Thursday 8:30 AM

March 2, 2017

Room: 11A

Location: San Diego Convention Ctr

Magnesium alloys use Al and Zn for precipitation hardening, and rare earth elements (Gd, Y, Nd, Ce and La) to randomize texture for improved formability and creep resistance. Controlling processing to obtain these microstructural changes requires accurate knowledge of solute transport. However, current theoretical models to predict diffusivity from atomic jump frequencies make uncontrolled approximations that affect their accuracy. Density-functional theory identifies nine different solute-vacancy configurations from which symmetry analysis determine 17 transitions states corresponding to a 27-frequency model. A new Green function approach computes diffusivity for 14 solutes using the density-functional theory data. We find significant differences for solute drag of Al, Zn, and rare earth solutes, and improved predictions of activation energies for diffusion. The differences with prior predictions can be directly attributed to missing jumps in the 8-frequency model.

Ordering and decomposition reactions play a critical role in many engineering materials. Order-disorder phenomena can be exploited to tune material properties such as creep resistance and mechanical strength. In such applications it is imperative to establish the crystal structure of the ordered intermetallic phases and to calculate accurate thermodynamic data at elevated temperatures. We will describe a first-principles approach to bridge gaps between modeling at the atomistic and mesoscale. This includes a general approach to construct rigorous order parameters and a first-principles statistical mechanics formalism to calculate free energies of the ordered and disordered phases as a function of order parameters. The calculated properties can be used to inform continuum phenomenological descriptions of ordering and decomposition reactions in multi-component alloys. The approach will be illustrated on important engineering alloys such as magnesium alloys as well as titanium and nickel aluminides.

Glass is attractive for high velocity impact resistance because of its low density, high strength, volume expansion following fracture and energy dissipation due to densification. At the present the engineering community lacks continuum models of Silica glass that characterize all of these effects, thereby limiting efforts to probe the ultimate utility of this glass in application. We model a representative volume element of the glass using molecular dynamics and measure elastic moduli pressure dependence, viscosity, and the effects of order parameters, particularly with respect to the densification. These efforts constitute a novel approach to modeling amorphous Silica. We hope they lead to better design of impact resistant structures such as spacecraft shielding from micrometeoroids.

This work utilizes the MOOSE Phase Field module to predict the evolution of strengthening precipitates in an Al-Cu alloy. The phase of interest is the thermodynamically metastable θ′ (Al2Cu), the primary strengthening precipitate in many commercial cast aluminum alloys. The upper limit of service temperature for these alloys is, in practice, related to the thermodynamic phase stability of θ′ precipitates. Phase Field simulations are performed incorporating ab initio calculations of interfacial energy and lattice strain for various levels of Cu content and microalloying additions to provide quantitative, verifiable predictions of microstructural evolution at elevated temperatures. These phase field simulations are compared to observed microstructural evolution in Al-Cu alloys. It is demonstrated that the structural and chemical contributions to the elevated temperature stability of the strengthening phase can be delineated with this approach.

A new phase field crystal model is developed to scrutinize the structural properties of binary two-dimensional materials with sublattice ordering. The model is parameterized to model hexagonal boron nitride (h-BN). We investigate the dislocation structures of both symmetric and asymmetric tilt grain boundaries and compute the grain boundary energies across the full range of misorientation angles, varying from zigzag to armchair boundaries and including the important case of inversion domains at 60-degree. At small angles the grain boundary energies are found to follow the Read-Shockley relation. Our results not only reproduce the types of dislocation cores observed in previous experiments and first-principles calculations such as penta-hepta (5|7) and square-octagon (4|8) pairs, but also predict some new defect structures for various grain boundary misorientations, particularly for the inversion domains which are of current interest in the study of 2D binary materials.

A theory for the epitaxial growth of Au on MoS

Simulations of phase transformations in planar geometries under various boundary conditions are performed. Ablation, accretion and self freezing under rarefied atmospheres and external heating are looked at for the ice-water-vapor and naphthalene systems. Consideration of ablation is important in applications with space shields in space flight under radiation and heat sources along with near vacuum conditions. Recent non invasive methods in cryogenic surgery also rely on production of extreme cold in subcutaneously by surface ablation. In this paper, sample calculations for water-ice and naphthalene give the velocities of the freezing and vaporization fronts under various parameter combinations. It is shown that considerable difference exists between the cases of self freezing, ablation and accretion. For instance in the case of water, rates of self ablation without heat sources and self accretion ( as in the formation of ice crystals directly from vapor) differ by an order of magnitude.

In recent experiment, metals with nanostructure such as nanohoneycomb, exhibit potential as artificial muscle upon voltage loading. In this work, a multi-scale, multi-field simulation is used to model the electrochemical actuation behavior. Molecular dynamics simulations with reactive force-field potentials and a modified charge-equilibrium method are used to calculate the surface stress built up in metal, Ni(100), surface in contact with water electrolyte due to a voltage applied across the interface. The calculated surface stress is then used as input in a meso-scale finite-element (FE) model of hexagonal unit and then a macro-scale FE model of bilayered cantilever to calculate the bending of an entire sample which replicates experimental conditions. Our investigation to simulate the electrochemical actuation of a real-sized, nano-porous metallic structure in an electrolytic environment will help understand the actuation mechanism from nano interface activity to macro scale deformation.

Currently most of superconducting Niobium (Nb) cavities are manufactured from fine grain Nb sheets. As-cast ingots go through a series of steps including forging, milling, rolling, and intermediate annealing, before they are deep-drawn into a half-cell shape and subsequently electron beam welded to make a full cavity. Tube hydroforming, a manufacturing technique where a tube is deformed into a die using a pressurized fluid, is an alternative to the current costly manufacturing process. A whole cavity can be made from a tube using tube hydroforming. This study focuses on deformation of large grain Nb tubes during hydroforming. The crystal orientation of the grains is recorded. The tube is marked with a circle-grid which is used to measure the strain after deformation. The deformation of the tube is modeled with crystal plasticity finite element. The results of the simulation and experiments are compared.

Similar as its counterpart graphene, one of the main obstacles of applying silicene in the modern electronics is the lack of energy band gap. In this work, we present a systematic study on structural and electronic properties of single and bi-layered silicon films under various in-plane strains using density functional theory. Energy band diagram, electron transmission efficiency, and the I-V curve were calculated. It turns out that bi-layered silicon film (BiSF) exhibits energy band gap as the applied tensile in-plane strain above 10.7%. The energy band gap of the BiSF reaches the maximum of about 168.0 meV as the tensile in-plane strain ~ 14.3%. BiSFs grown on various common semiconductor substrates have been modeled. By choosing proper substrate, the energy band gap of the bi-layered silicon film can be opened. This will open a new opportunity of applying 2D silicon structure in the main stream IC industry.