Computational Thermodynamics and Kinetics: Diffusion, Excitations and Rare Events II
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Computational Materials Science and Engineering Committee
Program Organizers: Nana Ofori-Opoku, Canadian Nuclear Laboratories; Jorge Munoz, University of Texas at El Paso; Sara Kadkhodaei, University Of Illinois Chicago; Vahid Attari, Texas A&M University; James Morris, Ames Laboratory
Wednesday 2:00 PM
February 26, 2020
Room: 33C
Location: San Diego Convention Ctr
Session Chair: Claire Saunders, California Institute of Technology; Camille Bernal, California Institute of Technology
2:00 PM Invited
Thermodynamics of Solids with Chemical, Magnetic and Displacive Degrees of Freedom: Anton Van der Ven1; 1University of California, Santa Barbara
Many crystals of technological interest contain multiple chemical species and are susceptible to symmetry lowering phase transformations upon changing temperature or composition. Symmetry breaking can arise due to the onset of chemical ordering, magnetic ordering or coordinated atomic shuffles. These phenomena can be described phenomenologically in terms of free energy descriptions that depend on suitably defined order parameters and kinetic rate equations. A continuing challenge is to connect the materials specific parameters that appear in phenomenological thermodynamic and kinetic theories to the electronic structure of the solid. I will describe statistical mechanics approaches that rely on effective Hamiltonians to extrapolate first-principles electronic structure methods within Monte Carlo simulations to predict generalized free energies and kinetic rate coefficients. The approach yields essential ingredients for continuum scale theories, such as phase field models, that predict phase transformations and microstructure evolution.
2:30 PM
Anharmonicity in BCC Chromium: Camille Bernal-Choban1; Hillary Smith2; Brent Fultz1; 1California Institute of Technology; 2Swarthmore College
Phonon densities of states (DOS) of bcc chromium were measured from 6-1493 K using inelastic neutron scattering and compared to ab-initio calculations performed within the temperature-dependent effective potential (TDEP) framework. The energies of the low transverse modes were observed to decrease approximately 12% over this range of temperature, giving a non-harmonic contribution to the entropy of more than 0.1 kB/atom. Non-harmonic behavior is also observed in the remaining phonon branches. Temperature-dependent anharmonicities (phonon-phonon interactions) were calculated with TDEP, which is based on density functional theory (DFT). Effects of higher-order excitations on the entropy, free energy, and isobaric heat capacity of chromium are also investigated, and the resulting agreement with experimental data is discussed.
2:50 PM
An Exact Formalism for Thermotransport in Liquid and Solid Alloys: Graeme Murch1; Irina Belova1; Tanvir Ahmed1; Rafal Kozubski2; Zi-Kui Liu3; William Wang4; Andreas Meyer5; 1University of Newcastle; 2Jagiellonian University; 3Pennsylvania State University; 4Northwestern Polytechnical University; 5German Aerospace Center
In this contribution, we first present a newly developed exact formalism for describing atomic/thermal diffusion in a temperature gradient (thermotransport or thermodiffusion) as well as determination of the various heats of transport in a liquid A-B-C ternary metallic alloy. This is illustrated with an example of a classical-potential molecular dynamics calculation for Ni-Al-Co liquid alloys. This formalism is then extended to develop a new exact formalism for the solid state crystalline ‘ternary’ metallic alloy A-B-Vacancies for the case of steady state and marker shift conditions. This formalism is illustrated with some results from some known models for describing diffusion in a binary alloy.
3:10 PM Invited
Direct Solution to the Space-time Dependent Peierls-Boltzmann Transport Equation using an Eigendecomposition Method: Chengyun Hua1; Lucas Lindsay1; Austin Minnich2; 1Oak Ridge National Laboratory; 2California Institute of Technology
Nonlocal thermal transport is generally described by the Peierls-Boltzmann transport equation (PBE). However, solving the PBE for a general space-time dependent problem remains a challenging task due to the high dimensionality of the integro-differential equation. In this work, we present a direct solution to the space-time dependent PBE with a linearized collision matrix using an eigendecomposition method. Furthermore, we show that there exists a generalized Fourier type relation that links heat flux to the local temperature, and this constitutive relation is valid from ballistic to diffusive regimes. Combining this approach with ab initio calculations of phonon properties, we demonstrate that the derived solution gives a more accurate description of thermal transport in crystals that exhibit weak anharmonicity than the commonly-used single-mode relaxation time approximation and thus will lead to an improved understanding of phonon transport in solids.
3:40 PM Break
4:00 PM Invited
Non-equilibrium Molecular Dynamics Studies of Shock-induced Phase Transitions: Ramon Ravelo1; 1University of Texas El Paso
At high-strain rates of deformation --as those induced by strong shock waves, structural transformations can proceed at pressures above the thermodynamic phase transition boundary resulting in the loading of the parent phase into metastable states from which the phase change proceeds at rates which for single crystal metallic systems, might depend on original dislocation densities and crystal orientation. These anisotropies have been studied via large-scale non-equilibrium molecular dynamics (NEMD) simulations. We will examine the role of defect nucleation in the kinetics of shock-induced phase transitions in two different but related examples: the strain-rate dependence of the Hugoniot elastic limit in shock wave propagation in metallic single crystals and the role of plastic deformation in the kinetics of a fcc → bcc structural phase transition.
4:30 PM
Phonons at High Pressure in FeTi: Bethuel Khamala1; Jorge Munoz1; Brent Fultz2; 1The University of Texas at El Paso; 2California Institute of Technology
The FeTi system is amenable to computational investigations due to its simple crystal structure and minimalist Fermi surface. A thermally-driven electronic topological transition that results in anomalous phonon softening was recently reported to occur in FeTi at elevated temperatures as new features appear in the Fermi surface and new spanning vectors increase electronic screening of particular phonon modes. Here we report results on the pressure dependence of the spanning vectors, which was investigated using DFT and a neural network to interpolate between k-points. We used the temperature-dependent effective potential (TDEP) methodology to investigate the screening effects on particular modes at high pressure and finite temperature and compared the mode Gruneisen parameters with the phonon dispersions calculated at 0K using atomic displacements. We observe that all phonon modes stiffen, but while the optical M5’ modes stiffen by 50% at a pressure of 47 GPa, the X1 modes stiffen by about 10%.
4:50 PM
Interplay Between Chemical Bonding and Anharmonicity in Cu2O: Claire Saunders1; Dennis Kim2; Olle Hellman3; Hillary Smith4; Douglas Abernathy5; Brent Fultz1; 1California Institute of Technology; 2University of California, Los Angeles; 3Linköping University ; 4Swarthmore College; 5Oak Ridge National Laboratory
We calculated and measured strong anharmonicity of the optical phonons in Cuprite, Cu2O. Both the broadenings and shifts of phonon energies cannot be explained by harmonic or quasiharmonic theory. Our ab initio calculations use the stochastic Temperature Dependent Effective Potential (s-TDEP) method to calculate phonon self-energies and spectral functions, which compare directly to experimental dispersions from single crystal inelastic neutron scattering measurements at 10, 300, and 700 K on ARCS at ORNL (post-processed using a data folding method). Thermal broadening of optical dispersions was especially prominent in the experimental data, indicative of a large cubic anharmonicity that is being assessed by s-TDEP. Preliminary calculations correctly show negative thermal expansion, but fail to account for the large thermal broadenings seen at 300 and 700 K. Both effects demonstrate some dependence on the bonding contributions. Broader implications for the interplay between anharmonicity and chemical bonds will be discussed.
5:10 PM
The Anharmonic Origin of the Large Thermal Expansion of NaBr: Yang Shen1; Claire Saunders1; Camille Bernal1; Douglas Abernathy2; Michael Manley2; Brent Fultz1; 1California Institute Of Technology; 2Oak Ridge National Laboratory
For most elements and simple compounds, the free energy originates from an elastic energy that changes with volume, and a vibrational entropy that depends on the occupancies and frequencies of phonon modes. In the quasiharmonic approximation (QHA), the phonon frequencies depend only on V, and on T only insofar as it alters V by thermal expansion, assuming that the T-dependence of phonon frequencies alone gives small corrections to the QHA result. Here we show how INS measurements on a single crystal of NaBr reveal an unqualified failure of the QHA to predict the thermal expansion. Ab initio computations with nuclear quantum effects successfully predicted both the phonon dispersions and the large thermal expansion of NaBr. The frequencies of longitudinal-optical phonon modes decrease significantly with temperature owing to the real part of the phonon self-energy. This originates from the cubic interactions associated with triplets along the direction of nearest-neighbor Na-Br bonds.