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The incorporation of cubic β/β0 phase improves significantly the ductility and workability of TiAl-based alloys. However, the ductility deteriorates with the precipitation of the ordered hexagonal ω0 phase in the β/β0 phase. To avoid the formation of the ω0 phase in alloy design, fundamentals about the β/β0 to ω0 phase transition and its alloying element dependence have to be known. In the present work, we investigated the phase transition and phase stability in Ti4Al3Nb and Ti4Al3Mo alloys using a first-principles method based on density functional theory. Our calculations indicate that for the β0 to ω0 transition, the "collapse-diffusion" pathway is easier than the "diffusion-collapse" one; however, both pathways are quite difficult due to the high-energy barriers. The pa to co" transition occurs through the "diffusion-collapse" pathway without an energy barrier (except for the activation energy for the short-range diffusion needed for atomic rearrangement). Comparing the total energies of different phases, we find that the ω0 phase of Ti4Al3Nb is more stable than β0 and vice versa for Ti4Al3Mo whereas the relative stability of ω" to β0 for Ti4Al3Nb is stronger than that for Ti4Al3Mo, indicating that the addition of Mo inhibits the transition from the β0- to the ω-related phases in Ti4Al3Nb. Our calculations demonstrate that ω" is more stable than ω0 for both Ti4Al3Nb and Ti4Al3Mo, in contrast to the previously proposed picture based on some experimental observations, the possible origins of which are discussed in detail. [ABSTRACT FROM AUTHOR]

Structure searches for new high-pressure phases of Y metal have been performed by using evolutionary algorithms in conjunction with a first-principles, pseudopotential plane-wave method based on density functional theory. The oFl6-Fddd and hP3-P3\2\ phases are predicted to be energetically favorable at pressures over 97 GPa. These two phases are shown to be dynamically stable by computing their phonon dispersions. We thus propose that oF\6-Fddd and hP3-P3]2l are the most probable crystal structures Y may take in the 97-206 GPa range. The superconducting critical temperatures (Tc) of the new phases are estimated using the Allen-Dynes formula. The Tc is predicted to decrease with increasing pressure over about 100 GPa, in sharp contrast to its observed monotonie increase under lower pressure. The electronic origins of the stabilities of the proposed high-pressure phases have also been investigated. [ABSTRACT FROM AUTHOR]

Abstract: Although the stacking fault energy (SFE) is a fundamental variable determining the minimum grain size (d min) obtainable in severe plastic deformation (SPD) processes, its accurate measurement is difficult. Here we establish the SFEs of binary Pd–Ag, Pd–Cu, Pt–Cu and Ni–Cu solid solutions using the axial interaction model and the supercell model in combination with first-principles theory. The two models yield consistent formation energies. For Pd–Ag, Pd–Cu and Ni–Cu, the theoretical SFEs agree well with those from the experimental measurements. For Pt–Cu no experimental results are available, and thus our calculated SFEs represent the first reasonable predictions. We discuss the correlation of the SFE and d min in SPD experiments and show that the d min values can be evaluated from first-principles calculations. [Copyright &y& Elsevier]

Density functional theory calculations are performed to investigate the electronic and magnetic properties of FeCrSi alloys with Cr concentration in the range of 0.25 ≤ x ≤ 2.75. The L2 phase is found to be a more stable one in comparison with the A15 phase for x ≤ 1.50 beyond which the A15 phase becomes more stable. Alloys with the stable L2 phase are found to be metallic for x ≤ 0.75, however, a half metallic behavior is found at x = 1.00, 1.25, and 1.50 with band gaps of 0.60, 0.24, and 0.21 eV, respectively. In contrast, all A15 structures are found to be metallic. The total magnetic moments are found to decrease for L2 phase from 14.4 μ/cell at x = 0.25 to zero at x = 2.00 with non-integer values for the metallic structures and integer values for the half-metallic. However, a monotonic decrease is found for the case of A15 phase with values larger than those of L2 phase. [ABSTRACT FROM AUTHOR]

The influences of doping elements (Ti, Zr, Hf, V, Ta, Cr, Mo, W and Al) on the heats of formation of Nb, Nb5Si3 and Nb3Si have been studied using first-principles pseudopotential plane-wave method based on density functional theory. Site preferences of dopants in Nb5Si3 and Nb3Si intermetallics are first determined by comparing the heats of formation of the systems with different site occupations of the doping atoms. The partitionings of dopants between the equilibrium phases Nb and Nb5Si3 of Nb-Si in situ composites are then discussed and compared with experimental results. Phase partitioning behaviours of the alloying elements are found to depend strongly on the number of their valence electrons. By calculating the enthalpy of reaction regarding the eutectoid decomposition of Nb3Si phase, we conclude that Ta is a Nb3Si stabiliser while other dopants destabilise Nb3Si at the investigated concentration. [ABSTRACT FROM AUTHOR]

The structural phase transitions of yttrium under high pressure up to 300 GPa have been studied by using the first-principles, pseudopotential plane-wave method based on density functional theory. The experimental rare-earth structure sequence with increasing pressure is successfully reproduced. The yet unknown distorted lace-centered cubic (dfcc) phase is determined to be of the R3̄m space group. In addition, the P6222 phase is predicted to be energetically more stable beyond the pressure range for dfcc. Accurate phonon-dispersion calculations demonstrate that the P6222 phase is dynamically stable beyond pressure of about 206 GPa. The corresponding projected band structures, density of states, and charge density differences are given in order to clarify the electronic origins of the stability of the P6222 phase under pressure. [ABSTRACT FROM AUTHOR]

Abstract: The interaction energies between substitutional rare earth (RE) atoms, between RE and interstitial C, N, O, H atoms, as well as between RE and vacancies in α-Ti are calculated via first-principles density-functional theory with projector augmented-wave (PAW) pseudopotentials. The results show that the RE–vacancy and RE–RE interactions are attractive due to the weaker RE–Ti bond than the host Ti–Ti bond. All of the RE atoms investigated in this paper are repulsive to C and N, but attractive to H. RE–O interactions are repulsive for the light RE atoms, though the interactions are very weak for the heavy RE atoms. The mechanism underlying the interactions and their possible influence on the properties of Ti alloys are discussed. [Copyright &y& Elsevier]

Density functional theory investigations indicate that carbon boronyls (CBO)n (n = 3–7) are considerably more stable in thermodynamics than their boron carbonyl isomers (BCO)n and exhibit aromaticity throughout the whole series. The extra stabilities of (CBO)n originate from their frontier π molecular orbitals delocalized over the Dnh Cn central rings which are absent in (BCO)n. It is expected that experimental characterization of these (CBO)n species may open a new branch of chemistry on carbon boronyls. © 2005 Wiley Periodicals, Inc. J Comput Chem 26: 799–802, 2005 [ABSTRACT FROM AUTHOR]

Ab initio theoretical evidence of hexacoordinate planar main group atoms centered in hexagonal hydrocopper complexes Cu6H6X (X = Si, P, As) is presented at the density functional theory level in this work. The results obtained extend the bonding capacity of silicon, phosphorus, and arsenic to planar hexacoordination in hydrocopper complexes which are important in fundamental research and may shed new insight into catalyst chemistry. [ABSTRACT FROM AUTHOR]