Ryohei Tanaka

Strengthening of nickel -base superalloys for nuclear heat exchanger application

Strengthening mechanisms of nickel-base superalloys have been discussed with the background of the Japanese research and development activities in this field. As candidates for materials of intermediate heat exchangers which will be used for a future programme of nuclear steelmaking systems, two kinds of alloys have successfully been developed in Japan. The designs of these alloys have been reviewed from metallurgical aspects including their composition and creep properties. In addition to the conventional methods to strengthen these alloys, such as solid solution hardening or particle precipitation hardening, a grain-boundary precipitation strengthening due to tungsten-rich α2 phase in the Ni-Cr-W system, would be expected as a further advanced method.

Nanoscale oxygen nonstoichiometry in epitaxial TiO2 films grown by pulsed laser deposition

Nanoscale control of oxygen nonstoichiometry in oxide thin films has become more and more important as the characteristic sizes of oxide devices have been reduced to the nanometer scale. We propose a new in situ electrochemical approach for nanoscale characterization of oxygen stoichiometry in epitaxial oxide thin films. For this purpose, we have developed a pulsed laser deposition system equipped with an electrochemical analysis cell. In situ characterization of homoepitaxial TiO2 thin films by a combination of reflection high-energy electron diffraction and Mott-Schottky analysis has revealed that the lattice oxygen in the TiO2 film subsurface region up to a depth of about 10 nm can be easily lost or gained, depending on temperature and the ambient oxygen pressure. This unavoidably results in a nanoscale inhomogeneous distribution of oxygen vacancies in TiO2 films.

Electrochemical impedance analysis of electric field dependence of the permittivity of SrTiO3 and TiO2 single crystals

Electric field dependence of the permittivity of Nb-doped SrTiO3(001) (Nb:STO) and Nb-doped TiO2 (110) (Nb:TiO2) were investigated by using an electrochemical impedance spectroscopy method. The plots for Nb:STO could be well fitted to extrapolate the flat-band potential and the donor density, provided that a model of the electric field dependence of the permittivity of STO is incorporated into the conventional Mott–Schottky equation. The constants that represents a degree of the electric field dependence of the permittivity were calculated from the fitting parameters, and they were in good agreement with the reported values for the Nb:STO-based solid-state Schottky junction. In a similar way, the electric field dependence of the permittivity of Nb:TiO2 was revealed but it was not so strong as the case of Nb:STO, which is a basis for the validity of the linear fitting approximation with the conventional Mott–Schottky equation for TiO2.

Suppression of grain boundary sliding by second phase particles

Confirmation des resultats theoriques par letude experimentale effectuee sur un alliage Cu−GeO 2

High-temperature deformation and fracture behaviour of Cu−SiO2 bicrystals

Bicrystals of CU-SiO2 dispersion-hardened alloys and of pure copper were tensile tested at various temperatures between 450 and 1050 K at a strain rate of 1.5 x 10−4 sec−1. In the case of pure copper bicrystals, elongation to fracture did not depend significantly on temperature and the fracture mode was invariably transgranular up to 850 K. On the other hand, the ductility of CU-SiO2 bicrystals decreased with increase in temperature and the transition in the fracture mode from transgranular to intergranular occurred at around 450 K. SiO2 particles on grain boundaries play an important role on intergranular fracture by suppressing grain-boundary sliding and also on the retardation of recrystallization during deformation. Two types of Cu-SiO2 bicrystals having different crystal orientation relationships show quite different deformation and fracture behaviour. This can be explained in terms of the contribution of lattice dislocations to the grain-boundary sliding.

Quenching defects in a nitrogen-containing austenitic stainless steel

Only a few studies have been made on the secondary defects in quenched austenitic stainless steels in contrast to a large number of studies on the secondary defects in quenched non-ferrous metals and alloys, especially in aluminum alloys. Recently Rowcliffe and Nicholson 1 have made a quench experiment of austenitic stainless steels and found a few types of secondary defects in a phosphorus bearing austenitic stainless steel. During the course of our investigation on the precipitation of dichromium nitride, CraN , in a high chromium-high nickel austenitic stainless steel containing more than 0.3 wt pct nitrogen, 2 new secondary defects other than those found by Rowcliffe and Nicholson were found in austenitic stainless steels. A prel iminary observation on these secondary defects is presented in this communication. The steel was melted and cast into an ingot of 3 kg under 10 atm pressure of nitrogen with commercial ly high purity in a h igh-pressure melting furnace, s The ingot was forged to a bar whose c ross section was 13 x 13 mm a. The composition of the steel is given below:

Effects of some carbide stabilizing elements on creep-rupture strength and microstructural changes of 18-10 austenitic steel

The creep rupture test has been carried out for 18Cr-10Ni-0.1 wt pct C stainless steels bearing individually Ti, Nb(Cb), and V, followed by the microstructural study. The highest value of 700°C-104 h rupture strength in a titanium and niobium series (the steel containing various amounts of titanium and niobium, respectively) has been obtained at Ti/C and Nb/C atomic ratio of 0.8 and 0.2 to 0.4, respectively. On the other hand, in a vanadium series, the creep rupture strength of the steel showed its maximum at V/C atomic ratio of about unity in the testing at the temperature of 700° and 800°C, but at 600°C, the strength increases monotonically with vanadium content up to 1.53 wt pct. Such high strength in the steels con-taining proper amount of Ti, Nb, and V is related mainly with the fine distribution of M23C6 precipitates which is caused by the acceleration of nucleation due to the foregoing precipi-tation of a MC type carbide within the austenite grains. And it has been deduced that the solid solution strengthening effect of the vanadium contributes also to the remarkable in-crease in the rupture strength of the vanadium steel at 600°C.

Precipitation of sigma phase in nickel-rich NiCrW alloys as an intermediate precipitate

En etudiant la precipitation dans des alliages a une seule phase γ riches en Ni on constate que la phase sigma qui se forme comme precipite metastable intermediaire apparait a des temperatures ou la phase sigma nexiste pas comme phase dequilibre. Etude experimentale sur Ni-33% Cr-14% W en poids

Photoelectrochemical epitaxy of silver oxide clathrate Ag7O8M (M = NO3, HSO4) on rutile-type Nb-doped TiO2 single crystals

Silver oxide clathrate Ag7O8M (M = NO3, HSO4) compounds were synthesized photoelectrochemically on rutile-type Nb-doped TiO2 single-crystal substrates. Epitaxial crystal growth was achieved for some clathrate compositions and substrate surface orientations, where commensurate growth is possible due to lattice matching between the pseudo lattice of the clathrate Ag6O8 cages and the TiO2 surface, similar to the well-known case of epitaxial C60 growth on single-crystal substrates. Particularly for the growth of Ag7O8NO3 on Nb-doped TiO2(110), fully (111)-oriented epitaxial crystallites without any other orientations were obtained. The selectivity for Ag7O8NO3 growth and the suppression of the formation of by-products, such as O2, were found to depend on the electrode potential. The highest selectivity was obtained at +0.2 V vs. Ag in a 0.01 M AgNO3 solution. An investigation of Ag7O8(MM′) (M = NO3, M′ = HSO4) depositions from solutions with different AgNO3 and Ag2SO4 mixing ratios showed that the growth of Ag7O8HSO4 is much faster than that of Ag7O8NO3. The process of incorporating monovalent M anions into the clathrate Ag6O8 cages was identified as the rate-limiting step for the growth of silver oxide clathrate compounds.