Masayasu Sugisaki

Observation of Hydrogen Distribution Around Non-Metallic Inclusions in Steels with Tritium Microautoradiography

Hydrogen distributions around non-metallic inclusions in steels are successfully characterized with high-resolution tritium autoradiography. The autoradiographs show that hydrogen accumulation characteristics around the inclusions depend on types of the inclusions. In the case of MnS, hydrogen was inhomogeneously distributed in the ferrite matrix surrounding the MnS inclusion, probably because hydrogen is trapped in defects formed around MnS. The inhomogeneous distribution of hydrogen may be originated from the asymmetric stress field produced by a contraction of the MnS phase in the heat treatment, i.e. the inhomogeneous volumetric change of MnS owing to its larger thermal expansion than that of the ferrite phase. In the case of Al2O3, hydrogen was intensely localized at boundary layers of the ferrite matrix surrounding the Al2O3 inclusion. This could be attributed to hydrogen trapping at defects introduced by a residual stress in the boundary layers of the ferrite matrix due to larger contraction of the ferrite phase than that of the Al2O3 phase on cooling. Similarly hydrogen was accumulated in the surrounding ferrite matrix but more widely distributed around Cr carbide probably because difference in the thermal expansion between the Cr carbide and ferrite phases is less than that between the Al2O3 and ferrite phases.

Atomic force microscopy of induction- and furnace-heating-tempered prestressed steels with different delayed fracture properties

Abstract Quantitative microstructure analyses by atomic force microscope and delayed fracture tests were performed for two types of prestressed steel with a tensile strength of 1470 MPa; an induction-heated-tempered specimen, and a furnace-heated-tempered specimen. Size distributions of cementite particles were measured to characterize the relationship between microstructures and delayed fracture properties.

Isotope effect in heat of transport of H, D and T in Nb

Abstract The thermal diffusion of hydrogen isotopes, H and D, in Nb was studied at an average temperature of 168°C. By analyzing the redistribution of hydrogen in Nb on the basis of the irreversible thermodynamics, the heat of transport Q∗ was determined for H and D as 9.5 kJ/mol and 16.0 kJ/mol, respectively. The large isotope dependence of Q∗ was concluded by comparing these values with the value of 18.8 kJ/mol for T, which was previously reported by the present authors. The diffusion coefficients of H and D were also determined from the transient process of redistribution and found to be in good agreement with those based on the Gorsky effect.

Influence of size distribution of Zr(Fe, Cr)2 precipitates on hydrogen transport through oxide film of Zircaloy-4

Abstract The hydrogen uptake behavior of two types of Zircaloy-4 specimens containing fine or coarse Zr(Fe,Cr)2 precipitates was studied at 623–723 K in the pre-transition period of oxidation in steam. The amount of hydrogen uptake of the specimen containing fine precipitates was smaller than that of the specimen containing coarse ones, and the former was independent of the oxidation temperature while the latter increased with decreasing temperature. These results were successfully explained with the model that the Zr(Fe,Cr)2 precipitates remaining unoxidized act as the short-circuiting route in the hydrogen transport through the oxide film. The amount of hydrogen uptake of the present Zircaloy-4 specimens containing coarse precipitates was larger than that of the Zircaloy-2 specimens containing comparable sizes of precipitates. This was attributed to the fact that the concentration of precipitates remaining unoxidized in the oxide film of Zircaloy-4 was higher than that of Zircaloy-2.

Thermal diffusion of tritium in Nb metal

Abstract Thermal diffusion phenomena of tritium in Nb metal are studied for temperatures between 100°–300°C. Some special types of apparatus and samples are developed to determine the distribution of tritium in the transient state of the thermal diffusion. By least squares fitting of the experimental data to the theoretical curve based on irreversible thermodynamics, the heat of transport, Q∗, and the diffusion coefficient, D, of tritium at an average temperature of 200°C are determined to be 18.0 kJ/mol and 1.6 x 10−5 cm2/sec, respectively. The importance of the thermal diffusion phenomena is discussed in connection with tritium permeation through the first wall of a nuclear fusion reactor.

Surface reaction and bulk diffusion of tritium in SUS-316 stainless steel

Abstract Diffusivities of tritium in SUS-316 stainless steel were measured by a gas-absorption method in the temperature range from 603 to 853 K. The measurements were carried out on clean and modified surfaces, which were carefully prepared by well characterized treatments. The diffusion behavior of the clean surface is successfully described by a simple diffusion equation and the bulk diffusion coefficient of tritium was determined as D(cm 2 /s) = 4.2 × 10 2 exp( −64 ( RT kJ ) ) . The diffusion process for the modified surface was not explained by a simple diffusion equation but by a diffusion equation having an induction period. The microscopic explanation of the induction period is given.

Tracer diffusion coefficient of tritium in vanadium and trapping effect due to zirconium impurity

The tracer diffusion coefficient of tritium in the α-phase of vanadium is measured in a temperature region from 323 to 523 K by making use of the glow discharge implantation method recently developed by the present authors. The obtained results are in good agreement with the data previously reported by Qi et al. (J. Phys. F: Metal Phys. 13 (1983) 2053). The trapping effect of zirconium on the diffusion of hydrogen isotopes is examined by using specimens containing a small amount of zirconium impurity. The existence of at least two kinds of trapping sources is deduced by analyzing the concentration dependence of tracer diffusion coefficient of tritium: one is due to zirconium and the other is due to a certain complex of zirconium with interstitial impurities such as oxygen and nitrogen. Trapping energies of 0.15 and 0.36 eV are assigned to the former and the latter, respectively.

Influence of surface impurities on plasma-driven permeation of deuterium through nickel

The plasma-driven permeation behavior of deuterium through nickel membrane was examined in the temperature range from room temperature to 723 K. The modification of its surface impurities by exposing it to the hydrogen plasma was examined with Auger electron spectroscopy. The influence of surface impurities on the penetration and recombination behavior of deuterium atoms at the nickel surface is discussed.

Tritium solubility in SUS-316 stainless steel

Tritium solubility in SUS-316 stainless steel was determined with a gas absorption method, in which tritium gas diluted by protium was used. The tritium absorption experiments were carried out at temperatures of 703, 804 and 903 K under pressures of 10, 30, 50 and 100 torr of tritiated hydrogen gas. The radioactivity of tritium dissolved in the specimen was measured by the method of liquid scintillation counting. The tritium solubility was derived from the experimental data by taking into consideration of isotopic equilibrium among H2, T2 and HT molecules. The determined tritium solubility can be expressed by the equation: CT=1.94×10−7exp−10.2RT/kJp12T2mol T2/cm3Pa12

Observation of the spatial distribution of hydrogen in Zircaloy-2 oxidized in H2O steam at 723 K by a technique of tritium microautoradiography

The spatial distribution of hydrogen in Zircaloy-2, which was oxidized in H2O steam at 723 K and quenched down to room temperature, was examined by a technique of tritium autoradiography. Tritium was introduced into the specimen through the oxide layer by the cathodic charging method using tritium water at room temperature. The radiograph of the cross-section was observed by a scanning electron microscope. Hydrogen was densely distributed on the grain boundary and in the grain of the inner base alloy, but it was scarcely distributed in the oxide layer. Hydrogen was locally distributed, however, in the region corresponding to the intermetallic precipitates embedded in the oxide layer. The mechanism of penetration of tritium through the oxide layer during the cathodic charging was discussed.