ORIGIN OF ORDERING IN B2-TYPE TRANSITION-METAL ALUMINIDES - COMPARATIVE-STUDY OF THE DEFECT PROPERTIES OF PDAL, NIAL, AND FEAL

The physical mechanism for ordering and the properties of lattice defects in B2-type transition-metal (TM) aluminides are presented. It is shown that the size effect is inadequate in explaining the defect structure (and ordering) in these intermetallics. Such is the case evident in the prototypical example of PdAl, which is found to exhibit all the physical characteristics of strongly ordered alloys even though the difference in atomic radii between constituent Pd and Al atoms is very small. We obtain a high heat of formation, high antiphase boundary energies,associated with the partial 1/2(111) slip, and the existence of triple defect (for the point defect structure) in PdAl. Comparative analyses are presented for PdAl, NiAl, and FeAl, and results reveal that the electronic structure at the TM sites plays a decisive role in the energetics (and types) of defects. The strong ordering in late TM aluminides of the B2 type is due to the lack of electronic screening from the TM d band to compensate the energetically unfavorable nearest-neighbor interaction between Al atoms when ordering is disrupted.