Presence Of Substitutional Atoms Research Articles

Density functional theory characterisation of cementite (Fe[formula omitted]C) with substitutional molybdenum (Mo) atoms

The presence of substitutional atoms, other than iron (Fe), in the cementite (Fe3C) phase in steels can have a beneficial influence on different mechanical and thermodynamic properties. For example, the addition of molybdenum (Mo), which tends to partition to cementite and stabilise it, can improve the hardness of the steel at higher temperatures. In order to better understand the effects of Mo as a substitutional element, an energetic, structural, electronic, and mechanical characterisation of cementite containing Mo atoms was performed. Formation enthalpies, densities of states, and the full stiffness tensors where calculated and analysed in terms of preferential occupation sites, stability, ductile/brittle behaviour, and isotropic and directional evolution of the elastic constants.

Influence of substitutional atoms on the diffusion of oxygen in dilute iron alloys

A multiscale approach including density functional theory (DFT) and atomistic kinetic Monte Carlo (AKMC) simulations is applied to investigate the diffusion of interstitial oxygen atoms in bcc Fe under the influence of substitutional foreign atoms (Al, Si, P, S, Ti, Cr, Mn, Ni, Y, Mo, and W). The substitutional atoms can be assumed to be immobile since their diffusion coefficient is much smaller than that of oxygen. First, jumps of oxygen in pure bcc Fe between first-, second-, and third-neighbor octahedral interstitial sites are investigated. It is found that the first-neighbor jump is most relevant, with the tetrahedral site as the saddle point. The second-neighbor jump consists of two consecutive first-neighbor jumps. The barrier for a direct third-neighbor jump is too high to be significant for the diffusion process. In the presence of substitutional atoms the most important migration paths are first-neighbor jumps between modified octahedral sites with modified tetrahedral sites as saddle points. Calculations show that Si, P, Ni, Mo, and W cause some modifications of the migration barriers of oxygen and their interaction with O is mainly repulsive. Al, Cr, and Mn have a significant influence on the barriers and they exhibit strong attractive interactions with O. The most important modification of the barriers is found for S, Ti, and Y where deep attractive states exist. Based on the migration energies obtained by DFT, AKMC simulations on a rigid lattice are employed to determine the diffusion coefficient of oxygen in a dilute iron alloy containing the different substitutional atoms. It is found that Si, P, Ni, Mo, and W have almost no influence on the diffusivity of O. The presence of Al, Cr, Mn, S, Ti, and Y causes a significant reduction of the mobility of oxygen. In these cases the temperature dependence of the oxygen diffusion coefficient shows considerable deviations from an Arrhenius law. These phenomena are discussed in detail by considering the occupation time for the different states. The present findings on the strong dependence on the kind of substitutional atoms change the picture of oxygen diffusion in dilute iron alloys substantially.

Dependence of magnetisation and magnetocrystalline anisotropy on site distribution of alloying elements in RE-TM phases with ThMn12 structure

The effect of site disorder (originating from the presence of substitutional atoms) on intrinsic magnetic properties of intermetallic phases derived from the ThMn12-type crystal structure has been studied using the tight-binding linear-muffin-tin-orbital atomic-sphere approximation (TB-LMTO-ASA) method of density functional theory (DFT). The focus was put on the analysis of magnetisation and magnetocrystalline anisotropy in 42 inequivalent configurations that differed in their arrangements of the substitutional atoms. The first-order anisotropy constant K1 turns out to be far more sensitive to the site occupancy than the magnetisation M. Addition of light interstitial atoms B, C or N gives rise to much higher K1 values but the same range of scattering is retained. As the anisotropy constant K1 is strongly increased by interstitial atoms, the influence of the atomic disorder is less pronounced for these systems.

Oxygen Diffusion in an Nb-Ta Alloy Measured by Mechanical Spectroscopy

When metals that present bcc crystalline structure receive the addition of interstitial atoms as oxygen, nitrogen, hydrogen and carbon, they undergo significant changes in their physical properties because they are able to dissolve great amounts of those interstitial elements, and thus form solid solutions. Niobium and most of its alloys possess a bcc crystalline structure and, because Brazil is the largest world exporter of this metal, it is fundamental to understand the interaction mechanisms between interstitial elements and niobium or its alloys. In this study, mechanical spectroscopy (internal friction) measurements were performed on Nb-8.9wt%Ta alloys containing oxygen in solid solution. The experimental results presented complex internal friction spectra. With the addition of substitutional solute, interactions between the two types of solutes (substitutional and interstitial) were observed, considering that the random distribution of the interstitial atoms was affected by the presence of substitutional atoms. Interstitial diffusion coefficients, pre-exponential factors and activation energies were calculated for oxygen in this alloy.

Effect of impurities on mechanical relaxation in niobium

Measurements of internal friction as a function of temperature were performed in samples of Nb containing different amounts of substitutional (Zr) and interstitial (O, N) solutes. These data were resolved using the method of successive subtraction, into a series of constituent Debye peaks corresponding to different interactions. For each relaxation process we obtained the height (Q-1max) and temperature (Tp) of the peak, the activation energy (E) and the relaxation time (t0). The height, shape and temperature of the peaks depend on the concentration of interstitial and substitutional elements. With the addition of substitutional solute one can observed interactions between the two types of solutes (substitutional and interstitial), showing that the random distribution of the interstitial atoms was affected by presence of substitutional atoms. The peaks observed were associated with matrix-interstitial (Nb-O, Nb-N) and substitutional-interstitial (Zr-O) interaction processes.

The investigation of Fe–Mn-based alloys with shape memory effect by small-angle scattering of polarized neutrons

The small-angle polarized neutron scattering (SAPNS) technique has been used to study a nuclear and magnetic homogeneity in the distribution of both substituent (Si, Cr, Ni) and interstitial (C, N) alloying elements on the mesoscopic range in Fe–Mn-based alloys with shape memory effect (SME). The four groups of alloys with various basic compositions: FeMn 18 (wt%), FeMn 20Si 6, FeMn 20Cr 9N 0.2 and FeMn 17Cr 9Ni 4Si 6 were investigated. It was found that the small-angle scattering of neutrons and depolarization on these alloys are very small altogether. The scattering did not exceed 1.5% from the incident beam and depolarization ∼2% for all samples. It means that these alloys are well nuclear and magnetically homogeneous on the scale of 10–1000 Å. However, the difference in the homogeneity depending on the compositions still takes place. Thus, the adding of Si in FeMn 18 and FeMn 20Cr 9N 0.2 alloys improves the homogeneity pronouncedly. At once, the effect of the doping by C or N atoms on the homogeneity in FeMn 20Si 6 and FeMn 17Cr 9Ni 4Si 6 alloys is multivalued and depend on the presence of substitutional atoms (Ni and Cr). The capability of SAPNS as a method for the study of mesoscopic homogeneity in materials with SME and testing of the quality of their preparation is discussed.

Effect of Texture on the Height of the Snoek Peak in Electrical Steels.

The most important factors affecting the Snoek peak height of carbon in commercial steel sheets are shortly reviewed. It is shown that the proportionality factor K Q + (=Q max -1 /C i )-for a given interstitial and a given internal damping technique-mainly is affected by the steel texture and the presence of substitutional atoms. The influence of texture and composition is illustrated for four non oriented electrical steels with Si-contents between 0 and 1.8%. This difference in Si-content, combined with differences in C-content between the internal damping specimens, resulted in significant texture changes, providing an interesting frame not only to apply the theory on the orientation dependence of the calculated proportionality factor K Q (developed in a previous paper), but also to discuss the effect of alloying elements on the Snoek peak height. The proportionality factors of the experimentally measured textures are calculated by making the convolution product of the orientation distribution function (ODF) of the measured texture and the distribution function of K Q . Doing so, the effect of texture on the experimentally measured K Q + can be separated from the direct effect of the substitutional elements (mainly Mn, Al and Si). The latter effect is discussed in terms of the elastic strain present around substitutional atoms due to their size difference with Fe atoms.