Zirconium Specimens Research Articles

Damage structure in zirconium alloys neutron irradiated at 573 to 923 K

Introduction. Zirconiun alloys are resistant to void formation and associated Swelling, but are subject to radiation-enhanced creep, radiation growth and radiation hardening, all of which are dependent on the damage microstructures. Information on the microstructures will aid in understanding these phenomena. We have examined specimens of zirconium, ZLrcaloy-2, and Zr-2.5 wt% Nb irradiated at each of 573,673,773, and 923 K to fluences of 1.3, 1.2, 1.0, and 0.7 x 1025 n.m-2, respectively, in the Oak Ridge Reactor. Foils were electropolished using a 4:1 ethyl alcohol:perchloric acid solution at 240 K and examined at 100 kV in a Phillips EM300G electron microscope using a double axis tilt stage.Observations. Annealed Crystal Bar Zirconium. Dislocation loops were present after irradiation at 573 and 673 K (Fig.1,2). The median diameter increased with irradiation temperature from 18.6 to 50 nm and corresponding loop concentrations decreased from 5 x 1021 to 5 x 1020m-3 respectively.

Effect of Fast-Neutron Irradiation on Twinning Deformation of Zirconium

Unirradiated zirconium specimens and specimens irradiated to a fluence of 4.1 ×1019 n/cm2 were subjected to tensile deformation at room temperature. During the straining, the specimens were removed at intervals from the testing machine for microphotographie inspection, to determine the progress of slip line density Ns, the twin volume fraction Vt and the twin density Ni, as function of strain. The effect of neutron irradiation on these values were studied in detail. The resulting data indicated that the neutron irradiation inhibited the growth of twins in the early stage of deformation, up to a plastic strain ϵp of about 11%, beyond which level the irradiation tended instead to inhibit nucleation. Between growth of twins and nucleation, it was the latter phenomenon that was found more effective in promoting slip deformation. The break in the slope of plots relating the reciprocal work hardening de/da to the true plastic stress σp gave the twin stress σt, which proved to represent a good approximation of the stress of twin nucleation.

Effect of hydrogen concentration and cooling rate on hydride precipitation in [formula omitted

Reactor-grade zirconium specimens containing from 50 to 640 ppm hydrogen have been cooled from solution-treatment temperatures at different rates by water-quenching, oil-quenching, air-cooling and furnace-cooling. Optical and electron metallography have shown that the average size of the hydrides, the proportion of the hydrides located at grain boundaries, and the proportion of the equilibrium δ-hydride increased as the hydrogen concentration was increased and/or the rate of cooling was decreased. The precipitate distributions and relative amounts of the γ and δ hydrides observed can be explained in terms of nucleation theory.

Amplitude-dependent damping in zirconium

Abstract The amplitude-dependent damping and modulus defect of high-purity poly-crystalline zirconium specimens have been measured, in the kilocycle frequency range in longitudinal vibrations, between liquid nitrogen and room temperatures. The influence of temperature strain amplitude, strain distribution and hydrogen content on the damping spectrum is considered. The results are interpreted in terms of an interaction between dislocations and hydrogen atoms and several parameters such as dislocation density, dislocation loop length, diffusion coefficient, dislocation damping constant and binding energy are obtained from the experimental curves.

The tensile stress diagram assuming an elastic hyperbolic sine strain rate law

The tensile stress-strain relations are considered in the microstrain region assuming that there exists an anelastic strain whose strain rate dependence varies as the hyperbolic sine of the relaxable strain. The equations giving the stress-strain curves for the cases of loading, unloading, and reloading are derived. The equations are compared with experimental curves obtained using zirconium specimens prestrained at 77°K to obtain an anelastic strain component.

Dynamic strain aging and ductility minima in zirconium

Tensile tests using coarse grained zirconium specimens were conducted at two strain rates, differing by 3 orders of magnitude, between 77° and 1032°K. At each strain rate, peaks were observed when the flow stress was plotted against the temperature. The temperature corresponding to a given peak was observed to rise with increasing strain rate. A pronounced minimum in the strain rate sensitivity of zirconium near 675°K can be explained in terms of the strain rate dependence of these peaks. At each strain rate, the zirconium tensile specimens also showed a minimum elongation at the hardening peak temperature. Since the reduction in area did not pass through a corresponding minimum, the elongation minima do not reflect a true ductility loss. What actually takes place is an increased tendency to neck at the hardening peak temperature. This tendency to promote a neck can be rationalized in terms of variations in the strain rate sensitivity caused by dynamic strain aging.

Rate controlling processes during the pre-transition oxidation of zirconium alloys

A study of the potentials developed across growing oxide films on zirconium alloys, and the current-voltage characteristics of these oxide films has shown that a simple treatment of the oxidation potential to give transport numbers for ions and electrons is not possible for these alloys. Many of the conditions for such a treatment to apply are not met. The shape of the current-voltage curves shows that the ionic current obeys the Tafel relationship, whereas the electronic current is best fitted by a Schottky emission process. The predicted effect of polarisation during oxidation (predicted from the current-voltage curve) was observed for zirconium specimens. An anomaly in the current-voltage curves for Zircaloy-2 resulted in similar predictions being obeyed only for very thin films. The cause of the anomaly was not established but its occurrence was correlated with a sharp change in the oxidation potential.

Some observations on the fracture characteristics of hydrided α-zirconium

A comprehensive examination of the polished surfaces of deformed Charpy impact specimens and the fracture surfaces of slow-bend specimens of hydrided zirconium indicates that fracture originates at broken hydrides resulting largely from dislocation pile-ups. The characteristics of the fracture surface suggest that propagation occurs in a similar manner in ductile and brittle samples and that microcracks link up by ductile failure of the intervening matrix.

The determination of oxygen in zirconium by radio activation with 14 MeV neutrons: E. Cerrai and F. Gadda, Energia Nucleare, 9 (1962), 317

The determination of oxygen in two zirconium specimens was carried out by radioactivation with 14-Mev neutrons through the O/sup 16/(n,p)N/sup 16/ reaction. Fast neutrons were produced by bombarding with a 150 k static accelerator a Ti-T target with deuterons. A source intensity of 10/sup 9/. neutrons/sec was obtained. Specimens in the form of 20 mm diameter disks were irradiated at the same time of reference standards. As a reference, SAP disks having a known content of oxygen were used. Due to the short half life of N/sup 16/ (7.4 sec) the oxygen measurements were done referring to the activity of aluminum (Mg/sup 27/) which had been previously calibrated towand oxygen. Therefore the same measurement method can be applied to aluminum. Specimens were rapidly transferred from the irradiation facility to the counting room by means of a pneumatic circuit 30 m long. The sensitivity of the method described is about 1 to 2 mg O/sub 2/ per g Zr and the accuracy within plus or minus 10%. With suitable corrections of measurements and a different counting method, sensitivity could be increased. (auth)

An autoradiographic technique for the study of tritium in metals and its application to diffusion in zirconium at 149° to 240° C

In recent years, hydrogen effects in zirconium and its alloys have been studied extensively, largely because of interest in nuclear applications of these alloys. In the present work, tritium has been used in a novel way to obtain diffusion coefficients in zirconium at 240°, 195° and 149° C, temperatures which are below any used heretofore in diffusion experiments. In addition, enhanced grain boundary diffusion was found not to be significant at 150° C. The specimens were prepared by electrolytically depositing a hydride layer from a tritiated water electrolyte on zirconium specimens. The 2 ml of electrolyte contained 220 curies of tritium. Details of the electrolytic cell and conditions of electrolysis are given. The tritium diffused into the specimens under closely controlled conditions of time and temperature and the distribution of tritium in the metal was then studied by autoradiographic techniques. Volume and grain boundary diffusion were studied and a number of segregation effects observed on the autoradiographs. The frequency factor for lattice diffusion of tritium in zirconium was found to be 1.53 × 10 −3cm 2/sec and the activation energy 9070 ± 600 cal/mole, compared with 7060 cal/mole reported for hydrogen in zirconium. The reason for the difference is discussed. The ratio of the diffusion coefficients D Boundary/ D Lattice at 149° C is less than 10 4. The significance of these and other findings is considered in the paper.

Electropolishing techniques for the preparation of zirconium and Zircaloy-2 specimens for transmission electron microscopy

A method for electropolishing zirconium and Zircaloy-2 thin foils for transmission electron microscopy is described. The foil, 1 in x 3/8 in x 0.001 to 0.003 in., insulated around the edges with polystyrene in trichloroethylene, is mounted in a spring clip as the anode. The cathodes are mounted separately from the anode and can be moved vertically without altering the position of one cathode relamaking a small hole at the bottom and at the top of the foil with the cathodes near the foil. Then the cathodes are brought to the center of the foil and moved about 1 cm apart and polishing is continued until the holes enlarge and meet. This takes about one to three minutes depending on the thickness of the foil. (N.W.R.)

Role of hydrogen in the high‐temperature corrosion of zirconium and its alloys. II Oxidation of unalloyed zirconium in steam and in oxygen

AbstractThe film capacities and weight gains of specimens of Kroll zirconium exposed to steam and to dry oxygen at 325° have been compared. The results show that the initial growth of uncracked oxide films is more rapid in steam, and that subsequent film cracking is also more severe, presumably as a result of effects due to hydrogen.Films on Van Arkel zirconium are more resistant to cracking in both steam and oxygen.At a late stage of oxidation in both steam and oxygen the film capacities reach constant values indicating that the mean thickness of the protective part of the film in each case also tends to a constant value. However, a comparison of the oxidation rates at this stage shows some discrepancies in relation to the initial oxidation rates and a modified picture of film growth and breakdown is proposed.

Some Elastic and thermal properties of zirconium and tungsten

Abstract The longitudinal and shear wave velocities of 10 Mc/s ultrasonic waves have been measured in polycrystalline specimens of zirconium and tungsten at temperatures between 14° and 300°k and the elastic constants calculated from these measurements. The difference between the specific heats at constant pressure and at constant volume in these metals at various temperatures has been calculated by means of the thermodynamic relation involving the bulk modulus of the metal; this difference is used in the examination of a suggested method of obtaining the γ values, and hence the electronic specific heats of metals. It is found that the γ values for zirconium and tungsten estimated by this method are greater by factors of about 1·3 and 2·8 respectively than the values obtained from low-temperature calorimetric determinations. For a number of reasons, which are discussed, close agreement is not to be expected but no satisfactory explanation for the very wide discrepancy has been found.

The Electrical Resistance of Oxide Films on Zirconium in Relation to Corrosion

The conductivity of anodic and corrosion films on zirconium was found to increase during heating in vacuum. The conductivity changes at 450°C were studied on anodic films which were 0.6, 0.75, 0.9, and 1.5 μ in thickness. The change in conductivity was attributed to the diffusion of anion vacancies into the oxide with an estimated diffusion coefficient of . Corrosion films produced in water at 350°C appeared to have higher diffusion coefficients. Among the materials zirconium, Zircaloy‐1, and Zircaloy‐2, specimens of zirconium had the highest corrosion rates and diffusivities (as judged by the conductivity changes). Consequently, an effect of vacuum heating on corrosion was expected and subsequently confirmed. Enhanced oxide growth was observed after corroded specimens were heated at 600°C for 1 min. The increase in weight gain was greater than would be expected from a simple restoration of dissolved oxide.

Rapid Measurements of Thermal Diffusivity

Abstract An apparatus and a technique were developed for determining the thermal diffusivity of metals by a periodic heat-flow method. Measurements with a probable error of ± 6.8 per cent were made on specimens of Armco iron, titanium, zirconium, and Haynes Stellite 25 at temperatures of 456, 564, and 672 R.