Zr Specimens Research Articles

Initial stages of oxide formation on the Zr surface at low oxygen pressure: An in situ FIM and XPS study

An improved methodology of the Zr specimen preparation was developed which allows fabrication of stable Zr nanotips suitable for FIM and AP applications. Initial oxidation of the Zr surface was studied on a Zr nanotip by FIM and on a polycrystalline Zr foil by XPS, both at low oxygen pressure (10−8–10−7mbar). The XPS data reveal that in a first, fast stage of oxidation, a Zr suboxide interlayer is formed which contains three suboxide components (Zr+1, Zr+2 and Zr+3) and is located between the Zr surface and a stoichiometric ZrO2 overlayer that grows in a second, slow oxidation stage. The sole suboxide layer has been observed for the first time at very early states of the oxidation (oxygen exposure ≤4L). The Ne+ FIM observations are in accord with a two stage process of Zr oxide formation.

Zr-based coating for the prevention of silver tarnishing

Purpose – This work aims to focus on evaluation of the ability of cathodically deposited ZrO2-based coatings to stabilise the surface of silver against tarnishing caused by S2−-based species and comparison of these coatings with Cr oxide-based coating. Design/methodology/approach – The coatings were evaluated using inductively coupled plasma optical emission spectroscopy (ICP-OES), scanning electron microscopy (SEM), impedance spectroscopy (EIS) and galvanostatic reduction of specimens exposed to H2S vapours. Findings – The kinetics of deposition were studied and optimum conditions for coating formation were determined. The options of manual application suitable for practical use were studied also. Zr specimens showed better barrier properties than did a chromated specimen. The best properties were observed on a specimen with a manually fast-applied coating. However, Zr coating does not provide as good protection from tarnishing as chromate. The structure of the Cr coating contains species of chromate that oxidize species of S2−. Originality/value – The paper provides new information regarding the anticorrosive properties of cathodically deposited ZrO2 coating as an alternative to Cr coating for the preservation of silver surfaces.

Twinning during recrystallization cooling in α-Zr alloy

Recently, twinning induced by intergranular thermal residual stresses (ITRSs) has been first reported in a Zr alloy after β→α furnace cooling. To further explore this kind of twinning behavior, microstructures and textures of the Zr alloy during α annealing between 650 and 750°C after β quenching were investigated in the present work, by use of X-ray diffraction, electron backscatter diffraction and electron channeling contrast imaging techniques. Results indicate that the ITRS-induced twins could also occur during cooling after α-recrystallization annealing in the Zr alloy, closely related to both the grain size and the texture. When the recrystallization texture is relatively random, high tensile ITRSs up to ~200MPa along c-axis could be accumulated. Tensile twins can only be induced by the tensile ITRS in the nearly texture-free Zr specimen with an average grain size larger than ~20μm.

Enhancement of Calcium Phosphate Formation on Zirconium by Combination of Simple Electrochemical Treatments

Electrochemical surface treatments of micro-arc oxidation (MAO) and following cathodic polarization were performed on zirconium (Zr) disks in this study to enhance the bioactivity of Zr. The surface oxide layers formed with electrochemical treatments on Zr disks were characterized using surface analyses; the calcium phosphate formation on the specimens after immersion in Hanks’ solution was evaluated. As a result, thick calcium phosphate layers formed on only Zr specimens that underwent both MAO treatment with a mixture of calcium glycerophosphate and magnesium acetate and subsequent cathodic polarization treatment with sodium sulfate solution, while no precipitate was observed without treatment. Thus, this technique was confirmed to be a promising method to improve the bioactivity of Zr.

Reactions between U–Zr alloys and Fe at 1003 K

In metal fuel for fast reactors, liquefaction at the fuel–cladding interface may occur in off-normal events and enhance cladding wastage. To provide the basis for understanding this liquefaction phenomenon, reactions in diffusion couples of U–23at.%Zr alloys and Fe at 1003K have been examined. The selected temperature is 5K higher than the eutectic point in the U–Fe binary system. The effect of lanthanide fission products on the liquefaction was evaluated by adding 1at.%Ce to some of the U–Zr specimens. The test results indicated that a liquid phase formed in the reaction zone. The phases in the reaction zone were identified based on electron probe microanalysis results and the calculated U–Zr–Fe phase diagram. The reaction zone growth rate decreased with time. The thickness of the reaction zones at 1003K in diffusion couples pre-annealed at 923K was less than those in diffusion couples without the pre-anneal. Zirconium in the fuel alloy has little influence on the threshold temperature for the liquefaction onset, and the addition of 1at.%Ce has little effect on the reaction of U–Zr alloy with Fe.

Enhancement of calcium phosphate formation on zirconium by micro-arc oxidation and chemical treatments

Surface treatments with various combinations of micro-arc oxidation (MAO) and chemical treatments were performed on zirconium (Zr) disks in this study to enhance the bioactivity of Zr. The surface oxide layers formed with MAO and chemical treatments on Zr disks were characterized using surface analyses; the calcium phosphate formation on the specimens after immersion in Hanks' solution was evaluated. As a result, thick calcium phosphate layers formed on Zr specimens that underwent MAO treatment with a mixture of calcium glycerophosphate and magnesium acetate and subsequent chemical treatment with sulfuric acid or sodium hydroxide solutions, while no precipitate was observed without treatment. The surface analyses revealed that the porous oxide layer formed by MAO treatment contained Ca, P, and Mg, which were incorporated from the electrolytes during the treatments. In addition, both the composition and crystal structure of the oxide layers uniquely changed as a result of the various chemical treatments.

Microstructure Changes in Zr-Based Metallic Glass Induced by Ion Milling

Abstract Four groups of transmission electron microscopy (TEM) specimens of Zr 52.5 Cu 17.9 Ni 14.6 Al 10 Ti 5 metallic glass were prepared by double-jet electropolishing, dimpling and ion milling. TEM observations indicate that the microstructures in electron-transparent areas change sensitively with ion milling conditions. Detailed comparisons between the selected area electron diffraction patterns of TEM specimens and the X-ray diffraction patterns of thermally annealed samples of the same composition show that significant heating effects exist in some severe milling processes, resulting in the formation of various ordered structures in thin areas. In TEM specimens prepared by intentionally long time ion milling, the peak temperature locally reached is estimated as high as 780 K, far above the glass transition temperature of this metallic glass. The results suggest that precautions must be taken to prepare TEM specimens of amorphous alloys by ion milling, particularly when the induced microstructure changes may lead to invalid judgments.

Friction and Wear Properties of Zr and TiC-Based Cermet Specimens in a Hydrogen Gas Atmosphere

In this study, Zr, Ti and TiC-based cermet specimens were prepared, and their friction and wear properties in various gas atmospheres were examined. The Zr specimens exhibited the lowest friction coefficients and the smallest volume change in the H2 gas atmosphere. This reason would have been due to the formation of the -type Zr hydride, which has a layered crystal structure. The Ti specimens exhibited similar friction and wear properties, although the TiC-based cermet specimens containing small amount of Mo, Ni and W exhibited the lowest friction coefficients in air, not in the H2 gas atmosphere. The low friction coefficients of the cermet specimens in air is likely to have been due to the formation of the low friction Mo oxides and W oxides.

Formation, Thermal Stability and Mechanical Properties of Bulk Glassy Alloys with a Diameter of 20 mm in Zr-(Ti,Nb)-Al-Ni-Cu System

Bulk glassy alloy rods with a diameter of 20 mm were produced for Zr61Ti2Nb2Al7:5Ni10Cu17:5 and Zr60Ti2Nb2Al7:5Ni10Cu18:5 by a tilt casting method. The replacement of Zr by a small amount of Ti and Nb caused a distinct increase in the maximum diameter from 16 mm for Zr65Al7:5Ni10Cu17:5 to 20 mm, accompanying the decrease in liquidus temperature and the increase in reduced glass transition temperature. The primary precipitation phase from supercooled liquid also shows a distinct change, i.e., from coexistent Zr2Cu, Zr2Ni and Zr6NiAl2 phases for the 65%Zr alloy to an icosahedral phase for the 61%Zr and 60%Zr alloys. These results allow us to presume that the enhancement of the glassforming ability is due to an increase in the stability of supercooled liquid against crystallization caused by the development of icosahedral shortrange ordered atomic configurations. The 60%Zr specimens taken from the central and near-surface regions in the transverse cross section at the site which is 15 mm away from the bottom surface of the cast glassy rod with a diameter of 20 mm exhibit good mechanical properties under a compressive deformation mode, i.e., Young’s modulus of 81 GPa, large elastic strain of 0.02, high yield strength of 1610 MPa and distinct plastic strain of 0.012. Besides, a number of shear bands are observed along the maximum shear stress plane on the peripheral surface near the final fracture site. The finding of producing the large scale Zr-based bulk glassy alloys exhibiting reliable mechanical properties is encouraging for future advancement of bulk glassy alloys as a new type of functional material. [doi:10.2320/matertrans.MER2008179]

The Role of Zr in the High-Temperature Oxidation of Fe<sub>3</sub>Al

The paper describes an examination of the effect of the addition of zirconium as a third element on the heat-resisting properties and explains the high temperature oxidation mechanism of Fe3Al intermetallic compounds. The Fe3Al and Fe3Al-0,05Zr specimens have been isothermally oxidized in the temperature range of 1173-1473 K in synthetic air for 100 hrs. The formed oxide layer, about 1,5-2 μm thick, was Al2O3. An examination of the cross-sectioned scales by SEM-EDS showed that the alumina layer consisted of a small inner columnar layer and an outer equiaxed grain layer. Additionally, very fine (50-150 nm) oxide grains rich in Zr, further identified as ZrO2, were found across the alumina scales. To understand the role of Zr on the growth mechanism of α–Al2O3 oxide scale on Fe3Al materials, two-stage oxidation experiments were performed (16O2/18O2), followed by SIMS and TEM-SAD observations. Particular attention was paid to the use of TEM in order to precisely characterize the products on samples prepared using the FIB (Focused Ion Beam) method. A combination of analytical techniques revealed that ZrO2 particles, most of which were formed along alumina grain boundaries, enhanced oxygen diffusion along grain boundaries due to oxygen-deficient composition of zirconium oxide (ZrO2-y).

Alpha to Omega Transition in Shock Compressed Zirconium: Crystallographic Aspects

In the present study, specimens of Zr were subjected to shock compression of 11.6 GPa. TEM examination of the recovered samples revealed that during shock compression the a phase has transformed into the w phase. The orientation relationships (ORs) between the a and w phase have been determined using both the stereographic projection method and the correspondence matrix method. Our ORs have been found to belong to the Variant I OR given by Usikov and Zilbershtein (UZ) for statically compressed Zr samples. Our ORs are the same as the one reported by Song and Gray (SG) on dynamically compressed samples. In the present paper it has been shown that the OR of SG is a subset of the OR of UZ and is not apart from it. The mechanism of a®w transition with respect to occurrence of an intermediate b (bcc) structure, during the transition has also been explored. We also show in this study that the amount of the transformed w phase decreases with increasing oxygen content in the samples that were shock loaded to the same peak pressure, as is revealed by both the TEM and XRD results.

Investigation of Homogenization Treatment in Al- Li- Cu- Zr Alloys

Aluminum-Lithium alloys were developed as major replacements for existing aluminum alloys to reduce the weight of aircraft and aerospace structures. Mechanical properties of Al-Li alloys greatly depend on solidification conditions. Other than reducing the microsegregation, homogenization treatment has other effects on the microstructure of Al-Li ingots. In this research, effects of homogenization treatment at constant temperature (500°C) on the precipitation in the microstructure of Al-1Li-3Cu-0.1Zr (wt %) and Al-2Li-3Cu- 0.1Zr (wt %) specimens have been investigated. Results show that homogenization at 500°C for 24 hours not only increases the hardness and phases precipitating in grain but also reduces microsegregation of Fe in grain boundaries.

Microstructure and corrosion characteristics of Zr–1.5Nb–0.4Sn–0.2Fe–0.1Cr alloy with a β-annealing

The corrosion characteristics of Zr-based alloys are very sensitive to their microstructures which are determined by the β-annealing conditions. Specimens of Zr–1.5Nb–0.4Sn–0.2Fe–0.1Cr alloy were investigated in order to obtain their optimized β-annealing conditions such as the β-annealing temperature and the cooling rate from the β-region. From the microstructural analysis, the prior β-grain size of this alloy was increased by increasing the β-annealing temperature. The martensitic structure was formed by a fast cooling rate of a water quenching from a range of 960–1200 °C and the Widmanstätten structure was formed by a slow cooling rate of an air cooling from 1050 °C. The corrosion behavior of the Zr–1.5Nb–0.4Sn–0.2Fe–0.1Cr alloy was not affected by the β-annealing temperature but it was changed by the cooling rate. The increased corrosion resistance was caused by a decreased the supersaturated Nb content in the matrix by a formation of the β-Zr phase. However, the formation of the β-Zr phase is not recommended in the β-solution annealing procedure of Nb-containing Zr-based alloy, because the aim of the β-solution annealing is to obtain a good homogeneity.

Potentiodynamic polarization studies on bulk amorphous alloys and Zr 46.75Ti 8.25Cu 7.5Ni 10Be 27.5 and Zr 65Cu 17.5Ni 10Al 7.5

Potentiodynamic polarization studies were carried out on virgin specimens of Zr 46.75Ti 8.25Cu 7.5Ni 10Be 27.5 (Vitreloy-4 or V4) and Zr 65Cu 17.5Ni 10Al 7.5 (Inoue alloy) bulk amorphous alloys in solutions of 0.5 M H 2SO 4, 0.5 M HNO 3, 0.5 M HCl and 0.5 M NaOH at room temperature. The corrosion current density ( I corr) of Zr 46.75Ti 8.25Cu 7.5Ni 10Be 27.5 bulk amorphous alloy has been found to be about two times than that for Zr 65Cu 17.5Ni 10Al 7.5 alloy in 0.5 M H 2SO 4, whereas the same is comparable in the other three media for both the alloys. A general comparison of the values of the I corr suggests that Zr 46.75Ti 8.25Cu 7.5Ni 10Be 27.5 alloy is more prone to corrosion than Zr 65Cu 17.5Ni 10Al 7.5 alloy in all the aqueous media. The differences in the corrosion behavior are attributed to the alloy constituents on the native surfaces of the two alloys.

Interface study of transition metal (Fe, Zr) on 4H–SiC(0 0 0 1)Si face: photoemission electron microscopy and soft X-ray fluorescence spectroscopy

We have studied interface electronic structure of transition metal film (Fe, Zr)/4H–SiC (substrate) contact systems by using a photoemission electron microscopy (PEEM) and a soft X-ray fluorescence spectroscopy (SXFS). PEEM can show surface micro- and/or nano-structures, whereas SXFS can tell details of chemical bonding states. For specimens of Fe(10 nm)/4H–SiC(0 0 0 1)Si face contact system annealed at 300–900 °C, PEEM images have shown dramatic change in surface morphology above 550 °C, where the Si L 2,3 fluorescence spectra can be explained by considering formation of iron silicides. On the other hand, PEEM images and the Si L 2,3 fluorescence spectra for specimens of Zr(film)/4H–SiC(0 0 0 1)Si face contact system annealed at 650–1100 °C have indicated that interfaces of specimens are stable up to 1100 °C without any reaction.

Tensile behaviour and acoustic emission in zirconium alloys after thermal aging and sodium exposure

Tubular specimens of Zircaloy-2 and Zr–2.5%Nb alloys were exposed to flowing sodium at 613 K for different durations. Similar tubular specimens of the two alloys were also given equivalent thermal treatment. Tensile testing of all the specimens was carried out at a nominal strain rate of 5 × 10 −4 s −1 and ambient temperature (298 K) along with recording of acoustic emission (AE) signals. Microhardness values of the specimens were determined. The tensile and microhardness results indicated marginal changes in the properties consequent to sodium exposure and thermal treatment. Higher AE generated in the sodium exposed and thermally treated specimens as compared to non-exposed/heat-treated specimens is attributed to decohesion of hydride platelets during tensile testing. While the tensile properties could not show any difference between the untreated and treated specimens, the AE results could very well bring out higher amount of decohesion of hydrides in sodium exposed as compared to thermally aged specimens.

Effect of β-Zr decomposition on the solubility limits for H in Zr–2.5Nb

The zirconium alloy Zr–2.5Nb (Zr–2.5 wt.% Nb) is used to fabricate pressure tubes for CANDU®11CANDU® (CANada Deuterium Uranium) is a registered trademark of AECL (Atomic Energy of Canada Limited). nuclear reactors. The microstructure of the as-extruded tubes consists of elongated h.c.p. α-Zr grains surrounded by a network of b.c.c. β-Zr containing at least 20 wt.% Nb. The β-Zr phase is meta-stable at temperatures below 900 K and gradually dissociates into its constituent elements α-Zr and β-Nb. The aim of this study was to determine the effect of β-Zr decomposition on the terminal solid solubility for dissolution (TSSD) for H in Zr–2.5Nb. Specimens of Zr–2.5Nb were charged to different hydrogen concentrations and aged at temperatures ranging from 673 to 773 K for 30 to 6000 min, to decompose their β-Zr component by different amounts. Hydride dissolution temperature and hydrogen concentration were determined using differential scanning calorimetry and hot vacuum extraction mass spectrometry, respectively. The results show that the as-extruded pressure tube structure, with the β-Zr almost intact, has the highest TSSD and that the TSSD for structures with fully decomposed β-phase is the lowest and approaches that of unalloyed Zr.

Measurements of hydrogen permeation and absorption in zirconium oxide scales

The hydrogen absorption and the permeation behavior through the oxide layer formed on modified Zircaloy-4 (Zry-4) alloys were investigated. The modified Zry-4 was prepared by altering the chemical composition of standard Zry-4. The tin content of Zry-4 (1.5 wt%) was reduced to 0.5 wt%, and alloying elements Si, O and Nb were added from 0.01 to 0.2 wt%. The oxide layers were grown in a static autoclave at 360 °C under 18.3 MPa for 150 days. Fick’s law was used to calculate the diffusivity of hydrogen after the steady state of the permeation flux was reached. The diffusivity of hydrogen in the 0.5Sn–0.1Nb–0.1Fe–0.2Cr–0.2O–Zr specimen was lower than that in the 1.5Sn–0.2Fe–0.1Cr–0.1O–0.01Si–Zr and Zry-4 specimens. As the area fraction of precipitates increased, the hydrogen diffusivity increased whereas an inverse relationship between the diffusivity and the amount of the tetragonal phase was observed. In addition to the oxide structural study, the effects of the microstructure of the zirconium alloys such as precipitates and grain boundaries on the hydrogen absorption were studied.

Fracture strength of hydride precipitates in Zr–2.5Nb alloys

The hydride precipitate fracture strength as a function of precipitate size and temperature (23–250°C) in smooth tensile specimens of Zr–2.5Nb pressure tube material (nominal hydrogen content 100 ppm by weight) was studied using the acoustic emission technique. At room temperature, this strength is sensitive to hydride length when the average hydride length is short, and is insensitive to the hydride length when the average hydride length is longer than 25 μm. The hydride fracture strength decreases slightly with temperature, but a more rapid decrease in the yield strength offsets this decrease, resulting in a brittle-to-ductile transition at 120°C to 140°C in smooth tensile tests. The presence of hydrides causes a decrease in the ultimate tensile strength of the material at low temperatures, and has no effect at high temperatures for these smooth tensile specimens.

Hydrogen solubility limits in α- and β-zirconium

Specimens of pure zirconium and Zr–20Nb alloy were charged to different hydrogen concentrations from the gas phase at 673 K. Before and after charging with hydrogen, the Zr–20Nb specimens were annealed in vacuum at 1123 K for 1 h and cooled to room temperature within about 30 min to produce a β-Zr structure. Some of these specimens were annealed further at 773 K for about 500 h to completely transform them to ( α-Zr+ β-Nb) phase. The specimens were analyzed for hydride transition temperatures using differential scanning calorimetry and for hydrogen concentration using hot vacuum extraction mass spectrometry. The hydrogen solubility limits found for pure Zr are in good agreement with the previously reported values. The solubility limits found in β-Zr are about two orders of magnitude higher than the solubility limits in α-Zr and the solubility limits found in the aged Zr–20Nb alloy are very close to those in α-Zr.