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Elastic constants of the high-temperature phases of the shape-memory alloys NiTi and PdTi are determined by means of full-potential linearixed augmented-plane-wave total-energy calculations. Thereby, insight into the driving mechanism of the martensitic transformation of these alloys can be obtained. In the case of NiTi, the c' and c44, moduli are found to be low and to decrease with rising pressure. We Snd that the (parent) PdTi phase is metastable with respect to tetragonal deformations. From these data, we can explain the different transformation behavior of the two alloys. In the case of PdTi, the Ti-Ti bonding seems to be responsible for the formation of the martensitic phases. Experimental data, if available, are found to be in good agreement with the results of our calculations.

The equilibrium geometries, formation energies, band structures, densities of states and charge densities of ordered titanium carbide phases of composition Ti 2 C-cubic F d3m-Ti 2 C and trigonal R 3m-Ti 2 C-were calculated selfconsistently by means of the full-potential linearized augmented-plane-wave method. The trigonal phase was found to be more stable than the cubic phase by 11.6 kJ/(mole of atoms) because it enables more efficient d-d bonding between Ti d states. The cubic phase is stabilized by the relaxation of the Ti atoms next to the vacancies towards their nearest-neighbour C atoms. In agreement with experiment, the maximum of the stabilizing relaxation energy (2.8 kJ/(mole of atoms)) is found for a relaxation of 0.04 Å. The formation energies are in good agreement with the available experimental values for TiC and F d3m-Ti 2 C. Calculations were also performed for two tetragonal phases of composition Ti 2 X found experimentally for the nitride but not for the carbide. All calculated ordered Ti 2 C phases are found to be stable against segregation into TiC and metallic Ti.

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