The role of chemical free energy and elastic strain in the nucleation of zirconium hydride (original) (raw)
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Effect of applied load on nucleation and growth of γ-hydrides in zirconium
Computational Materials Science, 2002
c-hydride precipitation and growth in zirconium were investigated by using a phase-field kinetic model. The orientation difference of hydride is represented by non-conservative structural field variables, whereas the concentration difference of hydrogen in precipitates and matrix is described by a conserved field variable. The temporal evolution of the spatially dependent field variables is determined by numerically solving the time-dependent Ginzburg-Landau equations for the structural variables and the Cahn-Hilliard diffusion equation for the concentration variable. It is demonstrated that a certain load level is required to completely reorient hydride precipitates and it is most effective to apply loads during the initial nucleation stage for producing anisotropic precipitate alignment.
Effect of applied load on nucleation and growth of [gamma]-hydrides in zirconium
Computational materials science, 2002
c-hydride precipitation and growth in zirconium were investigated by using a phase-field kinetic model. The orientation difference of hydride is represented by non-conservative structural field variables, whereas the concentration difference of hydrogen in precipitates and matrix is described by a conserved field variable. The temporal evolution of the spatially dependent field variables is determined by numerically solving the time-dependent Ginzburg-Landau equations for the structural variables and the Cahn-Hilliard diffusion equation for the concentration variable. It is demonstrated that a certain load level is required to completely reorient hydride precipitates and it is most effective to apply loads during the initial nucleation stage for producing anisotropic precipitate alignment.
Journal of ASTM International, 2008
Zirconium alloys are currently used in nuclear power plants where they are submitted to hydrogen pick-up. Hydrogen in solid solution or hydride precipitation can affect the behavior of zirconium alloys during service but also in long term storage and in accidental conditions. Numerical modeling at mesoscopic scale using a "phase field" approach has been launched to describe hydride precipitation and its consequences on the mechanical properties of zirconium alloys. To obtain realistic results, it should take into account an accurate kinetic, thermodynamic, and structural database in order to properly describe hydride nucleation, growth, and coalescence as well as hydride interaction with external stresses. Therefore, an accurate structural characterization was performed on Zircaloy-4 plates and it allowed us to identify a new zirconium hydride phase called . The phase has a trigonal symmetry and is fully coherent with hcp ␣Zr. The consequences of this new zirconium hydride phase on hydride transformation process and stressreorientation phenomenon are discussed. A first attempt to numerically model the precipitation of this new zirconium hydride phase has been undertaken using the phase field approach.
A Review on Hydride Precipitation in Zirconium Alloys
Journal of Nuclear Materials, 2015
Nucleation and formation of hydride precipitates in zirconium alloys have been an important factor in limiting the lifetime of nuclear fuel cladding for over 50 years. This review provides a concise summary of experimental and computational studies performed on hydride precipitation in zirconium alloys since the 1960's. Different computational models, including density functional theory, molecular dynamics, phase field, and finite element models applied to study hydride precipitation are reviewed, with specific consideration given to the phase field model, which has become a popular and powerful computational tool for modeling microstructure evolution. The strengths and weaknesses of these models are discussed in detail. An outline of potential future work in this area is discussed as well.
Computer Simulation of Hydride Precipitation in Bi-crystalline Zirconium
MRS Proceedings, 2001
ABSTRACTγ-hydride precipitation and growth in a zirconium bi-crystal were simulated using a phase field kinetic model. The temporal evolution of the spatially dependent field variables is determined by numerically solving the time-dependent Ginzburg-Landau equations for the structural variables and the Cahn-Hilliard diffusion equation for the concentration variable. The morphology evolution of γ-hydride with and without external load was simulated. It is demonstrated that nucleation density of the hydride at the grain boundary increases as compared to the bulk and favorable hydride precipitation at the grain boundary become weaker when an external load is applied.
Surfaces and Interfaces, 2019
A thin zirconium hydride layer was deposited electrolytically on a hot-rolled and annealed Zr-2.5 wt.% Nb material. Changes in the α-Zr phase associated with the hydride precipitation were studied by synchrotron X-ray diffraction. Strain variations and the dislocation density in the α-Zr matrix were estimated from changes in diffraction peak positions and using X-ray Line Profile Analysis, respectively. It was shown that the dislocation density increased measurably in the α-Zr matrix due to the precipitation of the hydrides which corroborates previous transmission electron microscopy (TEM) observations indicating that the α-Zr accommodates some of the volume misfit by plastic deformation. The probable strain profile in the Zr-H system as a function of tested layer thickness is discussed. 2. Experimental 2.1. Samples The composition of the base material was 2.5 wt.% Nb, ∼1200 ppm O, ∼950 ppm Fe, ∼110 ppm C and balance Zr. Ingots were forged to 112 mm thick at 1065 C and then hot rolled to 56 mm thick at about
Journal of physics. Condensed matter : an Institute of Physics journal, 2015
Structural, thermodynamic and elastic properties of the hydrogen-zirconium system including all major hydrides are studied from first principles. Interstitial hydrogen atoms occupy preferentially tetrahedral sites. The calculations show that a single vacancy in α-Zr can trap up to nine hydrogen atoms. Self-interstitial Zr atoms attract hydrogen to a lesser extent. Accumulation of hydrogen atoms near self-interstitials may become a nucleation site for hydrides. By including the temperature-dependent terms of the free energy based on ab initio calculations, hydrogen adsorption isotherms are computed and shown to be in good agreement with experimental data. The solubility of hydrogen decreases in Zr under compressive strain. The volume dependence on hydrogen concentration is similar for hydrogen in solution and in hydrides. The bulk modulus increases with hydrogen concentration from 96 to 132 GPa.
Effect of Number of Variants of Zirconium Hydride on Grain Growth of Zirconium
Metals
The microstructure characteristics of Zr-hydride in Zr are important concerns in metallurgy and nuclear engineering. In particular, it is known that the correlation between hydride and the grain boundary microstructure has a great influence on properties. In this study, a phase-field model was used to evaluate evolutions of the fractions of intra-granular and inter-granular hydride and multi-contacted hydride according to the number of structural variants of δ-hydride in the 3D system. The effect of the numbers of crystallographic variants of hydride on grain growth kinetics was also analyzed. We found that the pinning effect in 3D is minimized when hydrides have one crystallographic variant, which is contradictory observation with the 2D case. With grain structures with comparable average grain radii and quantities, we found that the fraction of the intra-granular and inter-granular hydrides increase as the number of crystallographic variants increases.
Journal of Nuclear Materials, 2014
Thermal cycling of Zr2.5%Nb pressure tubes specimens containing ~100 wt ppm H between room temperature and 400ºC produces the dissolution and re-precipitation of zirconium hydride, with a distinctive hysteresis between these two processes. In this work, we have found that the details of the precipitation and dissolution depend on the actual orientation of the α-Zr grains where hydride precipitation takes place. In-situ synchrotron X-ray diffraction experiments during such thermal cycles have provided information about hydride precipitation specific to the two most important groups of α-Zr phase orientations, namely crystallites having c-axes parallel (m Hoop) and tilted by ~20º (m Tilted) from the tube hoop direction. The results indicate that hydrides precipitate at slightly higher temperatures (~5ºC), and dissolve at consistently higher temperatures (~15ºC) in m Tilted grains than in m Hoop grains. Moreover, application of a tensile stress along the tube hoop direction results in two noticeable effects in hydride precipitation. Firstly, it shifts hydride precipitation towards higher temperatures, at a rate of ~(0.08±0.02)°C/MPa for hydrides precipitated in the m Hoop grains. Secondly, it produces a redistribution of hydrogen between grains of different orientations, increasing hydride precipitation on those α-Zr grains having their caxes stretched by the external load. A detailed analysis of the diffracted signal shows that such redistribution occurs during the precipitation stage, as a result of changes in the precipitation temperatures for different grain orientations.