First principles total energy local density method have addressed the problems of (i) bonding, cohesion and phase stability and (ii) the role of ternary additions, anti-phase boundaries (APB's) and other faults in determining the structural, electronic and mechanical properties of aluminum intermetallic alloys. A key goal has been to attempt to understand, at the electronic level, fundamental quantities that may be related to the crucial brittleness vs. ductility issue in high temperature Ni and Ti and other aluminides. Other contrasts between observed ductility properties of related systems (e.g., NiAl and RuAl) are related to their differing electronic and bonding properties, particularly the nature of p-d hybridization and the directional properties of their electronic charge distrubutions - especially for states near the Fermi energy.

Total energies of binary and ternary -metral trialminides in the L12DO22 Do23Structures and binary Transition- metal disilicides in the C1lb, C40, C54, and C49 structures have been obtained by ab initio band-structure calculations. In aluminides the tetragonal Do22 and Do23 structures are stabilized relative to cubic L12P and in silicides the hexagonal C40 structure is stabilized relative to orthorhombic C54 and tetragonal C11b relative to C40, as the transition-metal d-electron count increases. The observed easier stabilization of L12 in Ti(AI,Fe) 3 relative to Nb(AI,Fe)is justified by the calculations. Location of the Fermi level in a quasigap in the density of states distribution rationalizes the observed structural stabilities in aluminides but not in silicides.

We discuss how, in the linear-muffin-tin-orbitals (LMTO) method, the potential parameters for an intermetallic compound can be estimated from those of the constituents. For ordered alloys we illustrate the scheme by comparing the electronic density of states (DOS) obtained in self-consistent calculations with that obtained with the potential parameters transferred from the pure components. Several examples are discussed. We discuss how the scheme can be used in performing almost self-consistent calculations for disordered alloys, and present some results obtained by using this scheme with the LMTO-CPA(coherent potential approximation) method.

Improving the low temperature ductility of the intermetallic compound γ - TiAl by alloying with small concentrations of an additive is of great practical importance. The difference in site selection energy of the additive plays an important role in the stability and behavior of the alloy. For boron in L10 TiAl, we have calculated the site selection energy using linearized combination of muffin-tin orbitals (LMTO) total energy calculations. For pure γ - TiAl, we found the equilibrium lattice structure by minimizing the total energy, and obtained good agreement with the experimental values. With the introduction of boron, a relaxation of the lattice around the boron additive is expected. For boron additives, we have calculated the forces on each atom leading to the minimization of the total energy as a function of the ionic positions in order to obtain the “true” stable structure of the alloy.

The assessed phase diagram of Fe-V exhibits a continuous high temperature bcc solid solution intersected at lower temperatures by a complex sigma phase centered around equiatomic composition [1]. Slow kinetics of the bcc to sigma transformation make it possible to retain the bcc solid solution at low temperature. It has been observed that this metastable solid solution has a tendency to order with a CsCl type structure (B2) below 970 K [1,2]. As a first attempt to describe this behavior from an electronic structure approach, this paper will study the phase stability on the bcc lattice using a realistic tight-binding Hamiltonian. Details of the tight-binding description have been given elsewhere [3]. Main features are as follows: Element and structure specific Slater-Koster parameters are used [4] and lattice parameter effects are incorporated through scaling [5]. Charge transfer is set to zero by rigidly shifting the onsite energies of one constituent. The Coherent Potential Approximation (CPA) is invoked with four levels corresponding to states with s, p, t2g and eg like symmetry. Effects of off-diagonal disorder (ODD) have not been included, instead, an average alloy Hamiltonian was defined using the Slater-Koster parameters of the components weighted by concentration. At equiatomic composition the effect of this approximation has been evaluated by repeating the electronic structure calculation with inclusion of ODD effects (see also [6]). Effective pair interactions, as defined within the Generalized Perturbation Method (GPM) [7], have been computed and have been used to evaluate the ground states of configurational order on the bcc lattice in the Fe-V system. Furthermore, the theoretically derived energetic properties have been used to determine the phase diagram pertaining to the (metastable) bcc lattice with the Cluster Variation Method (CVM) [8] in the tetrahedron approximation

The contributions of the atomic and spin disorder to the resistivity of two intermetallic Co1−x Ptx (x=0.5 and 0.7) compounds have been previously determined from experimental studies and separated from the phonon contribution. Now, the atomic and magnetic contributions and their variation with temperature through the Curie and order-disorder transitions are analyzed using a statistical model. The components of the resistivity are described as sums of resistivities of individual tetrahedral clusters. The cluster probabilities are calculated in a magnetic (up and down spin model) CVM approximation, the parameters of which are those that reproduce the experimental Co-Pt phase diagram. The contribution of chemically (magnetically) ordered clusters to the resistivity due to the chemical (magnetic) disorder are taken equal to zero. It is the temperature dependence of all other cluster probabilities which determine the resistivity behavior. Although there are severe approximations in both the experimental analysis and the statistical model, the agreement between theory and experiment is very satisfactory, underscoring the strong interplay between chemical and magnetic order in such ferromagnetic compounds.

The free energy simulation method is employed to study segregation to Σ5 and Σ13 (001) twist grain boundaries and their free energies in ordered Ni3−xAl1+x. In the temperature range studied (300–900K), it is shown that there is almost no segregation, strong Al segregation, and weak Ni segregation to the grain boundary for the stoichiometric, Al-rich, and Ni-rich bulk compositions respectively. It is also shown that the segregation is limited to a few (002) planes around the grain boundary and its magnitude decreases with increasing temperature. For Al-rich bulk composition, it is demonstrated that segregation at low temperature substantially lowers the grain boundary free energy.

A detailed understanding of planar defects plays an important role in the search for a comprehensive description of the mechanical behaviour of metals and alloys. We present calculations for isolated stacking faults and grain boundaries using the layer Korringa-Kohn-Rostoker method including an assessment of the force theorem, which has already proven itself in evaluating defect energies for elemental close-packed metals. These ab initio total energy calculations will be supplemented by a study of the changes in bonding and local magnetic properties near a symmetric Σ5 (310) grain boundary in Fe

One of the most attractive attributes of intermetallics is their potential for alloying to achieve the desired balance of engineering properties via the creative use of in-situ composite processing. Following our initial investigation of 20 binary refractory intermetallics, selected primarily on the basis of structure type; seven systems were down selected for ternary alloying, based on a balanced consideration of room temperature toughness, high temperature creep strength, oxidation resistance and ultimate strength. These systems were Nb3Al and Cr3Si with A15 structure, CO2Nb and Cr2Nb Laves phases with C14/C15 structure, MoSi2 with C11b structure, and Nb2Al and Mo5Si3 sigma phases.

The alloying potential of each of these binary systems is evaluated with respect to solubility limits, phase relationships, melting point suppression with alloying, and potential for enhancement of creep resistance. The results show that, with alloyed intermetallics, realistic creep strength is attainable and several opportunities exist for in-situ processing of intermetallic based composite systems to improve room temperature toughness.

The influence of magnetism on atomic short range order is shown in a single crystalline FeAl19.sat% alloy in the disordered phase. The short range order has been measured in the paramagnetic state between 973 and 1573 K on D7 at ILL through neutron diffuse scattering in the [110] plane. The effective pair interaction potentials V1 , V2, V3 and V5 are calculated through an inverse cluster variation method. The results are compared to those obtained through X-ray diffuse scattering on the same alloy frozen in the ferromagnetic order state at 772 K. The effect of magnetic coupling on the atomic order is the most important on the 1st effective potential, which is smaller by about a factor 2 in the ferromagnetic phase than in the paramagnetic one.

Configurational energies have been calculated for equiatomic Fe-Cr and Fe-V alloys possessing the high temperature bcc crystalline structure, within a first principles electronic band structure approach. In agreement with experimental facts, a tendency towards order, with a B2 ordered structure of CsCI type, is found for FeV whereas phase separation characterizes FeCr. These results suggest that the nature of short range order in the high temperature bcc solid solution is not the primary driving force for describing the structural transformation from bcc to sigma which takes place in both alloys upon decreasing temperature.

Ordering of the disordered state (obtained by cold-working or by meltspinning) of L12 type Ni,AI,Fe alloys has been monitored by calorimetric, magnetic and X-ray diffraction measurements .It is found that the magnetic moment per atom of the Ni,Al,Fe alloys in the γ-disordered state is about half of that of the LI2 -γ'state. Magnetic measurements such as ours constitute a new tool for studyingthe γ→γ' transition below the Curie temperature.

Solid solution strengthening for stoichiometric and non-stoichiometric alloys is studied using a microscopic model that takes into account the micro-segregation of the alloying additions and of the host atoms to antiphase boundaries. Experimental results for four ternary intermetallic compounds, Ni-Al-Si, Ni-Al-Ti, Ni-Al-Hf and Ni-Al-V, are analyzed in the light of the theoretical model. It is found that the athermal increment of yield strength for both stoichiometric and non-stoichiometric alloys can be explained using solid solution strengthening theory provided that the local concentration at the {111} antiphase boundaries between superpartial dislocations is used.

We present the electronic density of states (DOS) in ordered and disordered Cu50Pd50 and Cu75Pd25 alloys calculated by using the linear muffin-tin orbitals (LMTO) method. For ordered alloys the results are obtained via self-consistent LMTO method. We then show how the LMTO parameters for pure Cu and pure Pd can be appropriately transferred to the ordered alloy calculation to produce results in excellent agreement with the self-consistent calculation. For disordered alloys we present the electronic DOS calculated via LMTO-recursion and LMTO-CPA methods, and study the effect of the deviation from the ideal lattice structure, due to different sizes of the Cu and the Pd atoms, on the DOS of these alloys. We also discuss how the scheme of transferring parameters from the pure components to the alloy can be used in the calculation for the disordered phase.

The occurrence of ordering in the Ni-Pt alloy system seems to violate theoretical predictions based on a tight-binding description of the electronic structure [1,2,3]. In these descriptions of transition metal alloys, ordering is predicted if the d-band is about half filled, and phase separation is predicted if the d-band is almost empty or almost full. This so-called bandfilling argument clearly fails in the case of Ni-Pt alloys, which order despite an almost filled d-band. The band filling rule is derived under the assumption that off-diagonal disorder (ODD), that is, the difference in d-bandwidths of the constituents, can be ignored. In Ni-Pt that is not the case, the d-bandwidths of the pure elements Ni and Pt are 0.32 Ry and 0.57 Ry, respectively [4]. However, it has been argued that taking ODD into account would result in an even stronger phase separation tendency in the theoretical prediction [2]. Magnetism is not a factor in equiatomic Ni-Pt alloys, and thus can not be invoked to correct the erroneous prediction. Other factors, such as spin orbit coupling in the relativistic description of Pt can change the prediction only if very large spin orbit splitting is assumed on the Pt atom [5].

Linear Muffin Tin Orbital and Linearly Augmented Plane Wave calculations of equations of state were performed for observed and hypothetical ordered structures in the NiAl-NiTi system. Total energies were parameterized in both the Connolly-Williams and ∈-G approximations, and the resulting parameters were used to calculate theoretical phase diagrams by the cluster variation method. Third nearest neighbor Al-Ti pairwise interactions are predicted to be strongly repulsive, and to be a major cause of observed B2+L21 two phase fields.

Total energy electronic structure calculations are performed for the elements and selected binary ordered compounds of the ternary system Nb-Ru-Zr. These calculations provide binding energies, atomic volumes, bulk moduli, Debye temperatures, and Grüneiesen constants for the selected structures and compounds. Volume dependent pair and many-body chemical interactions are also obtained from the total energy results which, in turn, are used to study partially ordered alloys at finite temperatures. The stability of all the binary intermetallic compounds experimentally observed at low temperatures is correctly predicted by the first-principles calculations. The solid state portion of the Nb-Ru binary phase diagram is calculated using the chemical interactions obtained from the total energy calculations, a Debye-Grüneisen model for the vibrational free energy and the cluster variation method (CVM) for the configurational entropy with a local volume relaxation scheme. The calculations reproduce the experimentally observed ordering temperature of the NbRu3 intermetallic to within 2%.

One way to calculate the coherent phase diagram of an alloy based on first principles methods is to compute the ground state total energy for various ordered configurations, from which many-body interactions can be calculated and employed in a thermodynamic model. If the Connolly and Williams method (CWM) is used to extract the many-body interactions from the calculated total energies, the resulting many-body interactions can exhibit a strong dependence on the choice of ordered configurations and multi-site clusters, and the accuracy and convergence of the CWM energy expansion is not assured. To overcome this difficulty, a successful systematic method for implementing the CWM is proposed. This approach is applied to a study of the fcc-based Ni-V and Pd-V substitutional alloys and these interaction parameters together with the cluster variation method (CVM) are used to calculate phase diagrams.

Recent studies have suggested a particular relationship between the degree of covalent bonding in TiAl and the mobility of dislocation[1,2]. Ultimately such electronic effects In ordered compounds must dictate the dislocation core structures and at the same time the dislocation mobility within a given compound. However, direct modelling of line defects Is beyond the capability of todays electronic structure techniques. Alternatively, significant steps toward extending our understanding of the flow behaviour of structural intermetallics may come through general application of empirical interatomic potential methods for calculating the structure and mobility of defects. Toward this end, we have constructed semi-empirical interatomic potentials within the embedded atom formalism for L1O and B2 type structures. These potentials have been determined by fitting to known bulk structural and elastic properties of TIAl and NiAl, using least squares procedures. Simple expressions that relate the parameters of the potentials to the bulk properties are used in the fitting procedure. Calculations of dislocation core structures and planar fault energies using these potentials are considered. The differences between the optimized bulk properties predicted from the potentials and the values for these properties are discussed in terms of non-spherical nature of the electron density distribution. Empirical methods which incorporate these effects into interatomic potentials are briefly discussed.

The electronic structures and cohesive properties of the intermetallics Ni3V, Co3V, and Fe3V in the L12 structure have been studied using the self-consistent total energy linear muffin-tin orbital method based on the local density approximation. The simple rigid-band concept appears to be adequate to explain the structural stability of these compounds. Further,the structural stability of the pseudobinary compounds (Ni,Co,Fe)3V has been investigated based on the rigid-band scheme. The correlation between the electronic concentration and the crystal structure is shown to be related to the fill-up of the bonding states.