Based On Density Functional Theory (dft) Ni <sub> 3 </ Sub> Of Al Ion Microscope Like The Theoretical Analysis

The intermetallic compounds, as a fire-new material, have attracted much interest of the researchers. Due to the high melting point and the yield strength increases with the increasing temperature, Ll₂-Ni₃Al will be a versatile high temperature structural material with great research value and application potential. In 1951, Erwin W. Muller fundamentally constructed a new type of microscope that he called a field ion microscope (FIM). It was the first technique by which individual atoms could be seen in history. In the FIM image of an ordered binary alloy, one atomic species is usually bright whereas the other is dim, or even invisible. The mechanism of FIM image has been a problem, which puzzled us for more than thirty years.In this paper, the sublimation energy of Al and Ni escaping from the (111) surface of Ni₃Al are calculated, respectively, by using generalized gradient approximation and ultrasoft pseudopotentials to describe the Ion-atom interaction. It is found that for Al and Ni, the increasing of total energies are nearly the same when escaping from the (111) surface of Ni₃Al, thus we can't conclude which species will be evaporated easily.Considering the temperature and curvature effects in sample surface, the structure of the medium-sized cluster Ni₃₆Al₁₉ is calculated by simulated annealing algorithm based on ab-initio method. It is observed that Al atoms are more likely tobe segregated on the surface of cluster Ni36Ali9 from the energy angle. Thus a theoretical support was proposed that Al would be the imaging species.In order to discuss the temperature and surface effects on the atom distribution, in the present work, we also calculate the radial distribution of Ni and Al in NiigoAUo by utilizing the pair potentials of Ni-Al obtained by Chen-Mobius lattice inversion formula and the annealing algorithm based on classical molecular dynamics. It is found that the results agree with that of Ni36Ali9 well, i.e., Al atoms are more like to appear in the surface of Nii8oAl6o cluster, which is in perfect agreement with the experiments.In conclusion, Al atoms will segregate from the needlelike field ion specimen due to surface effect of curvature and temperature effect during the process of manufacture. Accordingly, a theoretical support was brought forward which agree with the observation result of FIM image well.