Computational File Repositoryhttps://hdl.handle.net/11256/22019-06-18T22:27:07Z2019-06-18T22:27:07ZThermodynamic analysis of the topologically close packed σ phase in the Co-Cr systemWang, PeishengPeters, Matthew C.Kattner, Ursula R.Choudhary, KamalOlson, Gregory Bhttps://hdl.handle.net/11256/9852019-02-19T00:12:13Z2019-11-01T00:00:00ZThermodynamic analysis of the topologically close packed σ phase in the Co-Cr system
Wang, Peisheng; Peters, Matthew C.; Kattner, Ursula R.; Choudhary, Kamal; Olson, Gregory B
Density functional theory (DFT) calculations show that it is essential to consider the magnetic contribution to the
total energy for the end-members of the σ phase. A more straightforward method to use the DFT results in a
CALPHAD (Calculation of phase diagrams) description has been applied in the present work. It was found that
only the results from DFT calculations considering spin-polarization are necessary to obtain a reliable description
of the σ phase. The benefits of this method are: the DFT calculation work can be reduced and the CALPHAD
description of the magnetic contribution is more reliable. A revised thermodynamic description of the CoeCr
system is presented which gives improved agreement with experimental phase boundary data for the σ phase.
2019-11-01T00:00:00ZThermodynamic assessment of the Co-Ta systemWang, PeishengKoßmann, JörgKattner, Ursula RPalumbo, MauroHammerschmidt, ThomasOlson, Gregory Bhttps://hdl.handle.net/11256/9842019-02-19T00:11:41Z2018-12-01T00:00:00ZThermodynamic assessment of the Co-Ta system
Wang, Peisheng; Koßmann, Jörg; Kattner, Ursula R; Palumbo, Mauro; Hammerschmidt, Thomas; Olson, Gregory B
The Co-Ta system has been reviewed and the thermodynamic description was re-assessed in the present work.
DFT (density functional theory) calculations considering spin polarization were performed to obtain the energies
for all end-member configurations of the C14, C15, C36 and μ phases for the evaluation of the Gibbs energies of
these phases. The phase diagram calculated with the present description agrees well with the experimental and
theoretical data. Considering the DFT results was essential for giving a better description of the μ phase at lower
temperatures.
2018-12-01T00:00:00ZImpact of solutes on the lattice parameters and elastic stiffness coefficients of hcp Fe from first-principles calculationsFellinger, Michael R.Hector Jr., Louis G.Trinkle, Dallas R.https://hdl.handle.net/11256/9822018-12-16T20:59:06Z2018-12-16T00:00:00ZImpact of solutes on the lattice parameters and elastic stiffness coefficients of hcp Fe from first-principles calculations
Fellinger, Michael R.; Hector Jr., Louis G.; Trinkle, Dallas R.
The hexagonal close-packed (hcp) $\epsilon$-martensite phase in steels nucleates from the austenite parent phase during plastic straining and can be stabilized by solute additions. We compute the lattice parameters and elastic stiffness coefficients $C_{ij}$ of single-crystal hcp Fe as functions of solute concentration in the dilute limit for the substitutional solutes Al, B, Cu, Mn, and Si, and the octahedral interstitial solutes C and N. Solute strain misfit tensors determine the solute dependence of the lattice parameters, as well as the strain contributions to the solute-induced changes in the $C_{ij}$. We also compute chemical contributions to the changes in the $C_{ij}$ for each solute, and show that the sum of the strain and chemical contributions agrees with more computationally expensive direct calculations that simultaneously incorporate both effects. The computed data can be used to estimate solute-induced changes in polycrystalline elastic moduli and changes in mechanical properties such as strength and ductility, and can be directly incorporated into mesoscale simulations of multiphase steels to model solute effects on the $\epsilon$-martensite phase.
2018-12-16T00:00:00ZGeometries of edge and mixed dislocations in bcc Fe from first principles calculationsFellinger, Michael R.Tan, Anne Marie Z.Hector Jr., Louis G.Trinkle, Dallas R.https://hdl.handle.net/11256/9782018-11-28T23:51:04ZGeometries of edge and mixed dislocations in bcc Fe from first principles calculations
Fellinger, Michael R.; Tan, Anne Marie Z.; Hector Jr., Louis G.; Trinkle, Dallas R.
We use density functional theory (DFT) to compute the core structures of a_0[100](010) edge, a_0[100](011) edge, a_0/2[-1-11](1-10) edge, and a_0/2[111](1-10) 71 degree mixed dislocations in body-centered cubic (bcc) Fe. The calculations are performed using flexible boundary conditions (FBC), which effectively allow the dislocations to relax as isolated defects by coupling the DFT core to an infinite harmonic lattice through the lattice Green function (LGF). We use the LGFs of the dislocated geometries in contrast to most previous FBC-based dislocation calculations that use the LGF of the bulk crystal. The dislocation LGFs account for changes in the topology of the crystal in the core as well as local strain throughout the crystal lattice. The standard deviations of the dislocation Nye tensor distributions quantify the widths of the dislocation cores. The relaxed cores are compact, and the local magnetic moments on the Fe atoms closely follow the volumetric strain distributions in the cores. We also compute the core structures of these dislocations using eight different classical interatomic potentials, and quantify symmetry differences between the cores using the Fourier coefficients of their Nye tensor distributions. Most of the core structures computed using the classical potentials agree well with the DFT results. The DFT core geometries provide substitutional and interstitial sites for computing solute-dislocation interactions and can serve as inputs for mesoscale models of solute diffusion near dislocations.