Phase stability of Ni-Al nanoparticles

The phase stability of Ni-Al clusters of nanometer size was studied by usin g the embedded atom model and Monte Carlo simulation techniques. For temper atures 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 (L1(2)) phases were stable phases in the nanoparticle system; however, for deviations fro m ideal composition, the analysis revealed that, because of the surface eff ect, the composition of the clusters was not uniform. There was a core regi on in which the structure was ordered, B2 or L1(2), with a composition very close to the ideal, and a chemically disordered mantle region that allocat ed the deviations from ideal stoichiometries; in this way, a larger phase f ield appeared, indicating trends similar to those found in experiments on n anocrystalline Ni-Al powder [S.K. Pabi and B.S. Murty, Mater. Sci. Eng. A21 4, 146 (1996)]. For concentrations between 37 and 51 at.% Al, an intermedia te phase, similar to the tetragonal L1(2) martensite, appeared.