Showing papers on "Zirconium alloy published in 2002"

Shock wave response of a zirconium-based bulk metallic glass and its composite

Abstract: A zirconium-based bulk metallic glass, Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vit 1), and its composite, Zr56.3Ti13.8Cu6.9Ni5.6Nb5.0Be12.5 (beta-Vit), were subjected to planar impact loading. A surprisingly low amplitude elastic precursor and bulk wave, corresponding to the elastic response of the "frozen structure" of the intact metallic glasses, were observed to precede the rate-dependent large deformation shock wave. A concave downward curvature after the initial increase of the U-s-U-p shock Hugoniots suggests that a phase-change-like transition occurred during shock compression. Further, compression damage occurred due to the shear localization. The spalling in Vit 1 was induced by shear localization, while in beta-Vit, it was due to debonding of the beta-phase boundary from the matrix. The spall strengths at strain rate of 2x10(6) s(-1) were determined to be 2.35 and 2.11 GPa for Vit 1 and beta-Vit, respectively.

Hydride-induced embrittlement and fracture in metals—effect of stress and temperature distribution

Abstract: A mathematical model for the hydrogen embrittlement of hydride forming metals has been developed. The model takes into account the coupling of the operating physical processes, namely: (i) hydrogen diffusion, (ii) hydride precipitation, (iii) non-mechanical energy flow and (iv) hydride/solid-solution deformation. Material damage and crack growth are also simulated by using de-cohesion model, which takes into account the time variation of energy of de-cohesion, due to the time-dependent process of hydride precipitation. The bulk of the material, outside the de-cohesion layer, is assumed to behave elastically. The hydrogen embrittlement model has been implemented numerically into a finite element framework and tested successfully against experimental data and analytical solutions on hydrogen thermal transport (in: Wunderlich, W. (Ed.), Proceedings of the European Conference on Computational Mechanics, Munich, Germany, 1999, J. Nucl. Mater. (2000a) 279 (2–3) 273). The model has been used for the simulation of Zircaloy-2 hydrogen embrittlement and delayed hydride cracking initiation in (i) a boundary layer problem of a semi-infinite crack, under mode I loading and constant temperature, and (ii) a cracked plate, under tensile stress and temperature gradient. The initial and boundary conditions in case (ii) are those encountered in the fuel cladding of light water reactors, during operation. The effects of near-tip stress intensification as well as of temperature gradient on hydride precipitation and material damage have been studied. The numerical simulation predicts hydride precipitation at a small distance from the crack-tip. When the remote loading is sufficient, the near-tip hydrides fracture. Thus a microcrack is generated, which is separated from the main crack by a ductile ligament, in agreement with experimental observations.

Analysis of steady-state thermal creep of Zr-2.5Nb pressure tube material

Abstract: The steady-state thermal creep rate in the axial and transverse directions of Zr-2.5Nb of pressure tubes, used in CANDU nuclear reactors, was determined. The data were obtained both from tensile samples having their tensile axes cut along the axial and transverse directions of the pressure tubes and from small-sized, thin-walled tubes, i.e., “mini” tubes stressed either in torsion or by internally pressurizing capsules manufactured from the mini tubes, or by additionally applying an external, axial load on these internally pressurized capsules. The temperature range of the data was from 373 to 596 K (100 °C to 323 °C) and the duration of the tests was from about 1500 hours to over 12,000 hours. The tests were carried out over a sufficiently long time for the creep rate to be measurable in the steady-state creep regime. It was found that the steady-state creep rate depends on stress in a nonlinear fashion and the stress exponent over the entire temperature range was about four. This value is consistent with the values measured earlier on other zirconium alloys. The activation energy Q was found to be about 21 and 10 kcal/mol for temperatures above and below 475 K (∼ 200 °C), respectively. These values are lower than those measured by other investigators on the same material at higher temperatures but similar to values found on other Zr alloys at low temperatures. It appears that Q is dependent on temperature and its value is consistent with the presence of dynamic strain aging (DSA). The results of this study were analyzed with a polycrystalline, nonlinear self-consistent model that take into account the crystallographic texture of the material. This model was used to derive the values of critical resolved shear stress (CRSS), which are consistent with prismatic, basal, and pyramidal glide. By using these values and the apparent temperature dependence of Q, it was shown that this model predicts well the steady-state creep rate over the entire temperature range and under very different stress states.

A Study on the Chemical Forms and Migration Behavior of Carbon-14 Leached from the Simulated Hull Waste in the Underground condition

Abstract: The chemical forms of carbon leaching from carbon-containing Zr and Fe-based metallic materials have been investigated to improve the estimation of the contribution of C-14 in the performance assessment of TRU waste disposal. Both organic and inorganic carbons were identified in the leached solution with carbon containing zirconium and steel, and the concentrations of total carbon (organic plus inorganic) in the leached solutions increased with time. The carbon concentrations in the leached solution for both metallic samples were higher at higher pH. With High Performance Liquid Chromatography (HPLC), organic carbons were identified to be low-molecular weight alcohols, carboxylic acids and aldehydes. To explore the chemical state of carbon in the matrix materials, the leaching experiments were carried out also for ZrC, Fe3C, the powder mixtures of carbon and zirconium, and of carbon and iron. The low-molecular weight organic carbons were detected only in the case of carbides (ZrC and Fe3C). The chemical forms of carbon in the zirconium alloy were suggested to be carbide or carbon by H.D. Smith[1]. The present result suggests that the chemical forms of carbon in zirconium or iron are mainly in the form of carbide. In the interest of performance assessment, the distribution coefficients of the organic carbon species identified in the leached solution for cement were obtained. As expected, some of them were shown to be larger than the values assumed in the performance assessment of Progress Report on Disposal Concept for TRU Waste in Japan[2].

Development and validation of a processing map for zirconium alloys

Abstract: For the development of processing maps to zirconium alloys, a simple instability condition based on the Ziegler's continuum principles as applied to large plastic flow is extended for delineating the regions of unstable metal flow/occurrence of fracture or defects, utilizing the flow stress data of Zr-2.5Nb-0.5Cu. An attempt is made to fit the measured flow stress data in a constitutive equation, useful in the finite element process models. Instability maps at different strain levels were superimposed while delineating the unstable regions in the processing maps. This phenomenon takes into account the dependence of strain rate sensitivity and strain hardening coefficient of the material on the plastic instability during hot deformation. The applicability of the developed processing map has been examined by comparing with the reported microstructural observations of the deformed compression specimens of various zirconium alloys. It is found that the processing map is practically usable in the real fabrication process for the zirconium alloys.

Studies on the stability of anodic oxides on zirconium biocompatible alloys

Abstract: Studies on the stability of anodic oxides grown on zirconium and its biocompatible alloys Ti-50Zr at.% and Zr-2.5Nb wt.%, in aerated Ringer physiological solution, at 25 and 37 °C, were carried out by comparing their formation charge and their reconstruction charge after dissolution at open circuit in the physiological solution. The stability of oxides grown in the Ringer solution and in a 0.15 mol L-1 Na2SO4 solution was compared. The obtained results show that the stability of these oxides is increased by aging under potentiostatic conditions and can be decreased by the presence of chloride ions in the electrolyte during the anodization process.

Magnetically soft nanomaterials for high-temperature applications

Abstract: FeCo-based soft magnetic alloys of compositions Fe/sub 45/Co/sub 43/Cu/sub 1/B/sub 3.6/Zr/sub 7.4-x/AM/sub x/, where x=3.7 at.% and 7.4 at.%, and the alloying metals (AM) are Nb and Hf and were prepared in the form of amorphous ribbons by a melt-spinning technique. A two-phase microstructure, consisting of /spl alpha/-FeCo nanocrystals and amorphous matrix, was created by partial devitrification of the amorphous alloys during annealing for 1 h at temperatures 500/spl deg/C-600/spl deg/C. The crystallization process of the amorphous alloys was studied using differential scanning calorimetry, X-ray diffractometry, and transmission electron microscopy. The quasi-static hysteresis loop was measured using an inductive method. Partial replacement of zirconium by hafnium improved thermal stability of the amorphous structure and the magnetic properties of the alloys studied. The effect of niobium is opposite. The best magnetic properties were found in the alloy Fe/sub 45/Co/sub 43/Cu/sub 1/B/sub 3.6/Zr/sub 3.7/Hf/sub 3.7/. It was found that the nanocrystallization temperature strongly influences the thermal stability of the magnetic properties of the alloys studied.

On the embrittlement of zircaloy-4 under RIA-relevant conditions

Abstract: The extended use of Zircaloy cladding in light water reactors degrades its mechanical properties by a combination of irradiation embrittlement, coolant-side oxidation, hydrogen pickup, and hydride formation. The hydrides are usually concentrated in the form of a dense layer or rim near the cooler outer surface of the cladding. Utilizing plane-strain ring-stretch tests to approximate the loading path in a reactivity-initiated accident (RIA) transient, we examined the influence of a hydride rim on the fracture behavior of unirradiated Zircaloy-4 cladding at room temperature and 300 C. Failure is sensitive to hydride-rim thickness such that cladding tubes with a hydride-rim thickness >100 {micro}m ({approx}700 wppm total hydrogen) exhibit brittle behavior, while those with a thickness <90 {micro}m ({approx}600 wppm) remain ductile. The mechanism of failure is identified as strain-induced crack initiation within the hydride rim and failure within the uncracked ligament due to either a shear instability or damage-induced fracture. We also report some preliminary results of the uniaxial tensile behavior of low-Sn Zircaloy-4 cladding tubes in a cold-worked, stress-relieved condition in the transverse (hoop) direction at strain rates of 0.001/s and 0.2/s and temperatures of 26-400 C.

Method for manufacturing zirconium-based alloys containing niobium for use in nuclear fuel rod cladding

Abstract: The present invention relates to a method for manufacturing zirconium-based alloys containing niobium with superior corrosion resistance for use in nuclear fuel rod claddings. The method of this invention comprises melting of the alloy, β-forging, β-quenching, hot-working, vacuum annealing, cold-working, intermediate annealing and final annealing, whereby the niobium concentration in the α-Zr matrix decreases from the supersaturation state to the equilibrium state to improve the corrosion resistance of the alloy. Such zirconium-based alloys containing niobium are usefully applied to nuclear fuel rod cladding of the cores in light water reactors and heavy water reactors.

High-strength beryllium-free moulded body made from zirconium alloys which may be plastically deformed at room temperature

Abstract: The invention relates to high-strength, beryllium-free moulded bodies made from zirconium alloys which may be plastically deformed. Said moulded bodies are characterised in comprising a material, essentially corresponding to the following formula in composition: Zra (E1)b (E2)c (E3)d (E4)e, where E1 = one or several of Nb, Ta, Mo, Cr, W, Ti, V, Hf and Y, E2 = one or several of Cu, Au, Ag, Pd and Pt, E3 = one or several of Ni, Co, Fe, Zn and Mn, E4 = one or several of Al, Ga, Si, P, C, B, Sn, Pb and Sb, a = 100-(b+c+d+e), b = 5 to 15, c = 5 to 15, d = 0 to 15 and e = 5 to 15 (a, b, c, d, e in atom %). The moulded body essentially comprises a homogeneous, microstructural structure, which is a glass-like or nano-crystalline matrix with a ductile, dendritic, cubic body-centred phase embedded therein.