Tadatsune Okubo

A Systematic Survey of the Factors Affecting Zircaloy Nodular Corrosion

Out-of-reactor corrosion tests in 10.3-MPa steam at temperatures between 490 and 530°C have been carried out on Zircaloy-2 tube specimens of various fabrication histories including archive specimens of the same tube lots as were used in the commercial boiling water reactor (BWR) to get data for comparison between in- and out-of-reactor nodular corrosion performances. Test conditions, such as temperature and duration, as well as material parameters, such as chemical composition, texture, and precipitates, were examined to clarify the relation to the nodular corrosion susceptibility. The number density and size of nodules increased when tested at higher temperatures. Extending the test duration increased the nodule size, but not the number density. The archive tube lots showed different degrees of nodular corrosion in the out-of-reactor test and their relative rankings seemed to correlate with the visual ratings of in-reactor corrosion. A higher number density of precipitates tended to reduce the nodular corrosion susceptibility. It should be noted, however, that some lots constituted an exception to this, and the cause was sought in the tube manufacturing processes.

Fracture behaviour of boride-dispersed composites fabricated by hot-pressing amorphous Ni60Mo30B10 powder

Microstructural and mechanical property characteristics were investigated for three boride-dispersed composites fabricated by hot-pressing amorphous Ni60Mo30B10 powder. The first composite was tested in the as-hot-pressed condition (HP) whilst the other specimens were subjected to a solution treatment (ST) and further ageing (STA). X-ray diffraction showed the HP and ST composites to consist of Mo2NiB2 particles in a Ni-rich matrix whilst the STA material contained Mo2NiB2 particles in a NiB3Mo matrix. The hardness and fracture toughness decreased and increased, respectively, for the ST material compared to the HP case whilst the STA case showed increased and decreased hardness and fracture toughness, respectively, compared to the ST composite. These results were explained in terms of the brittle–ductile–brittle fracture modes for the HP–ST–STA specimens. In addition, the HP specimen showed only a 15% decrease in compressive strength at 973 K compared to 303 K.

THE INTEGRITY EVALUATION OF THE WELD JOINT OF SPHERICAL ROCKET MOTOR CASE

Abstract The Institute of Space and Astronautical Science (ISAS) is undertaking the launching of spacecraft for space observations and interplanetary explorations. The spherical rocket motor case of satellite launchers was made of Ti-6A1-4V alloy, however, to reduce the weight and fabrication cost, the material of the case was changed to Ti-15V-3Cr-3Sn-3AI alloy because of its good cold formability and the high strength. However, it was found that the hardness of weld joint was higher than that of base metal and the fracture toughness was lower when heat treated by a conventional aging process. This report describes the development of a new heat treatment process to obtain the high strength of base metal and the high fracture toughness of the weld joint, and the evaluation of the integrity of the weld joint by acoustic emission and radiography. The developed heat treatment process is as follows: after solution treatment, the motor case was aged at a relatively low temperature (preaging), welded, and then ...

Molecular Dynamics Simulations. Evaluation of Interatomic Potentials for Zr and Ti.

The embedding atom method (EAM) or the Finnis-Sinclair potentials of Zr and Ti by Pasianot, Oh, and Ackland are compared, and are used to simulate wide range of static and defect properties under the same conditions. The EAM potential for Zr based on Pasianot is determined by the authors with careful selection of experimental data necessary to define parameters in the potential. Selected potentials show similar static properties like lattice constants, cohesive energy, or elastic constants. However, more complicated properties that include interstitial formation energy and interstitial formation volume are not comparable, especially low in potentials that take account of the effect of internal strain. The reason for this tendency is attributed to the difference of deformation field around an interstitial.

A Study of Substructures in Welded Beta Titanium Alloy by Microbeam X-Ray Diffraction Analysis

Ti-15V-3Cr-3Sn-3Al alloy has been used for welded structures such as an upper stage motor case of launch vehicle for space-study satellite [1] because of its good formability and high strength [2]. The fabrication process of the case is as follows: Two half spherical shells, which are formed by cold deep drawing from hot rolled plate, are welded to make a sphere. The welded shell is aged to have desired strength and fracture toughness. The fracture toughness of weld joint is found not to meet the design criterion when the welded case was heat-treated conventionally.

On the Correlation between Microstructure and Mechanical Properties of Titanium Aluminide Matrix Composites

Titanium, Aluminum, and fine SiC or Al2O3 powders were used to synthesize Ti-Al intermetallicmatrix composites by hot press reactive sintering. In the SiC reinforced composites, the exothermic heat during sintering caused a reaction between SiC powders and Ti, producing Ti-Si compounds. The compounds increased the hardness of the composites, and existed mainly between the Ti3Al grain and the TiAl lamella. The fractured surface of the SiC reinforced composites showed much smaller cleavage than that found in the non-reinforced Ti-Al intermetallic. The change in size of microstructure was correlated with the size of fracture unit by the result of microbeam X-ray analysis.

Stress versus strain rate characteristics of Zircaloy cladding tubes subjected to various deformation paths

Load relaxation properties of unirradiated Zircaloy nuclear fuel claddings were measured at 648 K. Deformation paths preceding the relaxation were altered in order to study the factors affecting relaxation properties. Thehistories chosen were (1) constant strain rate tensile deformation. (2) constant load creep deformation, and (3) biaxial followed by uniaxial creep. Stress versus time records were obtained from the relaxation runs. Stress versus strain rate curves were further derived and effects of prior strain rate, strain, transient deformation and strain hardening on relaxation properties were discussed. It was found from the experimental results that after strain hardening and transient deformation saturates, the strain rate just prior to relaxation has a major effect on the stress versus strain rate characteristics. This finding holds both for the relaxation after tension and creep. Existing models could not describe these findings.