Accepted Manuscripts

Additive manufacturing has enabled a transformational ability to create cellular structures (or foams) with tailored topology. Compared to their monolithic polymer counterparts, cellular structures are potentially suitable for systems requiring materials with high specific energy-absorbing capability to provide enhanced damping. In this work, we demonstrate the utility of controlling unit-cell topology with the intent of obtaining a desired stress-strain response and energy density. Using mesoscale simulations that resolve the unit-cell sub-structures, we validate the role of unit-cell topology in selectively activating a buckling mode and thereby modulating the characteristic stress-strain response. Simulations incorporate a linear viscoelastic constitutive model and a hyperelastic model for simulating large deformation of the polymer under both tension and compression. Simulated results for nine different cellular structures are compared with experimental data to gain insights into three different modes of buckling and the corresponding stress-strain response.

Understanding the phononic origin of the infrared dielectric properties of yttria (Y2O3) and other rare-earth sesquioxides (RE2O3) is a fundamental task in the search of appropriate RE2O3 materials that serve particular infrared optical applications. We herein investigate the infrared dielectric properties of RE2O3 (RE = Y, Gd, Ho, Lu) using DFT-based phonon calculations and Lorentz oscillator model. The abundant IR-active optical phonon modes that are available for effective absorption of photons result in high reflectance of RE2O3, among which four IR-active modes originated from large distortions of REO6 octahedra are found to contribute dominantly to the phonon dielectric constants. Particularly, the present calculation method by considering one-phonon absorption process is demonstrated with good reliability in predicting the infrared dielectric parameters of RE2O3 at the far-infrared as well as the vicinity of mid-infrared region, and the potential cutoff frequency/wavelength of its applicability is disclosed as characterized by the maximum frequency of IR-active longitudinal phonon modes. The results deepen the understanding on infrared dielectric properties of RE2O3, and aid the computational design of materials with appropriate infrared properties.

Anharmonic inter-atomic potential ∼ |x| ⁿ , n > 1 , has been used in
molecular dynamics (MD) simulations of stress dynamics of FCC oriented crystal.
The model of the chain of masses and springs is found as a convenient and accurate
description of simulation results, with masses representing the crystallographic planes.
The dynamics of oscillations of two planes is found analytically to be given by
Euler's beta functions, and its scaling with non-linearity parameter and amplitude
of oscillations, or applied shear pressure is discussed on examples of time dependencies
of displacements, velocities, and forces acting on masses (planes). The dynamics of
stress penetration from the surface of the sample with multiply-planes (an anharmonic
crystal) towards its interior is confirmed to be given exactly as a series of Bessel
functions, when n=2 (Schrödinger and Pater solutions). When n ̸= 2 the stress
dynamics (wave propagation) in bulk material remains qualitatively of the same nature
as in the harmonic case. In particular, results suggest that the quasi-linear relationship
between frequency and the wave number is preserved. The speed of the transverse
sound component, dependent on sound wave amplitude, is found to be a strongly
decreasing function of n. The results are useful in the analysis of any MD simulations
under pressure, as they help to understand the dynamics of pressure retarded effects,
as well as help design the proper methodology of performing MD simulations in cases
such as, for instance, studies of the dynamics of dislocations.

Bioresorbable Mg-based alloys with low density, low elastic modulus, and excellent biocompatibility are outstanding candidates for temporary orthopedic implants. Coincidentally, metal additive manufacturing (AM) is disrupting the biomedical sector by providing fast access to patient-customized implants. Due to the high cooling rates associated with fusion-based AM techniques, they are often described as rapid solidification processes. However, conclusive observations or rapid solidification in metal AM — attested by drastic microstructural changes induced by solute trapping, kinetic undercooling, or morphological transitions of the solid-liquid interface — are scarce. Here we study the formation of banded microstructures during laser powder-bed fusion (LPBF) of a biomedical-grade Magnesium-rare earth alloy, combining advanced characterization and state-of-the-art thermal and phase-field modeling. Our experiments unambiguously identify microstructures as the result of an oscillatory banding instability known from other rapid solidification processes. Our simulations confirm that LPBF-relevant solidification conditions strongly promote the development of banded microstructures in a Mg-Nd alloy. Simulations also allow us to peer into the sub-micrometer nanosecond-scale details of the solid-liquid interface evolution giving rise to the distinctive banded patterns. Since rapidly solidified Mg alloys may exhibit significantly different mechanical and corrosion response compared to their cast counterparts, the ability to predict the emergence of rapid solidification microstructures (and to correlate them with local solidification conditions) may open new pathways for the design of bioresorbable orthopedic implants, not only fitted geometrically to each patient, but also optimized with locally-tuned mechanical and corrosion properties.

The current approach to modeling viscoelastic materials in most
commercial finite element packages is based on the General Maxwell Model, which
views these materials as combinations of spring and dashpot elements. However, the
data can be incorporated more directly into a transient finite element study by direct
interpolation of the relaxation function. This work explores a linear interpolation
scheme to the inclusion of viscoelastic relaxation functions on an example problem.
The results show several benefits over the General Maxwell Model for transient
studies. Included in the analysis are displacement solutions utilizing both approaches,
relaxation function error calculations for both approaches, and parametric runtime
studies comparing speed of calculation. The variation in computational flop counts is
considered and an argument is made for the preference of the proposed approach.

Through heat treatment experiments and numerical simulations, the effects of the heating temperature (1313–1423 K) and holding time (10–240 min) on the grain growth behavior of the extruded FGH96 alloy were investigated. A two-dimensional Cellular Automata (CA) model that considered the dissolution of the γ´ phase over time and the distribution characteristics with different sizes was developed to explore the grain growth behavior above the γ´ phase over-solution temperature (1423 K) and below the γ´ sub-solution temperature (1383 K), respectively. The results showed that the rate of grain growth of FGH96 alloy was obviously enhanced when the heating temperature exceeded 1363 K, which was mainly related to the dissolution of the γ´ phase, and the grain growth of FGH96 alloy mainly occurred during the initial stage of insulation. The grain growth model of the extruded FHG96 alloy could accurately predict the grain growth behavior, and the simulation results were in good agreement with the experimental results at over-solution temperature or sub-solution temperature. The effects of volume fraction and radius of γ´ phase on the grain growth behavior of FGH96 alloy were studied by simulating the grain growth behavior of FGH96 alloy under different sizes and volume fractions of γ´ phase. The results follow the Zener relation, and the coefficient n in the Zener relation was determined by fitting the simulation results.

The metal casting process, which is one of the key drivers of the manufacturing industry, involves several physical phenomena occurring simultaneously like fluid flow, phase change, and heat transfer which affect the casting yield and quality. Casting process modeling involves numerical modeling of these phenomena on a computer. In recent decades, this has become an inevitable tool for foundry engineers to make defect-free castings. To expedite computational time Graphics Processing Units (GPUs) are being increasingly used in the numerical modeling of heat transfer and fluid flow. Initially, in this work a CPU based implicit solver code is developed for solving the 3D unsteady energy equation including phase change numerically using Finite Volume Method (FVM) which predicts the thermal profile during solidification in the metal casting process in a completely filled mold. To address the computational bottleneck, which is identified as the linear algebraic solver based on the Bi-Conjugate Gradient Stabilized (BiCGSTAB) method, a GPU-based code is developed using CUDA toolkit and was implemented on the GPU. The CPU and GPU based codes are then validated against a commercial casting simulation code FLOW-3D CAST® for a simple casting part and against in-house experimental results for gravity die casting of a simple geometry. Parallel performance is analyzed for grid sizes ranging from 10x10x10 to 210x210x210 and for three time-step sizes. The performance of the GPU code based on occupancy and throughput is also investigated. The GPU code exhibits a maximum speedup of 308x compared to the CPU code for a grid size of 210x210x210 and a time-step size of 2s.

Nowadays, anodized coating on additively manufactured (AM) or 3D printed Al-10Si-Mg alloy are used for various components in spacecraft such as antenna feeds, wave guides, structural brackets, collimators, thermal radiators etc. In this study, Artificial Neural Network (ANN) and Power law-based models are developed from experimental nanoindentation data for predicting elastic modulus and hardness of anodized AM Al-10Si-Mg at any desired loads. Data from nanoindentation experiments conducted on plan- and cross-sections of anodized coating on AM Al-10Si-Mg alloy was considered for modelling. Apart from nanomechanical properties, load and displacement curves were predicted using Python software from ANN and the Power law model of nanoindentation. It is observed that the ANN model of 50 mN nanoindentation experimental data can accurately predict the loading pattern at any desired load below 50 mN. Elastic modulus and hardness of anodized AM Al-10Si-Mg computed from ANN and the power law model of the unloading curve are also comparable with the values obtained from Weibull distribution analysis reported elsewhere. The derived models were also used to predict nanomechanical properties at 25 and 35 mN, for which no experimental data was available. The computed hardness of plan section of the anodic coating is 3.99 and 4.02 GPa for 25 and 35 mN, respectively. The computed hardness of cross-section of the anodic coating of is 7.16 and 6.61 GPa for 25 and 35 mN, respectively. Thus, the ANN and Power law model of nanoindentation can predict elastic modulus and hardness at different loads by conducting the minimum number of experiments. The novel approach to predict nanomechanical properties using ANN resulted in determining realistic and design specific data on hardness and modulus of the anodized coating on AM Al-10Si-Mg alloy.

We examine the influence of grains size on the stability of polycrystalline coherent binary solid solutions. By assuming that the grains are isotropic, we find that the tendency for instability decreases as the radius of the grains decrease. We also find that a temperature-dependent critical grain radius exists below which the tendency for instability vanishes and the grains are stable, with respect to infinitesimal composition fluctuations, for any initial composition. We find that the critical grain radius decreases monotonically as the temperature decrease. If the radius of the grains is smaller than the minimum critical grain radius the grains are stable for any temperature and initial composition.

A method for identifying dislocation motion in atomistic simulations is presented. While identifying
static and moving dislocations within a single crystal or a combination of such is well established,
the method described here is tailored to identify dislocation motion by correlating the displacements
of individual atoms. This facilitates the identification of dislocation motion in complex structural
arrangements, and allows the specific contribution to plastic deformation, due to dislocation motion,
to be separated from that of other mechanisms. The method is applied to test cases in crystals and
grain boundaries, in which irradiation-induced creep was induced. It is shown that the method
singles out the moving dislocations from among the dislocation forest at grain boundaries, thus
identifying the specific reactions driving the distortion at any given time. This enables the study
of dislocation processes in the presence of realistic obstacles, and the study of the effects of microstructure
on dislocation mobility. As an example of such a study, the method is applied to rule
out intragranular slip, and to estimate the contribution of dislocation motion to strain, in a NC
undergoing irradiation-induced creep.