  The phase stability of NiAl clusters of nanometer size was studied by using
the embedded atom model and Monte Carlo simulation techniques. For temperatures
of 500 and 1000 K and for a range of compositions below 70 at.% Al, the
equilibrium structures of the system were determined and compared with the bulk
results. We found that the bulk NiAl (B2) and Ni3Al (L12) phases were stable
phases in the nanoparticle system; however, for deviations from ideal
composition, the analysis revealed that, because of the surface effect, the
composition of the clusters was not uniform. There was a core region in which
the structure was ordered, B2 or L12, with a composition very close to the
ideal, and a chemically disordered mantle region that allocated the deviations
from ideal stoichiometries; in this way, a larger phase field appeared,
indicating trends similar to those found in experiments on nanocrystalline NiAl
powder [S.K. Pabi and B.S. Murty, Mater. Sci. Eng. A214, 146 (1996)]. For
concentrations between 37 and 51 at.% Al, an intermediate phase, similar to the
tetragonal L10 martensite, appeared.
