[Show abstract][Hide abstract]ABSTRACT: An in-depth understanding of the phase stability in ZrO2 film is very important for improving the corrosion resistance of zirconium alloys. ZrO2 film formed on Zr–2.5Nb alloys was characterized by grazing incidence X-ray diffraction. It is shown that in comparison to the monoclinic phase crystallites, the tetragonal phase crystallites undergo larger compressive strain, indicating that there are some regions of local stress concentration in ZrO2 film. These local stresses have an important influence on the phase stability in ZrO2 film. In this paper, the correlation among the macroscopic compressive stress, the grain size and the local stress was analyzed qualitatively from a thermodynamic viewpoint, showing that the tetragonal phase may be partly stabilized by the local stresses. The local stresses mainly come from three sources: the local transformation strain produced by the tetragonal→monoclinic phase transformation, the oxidization of intermetallic precipitates and the different matching degrees between the ZrO2 and Zr-based crystallites.
No preview · Article · Jun 2007 · Journal of Alloys and Compounds
[Show abstract][Hide abstract]ABSTRACT: A thermodynamic model is developed to understand the origin of variation in the microstructure of ZrO2 film formed on zirconium alloys and its effects on corrosion resistance. The correlation among the tetragonal phase fraction, the stress (macroscopic and internal one), the ZrO2 grain size and the microstructural change of oxide film is formulized, and then analyzed. The results show that many complicated factors simultaneously govern the microstructure of oxide film. The tetragonal phase content near the oxide/metal interface, the macroscopic compressive stress near the interface, the decline gradient of macroscopic compressive stress and the internal stress induced by the transformation from the tetragonal to the monoclinic phase have very important influences on the transition from columnar grains to equiaxed grains, the crack formation and the degradation of oxidation resistance. The presence of intermetallic precipitates in oxide film may effectively relax the internal stress caused by transformation strain, stabilize the columnar-grain structure and reduce the probability of crack formation. How to reduce the transformation stress in the oxide film is a key to improve the corrosion resistance of zirconium alloys.
[Show abstract][Hide abstract]ABSTRACT: Understanding of diffusion processes in polycrystalline solids is of importance for studying processes like oxidation, precipitation, creep, superplastic forming, annealing and many other processes. In this paper, we will introduce our latest software that is able to simulate the diffusion process in poly and nano-crystal line solids. The diffusion process is simulated based on Random Walk theory. The diffusion matrix can be computer generated or obtained from the experimental measurement using Orientation Imaging Microscopy. The software describes microstructure and incorporates the effects of the material's texture, grain size and shape, grain boundary character distribution, statistical information on CSL boundary distributions, contribution from triple junctions and interfaces, the trapping of diffusing atoms and interaction of atoms with second phases and voids. A built-in database of diffusivities of various diffusing species and the user friendly interface make the software easy to use. The software is also applicable to thin films and multilayer structures. The output of simulation can be presented as a normalized concentration profile, a two-dimensional contour map of diffusing species, and also using many other statistical representations.
[Show abstract][Hide abstract]ABSTRACT: The effects of Pd and Pd77Ag23 alloy membrane microstructures on hydrogen permeation have been investigated using the electrochemical permeation method. Deformation and annealing have been used to modify the membrane microstructures. The delay of hydrogen permeation in palladium increases when the degree of deformation is increases. The annealing of deformed palladium and Pd77Ag23 accelerates hydrogen diffusion. The annealed metals release hydrogen more rapidly than deformed metals. Also, the Pd77Ag23 alloy needs more time to release hydrogen than pure palladium. The solubility of hydrogen increases in both palladium and Pd77Ag23 when the amount of deformation is increased. A hydrogen trapping model was developed, which allows the prediction of trends of hydrogen permeation through membranes having different structures.
No preview · Article · Aug 2004 · Materials Science and Engineering A