Chong Long Fu

Phase stability, bonding mechanism, and elastic constants of Mo5Si3 by first-principles calculation

We present results of first-principles local-density-functional calculations of the structural and elastic properties of Mo5Si3. Among the three different structures (D8m, D88, and D8l), the D8m structure (referred to as the T1 phase) has the greatest binding with a high heat of formation of −3.8 eV/formula unit. The bonding in Mo5Si3 is found to have pronounced covalent components, characterized by the planar Mo–Si–Mo triangular bonding units on the (001) plane and by the unusually short Mo–Mo bonds directly along the c-axis. The calculated six elastic constants of the D8m structure are in excellent agreement with the experimental values. While the bonding in the (001) basal plane is stronger than the bonding along the [001] direction (i.e. C11+C12>C33 and C66>C44), the crystal anharmonicity is found to be higher along the [001] direction. The implication of our results on the anisotropy of thermal expansion coefficients is briefly discussed.

Site preference of ternary alloying additions in FeAl and NiAl by first-principles calculations

Abstract First-principles calculations have been performed to investigate the site preference of ternary alloying additions in FeAl and NiAl. In FeAl, Cr and Ti are found to occupy the Al sublattice whereas Ni has a distinct preference for the Fe sublattice. The site substitutional behavior of 3d ternary elements in FeAl can be explained in terms of the trends in the heat-of-formation. In Al-rich NiAl, Fe atoms occupy exclusively the Ni sublattice. In Ni-rich NiAl, because of the small enthalpy difference between Fe occupying Al and Ni sublattices (i.e. less than 0.1 eV with a preference of Fe for Al sites at 0 K), the site distributions of Fe in these alloys are found to vary with alloy composition and temperature. Due to the large difference in the local magnetic moments between Fe atoms occupying Al and Ni sublattices (with values of 2.4 μ B and less than 0.1 μ B , respectively) in NiAl, magnetic susceptibility measurement should be the most effective way to measure the site distributions of Fe in NiAl.

Phase stability and elastic moduli of Cr2Nb by first-principles calculations

The phase stability and elastic moduli of Cr2Nb are investigated by first-principles calculations. Heats of formation are calculated and compared for the three Laves phases (C15, C14, and C36). It is found that the C15 phase is the ground-state structure with the lowest energy and the C36 phase is an intermediate state between C15 and C14. These three phases, however, are very close in energy, indicating low stacking fault energies in this system. For the ground-state C15 phase, we calculate three elastic constants from which the shear and Young’s moduli are obtained. It is found that these calculated moduli are smaller than the experimental values obtained from polycrystals.

Point defects and the binding energies of boron near defect sites in Ni3Al: A first-principles investigation

Abstract First-principles local-density-functional calculations have been used to investigate the equilibrium point defect structure and boron-defect interactions in Ni 3 Al. The dominant point defect types in off-stoichiometric Ni 3 Al are substitutional antisite defects on both sublattices. The boron binding energy is dependent on lattice coordination; it is strongest near vacancy sites with a nearst-neighbour nickel coordination number of about four (instead of six as in the defect-free interstitial site) and with no aluminium atom nearest-neighbours. This suggests that boron tends to segregate to “open” defect sites and to enhance cohesion through the formation of localized Ni-B covalent bonds. Comparison of the binding energies of boron and carbon in Ni 3 Al shows that boron has a stronger tendency to segregate to “open” sites than carbon.

Theoretical calculations and experimental measurements of the structure of Ti5Si3 with interstitial additions

Abstract The equilibrium structural parameters, enthalpies of formation and partial densities of state for Ti5Si3 and Ti5Si3Z0.5 (Z= B, C, N or O) were calculated based on first-principle techniques. Enthalpy of formation calculations suggest that of the known structures for transition metal (TM) silicide compounds containing TM5Si3 (D88, D8l and D8m) the D88 structure is the most stable form of Ti5Si3, and the stability of the structure increases as Z atoms are added. The theoretically determined structural trends as a function of interstitial element, Z, agreed well with experimentally determined values. Both indicate bonding between Ti and Z atoms based on contraction of Ti–Z separations. The calculated partial densities of state suggest that p(Si)–d(Ti) and d(Ti)-d(Ti) interactions are responsible for most of the bonding in pure Ti5Si3, which agrees with previous studies. As Z atoms are added, p(Z)–d(Ti) interactions become significant at the expense of weakening some of the d(Ti)–d(Ti) interactions.

Mechanistic modeling of deformation and fracture behavior in TiAl and Ti3Al

Abstract Phase stability and bulk properties of TiAl and Ti 3 Al are investigated based on the first-principles local-density-functional approach, and deformation and fracture behavior of two-phase lamellar γ-TiAl alloys are analyzed using the linear elasticity theory applied to interfaces, dislocations, and cracks. In terms of the calculated elastic constants, Youngs and shear moduli are higher in Ti 3 Al than in TiAl. The coupling effect due to elastic incompatibility reduces the misfit strain at an interface when a tensile stress is applied normal to it. The mode mixity of stress concentration by a dislocation pile-up plays an important role in determining whether slip transfer or microcracking occurs at the interface. The fracture morphology reported in notched polysynthetically twinned crystals is explained in terms of the elastic interaction of a (11 2 )[1 1 0] crack with an α 2 -Ti 3 Al plate. Additional factors that are involved in the brittle-to-ductile transition behavior of TiAl are discussed.

Phase stability of intermetallics in the AlTi system: A first-principles total-energy investigation

The phase stability of ordered Ti3Al, TiAl, and TiAl3 alloys is studied in various structures with first-principles total-energy calculations. The observed structures of these alloys are found to have lower energies than other competing structures. The calculated heats of formation are in good agreement with experiment. The most significant finding of these calculations is that for Ti3Al the cubic L12 structure is only slightly higher in energy than the observed hexagonal DO19 structure, i.e. 0.01 eV/atom. Based on the total-energy calculations, it is suggested that additions of ternary elements may be able to stabilize Ti3Al into the cubic L12 structure, which will presumably improve the ductility.

Elastic constants and planar fault energies of Ti3Al, and interfacial energies at the Ti3AlTiAl interface by first-principles calculations

Abstract We have presented results of first-principles calculations of the elastic constants of Ti3Al, and the planar fault energies in Ti3Al and at the TiAl Ti 3 Al interfaces. Similar to those found in Ti, the bonding in Ti3Al is dominated by the multi-centered bonding interactions, but is enhanced by the presence of Al atoms. The bonding enhancement is manifested in stiffer elastic moduli and considerably high APB energies (even in the case where the nearest-neighbor coordination of atoms is not altered by the presence of APB, i.e., APB-I on the prism plane). The APB and CSF energies are reduced at the TiAl Ti 3 Al interface from those of either constituent phase, since the long-range bonding interactions are disrupted at the interface.

Bonding mechanisms and point defects in TiAl

Abstract The bonding mechanism and point defect structure in TiAl were investigated using a first-principles quantum mechanical calculation. The most remarkable feature found in the calculated binding charge density is the polarization of the p-electron at the aluminum sites, which gives rise to a large bond bending force between the Ti and Al layers. For the point defects, the absence of structural vacancies is predicted in TiAl, and the deviations from stoichiometry are accommodated by the substitutional antisite defects on both sublattices. High vacancy formation energies are obtained, which are closely related to the strong TiAl bonding and the similar atomic radii of Ti and Al.

Interfacial energies in two-phase TiAl-Ti3Al alloy

Abstract The intrinsic values of interfacial energies based on first-principles calculations, including atomic relaxation, were obtained for the three types of γ/γ interfaces and the α 2 /γ lamellar boundary in two- phase TiAl alloy. The pseudo-twin boundary energy is highest, Γ P = 270 mJ/m 2 , and the true-twin boundary energy is lowest, Γ P = 60 mJ/m 2 . Planar fault energies at pseudo-twin and 120°-rotational interfaces are markedly different from those in the bulk of the γ-phase, i.e., approximately, E APB and E CSF decreases by more than 30% and E SISF increases by threefold. Enhanced mobility of ordinary dislocations along the γ/γ and α 2 /α interfaces (except true twin boundaries) is predicted based on the reduced E CSF values at the interfaces.