A3B Intermetallics with L1 2-, DO3-, DO19-Structures

The increasing importance of intermetallic compounds of the type A3B was pointed out in the original proposal. Especially, Fe3AI, MAI, and Ti3AI draw more and more attention from industry due to their many attractive properties, like low density, good wear resistance, excellent oxidation and corrosion resistance, and others. Many of the properties of intermetallics are determined by the types and concentrations of point defects that are present in the crystal lattice and their corresponding distribution over the different sublattices. Although different experimental and theoretical methods are available to determine defect concentrations in such compounds, each of them has its shortcomings and may break down under certain circumstances. It has been shown by the applicant that the combination of accurate thermodynamic measurements with appropriate statistical- thermodynamic models can be a very powerful tool to determine the types of point defects present in a nonstoichio-metric intermetallic compound and the variation of their concentrations with composition and temperature. Furthermore, it is possible to derive values for the corresponding energies of formation of the different point defects. Therefore, in continuation of the original project where due to the imposed financial and time restrictions the investigations had to be restricted to intermetallics with 1,12-Structure, it is proposed to study now several intermetallic phases with D03- and D019-structure. In detail, the following work is proposed: a) Derivation of statistical-thermodynamic models for nonstoichiometric intermetallic phases with D03- and D019- structure; b) Experimental determination of thermodynamic activities, enthalpies of formation, and heat capacities as a function of composition and temperature in different intermetallic compounds with D03- and D019-structure; for this purpose an emf-method (based on CaF2 as solid electrolyte) will be used for D03-Fe3Al and D019-Ti3Al, an isopiestic vapor pressure method will be used for D03-Ni3Sb, and a Knudsen-cell mass spectrometric method will be used for D03-Fe3Si; c) Evaluation of the composition dependence of the thermodynamic activities in terms of the statistical- thermodynamic model equations to obtain in this way types and concentrations of point defects over the entire composition range in the different intermetallic compounds as well as the corresponding defect formation energies; d) Comparison of the obtained results with literature values obtained from theoretical considerations; e) Investigation of further interrelationships between defect concentrations and various physical properties.