Papers by Keyword: Site Preference

Abstract: Following nuclear decay, a daughter atom in a solid will "stay in place" if the recoil energy is less than the threshold for displacement. At high temperature, it may subsequently undergo long-range diffusion or some other kind of atomic motion. In this paper, motion of ¹¹¹Cd tracer probe atoms is reconsidered following electron-capture decay of ¹¹¹In in the series of In₃R phases (R= rare-earth). The motion produces nuclear relaxation that was measured using the method of perturbed angular correlation. Previous measurements along the entire series of In₃R phases appeared to show a crossover between two diffusional regimes. While relaxation for R= Lu-Tb is consistent with a simple vacancy diffusion mechanism, relaxation for R= Nd-La is not. More recent measurements in Pd₃R phases demonstrate that the site-preference of the parent In-probe changes along the series and suggests that the same behavior occurs for daughter Cd-probes. The anomalous motion observed for R= Nd-La is attributed to "lanthanide expansion" occurring towards La end-member phases. For In₃La, the Cd-tracer is found to jump away from its original location on the In-sublattice in an extremely short time, of order 0.5 ns at 1000 K and 1.2 ms at room temperature, a residence time too short to be consistent with defect-mediated diffusion. Several scenarios that can explain the relaxation are presented based on the hypothesis that daughter Cd-probes first jump to neighboring interstitial sites and then are either trapped and immobilized, undergo long-range diffusion, or persist in a localized motion in a cage.

Abstract: The structural properties of GdFe_2-xT_x (T=Ti, Al) and their hydrides are studied by using inter-atomic potentials based on Chens lattice inversion technique. The results show that GdFe_2-xAl_x crystallizes with the MgZn₂ type phase in the range 0.35x0.7. And GdFe2-xTix crystallizes with the MgZn₂ type phase has the lowest energy for 0x<0.17 and="" the="" tolerance="" is="" acceptable="" gdfe="" sub="">2-xTi_x compounds are stabilized in MgZn₂ type phase with different Ti atoms content in the range 0.17<x0.6. Moreover, the calculated lattice constants coincide quite well with experimental values. All the results indicate the potentials are valid for studying the structural properties of the intermetallics.

Abstract: An atomistic simulation of the structural properties of the Rh7-xTxB3 series, where T is Fe, Cr, Mn, has been carried out using interatomic pair potentials based on the lattice inversion method. Calculated results show T atoms can stabilize Rh7-xTxB3 with Th7Fe3-type structure, and T atoms substitute for Rh with a strong preference for the 2b sites. The phase stability of the intermetallics Rh7-xTxB3 is tested by many means including random atom shift, global deformation and high temperature disturbance under the control of the pair potentials. Calculated unit-cell parameters for Rh7-xTxB3 agree with the experimental data very well. All the above results indicate that the potentials are valid for studying the structural properties of these kinds of complex structure of transition metal boride.

Abstract: The phase stability and site preference of transition metal carbides Cr in Fe7-xCrxC3 are studied based on the pair potentials obtained by the lattice inversion method. The lattice constants and cohesive energy of Fe7-xCrxC3 with the content x are calculated. The results show that Cr atoms substitute for Fe with a strong preference for the 6c1 sites and the order of site preference is 6c1, 6c2 and 2b. Calculated lattice parameters are in good agreement with the experimental data. Moreover, the total and partial phonon densities of states are first evaluated for the Fe7-xCrxC3 compounds with the hexagonal structure. We also provide some information on the vibrational properties of transition metal carbides, such as the specific heat and Debye temperature were also evaluated.

Abstract: A first principles calculation method was used to investigate the site preference of Ruthenium (Ru) at the γ/γ′ interface in Ni-based single-crystal superalloys. The calculation results show that the addition of Ru can decrease the total energy and the binding energy of γ/γ′ interface, which may result in an improved microstructure stability of Ni-based single-crystal superalloys. Moreover, by calculation, it is also found that Ru can stabilize both γ and γ′ phases and have a preference for Ni site at the coherent γ/γ′ interface. When Ru substitutes the central Ni at the γ/γ′ interface, a reverse partitioning of W, Re and Cr occurs; while the partitioning behavior of Mo is not affected. The influence of Ru on the partitioning behavior of W, Re and Cr in γ′-Ni3Al was studied by Dmol3 calculation as well. The calculation results show that W, Re and Cr have a preference for Ni site in γ′- Ni3Al with Ru alloying. When Ru substitutes the central Ni atom, the site preference of W, Re and Cr varies accordingly. Furthermore, electronic structure analysis of γ/γ′ interface and γ′-Ni3Al in terms of Mulliken population and partial density of states (PDOS) was performed to understand the alloying mechanism of Ru in Ni-based single-crystal superalloys. The results show that the strengthening effect of Ru alloying is mainly due to the reduction in binding energy of Ru as well as a p-orbital hybridization between Ru and the host atoms.

Abstract: First-principles supercell calculations based on density functional theory were performed to study the site preference behavior and elastic properties of 3d (Ti-Cu) transition-metal elements in B2 ductility YAg alloy. It is found that Ti occupies the Y sublattice, while V, Cr, Co, Fe, Ni and Cu tend to substitute for Ag site. All alloying elements can decrease the lattice parameters of Y8Ag8, among which Y7Ag8Ti shows the largest change. Furthermore, the calculated elastic constants show that Cr, Fe, Co and Cu can improve the ductility of YAg alloy, and Y8Ag7Fe presents the most ductility among these alloy, while Ti and Ni alloying elements reduce the ductility of YAg alloy, especially, V transforms ductile into brittle for YAg alloy. In addition, both V and Ni can increase the hardness of YAg alloy, and Y8Ag7V is harder than Y8Ag7Ni.

Abstract: The effect of cobalt on the structural properties of intermetallic Tb₃(Fe_28-xCo_x)V_1.0 with Nd₃(Fe,Ti)₂₉ structure has been studied by using interatomic pair potentials obtained through the lattice inversion method. Calculated results show that the order of site preference of cobalt is 8j(Fe8), 4e(Fe11) and 2c(Fe1) which is in good agreement with experimental results. And the calculated lattice constants coincide quite well with experimental values. All these prove the effectiveness of interatomic pair potentials obtained through the lattice inversion method in the description of rare-earth materials.

Abstract: The method of perturbed angular correlation (PAC) was used to determine lattice locations of ¹¹¹In impurity probe atoms present in extreme dilution in the intermetallic compound FeGa₃. In slightly Ga-poor samples, probes were found to strongly prefer one of two inequivalent Ga-sites. In slightly Ga-rich samples at room temperature, 293 K, the PAC spectrum exhibited an unperturbed quadrupole interaction signal that is consistent with indium probes dissolved in small liquid pools of the excess Ga. A myriad of such pools are probably located along grain boundaries in the sample. Cooling from 293 K down to 12 K, the site fraction of indium in liquid decreased, being offset by the increase in a signal attributed to indium solutes in precipitates with other impurities at the sides of the Ga pools. However, these changes were completely reversible upon heating, and no crystallization of the liquid gallium pools was observed down to 12 K. This is attributed to the extraordinarily small volumes for the pools, which, while not measured directly, are orders of magnitude smaller than cubic microns. The measured temperature dependence of the site fraction of indium in the liquid was used to extend the metastable solubility curve for indium in liquid gallium down to a temperature of 150 K, much lower than the eutectic temperature of Ga-In at 288.5 K.

Abstract: First-principles supercell calculations, which are based on density functional theory, were performed to study the site preference behavior and elastic properties of 5d (Hf-Au) transition-metal elements in B2 ductility YAg alloy. It is found that all alloying elements Hf, Ta, W, Re, Os, Ir, Pt and Au occupy the Y sub-lattice. Micro-alloying transition metals W, Re, Os, Ir and Pt decrease the lattice parameters of Y8Ag8 except Hf, Ta and Au, among which Y8Ag7Hf shows the largest variance. Furthermore, the calculated elastic constants show that Hf, Ta, W, Re, Os, Pt and Au improve the ductility of YAg alloy, and Y8Ag7Hf presents the most ductility among these alloy, while Ir transforms ductile into brittle for YAg alloy. In addition, Os alloying element increases the hardness of YAg alloy.

Abstract: The site preference and thermodynamic properties of UT_xAl_12-x (T = Zr, Nb, Mo and Fe) and their related hydrides are studied based on the pair potentials obtained by the lattice inversion method. The calculated result demonstrates that the stabilizing elements Zr, Nb, or Mo prefer to substitute for Al in 8i sites; and Fe atom preferentially substitutes for Al in the 8f site. The interstitial H atoms only occupy 2b interstitial sites in UT_xAl_12-x. The calculated lattice parameters coincide with the experimental values. In addition, the total and partial phonon densities of states are first evaluated for these compounds.