Showing papers on "Zirconium alloy published in 2007"

Cooling-rate induced softening in a Zr50Cu50 bulk metallic glass

Abstract: Contrary to the cooling-rate induced hardening observed in crystalline metals, the authors report here an unexpected surface softening in rapidly solidified Zr50Cu50 bulk metallic glass. A soft layer ∼500μm thick was detected near the surface with both hardness and elastic modulus increasing from the surface to the interior. To understand the reason for this, a correlation between cooling rate and defect concentration was derived. Defect concentration was found to increase as the cooling rate increased, suggesting that surface softening may be the result of freezing-in of excess defects, induced by a faster cooling rate near the surface compared to the interior.

Electrochemical Impedance Spectroscopic Study of Passive Zirconium I. High-Temperature, Deaerated Aqueous Solutions

Abstract: The development of deterministic models for predicting the accumulation of corrosion damage to zirconium and Zircaloys in pressurized water reactor (PWR) coolant environments requires the acquisition of values for various model parameters. In the present work, the point defect model (PDM) was further developed to account for the properties of passive films comprising hydride barrier layers and porous oxide outer layers that form on zirconium and Zircaloys in high-temperature, hydrogenated aqueous solutions. The model parameter values were extracted from electrochemical impedance spectroscopic data for zirconium in hydrogenated, borate buffer solution [0.1 M B(OH) 3 + 0.001 M LiOH, pH 6.94] at 250°C by optimization. The results indicate that the corrosion resistance of zirconium in high-temperature, hydrogenated aqueous solutions is dominated by the outer layer, as was found to be the case for nonhydrogenated solutions where a defective oxide barrier layer forms. The impedance model based on the PDM provides a good account of the growth of the bi-layer passive films described above, and the extracted model parameter values might be used, for example, for predicting the accumulation of general corrosion damage to Zircaloy fuel cladding in PWR operating environments.

Corrosion behavior and glass-forming ability of Cu-Zr-Al-Nb alloys

Abstract: Amorphous Cu50−xZr45Al5Nbx (x = 0, 1, 3 and 5 at.%) samples with critical diameters of 1.54–2.05 mm were formed by copper-mold suction casting. The effects of Nb on the glass formation ability were evaluated with differential scanning calorimetry and the corrosion resistances of the alloys with the addition of Nb were evaluated in 1 N HCl, 3% NaCl and 0.5 N H2SO4 solutions, respectively, by weight loss and polarization curve measurements. The samples surfaces were examined after the corrosion test with SEM. It was found that the addition of Nb causes a decrease of ΔTx of Cu50−xZr45Al5Nbx alloy from 54 K to 45 K. On the other hand, the corrosion resistances in various solutions were improved with the addition of Nb.

Quantitative hydrogen analysis of zircaloy-4 using low-pressure laser plasma technique.

Abstract: It is found in this work that variation of laser power density in low-pressure plasma spectrochemical analysis of hydrogen affects sensitively the hydrogen emission intensity from the unwanted and yet ubiquitous presence of ambient water. A special experimental setup has been devised to allow the simple condition of focusing/defocusing the laser beam on the sample surface. When applied to zircaloy-4 samples prepared with various hydrogen impurity concentrations using low-pressure helium surrounding gas, good-quality hydrogen emission lines of very high signal to background ratios were obtained with high reproducibility under weakly focused or largely defocused laser irradiation. These measurements resulted in a linear calibration line with nonzero intercept representing the residual contribution from the recalcitrant water molecules. It was further shown that this can be evaluated and taken into account by means of the measured intensity ratio between the oxygen and zirconium emission lines. We have demonstrated the applicability of this experimental approach for quantitative determination of hydrogen impurity concentrations in the samples considered.

Correlation between mechanical, thermal and electronic properties in Zr–Ni, Cu amorphous alloys

Abstract: We show that mechanical properties (stiffness and hardness) of Zr–Ni, Cu amorphous alloys increase linearly with Ni, Cu content over a wide composition range (22 ≤ x Ni,Cu ≤ 65 at%). This correlates with the observed increase in the Debye temperatures and densities with x and shows that the strength of interatomic bonding increases with x in these alloys. Accordingly, the thermal stability (e.g. the crystallization and glass transition temperatures) of these alloys also increases with x. Since the electronic density of states at the Fermi level decreases linearly with x within the same x-range, a very simple relationship exists between the electronic structure and mechanical and thermal properties. We also deduce the mechanical properties of hypothetic amorphous Zr and briefly discuss the possibility of its preparation.

Thermal and elastic properties of Cu–Zr–Be bulk metallic glass forming alloys

Abstract: The compositional dependence of thermal and elastic properties of Cu–Zr–Be ternary bulk metallic glass forming alloys was systematically studied. There exists a linear relationship between the glass transition temperature Tg and the total Zr concentration. G decreases linearly with increasing Zr concentration as well. The results also show that Tg, shear modulus G, and Poisson's ratio nu are very sensitive to changes in compositions. Low Tg, low G, and relatively high nu can be achieved with high Zr and Ti concentration.

Influence of Zr addition on the microstructure evolution and thermal stability of Al-Mg-Mn alloy processed by ECAP at elevated temperature

Abstract: Two Al–Mg–Mn alloys having similar compositions but with and without Zr addition were subjected to equal channel angular pressing (ECAP) at 350 °C using route Bc and a 90° die, followed by water quenching or air cooling. A series of annealing experiments were conducted at various temperatures from 400 to 460 °C on the water-quenched alloys. Fine structures with grain sizes of about 1–2 μm were obtained in these alloys after six passes. The water-quenched alloy containing Zr exhibited finer structure and higher strength compared to the Zr-free alloy in the same cooling condition, which was due to the existence of Al 3 Zr dispersoids. And in air cooling condition, the difference of strength between these two alloys increased further. This was because that the static recovery and static recrystallization during the air cooling process decreased the strength of the Zr-free alloy, while the fine Al 3 Zr dispersoids inhibited the restoration in the alloy containing Zr. In addition, in the annealing experiments, secondary recrystallization took place in the Zr-free alloy annealed at 410 °C for 1 h, and it was completed when annealed at 460 °C for 1 h. While the alloy containing Zr kept stable up to 460 °C for the pinning effect of the Al 3 Zr dispersoids. Tensile tests showed that the strength of the Zr-free alloy decreased dramatically after annealing at 460 °C for 1 h, while the alloy containing Zr exhibited almost no decline at the same condition.

Mechanical properties and ion irradiation of bulk amorphous Zr55Cu30Al10Ni5 alloy

Abstract: The bulk amorphous Zr55Cu30Al10Ni5 alloy is one of the widely studied Zr-based alloys due to very attractive thermal and mechanical properties. Alloy ribbons and bulk samples were synthesized by Cu mould casting and characterized by SEM/EDS, DSC and XRD. Mechanical properties like Vicker's hardness, nanohardness and elastic modulus etc. were measured. Ion irradiation of the samples was carried out to enhance the surface properties without altering the amorphous nature of the bulk material. Ion irradiation by singly charged Ar+ enhanced the hardness as well as elastic modulus considerably. (C) 2007 Elsevier B.V. All rights reserved.

Grain refinement of commercially pure zirconium by ECAP and subsequent intermediate heat treatment

Abstract: Microstructural evolution of commercially pure zirconium (Zr702) processed by equal channel angular pressing (ECAP) and subsequent intermediate heat treatment was investigated. ECAP was carried out at room temperature, and the sample was rotated 90° in the same sense in each pass. Also, intermediate heat treatment of the ECAP samples processed by was performed at 773 and 873 K for their recovery and recrystallization temperatures. When the samples were annealed at recrystallization temperature of 873 K, the mean grain size of these samples decreased from ∼40 to ∼3 μm by the further four passes of ECAP. However, when the samples were annealed at recovery temperature of 773 K, the mean grain size was found to be 0.2 μm after the same ECAP process. A difference in microstructural evolution with annealing processes indicated that the rate of formation of new grains for ultra-fine-grained Zr702 was accelerated due to the high fraction of subgrains into the matrix of Zr702 after recovery.