THE BONDING CHARGE-DENSITY OF BETA'NIAL

The (100), (110), (111) and (200) low-angle structure factors of beta-'NiAl have been accurately measured by high energy electron diffraction (HEED) and it was found that, except for the (200) reflection, these agreed closely with the values deduced by interpolation between the best pure element crystal atomic scattering (form) factors; the (200) structure factor was found to be about 2% lower than the best equivalent pure element value. The trend of these results is somewhat similar to those obtained from powder X-ray diffraction measurements made on beta-'NiAl over 25 years ago and indicates that there is no ionic component in the bonding scheme for this alloy. This was confirmed by the generation of deformation electron density distributions (DEDDS) from the structure factor data which showed that bonding is this alloy is achieved by the depletion of electrons from both the nickel and aluminum atom sites with extensive charge build-up at the mid-points between nearest-neighbor (n.n.) Ni-Al atoms ([111] directions). This shows that bonding in this intermetallic compound is essentially covalent with some metallic character and no ionic component. The DEDDs also showed a large depletion of electrons between second n.n. Al-Al atoms ([100] directions) which also occurred, but to a lesser extent second n.n. Ni-Ni atoms. This suggests that as well as the atomic size difference, there are electronic reasons why n.n. and second n.n. Al-Al bonds are not favored in this B2 alloy type. This explains why beta-'FeAl, beta-'CoAl and beta-'NiAl are usually fully ordered and do not easily tolerate Al antistructure atoms on their transition metal sublattices. The build-up of electrons between n.n. atoms along [111] and depletion between second n.n. Ni-Ni and Al-Al atoms along [100] also gives indications of the electronic origins of the omega phase (collapse of atoms along [111] directions towards vacancy or antistructure atom defects) and the elastic softening associated with shear along [011BAR] on (011), which occurs in beta-'NiAl containing more than about 53 at.% Ni.