Engineering topics

Ahzi, Said

Publications and source records attributed to Ahzi, Said.

11 records

DOE OSTI2020

Understanding the Nature of Capacity Decay and Interface Properties in Li//LiNi 0.5 Mn 1.5 O 4 Cells by Cycling Aging and Titration Techniques

The spinel structure LiNi 0.5 Mn 1.5 O 4 (LNMO) is a propitious cathode material for next-generation lithium-ion batteries for fast charge–discharge applications, but its capacity decay mechanism and rate-limiting process are not yet well understood. In this work, electrochemical impedance spectroscopy (EIS) with galvanostatic intermittent titration (GITT) and cycling aging techniques were employed to investigate the nature of capacity decay in disordered-phase LNMO. Different resistive components were separated after every 10 cycles. Cell overvoltages (ΔVs) due to ohmic conduction, charge transfer (CT), and concentration polarization (CP) were individually determined. Results revealed that the cell exhibited a higher ΔV at a higher discharged state. However, the ΔV value for CP was higher at a higher state of charge (SOC), and the overall LNMO/electrolyte interface played a major role in the rate-determining step. Battery life was estimated based on the results. Battery calendar life was found to be more vulnerable than cycle life. Furthermore, results also indicated that the working SOC range could be optimized based on the resistance analysis by avoiding those SOCs that have the most detrimental impact (e.g., heat generation and fire hazard).

25 ENERGY STORAGE↗

DOE OSTI2014

Modeling of deformation behavior and texture evolution in magnesium alloy using the intermediate $\phi$-model

The viscoplastic intermediate phi-model was applied in this work to predict the deformation behavior and texture evolution in a magnesium alloy, an HCP material. We simulated the deformation behavior with different intergranular interaction strengths and compared the predicted results with available experimental results. In this approach, elasticity is neglected and the plastic deformation mechanisms are assumed as a combination of crystallographic slip and twinning systems. Tests are performed for rolling (plane strain compression) of random textured Mg polycrystal as well as for tensile and compressive tests on rolled Mg sheets. Simulated texture evolutions agree well with experimental data. Activities of twinning and slip, predicted by the intermediate $\phi$-model, reveal the strong anisotropic behavior during tension and compression of rolled sheets.

$\phi$-Model↗

DOE OSTI2014

Effects of Morphology and Geometry of Inclusions on Two-Point Correlation Statistics in Two Phase Composites

A spectral form of the two-point correlation functions for two-phase composites is presented and the effects of secondary phase morphology, geometry and volume fraction on the coefficients of the corresponding series are studied. Initially, a principal component analysis is performed on the coefficients of the series and then, the effects of different morphological and geometrical parameters of the second phase inclusions on two-point correlation functions are examined. A direct linkage between the principal components of the coefficients with the morphological features (inclusion shape, aspect ratio and size) is established and the influence of preferred orientation of the anisotropic inclusions is also investigated. Finally a composite hull is constructed using the principal components of the coefficients in a three-dimensional space, which helps in determining the interrelation of properties and microstructures of any composite containing these types of inclusions.

97 MATHEMATICS AND COMPUTING↗

DOE OSTI2013

Yield asymmetry design of magnesium alloys by integrated computational materials engineering

Deformation asymmetry of magnesium alloys is an important factor on machine design in the automobile industry. Represented by the ratio of compressive yield stress (CYS) against tensile yield stress (TYS), deformation asymmetry is strongly related to texture and grain size. A polycrystalline viscoplasticity model, modified intermediate Φ-model, is used to predict the deformation behavior of magnesium alloys with different grain sizes. Validated with experimental results, integrated computational materials engineering is applied to find out the route in achieving desired asymmetry via thermomechanical processing. For example, CYS/TYS in rolled texture is smaller than 1 under different loading directions. In other textures, such as extruded texture, CYS/TYS is large along the normal direction. Starting from rolled texture, asymmetry will increase to close to 1 along the rolling direction after being compressed to a strain of 0.2. Our modified Φ-model also shows that grain refinement increases CYS/TYS. Along with texture control, grain refinement also can optimize the yield asymmetry. After the grain size decreases to a critical value, CYS/TYS reaches to 1 because CYS increases much faster than TYS. By tailoring the microstructure using texture control and grain refinement, it is achievable to optimize yield asymmetry in wrought magnesium alloys.

36 MATERIALS SCIENCE↗

DOE OSTI2013

A new smoothed particle hydrodynamics non-Newtonian model for friction stir welding: Process modeling and simulation of microstructure evolution in a magnesium alloy

We present a new smoothed particle hydrodynamics (SPH) model for friction stir welding (FSW). FSW has broad commercial application in the marine, aerospace, rail, and automotive industries. However, development of the FSW process for each new application has remained largely empirical. Few established numerical modeling techniques have been developed that can explain and predict important features of the process physics involved in FSW. This is particularly true in the areas of material flow and mixing mechanisms. In this paper, we present a novel modeling approach to simulate FSW that may have significant advantages over current finite element or finite difference based methods. Unlike traditional grid-based methods, Lagrangian particle methods such as SPH can simulate the dynamics of interfaces, large material deformations, and the material’s strain and temperature history without employing complex tracking schemes. Three-dimensional simulations of FSW on AZ31 Mg alloy are performed. The temperature history and distribution, grain size, microhardness as well as the texture evolution are presented. Numerical results are found to be in good agreement with experimental observations.

Pan, Wenxiao↗

DOE OSTI2013

Dynamic Composition Determination in Heterogeneous Ensembles using Angular Autocorrelation Functions as Signatures

Signatures of heterogeneous ensembles composed of multiple types of random oriented particles are represented by the angular autocorrelation functions of the systems. These signatures are retrieved by averaging oversampled angular correlation functions calculated from high throughput X-ray diffraction patterns. Using a component signature matrix, the composition of the heterogeneous system will be directly calculated from the signature of the ensemble. This is the first attempt to determine dynamic composition of a heterogeneous ensemble by fluctuation X-ray scattering using the angular autocorrelation functions as signatures.

composition determination, angular correlation fun↗

DOE OSTI2013

Simulation of cooling and solidification of three-dimensional bulk borosilicate glass. Effect of structural relaxations

The modeling of the viscoelastic stress evolution and specific volume relaxation of a bulky glass cast is presented in this article and is applied to the experimental cooling process of an inactive nuclear waste vitrification process. The concerned borosilicate glass is solidified and cooled down to ambient temperature in a stainless steel canister, and the thermomechanical response of the package is simulated. There exists a deviant compression of the liquid core due to the large glass package compared to standard tempered glass plates. The stress load development of the glass cast is finally studied for different thermal load scenarios, where the cooling process parameters or the final cooldown rates were changed, and we found a great influence of the studied cooldown rates on the maximum stress buildup at ambient temperature.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗

DOE OSTI2012

Electro-Hydraulic Forming of Sheet Metals: Free-forming vs. Conical-die Forming

This work builds upon our recent advances in quantifying high-rate deformation behavior of sheet metals, during electro-hydraulic forming (EHF), using high-speed imaging and digital image correlation techniques. Following recent publication of an earlier manuscript, resulting from this project, in the Journal of Materials Processing Technology, this manuscript further details our results and compares forming behavior when the process is carried out inside an open-die or a conical die. It is anticipated that quantitative information of the sheet deformation history, made possible by the experimental technique developed in this work, will improve our understanding on the roles of strain-rate and sheet-die interactions in enhancing the sheet metal formability during high-rate forming. This knowledge will be beneficial to the automotive industry and enable them to fabricate light-weight sheet parts out of Al and advanced high strength steels.

36 MATERIALS SCIENCE↗

DOE OSTI2012

Three-dimensional Reconstruction and Homogenization of Heterogeneous Materials Using Statistical Correlation Functions and FEM

In this study, a previously developed reconstruction methodology is extended to three-dimensional reconstruction of a three-phase microstructure, based on two-point correlation functions and two-point cluster functions. The reconstruction process has been implemented based on hybrid stochastic methodology for simulating the virtual microstructure. While different phases of the heterogeneous medium are represented by different cells, growth of these cells is controlled by optimizing parameters such as rotation, shrinkage, translation, distribution and growth rates of the cells. Based on the reconstructed microstructure, finite element method (FEM) was used to compute the effective elastic modulus and effective thermal conductivity. A statistical approach, based on two-point correlation functions, was also used to directly estimate the effective properties of the developed microstructures. Good agreement between the predicted results from FEM analysis and statistical methods was found confirming the efficiency of the statistical methods for prediction of thermo-mechanical properties of three-phase composites.

36 MATERIALS SCIENCE↗

DOE OSTI2012

Bridging Microstructure, Properties and Processing of Polymer Based Advanced Materials

This is a guest editorial for a special issue in Journal of Engineering Materials and Technology. The papers collected in this special issue emphasize significant challenges, current approaches and future strategies necessary to advance the development of polymer-based materials. They were partly presented at the symposium of 'Bridging microstructure, properties and processing of polymer based advanced materials' in the TMS 2011 annual conference meeting, which was held in San Diego, US, on Feb 28 to March 3, 2011. This symposium was organized by the Pacific Northwest National Laboratory (USA) and the Institute of Mechanics of Fluids and Solids of the University of Strasbourg (France). The organizers were D.S. Li, S. Ahzi, and M. Khaleel.

36 MATERIALS SCIENCE↗

DOE OSTI2011

Three-Phase Solid Oxide Fuel Cell Anode Microstructure Realization Using Two-Point Correlation Functions

A Monte Carlo methodology is developed as a means for three-dimensional (3D) reconstruction of the microstructure of a three-phase anode used in solid oxide fuel cells, based on two-point statistical functions. The salient feature of the presented reconstruction methodology is the ability to realize the 3D microstructure from its 2D SEM image for a three-phase medium extendable to n-phase media. In the realization procedure, different phases of the heterogeneous medium are represented by different cells which are allowed to grow. The growth of cells, however, are controlled via several optimization parameters related to rotation, shrinkage, translation, distribution and growth rates of the cells. Indeed, the proposed realization algorithm can be categorized as a member of dynamic programming methods and is designed so comprehensive that can realize any desired microstructure. To be more specific, at first the initial 2D image is successfully reconstructed and then the final optimization parameters are used as the initial values for the initiation of the 3D reconstruction algorithm. This paper presents a novel hybrid stochastic methodology based on the colony and kinetic algorithm for the simulation of the virtual microstructure. The simulation procedure involves repeated realizations where each realization in turn consists of nucleation and growth of cells. For each of the subsequent realizations, the controlling parameters get updated by minimization of an objective function at the end of the preceding realization. Here, the objective function is defined based on the two-point correlation functions from the simulated and real microstructures. The kinetic growth algorithm is established on the cellular automata approach which facilitates the simulation procedure. Comparison of the two-point correlation functions from different sections of the final 3D reconstructed microstructure with the initial real microstructure shows a satisfactory agreement which confirms the proposed methodology.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗