 1), as determined by TDA, decreases in maximum stress and plastic elongation with increasing occlusion time of peak 1 hydrogen. Thus the trapping state of peak 1 hydrogen is directly associated with environmental degradation. The trap activation energy for peak 1 hydrogen is 23.4 kJ/mol, so the peak 1 hydrogen corresponds to weaker binding states and diffusible states at room temperature. In contrast, the high-strength steel occluding only hydrogen desorbed at high temperature (peak 2), by TDA, maintains the maximum stress and plastic elongation in spite of an increasing content of peak 2 hydrogen. This result indicates that the peak 2 hydrogen trapping state is innocuous to environmental degradation, even though the steel occludes a large amount of peak 2 hydrogen. The trap activation energy for peak 2 hydrogen is 65.0 kJ/mol, which indicates a stronger binding state and nondiffusibility at room temperature. The trap activation energy for peak 2 hydrogen suggests that the driving force energy required for stress-induced diffusion during elastic and plastic deformation, and the energy required for hydrogen dragging by dislocation mobility during plastic deformation are lower than the binding energy between hydrogen and trapping sites. The peak 2 hydrogen, therefore, is believed to not accumulate in front of the crack tip and to not cause environmental degradation in spite of being present in amounts as high as 2.9 mass ppm.

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The simultaneous measurement of both the relative electrical resistance and the equilibrium hydrogen and deuterium pressure as a function of composition of Pd-H and Pd-D systems have been carried out at temperatures between 273 and 323 K at H 2 (D 2 ) pressures up to about 3.3 MPa. The relative resistance, R/R 0 , in the (α +β) two-phase region for the absorption processes shows a very small and almost linear increase with increasing H(D) content, especially for the Pd-H system, compared to the larger changes previously observed by the electrolysis method. The resistance behaviour is quite similar to the shape of p-c isotherm relationships. The relative resistance increments per unit change of H(D)/Pd content at 298 K, (R/R 0 )/ r, in the (α + β) two-phase region are about 1.5 and 2.1 times larger for the Pd-H and Pd-D systems, respectively, compared to the changes in the relative lattice parameters with H(D)/Pd content, (a/a 0 )/ r, within the two-phase region, where a 0 is the lattice parameter of H(D)-free Pd and r is the atom ratio. On the other hand, the resistance increment in the α single solid solution phase and β single phase, except for the higher-H(D)-content region, is significantly larger compared to the changes of the lattice expansion due to dissolved hydrogen and deuterium. Thus, the variation in resistance with hydrogen and deuterium content in the (α + β) twophase region may be mainly associated with an incoherent formation of β hydride within the α phase. The relative resistance for the subsequent desorption processes from the absorption up to about 3.3 MPa at 298 K in both Pd-H and Pd-D systems exhibits almost the same maximum as that of the absorption processes, i.e. (R/R 0 ) H,max 1.87 at about H/Pd = 0.76 and (R/R 0 ) D,max 2.07 at about D/Pd = 0.75, and then the R/R 0 values decrease gradually with decreasing H(D) content up to the β min phase boundary composition; on entering the (α+β) two-phase region, the R/R 0 values remain almost constant, i.e. (R/R 0 ) (α+β) 1.76 for the Pd-H system and (R/R 0 ) (α+β)1.89 for the Pd-D system. This large hysteresis of resistance can be attributed to the creation of 'lattice strain deformations' accompanied by dislocation formation from β hydride (deuteride) formation and by further highly dissolved hydrogen and deuterium in the β phase region.