Structural, electronic and mechanical properties of high-temperature ordered intermetallic compounds

Although they are superior to the widely used superalloys in many aspects, the severe brittleness at ambient temperature observed in most intermetallic compounds has so far prevented their use as high temperature structural materials. To investigate the possibility of ductilizing intermetallic compounds, first principle total energy quantum mechanical calculations were employed to study the phase stability, electronic structure and mechanical properties of several important aluminides such as TiAl{dollar}sb3{dollar} Ti{dollar}sb3{dollar}Al, NiAl and FeAl.; In general, the results obtained from the calculations are in good agreement with those available experimentally. For example, the naturally observed phase of TiAl{dollar}sb3{dollar} and Ti{dollar}sb3{dollar}Al is the one with the lowest calculated total energy; and the lowest calculated density of states (DOS) at the Fermi energy corresponds to the stable phase for the compounds.; To increase slip systems in intermetallic compounds that crystallize in low crystal symmetry to accommodate extensive plastic deformation, stabilizing the compounds into the cubic phase with high symmetry is of particular interest. Cu as an element was found to stabilize TiAl{dollar}sb3{dollar} (with tetragonal symmetry) into the cubic phase, as suggested experimentally. Moreover, the calculated charge density of the Cu-modified systems showed planar character--an effect referred to as "pseudo-tetragonal distortion".; To increase slip systems in intermetallic compounds with B2 structure that deform by {dollar}langle 001rangle{dollar} dislocations (like NiAl), activation of {dollar}langle 111rangle{dollar} slip is the most direct approach. Following this consideration, the anti-phase boundary (APB) energies for pure NiAl and for NiAl with selected ternary additions were calculated. Our calculations gave extremely high APB energies in pure NiAl which implies activation of {dollar}langle 111rangle{dollar} to be a formidable task. Significantly reduced APB energies in NiAl with ternary additions of V and Cr were obtained, which stimulated the experimental finding that Cr additions increase the tendency for {dollar}langle 111rangle{dollar} slip in NiAl.; Finally, the effect of stoichiometry on the APB energies in NiAl and FeAl were calculated. Through a simple model, the calculated stoichiometric dependence of the APB energies is in good agreement with that found by extrapolating the experimental results.