Theoretical investigation of magnons in Fe-Ga alloys

Fe-Ga alloys show an unusually large increase in magnetostriction compared to pure Fe and are one of the most interesting Fe-based alloys for this reason. However, the origin of the large magnetostriction and its relation to the chemical ordering on the underlying bcc phase is still under debate. To gain further understanding of the extraordinary magnetoelastic characteristics of this system, we investigate the effect of Ga-concentration and ordering on the spin-wave spectra and stiffness. The magnetic interactions in the Fe-Ga alloys are obtained by ab initio electronic structure calculations and the magnon spectra are modeled using atomistic spin dynamics modeling. Our results agree with available experimental data and show softening of the magnon modes with increasing Ga-concentration and a strong reduction of the spin-wave stiffness due to atomic ordering.Validerad;2024;Nivå 2;2024-01-18 (signyg);Funder: Swedish National Infrastructure for Computing (SNIC 2020/5-415);Full text license: CC BY</p

Ab-initio based modeling of precipitation in Al–(Sc,Zr) alloy. Formation and stability of a core–shell structure

Statistical alloy theory based on the Master Equation approach with ab initio calculated interatomic interactions is employed to investigate the growth of precipitates at the early stages of solid solution decomposition, as well as the dissolution of small precipitates during the coarsening stage, upon simulated annealing of ternary Al–Sc–Zr alloys. We show, in agreement with previous studies, that the Zr alloying to Al–Sc alloys promotes the formation of core–shell nanoparticles whose structure is found to be very sensitive to the parameters characterizing the solute diffusion rates in the alloy. We demonstrate that the core–shell structure of precipitates slows down the dissolution of small particles, thus hampering the microstructure coarsening at elevated temperatures.</p

In Situ Bulk Observations and Ab Initio Calculations Revealing the Temperature Dependence of Stacking Fault Energy in Fe–Cr–Ni Alloys

The dependence of stacking fault energy (γSFE) on temperature in austenitic Fe–Cr–Ni alloy powders was investigated by in situ high energy synchrotron X-ray diffraction and ab initio calculations in the temperature range from − 45 °C to 450 °C. The X-ray diffraction peak positions were used to determine the stacking fault probability and subsequently the temperature dependence of γSFE. The effect of temperature on the diffraction peak positions was found to be mainly reversible; however, recovery of dislocations occurred above about 200 °C, which also gave an irreversible contribution. Two different ab initio-based models were evaluated with respect to the experimental data. The different predictions of the models can be explained by their respective treatment of the magnetic moments for Cr and Ni, which is critical for the alloy compositions investigated. Ab initio calculations, taking longitudinal spin fluctuations (LSF) into consideration within the quasi-classical phenomenological model, predict a temperature dependence of γSFE in good agreement with the experimentally evaluated trend of increasing γSFE with increasing temperature: | Δ γSFE/ Δ T| = 0.05 mJm - 2/ K. The temperature effect on γSFE is similar for all three investigated alloys: Fe–18Cr–15Ni, Fe–18Cr–17Ni, Fe–21Cr–16Ni (wt pct), while their room temperature γSFE are evaluated to be 22, 25, 20 mJ m−2, respectively.</p

In Situ Bulk Observations and Ab Initio Calculations Revealing the Temperature Dependence of Stacking Fault Energy in Fe–Cr–Ni Alloys

The dependence of stacking fault energy ($γ_{SFE}$) on temperature in austenitic Fe–Cr–Ni alloy powders was investigated by in situ high energy synchrotron X-ray diffraction and ab initio calculations in the temperature range from − 45 °C to 450 °C. The X-ray diffraction peak positions were used to determine the stacking fault probability and subsequently the temperature dependence of $γ_{SFE}$. The effect of temperature on the diffraction peak positions was found to be mainly reversible; however, recovery of dislocations occurred above about 200 °C, which also gave an irreversible contribution. Two different ab initio-based models were evaluated with respect to the experimental data. The different predictions of the models can be explained by their respective treatment of the magnetic moments for Cr and Ni, which is critical for the alloy compositions investigated. Ab initio calculations, taking longitudinal spin fluctuations (LSF) into consideration within the quasi-classical phenomenological model, predict a temperature dependence of $γ_{SFE}$ in good agreement with the experimentally evaluated trend of increasing $γ_{SFE}$ with increasing temperature: $|Δγ_{SFE}/ΔT|$=0.05mJm$^{−2}$/K. The temperature effect on $γ_{SFE}$ is similar for all three investigated alloys: Fe–18Cr–15Ni, Fe–18Cr–17Ni, Fe–21Cr–16Ni (wt pct), while their room temperature γSFE are evaluated to be 22, 25, 20 mJ m$^{−2}$, respectively

Effective interactions and atomic ordering in Ni-rich Ni-Re alloys

Interatomic interactions and ordering in fcc Ni-rich Ni-Re alloys are studied by means of first-principles methods combined with statistical mechanics simulations based on the Ising Hamiltonian. First-principles calculations are employed to obtain effective chemical and strain-induced interactions, as well as ordering energies and enthalpies of formation of random and ordered Ni-Re alloys. Based on the nonmagnetic enthalpies of formation, we speculate that the type of ordering can be different in alloys with Re content less than 10 at.%. We demonstrate that effective chemical interactions in this system are quite sensitive to the alloy composition, atomic volume, and magnetic state. In statistical thermodynamic simulations, we have used renormalized interactions, which correctly reproduce ordering energies obtained in the direct total energy calculations. Monte Carlo simulations for Ni 0.91 Re 0.09   alloy show that there exists a strong ordering tendency of the (112 0)  type leading to precipitation of the D1 a   ordered structure at about 940 K. Our results for the atomic short-range order indicate, however, that the presently applied theory overestimates the strength of the ordering tendency compared to that observed in the experiment.QC 20160721</p

First-principles investigation of the (CrMnFeNi)_1-xCo_x (0≤x≤0.2) alloy

The (CrMnFeNi)1−xCox high-entropy alloy is investigated for 0≤x≤0.2 by density functional theory calculations. All calculations are performed in theparamagnetic fcc-phase. It is shown that the exact muffin-tin orbital formalismcombined with the coherent potential approximation can reproduce experimentalvalues of equilibrium volume and magnetic moment. The thermal expansion isinvestigated using the Debye-Grüneisen model. Experimental results of the thermalexpansion coefficient and lattice parameter are reproduced only when including bothelectronic and magnetic contribution to the free energy. The investigated alloysshow anti-invar behaviour with a large increase in thermal expansion parameter withtemperature. For reduced Co-concentrations, the thermal expansion coefficient andlattice parameter are seen to increase, leading to slightly lower values of the elasticconstants. The stability of the alloys is discussed in terms of stacking fault energy andmixing energy.</p

Interactions and phase stability in Ni-rich Ni-W alloys

Interatomic interactions and phase transformation in Ni-rich Ni-W alloys are investigated using rst-principlesmethods and statistical thermodynamics simulations. The formation enthalpies of fcc and bcc random as wellas some fcc-based ordered structures are determined in the ferromagnetic and nonmagnetic states. The effective interactions are calculated in supercell ab initio calculations and using screened generalized perturbation method(SGPM). We find the stable fcc-based ordered structures are D1a, DO22 and Pt2Mo phases and they can be observed in the Ni-25 at.% W, Ni-25 at.% W and Ni-33 at.% W alloys, respectively. The calculated atomic short-range order results are in reasonable agreement with experiments and other theoretical investigations.</p

Simulation of fuel bed ignition by wildland firebrands

A 3-D mathematical model of fuel bed (FB) ignition initiated by glowing firebrands originating during wildland fires is proposed. In order to test and verify the model, a series of experiments was conducted to determine the FB ignition time by a single pine bark and twig firebrand (Pinus sylvestris). Irrespective of the pine bark sample sizes and experimental conditions, the ignition of the FB was not observed. Conversely, pine twigs, under certain parameters, ignited the FB in the range of densities (60–105 kg m−3) and with the airflow velocity of ≥2 m s−1. The results of the mathematical modelling have shown that a single pine bark firebrand ≤5 cm long with a temperature of T ≤ 1073 K does not ignite in the flaming mode the FB, and only the thermal energy of larger particles is sufficient for flaming ignition of the adjacent layers of the FB. The analysis of the results has shown that the firebrand length is a major factor in the initiation of ignition. Comparison of the calculated and observed FB ignition times by a single firebrand have shown that our modelling accords well with the experimental results