Hualong Li

A model of texture formation in ZrO2 films

Abstract A model for predicting the texture development in monoclinic ZrO 2 films grown on Zr alloys has been proposed. The model is based on assumptions of selective nucleation and anisotropic oxide grain growth. In this model during the stage of oxide nucleation, oxide and substrate lattice matching is the main factor influencing the orientation of nuclei. In the stage of oxide grain growth, the matching stress at oxide–metal interface plays a major role. The predicted oxide textures have been compared with the results from oxidation experiments performed on three samples obtained from different faces of the Zr–2.5% Nb pressure tube. A good agreement between the predicted oxide texture and the measured texture is observed. Texture analysis of the monoclinic oxide is based on the orientation distribution function (ODF).

Monte-Carlo simulation of texture and microstructure development in nanocrystalline electrodeposits

Abstract A computer model has been proposed to describe the development of texture and microstructure in nanocrystalline electrodeposits of Ni-20%Fe. The main driving force for this process is the minimization of free energy of the system. The results obtained from the simulation show that the texture and microstructure are strongly influenced by the crystallographically anisotropic surface energy and nucleation rate.

Computer modelling the diffusion of Ni in NiO at high temperatures

A computer model was developed to simulate the diffusion of Ni through both the grain boundaries and lattice in NiO at high temperatures, using a random walk method. The grain boundary network was defined by the shape and size of oxide grains and by the grain boundary character distribution (GBCD). These were derived from the experimental data for NiO grown at 1073 K on (100) and (111) faces of Ni single crystal. The simulation shows that GBCD strongly affected the Ni diffusion and the resultant oxidation rate. For GBCD, specific for (100)Ni substrate, the influence of NiO grain size on the oxidation rate was also tested. The NiO growth parameters obtained during the simulation were verified by comparing them with the experimental oxidation kinetics.

Computer Model of Recrystallization Texture in Aluminum Alloys

During recrystallization, frequency of occurrence of nuclei with certain orientation depends on the location of the nucleation site, the energy stored at that location and the rate of release of stored energy. A computer model of nucleation of recrystallization process in cold rolled aluminum is developed based on the analysis of misorientation distributions around the nuclei of different orientations and the measured orientation dependent elastic energy stored in the specimen. Comparison of selected locations of nucleation sites with experiment shows that Cube nuclei are located much more randomly in the cold rolled matrix than the rolling texture components, namely, copper, brass, and sulfur. A specific range of rate of release of stored energy increases the frequency of occurrence of the Cube nuclei. Information on the stored energy after cold deformation and stress relieving provides a great deal of understanding on the mechanism that controls the primary recrystallization in deformed metals. From the diffraction peaks that are measured using X-rays in 88% cold-rolled and stress-relieved can body aluminum alloy, stored energy as a function of crystal orientation is calculated. The rate of reduction of stored energy plays a major role in deciding the rate of nucleation of new grains with different orientations. The stored energy of deformation and unique misorientation distribution that cube grains have in the cold rolled matrix is responsible for the enhanced growth rate of the nucleated cube grains. The simulated results are compared with the experimental data. Agreement of these results suggests that in the development of the cube texture, both oriented nucleation and oriented growth play an important role. Software was developed to simulate texture and microstructure transformation during the annealing process.

Thermal Stability Of Electrodeposited Nanocrystalline Ni-45%Fe Alloys

Electrolytically deposited nanocrystalline Ni-45%Fe alloys, composed of γ (f.c.c.) phase, were annealed at 573 and 673 K for time periods up to 20 h in an inert atmosphere of argon. The evolutions of crystallographic texture, grain size, and microhardness were analyzed as a function of annealing parameters. The initial grains, approximately 5 nm in size increased after annealing up to 10–20 nm, however, the growth kinetics and the final grain size depended on the temperature. A correlation was found between the grain size and alloy microhardness. The phase composition was unstable and during annealing the α(b.c.c) phase was detected to precipitate from the γ matrix. The texture of γ phase, consisted of two fibre components of and , and evolved at high temperatures. Although the detailed changes depended on the annealing temperature, in general, the component became stronger at the expense of the component.

Anisotropic Hydrogen Permeation in Nano/Poly Crystalline-Nickel Membranes

Bilayer nickel membranes have been prepared using electrodeposition to grow polycrystalline nickel on a nanocrystalline nickel substrate. When hydrogen is charged from the nano-Ni side of the nano-Ni and poly-Ni composite membrane, the permeation current increases rapidly, then the membrane releases hydrogen faster during decay. When hydrogen is charged from the poly-Ni side of the same composite membrane, the permeation current rises gradually and takes a longer time to reach steady state. Also the permeability of nano-poly-Ni membrane is eight times higher than that of poly-nano-Ni membrane. The diffusivity for the nano-Ni side charging in a nano-poly-Ni membrane is two times higher than that of poly-Ni side charging of the same membrane. The diffusivity and permeability of nano-poly-Ni membranes are smaller than those for nano-Ni membranes, but larger than those for poly-Ni membranes. Using this anisotropic behavior, one can manipulate hydrogen permeation through composite membranes. A hydrogen permeation model for bilayer membranes is proposed to simulate diffusion in a nano-Ni and poly-Ni bilayer membrane in two-directions of charging. The experimental data is in good qualitative agreement with the model.

Statistical analysis of boundaries in NiO films grown on Ni single crystals

An attempt was made to apply the statistical analysis of NiO grain boundaries to explain the dependence of NiO growth kinetics on the crystallographic orientation of Ni substrate. Study was conducted for two crystals faces of Ni, (100) and (111), which exhibits a difference in oxidation rate constants at 1,073 K, over one order of magnitude. The orientation distribution functions (ODF), calculated from X-ray measurements for NiO grown on both Ni faces, were analyzed numerically to assess the grain boundary character. The grain boundary misorientation distribution (GBMD) and grain boundary character distribution (GBCD), used to describe oxide structure, were derived using three different assumptions regarding the spatial correlation in orientation of neighboring grains. Grain boundary network was created to analyze the high temperature diffusion properties through the oxide using diffusion constants available in the literature under assumption of various contribution of bulk and grain boundary diffusion paths to overall transport of ionic species. The growth parameters obtained were finally verified by comparison with experimentally measured oxidation kinetics.

Three dimensional simulation of the microstructure development in Ni-20%Fe nanocrystalline deposits

A Monte-Carlo computer model was applied to simulate a development of the three dimensional microstructure during electrodeposition of nanocrystalline alloys. The driving force for this process was the minimization of free energy of the system. For a particular deposit of Ni-20%Fe, the influence of the overpotential and current density on the grain size was tested. A strong decrease in grain size with increasing overpotential and current density obtained from the simulation is in qualitative agreement with the experimental data.