General procedures for computing alloy phase equilibria from ab initio electronic structure calculations are reviewed and applied to the Al-Li phase diagram. Free energies were calculated by the cluster variationm.method (CVM) in the tetrahedron approximation for the fcc and bcc lattices and ordered superstructures. Input was provided by first principles FLAPW calculations. The computed phase diagram for both stable and metastable structures agrees remarkably well with the experimental one.

At stoichiometry, DO22 is the observed ground state of Al2Ti and has a C/A ratio of 2.23, but as a function of both concentration and temperature other ordered arrangements of APB's are observed. These phase transitions have sparked many studies in which the energy has been treated as that of chemical rearrangement on an fcc lattice. We have found that at the ideal C/A ratio, the Ll2 structure is lower in energy, but as the tetragonal distortion increases the DO22 energy drops below that of Ll2. The critical role played by the tetragonal distortion in the balance between Ll2 and DO22 energies precludes the use of any model based on the undistorted lattice.

The major impediment to the development of Al3Ti as a high-temperature material is its lack of ductility. The standard approach is to make alloy additions which transform the structure to Ll2. An alternate approach is to work toward the enhancement of ductility in the DO22 phase. As a first step we have calculated the twinning energy in Al3Ti.

Structural modifications resulting from changes in the sequence of stacking of close-packed layers are very common in A3B-type transition-metal alloys. In the present work, a one-dimensional model of stacking has been explored. We found that the stacking changes observed in these alloys can be interpreted in terms of interactions between a few adjacent layers.

Calculations of the surface free energy and anti-phase boundary energy as a function of low index orientations and temperature have been determined for equiatomic perfectly ordered bcc NiAl via molecular dynamics computer simulations. The simulations utilized an Embedded Atom Method calculation of the interatomic potentials and volume forces in the Ni-As alloy system. Values of about 0.95, 1.6, 1.9 and 2.0 J/m2 were found for surface energies of the {100}, {110}, {112} and {111} orientations:, respectively. APB energies of about 0.24 and 0.38 J/m2 were determined for {110} and {112} boundaries, respectively. In addition, we have examined the phase stability and relative energies of the ordered bcc, fcc and bct phases at low temperature, and find a bct phase with c/a = 1.32 slightly lower in energy than the bcc, presaging the martensitic transformation that occurs at finite temperatures in more nickel rich alloys.

Rare-earth elements including Y, Er and Sc were added to AlTi3 for stabilizing the Ll2 ordered crystal structure, as predicted by the AB3 structure map. The crystal structure and phase composition in the AlTi3 alloys were studied by electron microprobe analysis, X-ray diffraction and TEM. The solubility limit of the rare-earth elements were determined and correlated with the atomic size factor. The results obtained so far indicate that rare-earth additions are unable to change the crystal structure of AlTi3 from DO19 to Ll2. The inability to stabilize the Ll2 structure demonstrates the need to characterize the structure map domains with a further period-dependent parameter.

Alloy compositions around Ti2 AINb were studied to establish phase equilibria and transformations during cooling from 1100°C and 1400°C. In addition to general results obtained on a wide range of compositions, which include evidence for a broad B2 phase field, transformation of BCC Ti4 Al3 Nb to a phase with an omega-type structure is reported. Detailed analysis indicates that this phase has the B82 structure after annealing at 700°C.

Alloying transition metal trialuminides such as Al3Nb with appropriate ternary elements may stabilize the ordered cubic Ll2 structure relative to the DO22 structure. This may enhance the symmetry related contributions to plastic flow, thereby aiding in overcoming the low ductility in the trialuminide systems. The present investigation was directed towards exploring the potential for obtaining new ternary Al3Nb-based intermetallics with the Ll2 structure. The effects of various ternary elements on the phase stability of A13Nb were studied by SEM, XRD, and EPMA. Results of the A13 Nb-X phase relationship studies are presented. Preliminary microhardness results on the ternary DO22 Al3Nb-X phases are also reported. The predictive capabilities of Pettifor's structural stability scheme are evaluated in the light of the present investigation.

Many questions still remain about the Ti-Al phase diagram, particularly for the compositions between the intermetallic compounds Ti3Al and TiAl. In an experimental study of the phase equilibria, titanium-aluminum alloys with 44, 46, 48, 50 at.% aluminum were produced by drop casting, HIP, and a double forging process method. Differential thermal analysis (DTA), optical metallography, and residual oxygen analysis were performed in order to characterize the low and high temperature phase equilibria of the alloys. The experimental results are compared with the calculated Ti-Al phase diagram which is being modeled concurrently. For the bcc, hcp and liquid phases, the Margules type of equations are used to represent the excess Gibbs energies. A maximum of six parameters are used for each of the phases. For the TiAl (Llo) and Ti3Al (D019 ) phases, the Wagner-Schottky type of equations are used to represent the Gibbs energies. All of the other phases are treated as line compounds. Values of the solution parameters were obtained by optimization using existing thermochemical and phase boundary data reported in the literature. The calculated results show that the high temperature hcp phase field is stable between approximately 34 at% Al, in equilibrium with β and α2 up to about 48 at% Al in equilibrium with γ and L and forms from the liquid by a peritectic reaction β+L-α. The experimental results obtained to date for the four alloys are consistent with the calculated equilibria which is being refined and also allows for an estimate of the metastable equilibria.

The Cluster Variation Method is used to study metastable phase equilibrium and possible ordering reactions in the Ni2AlTi⇆;NiAl dual phase alloy system. The Cluster Variation Method was also used to model the corresponding binary alloys and the results were extrapolated to the ternary system. The occurrence of a continuous ordering reaction in the Ni-Al-Ti system is proposed and possible decomposition reactions are discussed.

Studies consisting of both bulk sample and diffusion couple experiments have been conducted in order to establish an isothermal section of the Ti-Nb-Al system. Bulk samples were prepared by arc-melting and then annealed in a gettered, flowing, argon atmosphere for extended equilibration periods. The results obtained so far have established the existence of two ternary phases with the following approximate compositions, TI (Ti-18 at% Nb-34 at% Al) and T2 (Ti-ll at% Nb-44 at% Al). Moreover, several of the binary phases were found to have extensive solubilities, including the TiAl phase. The phases TiAl3 and NbAl3 form a continuous solid solution. Several heterogeneous equilibria were also determined. Determination of these phase fields was accomplished through the use of electron microprobe analysis (EMPA), x-ray diffraction, microstructural observations and backscatter electron imaging (BEI).

The possible origin of the high degree of brittleness (i.e., low ductility) of the Ni-Al-based alloys in the B2 structures is investigated by means of all-electron self-consistent total energy LMTO calculations. Using a supercell approach, the energetics of the two simplest anti-phase boundaries (APB) for NiAl in the B2 structure - namely the ½<111> on {110} and ½<111> on {112} - are calculated for the first time assuming no relaxation at the interface. We find APB energies of order of 800 ergs/cm2 for both cases. Since the calculated APB energies are very high, <111> slip is hardly likely to occur - as suggested experimentally. By substituting Ni or Al with V, Cr or Mn at the APB interface plane, remarkably decreased APB energies are obtained. These first results on these simplified model systems may suggest a way to decrease the APB energy contribution to the ductility of NiAl-based alloys.

First-principles calculations of the stacking fault energies of Ni3Al, and the linear elastic constants of Ni3Al and Pt3Al are presented. The anomalous (positive) temperature dependence of flow stress in Ni3Al and its absence in Pt3Al are fully rationalized in terms of the present results and cross-slip pinning mechanism. It is found that the elastic shear anisotropy factor plays an equally (or even more) important role than the anisotropy of antiphase-phase boundary energy in determining the plastic flow behavior of L12-ordered alloys.

In alloys containing 0.24% boron, atom-probe field-ion microscopy (APFIM) revealed the presence of boron clusters in Ni-25 at. % Al and Ni-26 Al but not in Ni-24 Al. The observed boron clusters generally consisted of two to three boron atoms with a maximum size of 10 atoms. Quench rates that ranged from rapid solidification to furnace cooling had little effect on the clustering. The occurrence of the clustering coincides with a higher rate of boron strengthening as measured by an increase in the yield stress per atomic percent boron, and it also coincides with a reduced amount of boron segregation to grain boundaries. The levels of nickel and boron were highly variable on grain boundaries in rapidly solidified material and therefore no consistent indication of nickel enrichment at the grain boundaries associated with boron segregation was found. This result suggests that cosegregation of nickel with boron may not be necessary for the ductilization of Ni3Al by boron, since the rapidly solidified material is also ductilized by boron and exhibits segregation of only boron to the grain boundaries.

Twinning modes in topologically close-packed A-15 compounds are examined from a geometrical viewpoint. The crystallographically possible twinning modes with a shear less than a certain maximum are tabulated along with the percentage of shuffles involved for each of the modes. Based on some simple criteria, two of the most likely modes for deformation twinning are discussed in detail. The shuffle parameters usually employed to determine the percentage of lattice shuffles for a given mode have not been found to be useful in these structures because the primitive unit cell typically contains more than one formula unit. One has to examine the distribution of atoms in detail in order to determine the percentage of shuffles involved.

Atomic structures of grain boundaries in two L12 ordered alloys with very different ordering energies have been calculated. The interatomic forces are represented by many-body empirical potentials based on the second moment approximation to the tight-binding density of states. Grain boundaries in strongly ordered alloys show very little relaxation. This results in the presence of columns of cavities in the boundaries. On the other hand boundaries in weakly ordered alloys, and also in pure f.c.c. materials, show extensive relaxations leading to much more homogeneous structures. We suggest that these structural features may be responsible for the intrinsic brittleness of grain boundaries in compounds such as Ni3Al in contrast with boundaries in pure f.c.c. metals or ordered alloys such as Cu3Au which are only embrittled by segregation.

Single crystals of stoichiometric NiAl and NiAl+V alloys were tested in compression and tension from room temperature to 871°C to determine deformation behavior. The dislocations were predominately <100> in the plastically deformed specimens. Attempts to ductilize NiAl by the addition of vanadium are described. The lowering of the anti-phase boundary energy by vanadium addition to NiAl, believed to promote the formation of <111> dislocations, was predicted by the all electron self consistent total electron band structure calculations. The vanadium additions caused considerable solid solution strengthening in NiAl, rendering the ternary alloys more brittle than stoichiometric NiAl.

The microsegregation to (111) anti-phase boundaries of the constituents and of the ternary atoms in monolithic γ′ phase is modeled using the tetrahedron approximation of the Cluster Variation Method. Similar to the binary case, alloy stoichiometry and temperature play an important role in determining the type and degree of atomic segregation to anti-phase boundaries. For a given composition, the segregation phenomenon is also found to be dependent on the site preference of the ternary additions. The results of our calculations are also used in order to assess the contribution of anti-phase boundary dislocation dragging to the strength of γ′ intermetallics.

Information on APB energy in ordered alloys is important not only to understand their mechanical properties but also to analyse conditions for phase transformation. APB surface energies are determined from dissociation distances of superdislocations both in TiAl and Al3Ti Medium and high APB energy anisotropies are found in TiAl and Al3Ti respectively. These results are consistent with phase diagram studies which predict that the APB energy vanishes in the (001) plane near the Al3Ti composition. Examples of decompositions of perfect dislocations into perfect dislocations are discussed.

High resolution electron microscopy has been used to study the core structure of a/2[101] and a/3<112> dislocations in Ni3Al deformed in the range of increasing strength with temperature. a/3<112> coupled SISFs were found to lie on (111) and their structure agreed well with theoretical predictions. a/2[101] superpartials were always dissociated on (111) or (111) planes while the APB plane was found to be (010). Computer simulation of dislocation core structures were found to agree well with the observed dissociations. The APB width was found to increase significantly with increasing deformation temperature near the peak yield strength temperature.

Various modeling techniques have been used to study the structure of grain boundaries in ordered compounds, particularly Ni3Al. The techniques include computer simulation, group theoretical analysis and the cluster variation method for high temperature effects. A multiplicity of possible grain boundary structures was analyzed for different misorientations and grain boundary plane location. 1The influence of several alloy properties like atomic size differences and deviation from stochiometry on the occurrence of these different structures is analyzed. The implications of these results for grain boundary misorientation distribution, grain boundary plane location and faceting behavior are discussed and compared to experimental findings. The possibility of orderdisorder transitions in the grain boundary region is also discussed.