Hexagonal Zirconium Research Articles

Synthesis of Highly Ordered Thermally Stable Cubic Mesostructured Zirconium Oxophosphate Templated by Tri-Headgroup Quaternary Ammonium Surfactants

Highly ordered cubic mesostructured zirconium oxophosphate with Pm3n symmetry analogous to SBA-1 silica is first obtained by using tri-headgroup quaternary ammonium surfactant CH3(CH2)16CH2N(CH3)2CH2CH2N(CH3)2CH2CH2CH2N(CH3)3Br3 (C18-2-3-1) as a structure-directing agent and zirconium sulfate as an inorganic precursor under hydrothermal condition. X-ray diffraction (XRD), transmission electron microscopy (TEM), and 31P magic angle spinning nuclear magnetic resonance (MAS NMR) have been used to characterize these mesostructured zirconium-based composites. The cubic mesostructured zirconium oxophosphates are thermally stable up to 500 °C. In addition, a phase transition from Pm3n through mixed mesophase to p6m symmetry is observed through finely tuning the spacer methylene chain length among the hydrophilic headgroups in the multi-charged amphiphile (from C18-2-3-1 through C18-3-3-1 to C18-3-4-1). 31P MAS NMR spectra show that phosphorus has four types of chemical environments in the cubic mesostructured zirconium oxophosphate framework and three types of chemical environments in mixed or hexagonal zirconium oxophosphate mesophases.

A study of the stress state associated with twin nucleation and propagation in anisotropic materials

Abstract In this work we are concerned with the study of the stress state associated with the activation of twinning in elastically anisotropic materials, in an attempt to achieve a better understanding of the contribution that twinning makes to plastic deformation and to texture development during forming operations. The assumption usually made is that twinning is activated by a critical resolved shear stress on the twinning plane and in the twinning direction, although the experimental evidence suggests that twinning depends in a more complex way upon the other stress components present in the medium. Here we use a continuum approach to model the twin lamella as a flat inclusion of elliptic section embedded in an elastically anisotropic medium being acted upon by externally applied stresses. We calculate the Gibbs free energy of the system as the sum of an elastic term associated with size and inhomogeneity effects, and a surface energy term, associated with the twin-matrix interface. The minimization of the total energy with respect to the dimensions of the lamella is relevant both to twin nucleation and to twin propagation; in the former case, it gives the condition for when twin nucleation is energetically favourable with respect to a homogeneously strained matrix. In the latter case, it gives the condition of instability of an existing twin embryo, much the same as the Griffith's criterion does for crack extension. We derive explicit results for the twinning systems active in silicon-iron, calcite and hexagonal zirconium, titanium and zinc. We find that, although the critical condition for twin activation depends on all six independent stress components, the resolved shear dominates and the influence of the other stress components is about two orders of magnitude weaker. We conclude that any substantial dependence of twinning upon stress components other than the resolved shear must come from the dependence of the twin-boundary energy upon stress. Possible atomistic mechanisms which may be influenced by a general stress field are discussed qualitatively.

An X-ray Fourier line shape analysis in cold-worked hexagonal zirconium

Detailed Fourier analysis of line shapes has been carried out in hexagonal zirconium metal in the cold-worked and partially recovered states. X-ray diffraction profiles from the fault-unaffected 10.0, 00.2, 11.0, 11.2 and 00.4 and fault-affected 10.1, 10.3, 20.1, 10.2 and 20.2 reflections have been recorded using a counter diffractometer for this purpose. The line shape analysis has shown small anisotropy as regards the domain sizes and strains, the average values being of the order of 280 A and 2.08×10−3 respectively in the cold-worked state with a small recovery at a higher temperature. Least-square solution applied to fault-affected reflections has yielded very small concentration of deformation faults with a negligible proportion of growth faults. Results are found to be consistent with earlier X-ray measurements and recent electron microscope investigations on zirconium.

Thermodynamic properties of zirconium-cadmium and certain other solid solutions

The experimental work reported here has to do with the zirconium-rich zirconium-cadmium alloys. The phase diagram has been determined up to a concentration of 20 at. % Cd. Lattice parameters have been measured for the hexagonal phase. The vapor pressure of cadmium in equilibrium with the solid solution has been measured for both the hexagonal and cubic phases as a function of temperature and concentration. These data are considered together with earlier data on the Zr-Ag, Zr-In, Zr-Sn and Zr-Sb primary substitutional solid solutions. Empirical relations are found between the hexagonal ( c a ) ratio, the temperature of the transformation from cubic to hexagonal zirconium and the electronic specific heat coefficient on the one hand and the concentration of principal quantum number 5 electrons on the other. These simple relations suggest the possibility that the principal quantum number 5 electrons are entering a rigid conduction band in zirconium. In order to investigate this possibility, we have calculated the partial molal energy of cadmium in zirconium from the vapor pressure measurements. We have also done this for certain other solutions for which vapor pressure data have been published. We conclude that the rigid band model is a bad approximation and point out that this is in accord with recent quantum mechanical calculations. Furthermore, the effect of cadmium on the transformation temperature of zirconium is the resultant of two opposite and nearly balanced effects—an absolute zero energy effect which tends to stabilize the cubic form and an entropy effect which tends to stabilize the hexagonal form. Finally, it is shown that, for most of the solutions considered here, the energy of a Wigner-Seitz polyhedron is independent of concentration within experimental error.

Yield loci of Zircaloy tubing with different textures

Abstract Yield loci of four batches of Zircaloy-4 canning tubes with different textures were measured at room temperature for quadrants I and IV (axial tensile or compressive stresses in combination with internal pressure). The circumferential and axial strains were continuously recorded using applied strain gauges and maximum plastic strains of about 0.2%.A comparison with the yield loci determined by Knoop hardness measurements gave marked differences in the shape of the yield loci. The experimental results showed that the shape of the yield loci of Zircaloy tubing can be altered by texture variations, mainly by changes in the tilt angle of the basal poles of the hexagonal zirconium crystals from the radial towards the tangential direction of the tubes. The yield loci of tubes with a sharp ideal orientation can be calculated successfully for all four quadrants on the basis of the active slip or twinning systems for specific biaxial loading conditions. It is shown that the use of the Von Mises yield crit...

Lattice dynamics, third order elastic constants and low temperature lattice thermal expansion of zirconium

The lattice dynamics and the anharmonic properties of the hexagonal Zirconium are worked out using Keating's approach. The dispersion curves are fitted using twelve second order parameters and the six second order elastic constants are evaluated. The ten third order elastic constants are calculated using five third order parameters. The experimental measurements on the pressure derivatives of the second order elastic constants in Zirconium are in good agreement with the calculated values. The low-temperature limit of the lattice thermal expansion is calculated which agrees well with the value obtained from thermal expansion data. The variation of the generalised GPs of the elastic modes with the direction of propagation is illustrated by polar diagram.

On indexing electron diffraction patterns for hexagonal zirconium alloys

On indexing electron diffraction patterns for hexagonal zirconium alloys

Evolution structurale, au cours de trempes ou de revenus, d'alliages zirconium-cuivre a teneur en cuivre inferieure a 5% pds

Various zirconium-copper alloys with 1.1–1.6 or 2.4 wt % copper were quenched from the hightemperatureβ phase and their associated phase transformation studied. According to the quenchingrate, the β phase gives rise to various metastable structures which can be subsequently decomposed by tempering. The fastest quenching-rates give rise to a ω phase of hexagonal structure ( c/ a = 0.624) which forms martensitically. If the ω phase is reheated, a linear expansion anomaly of + 0.34% is found in the range 418–455 °C approximately; this corresponds to the decomposition of the ω phase into an α s solid solution, which is a supersaturated solution of copper in hexagonal zirconium, similar to the α equilibrium solid solution. On isothermal tempering or on heating to higher temperatures, α s decomposes into equilibrium α and a precipitate of Zr 2Cu. Only the most zirconium-rich alloys among those studied give rise to the metastable ω phase. The TTT curves for the decomposition of ω have been determined over a wide temperature range.

Specific Heats of Zirconium Alloys at Low Temperatures

The specific heats of dilute alloys of silver, cadmium, indium, tin, and antimony in hexagonal zirconium were measured from 1.2 to 4.5 deg K. For each alloy the specific heat obeyed the relation c = gamma T + BETA T/sup 3/ within the experimental error. All of these solutes increased gamma linearly with concentration and d gamma /dx was linearly related to the chemical valence of the solute. The increase in gamma in the tinzirconium alloys showed that all electrons outside closed shells cannot be treated as equivalent in the rigid band model of alloying. All of these solutes increased the density af states of zirconium, providing evidence for a zone overlap in the zirconium d band and for a small number of electrons in a new band. All of the solutes increased the lattice specific heat and the Debye temperature was a linear function of solute concentration for each of these alloy systems. The function d theta /dx was not simply related to the solute valence but showed a close correlation with the magnitude of the rate of change of the distance between atoms at 000 and 1/3, 2/ 3, and 1/2 of the hexagonal cell. Additions of tinmore » to zirconium did not change the average interaction potential of the BCS theory appreciably. (auth)« less

Coefficients of Thermal Expansion for Zirconium

The expansion coefficients of hexagonal (α) zirconium have been calculated from the lattice parameters of both low and high hafnium alloys in the range 0° to 600°C. It is found that the coefficients are straight-line functions of the temperature and that the effect of hafnium is to depress the mean coefficients by about 1 pct at room temperature and 9 pct at 600°C. The cubic (β) form was stabilized so that its coefficient could be determined from 400° to 900°C.

Thermal Expansion of Zirconium between 298°K and 1600°K

The average coefficients of thermal expansion of hexagonal zirconium between 298°K and 1143°K are 5.5×10−6 deg−1 along the a axis, 10.8×10−6 deg−1 along the c axis, and 7.2×10−6 deg−1 for a randomly oriented polycrystalline sample. The average value of the linear coefficient of expansion of cubic zirconium between 1143°K and 1600°K is 9.7×10−6 deg−1. At the transition there is a decrease in volume of 0.66 percent.