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The secondary phase, such as Ni3Al-based L12 γ′, is crucially important for the precipitation strengthening of superalloys. Composition–structure–property relations provide useful insights for guided alloy design. Here we use density functional theory combined with the multiple scattering theory to compute dependencies of the structural energies and equilibrium volumes versus composition for ternary Ni3(Al1−xXx) alloys with X = {Ti, Zr, Hf; V, Nb, Ta; Cr, Mo, W} in L12, D024, and D019 phases with a homogeneous chemical disorder on the (Al1−xXx) sublattice. Our results provide a better understanding of the physics in Ni3Al-based precipitates and facilitate the design of next-generation nickel superalloys with precipitation strengthening. [ABSTRACT FROM AUTHOR]

We present a procedure to determine temperature-dependent thermodynamic properties of crystalline materials from density functional theory molecular dynamics (DFT-MD). Finite temperature properties (structural, thermal, and mechanical properties) of the phases (ground state monoclinic B33, martensitic B19', and austenitic B2) of the shape memory alloy NiTi are investigated. Fluctuation formulas and numerical derivatives are used to evaluate mechanical and thermal properties. A modified version of thermodynamic upsampling is used to predict properties in the high DFT convergence threshold limit from low threshold simulations. DFT convergence thresholds. In addition, a thermodynamic integration expression is developed to compute free energies from isobaric DFT-MD simulations that accounts for volume changes. Structural parameters, elastic constants, volume expansion, and specific heats as a function of temperature are evaluated. Phase transitions between B2 and B19' and between B19' and B33 are characterized according to their thermal energy, entropy, and free energy differences as well as their latent heats. Anharmonic effects are shown to play a large role in both stabilizing the austenite B2 phase and suppressing the martensitic phase transition. The quasiharmonic approximation to the free energy results in large errors in estimating the martensitic transition temperature by neglecting these large anharmonic components. [ABSTRACT FROM AUTHOR]

The stretching of a single chain of phenolic polymer is studied using density functional theory (DFT) and the reactive force field (ReaxFF) potential. The most detailed studies are performed at 0 K, which shows the breaking of the C(ring)-CH 2 bridge bond can be delayed to longer polymer length by a ring opening mechanism that relieves the strain. Molecular dynamics (MD) simulations, using the DFT approach, show that C–CH 2 bridge bond breaking occurs much more frequently than the ring opening. The stretching potentials determined using ReaxFF compare favorably with the DFT results. Using ReaxFF, stretching of cross-linked systems were considered and compared to single chains. The effect of changing the linking group on the bond stretching portion of the potential is found to be relatively small, but the effect on the bond energies can be significant. [ABSTRACT FROM AUTHOR]

A systematiccomparison of atomistic modeling methods including density functionaltheory (DFT), the self-consistent charge density-functional tight-binding(SCC-DFTB), and ReaxFF is presented for simulating the initial stagesof phenolic polymer pyrolysis. A phenolic polymer system is simulatedfor several hundred picoseconds within a temperature range of 2500to 3500 K. The time evolution of major pyrolysis products includingsmall-molecule species and char is examined. Two temperature zonesare observed which demark cross-linking versus fragmentation. Thedominant chemical products for all methods are similar, but the yieldsfor each product differ. At 3500 K, DFTB overestimates CO production(300–400%) and underestimates free H (∼30%) and smallCmHnO molecules(∼70%) compared with DFT. At 3500 K, ReaxFF underestimatesfree H (∼60%) and fused carbon rings (∼70%) relativeto DFT. Heterocyclic oxygen-containing five- and six-membered carbonrings are observed at 2500 K. Formation mechanisms for H2O, CO, and char are discussed. Additional calculations using a semiclassicalmethod for incorporating quantum nuclear energies of molecules werealso performed. These results suggest that chemical equilibrium canbe affected by quantum nuclear effects at temperatures of 2500 K andbelow. Pyrolysis reaction mechanisms and energetics are examined indetail in a companion manuscript. [ABSTRACT FROM AUTHOR]

The Li+–BF4–andBF4––BF4–interactions are studied using second order perturbation theory(MP2) and coupled cluster singles and doubles approach, includingthe effect of connected triples, CCSD(T). The MP2 and CCSD(T) resultsare in excellent agreement. Using only the MP2 approach, the interactionsof Li+with bis(trifluoromethane)sulfonimide anion (TFSI)and Li+with bis(fluorosulfonyl)imide anion (FSI) are studied.The results of these high level calculations are compared with densityfunctional theory (DFT) calculations for a variety of functionalsand with the APPLE&P force field. The B3LYP approach well reproducesthe accurate calculations using both a small and large basis set.The M06 and M06L functionals in the larger basis set are in good agreementwith the high level calculations. While the APPLE&P force fielddoes not outperform the best functionals, the APPLE&P resultsagree better with the accurate results than do some of the functionalstested. [ABSTRACT FROM AUTHOR]

Reaction paths for the loss of CO,H2, and H2O from atomistic models of phenolicresin are determined using the hybrid B3LYP approach. B3LYP energeticsare confirmed using CCSD(T). The energetics along the B3LYP pathsare also evaluated using the PW91 generalized gradient approximation(GGA), the more approximate self-consistent charge density functionaltight binding (SCC-DFTB), and the reactive force field (ReaxFF). Comparedwith the CCSD(T)/cc-pVTZ level for bond and reaction energies andbarrier heights, the B3LYP, PW91, DFTB(mio), DFTB(pbc), and ReaxFFhave average absolute errors of 3.8, 5.1, 17.4, 13.2, and 19.6 kcal/mol,respectively. The PW91 is only slightly less accurate than the B3LYPapproach, while the more approximate approaches yield somewhat largererrors. The SCC-DFTB paths are in better agreement with B3LYP thanare those obtained with ReaxFF. [ABSTRACT FROM AUTHOR]