Embrittlement in Neutron Irradiated Niobium

This paper discusses the effect of neutron irradiation on the embrittlement of niobium. The irradiation was carried out at about 100℃ to a neutron does of 2.9×10^n/cm^2 (<1 MeV) and measurements were made of the yield stress, fracture stress and fracture strain in the temperature range from liquid nitrogen to room temperature. The interaction of dislocations produced by deformation with irradiation-induced defects was studied by transmission electron microscopy. The fracture surfaces were examined using a scanning electron microscope. The brittle fracture stress was evaluated under theory of the dislocation and by the strain-energy method. Based on the results obtained, a possible mechanism for the fracture in neutron irradiated niobium was discussed

Nucleation of Dislocation Around Inclusions in Iron Crystals

Nucleation of dislocations around inclusions, small particles of FeO, Al_2O_3 or ZrO_2, in iron was investigated by means of transmission electron microscopy. Dislocation loops produced around inclusions by the prismatic punching were observed in iron containing FeO or ZrO_2 particles when annealed for 100 min at 800℃ and furnace cooled. These loops seem to be produced by thermal stresses arising, during cooling, from the difference in the thermal expansion coefficient between the inclusion and the matrix. Generation of dislocations was also observed around inclutions and precipitates in iron deformed by 2% at room temperature. The nucleation mechanism of dislocation loops around inclusions in iron was discussed. The nucleation stress of the dislocation loop calculated from the present model is μ/22～μ/47 at room temperature where μ is the shear modulus of the matrix, which is compatible with the experimental observation of dislocation around the ZrO_2 inclusion. The elastic shear stress distribution around a spherical inclusion in an isotropic elastic medium during the application of tension was considered. In this case, the stress concentration around the inclusion was too small to nucleate a dislocation loop. It is deduced that the residual stress around the inclusion thermally induced during cooling, for example, plays a major role in the nucleation of dislocations by mechanical deformation

Structural Changes in Amorphous Pd_<80>Si_<20> by Neutron Irradiation(Metallurgy)

Amorphous Pd_Si_ was irradiated with fast neutrons ( neutrons/cm^2. X-ray scattering intensities were measured before and after the irradiation with monochromatic Cu-Kα_1 rays. Scattered intensities for s>0.4A^ (s=2sinθ/λ) proved unaffected, while intensities were found remarkably enhanced for s>0.4A^ after the irradiation, i.e., in the small-angle region and the leading edge of the first halo . The results are discussed in relation to the structural anomalies in amorphous solids

New Ternary Hydride Formation in U-Ti-H System

Hydrogen absorption properties of two titanium-rich uranium alloys, UTi_2 and UTi_4, were studied in order to prepare and identify the recently found ternary hydride. They slowly reacted with hydrogen of the initial pressure of 10^5Pa at 873K to form the ternary hydride. The hydrogenated specimen mainly consisted of the pursued ternary hydride but contained also U(or UO_2), TiH_x, and some transient phases. X-ray powder diffraction and Electron Probe Micro Analysis proved that it was the UTi_2H_x with the expected MgCu_2 structure, though all the X-ray peaks were broad probably because of inhomogeneity. This compound had extremely high resistance to powdering on its formation, which showed high potential utilities for a non-powdering tritium storage system or for other purposes

The Role of the Dislocation Nucleation Around Precipitates in the Mechanical Properties of a Fe-C Alloy

The effect of the dislocation nucleation around the precipitates on the mechanical properties of a Fe-0.02 wt%C alloy has been investigated by tensile testing and by means of transmission electron microscopy, and an attempt has been made to correlate tensile data with the observed dislocation configuration. After ageing for 30 min at 400℃, dislocation loops or helical dislocations were seen around many precipitates. These loops seemed to be produced by the prismatic punching from the precipitate-matrix interface, because the mean atomic volume of the precipitate is larger than that of the matrix. When the ageing temperature decreased to 150℃, dislocation loopes were seen only around some precipitates, and herical dislocations were rarely detected. The variation of the yield drop which is deformed at room temperature with ageing temperature has a minimum at 400℃ after ageing for 30 min. It has been suggested that the yield drop is associated with the dislocation nucleation around the precipitates when ageing or straining. In deformation at liquid nitrogen the specimens either fractured before yielding or necked immediately after yielding and the specimens containing large precipitates were less ductile than those with small precipitates. The stress concentration due to dislocations nucleated around the precipitates during straining will cause a cleavage crack of a precipitate particle and this cracking will initate cleavage microcracks in the adjacent matrix

Vickers Microhardness Testing with Miniaturized Disk Specimens

The microhardness technique has been increasingly important for testing irradiated materials because of the necessity of small-scale specimen technology. In order to establish Vickers microhardness testing over a wide temperature range using miniaturized specimens such as transmission electron microscopy (TEM) disks, an apparatus that permits the measurements in the temperature range of well below liquid nitrogen temperature to well above room temperature is developed. Effects of indentation size, disk-specimen thickness, specimen setting and test temperature on the Vickers microhardness data of an unirradiated iron alloy are described

Low Temperature Ultrasonic Attenuation in Magnesium and Magnesium Alloys

The ultrasonic attenuation in pure Mg and Mg-Li and Mg-N alloys was studied in the temperature range from 4.2°K to 300°K. In Mg, four peaks were observed in the attenuation vs temperature curves. The peaks were named as P_1, P_2, P_3 and P_4 from the low temperature side. P_1 was observed at about 20°K and the origin of which was confirmed to be the interaction between sound waves and conduction electrons. The activation energies of the relaxation processes accompanied with P_2 and P_3 were obtained as 0.009 eV and 0.09 eV, respectively. The ratio between the activation energies for P_2 and P_3 agrees well with that calculated from Seeger\u27s theory making use of the values of the critical resolved shear stress for the basal slip and the non-basal slip. Therefore, the relaxation processes related to P_2 and P_3 are confirmed to be dislocation movements in the basal plane and in the non-basal plane, respectively. In Mg-Li alloys, the activation energy increased for P_2 but decreased for P_3. In Mg-N alloys, the activation energy for P_2 was comparable with that of pure Mg. The activation energy for P_4 was about 0.5 eV, and the value was considerably higher than that of the other peaks. Therefore, the origin of P_4 probably differs from that of the other peaks