Late News Poster Session: Modeling
Program Organizers: MS&T19 Administration, MS&T PCC

Tuesday 11:00 AM
October 1, 2019
Room: Exhibit Hall CD
Location: Oregon Convention Center

P1-92: Crystal Structure Prediction through Density Functional Theory Combined with Unsupervised Machine Learning: A Vitamin B2 Case Study: Thiago Henrique da Silva1; Matthew King1; 1Boise State University
    First-principles density functional theory (DFT) calculations were performed to predict the unknown crystal structure of riboflavin, also known as vitamin B2. Assessment of the optimal structure of vitamin B2 was done in three steps: initial structure generation, crystal geometry optimizations using DFT and implementation of an unsupervised learning algorithm to select the fittest lowest-energy structures. Structural candidates were created by implementing changes in the rotational and translational elements of the molecule inside of the initial unit cell. The DFT self-consistent field method (SCF) was applied using the CRYSTAL14 software for all structures utilizing an atom-centered basis at the PBE/6-31G(d,p) level. Lowest-energy structures were selected through an unsupervised learning algorithm and the corresponding structures were then submitted to a full geometry optimization. Through this method a structural candidate was obtained that led to an accurate prediction of the crystal structure, verified experimentally by PXRD and low-frequency vibrational THz spectra.

P1-93: Development of EAM Interatomic Potentials of Aluminides and Carbides for Ni-based Superalloys: Muztoba Rabbani1; Sabila Kader Pinky1; Nirmal Baishnab1; Tyler McGilvry James1; Ridwan Sakidja1; 1Missouri State University
    We initiated the development of multi-component EAM potential for Aluminides and Carbides Ni-based Superalloys. The goal is to utilize the MD simulation to understand the deformation dynamics that contribute to the formation of voids and creep initiation. For this purpose, we constructed the raw data from ab-initio (molecular dynamics) MD simulations fed into the potential development code and used Nickel as the base metal with the addition of a number of various elements including Aluminium, Chromium, Tungsten. We then developed the EAM potentials for the aluminide and carbide phases using the force-fitting code MEAMfit. Our generated potential reproduces the fundamental properties of the Ni3Al and M23C6 phases. We verified further the EAM potential through the thermal stability test at different temperatures and by reproducing the elastic constants consistent with the experimental values. We gratefully acknowledge the support from DoE’s NETL (DE-FE0031554) and the computing support from NERSC.

P1-94: Effect of Virtual Spherical Indenter Stiffness on Atomistic Simulations of Nanoindentation: Zhenhai Xu1; Rui Xi1; Debin Shan1; 1Harbin Institute of Technology
    The atomistic simulation of nanoindentation is a well-established method to study mechanical characterization of small volumes of materials from the atomic view. The indenter is usually modelled as a virtual sphere without any atomic detail. The indenter stiffness k is often set by a compromise, inducing its diversity in different studies. In this work, nanoindentation simulations with different indenter parameters were performed on Cu(111) surfaces. Only when k is large enough, the critical load and critical depth for the initial dislocations emission become physical quantities to characterize the real response of indented materials to the nanoindentation. The reduced elastic modulus fitting to the load-nominal indentation depth curve is dependent on k and upper limit for fitting. The larger k cost larger computational time. A study with nanoindentation simulation should indicate value of k explicitly.

P1-95: Modeling Ductile Failure of High Strength Aluminum Alloy: Balaji Selvarajou1; Mark Jhon1; Siu Sin Quek1; 1Institute of High Performance Computing
    Some classes of AA2xxx/AA7xxx aluminum alloys exhibit fracture orthotropy despite having relatively isotropic elastic and yield properties. Quantitative measures of fracture (e.g. elongation to fracture) can vary significantly with direction of loading with respect to the rolling direction. This orthotropy similarly manifests in the varying contribution of intergranular and transgranular failure with different loading directions. The origins of this orthotropy lies in microstructural features such as grain shape, size, and the grain boundary characteristics. In order to systematically assess the microstructural underpinnings of this orthotropy, we develop a crystal-plasticity finite element model coupled to fracture models within the grains and along the grain boundaries. Transgranular fracture is modeled by a Gurson-type model, while intergranular fracture is modeled using a cohesive law. We integrate our model with experiment using microstructural analyses with EBSD to construct statistically equivalent grain microstructures, enabling our model to capture realistic distributions of grain sizes and textures.

P1-96: Modeling of Phase Transformation Kinetics in Post Heat-treated Resistance Spot Welds of AISI 1010 Mild Steel: Feujofack Kemda Bleriot Vincent1; Noureddine Barka1; Mohammad Jahazi2; Denis Osmani3; 1Université du Québec à Rimouski; 2École de Technologie Supérieure; 3AMH Canada Ltée
    The usage of transformation induced plasticity (TRIP) steels and boron steels in some auto body parts have become a necessity because of their lightweight. However, in resistance spot welding (RSW), the resulting weld nugget is generally fully martensitic, especially in the case of TRIP and boron steels but also for plain carbon steels as AISI 1010 which is extensively used in auto body inner parts. Martensite is the principal source of brittleness in weld nugget and must be avoided as much as possible. Thus, this work aims in finding means to reduce martensite fraction in weld nugget. Prediction of phase transformation was done through a modeling of the whole welding process and post weld heat treatment (PWHT) have been applied in order to reduce martensite fraction in the weld. Simulation results show that application of PWHT leads to a reduction in martensite and an increase in ferrite and bainite fractions in the weld nugget.

P1-97: Numerical Methods Applications in Crystal Plasticity Finite Element Method: Theodore Zirkle1; Bill Locke2; Ben Anglin2; Clint Geller2; David McDowell1; 1Georgia Institute of Technology; 2Bettis Laboratories
    Complex crystal plasticity models used in finite element applications generally require implicit integration in order to update the internal state variables while maintaining global force equilibrium. Implicit integration schemes require an understanding of the state variables at a future time increment, necessitating the application of root-finding numerical methods. The numerical method generally used in the iterative implicit integration scheme is Newton’s method coupled with a mathematically derived Jacobian of the slip system level flow rule. However, in the case where the flow rule is too complex or involves non-general terms, the Jacobian calculation can become unwieldy and computationally expensive. Here, we present modifications to the traditional numerical methods that increase the accuracy and computational speed of the implicit integration through the use of Broyden’s method coupled with a numerically evaluated Jacobian.

P1-98: Simulation and Optimization of Magnetically-assisted Welding of Stainless Steel 316L: Kevin Carpenter1; Ali Tabei1; Saereh Mirzababaei1; Somayeh Pasebani1; 1Oregon State University
    Recently, the application of magnetic fields by permanent magnets during welding has been reported to improve the quality of welded joints. It has been shown that magnetically-assisted welding leads to homogeneous solidification, and improvements in crystallographic orientations and grain size, resulting in enhanced mechanical properties and reduced part distortion. It is known that ferromagnetic permanent magnets demagnetize as temperature increases, up to the Curie temperature when they become paramagnetic. So far, the reported simulations in the literature do not account for temperature-induced demagnetization in magnetically-assisted welding, which is a significant phenomenon due to typically compact fixture arrangements. This work simulates magnetically-assisted welding in Stainless Steel 316L, while accounting for all the mechanisms of heat transfer and subsequent demagnetization. In addition, the necessary magnetization and arrangement of permanent magnet(s) are optimized in order to get the required field in the melt pool.

P1-100: Void Growth in Bicrystalline and Polycrystalline Ni-based Superalloy: Atomistic Calculation: Sabila Kader Pinky1; Ridwan Sakidja1; 1Missouri State University
    This study evaluates the microstructure-sensitive evolution of the deformation mechanisms in Ni-based superalloy with the emphasis on evaluating the role of voids within the microstructures. We assessed the effect of the intergranular void formation toward the overall deformation behaviours by using the Molecular Dynamics (MD) simulations. We varied the positions of voids as well as the grain size and simulated the compression tests from ambient conditions to 1000 °C to determine these factors as a function of temperature on the dislocation dynamics. As deformation accumulation increases, the intergranular voids would grow and coalesce, forming grain boundary cavities, leading to intergranular failures and a catastrophic tertiary creep at high temperatures. In addition, the machine learning method applied to the simulation results provides us with an in-depth correlation between the dislocation dynamics and deformation behaviour. The support from NETL (Crosscutting Research Program) FE0031554 is gratefully acknowledged.