Dongsheng Li

Enhancement of thermal and electrical properties of carbon nanotube polymer composites by magnetic field processing

We show that the thermal and electrical properties of single wall carbon nanotube (CNT)-polymer composites are significantly enhanced by magnetic alignment during processing. The electrical transport properties of the composites are mainly governed by the hopping conduction with localization lengths comparable to bundle diameters. The bundling of nanotubes during the composite processing is an important factor for electrical, and in particular, for thermal transport properties. Better CNT isolation will be needed to reach the theoretical thermal conductivity limit for CNT composites.

Evolution of crystal orientation distribution coefficients during plastic deformation

A general methodology is developed to model the texture evolution of polycrystalline materials during mechanical processing. The methodology is based on the conservation of texture volume as proposed by Bunge. One of the interesting features of the formulation is that the texture coefficients can be predicted for each processing path using a sixth rank tensor. The range of validity for this methodology is investigated using a set of input data predicted by a crystal plasiticity model based on Taylor.

A study of the hot and cold deformation of twin-roll cast magnesium alloy AZ31

Recent advances in twin-roll casting (TRC) technology of magnesium have demonstrated the feasibility of producing magnesium sheets in the range of widths needed for automotive applications. However, challenges in the areas of manufacturing, material processing and modelling need to be resolved in order to fully utilize magnesium alloys. Despite the limited formability of magnesium alloys at room temperature due to their hexagonal close-packed crystalline structure, studies have shown that the formability of magnesium alloys can be significantly improved by processing the material at elevated temperatures and by modifying their microstructure to increase ductility. Such improvements can potentially be achieved by processes such as superplastic forming along with manufacturing techniques such as TRC. In this work, we investigate the superplastic behaviour of twin-roll cast AZ31 through mechanical testing, microstructure characterization and computational modelling. Validated by the experimental results, a novel continuum dislocation dynamics-based constitutive model is developed and coupled with viscoplastic self-consistent model to simulate the deformation behaviour. The model integrates the main microstructural features such as dislocation densities, grain shape and grain orientations within a self-consistent viscoplasticity theory with internal variables. Simulations of the deformation process at room temperature show large activity of the basal and prismatic systems at the early stages of deformation and increasing activity of pyramidal systems due to twinning at the later stages. The predicted texture at room temperature is consistent with the experimental results. Using appropriate model parameters at high temperatures, the stress–strain relationship can be described accurately over the range of low strain rates.

Semi-inverse Monte Carlo reconstruction of two-phase heterogeneous material using two-point functions

A new Monte Carlo (MC) methodology using ant Colony and kinetic growth models is developed to reconstruct the microstructure of two-phase composites using correlation statistics. After the initial patterns are generated, the new patterns evolved by introducing optimisation parameters for rotation, shrinkage, transportation and distribution. The realisations are then optimised using kinetic growth rate applied to each representative cell. For each realisation, the final microstructures are then compared to the experimental results through minimisation of the error function. Finally an optimisation methodology was developed to introduce initial input parameters for MC simulation to minimise the error function.

Magnetic-field-induced crystallographic texture enhancement in cold-deformed FePt nanostructured magnets

This paper reports a unique approach to the fabrication of magnetically anisotropic nanostructured FePt magnets: cyclic sheath cold rolling and subsequent magnetic annealing. High magnetic fields enhance both crystallographic texture and magnetic properties of cold-deformed FePt nanostructured magnets. Magnetic annealing increases (001) out-of-plane texture of the FePt hard phase by 50% and introduces magnetic anisotropy in the annealed samples. It is suggested that pretextured Fe and Pt nanocrystals provide favorable nucleation sites for magnetic-field-induced nucleation and growth of textured FePt grains. It is argued that the enhancement of texture of the FePt hard phase and promotion of the solid-state phase transformation by magnetic annealing are the reasons for improvement of the magnetic properties.

Crystallographic texture evolution in high-density polyethylene during uniaxial tension

Abstract This paper presents experimental measurements of crystallographic texture evolution in high-density polyethylene subjected to very large strains in uniaxial tension (up to a true strain of 2.1). The measurements presented here differ from prior studies in three important aspects: (1) The initial texture in the sample is quite strong with a large fraction of the crystallites oriented in an unstable orientation with the crystal c-axis perpendicular to the tensile axis of the sample. (2) Rigorous methods of texture analyses, based on spherical harmonics, have been applied to produce “complete, recalculated” pole figures based on diffraction data from five incomplete pole figures. (3) The measurements were performed while the samples were kept in the deformed state. The results presented here provide several new insights into texture development in tensile straining of high-density polyethylene to large strains. There are at least three distinct preferred orientations: (i) a component with (001) aligned along the extension axis, (ii) a component with (011) aligned close to the extension axis, and (iii) a component with (010) aligned along the extension axis. Note that only the first component has been reported to be stable at high strains in previous studies. The rate of texture evolution in the present study is significantly lower than that reported in previous studies. It was also observed that the natural relaxation of strain following the tensile loading had a significant impact on the texture in the sample. It was observed that the relaxation process mitigated or eliminated the second and third preferred texture components described above, while strengthening the first.

Simulation of heterogeneous atom probe tip shapes evolution during field evaporation using a level set method and different evaporation models

Abstract In atom probe tomography (APT), accurate reconstruction of the spatial positions of field evaporated ions from measured detector patterns depends upon a correct understanding of the dynamic tip shape evolution and evaporation laws of component atoms. Artifacts in APT reconstructions of heterogeneous materials can be attributed to the assumption of homogeneous evaporation of all the elements in the material in addition to the assumption of a steady state hemispherical dynamic tip shape evolution. A level set method-based specimen shape evolution model is developed in this study to simulate the evaporation of synthetic layered-structured APT tips. The simulation results of the shape evolution by the level set model qualitatively agree with the finite element method and the literature data using the finite difference method. The asymmetric evolving shape predicted by the level set model demonstrates the complex evaporation behavior of heterogeneous tip and the interface curvature can potentially lead to the artifacts in the APT reconstruction of such materials. Compared with other APT simulation methods, the new method provides smoother interface representation with the aid of the intrinsic sub-grid accuracy. Two evaporation models (linear and exponential evaporation laws) are implemented in the level set simulations and the effect of evaporation laws on the tip shape evolution is also presented.

Comparison of Different Upscaling Methods for Predicting Thermal Conductivity of Complex Heterogeneous Materials System: Application on Nuclear Waste Forms

To develop strategies for determining thermal conductivity based on the prediction of a complex heterogeneous materials system and loaded nuclear waste forms, the computational efficiency and accuracy of different upscaling methods has been evaluated. The effective thermal conductivity, obtained from microstructure information and local thermal conductivity of different components, is critical in predicting the life and performance of waste forms during storage. Several methods, including the Taylor model, Sachs model, self-consistent model, and statistical upscaling method, were developed and implemented. Microstructure-based finite-element method (FEM) prediction results were used to as a benchmark to determine the accuracy of the different upscaling methods. Micrographs from waste forms with varying waste loadings were used in the prediction of thermal conductivity in FEM and homogenization methods. Prediction results demonstrated that in term of efficiency, boundary models (e.g., Taylor model and Sachs model) are stronger than the self-consistent model, statistical upscaling method, and finite-element method. However, when balancing computational efficiency and accuracy, statistical upscaling is a useful method in predicting effective thermal conductivity for nuclear waste forms.

Processing Path Model to Describe Texture Evolution during Mechanical Processing

Using a processing path model based on the conservation principle in the orientation space explicit solutions can be formed linking any final (desired) microstructure to a given initial state for polycrystalline materials. The model uses texture coefficients in spherical harmonics expansion to as materials descriptors to represent the texture state of polycrystalline materials. In this work, the effect of increasing the maximum number of texture coefficients used in the series expansion (represented by Lmax) on the prediction of texture and its accuracy is fully studied.

Effects of Pore Distributions on Ductility of Thin-Walled High Pressure Die-Cast Magnesium

In this paper, a microstructure-based three-dimensional (3D) finite element modeling method is adopted to investigate the effects of porosity in thin-walled high pressure die-cast (HPDC) Magnesium alloys on their ductility. For this purpose, the cross-sections of AM60 casting samples are first examined using optical microscope and X-ray tomography to obtain the general information on the pore distribution features. The experimentally observed pore distribution features are then used to generate a series of synthetic microstructure-based 3D finite element models with different pore volume fractions and pore distribution features. Shear and ductile damage models are adopted in the finite element analyses to induce the fracture by element removal, leading to the prediction of ductility. The results in this study show that the ductility monotonically decreases as the pore volume fraction increases and that the effect of ‘skin region’ on the ductility is noticeable under the condition of same local pore volume fraction in the center region of the sample and its existence can be beneficial for the improvement of ductility. The further synthetic microstructure-based 3D finite element analyses are planned to investigate the effects of pore size and pore size distribution.